Recent advances in flexible and wearable OLEDs for biomedical applications: a review

Abstract

Light-based therapies and diagnostics have gained prominence in medicine due to their non-invasive approach and therapeutic efficacy. Among these technologies, organic light-emitting diodes (OLEDs) are emerging as promising platforms in healthcare, offering advantages such as mechanical flexibility, lightweight construction, and efficient surface emission. Initially developed for display applications, OLEDs have been adapted for biomedical use, enabling conformal integration onto the skin or within the body. Here, we present recent advances in OLED technologies for biomedical applications, focusing on the development and optimization of OLEDs to meet the specific requirements of biomedical use. Developments in device platforms—utilizing flexible substrates and free-form architectures—are discussed, enabling the realization of wearable and implantable systems. Applications are categorized based on functional mechanisms of light-based stimulation and sensing, including photobiomodulation (PBM), photodynamic therapy (PDT), optogenetics, and biosensing. Finally, we conclude by outlining key remaining challenges in the development of OLEDs for biomedical applications.

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Wider impact

Organic light-emitting diodes (OLEDs) have evolved from conventional display technologies into versatile biomedical platforms. Significant advancements include the development of flexible and implantable OLED systems that enable various forms of light-based therapies, such as photobiomodulation, photodynamic therapy, and optogenetics. The convergence of materials science, device engineering, and healthcare has facilitated the creation of miniaturized, biocompatible systems tailored specifically to biological tissues. This field is widely important due to the rising global demand for wearable, personalized healthcare devices. OLED technologies uniquely address these demands with their inherent mechanical flexibility, ultra-thin form factors, lightweight structures, and customizable emission spectra, distinguishing them from traditional rigid and bulky light sources. Looking forward, OLED-based biomedical devices are anticipated to significantly influence the development of advanced non-invasive diagnostics and targeted therapeutic interventions. By synthesizing recent progress in OLED device design and biomedical application strategies, this review aims to provide critical insights, thereby guiding future innovations in materials and promoting system-level integration within the rapidly expanding field of bio-optoelectronics.

1. Introduction

Biomedical therapy and diagnostic techniques using light have a very long history. The ancient Egyptians used sunlight to treat diseases,¹ and the basis for modern phototherapy was first written in the 19th century by the German poet and writer Johann Wolfgang von Goethe (1749–1832).¹ After the invention of light bulbs, LEDs, and lasers, advances were made in the 20th century to treat neonatal jaundice through artificial methods,² depression,³ and retinal diseases.⁴ In addition, diagnostic technology using light began in 1895 when Wilhelm Conrad Röntgen discovered X-rays, and human body diagnosis using X-ray imaging began in 1896.⁵ In 1937, Alrick Hertzman first described pulse measurement using light, and in the 1970s, Takuo Aoyagi developed a pulse oximeter.⁶

The field of utilizing light for the treatment of various diseases and medical conditions has significant research value due to its non-invasive nature and its ability to prevent the development of resistance without the need for surgery or pharmaceutical interventions.^7–9 It is also effective in reducing inflammation, alleviating pain, and promoting tissue regeneration or combating diseases.^10–13 As optical devices are gradually improved and developed, light therapy is being studied to become simpler and easier to use beyond sunlight therapy, including lamps,^14,15 lasers,^16,17 and photodiodes.^16,18,19 However, these devices are still large and rigid, so there are spatial restrictions during treatment.

Organic light-emitting diodes (OLEDs), first demonstrated in 1987 by Dr Ching W. Tang,²⁰ were commercialized by Samsung in 2007. Since then, OLED technology has been widely adopted in applications such as televisions, mobile phones, and monitors, effectively replacing conventional liquid crystal displays (LCDs). And now, OLEDs are expanding their position beyond a simple display device to a human-friendly light therapy platform.²¹ OLEDs, which are organic-based light-emitting devices, offer significant advantages over other display technologies in terms of size and weight. These advantages stem from their unique properties, including the ability to utilize deposition methods, flexibility, self-luminosity, and surface-emitting characteristics.^22–25 These OLED devices are evolving into smaller and more human-friendly forms.

The human-friendly properties of OLEDs, enabling their attachment to the skin or insertion into the human body, stem from their flexibility and the ability to be manufactured in free-form designs on diverse substrates.²⁶ OLEDs can be produced on a variety of substrates, including PETs,^26,27 fibers,^28–31 paper,³² and others,^33,34 using a variety of methods, including vacuum evaporation deposition,^26,28 dipping,^35,36 and spin coating,³⁶ allowing experimenters to develop devices in the desired form. At this time, various achievements are required to produce wearable OLEDs for biomedical applications.²¹ This requires securing a certain level of light emission intensity and lifespan of the device (for example, 10 times more than that of a display device), securing a level of flexibility that allows attachment to the skin, and implementing an encapsulation layer that can stably block moisture and oxygen. In addition, in the case of implantable OLEDs that are inserted into the human body, a design is required that prevents risks related to operating temperature and to prevent human toxicity.³⁷

In addition, various attempts are being made to improve the performance of OLEDs or to improve their form factor and platform. This approach could further position OLEDs for biomedical applications. Examples of such studies include studies on encapsulation films with high flexibility and improved stability against water and oxygen,^38,39 and studies on fabricating mechanically robust OLEDs.⁴⁰

OLEDs can be manufactured as patch-type OLEDs (as shown in Fig. 1) which can be implemented as band aids or attached directly to the skin to implement electronic skin (e-skin) that has a display. Moreover, wearable OLEDs can be used in versatile forms, such as fabric-based clothing for external applications. Additionally, they can be designed as insertable platforms for biomedical use, including catheters for placement in blood vessels or intestines, or as specialized devices for patching into joints. These platforms can be utilized in a variety of ways for treatment and diagnosis, including photodynamic therapy (PDT) for anticancer cells and bacteria, photobiomodulation (PBM) for cell proliferation and treatment, optogenetics for neural activity modulation, and diagnostic platforms for self-health monitoring.

Fig. 1 Schematic diagram of promising fields where flexible wearable healthcare OLEDs are particularly influential.

These OLEDs have various advantages in biomedical applications compared to other light sources (e.g. QLEDs, LEDs, and LASERs). First of all, LASERs and LEDs have strong output, can provide long wavelengths in the IR range, and have a long lifespan, which are the biggest advantages of the biomedical application platform using LASERs and LEDs. However, in the case of LASERs and LEDs, they may not be suitable for wearable devices due to problems with local heat generation and uniformity caused by the array method. In addition, an array is essential for providing uniform treatment, and accordingly, an appropriate distance must be maintained to provide uniform treatment over a wide area. This causes a light output attenuation over distance. However, OLEDs can be used as a surface light source by directly attaching them to the skin.

Table 1 summarizes the key requirements for each medical application, including optical output, spectral characteristics, thermal and operational stability, and biocompatibility. In the case of PBM, sufficient treatment can be provided even at the output level of 1–10 mW cm⁻², and wavelength control can also be appropriately induced by microcavity, organic synthesis, etc. When treatment is performed at an intensity of 1–10 mW cm⁻², the treatment time is approximately 30 minutes, so the lifespan is sufficient. Additionally, OLEDs do not require arrays as a surface light source and have very high uniformity over the treatment area. In many studies, they exhibit biocompatible performance in the form of patches, fabrics, catheters, etc. Finally, it can be confirmed that the operating temperature is driven at a temperature of around 40 degrees in various studies. This confirms that OLEDs are highly suitable for the PBM field.

Table 1 Suitability of various optical equipment for biomedical applications. (◎ = exellent, ○ = good, △ = not good, × = poor)

PBM	Intensity	Wavelength control	Device lifetime	Uniformity	Biocompatibility (wearable, implantable, patchable etc.)	Driving temperature (<43 °C)
OLED	◎	○	○	◎	◎	◎
QLED	◎	◎	△	◎	○	○
LED	◎	△	◎	△	△	△
LASER	◎	△	◎	×	×	×

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PDT	Intensity	Wavelength control	Device lifetime	Uniformity	Biocompatibility (wearable, implantable, patchable etc.)	Driving temperature (<43 °C)
OLED	○	○	○	◎	◎	○
QLED	◎	◎	△	◎	○	○
LED	◎	△	◎	△	△	△
LASER	◎	△	◎	×	×	×

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Optogenetics	Intensity	Wavelength control	Device lifetime	Resolutions (pixel size)	Biocompatibility (implantable)	Driving temperature (<43 °C)
OLED	△	◎	○	◎	◎	○
QLED	○	◎	△	○	○	○
LED	◎	○	◎	○	△	△
LASER	◎	△	◎	×	×	×

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In the case of PDT, treatment is possible at an output of 5 mW cm⁻² or higher, but the stronger the output, the more ROS are formed. In that respect, the output of OLED is still insufficient. Wavelength control, device life, uniformity, and biocompatibility are the same as those explained in PBM. However, in the case of OLEDs, the temperature increases as the output increases. This is a problem that researchers studying OLEDs will have to solve in the future.

Finally, in the case of optogenetics, a very high level of optical intensity is also required. However, OLEDs, which can be implemented at the micro level, can be said to be far superior to other devices in terms of resolution in optogenetics. Nevertheless, there is still room for improvement in terms of output.

However, OLEDs still have a number of challenges to overcome in order to advance: (1) materials engineering challenges, such as implementing beyond the visible light range to UV and IR, or creating high-power light-emitting materials; (2) improving biosafety when implanted inside a human body to provide treatment; and (3) implementing a high-power, high-reliability platform for the certainty of new biomedical applications. If these challenges are solved: (1) it will be possible to provide treatment in the range beyond the visible light range (infrared, IR) that is imperceptible to humans, and to improve the internal and external photon efficiency of the device, thereby increasing the intensity of the output and improving the lifespan of the device, and the efficiency of PDT, PTT, PBM, etc. (2) It will be possible to provide treatment without toxicity and affecting the lifespan of the device even inside the human body, and to implement a freer platform by operating without connecting an external power line. (3) It will be possible to improve performance to provide treatment that is only possible with existing platforms such as LEDs and lasers.

In this review, we provide a comprehensive overview of the technological advancements in OLED devices and explore their potential for integration into diverse biomedical platforms. We also examine the future technologies that scientists may develop to further expand this field. The performance of OLED devices is evaluated based on the materials used in their construction and the effectiveness of their encapsulation films, assessing their suitability as attachable, wearable, and implantable platforms. Furthermore, we introduce various methods for controlling light sources and discuss their applications in biological fields, including photodynamic therapy (PDT), photothermal therapy (PTT), photobiomodulation (PBM), optogenetics, and health monitoring.

2. High-reliability OLEDs for biomedical applications

2.1. Development of emissive materials

OLEDs are designed as multilayer structures, each playing a critical role in charge injection, transport, and light emission. The fundamental OLED structure consists of an anode, hole injection layer (HIL), hole transport layer (HTL), emissive layer (EML), electron transport layer (ETL), electron injection layer (EIL), and cathode (Fig. 2a). When an external voltage is applied, holes are injected from the anode through the HIL and HTL, while electrons are injected from the cathode through the EIL and ETL (Fig. 2b). These charge carriers migrate towards the emissive layer, where recombination occurs, resulting in electroluminescence. Since OLEDs are composed of organic material-based multilayer structures, the selection of organic materials with appropriate energy band gaps and precise control over the thickness of each layer are essential for achieving optimal device performance.^41–45

Fig. 2 Design strategies for high-efficiency and NIR OLEDs. (a) Schematic illustration of the OLED structure. (b) Energy band diagram of the OLED. (c) Molecular structure of Ir complex-based emitters with various ligands for enhanced EQEs. (d) Horizontal dipole orientation of emitters depending on ligand structures. (e) EQE comparison of each emitter. (c)–(e) Reproduced with permission.⁵⁶ Copyright © 2014, Wiley-VCH GmbH. (f) Molecular structures of Pt(II) complexes designed to achieve NIR emission. (g) Electroluminescence spectra of NIR OLEDs based on the emitters. (h) Current density–voltage–radiance (J–V–R) characteristics of the NIR OLEDs. (f)–(h) Reproduced with permission.⁷⁰ Copyright © 2020, Wiley-VCH GmbH.

To achieve high-efficiency light emission, it is essential to ensure that recombination predominantly occurs within the EML. Uncontrolled charge transport can lead to exciton quenching and undesired exciplex formation at transport interfaces, reducing overall efficiency. The design of injection and transport layers is thus critical to confining charge carriers within the EML while maintaining a balanced charge transport.

Furthermore, efficient extraction of emitted photons from the emitter to the air is critical, as a substantial fraction of the generated light is typically confined within the multilayer OLED architecture due to optical losses such as waveguiding and total internal reflection. One strategy to enhance external quantum efficiency (EQE) is the synthesis of emitters with high horizontal dipole orientation, which improves light outcoupling efficiency and minimizes internal losses. An emitter with a high horizontal dipole ratio minimizes coupling to non-radiative modes and maximizes light extraction, thereby enhancing the EQE. In a typical bottom-emitting OLED stack, only about 18% of the photons generated inside the organic layer actually escape into the air, while roughly 22% remain confined within the substrate, 20% couple into waveguide modes, 36% are lost through surface plasmon polaritons (SPPs), and about 4% are absorbed. When the transition dipole moments (TDMs) are preferentially oriented parallel to the OLED plane (i.e., exhibiting a high horizontal dipole ratio), the overlap with SPP and waveguide modes is substantially reduced.⁴⁶ This orientation lowers the fraction of photons that become trapped at metal–organic interfaces or propagate through the device as guided modes, leading to a greater proportion of the generated light escaping into air. Consequently, by optimizing the horizontal orientation of the TDMs the device can effectively harness this photoluminescence for enhanced light outcoupling and, ultimately, improved EQE.^47–50

The challenge is that most molecules do not naturally adopt a perfectly horizontal orientation in amorphous thin films. One effective approach to achieving high horizontal dipole orientation is molecular structure engineering by incorporating additional ligands.^51–55 It has been discovered that the molecular shape and symmetry of the emitter play a crucial role. For example, heteroleptic iridium(III) complexes with two cyclometalating ligands and one ancillary ligand can achieve a more planar, disk-like shape that tends to lie flat in a host film. Kim et al. showed that by changing the ancillary ligand on Ir(ppy)₂ complexes, one can go from an isotropic orientation with Θ ≈ 0.67 for homoleptic Ir(ppy)₃, to a strongly horizontal orientation with Θ ≈ 0.78 for Ir(ppy)₂tmd, where tmd is an elongated diketonate ligand (Fig. 2c and d).⁵⁶ This demonstration resulted in an OLED based on Ir(ppy)₂tmd achieving an EQE of 32.3%, marking a significant improvement over the ∼26% EQE obtained with the less oriented Ir(ppy)₃ (Fig. 2e).

Another strategy for achieving high horizontal dipole orientation is controlled crystallization in thin films.^57–60 One notable example is a series of square-planar Pt(II) complexes that pack into a highly ordered, quasi-2D crystal structure in neat films, where the entire film functions as an oriented single crystal with molecular planes aligned parallel to the substrate. Kim et al. demonstrated this approach using platinum-based phosphors, incorporating trifluoromethyl-substituted pyrazolate ligands to induce strong Pt–Pt and π–π stacking interactions, leading to self-assembled crystalline thin films.⁶¹ This method resulted in a horizontal dipole ratio of Θ ≈ 0.93 in a vacuum-deposited film. A Pt(II) complex, [Pt(fppz)₂], in a non-doped crystalline thin-film emitter layer exhibited Θ ≈ 93% horizontal orientation and an exceptionally high photoluminescence quantum yield (PLQY) of ∼96%. OLEDs utilizing this material achieved an EQE of 38.8%.

Horizontally oriented emitters have now enabled >30% EQE in multiple colors (blue, green and red) through phosphorescence and even thermally activated delayed fluorescence (TADF) emitters.^49,62–65

While horizontal dipole orientation plays a key role in maximizing light outcoupling efficiency, the suppression of non-radiative decay pathways is equally important for improving overall device performance. Among these pathways, Dexter energy transfer and reverse energy transfer are particularly detrimental, as they contribute to exciton quenching through short-range intermolecular interactions and back-transfer of triplet excitons to non-emissive host states, respectively. These mechanisms can be effectively suppressed by employing host–guest systems, which provide spatial separation between emitter molecules and facilitate exciton confinement on the guest sites via appropriate energy level alignment.^66,67

Beyond the conventional host–guest framework, alternative strategies based on molecular design have emerged to address these quenching processes in non-doped architectures. In particular, Pt(II) emitters exhibit these molecular design characteristics due to their square-planar geometry, which promotes directional stacking and facilitates strong intermolecular interactions.^68,69 The square-planar configuration arises from the d⁸ electronic configuration of Pt(II), leading to ligand field stabilization that favors planar coordination. This geometry enables overlap of d_z² orbitals between adjacent Pt centers, giving rise to metal–metal-to-ligand charge transfer (MMLCT) states. These states are highly emissive and contribute to strong spin–orbit coupling, which facilitates efficient intersystem crossing and phosphorescent emission. These characteristics allow Pt(II) complexes to serve as efficient emissive materials without the need for a host matrix, enabling the development of high-performance OLEDs in non-doped configurations.^70–73

In addition to efficiency improvements, NIR emitters are crucial for expanding the capabilities of OLED-based biomedical devices. NIR light is particularly advantageous due to its deep tissue penetration depth, enabling therapeutic and diagnostic functionalities for various applications.⁷⁴

Despite these advantages, achieving efficient long-wavelength emission remains challenging with conventional organic materials. The challenge is the energy-gap law caused non-radiative decay rates to increase exponentially as the emission wavelength shifts towards NIR, reducing efficiency.^71,75,76 Additionally, exciton quenching occurs as NIR emitters tend to aggregate, leading to concentration quenching and a reduced PLQY.

To address these challenges, recent advances have proposed several strategies, one of which is utilizing the heavy atom effect. The introduction of heavy atoms such as Pt(II),^71,77–80 to leverage spin–orbit coupling, enables efficient triplet exciton utilization and improved radiative decay rates. Furthermore, transition metal complexes, including Ir(III),^81–84 and osmium(II),^85–87 have demonstrated NIR electroluminescence (EL) characteristics.

A previous study reported remarkable characteristics of Pt(II)-based emitters, achieving up to 24% EQE, attributed to strong MMLCT interactions.⁷³ These strategies are evident in the molecular structures, where Pt(II) complexes are fine-tuned for long-wavelength emission through ligand field engineering and MMLCT states. The complex's PL characteristics have a peak at 740 nm. The NIR OLED based on complex 1 demonstrates performance, with a maximum radiance around 3.6 × 10² W m⁻² sr⁻¹.

To achieve long-wavelength emission, a recent study explored heteroleptic Pt(II) complexes incorporating pyridyl pyrimidinate and functional azolate chelates (Fig. 2f).⁷⁰ These complexes exhibit photoluminescence spectra ranging from 776 to 832 nm. OLEDs fabricated using these heteroleptic Pt(II) complexes achieve a maximum EQE of 10.61% at 794 nm and 9.58% at 803 nm, respectively (Fig. 2g). Notably, these devices display a high maximum radiance nearing 10² W m⁻² sr⁻¹, accompanied by significantly suppressed efficiency roll-off at elevated current densities (Fig. 2h).

Efforts to develop NIR emitters continue to focus on achieving both higher efficiency and longer emission wavelengths, aiming to overcome the limitations of the energy-gap law. The scope of NIR OLED research has advanced beyond the conventional NIR-I window (700–900 nm), extending into the NIR-II region (beyond 1000 nm),^{71,76,80,88,89} which offers promising potential for biomedical applications requiring deeper tissue penetration.

2.2. Encapsulation requirements for OLED stability in biological environments

In biological environments, contact with bodily fluids or moisture is inevitable, so the water and oxygen barrier properties of the encapsulation layer are critical for preventing the degradation of organic electronic devices. The water vapor transmission rate (WVTR) is typically required to be below 10⁻⁶ g m⁻² day⁻¹. Additionally, the biological environment is dynamic, as it includes fluctuating factors such as body temperature, humidity, and pH levels. Thus, the development of multifunctional encapsulation layers capable of adapting to environmental changes is an important challenge.

ALD (atomic layer deposition)-based inorganic thin films are widely used in recent encapsulation development processes due to their ability to achieve precise nanoscale thickness control and form densely packed thin layers. Al₂O₃ (aluminum oxide) has been extensively studied as an inorganic layer due to its excellent moisture and oxygen barrier properties.^90–92 However, a single material or structure alone often fails to meet the diverse requirements of biological environments. To address this challenge, hybrid structures that alternate organic and inorganic layers have been introduced, enabling the simultaneous achievement of mechanical stability and chemical durability. In particular, S–H nanocomposites are utilized as the organic layer in hybrid structures, integrating with inorganic layers to enhance the durability and flexibility of the encapsulation. Additionally, materials such as polydimethylsiloxane (PDMS) and silica-based polymers can also be incorporated into these structures.^93,94 This hybrid approach allows the encapsulation layer to withstand strains exceeding 1% without mechanical failure. Furthermore, when designed as a multilayer structure, it can achieve the WVTR of 10⁻⁶ g m⁻² day⁻¹ required for OLED applications (Table 2). Fig. 3a presents a study demonstrating the use of hybrid structures to achieve excellent water vapor barrier properties and waterproofing for PSC and OLED devices on flexible substrates such as textiles. While Al₂O₃ thin films provide superior moisture and oxygen barrier properties, they are susceptible to phase transformation into boehmite (AlOOH) under high-humidity conditions, which leads to increased surface roughness and degradation of barrier performance. To mitigate this issue, integrating Si-based polymers to form Si–O–Al structures can effectively suppress the phase transformation of Al₂O₃, maintain surface smoothness, and ensure long-term moisture barrier performance (Fig. 3b and c). In addition to Al₂O₃ and TiO₂,^95,96 other materials such as HfO₂⁹⁷ are also commonly deposited using ALD for encapsulation applications.

Table 2 Encapsulation types and their corresponding WVTR characteristics

Structure/material	Total thickness	Bending strain (radius)	WVTR [g m^−2 day^−1]	Deposition technique	Remark	Ref.
Al2O3/ZnO nanolaminate + SiO2–polymer composite	1.3 μm (3 dyads)	0.25% (3 mm)	1.18 × 10^−5	ALD + spin coating	20 times washable (30 days)	90
Al2O3/TiO2 nanolaminate + S–H composite	380 nm (1 dyad)	1.7% (1.5 mm)	≤×10^−5	ALD + spin coating	Immersion in water for 1440 min	96
Al2O3/TiO2 nanolaminate + pV3D3	115 nm (1 dyad)	1.7% (1.5 mm)	9.94 × 10^−6	ALD + iCVD	Fabrication temperature 40 °C	95
ZnO/Al2O3/MgO nanolaminate + S–H nanocomposite	240 nm (1.5 dyads)	1.25%	2.44 × 10^−6	ALD + spin coating	50 nm ZAM layer 1.92 × 10^−5	91
Al2O3/MgO nanolaminate + S–H nanocomposite	330 (2.5 dyads)	1.25% (10 mm)	1.70 × 10^−5	ALD + spin coating	2.71 × 10^−4@harsh conditions 60 °C/90%	92
Al2O3 + silamer	1770 nm (2.5 dyads)	1%	≤×10^−5	ALD + spin coating	Freestanding 2.8% elongation	303

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Fig. 3 Multifunctional encapsulation for improving OLED stability. (a) OLED and PSCs that operate without damage during and after washing with a waterproof encapsulation layer. (b) Mechanism for forming strong bonds through protonation–deprotonation reactions between Al₂O₃ and SiO₂. (c) Waterproof WVTR test according to SiO₂ content. (a)–(c) Reproduced with permission.⁹⁰ Copyright 2019, Energy and Environmental Science. (d) Illustration of flexible OLEDs and sandwich-type encapsulation stacking structures applicable to the real-world finger curvature. (e) Device driving current stability according to immersion time in PBS and cell culture medium. (d) and (e) Adapted with permission. Copyright 2020, Nature Communications. (f) Multifunctional encapsulation stacking structure that blocks UV and heat, and actual TEM image. (g) Transmittance change before and after 48 hours of storage at 60 °C and 90% humidity. (h) Comparison of UV transmittance intensity according to film type. (f)–(h) Reproduced with permission.¹⁰⁸ Copyright 2019, ACS Applied Materials and Interfaces.

The encapsulation material should exhibit biocompatibility, meaning it must not induce toxicity or cause inflammatory reactions even with prolonged contact with biological tissues. Biocompatible materials are designed to maintain stable interactions with cells and tissues while ensuring functional performance in biomedical applications and electronic devices.^98,99 In particular, parylene-C possesses a chemically stable and highly inert structure, with minimal surface charge, which effectively reduces nonspecific protein and cell adhesion.^100–103 This characteristic prevents immune and inflammatory responses, making it a highly regarded material in the biomedical field. As shown in Fig. 3d, parylene-C is frequently used as the outermost encapsulation layer for skin-adherent bioelectronic devices, forming a sandwich structure to prevent the device from affecting surrounding cells. Furthermore, when tested in phosphate-buffered saline (PBS, pH 7.2–7.4), an isotonic solution mimicking biological fluids, OLEDs encapsulated in this sandwich structure maintained their initial current performance for over 350 hours (Fig. 3e). These results demonstrate that parylene-C not only provides biocompatibility but also plays a crucial role in protecting bioelectronic devices from physiological environments, ensuring stable long-term operation.

In addition to biocompatibility, biodegradable encapsulation materials have also been extensively studied.^104–106 Biodegradable encapsulation layers are designed to degrade naturally within the body, eliminating the need for device removal following chronic treatments, thereby reducing the burden of additional surgical procedures for patients.

Furthermore, OLEDs used in biomedical application require high optical efficiency, making the optical properties of the encapsulation layer a critical design factor. To achieve this requirement, strategies incorporating nanoparticles or nanofibers to enhance light scattering effects have been explored. Studies optimizing TiO₂ nanoparticle content have demonstrated that total transmittance can be maintained at approximately 70%, while achieving a WVTR below 10⁻⁵ g m⁻² day⁻¹.¹⁰⁷ When applied to OLEDs, these enhancements resulted in a 23.78% increase in electroluminescence and a 32.31% improvement in external quantum efficiency (η_ext).

Additionally, the UV-blocking properties of the encapsulation layer play a crucial role in ensuring long-term stability in biomedical environments. UV exposure accelerates the degradation of organic layers in OLEDs and can lead to protein damage and cell apoptosis. Studies have also explored encapsulation layers that simultaneously maintain high transmittance in the visible region while effectively blocking UV radiation.¹⁰⁸ Adjusting the type and thickness of the inorganic layer can enhance thermal dissipation, while refractive index matching enables approximately 80% transmittance of visible light while blocking up to 96.82% of UV radiation in the 300–400 nm range (Fig. 3f–h). This encapsulation structure improves OLED stability, and protects biological tissues.

3. Structural innovations for biomedical applications

3.1. Platform design

The development of diverse physical platforms, including attachable devices (e-skin and patches), wearable textiles, and implantable probes, is critical to fully exploit the advantages of OLED technology in biomedical applications. To effectively address a wide range of biomedical applications—such as health sensors, neural stimulation devices, therapeutic clothing, and more—OLEDs must be fabricated into various structural forms tailored to specific needs. Attachable platforms like e-skins and patches enable intimate contact with the skin, facilitating continuous and precise health monitoring, therapy, and real-time bio-signal feedback. Wearable textile platforms provide enhanced comfort, prolonged usability, and seamless integration into daily life, making them ideal for sustained healthcare interventions. Lastly, implantable OLED probes can deliver targeted therapeutic effects directly within the body, supporting minimally invasive treatments with high precision. Accordingly, research on diverse platforms is essential for the multifunctional capabilities of OLEDs.

3.1.1. Attachable platform. OLED-based skin-attachable devices like e-skin and patches have attracted significant attention in biomedical applications due to their great mechanical properties and human-friendly design. Leveraging the ultrathin, lightweight, and flexible nature of OLED devices, these platforms can naturally conform to the complex contours and dynamic movements of the human body and skin, ensuring stable performance and user convenience even during long-term wear.^50,109,110Table 3 summarizes representative studies in which OLED devices were integrated with various substrate materials and platform types.

Table 3 Comparison of mechanical and optical properties of representative studies applying OLED devices to different platform types

Platform type	Main substrate	Flexibility or stretchability	Long-term biostability	Optical performance	Ref.
Attachable	PET	Bending radius 350 μm	Washing reliability up to 150 h	Max. CE: 79.4 cd A^−1	26
Attachable	PET	Bending radius 20 mm, strain 0.33%	Air condition up to 329 h	Max. power: 18.31 mW cm^−2	149
Attachable	PI	Bending radius 55 mm to 2 mm	—	Max. PE: 68.3 lm W^−1	117
Attachable	CPI	Bending strain 4%	—	Max. CE: ∼70 cd A^−1	118
Attachable	Parylene-C	Bending radius 0.2 mm, strain 3%	Washing reliability up to 350 h	Max. PE: ∼40 lm A^−1	102
Attachable	Parylene-C	Bending radius 1 mm	Washing reliability up to 150 h	Max. power: 15 mW cm^−2	103
Stretchable/pillar	SU8/PDMS	Stretching strain 35%	Stretching, air condition up to 50 h	Max. CE: 3 cd A^−1	143
Stretchable	SU8/PDMS	Stretching strain 140%	—	EQE ∼ 2.0%, 2500 cd m^−2	120
Stretchable/kirigami	PET/PDMS	Stretching strain 95%	1 month water, air condition up to 753 h	Max. CE: 15.7 cd A^−1, 6760 cd m^−2	144

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A critical factor determining the performance and durability of attachable platforms is the choice of substrate. Common substrates include polyethylene terephthalate (PET),^111–113 parylene,^114,115 polyimide (PI),^116–118 and polydimethylsiloxane (PDMS).^119,120 Among these, PET offers advantages in cost, transparency, and mechanical flexibility, making it suitable for large-area and mass production applications.¹²¹ Parylene exhibits excellent chemical stability, biocompatibility, and low moisture permeability, which effectively protects the OLED organic layers while also demonstrating remarkable mechanical properties.¹²² Polyimide (PI) is prized for its excellent thermal stability, mechanical strength, and chemical resistance despite its inherent coloration and higher cost. PDMS is valued for its superior flexibility, optical transparency, and biocompatibility, making it ideal for soft devices.^121,123

Among skin-attachable platforms, the e-skin fabricated by combining OLED light sources with flexible substrates has been actively researched for various applications such as healthcare and real-time bio-signal feedback systems.^124–127 In particular, the active matrix driving system that integrates TFTs (thin film transistors) with OLEDs is a critical technical approach for maximizing the performance of e-skin. TFT-based active matrices enable independent pixel control, which is essential for achieving precise light modulation in biomedical applications. For example, integrating OLEDs with TFT-based active matrix systems in e-skin applications enables the development of high-resolution, wearable displays that can conform to the human body, providing real-time visual feedback for health monitoring and interactive interfaces. Additionally, this integration facilitates the creation of multifunctional electronic skins capable of simultaneously sensing and displaying bio-signals, thereby enhancing personalized healthcare and human–machine interactions.^128–131

Materials exhibiting high mobility and mechanical flexibility such as carbon nanotubes,^131–133 MoS₂,^134,135 and organic semiconductors^136,137 are frequently employed as the backplane TFT components connected to OLEDs for constructing flexible, skin-attachable e-skins. To integrate backplane systems with OLEDs, several challenges must be addressed, including mechanical mismatch at material interfaces, fatigue caused by cyclic mechanical deformation, and degradation in contact resistance. To overcome these issues, various approaches have been proposed in previous studies, such as adoption of intrinsically flexible and high-mobility electrodes,^131–133 all-organic device designs,^136,137 and the implementation of thin-film encapsulation layers.^134,135

Wang et al. employed a carbon nanotube-based TFT to fabricate an active matrix OLED e-skin capable of bending to a curvature radius of up to 4 mm. The TFT exhibited a mobility of 20 cm² V⁻¹ s⁻¹ and a high on-current of 3.6 mA, while the corresponding OLED pixel exhibited luminance exceeding 5000 cd m⁻² at a 10 V bias and a response time of approximately 1 ms.¹³¹ Xue et al. developed an all-organic and fully conformable transparent OLED (TC-OLED) system for skin-attachable invisible displays.¹³⁶ By designing highly transparent and mechanically robust composite electrodes, they demonstrated a device with up to 85% transmittance at 400 nm and a total brightness of 10 217 cd m⁻². The device exhibited excellent optoelectronic performance with a peak current efficiency of 56.8 cd A⁻¹ and EQE of 17.1%. Furthermore, they successfully integrated the TC-OLED with a transparent organic thin-film transistor (TC-OTFT), realizing the first all-organic, all-conformable active-matrix invisible display.

Two-dimensional material-based backplane transistors, such as those based on MoS₂, have attracted significant attention due to their inherent flexibility and high electron mobility. Choi et al. fabricated a MoS₂-based TFT backplane on an ultrathin 7 μm PET substrate and integrated it with an RGB OLED to realize a flexible and thin OLED e-skin (Fig. 4a).¹³⁵ Building on prior work demonstrating that Al₂O₃ encapsulation significantly reduces interfacial trap density and contact resistance in MoS₂ TFTs, while simultaneously enhancing mechanical stability under bending,¹³⁴ this study applied a similar architecture to achieve robust OLED–TFT integration. The resulting e-skin, operating under an active matrix driving scheme, enabled precise current control of individual pixels and maintained uniform light output and stable performance even on curved surfaces and in dynamic environments. As shown in Fig. 4b, the system exhibited a fast switching speed of 2.5 ms, indicating its capability for high refresh rates and real-time signal responses. In the off state, no gate modulation was observed, and the device operated stably without leakage current. Increasing the gate voltage from 6 V to 9 V in the on state resulted in higher pixel currents and luminance, with a maximum luminance of 791 cd m⁻² achieved at a gate bias of 9 V (Fig. 4c). Furthermore, the overall structure maintained a thickness of around 7 μm and demonstrated changes within about 8% in the electrical and optical characteristics of the OLED pixels under compressive and tensile deformations.

Fig. 4 Skin-attachable platform technologies for biomedical OLEDs. (a) Photograph of the e-skin in actual attachment. (b) Switching properties under a gate bias of −10 and 10 V at fixed data biases of 4 V (red) and 10 V (blue). (c) I–V–L characteristics of the e-skin OLED at fixed gate bias of 6 V and 9 V.¹³⁵ From M. Choi, S.-R. Bae, et al. Sci. Adv., 2020, 6, eabb5898. © The Authors, some rights reserved, exclusive licensee AAAS. Distributed under a CC BY-NC 4.0 license. Reprinted with permission from AAAS. (d) Schematic illustrations of the SOLEDs fabrication process using a laser patterning process (SOLED – stretchable OLEDs). (e) Electro-optical characteristics of the red SOLEDs. (f) Normalized resistance and luminance of the red SOLED during 100 000 repeated stretching tests at 50% strain. Adapted with permission under a CC BY 4.0.¹⁴⁴ (g) Schematic diagram of the OLED patch and a photograph of its actual attachment.¹⁴⁹ Reprinted from Advanced Materials Technologies, “A Wearable Photobiomodulation Patch Using a Flexible Red-Wavelength OLED and Its In Vitro Differential Cell Proliferation Effects,” © 2023 Wiley-VCH GmbH. Reproduced with permission. (h) Current efficiency before and after transfer of the sandwich-structure transferable OLED onto various substrates. (i) Characteristics before and after bending tests for the transferable OLED transferred onto textile.²⁶ Reproduced from Jeon Y. et al., Light: Science & Applications, 2019, 8, 102. © The Author(s), licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0). https://creativecommons.org/licenses/by/4.0/.

Stretchable OLEDs (SOLEDs) are a key technology for next-generation wearable devices and personalized medical services, with various fabrication strategies being explored to maintain mechanical robustness and performance under repeated deformation.^138–140 However, maintaining stable optoelectronic performance while ensuring mechanical durability under repeated deformation remains a significant challenge.^141,142 To address this issue, recent studies have proposed innovative fabrication strategies, including stress-relief structures and laser-patterned interconnections.

Conventional SOLEDs exhibited low operational reliability, with lifetimes reported to be around 10 hours. To overcome this limitation, Lim et al. introduced a pillar structure and developed a stretchable OLED fabrication method utilizing a conventional OLED manufacturing process.¹⁴³ By incorporating pillar arrays, they designed a unique stretchable substrate that effectively reduces stress applied to the active area during stretching. The proposed substrate exhibited biaxial stretchability and remained stable under 35% strain, extending the operational half-life of SOLEDs to 50 hours. Nam et al. applied laser cutting technology to fabricate highly reliable and stretchable OLEDs.¹⁴⁴ They deposited OLEDs on a flexible and transparent PET film (12 μm) and then used CO₂ laser cutting to align and pattern the device (Fig. 4d). This method formed light-emitting islands connected to kirigami stretchable electrodes. The SOLED fabricated using this technique demonstrated a turn-on voltage of 2.25 V and achieved performances comparable to OLEDs fabricated on glass substrates (Fig. 4e). Furthermore, the device maintained stable operation under 50% strain. Additionally, the non-selective laser cutting process prevented OLED exposure to external stress, ensuring enhanced mechanical stability. The SOLED maintained stable performance even after 10⁵ cycles of repeated stretching at 50% strain, demonstrating exceptional mechanical durability for stretchable and wearable applications (Fig. 4f). Kim et al. proposed a hybrid platform consisting of a rigid island array and serpentine-shaped interconnectors on a bilayer elastomer substrate for stretchable OLED fabrication.¹²⁰ This design effectively mitigated mechanical stress, thereby reducing the strain applied to the OLEDs. By employing thermal evaporation, a conventional OLED fabrication technique, they successfully developed SOLEDs capable of withstanding 140% cyclic strain while maintaining their original performance. Even after 1000 stretching cycles, the SOLEDs retained similar optoelectronic characteristics.

In a similar vein, patch platforms, which were designed for photobiomodulation (PBM)^26,145–149 and photodynamic therapy (PDT),^113,150 have also been the subject of active research. Jeon et al. achieved the integration of a flexible OLED module, multi-layer thin-film encapsulation, a heatsink for thermal management, and an ultrathin battery module into a single laminated structure, as illustrated in Fig. 4g.¹⁴⁹ Due to this integrated design, the patch maintains a thickness of less than 1 mm and a weight of less than 1 g, allowing for natural attachment to the body without burden. The fabricated OLED patch demonstrated stable electrical and optical properties under mechanical deformation, particularly during bending and folding.

In a more advanced approach, a sandwich-structure transferable OLED with an overall thickness of less than 10 μm was developed.²⁶ The devices encapsulated an OLED between transferable barriers, which consisted of a hybrid organic/inorganic ultrathin multilayer, enabling transfer onto various substrates such as textiles and paper. Fig. 4h and i show that post-transfer, the electrical and optical properties of the OLED were maintained, and its driving characteristics remained stable even after 1000 repeated folding tests with a bending radius of 350 μm. The superior flexibility of the transferable OLED is attributed to its sandwich structure, which utilizes upper and lower transferable barriers to position the neutral axis within the OLED layer, thereby ensuring reliability for attachable applications.

The patch platform has been expanding beyond wound healing to include cancer treatment patches, hair loss treatment patches, and other biomedical applications, through various forms. Kim et al. developed a flexible near-infrared QD-OLED patch that was successfully used for hair-loss treatment.¹⁴⁷ Jeon et al. demonstrated patch-based high-power OLEDs to destroy a melanoma cancer.¹¹³

3.1.2. Wearable platforms. The textile platform, recognized as the ultimate form factor for wearable displays, provides key advantages such as thinness, flexibility, comfort, and multifunctionality, establishing itself as a major focus of recent research. Unlike conventional flat substrates such as glass or PET, textiles possess an interwoven fiber structure that enhances flexibility and minimizes mechanical stress on devices during deformation, even when compared at the same thickness (Fig. 5a).^151,152

Fig. 5 Textile platform for wearable OLEDs. (a) The difference in strain distribution under deformation between a PET substrate and a textile platform of the same thickness. (b) Textile platforms incorporating wearable OLED technology. (c) The components of the textile platform for textile-based OLEDs. (d) JVL characteristics of the textile-OLEDs fabricated on five different types of textiles, with the OLED on a glass substrate. (e) Schematic illustration of the cantilever test (left), an image of the cantilever tester (middle), and graphs showing the cantilever length and calculated bending (right). (a) and (c) Reproduced with permission.¹⁵² Copyright © 2024, The Authors, licensed under CC BY-NC-ND 4.0. (b), (d) and (e) Reproduced with permission.¹⁵³ Copyright © 2020, The Authors. Published under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

The textile platform exhibited lower surface strain than a PET substrate of similar thickness. For the 250 μm PET film, a stretch ratio of 1.05 resulted in ∼5.0% surface strain, while the 242 μm textile substrate showed ∼3.5% strain under the same condition, reflecting a 30% reduction (Fig. 5b).¹⁵³ This difference stems from the textile's woven structure, which allows localized deformation such as fiber slippage and void compression.

The fabrication of textile-based OLEDs, however, encounters challenges due to the surface irregularities and gaps between fibers in textiles, making them unsuitable for direct thermal evaporation, which is commonly used for OLED deposition. Unlike glass, textiles exhibit a high degree of surface roughness and fiber misalignment, both of which can lead to uneven film deposition, reduced device performance, and inconsistencies in electrical and optical characteristics. To resolve this issue, a planarization layer is applied to smooth the textile surface and create a suitable base for OLED fabrication.^154–157 While an adhesive buffer layer is introduced between the bare textile and the planarization layer, it ensures proper attachment by compensating for surface roughness and filling gaps. Its inherent shear deformations also reduce the transfer of mechanical stress from the textile to the OLED structure.^153,158,159 This strain relief improves the mechanical durability of textile-based OLEDs. The OLEDs maintained their luminance after 1000 bending cycles from a 10 mm to a 2 mm radius, whereas PET-based OLEDs began to show a decrease in luminance around a 9 mm radius. Despite similar thicknesses, textile substrates impose less strain and support greater bending tolerance.^152,153

Through this multilayered configuration, a finalized and functional textile platform is established for reliable OLED integration (Fig. 5c), which can be readily incorporated into existing OLED fabrication processes and exhibits electrical and optical characteristics comparable to those of OLEDs fabricated on glass substrates.^152,153 Furthermore, to assess compatibility with various textiles, OLED devices were fabricated on five types of textiles—polyester, cotton, linen, wool, and leather—without additional surface treatments. For all kinds of textile, the devices exhibited comparable current density–voltage–luminance (J–V–L) characteristics. The maximum luminance reached over the 1000 cd m⁻², and turn-on voltages remained within the range of 2.5–3.0 V, showing no significant deviation from the reference OLED fabricated on a glass substrate (Fig. 5d). The ability to maintain comparable performance to glass-based OLEDs is significant, as it demonstrates that wearable OLED technology can be adopted without substantial modifications to current manufacturing processes.

Additionally, despite the added thickness from the planarization and adhesive buffer layers, mechanical property evaluations have demonstrated that, when these layers are kept sufficiently thin, the bending rigidity remains similar to that of bare textiles. To evaluate the applicability of the textile platform, mechanical flexibility and substrate compatibility were quantitatively assessed. Mechanical characteristics of the platform were measured using standardized cantilever testing according to ASTM D:1388 (Fig. 5e). The textile platform exhibited bending rigidity and Young's modulus values comparable to those of the unmodified textile: 0.50 and 0.28 MPa for the textile platform, and 0.52 and 0.64 MPa for the unmodified textile.^160,161 These results indicate that the added planarization and strain-buffering layers did not significantly alter the inherent flexibility of the textile.

Maintaining a balance between mechanical stability and flexibility is crucial in wearable electronics, as excessive rigidity can lead to discomfort and reduced user acceptance. To assess this balance, the influence of buffer layer thickness on the mechanical response of textile-based platforms was investigated through strain distribution analysis and bending rigidity measurements. As the buffer layer thickness increased from 12 μm to 60 μm, the maximum surface strain observed on the textile platform under bending decreased from approximately 1.6% to 1.1%, as determined by digital image correlation.¹⁵²

However, cantilever test-based evaluation of bending rigidity further confirms that a thicker buffer layer significantly stiffens the textile platform. This increase in mechanical stiffness can diminish one of the key functional attributes of wearable electronics—flexibility. As the buffer layer becomes excessively thick, the platform loses its ability to deform along body contours and skin curvature, ultimately making the device feel stiff and bulky. This stiffness not only reduces comfort but also disrupts the intended functionality of the wearable system. As such, while a thicker buffer layer improves strain mitigation, it also introduces trade-offs in device compliance that are critical for wearable applications. Optimization of buffer thickness is therefore essential to simultaneously ensure mechanical robustness and maintain the soft, conformal properties required for practical deployment in wearable OLED systems.

3.1.3. Implantable platform. In cases where light irradiation is required not just for skin cells but for internal tissues such as organs, nerves, and the brain, an implantable platform becomes essential. The insufficiency of light exposure from the skin surface is due to the limited optical windows of irradiation.^162,163 These optical limitations result in significant absorption of light by surrounding tissues, making it challenging to deliver adequate light to targeted internal tissues. Therefore, implantable devices that can position light sources near the target tissues for direct irradiation are being developed.

Many studies are being conducted, starting with indirect irradiation through platforms that combine optical fibers and lasers, to implantable phototherapy using LED needles.^164,165 Breaking away from rigid platforms such as silicon-based electronics, researchers have explored the integration of flexible substrates with OLEDs as a light source due to their inherent flexibility. However, ensuring the biocompatible durability required for long-term operation under inner body conditions presented challenges.²¹

The introduction of a hybrid film structure, where organic thick films (∼3 μm) and inorganic thin-film encapsulation (∼30 nm) are alternately layered in a sandwich structure on both the top and bottom of the light source, has enabled the development of a stable and flexible implantable OLED platform (Fig. 6a).¹⁰³ Among various biocompatible organic materials (e.g., PET, PI, SU-8, and parylene-C), parylene-C has been widely utilized as the main substrate for OLED integration due to its FDA-approved biocompatibility and low modulus of elasticity (∼2.8 GPa).¹⁶⁶ There are several studies to illustrate a representative large-scale implantable OLED platform utilizing this approach. Kim et al. first reported the development of a planar-type OLED-based optogenetic stimulator using parylene-C, which was applied to surgically exposed neural surface tissues.¹¹⁵ Another notable example, as demonstrated by Sim et al., presents an implantable catheter incorporating OLED films based on the two-dyad multi-barrier structure, enabling its use within the inner body (duodenum) through a minimally invasive incision.

Fig. 6 Strategies for utilizing OLED light sources in implantable platforms. (a) Structure design of an organic–inorganic hybrid sandwich OLED, integrated into a cylindrical fiber to form a catheter device. (b) Verification of its mechanical durability under physical bending. A clear light emission image is shown even at a minimum bending radius of 1 mm. (c) Evaluation of operational stability under conditions similar to the in vivo environment. There is no difference in radiance between PBS solution (38 °C) and room temperature conditions. (a)–(c) Reproduced with permission.¹⁰³ Copyright 2023, American Association for the Advancement of Science. (d) Representative images of Si-based micro-OLED probes integrated with CMOS technology for optogenetic applications. (e) Evaluation of their spatial resolution performance. The device exhibits high-precision resolution capable of stimulating at the scale of a single neuron. (f) Histogram of multi-array (blue and orange) OLED pixel operation. (d)–(f) Reproduced from ref. 168 under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0). (g) Schematic design of a flexible thin optogenetic probe. Eight independent blue micro-OLEDs are integrated on the flexible shank tip. (h) Actual operational image of the flexible probe, demonstrating its transparency and flexibility. (i) Electrical and optical performances evaluation of this platform for different pixel sizes (10 × 10 μm² to 100 × 100 μm²). (g)–(i) Reproduced with permission.¹⁶⁹ Copyright 2024 Wiley-VCH.

The OLED film incorporating this hybrid organic–inorganic film structure meets various durability requirements essential for bio-applications.¹⁰³Fig. 6b validates the high flexibility of this film by demonstrating that it maintains identical J–V characteristics in both its original state and bending state at a minimum curvature radius of 1 mm. As shown in Fig. 6c, the long-term stability of the OLED in aqueous environments has also been demonstrated. Even under conditions simulating in vivo environments (1× PBS at room temperature) and actual physiological conditions (cell culture medium at 37 °C, 5% CO₂, and 99% relative humidity), the device exhibited no degradation for over two weeks. The combination of an organic layer with high flexibility and low moisture permeability and an inorganic TFE layer with contrasting properties enables the development of an implantable film that preserves its advantages while compensating for each material's limitations. Another critical aspect for insertion platforms is their low heat generation. From a biological perspective, it is crucial to maintain an operational temperature below 45 °C to ensure a no-thermal-injury zone. In the same study, this OLED-based platform achieves this condition by delivering sufficient optical power for therapeutic applications while maintaining a temperature lower than that of normal body temperature in rodents. In this case, after 10 minutes insertion of light irradiation followed by device removal, a 4-week observation period showed no significant signs of tissue damage, suggesting minimal mechanical stress between the device and surrounding tissues. Additionally, the OLED device, composed entirely of flexible and non-magnetic materials, minimizes physical damage to surrounding tissues post-implantation. Unlike GaN-based LEDs, it also ensures interference-free MRI imaging, producing clear diagnostic images.¹¹⁵ These mechanical and environmental stabilities emphasize the feasibility of OLEDs as an implantable platform.

Along with the previously mentioned platforms designed for investigating localized tissues, micro-OLED implantable probes targeting single-cell structures (e.g., single neurons) are also being developed. These probes are primarily utilized in optogenetics and require the ability to deliver light with sufficiently high intensity to specific single-cell regions, with high spatial and temporal precision, to achieve clear biological effects.¹⁶⁷ Focusing on this aspect, Taal et al. developed a Si-based probe integrating 1024 micro-OLEDs of two types (orange and blue) at a scale of approximately 20 μm, arranged with a 24.5 μm pitch (Fig. 6d).¹⁶⁸ By utilizing monolithic integration with CMOS technology, these micro-OLEDs can be independently driven, enabling the selective optogenetic activation of individual neurons. As shown in Fig. 6e, the device demonstrates the light intensity received by neurons at different distances when the blue OLED operates at 7 V. When a single OLED is activated, the average optical output density is 0.25 mW mm⁻², which is considered sufficient for optogenetic stimulation. In addition, in cases where higher light intensity is required, multiple OLEDs can be driven simultaneously, allowing both the stimulation volume and intensity to be increased. This capability shows the advantage of enabling stimulation under various spatial resolution conditions. Furthermore, in Fig. 6f, the histogram of multi-array OLED pixel operation demonstrates that almost all pixels turn on. This suggests the development of a platform capable of high spatial resolution and large-field-of view optogenetic stimulation.

Advancing beyond Si-based probes, recent research has focused on the flexibility of OLEDs by integrating into flexible substrates. Lee et al. developed a flexible thin optogenetic probe incorporating eight independently driven micro-OLEDs, in the range of 10–100 μm.¹⁶⁹ As shown in Fig. 6g, a schematic illustration of the flexible probe features a 260 μm-wide and 5 mm-long shank micro-patterned using SU-8 and parylene-C as the main organic substrates. This probe consists of a hybrid organic–inorganic film, similar to the structure used in large-area OLEDs. Fig. 6h presents an actual operational image of the flexible probe, demonstrating that one of the eight blue OLED pixels is independently activated at the tip. The electrical and optical performance of the platform is evaluated according to different pixel sizes (Fig. 6i). The study confirmed that across the entire OLED size range, the platform achieves a light intensity exceeding 1 mW mm⁻² in the blue wavelength range, sufficient for optogenetic stimulation. A notable observation is that micro-OLEDs demonstrate their advantages as smaller scale OLEDs exhibit nearly twice the maximum luminance range (∼2 mW mm⁻²) compared to larger scale. This enhancement is attributed to a reduction in leakage paths, which improve efficiency. Also, by finding the low Young's modulus values of the flexible probe, the study highlights its potential for minimally invasive deep-brain insertion. Finally, an extremely low water vapor transmission rate (WVTR) was observed, confirming the feasibility of chronic in vivo testing.

3.2. Optical design

Light sources used in phototherapy have a variety of wavelengths beyond the visible light range, including UV and IR, or even wider. At this time, the purpose of each wavelength (UV, visible light, NIR, IR, etc.) will vary depending on the wavelength used in biomedical applications. UV wavelengths are involved in sterilization¹⁷⁰ and vitamin synthesis,¹⁷¹ while NIR and IR are used in skin care,^172,173 cell regeneration,¹⁷⁴ and various diagnostic and imaging applications.^175,176 The visible light range is also used in various aspects such as sterilization, regeneration, beauty, and diagnosis depending on the wavelength.^177–180 That is, optical access, encompassing key parameters like wavelength, intensity, and illumination duration, is regarded as a crucial factor in optimizing the efficacy of biomedical applications. Therefore, to enable more precise and high-performance treatment and diagnosis using OLED technology in biomedical applications, it is imperative to explore and address methods for effectively controlling the wavelength of the device.

One of the easiest and most common methods for precisely controlling the wavelength of a light source is to use color filters to adjust the wavelength.^181,182 There are several types of color filters available, each leveraging distinct mechanisms to achieve selective light transmission. For instance, filters utilizing organic dyes can transmit specific colors while absorbing the remainder of the light spectrum.¹⁸² Other approaches include plasmonic nanoresonators, which exploit localized surface plasmon resonance to filter light,^183,184 and distributed Bragg reflector (DBR) filters, which rely on Bragg reflection to achieve wavelength selectivity.^185,186 This is known to be the most effective and easiest way to develop display devices. These color filters extract only a portion of the wavelength from the entire white light source and use it, or the output of the light source inevitably decreases because they absorb light of other wavelengths.¹⁸⁷ Therefore, for optical devices for medical applications, other methods are more intuitive to prepare for the decrease in light output.

Attempts to control the wavelength of OLEDs mainly start from the development of organic semiconductors. However, this method requires a process of combining hundreds of atoms to develop a light-emitting layer,^188,189 and it is necessary to analyze the orbitals and mechanisms of organics through density functional theory (DFT) calculations to analyze the light-emitting characteristics.^188,190 In addition, since the HOMO and LUMO of the organic compounds constituting each layer must be matched and the diffusion speeds of electrons and holes must be considered,^191,192 the technical and time requirements are very high.

Therefore, the easiest and most common way to control and convert the wavelength of OLEDs is to use the microcavity effect to control the wavelength of OLEDs.^193,194 Microcavity is a method to enhance the constructive interference effect of waves by using the resonance of the waves.¹⁹⁵ By employing two reflectors, a light resonator can be formed, which enhances the amplitude of emitted light, resulting in higher output intensity. This not only improves the color purity of the light source but also increases its overall efficiency. This can be used to control the wavelength of OLEDs, shift the wavelength peak, or obtain a stronger emission wavelength. At this time, the main factors for wavelength control are determined by three parameters: two-beam interference, Fabry–Pérot resonance, and light radiation. This is determined based on the equation below.

I_out(λ) = G_cav(λ)I_EML(λ) (1)

In eqn (1), I_out(λ) represents the intensity of the light output by wavelength, G_cav(λ) represents the microcavity effect, and I_EML(λ) represents the intensity by wavelength of the emission spectrum of the OLED.

G_cav(λ) = f_FP(λ) × f_TI(λ;z₀) (2)

In eqn (2), f_FP(λ) stands for Fabry–Perot resonance, and f_TI(λ) stands for two-beam interference. That is, when combining the two equations, the intensity of light at each wavelength that is output is the product of the emission intensity of the OLED element, the Fabry–Perot resonance, and the two-beam interference. At this time, in the case of Fabry–Perot resonance and two-beam interference, it is determined by the value of the refractive index for each wavelength that each material has, which follows the equation below.

In eqn (3), R₁ is the reflectance of the electrode that undergoes total reflection, and R₂ is the reflectance of the electrode of the transmitting interface. For T₂, it is the transmittance of the penetrating electrode, and Φ₁ and Φ₂ are the phase shift angles at each electrode. Also, z₀ is the distance from the emitting layer to electrode 1, n_org is the refractive index of the material, and d_org is the thickness of the constituent material.

In Fig. 7a–c, E. H. Cho et al. developed a wavelength-controllable OLED device that emits light between NIR light sources (730–800 nm) for hair growth by utilizing the microcavity effect.¹⁴⁶ In this study, a PT(II) metal mixture, which is a highly efficient light-emitting material with EL in the 700–800 nm range, was used, and a device capable of tunable wavelength was developed by adjusting the thickness of NPB used in the HTL. In addition, wavelength control using the microcavity effect has been confirmed in several studies.^196–198 At this time, in order to achieve optimal luminescence performance beyond the wavelength control aspect, the recombination layer of electrons and holes must be identical to the EML of the device.¹⁹⁹ If this is not considered, exciplexes may form in layers outside the EML, which can alter the luminescence properties of the device. Therefore, the simulated thickness in Fig. 7c simply considers the three main elements of the microcavity, but if a new term is included to match the position of the electron–hole recombination layer, it will be possible to improve not only the device wavelength control but also the luminescence performance characteristics.

Fig. 7 Optical tuning strategies. (a) Schematic diagram of wavelength control in the NIR using microcavity technology. (b) Actual experimental results of the tuning spectrum. (c) Optimal thickness design through simulations. Reproduced with permission¹⁴⁶ Copyright 2023, American Chemical Society. (d) Schematic and photograph of wavelength control technology using parallel-stacked OLED technology. (e) Wavelength control results graph of parallel-stacked OLEDs. (f) Wavelength control results graph of AC-driven parallel-stacked OLEDs. Reproduced with permission.¹¹³ Copyright 2020, American Chemical Society. Reproduced with permission.²⁸ Copyright 2024, American Chemical Society. (g) Schematic of a real-time wavelength conversion method using QD films. (h) Actual experimental result of converting wavelength using QD films. Reproduced with permission.¹⁴⁷ Copyright 2024, Elsevier B.V.

The recombination site is typically determined based on the hole and electron mobilities of the materials that make up OLEDs, allowing light to be emitted from the EML. As is well known, in organic matter, the mobility of holes is faster than that of electrons, and as a result, the thickness of the HTL generally tends to be thicker than that of the ETL. In conclusion, based on the constant and thickness values for this mobility, the desired recombination region can be implemented according to the EML layer that is intended to be the recombination layer.

There is also a method of changing the wavelength by providing light of different wavelength bands from a single pixel. An example of this is the method of stacking elements in tandem in series or in parallel to emit multiple wavelengths from a single pixel. The concept for this originated from a vertical stacking device proposed by Forrest et al. in 1996.²⁰⁰ This technology has made it possible to design OLED devices that can emit light of completely different wavelengths in a single manufacturing process. However, it requires advanced technology for the microcavity effect because it involves more complex vertical stacking of organic materials and electrodes.

Y. Jeon et al. reported parallel-stacked OLEDs that operate in parallel using tandem technology with different advantages over conventional serial connections in 2020.¹¹³ The conventional serial tandem stacking method can increase brightness and luminous efficiency by stacking multiple light-emitting layers. However, although this technology can increase luminous efficiency, the driving voltage increases as the electric field inside the device increases, which may not be suitable in terms of wearability (portable driving) and biocompatibility safety. However, this paper was manufactured to obtain an intensity of 100 mW cm⁻² at a low voltage of 8 V, which was difficult to achieve with existing single stack OLEDs and serial tandem structures, and to simultaneously emit blue and red light wavelengths. In addition, Y. Cho et al. reported fiber-based color-tunable OLEDs in 2024,²⁸ and both studies tuned the color within a single pixel as shown in Fig. 7d, and Fig. 7e shows the wavelength for the OLED developed by Y. Jeon et al. The wavelength was controlled by adjusting the thickness of the NPB, which is the HTL of the red OLED among the stacked OLEDs, and it was confirmed that two wavelengths were emitted: the blue OLED with a peak of 465 nm and the red OLED with a peak in the 620–690 nm region. It was expected to play a role as a white wearable display by displaying two wavelengths at once. However, it has a clear weakness as a white display due to the lack of wavelengths in the green light range. In addition, it may be difficult to apply it to multi-functional biomedical applications because it displays light of different wavelengths at once.

In 2015, M. Fröbel et al. introduced a device that can control the wavelength by stacking two wavelengths of blue light and yellow light in parallel and using an AC/DC operation.²⁰¹ Y. Cho et al. developed a fiber-based AC-driven OLED based on this technology.²⁸Fig. 7f shows that the wavelength of the fiber-based AC-driven tandem stacked devices which Y. Cho et al. developed. When AC driven, there is an advantage in that the intensity of OLEDs of different wavelengths can be controlled by adjusting the voltage and current of the forward and reverse basis, respectively. In addition, if these two wavelengths are implemented in one pixel, it will be possible to form a platform that targets different applications (antibacterial activity, cell proliferation, PDT, etc.) for each different wavelength at once. In addition, studies that adjust the wavelength using AC driving are gradually being reported.^202–204

In 2024, Y. W. Kim et al. reported wavelength conversion OLEDs using QDs (Fig. 7g).¹⁴⁷ This study developed a wavelength conversion device corresponding to 630–730 nm using blue OLEDs with strong luminescence, and the wavelength results are shown in Fig. 7h. At this point, they solved the limitations of existing QD-OLEDs, such as blue light leakage and difficulties with high-output devices, by using a DBR filter layer that can reflect blue light and a parallel-stacked OLED. Additionally, in 2024, Y. W. Kim et al. also implemented a pure white display using QD-OLEDs.²⁰⁵ In addition, studies on wavelength-controlled OLEDs using QDs have been reported,^206–208 and they are emerging as a method to solve the problem of OLEDs having difficulty in implementing high-output NIR or to simply control the wavelength of OLEDs. However, the low lifetime and EQE of blue-light OLEDs and the PLQY of QDs are still challenges to be solved.

4. OLED-based biomedical applications

4.1. PDT/PTT

PDT (photodynamic therapy) and PTT (photothermal therapy) are non-invasive phototherapies that destroy cells by generating reactive oxygen species (ROS) or heat through the absorption of light by photosensitizers (PS) or photothermal agents (PTA).²⁰⁹ They are primarily used in cancer treatment, skin diseases, and antibacterial therapies.^210–212Fig. 8a shows the basic mechanisms of PDT and PTT, both centered on the interaction between PSs or PTA and light sources. PDT and PTT start when PSs or photothermal agents are introduced into the body or applied to the skin surface, where they gradually accumulate selectively in cancer cells or infected areas. When a light source irradiates light that matches the specific absorption wavelength of the photosensitizer, the photosensitizer absorbs energy and transitions from the ground state to the excited state. The excited photosensitizer rapidly transitions to the triplet state, during which it either releases energy or reacts with surrounding oxygen. In the triplet state, the photosensitizer generates ROS through two pathways such as electron transfer reactions and energy transfer reactions (Fig. 8b).²¹³

Fig. 8 Mechanisms and applications of PDT and PTT. (a) Schematic illustration of the therapeutic application of the PDT and PTT. Adapted with permission under a CC BY-NC-ND 4.0.³⁰⁴ (b) Schematic diagram of the mechanism of the PDT and PTT. Adapted with permission under a CC BY-NC-ND 4.0.³⁰⁵ (c) Wearable OLED-based photodynamic therapy using high power OLEDs. (d) Wavelength-tunable OLED designed to match the absorption peak wavelength of the photosensitizer. (e) Graph showing the singlet oxygen generation rate as a function of radiant emittance. (f) Concept of a wearable OLED-based PDT platform. Adapted with permission. Copyright 2020, ACS nano.¹¹³ (g) Photodynamic therapeutic effect depending on OLED irradiation time and photosensitizer concentration. Adapted with permission under a CC BY 4.0.¹⁵⁰ (h) Comparison photodynamic therapy effect between OLED and LED for L. amazonensis. Adapted with permission under a CC BY 4.0.²²⁵

In PDT, PSs react with oxygen molecules to produce reactive oxygen species such as singlet oxygen, hydroxyl radicals, and superoxide anions. These ROS oxidize the cell's membrane lipids, proteins, and DNA, leading to apoptosis or necrosis, effectively destroying cancer cells, pathogenic bacteria, and fungi.^113,214 In contrast, PTT involves PTAs absorbing light and converting it into heat, elevating the local temperature of tumor tissues. Since tumor cells are more sensitive to high temperatures than normal cells, this results in cell damage and death. Structural changes in the cell membrane, protein damage, and DNA deformation halt tumor growth or induce cell death. Local tissue temperatures between 42 °C and 45 °C trigger apoptosis, while temperatures above 50 °C cause necrosis and direct cell destruction.²¹⁵

The light sources used to activate PSs, including lasers, LEDs, and OLEDs, are selected to emit light at wavelengths appropriate for the absorption spectrum of the PSs. Initially, lasers were used to provide focused light to localized areas, but LEDs have become commercially available for hospital use due to their suitability for treating larger areas.^113,216,217 However, lasers and LEDs, being point light sources, often suffer from low uniformity, causing localized concentration of heat or light.^218,219 To address this, recent research has focused on surface light sources like OLEDs (Fig. 8c). Wearable OLED-based light therapy devices offer treatment without spatial or temporal constraints.^148,220,221

The critical factors for effective PDT and PTT therapies are the wavelength and energy dose of the light emitted from the source. The skin penetration depth depends on the wavelength, covering the UV to IR regions. Wavelengths in the red to NIR range are necessary for sufficient penetration into the skin. Specifically, light in the 600–800 nm range can penetrate deep tissues, making it effective for cancer and deep-tissue infection treatments.²⁰⁹ When the light emission spectrum overlaps with the absorption spectrum of the PSs, the efficiency of PDT increases (Fig. 8d).^150,222 OLEDs can be designed to match the absorption spectrum of PTA or PS by adjusting the thickness of the organic layers, allowing precise control over the EL spectrum. By adjusting the OLED's emission spectrum to match the PS's absorption spectrum, the therapeutic effect can be maximized. An OLED structure and emitter capable of emission from the red to the NIR region are being developed.^146,149,223

A minimum amount of light irradiation energy is needed to effectively excite the PSs, and the amount of ROS generated depends on the magnitude of the irradiation energy. OLEDs generally have lower light intensity compared to lasers, LEDs, and QLEDs. However, by employing parallel-stacked OLEDs (PAOLEDs) or tandem OLEDs, high-output OLEDs capable of meeting PDT requirements can be developed.^113,224 The rate of singlet oxygen generation increased in proportion to the intensity and duration of the OLED output. When the photosensitizer was irradiated at an intensity of 5 mW cm⁻² for 35 minutes (10.5 J cm⁻²), the singlet oxygen generation rate increased by approximately 1.8 times compared to the reference. In contrast, when irradiated at 35 mW cm⁻² for 35 minutes (73.5 J cm⁻²), the generation rate improved by 3.8 times compared to the reference (Fig. 8e). In addition, the PDT-based wearable OLED can be fabricated into a patch-type platform, as shown in Fig. 8f, allowing it to be closely attached to the skin and generate ROS to eliminate target cells.

PDT offers localized treatment with minimal damage to normal cells and is effective against drug-resistant bacteria. Recent research using OLEDs aims to overcome the limitations of conventional light sources, enhancing treatment uniformity and portability. Continuous low-level light irradiation minimizes photothermal damage and allows for gradual tumor elimination using metronomic therapy approaches.^149,223

Cheng Lian et al. applied photodynamic therapy (PDT) using flexible OLEDs and demonstrated an antibacterial effect of over 99% against S. aureus.¹⁵⁰ Unlike conventional lasers or LEDs, they confirmed that S. aureus could be effectively inactivated even at a low irradiance level of a few mW cm⁻². OLEDs with an irradiance of 6 mW cm⁻² were used to perform light irradiation for 1, 3, and 6 hours, corresponding to radiant exposures of 18, 54, and 108 J cm⁻², respectively. In addition, under OD 0.005 conditions, the group treated with 1.25 μg mL⁻¹ MB and OLED irradiation showed bacterial survival rates below 40% after both 3 and 6 hours, while the group treated with 5 μg mL⁻¹ MB and OLED irradiation showed bacterial survival rates below 10%, confirming a high antibacterial efficacy (Fig. 8g).

Cabral et al. explored the use of OLED-based PDT for treating cutaneous leishmaniasis. In experiments with drug-resistant Leishmania amazonensis, the DMMB was used alongside 670 nm red OLED light irradiation at 7.8 J cm⁻², exhibiting effective antibacterial effects.²¹⁶ Similarly, antibacterial activity was observed against L. major and L. amazonensis using methylene blue, new methylene blue, and 1,9-dimethyl-methylene blue, under OLED light at 6.5 mW cm⁻² with a total irradiation dose of 50 J cm⁻².²²⁵ Moreover, the thin and flexible OLED light source can deliver uniform illumination while being placed close to the skin, leading to superior therapeutic effects compared to LEDs under an identical light energy dose (Fig. 8h).

In conclusion, PDT is a promising technology for treating cancer and infections, with the optimization of nanomaterials and the development of NIR light sources playing crucial roles in successful therapy. Additionally, the PDT methods using OLEDs that have been shown so far maintain a temperature of less than 40 °C at the light output required for phototherapy and are driven by a DC power supply of less than 10 V, indicating that they are highly biocompatible platforms. Although PTT is still in its early research phase, integrating its light irradiation conditions into OLED-based systems is expected to yield effective therapeutic outcomes.

4.2. Photobiomodulation (PBM)

Phototherapy, also known as PBM, is a method that uses light to optically stimulate cells to accelerate the cell regeneration process.²²⁶ This method uses light to stimulate mitochondria in cells to increase cell growth rate, or stimulate cell motility to promote wound healing and pain reduction. In 2021, Y. Kim et al. analyzed the molecular pathways and mechanisms that occur within cells when PBM occurs when light enters the cell using an OLED.²²⁷ When 610 nm wavelength light enters the mitochondria, cytochrome C oxidase (CCO), the electron transport chain complex IV in the mitochondria, is stimulated, and cellular respiration increases. As a result, the ATP production level increases,²²⁸ which can increase the cell proliferation rate. This is explained in detail in Fig. 9a.²²⁷

Fig. 9 Principles and results of various photobiomodulation effects using OLEDs. (a) Schematic diagram of the principles of CCO and cell proliferation that occur when light reaches a cell. Reproduced with permission.²²⁷ Copyright 2021 John Wiley and Sons. (b) The PBM effect was confirmed by irradiating wearable red OLEDs at 5 mW cm⁻² on cryo-wounds on mouse skin. Reproduced with permission.²⁶ Copyright 2019, The Author(s), Springer Nature. (c) A result of determining the increased cell regeneration ability by irradiating green OLEDs to hADSCs and using them to improve tissue regeneration ability with injecting hADSCs into mouse skin. Reproduced with permission.²³⁹ Copyright 2022, The Author(s), Springer Nature. (d) Photograph of a catheter-type OLED platform that can be inserted into humans. (e) Results of intravitreal phototherapy via OLED catheter inserted into the duodenum of rats demonstrate that insulin does not stimulate glucose disposal. (f) Evaluation of intrahepatic serum cholesterol levels after intravitreal phototherapy using an OLED catheter. Reproduced with permission.¹⁰³ © The Authors, some rights reserved; exclusive licensee AAAS. Distributed under a CC BY-NC 4.0 license https://creativecommons.org/licenses/by-nc/4.0/”. Reprinted with permission from AAAS.

Historically, the most common phototherapy was a dermatological treatment using NIR laser to ablate the skin and promote regeneration.^172,173,229 Although this treatment provided powerful and effective treatment, the cost of the laser treatment equipment was too expensive for the household, and it could cause side effects such as infection, erythema, pain, and burns after treatment.^230,231 This type of light therapy can provide non-invasive treatment using only light without surgical or injection-based treatments, and has now been expanded into a lifestyle-friendly PBM platform using wearable OLEDs. The lifestyle-oriented PBM platform expanded with wearable OLEDs is more affordable, has reduced side effects, and enables treatment to be provided by simply attaching and wearing it at home without visiting a hospital.

In the previous section, it was reported that cell proliferation efficiency changes depending on the wavelength. At this time, cell proliferation is affected not only by the wavelength but also by the intensity of the light source (mW cm⁻²) and the total dose of the applied light source (J cm⁻²). In 2009, Y. Y. Huang et al.²³² and in 2012, Chung et al.¹⁰ analyzed this in detail. That is, if the intensity of the light source is not sufficient for cell proliferation or the irradiation time is too short, the responsiveness to cell stimulation may be suppressed,^233–235 and this dosage is also likened to a method known as the Arndt–Schultz law.^236,237 Therefore, for cell proliferation, it is necessary to first find the appropriate intensity and dosage for the cells in vitro, and then apply them to animal experiments accordingly. Based on this, various papers have confirmed that PBM treatment using OLEDs is most effective in promoting cell proliferation when phototherapy is performed for about 30 minutes at a light output intensity of 1–10 mW cm⁻². As a result, OLEDs can be safely used in PBM because they can utilize their characteristics such as low operating temperature and low voltage.

Among them, the research results for the treatment of cryo-wounds in mice using wearable OLEDs are shown in Fig. 9b.²⁶ In a study conducted in 2019, Jeon et al. verified the cell proliferation effect and toxicity of OLEDs by proliferating normal human fibroblasts by more than 26% in vitro using autonomously transferable OLEDs with a wavelength of 629 nm. Afterwards, the platform was applied to a cryogenic wound model extracted from rat skin and irradiated with light of 670 nm wavelength. At this point, no wound regeneration occurred in the control group, whereas 21% of wounds that underwent therapy were reported to have regenerated. In addition, studies on PBM using OLEDs^{28,146,148,238} and PBM studies using QD-OLEDs^147,206 have been reported.

S. H. Lee et al. reported a study on expanding human adipose-derived stem cells (hADSCs) via green OLEDs in 2022.²³⁹ Rather than directly applying OLEDs to the wound site, this study promoted the culture of hADSCs using OLEDs and directly injected the cultured hADSCs into the skin layer requiring treatment to see the therapeutic effect (Fig. 9c). Conventional hADSCs have exhibited shortcomings in therapeutic effects due to low survival rates, immune responses, etc.^240–242 However, this study complements these shortcomings by increasing the growth factor secretion, proliferation, mobility, and adhesiveness of hADSCs through PBM using OLEDs. This study is a new method that was created by not directly applying OLEDs to wounds to see the healing acceleration effect. In addition, this study reports that CCO due to green light sources has a different mechanism from red and NIR light sources. However, based on the papers that induced the proliferation of hADSCs using red and NIR light sources,^243,244 it is thought that the proliferation effect due to CCO has a higher advantage in the red to NIR range, so analysis using a red light source is also needed to support it.

In 2023, JH Sim and colleagues designed OLEDs that could be applied to catheters with a bending radius of 1.8 mm beyond the patch form (Fig. 9d).¹⁰³ They evaluated glucose tolerance by inserting the device into the duodenum of diabetic rats. Rats with the catheter inserted into the duodenum showed a −11.25% decrease compared to the control group that did not receive PBM treatment. To summarize, Fig. 9e shows that in the case of mice that received OLED PBM, the homeostasis model assessment of IR (HOMA-IR) (an indicator of the inability of insulin to stimulate glucose processing) significantly decreased (p < 0.005), and it was restored after 4 weeks. In addition, it shows better HOMA-IR than the control group. Fig. 9f shows the collagen accumulated in the cross-section of the liver, and a significantly lower level of collagen was detected in the mice that received OLED PBM than in the control group. At this time, the higher the cholesterol level, the higher the incidence of steatohepatitis and liver fibrosis.²⁴⁵ As a result, it is known through this study that PBM in the duodenum using OLEDs is effective in reducing cholesterol levels and treating diabetes. Additionally, this study also demonstrated the stability of OLED catheters when inserted into the body of a rat. The authors experimentally confirmed that OLEDs were operated at a temperature below 35.8 °C, which is the body temperature of a rat, when operated for 10 minutes at the PBM condition of 1.33 mW cm⁻² in the paper.

In essence, OLEDs can provide real-time treatment that conventional phototherapy devices, such as lasers and diodes, could not provide, and are showing promise in wearable and implantable devices. However, there are still issues that need to be analyzed and resolved, such as electrical problems for the human body that occur when power is supplied to implantable devices, and the provision of NIR and IR light sources for more diverse applications in skin-attached treatment devices. Therefore, we need to gradually conduct electrical engineering research that can wirelessly supply power to OLEDs, and material engineering research that implements NIR and IR.

4.3. Optogenetics

Optogenetics is an advanced neuroscience approach that combines light-sensitive proteins (called opsins) with light stimulation, enabling precise control of neural signals.²⁴⁶ By allowing high-resolution neural circuit mapping and precise regulation of neuronal activity, optogenetics presents an innovative solution for addressing currently incurable neurological disorders, such as neurodegenerative diseases.^247–250 The fundamental mechanisms of optogenetics are illustrated in Fig. 10a. Genetically targeted, light-sensitive proteins embedded in the neuronal cell membrane absorb specific wavelengths of light, triggering photochemical reactions that regulate neuronal action potentials through ion transport. Generally, depending on the type of ion and its directional movement, these interactions can generate either activation or inhibition signals. As shown in Fig. 10b, the absorption spectrum of commonly used activation opsins is presented. For instance, ChrimsonR exhibits peak activation in the red spectrum (590 nm), whereas channelrhodopsin-2 (ChR2) is activated in the blue spectrum (470 nm).²⁵¹

Fig. 10 Optogenetics based on OLEDs. (a) Illustration of the fundamental mechanisms of optogenetics. Created with Biorender.com. (b) Absorption spectrum of commonly used activation opsins. Reproduced from ref. 251 under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0). (c) Application of OLED on the surface of the gracilis muscle to evaluate motor system stimulation. (d) Electromyography (EMG) measurement results from the gracilis group. Both single stimulation and 10 Hz frequency stimulation show their corresponding evoked responses. (e) Electrical potential evoked in other tissues, with the hindlimb selected as a representative sensory system. (c)–(e) Reproduced with permission from ref. 115. Copyright 2020 Proceedings of the National Academy of Sciences. (f) Photograph of the neural insertion of the Si-based micro-OLED probe. Microscope images show the multi-array micro blue OLEDs integrated on the probe's shank (25 μm scale and 24 μm pitch). (g) Power density graph of OLEDs at different wavelengths as a function of neuron distance. The required power density range for each opsin is indicated as an area. (h) Hyperlocalized resolution characteristics based on in vivo optogenetic stimulation results. The graph (left) shows spike images where single-unit cells at corresponding locations respond to sequential activation of micro-OLED cells. Additionally, a histogram matrix (right) provides a detailed analysis of each neuron's response, enabling the calculation of neuronal activity in response to OLED stimulation. (f)–(h) Reproduced from ref. 168 under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0).

Thus, when utilizing optogenetics, selecting an appropriate light source is crucial, as different opsins require specific wavelengths to perform the same function effectively. The initial light source platforms used in optogenetics were optical fibers with lasers or LEDs.^252–254 Currently, with advancements in optogenetic biological tools, one of the emerging trends is utilizing OLEDs as a light stimulator. These flexible light sources use the microcavity effect to precisely match the opsin absorption spectrum, enhancing stimulation accuracy.

Based on these findings, research has been conducted to apply OLED-based optogenetics at the single cell level. A key advantage of using this light source is that it eliminates the need for an external light source, microscope, or diffusion lenses due to its intrinsic emitter. This self-emissive property allows direct illumination on the cell surface, enabling optogenetic investigations with high spatial and temporal resolution.

For the first time, Steude et al. utilized a micro-OLED array panel, with individual pixels measuring 6 μm × 9 μm, to precisely control the movement of Chlamydomonas reinhardtii, which naturally expresses ChR2.²⁵⁵ Their study further demonstrated the applicability of OLED-based stimulation in human embryonic kidney cells, where optical stimulation successfully induced cellular electrical activity.²⁵⁶ Finally, they extended this research to neurons, successfully monitoring their electrical activity in response to light stimulation by measuring membrane potential changes.²⁵⁷

Recent trends in OLED-based single-cell optogenetics focus on validating the feasibility of different opsin & light source combinations for neuronal applications. The optical stimulation platform is also evolving through approaches such as multi-array configurations and high-efficiency OLED integration. These studies have established a basis for applications in multicellular organisms and implantable optogenetics.

To extend this research to multicellular organisms, Murawski et al. investigated the functional connectivity of neural circuits by analyzing movement responses to specific neural segment activation in Drosophila larvae, a model with a relatively simple neural structure.²⁵⁸ A recent study in this area, conducted by Ciccone et al., demonstrated the potential of using a dual pin-OLED structure to independently control two different-colored light sources at the same location, enabling the simultaneous activation and inhibition within the different neural segment. This study presents representative behavioral experiments confirming that stimulation of the anterior neural segment had a more dominant influence on movement responses compared to posterior segment stimulation.

In the case of flexible implantable stimulators, Kim et al. demonstrated the feasibility of neural stimulation by attaching OLEDs to various curved biological tissues in W-TChR2V4 transgenic rats.¹¹⁵ As shown in Fig. 10c, OLEDs were applied to the gracilis muscle in the hindlimb to induce neural stimulation. Electromyography (EMG) measurements from the corresponding muscle groups revealed immediate potential responses 4–5 ms after optical stimulation (Fig. 10d). Through this experiment, evoked electromyograms were consistently observed even under periodic stimulation at approximately 10 Hz, confirming that the voltage fluctuations and muscle contraction responses remained stable and synchronized with the optical stimulation cycle. Additionally, similar results were obtained when OLEDs were applied to various tissues, including the sciatic nerve, plantar skin, and the surface of the sensorimotor cortex (Fig. 10e). These findings establish a reliable and minimally invasive optogenetic solution for multi-fascicle neural stimulation.

For neural circuitry research, effectively delivering light through a deep brain insertion probe designed for independent single-neuron stimulation is crucial. Taal et al. developed an OLED-on-CMOS-based probe, reducing individual OLED pixels to single-neuron to enable selective stimulation of specific neurons.¹⁶⁸Fig. 10f illustrates the insertion of the probe shank into mouse neural tissue. To allow simultaneous neural signal recording during optical stimulation, a flexible microelectrode array (MEA) was co-inserted with the probe. The activation of the blue OLEDs, combined with a 25 μm pitch, provided high spatial resolution. The optical properties of the probe confirm that it provides sufficient light intensity to stimulate neurons located 100–200 μm away for each wavelength range, achieving 0.1–0.5 mW mm⁻² for blue and 5–50 μW mm⁻² for orange (Fig. 10g). As shown in Fig. 10h, the left-side graph displays electrode signals from three distinct neurons across vertical columns. As individual OLED pixels were sequentially activated in each row, the propagation of stimulation along the neuronal network was observed. Furthermore, the bottom-right graph shows spike waveform analysis identifying neuron 1 (green) and neuron 2 (blue) as interneurons, while neuron 3 (red) was classified as a pyramidal neuron. The histogram on the right confirms that specific neurons only generated spikes when particular OLED groups were activated, demonstrating the capability of single OLED stimulation to selectively activate target neurons.

In its current state, OLED-based optogenetics offers large-area stimulation that integrates with tissue flexibility, allowing uniform light delivery across multiple fascicles to facilitate localized motor response control. At the micro-scale, independent single-neuron activation enables precise neuronal tracking and control, advancing neuroscience research on functional neural connectivity. Additionally, current research trends focus on developing optogenetic micro-OLED probes as flexible and ultra-thin devices, with in vivo studies expected to confirm their biological applicability and enable highly biocompatible implantable platforms.¹⁶⁹

4.4. Health-monitoring

Detecting vital signs and blocking diseases and physical risks early is very intuitively linked to human survival rates. Among these, cardiovascular problems in particular are much easier to prevent if diagnosed early.^259,260 That is, a platform capable of continuously and real-time monitoring of biosignals is required, and the industry uses devices such as finger clips, smart watches, and smart rings to diagnose and evaluate biosignals. These are compact enough and sufficient as wearable biosignal diagnosis devices, but their size and flexibility are still large and rigid compared to simpler wearable OLED devices.²⁶¹ Wearable OLEDs are being applied to textiles,^29,30,95,262 enabling treatment and diagnosis from fabrics, etc. Among them, a study published in 2024 by Y. Cho et al.²⁸ was conducted on applying OLEDs and OPDs to textiles to measure heart rate. That is, this shows the implementation of a lighter and more human-friendly biosignal diagnosis platform using OLEDs.

The principle of diagnosing heart rate is to detect the difference in the amount of light absorbed and reflected by hemoglobin and deoxy-hemoglobin when light is irradiated to blood vessels passing through the skin (Fig. 11a). According to a paper published by H. Wang et al. in 2023,²⁶¹ there is a difference in the wavelengths absorbed by hemoglobin and deoxy-hemoglobin for each wavelength (Fig. 11b). This study measured oxygen saturation using one wavelength of near-infrared OLED, and is a study that further improved portability, which is quite different from the existing general oxygen saturation measurement method (method using two different wavelengths with different absorbance).^263,264 However, since wearable OLEDs for detecting oxygen saturation can be sufficiently miniaturized and arrayed even when using two light sources, questions still arise about using a single wavelength when considering the accuracy of oxygen saturation and mechanical errors.

Fig. 11 Research on healthcare devices using flexible and wearable OLEDs. (a) Schematic diagram of pulse oximetry and PPG with OLEDs via human skin. (b) Wavelength-dependent absorbance changes of hemoglobin and deoxy-hemoglobin. Reproduced with permission.²⁶¹ Copyright 2023, Elsevier B.V. (c) Analysis of the effect of geometric arrangement of OLEDs on blood oxygen concentration measurement. Reproduced with permission.¹⁷⁶ Copyright 2019, IEEE. (d) Photo of an oxygen saturation meter and display utilizing ultra flexible photonic skin. (e) PPG measurement and oxygen saturation results using ultra flexible photonic skin. Reproduced with permission.²⁶⁷ © The Authors, some rights reserved; exclusive licensee AAAS. Distributed under a CC BY-NC 4.0 license https://creativecommons.org/licenses/by-nc/4.0/”. Reprinted with permission from AAAS. (f) Schematic of PPG measurement and health information display device using flexible standalone array OLEDs. (g) Photo of standalone array OLED devices for implemented healthcare monitoring. Reproduced with permission.²⁶⁸ © The Authors, some rights reserved; exclusive licensee AAAS. Distributed under a CC BY-NC 4.0 license https://creativecommons.org/licenses/by-nc/4.0/”. Reprinted with permission from AAAS.

In 2019, Y. Khan et al. analyzed the accuracy and mechanical error through arrays using OLEDs and OPDs using three geometric structures (Fig. 11c).¹⁷⁶ In this study, the sensitivity of the oxygen saturation sensor with a square geometry was evaluated for the bracket geometry and circular geometry using the reference. In the red channel and NIR channel, the bracket geometry obtained a signal size that was 39.7% and 18.2% higher than the square geometry, and the circular geometry showed an improvement of 48.6% and 9.2%. It can be seen that the influence of geometric structures is very important in biosignal detection. In addition, biosignal detection systems utilizing various geometric structures have also been reported.^265–267

When detecting biosignals, it is also important to visually convey them to the user. T. Yokota et al. presented an ultra-thin e-skin skin-attachable health monitoring organic device as shown in Fig. 11d.²⁶⁷ We fabricated the device on an extremely thin 1 μm parylene film to realize a highly flexible platform that maintains device performance even under compression of 0–67%. Using this device, oxygen saturation was detected within the range of 90% and 99% (Fig. 11e). In addition, Y. Lee et al. also presented a platform capable of independent real-time heart rate monitoring (Fig. 11f and g).²⁶⁸ Both of these studies confirmed that it can be attached to the skin in the form of e-skin and can measure heart rate to indicate a health signal status to the user. However, if the bulky size and weight of the driving unit cannot be reduced to the level of a wearable device, it will be difficult to immediately replace existing smart wearable devices. In other words, research on the design of a light and flexible wearable driving circuit is necessary.

4.5. Emerging applications

The application of photomedicine using light sources is not limited to treatment, diagnosis, and optogenetics, and is expected to be utilized in a wider range of fields. Examples include studies in the past using rigid light sources to treat jaundice^269,270 and depression,^271–273 and OLEDs are also improving existing limitations with their excellent capabilities in these fields.

In 2022, S. Choi et al. laid the groundwork for treating infant jaundice using fiber-based blue-light OLEDs, and Fig. 12a shows a schematic diagram of this.²⁹ It has been reported that when jaundice occurs, the bilirubin level increases, causing brain damage or neurological problems,^274–276 so treatment for jaundice in newborns should be done early and quickly. In the case of treating existing infant jaundice using optical methods, it was performed using fluorescent lamp^277,278 and LEDs²⁷⁹ in an incubator, and among them, blue light was confirmed to be the most effective.²⁸⁰ S. Choi et al. fabricated a fiber-based blue-light OLED and presented a platform that is safe for newborn eyes and can be used in everyday life. Fig. 12b is the result of actually examining the concentration of bilirubin in a test tube using fiber-based OLEDs. At this time, it was shown that the bilirubin level decreased linearly as time increased, and the bilirubin level was normalized after about 3 hours of treatment.

Fig. 12 Emerging applications utilizing OLEDs. (a) Schematic diagram of textile-based wearable blue-light OLEDs for neonatal jaundice treatment and bilirubin decomposition via blue light irradiation. (b) Results of bilirubin decomposition using textile-based wearable blue-light OLEDs. Reproduced with permission.²⁹ Copyright 2022, The Authors, Advanced Science published by Wiley-VCH GmbH. (c) Schematic diagram of a platform design for overcoming sleep disorders using blue light OLEDs. (d) Schematic diagram of a solution using blue light OLEDs stimulated at specific times. (e) Measurement of improved NREM sleep quality when OLEDs are stimulated using a mouse model. Reproduced with permission.²⁸¹ Copyright 2023, The Authors. Advanced Science published by Wiley-VCH GmbH. (f) Schematic diagram of the OLEDs therapy box to increase the lifespan of elderly rats and photos of the actual health status of the rats. (g) Results of comparing the lifespan of mice that received OLED therapy and the control group that did not receive OLED therapy. (h) Photograph of bone density in mice that received OLED therapy and in a control group that did not receive OLED therapy. Reproduced with permission.²⁸⁶ Copyright 2024, The Authors, Published by Elsevier B.V.

Fig. 12c shows a schematic diagram of the study conducted by H. Chae and colleagues in 23 years to solve sleep disorders using OLEDs.²⁸¹ In the past, researchers studied the treatment of sleep disorders through light stimulation,^282,283 and it was found that melanopsin is affected by blue light.^284,285 Therefore, in this study, blue light OLEDs were attached to the four sides of the mouse cage for about 2 hours immediately before sleep in the mouse model to stimulate it at a level of 100 Lux (Fig. 12d). As a result, the non-rapid eye movement (NREM) sleep time of the mouse models that received light stimulation was shown to decrease by approximately 14.1% (P < 0.001, N = 8) (Fig. 12e). These results indicate that mice can obtain high-quality sleep when they receive light stimulation through OLEDs.

Research results have shown that exposure to light using OLEDs can extend the lifespan of mice.²⁸⁶ Y. Deng et al. used a 10-month-old accelerated aging mouse model (prone 8 mice) in 2024 and attached OLEDs to the mouse cage and evaluated the lifespan compared to a control group without attachment (Fig. 12f). Previous studies have shown that oxidative stress affects the expression of regulator genes (CDKN2A, CDKN2D, and TP53) and aging molecular agonists such as SIRT1 and c-FOS.^287,288 This study aimed to address this by reducing oxidative stress through optical therapy. Rats were exposed to a light source with an intensity of 20 mW cm⁻² for 5 minutes per week, and treatment continued for 4 months. As a result of comparing the survival rates of the two groups (Fig. 12g), the group treated with OLEDs reported a lifespan extension of more than 80%. In addition, the analysis of the skeletons of mice reported that OLED therapy inhibited bone degradation and resulted in higher bone density (Fig. 12h).

In some fields where diagnosis is possible using existing rigid light sources (e.g. fNIRs,²⁸⁹ bilirubin diagnosis,²⁹⁰ cancer diagnosis²⁹¹), as far as we know, no diagnostic technology (fNIRs, bilirubin diagnosis, cancer diagnosis) utilizing OLEDs has been reported. In addition, with the use of rigid light sources in mental health care such as in depression treatment^292–294 and dementia treatment,^295,296 it is expected that OLEDs, which are good light sources for everyday use, will advance research into the fields of diagnosis and treatment that were not previously listed.

Research on biomedical applications using OLEDs is now in its infancy. We reviewed the usability of OLEDs in terms of industrialization in each field. The field where this device can be most easily utilized in the industry is considered to be healthcare monitoring. In the healthcare monitoring field, biometric information can be easily detected even with very low output light. As a result, wearable healthcare monitoring OLEDs will have a long lifespan and will be able to reach many areas in terms of mass production.

The next most advantageous field for industrialization is PBM using wearable and patchable OLEDs. Currently, many studies using wearable OLEDs are being reported in the fields of skin care, burn treatment, frostbite treatment, and hair growth. Although they use high outputs of about 5 mW cm⁻², they have a lifespan of tens to hundreds of hours and have been proven to have mechanical properties such as washing tests and bending tests that are usable in real life. In addition, they are being studied to make them even more user-friendly by applying them to fabrics, bandages, etc. so that they can be used in real life without inconvenience or external cumbersomeness.

However, there are still many areas that need improvement in areas such as PDT and optoelectronics. As we discussed earlier, more research is needed on OLED emitters to improve the still insufficient emission efficiency, on implantable platforms, and on optical design. If the insufficient optical output is improved and research on platforms progresses, these will also be able to get closer to industrialization.

5. Discussion and conclusions

OLEDs have rapidly evolved from display-oriented devices into versatile biomedical platforms, owing to their intrinsic flexibility, lightweight structure, and tunable optical properties. Recent studies have demonstrated the potential of OLEDs in various biomedical contexts, including PBM, PDT, optogenetics, and biosensing, illustrating their expanding role beyond traditional display technologies.

In this review, we discussed key technological developments required for biomedical applications of OLEDs.

Improvements in OLED efficiency through novel emitter materials were discussed, with an objective of achieving high-efficiency emission across a wide spectral range, including visible and NIR regions extending beyond 1000 nm into the NIR-II spectrum. Additionally, developments in flexible encapsulation technologies were presented, aimed at enhancing OLED operational stability.

We also reviewed tailored OLED designs required for specific biomedical applications. Platform designs include attachable formats such as e-skin and therapeutic patches, wearable systems utilizing textile substrates, and implantable devices such as optical probes and insertion-based systems.

In addition, precise optical tuning of OLEDs is essential for achieving therapeutic outcomes, and methods such as microcavity structures, integration with QD films, and multi-stack OLED configurations have been explored to optimize spectral and intensity profiles.

Lastly, we summarized current research on OLED-based biomedical applications. OLED technology has demonstrated its applicability across a broad spectrum of medical applications such as wound healing, hair loss treatment, tumor therapy, PPG, and jaundice management, highlighting the versatile and promising potential of OLEDs in achieving measurable biomedical outcomes.

Looking forward, we suggest several research directions to advance OLED biomedical applications.

Integration of artificial intelligence (AI) into OLED design processes could enhance reliability and performance. Machine learning can predict critical parameters such as charge transport behavior, recombination zones, and optical-electrical trade-offs within OLED structures. Applying AI methods to organic material selection and device architecture optimization can facilitate the development of high-performance, durable OLEDs suitable for biomedical use.^297–299

Further investigation into NIR OLED applications is needed. Although NIR emitters have advanced, biomedical applications utilizing NIR OLEDs remain limited, with only a few previous studies.^{146,147,176,300} Given the potential of NIR light to penetrate deep into biological tissues, which presents opportunities for broader and more effective therapeutic applications, additional research based on NIR OLEDs is needed to demonstrate their utility. Therefore, systematic research into the integration and functional validation of NIR OLEDs in biomedical contexts is needed.

Power integration is another important area for future research. To achieve fully flexible devices, flexible and reliable power solutions, such as thin-film batteries and wireless energy transfer systems, should be developed, enabling autonomous and wearable biomedical OLED devices.^301,302

Finally, clinical translation depends on preclinical data. Comprehensive evaluations of OLED-based therapies through systematic in vitro and animal studies are necessary to gather evidence for regulatory approval and investigational new drug (IND) applications. Detailed assessments of efficacy, biocompatibility, safety, and long-term performance will support the transition to clinical trials and facilitate industry adoption of standardized OLED technologies in medical applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

This review article does not present any original experimental data. All data and findings discussed herein are derived from previously published literature, which has been properly cited throughout the manuscript.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00742a

‡ These authors contributed equally to this work.

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