Conducting gels for wearable bioelectronic devices

Electronic devices embedded in our clothes, garments, and even on our skin or inside tissue are becoming a reality with continuous advances in materials research. The main aim of these devices is to monitor vital signs and inform us if an intervention is necessary. Such sensors will collect chronic data which will be used for early diagnosis, reduce the number of hospitalizations, and aid in determining the required type and dose of medication. For example, sensors attached to the skin will record metabolite concentrations from sweat. The data acquired from these sensors over several days will give the clinician enough data to determine the correct dose of medication required. On the other hand, strain sensors attached to the skin will monitor different types of human motion, such as movement, breathing, and phonation. Such devices imitate skin characteristics such as flexibility and stretchability, hence will move as the body moves. Similar to the skin that encompasses receptors, these skin-compliant sensors can also be used to fine-control motion in prosthetics, robotics, and virtual reality, forming a component of brain–machine interfaces.¹

However, such wearable electronic devices will be a part of our lives only when we have access to functional materials. For example, skin-compatible electronics require conducting materials that are biocompatible and stretchable under a wide tensile range. However, most conducting materials are either too stiff to be bent over curved surfaces or fail when they are subject to mechanical forces that the skin experiences. The ability to conform, cover, and bend over movable and curvilinear surfaces while retaining conductivity or reversibly changing conductivity can only be attained with new materials and processing conditions. For building skin-compliant sensors, materials need to change conductivity properties in response to the stimuli (e.g., stress, strain, temperature, analytical species) while conforming to the skin. One approach to making wearable electronics is to disperse conducting particles inside inherently stretchable substrates typically made of polymers. For instance, Zhang et al. (https://doi.org/10.1039/D0TB00954G) distributed Ag nanowires inside polydimethylsiloxane (PDMS) to generate a conducting elastomer. The authors reported that the flexibility and the strain response of the Ag-embedded PDMS increased as they introduced micropillars, increasing with a decrease in the size of the microfeatures. The template laser casting method used to develop these substrates allowed easy generation of various geometries of microstructures on the stretchable substrate.

Developing new substrate materials may, however, better control the device’s conductivity properties and skin adhesion. Li et al. (https://doi.org/10.1039/D1TB01441B) deposited silver nanoparticles on electrospun, polydopamine (PDA) coated polyurethane (PU) fibers. The authors used solvent immersion to generate wave-like curls to make the elastomer fibers stretchable. The films showed reversible resistance changes to tensile strains applied at various frequencies thanks to the excellent adhesion between the conducting nanoparticles and the substrate, established by the adhesiveness of the PDA. As an alternative to PDA, Kong et al. (https://doi.org/10.1039/D0TB02460K) developed poly(acrylamide–acrylic acid) hydrogels incorporating amylopectin. The starch allowed the hydrogel to adhere well to various surfaces, including the skin, which is extremely important for the mechanical durability of any sensors relying on these materials.

A reversible resistance change to mechanical stress or strain is only possible if the interactions between the conducting component and the stretchable or flexible matrix are stable over time and reversible upon the release of the stimulus. PDA can covalently immobilize graphene oxide (GO) and simultaneously reduce it to the conducting form. Zhang et al. (https://doi.org/10.1039/D0TB02100H) prepared a physically crosslinked conductive PDA-rGO embedded polyvinyl alcohol (PVA) composite through a freezing-thawing process. The resulting network was closely associated through dynamic hydrogen (–H) bonding interactions between the amine and hydroxyl (–OH) groups of PDA-rGO and the –OH groups of PVA. These components and their interactions endowed the hydrogel with softness, operational stability, and response to force and strain. The authors assembled this hydrogel in an array to detect the 2D distribution of force or strain. The sensitivities of such materials can be further improved. Zhang et al. (https://doi.org/10.1039/D0TB01926G) generated a PVA/carbon nanotube (CNT)/graphene composite, combining the features of two independent hydrogels. The PVA/CNT hydrogel acted as a bridge to connect the PVA/graphene hydrogel islands during stretching. Borate ions and PVA formed H-bonds that could be reestablished even if mechanical disintegration occurs. The composite hydrogel exhibited a response to a broad range of strains (1–600%) and mechanical pressure up to 10 kPa with high durability and repeatability.

Instead of adding conducting fillers (e.g., Ag nanoparticles, CNT, GO) inside the stretchable matrix, the stretchable gel can be generated around conducting thin films to ensure good adhesion between the components. Graphene, for example, is the thinnest two-dimensional (2D) carbon and ideal for use in flexible sensors; however, coating graphene on flexible films may not be straightforward, and its poor dispersibility inside hydrophilic gels hinders sensor performance. Wu et al. (https://doi.org/10.1039/D1TB00082A) prepared a strain sensor via in situ polymerization of PVA/poly(acrylic acid) (PAA) between two graphene-deposited layers. The sensors could detect large and subtle human body movements, such as limb joint movements, breathing, speaking, coughing, and swallowing. The PVA/PAA gel also contained glycerol so the sensor could maintain its moist state in a cold environment (−15 °C). Moreover, the dynamic ionic coordination bonds between Fe³⁺ ions and the carboxyl groups of the PAA endowed the sensor with self-healing properties.

PVA has been a commonly used scaffold material to make conducting gels. To make pressure sensors on a flexible substrate that can conform to the skin, Xia et al. (https://doi.org/10.1039/D1TB02578C) used filter paper that is coated with PVA as the substrate. The PVA dispersed the conductive fillers, CNTs, and MXene. While CNTs in this work contain hydrophilic carboxylic functional groups on the sidewalls and ends of nanotubes, Mxenes have inherently functional groups (–OH, –F, –O) on their surface. These chemical units allow the conductors to disperse better inside PVA while enhancing the gel’s hydrophilicity and mechanical properties.

With their ability to form H-bonds with the surrounding network and large specific surface area, MXenes have been increasingly used as conducting fillers inside hydrogels as they lead to repeatable and durable sensor biointerfacing applications but also in other bio-interfaces.² Sun et al. (https://doi.org/10.1039/D1TB01769A) generated an MXene-integrated waterborne PU hydrogel that formed an interpenetrating network with polyacrylamide. Such a network led to excellent mechanical properties (e.g., tensile ratio >600%, tensile strength 639 kPa, 1000 stretching cycles, and a self-recovery rate of 93.7%). Adepu et al. (https://doi.org/10.1039/D1TB00947H) used MXene to develop piezoresistive multifunctional sensors that mimic skin. They deposited MXene on PU using dip coating, and the conducting gels showed high sensitivity and gauge factor values. The pressure and strain sensing mechanisms were associated with a reversible interconnection of the unconnected branches of PU upon applying the stimulus, which increased the number of conduction paths. When fabricated on a cellulose paper substrate, MXene was reported to detect temperature changes with a higher sensitivity than existing platinum-based sensors. Note that other carbon-based flexible temperature sensors have been reviewed recently by Chen et al. (https://doi.org/10.1039/D0TB02451A), who compared their performances (sensitivity and range of detection). While they have attractive properties for biointerfacing, MXenes can be challenging to integrate inside an elastomeric matrix due to the accumulation of the flakes. Liu et al. (https://doi.org/10.1039/D1TB01798E) overcame this problem by first dispersing negatively charged MXene flakes in a positively charged chitosan solution. Polymerizing polyacrylamide around the chitosan-separated flakes generated a hydrogel with very high stretchability (of up to 1900%) and reversible resistance response to finger, wrist, or elbow bending, as well as swallowing. Zhu et al. (https://doi.org/10.1039/D1TB02759J) reviewed other wearable sensor applications where MXenes can provide compelling advantages due to their versatile chemistry, hydrophilicity, and conductivity.

The role that conducting hydrogels play in the development of wearable devices seems very important. The applications of such conducting hydrogels in smart wearable devices have recently been reviewed by Chen et al. (https://doi.org/10.1039/D0TB02929G). They classified these materials under electronic or ionic conductors and discussed their fabrication methods and applications. Another recent and excellent review in this area by Li et al. (https://doi.org/10.1039/D1TB00523E) reports conducting hydrogels with a focus on the necessary characteristics of tissue adhesiveness for on-skin or implantable applications. While most of these materials are currently designed as physical sensors, they can be used for monitoring electrophysiological signals or detecting biochemical species. The latter topic has been under the focus of Yoon et al. (https://doi.org/10.1039/D0TB01325K) who reviewed materials and fabrication methods used in some of the flexible electrochemical sensors designed for wearable, real-time biomarker monitoring applications. Lastly, it is essential to realize that electronics even in such niche applications can be designed considering sustainability, with the aim to degrade them in green or biological media. The ongoing developments in degradable electronic components have been summarized in the review of Lee et al. (https://doi.org/10.1039/D2TB01475K) who considered in vivo systems, for which degradability properties combine with much more challenging conditions than for wearable devices.

This collection lists only some of the many recent and inspiring works published in the Journal of Materials Chemistry B on wearable electronics. I hope it can guide readers by providing a summary of these selected publications and inspire them to develop extraordinary materials which will bring us a step closer to wearable sensors and sensor-integrated prosthetics and robotics technologies.

References

1. T. Yamada, Y. Hayamizu and Y. Yamamoto, et al., A stretchable carbon nanotube strain sensor for human-motion detection, Nat. Nanotechnol., 2011, 6, 296–301,  DOI:10.1038/nnano.2011.36.
2. Abdulelah Saleh, et al., Inkjet-printed Ti₃C₂T_x MXene electrodes for multimodal cutaneous biosensing, J. Phys. Mater., 2020, 3, 044004,  DOI:10.1088/2515-7639/abb361.

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