A DMSO-modified porous organogel with breathability and degradability for wearable electronics

Abstract

Wearable electronics for real-time monitoring of the physical status of the human body have been significantly developed recently. However, the substrates of wearable electronics still suffer from the challenge, including weak mechanical properties, low breathability and weak degradation capabilities. Herein, a dimethyl sulfoxide (DMSO)-modified agar organogel (DSAO) with porous microstructures has been reported as a breathable and degradable substrate for wearable electronics. The DSAO exhibits excellent mechanical properties, where the fracture strength is as high as 34.21 MPa. In addition, DSAO exhibits excellent moisture permeability due to many tiny pores inside and can be degraded in 30 days. The devices constructed on the basis of DSAO exhibit superior pressure resolution with a pressure response of 50 mN and respond well to different pressure levels and frequencies, which demonstrates potential applications in wearable healthcare monitoring and intelligent robots.

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Introduction

Nowadays, with the growing concern for personal health, wearable devices for real-time monitoring of the body's physical condition have developed considerably.¹ Currently, common substrate materials for wearable devices are polydimethylsiloxane (PDMS),^2–4 polyethylene terephthalate (PET),^5–7 polyimide (PI),^8,9 polyaniline,^10,11 and silicone rubber.^12,13 However, these substrate materials have problems of insufficient mechanical properties and poor breathability, leading to their reduced stability during use and affecting the wearer's comfort.^14–16 Moreover, discarded substrate materials cannot be naturally degraded, which is harmful to the environment.¹⁷ Therefore, researchers have been pursuing materials that can mimic the properties of human skin to improve the wearing comfort of wearable devices, including breathability, mechanical stability and degradability.

Organogels are gels with organic liquids as the liquid phase and have been widely used in wearable devices.^18–23 Compared to hydrogels,^24–26 which are already widely used in wearable sensor devices, organogels have the unique advantage of having different functions and properties that can be obtained by choosing different organic solvents and gel compositions. For example, organogels infused with high-boiling-point polar liquids can exhibit excellent mechanical properties.^27,28 In addition, organogels with polymer skeletons have excellent degradation properties.^29,30 However, combining mechanical properties with excellent degradability remains a challenge.

Herein, we present a dimethyl sulfoxide (DMSO)-modified agar organogel (DSAO) with good mechanical properties, degradability and breathability that can be used as a substrate material for wearable electronics. DSAO is prepared by modifying agar with DMSO via heat blending. The cross-linked network structure formed by hydrogen bonding between agar and DMSO inside the organogel endows the material with excellent mechanical properties, enabling it to carry heavy loads of up to 60 kg. Moreover, there are many tiny pores inside the organogel, which greatly improves its moisture permeability. Moreover, DSAO can be degraded within 30 days. The degradation product is the raw material, agar, thus avoiding environmental pollution through degradation after device failure. Triboelectric nanogenerators (TENGs) constructed on the basis of this organogel can be used to collect low-frequency and micromechanical energy and efficiently convert it into electrical energy, realizing the dual functions of wearable sensing and energy harvesting (Fig. 1a).

Fig. 1 The applications, synthesis and structure of DSAO. (a) Schematic application of a DSAO for wearable devices. (b) Schematic of the fabrication process of DSAO and the SEM images of the DSAO section and surface. (c) Internal network structure and the Fourier transform infrared spectra of DSAO.

Results and discussion

DSAO is fabricated via the bubble gel method (details are provided in the Experimental section). From the scanning electron microscopy (SEM) image (Fig. 1b), a network structure with many pore structures is visible in the organogel. Fourier transform infrared (FTIR) spectroscopy revealed hydrogen bonding interactions between DMSO and the agar polymer chains (Fig. 1c). The peak at 3338 cm⁻¹ corresponds to the O–H stretching vibration, whereas the peak at 1388 cm⁻¹ is due to the presence of sulphate groups in the agar, and the peak at 1151 cm⁻¹ originates from ester sulphate of the finger galactose bond vibration. The O–H absorption peak of DSAO shifted from 3429 cm⁻¹ in agar to 3338 cm⁻¹ at lower wavelengths, indicating hydrogen bond formation between the agar and DMSO.³¹

DSAO has excellent mechanical properties. As shown in Fig. 2a. DSAO, with a thickness of 2 mm, a width of 3 cm, and a length of 20 cm, is capable of carrying a person with a weight of 60 kg without any damage, demonstrating its extremely high load-bearing capacity. The excellent mechanical properties are further demonstrated via tensile stress–strain tests at 70% relative humidity (RH) and 25 °C. The breaking strength of DSAO is 34.21 MPa (Fig. 2b), and the Young's modulus is 869.3 MPa. Its mechanical strength is much greater than that of other organogels reported in the literature (Table S1†). The excellent mechanical properties are further demonstrated by loading weights. Subsequently, cyclic loading tests (Fig. 2c) were carried out on the DSAO. As shown in Fig. 2c and Fig. S1,† the mechanical properties of the DSAO are essentially unchanged during seven loadings of heavy weight, even 30 loadings, indicating excellent durability.

Fig. 2 Characterization of mechanical properties, permeability and degradability of DSAO. (a) CCD load-carrying image of DSAO. The inset figure is the optical image for the DSAO production. (b) Stress–strain curve of DSAO. The inset photograph is the DSAO loading test system. (c) Stress–strain curve of DSAO under cyclic loading. (d) Fourier transform infrared absorption spectra and (e) stress–strain curves of organogels composed of different ratios of agar/DMSO. (f) Schematic diagram of the moisture permeability of DSAO. (g) Comparison of the moisture permeability of DSAO with that of conventional substrate materials. (h) CCD images of DSAO after placement in hydrochloric acid solution for 0 and 30 days. NMR spectra of DSAO (i) before and (j) after degradation.

The excellent mechanical properties result from the hydrogen-bonding interaction within the polymer network structure in DSAO.^32–34 We analysed the FTIR spectra (Fig. 2d) and mechanical properties (Fig. 2e) of DSAO composed of different ratios of agar/DMSO. At low agar contents (the ratio of agar to DMSO mass was less than 4 : 100), the fracture plateau of the DSAO obtained was not obvious and exhibited significant fragility. The FTIR absorption peak of the band at 3750–3000 cm⁻¹ is at a relatively high wavenumber, which is attributed to the fact that at this ratio, relatively few hydrogen bonds are formed by cross-linking between the agar polymer chains. After increasing the proportion of agar added, the absorption peak of DSAO at this band shifted in the short band direction, which was attributed to the stronger hydrogen bonding interactions between the agar polymer chains. When the proportion of agar was increased to 6 : 100, the hydrogen bonding interactions within the organogel reached the maximum value, i.e., the position of the O–H peak reached its lowest wavenumber, at which point the organogel had the best mechanical properties (Fig. 2e). As the proportion of agar continues to increase, the agar cannot be fully dissolved in the DMSO solution, which hinders the hydrogen bond formation between the polymer chains in the interior. This ultimately leads to a decrease in the mechanical properties of the DSAO which is more likely to be deformed or even fractured when subjected to an external stretching force.

DSAO exhibits excellent permeability as shown in Fig. 2f. As shown in Fig. 2g, the water vapor transmission rate of DSAO is higher than that of other membrane materials (PET, PI, and BOPP) with the same thickness (120 μm) and that in the literature (Table S2†). This effect may be due to the multilayered pore structure with many micron-sized channels (Fig. 2f). Water molecules on the skin can flow quickly and continuously along these channels from the organogel–skin contact surface to the surface layer.^35,36 Thus, the large surface area of the organogel allows for the rapid evaporation of water.

DSAO also presents excellent degradability. When DSAO is stirred in a hydrochloric acid solution (pH = 6.0), its mechanical structure is gradually subjected to breakage (Fig. 2h). After 30 days, the structure of DSAO was not visible, indicating that the organogel was completely degraded (Fig. S2†). In comparison, other common substrates, such as PET and PI (Fig. S3†), cannot be fully degraded. Note that, as shown in the NMR spectra (Fig. 2i and j), the peaks after degradation of the DSAO are the same as those of agar, which is a natural polymer.

To demonstrate potential applications in wearable devices, we have added graphene in DSAO to increase its conductivity³⁷ (graphene/DSAO) to construct TENG (Fig. 3a). The structure of graphene/DSAO shows no obvious change (Fig. S4†). The graphene/DSAO still shows excellent degradation performance, mechanical properties and air permeability³⁸ (Fig. S5 and S6†). In addition, the conductivity improves with the increase of graphene content (Fig. S7†). 50 mg of graphene was chosen to add due to the consideration of its mechanical properties as well as its permeability. This wearable device consists of two parts: the upper part is composed of DSAO as the upper substrate material and graphene/DSAO as the top electrode. The lower part has a sandwich structure. The upper layer is a microstructured DSAO triboelectric layer. The middle layer (Fig. S8†) is graphene/DSAO as the bottom electrode, and the lower substrate material is DSAO as well. The finite element simulation results show the potential and pressure distributions of the upper and lower layers of the device during the contact–separation processes (Fig. 3b). The simulation results indicate that the potential difference between the two electrodes of the TENG gradually increases with increasing external pressure (Fig. S9†), so the external pressure can be effectively detected by measuring the output voltage of the device. The working principle is shown in Fig. S10.†

Fig. 3 Performance and characterization of DSAO-based TENGs. (a) Schematic structure of a DSAO-based TENG. (b) Finite element simulation of the relationship between the pressure and potential difference between the upper and lower layers. (c) Pressure sensing sensitivity of the TENG. (d) Sensing electrical signal curves of the flexible TENG under different pressures. (e) Sensing electrical signal curves of the TENG at different frequencies. (f) Sensing response time of the TENG.

The output voltages under different loads are measured by applying different magnitudes of pressure to the device, and the electrical signals exhibit gradient changes with increasing pressure and two different linear trends upon two pressure ranges (Fig. 3c). Specifically, the sensitivity of the TENG-based sensor is 0.015 kPa⁻¹ when the pressure is less than 230 kPa, whereas the sensitivity of the sensor is 0.0035 kPa⁻¹ at pressures above 230 kPa. In the high-pressure range, as the pressure increases, the voltage variation decreases compared with that in the low-pressure range. This is because at high pressures, the microstructural deformation of the material surface gradually approaches its limit, resulting in a smaller change in the amount of deformation of the material under compression, thus leading to a decrease in the overall sensitivity.

Fig. 3d further shows the responsiveness of TENG-based sensors to different magnitudes of pressure. As seen from the results in the figure, the output voltage exhibits different amplitudes as different pressures are applied to the device surface at the same frequency. Specifically, as the applied pressure increases, the output voltage of the device increases simultaneously, which indicates that the device has excellent pressure response characteristics. However, the output signals depicted here are small due to the limited sensing area. By expanding the sensor's effective area, higher output signals can be achieved, as shown in Fig. S11.† In addition, TENG-based sensors were tested for their response of electrical signals to pressures at different frequencies, and the devices exhibit excellent responses to pressure at different frequencies (Fig. 3e). When the same pressure is applied to the surface of the device at different frequencies, the output voltage of the device not only has the same amplitude change, but also has the same frequency characteristics as the pressure. In addition, the TENG-based sensor has excellent response and recovery times. The test results (Fig. 3f) indicate that the response and recovery times of the sensor based on DSAO are 50 ms and 155 ms, respectively, indicating that the device has a fast response time. In summary, the TENG-based sensor prepared from graphene/DSAO has excellent friction electrical output performance and a fast response time and can potentially convert human body motion into stable electrical signals.

A timely and accurate understanding of the physical status is essential for the maintenance of human health in daily life.^39,40 By effectively collecting and analysing the movement status, comprehensive monitoring of physiological information can be achieved, thereby improving the understanding of the health status.^41,42 Such monitoring can help detect physical abnormalities in advance so that appropriate measures can be taken to maintain health (Fig. 4a). We compared the difference in wearing comfort between a TENG-based sensor and a typical wearable device's base material (polyimide (PI)) tape (Fig. 4b). When the TENG-based sensor and the PI tape are each worn on the right arm and exercised for thirty minutes, followed by the removal of the two materials, the part of the arm wearing the PI tape is reddened due to the impermeability of the tape. In contrast, as the TENG-based sensor has good moisture permeability, the state of the part of the skin to which it is attached is almost the same as that in the natural state, which proves that the wearing comfort of the wearer is greater when TENG-based sensors are used for the preparation of wearable devices. To further verify the feasibility of the devices in monitoring human motion, the TENG-based sensors are attached to the human index finger, elbow and knee joints for testing. As shown in Fig. 4c, the sensors are able to generate electric signals with different voltage magnitudes when the fingers are bent at different amplitudes. The pressure applied to the sensors increases accordingly as the degree of bending of the fingers increases, which leads to an increase in the friction electric signals. In addition, Fig. 4d shows that the sensor is likewise capable of capturing an increasing friction electrical signal as the elbow is gradually flexed from extension by 45°, 90° and 135° at a fixed frequency. Similarly, the sensor was affixed to the knee joint to effectively monitor knee flexion (Fig. 4e). The friction electrical signal shows a gradient increase as the degree of knee flexion increases. Under the same bending degree, the friction electric signal output from the sensor has high repeatability (Fig. 4f). The above experimental results show that the flexible TENG-based sensor is capable of accurately monitoring and recording human movement. To further evaluate the sensor's ability to monitor at higher pressures, a TENG sensor based on graphene/DSAO was attached to the sole of the foot for testing (Fig. 4g). Since the pressure on the sole of the foot varies in different motion states, different motion states can be identified by analysing the friction electric signals generated by the sensor. On the basis of the stable periodic waveforms presented by the device in different states, the walking and running states can be accurately identified by the frequency of the friction electrical signal. In addition, the number of cycles of the periodic waveform represents the number of steps taken by the wearer, so the sensor also has a pedometer function that can record the number of steps taken by the wearer in real time.⁴³ These experimental results fully demonstrate the excellent performance of the sensor in human motion monitoring.

Fig. 4 DSAO-based TENG sensor for physiological signal detection. (a) Schematic diagram of a wearable TENG-based sensor for integrated monitoring of physiological information. (b) Moisture permeability testing of TENG-based sensors and PI. (c)–(e) Physiological signal detection of knuckle, elbow, and knee flexions. (f) Stability verification of the periodic knee flexion. (g) Detection evaluation under a higher pressure range through walking and running.

On the basis of the surface microstructure of graphene/DSAO, its internal pore design increases the contact area and thus significantly improves the sensitivity of piezoresistive sensors. A piezoresistive pressure sensor with cone and porous microstructures was prepared using graphene/DSAO as the active layer material (Fig. 5a). This design enables the sensor to produce a more pronounced resistance change when subjected to pressure, thus achieving highly sensitive detection of pressure. This cone microstructure produces stress concentrations and local deformation at the point of contact when subjected to pressure. The deformation results in a significant increase in the contact area between the arrays of cone microstructures, which affects the resistance at the contact points. As shown in Fig. 5b, when the applied pressure increases, the resistance of the sensor increases accordingly. The pressure sensor based on graphene/DSAO exhibits two different segments of linear response in its measured pressure range (Fig. 5c), which is reflected in two sensitivity values. Notably, in the pressure range above 25 kPa, the sensitivity of the sensor is 2.76 times greater than that when it is below 25 kPa. The performance of the sensor is highly dependent on the degree of change in the geometry of the graphene/DSAO; therefore, the relative increase in the sensitivity of the sensor can be attributed to the larger dimensional deformation due to the inner porous microstructures of the graphene/DSAO under the action of greater pressures.

Fig. 5 Graphene/DSAO-based flexible piezoresistive pressure sensor for signal detection. (a) Schematic of a flexible piezoresistive pressure sensor based on graphene/DSAO. (b) Relative resistance change of a piezoresistive pressure sensor during staged loading. (c) Relative resistance change of the pressure sensor along the Z direction under external pressure. (d) Pressure sensor arrays based on graphene/DSAO. Response testing of pressure sensor arrays to the touch of (e) a single point and (f) two points. (g) Finger tactile sensing device based on the fabricated piezoresistive pressure sensor. (h) Touch response testing of the finger tactile sensing device. (i) Finger tactile sensing device touch test results.

A 4 × 4 sensing array was constructed via the structural design of pressure sensors (Fig. 5d). When a weight is placed on the surface of the sensor, the sensor located below the weight is immediately triggered as shown in Fig. 5e. When two weights are placed on the sensor array, the two corresponding sensor points show immediately the weights, as shown in Fig. 5f. This finding demonstrates that the constructed pressure sensor array is capable of accurately sensing the position of pressure and effectively sending this information to the outside.

We constructed a tactile sensing array system by integrating a pressure sensor based on graphene/DSAO onto a robotic finger (Fig. 5g). This design enables the robotic hand to have tactile sensing function when grasping objects, significantly enhancing its perception and manipulation capabilities. To ensure a proper fit between the sensors and the manipulator, the sensing array was customized according to the contours of the manipulator. The gripper of the manipulator has several degrees of freedom. When the finger touches the object, it sends a command to pull it taut, thus enabling the manipulator to grasp the object (Fig. 5h). As the finger touches the object, the pressure sensor located at the contact position is able to sense a large increase in the resistance to squeezing. As soon as the finger releases the object, the resistance of the sensor quickly returns to its initial value. During the grasping of a heavy object, the pressure sensor is able to clearly feel the pressure signal (Fig. 5i), which proves the feasibility of the designed finger haptic sensing device in practical applications.

Conclusions

In summary, we successfully developed a DMSO-modified agar organogel as a substrate material for wearable devices. DSAO exhibits unique mechanical properties, with a fracture strength of 34.21 MPa, and can carry a person up to 60 kg due to the hydrogen bond formation. The large number of pores inside DSAO results in good moisture permeability. In addition, DSAO can be completely degraded to agar in 30 days, which is not harmful to the environment. To demonstrate the potential applications in wearable devices, DSAO was assembled into TENG-based sensors and piezoresistive pressure sensors, respectively. The TENG sensor can recognize motion states such as walking and running by output voltage signals. In addition, a sensing array based on graphene/DSAO piezoresistive pressure sensors presents tactile sensing capability, which shows promising potential in wearable medical monitoring and intelligent robotics.

Experimental section

Materials

Agar was purchased from Shanghai Yuanye Biotechnology Co., Ltd. Graphene and DMSO were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Hydrochloric acid was purchased from Sinopharm Chemical Reagent Co. Polyimide was purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.

Fabrication of DSAO

First, 0.6 g of agar powder was added to 10 mL of DMSO and stirred well to form the precursor solution. The beaker containing the precursor solution was placed on a magnetic stirrer, and N₂ was continuously injected into the precursor solution at a gas flow rate of 5 ml min⁻¹ so that the inside of the precursor solution was filled with air, and the air holes became denser after stirring at 90 °C for 3 h. After the solution cooled to room temperature, a spin coater was used to apply the solution to the precursor solution. When the whole solution was cooled to room temperature, the solution was uniformly spin-coated on a slide. The slides were subsequently placed in a fume hood and allowed to dry naturally for 24 h to obtain porous DSAO. The thickness of the film can be effectively controlled by controlling the rotational speed and time of the spin coater.

Preparation of graphene/DSAO

50 mg of graphene was added to the precursor solution with stirring at 90 °C for 3 h. After cooling the solution to room temperature, it was uniformly spun-coated on the slides via a spin coater. The slides were subsequently placed in a fume hood and allowed to dry naturally for 24 h. Similarly, the thickness of the film can be effectively controlled by controlling the rotational speed and time of the spin coater.

Characterization of DSAO

CCD images: digital pictures of the material are captured using a Nikon z 7II digital camera with a charge-coupled device (CCD) image sensor. SEM images: a 2 kV high-resolution field emission scanning electron microscope (Zeiss G500) was used to observe the micromorphological features of the agar and modified agar organogel. The surface of the organogels was observed at magnifications of ×2000 and ×8000. Fourier transform infrared (FTIR) spectroscopy: the modified agar organogel was ground into fragments, mixed with KBr in an onyx mortar and then compressed into thin flakes. The peak at 400–4000 cm⁻¹ was subsequently measured via an FTIR spectrometer (V70–H1000) with a resolution of 0.7 cm⁻¹. Mechanical properties: the mechanical strength of the modified agar organogel was measured via a universal material testing machine (34TM-50). The measurements were based on GB/T 1040.3-2006. Before the measurements, the samples were cut into fixed sizes via a mold, and their thicknesses were measured via a contact thickness gauge. The measurement rate, clamping distance and sensor specifications of the universal material control tester were 100 mm min⁻¹, 20 mm and 500 kgf, respectively. Water vapor permeability: the water vapor permeability of the modified agar was measured using a water vapor transmission rate tester (W3/060, Labthink Mechatronics Technology Co., Ltd). The measurements were based on GB/T 1037-1988. First, deionized water (10 mL) was added to a sheet cup (Φ 70 mm), which was subsequently sealed with modified agar organogel and placed in the test chamber. The tests were conducted at 25 °C with a relative humidity of 90%. Degradation characteristics of the composite film in hydrochloric acid solution: two grams of modified agar organogel were placed in a hydrochloric acid solution (pH = 6.0) and stirred at 400 rpm, and the morphology of the film was observed after thirty days of continuous stirring.

Preparation of TENG-based sensors and piezoresistive sensors

A mold with a cone microstructure pattern inside the PMMA was fabricated using laser energy. By adjusting the power and motion modes of the laser machine, microstructures with different sizes and shapes can be obtained. By spin coating DSAO on a PMMA mold, a DSAO film with a cone microstructure can be obtained. To assemble electrodes with a DSAO film, sensor devices can be prepared.

Performance characterization

To measure the electrical output of the TENG-based and piezoresistive sensors, a commercial linear actuator (Winnemotor, WMUC512075-06-X) is used to apply an external contact force, whereas a digital force gauge (Chatillon, DFS II) is used to monitor the force applied. Programmable electrometers (Keithley models 6514 and 2400) capture the electrical output signals. Simulations of the triboelectric potential distribution are performed via COMSOL Multiphysics software. The formula for calculating sensitivity is shown below:where ΔV is the relative change in the output voltage, V₀ is the saturation voltage, and P is the pressure applied.

Author contributions

Z. Y. and H. L.: investigation, methodology, visualization, writing, and editing. P. Z. and Y. L.: formal analysis, visualization, writing and editing. Y. L. and J. C.: conceptualization and methodology. J. J.: conceptualization, methodology, writing, review and editing. B. D. and Z. W.: conceptualization, methodology, funding acquisition, writing, review and editing.

Data availability

All relevant data are within the manuscript and its additional files.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science and Technology Major Project from Minstry of Science and Technology of China (Grant No. 2018AAA0103104), the National Natural Science Foundation of China (Grant No. 22173068, 62304184, 62174115, U21A20147), the Natural Science Foundation of Jiangsu Province of China (No. BK20240152), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 24KJA430011), and the Research Development Fund of Xi'an Jiaotong-Liverpool University (No. RDF-SP-102, REF 2301006). It was also supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Fund for Excellent Creative Research Teams of Jiangsu Higher Education Institutions and Suzhou Key Laboratory of Surface and Interface Intelligent Matter (SZS2022011). This work is supported by the Suzhou Key Laboratory of Functional Nano & Soft Materials, Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nr00403a

‡ These authors contributed equally to this work.

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