Wearable electronics can be integrated with the human body for monitoring physical activities and health conditions, for human-computer interfaces, and for virtual/augmented reality. We here report a multifunctional wearable electronic system that combines advances in materials, chemistry, and mechanics to enable superior stretchability, self-healability, recyclability, and reconfigurability. This electronic system heterogeneously integrates rigid, soft, and liquid materials through a low-cost fabrication method. The properties reported in this wearable electronic system can find applications in many areas, including health care, robotics, and prosthetics, and can benefit the well-being, economy, and sustainability of our society.
Stretchable/flexible electronics has attracted tremendous attention in the past two to three decades due to the combination of its superior mechanical attributes and electrical performance (1–4). It can be applied in places that are not accessible by traditional rigid printed circuit boards (PCBs), such as seamless integration with soft tissues and organs of human body for health care (5–7), bioinspired curvilinear imagers (8–11), and artificial skins that mimic the mechanical and electrical properties of natural skin (12–14). Among all the exciting applications, wearable electronics represents one of the most important, as it is the most accessible to people, and can be integrated onto the surface of human body to provide many useful functions, including physical activity tracking (15, 16), health monitoring (17, 18), drug delivery (19), human-computer interface (20, 21), and virtual/augmented reality (22). To achieve excellent mechanical compliance and electrical performance simultaneously, integration techniques that combine stretchable substrates, interconnects, and hard commercial chip components were developed, including soft microfluidic assemblies (15), solderable and stretchable platforms (23–25), and liquid metal (LM)–based systems (26, 27). More recently, various chemistry and mechanisms have been explored to enable self-healability (28–32) and degradability (7, 33) in wearable electronics. For example, microvascular agents (28), dynamic covalent bonds (31), metal-ligand supramolecular chemistry (34), and hydrogen bonds (35, 36) have been studied to realize self-healable electronics, for improved reliability, durability, and cost (37, 38). Among these mechanisms, polymer networks with covalent bonding demonstrate advantages in robustness, stability, and mechanical strength when compared with noncovalent chemistry (39).
All these developments could lead to an exciting bright future of applying technological advancements to improve the well-being of people and the society. However, on the other hand, mass production and application of electronics generate a large amount of electronic waste. By 2021, the total electric waste is estimated to reach 52.2 million metric tons, and the majority of the waste cannot be appropriately recycled (40). The consequence is that a large amount of heavy metals and other hazardous substances have been entering the ecosystem, causing serious environmental problems and human health issues (41, 42). To resolve this issue, dynamic covalent thermoset polyimine has been developed, which can not only be self-healable/rehealable but also be recyclable (43–47). On the basis of the polyimine chemistry and its nanocomposites, we have recently reported a rehealable, fully recyclable, and malleable electronic skin (29). However, the electronic skin reported in this work has limited compliance and stretchability (failure strain, ~4%; Young’s modulus, ~1 GPa), and the resistivity (~1 × 10−3 ohm∙cm) of the nanocomposite is too high for high-performance electronics. Furthermore, the rehealing process requires heat press, which is not ideal for wearable devices. Thus, this previously reported polyimine and its nanocomposites are not optimal for stretchable, compliant, and self-healable electronics.
Here, we report a multifunctional wearable electronic system that can simultaneously provide full recyclability, excellent mechanical stretchability, self-healability, and reconfigurability based on modified polyimine chemistry. This wearable electronics is achieved by heterogeneous integration of rigid (chip components), soft (dynamic covalent thermoset polyimine with new formula), and liquid (eutectic LM) materials through advanced mechanical design and low-cost fabrication method. In this wearable electronic system, off-the-shelf chip components provide high-performance sensing and monitoring of the human body, including physical motion tracking, temperature monitoring, and sensing of acoustic and electrocardiogram (ECG) signals. They are interconnected by intrinsically stretchable and robust LM circuitry and encapsulated by dynamic covalent thermoset polyimine matrix. Bond exchange reactions in the polyimine network, together with the flowability of LM, enable the wearable electronics to self-heal from damages and to be reconfigured into distinct configurations for different application scenarios. Furthermore, through transimination reactions, the polyimine matrix can be depolymerized into oligomers/monomers that are soluble in methanol and are separated from the chip components and LM. All recycled materials and components can be reused to fabricate new materials and devices.
The multifunctional wearable electronics incorporates three sensing modules: (i) ECG sensor implemented by operational amplifiers and resistors, (ii) thermometer sensor for body temperature sensing, and (iii) accelerometer for acoustic and motion sensing (Fig. 1A). Figure 1B provides an exploded view of the schematic illustration of the design, which includes eutectic gallium-indium (EGaIn) LM alloy, and commercial small-scale chip components encapsulated between two thin elastomeric polyimine membranes (fig. S1A). The LM serves as electrical interconnects between chips, and each chip’s pins are mounted precisely onto the patterned LM contact pads, similar to PCBs but without the need of welding (fig. S1, B and C). The device in this case includes 12 chip components, the specifications of which are summarized in table S1. The LM circuitry in this multifunctional wearable electronics provides a combination of excellent electrical conductivity (3.4 × 106 S/m) (48) and superior intrinsic mechanical softness and deformability, so that the device can be crumpled when attached on the human skin (Fig. 1C, top left), bended (Fig. 1C, top right), twisted (Fig. 1C, bottom left), and stretched (Fig. 1C, bottom right).
The stretchable substrate, insulation, and encapsulation films in this wearable electronics are made of polyimine, which can be synthesized using three commercially available monomers: terephthalaldehyde, 3,3′-diamino-N-methyldipropylamine, and tris(2-aminoethyl)amine (fig. S2A). As schematically illustrated in Fig. 1D, the cross-linked polyimine functions are based on the dynamic covalent chemistry principle (39). A model reaction as shown in fig. S2B indicates that the imine bond exchange reaction is efficient at both room temperature and 80°C. The Fourier transform infrared (FTIR) spectra of terephthaldehyde and polyimine shown in fig. S2C further prove the full conversion of aldehyde into imine bonds. Under appropriate environmental conditions, bond exchange reactions in polyimine network lead to effective stress relaxation, enabling excellent malleability and reprocessibility (45–47). At the interface between two polyimine films, bond exchange reactions produce new covalent bonding across the interface, leading to effective interfacial bonding or healing. Another interesting characteristic of the polyimine network is that when excessive diamine and/or triamine monomers are introduced, transimination reactions lead to depolymerization of the polyimine network into oligomers/monomers soluble in methanol. This mechanism enables 100% recycling of the cross-linked polyimine. As shown in fig. S2C, the high similarity between the FTIR spectra of the original and recycled polyimine supports the notion that these two films are chemically identical. Because of these properties of polyimine, the multifunctional wearable electronics is self-healable and fully recyclable (Fig. 1E). When the device is damaged, the broken pieces can be brought into contact, and bond exchange reactions can generate new covalent bonds at the interface. It should be noted that different from those self-healing materials using noncovalent supramolecular bonding interactions (e.g., hydrogen bonding, metal-ligand coordination, etc.), the polyimine matrix used herein consists of covalent bonds, thus making the devices much more robust and can operate under a broad range of conditions. The self-healed device regains both mechanical and electrical functions. When necessary, press can be used to improve interfacial contact and to accelerate bond exchange reactions for more effective interfacial healing. When the device is severely damaged or not needed, it can be fully recycled by soaking in the recycling solution, which is 3,3′-diamino-N-methyldipropylamine and tris(2-aminoethyl)amine dissolved in methanol. When polyimine is completely depolymerized, the electrical components, including the chips and EGaIn LM, can be easily separated from the solution. The polymer solution and the electrical components can be reused for making new devices.
Fabrication of the multifunctional wearable electronics is schematically illustrated in Fig. 2A. It starts with mounting a silicon paper mask onto a synthesized polyimine film substrate. Screen printing method is adopted to brush EGaIn LM against the mask, and peeling off of the mask leaves patterned LM traces on the polyimine substrate. At each intersection of LM traces, a polyimine insulation layer is added. Then, chip components are pick-and-placed onto the predesigned locations with LM contact pads. Applying and curing uncured polyimine solution onto the device provide encapsulation and protection to the LM circuitry and chip components. This process is very low cost and compatible with printing technology.
The multifunctional wearable electronics integrates a thermal sensor (MCP9700, Microchip Technology), a triaxial accelerometer (ADXL335, Analog Devices Inc.), and a ECG sensor (AD8505, Analog Devices Inc. and Resistors, Bourns Inc.), so that it can provide real-time monitoring of body temperature, physical activities, and health state and can also serve as human-machine interfaces, as demonstrated in Fig. 2 (B to E). The multifunctional wearable electronics can be worn on the wrist to provide monitoring of physical activities, as shown in Fig. 2B. It can detect the different amplitude and frequency of walking, running, and jumping. The thermal sensor can provide accurate measurement of body temperature in different environments and at different body locations. Figure 2C shows the measured forehead temperature when the participant stayed indoors (35.4°C) and outdoors (28.7°C) (top frame) and the different indoor temperatures on hand (33.6°C), lower abdomen (34.8°C), and forehead (35.4°C) (bottom frame). These measurements are consistent with the data obtained using a commercial thermometer (fig. S3A) (49). When attached on the throat, the acoustic sensor can capture the vibration characteristics of the vocal cords and serve as human-machine interfaces. As exhibited in Fig. 2D, the data captured show distinct vibration signatures of different verbal commands, which are consistent with those obtained using a commercial PC microphone (fig. S3B). The ECG sensor monitors the electrocardiogram of the heart. The ECG data of the participant at rest and after exercising for 13 and 34 s are presented in Fig. 2E. They correspond to the heart rate of 72, 96, and 114 per min, respectively. Refer to movie S1 for the real-time recording of the ECG data during exercising. To test biocompatibility, fig. S4 shows optical images comparing the skin conditions before (top left) and after wearing a polyimine film for 12 hours (bottom left) and 24 hours (bottom right), respectively. No noticeable adverse effects on the skin can be observed.
To ensure superior stretchability of the wearable electronics, both elastic polymer substrates and highly stretchable conductors are essential. The mechanical properties of the polyimine film are demonstrated in fig. S5. The uniaxial stress-strain curve as shown in fig. S5A exhibits a hyperelastic behavior of polyimine, and the initial slope gives the Young’s modulus to be 2 MPa. Figure S5B shows the loading and unloading stress-strain curves of polyimine under cyclic loading test, which exhibits good elasticity with slight hysteresis. For the conductor, as LM is a liquid and can flow, it does not add rigidity to the wearable device and provides excellent deformability to the electronic system. How much the LM conductor can be stretched is limited by the polymer encapsulation. As shown in fig. S6A the LM conductor encapsulated by polyimine can be stretched by ~200%. Compared with stretchable conductors based on serpentine metal filamentary interconnects or buckled arc-shaped bridges (12, 15), LM circuitry does not have fatigue issues, encapsulation does not affect the mechanical performance, and there is no stress concentration in the conductor at the interfaces between the rigid chip components and the soft substrate (fig. S1C). Furthermore, the LM interconnects are robust and reliable under mechanical disturbances. As shown in fig. S7 (A and B), the resistance of a polyimine encapsulated LM conductor subjected to repeated rubbing and pressing recovers to the original value when the mechanical forces are removed. These properties of the LM circuitry lead to superior mechanical stretchability, robustness, and reliability in the multifunctional wearable electronics. Figure 3A shows an as-fabricated, undeformed multifunctional wearable device (left) and its finite element analysis (FEA) model (right). Figure 3 (B and C) exhibits the multifunctional wearable device being uniaxially stretched by 60% along vertical and horizontal directions, respectively. The FEA results show that the maximum strains in the chip components are below 0.01%, far below the typical failure strain of silicon (~1%). Figure 3D presents the results of the device being stretched by 30% in both directions (areal strain, ~70%). From FEA simulation, the maximum strain in the chip components is below 0.004%. The strain distributions in the polyimine substrate when the device is under different loading conditions are presented in fig. S8. Figure S7C shows the resistance data of an LM conductor encapsulated by polyimine subjected to cyclic 60% strain. Since the resistance of the LM conductor is several orders of magnitude smaller than that of the electronic components shown in table S1, the resistance change of the LM conductor during stretching should have negligible effect on the performance of the wearable device. Figure 3E exhibits the ECG data obtained by the wearable device under 30% biaxial stretching and 60% uniaxial stretching along vertical and horizontal directions. The sensing performance of the other devices under different loading conditions is shown in fig. S9. No noticeable influence of mechanical stretching on the sensing performance of the device is observed.
In addition to superior mechanical properties, the LM circuitry in polyimine matrix also provides excellent self-healing capability for the multifunctional wearable electronics. As demonstrated in Fig. 4A, when the LM circuit is cut broken by a razor blade (top frame), the ECG signal is lost (Fig. 4B, top). The top inset in Fig. 4A illustrates a magnified view of the broken LM conductor. After slightly pressing the two sides of the cut location, the LM conductor is self-healed, and the ECG sensor regains sensing capability (Fig. 4B, top, and movie S2). After the device self-heals for 13 min, the cut is optically invisible (Fig. 4A, middle), even under the optical microscope, as shown by the inset. After 48 hours of self-healing, bond exchange reactions at the interface lead to reformation of sufficient covalent bonds and effective interfacial healing. The mechanical strength of the healed interface is comparable to the original polyimine, as demonstrated by the tensile test results of self-healed LM conductors shown in fig. S6A. Therefore, the self-healed multifunctional wearable electronics can be stretched by 60% (Fig. 4A, bottom, and movie S3), and the sensing performance is not affected (Fig. 4B, bottom). To effectively improve contact at the interface and to accelerate self-healing, some pressure can be applied on the broken device at room temperature. As shown in fig. S6 (C to G), by introducing a 90-g weight on the self-healing device, the self-healing time can be reduced to 144 min. When the weight is increased to 1000 g, the self-healing time can be further reduced to 13 min. All the self-healed devices show mechanical robustness and stretchability comparable to the original device (fig. S6A). The sensing performances of the other devices, including motion, temperature, and acoustic sensing, are shown in fig. S10A, which are also comparable to the original sensors. To further demonstrate the self-healing performance, the cutting and self-healing process at the same location of a polyimine encapsulated LM conductor was repeated five times. As shown in fig. S6B, the resistance recovers to the original value after five times cutting and self-healing. The stress-strain curve of the self-healed device shows elasticity comparable to the original device (fig. S6A). As demonstrated in fig. S6H, the LM conductor can still be stretched by 100% after five times self-healing.
Not only self-healable, the multifunctional electronics is also fully recyclable. When the multifunctional device is soaked in the recycling solution (Fig. 4C, top left), consisting of 3,3′-diamino-N-methyldipropylamine and tris(2-aminoethyl)amine in methanol, the excessive amine monomers can react with the imine bonds inside the polyimine network. Transimination reactions lead to depolymerization of polyimine into oligomers/monomers that are soluble in methanol. The top right and bottom right frames in Fig. 4C show the multifunctional device after being soaked in the recycling solution for 30 and 47 min. After 47 min, the polyimine matrix is completely depolymerized into oligomers/monomers and dissolved into methanol, the chip components and LM sink to the bottom of the container, which can be easily separated from the solution using a paper filter. When an appropriate amount of terephthalaldehyde is added to rebalance the stoichiometry between the amine and aldehyde reactants, the recycled solution, chip components, and LM can all be reused to make a new multifunctional device (Fig. 4C, bottom left). The recycling process was repeated three times, and the electronic and mechanical performances of the recycled device are shown in Fig. 4 (D and F) and fig. S10B. The new motion sensor fabricated using the recycled materials shows sensing performance comparable to the original device (Fig. 4D and movie S4). The sensing performances of the other sensors in the recycled multifunctional device are shown in fig. S10B. Figure 4E compares the mechanical property of the self-healed multifunctional electronics with the original device, along both horizontal and vertical directions, which show almost identical curves. The comparison between the recycled multifunctional electronics and the original device in Fig. 4F also shows almost identical mechanical properties in both directions.
The bond exchange reactions in the dynamic covalent polyimine network enable not only effective self-healing at the interfaces but also stress relaxation in deformed material (29, 45), which can be used to realize reconfigurable multifunctional electronics. This capability also partially attributes to the fluid behavior of the LM circuitry, as it does not add any rigidity or residual stress to the multifunctional wearable electronics. The time- and temperature-dependent relaxation behavior of the polyimine film can be characterized using dynamic mechanical analyzer (Model Q800, TA Instruments, New Castle, DE, USA). Figure S11A shows the dependence of storage modulus, loss modulus, and tan δ over temperature, and it gives the glass transition temperature of polyimine as 21°C. Figure S11B gives the results of a series of relaxation tests over a wide range of temperatures (12° to 80°C). Specifically, at 60°C, the relaxation modulus decreases from 1 to 0.034 MPa in 10 min, which corresponds to 96.6% relaxation of the internal stress in the dynamic covalent network due to bond exchange reactions. By harnessing the effective stress relaxation in the polyimine network at elevated temperature and the fluid behavior of the LM circuitry, the multifunctional wearable electronics can be reconfigured into distinct configurations that suit for different application scenarios. As demonstrated in Fig. 5A, an initially flat multifunctional wearable device (160 mm in length, the first configuration; top left) can be worn on the wrist by relaxing the stress induced by bending (bottom left). Then, the device is stretched by 45% and heated to 60°C to relax the stress, which yields the second flat configuration of the multifunctional device (232 mm in length; top middle). It can be further wrapped onto the ankle, and stress relaxation keeps the bent state of the device on the ankle (bottom middle). Further stretching the device by 112% and applying heat result in the third flat configuration of the multifunctional device (340 mm in length; top right). It is then bent against the neck, and bending-induced stress can also be relaxed to make sure the device stays on the neck for reliable sensing (bottom right). This process represents totally six different configurations. Optical microscope images in fig. S1C prove robust connection between the LM circuitry and the rigid chip components under extreme deformation. Here, relatively thick polyimine films were used for the reconfigurable device; however, much thinner polyimine films can also be adopted, as shown in fig. S5C, for improved flexibility and conformability. Figure 5 (B to D) presents the FEA simulation results of the flat states of the three different flat configurations. The maximum strains in the chip components are less than 0.01%, far below the failure strain of silicon (~1%). Figure 5E shows the ECG measurement from the device in its first configuration, worn on the wrist. Figure 5F presents the motion tracking data of the accelerometer in the multifunctional wearable electronics when it is worn on the ankle in the second configuration (movie S5). Figure 5G exhibits the acoustic sensing of the device in its third configuration, when wrapped around the neck (movie S6). The temperature monitoring at the wrist, the ankle, and the neck in three different configurations of the same multifunctional wearable device is shown in Fig. 5H, indicating different temperatures at different parts of the body, which agrees well with data obtained from a commercial thermometer. The sensing data of all the sensors in the wearable device in different configurations are presented in fig. S12, proving robust sensing performance of the device during reconfiguration. Here, totally five times reconfiguration processes have been demonstrated. However, if the applied total stretch is kept below 200%, then the device can be reconfigured many times.
We have demonstrated a highly stretchable, self-healable, recyclable, and reconfigurable multifunctional wearable electronic system that can provide physical motion tracking, body temperature monitoring, and sensing of acoustic and ECG signals. The wearable electronics is realized by electrically interconnecting high-performance, off-the-shelf chip components using LM circuity and encapsulated with a dynamic covalent thermoset polyimine matrix. The wearable electronics can be stretched uniaxially by 60% in both horizontal and vertical directions and biaxially by 30% without affecting the electrical sensing performance. Enabled by the bond exchange reactions in the polyimine network and fluid behavior of the LM circuitry, the wearable electronics can self-heal from damages and can be reconfigured into distinct configurations for different application scenarios. When severely damaged or no longer needed, the wearable electronics can be fully recycled, leading to oligomers/monomers dissolved in methanol, and chip components and LM can be separated from the solution. The recycled solution, chip components, and LM can be reused to make new-generation materials and devices. This work can find applications in many areas, including health care, prosthetics, robotics, and human-computer interfaces, and can benefit the well-being, economy, and sustainability our society.
MATERIALS AND METHODS
To synthesize polyimine, terephthalaldehyde (1.5 g, 11.18 mmol), 3,3′-diamino-N-methyldipropylamine (1.251 g, 8.61 mmol), and tris(2-aminoethyl)amine (0.252 g, 1.72 mmol) were mixed in methanol (20 ml), as shown in fig. S2A. The solution was vigorously stirred and poured into a silicon paper mold, followed by evaporating methanol in a fume hood for 12 hours at room temperature and heat pressing at 80°C and 8.5 kPa for 12 hours. For self-healable polyimine films, the solution was placed in fume hood for evaporating methanol for 2 hours at room temperature.
The LM used in this study is eutectic metal alloy consisting of gallium (75%) and indium (25%) (EGaIn, Sigma-Aldrich). Its melting temperature is 15.7°C and therefore maintains liquid state at room temperature. The resistivity of EGaIn is 29.4 × 10−6 ohm∙cm (48).
Fabrication of multifunctional wearable electronics
A silicon paper mask, made by laser cutting (LIDE laser cutting machine) a 0.2-mm-thick silicon paper film (RUSPEPA nonstick silicone paper), was laminated over a polyimine film. Then, LM was screen-printed over the silicon paper mask using a razor blade. The LM solidified after been cooled down to below 15.7°C. Removing the silicon paper mask left solidified LM traces on the polyimine film. After the temperature was raised to room temperature, the chip components were pick-and-placed on the predefined contact pads. The placement accuracy was ensured using an optical microscope. Pouring polyimine solution on top and curing at room temperature for 48 hours, the LM circuitry and chip components were encapsulated. For intersections, insulating polyimine was introduced to separate two crossing LM interconnects (Fig. 2A and fig. S13A). The fabricated multifunctional electronics was then connected to external power source and data acquisition system using copper wires (fig. S13B).
Self-healing and recycling process
To heal the broken polyimine device, a 1-kg weight was used to press polyimine for 13 min at room temperature to ensure good contact at the interface. Then, the weight was removed, and the device was placed in a chemical hood for 48 hours.
The recycling solution was prepared by mixing 3,3′-diamino-N-methyldipropylamine (0.417 g, 2.87 mmol) and tris(2-aminoethyl)amine (0.084 g, 0.574 mmol) in methanol. The device was then soaked in the recycling solution for 47 min to depolymerize the polyimine network into oligomers/monomers that can dissolve in methanol. After separating the LM and the chip components from the polymer solution, terephthalaldehyde (0.5 g, 3.72 mmol) was added into the polymer solution for synthesis of a new polyimine film. Diluted hydrochloric acid was used to remove the oxide layer on the surface of the LM, leading to congregation of LM (50). Then, both the LM and chip components can be cleaned using methanol.
Tension mechanical testing
Multifunctional wearable devices were tested using an Instron mechanical testing system, under quasi-static tension conditions (51, 52). For multifunctional sensing devices, the samples were cut into rectangular (40 mm by 55 mm) shapes.
To investigate the mechanical performance of the multifunctional wearable electronics, FEA was conducted using a commercial software package Abaqus. The polyimine and LM were modeled as Neo-Hookean hyperelastic material using three-dimensional (3D) hybrid stress elements (C3D8H), and the chip components were modeled using elastic isotropic material with 3D stress elements (C3D8). The Young’s moduli and Poisson’s ratios were 2 MPa and 0.35 for polyimine and 160 GPa and 0.4 for the chip components (53, 54). For simplicity, the LM circuitry was modeled as an extremely soft solid, with a Young’s modulus of 10 Pa and a Poisson’s ratio of 0.5, which does not noticeably affect the mechanical behavior of the wearable electronic system, as the Young’s modulus is five orders of magnitude smaller than polyimine.
Acknowledgments: Funding: Financial support from the NSF (CMMI-1762324) is acknowledged. C.S. acknowledges financial support from the China Scholarship Council (file no. 201706260019). Author contributions: C.S., Z.Z., W.Z., and J.X. designed the experiments. C.S., Z.Z., Z.L., and P.Z. performed the experiments and analysis. C.S., Z.Z., Z.L., W.Z., and J.X. wrote the paper. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data are available in the main text or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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