Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics — TEI-tagged view

Peng TanpersName, Haifei WangpersName, Furui XiaopersName, Xi LupersName, Wenhui ShangpersName, Xiaobo DengpersName, Huafeng SongpersName, Ziyao XupersName, Junfeng CaopersName, Tiansheng GanpersName, Ben WangpersName, Xuechang ZhoupersName (2022date). “Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics.” Nature Communicationstitle·j.

Object of study — a self-adhesive conductive PEDOT:PSSterm composite for soft electronicsterm·dom

Research question — how to combine low modulus, strong adhesionterm·key and high conductivity in one conductor

Contribution — SACPterm, a solution-processable soft self-adhesive conductor, and bioelectronic devices built from it

Full text — tagged

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Solution-processable, soft, self-adhesive, and conductive polymer compositesterm·dom for soft electronicsterm·dom Abstract Soft electronicsterm·dom are rising electronic technologies towards applications spanning from healthcare monitoring to medical implants. However, poor adhesionterm·key strength and significant mechanical mismatchesterm·dom inevitably cause the interface failureterm·dom of devices. Herein we report a self-adhesive conductive polymerterm that possesses low modulus (56.1-401.9 kPa), high stretchabilityterm·key (700%), high interfacial adhesionterm·key (lap-shear strength >1.2 MPa), and high conductivity (1-37 S/cm). The self-adhesive conductive polymerterm is fabricated by doping the poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) composite with a supramolecular solventterm (β-cyclodextrin and citric acid). We demonstrated the solution process-based fabrication of self-adhesive conductive polymer-based electrodes for various soft devices, including alternating current electroluminescent devices, electromyography monitoring, and an integrated system for the visualization of electromyography signals during muscle training with an array of alternating current electroluminescent devices. The self-adhesive conductive polymer-based electronics show promising features to further develop wearable and comfortable bioelectronic devices with the physiological electric signals of the human body readable and displayable during daily activities.

Introduction Soft electronicsterm·dom with abilities to withstand various mechanical deformations are essential in the fields spanning from wearable healthcare monitoring to medical implants 1 – 4 . Reliability of such devices particularly relies on electrodes/interconnects that bridging different active components as well as biological tissues, which are expected to form conformal attachment on irregular and non-flat surfaces and maintain durable stability upon deformations 5 – 7 . To date, various soft materials and flexible structures have been developed to construct reliable and sensitive electrodes for purpose of monitoring physiological signals in real-time fashion 8 , 9 , including hydrogels 10 , carbon-based composites 11 , liquid metal composites 12 , metal composites 13 , 14 and conducting polymers 15 . However, biological tissues, e.g., skins and muscles, are typically soft (with a mechanical modulus of 60–850 kPa), and the contact areas interfacing with electrodes are always irregular and even dynamic 16 . Such significant mechanical mismatchterm·dom and weak interfacial adhesionterm·key strength may increase the noise and decrease the sensitivity of physiological signals, or even cause the failure of devices. Reliable interfacial connections between electrodes and biological tissues are of increasing importance for soft electronicsterm·dom 17 .

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSSterm) is a commercially available conductive polymer compositeterm·dom with good solution processability, tunable electrical property, and biocompatibility 18 , 19 . As a proper candidate for bioelectronics, PEDOT:PSSterm films process high rigidity (young’s modulusterm·dom > 500 MPa), low stretchabilityterm·key (about 5%), and poor interface adhesionterm·key strength, which may induce uncomfortability and even immune response during their bio-applications 20 , 21 . To reduce the modulus and enhance the stretchabilityterm·key, various methods have been developed, including small molecular doping, polymer blending, and hydrogel network construction 22 – 25 . However, the rigidity and modulus of as-doped PEDOT:PSS-based composites are still higher than that of human tissues. Moreover, undesirable plastic deformationterm·dom is another issue that may cause incomplete recovery after deformation. Furthermore, the potential leakage of dopantsterm (ionic liquids) may depress the performance of materials and cause a critical health issue to surrounding tissues. Hydrogels are commonly applied to fabricate ultrasoft PEDOT:PSS-based composites 17 . The limited conductivity (<10 S cm −1 ) of PEDOT: PSS-based hydrogels is still a critical issue as a trade-off exists between softness and conductivity 26 – 28 . Apart from the mechanical mismatchterm·dom, interface mechanical adhesionterm·key also needs to be improved. Approaches like electrogelation, electrografting, and the introduction of an adhesionterm·key layer have been reported to enhance interface adhesionterm·key stability of PEDOT 29 – 31 . However, these approaches usually require complicated interface modification or specific electrochemical deposition process and the adhesionterm·key of the PEDOT layer to the substrate is disposable and non-recyclable.

Herein, we developed a self-adhesive conductive polymerterm (SACPterm) composite for soft electronicsterm·dom by doping the rigid and nonstick PEDOT:PSSterm composites with a biocompatible supramolecular solventterm (SMSterm). SACPsterm are prepared by the mixing and drying of a mixture containing SMSterm (i.e., citric acid and cyclodextrin), elastic polymer network (poly (vinyl alcohol) (PVA) covalently crosslinked by glutaraldehyde (GA)), and conductive polymerterm·key (PEDOT:PSSterm) (Fig. 1a ). The SACPterm composites possess exceptional mechanical flexibility (low modulus, low residual strain, and high stretchabilityterm·key), high conductivity, and strong interface adhesionterm·key strength. SACPterm films and patterns can be fabricated by solution processing (i.e., drop-casting, spin-coating, microfluid molding, and even transfer printing) on diverse substrates, such as metals, glass, polytetrafluoroethylene (PTFE), polyimide (PI), polydimethylsiloxane (PDMS), etc. The SACPterm interface shows high adhesionterm·key strength (e.g., lap-shear strength up to 1.2 MPa on PI substrate), low interface electrical impedance, and good reusability (over 100 repetitions). We demonstrated the application of SACPsterm in a series of soft electronicterm·dom devices, including alternating current electroluminescent (ACEL) devices (<150 μm in thickness), adhesive bioelectrodesterm·key for electromyography (EMG) monitoring, and an integrated system combing EMG sensors with an array of ACEL devices for the visualization detection of EMG signals during muscle training. Fig. 1 Fabrication of self-adhesive conductive polymersterm (SACPsterm) with supramolecular solvent-doped PEDOT: PSS composites and demonstration of their key features. a Schematic illustration of the fabrication, molecular structures, and interactions of the SACPsterm, including chemical structures of PEDOT, PSS, citric acid (H3Cit), poly (vinyl alcohol) (PVA), glutaraldehyde (GA), and β-cyclodextrin (β-CD), and the involved interactions enabled by covalently crosslinked elastic networks, electrostatic interaction, and supramolecular solventsterm. b Images of the aqueous SACPterm ink, droplets on PI (CA 65.9 o ) and PTFE (CA 99.8 o ) substrates, and the drying process of a droplet on PI substrate. No coffee ring effect is observed during the drying process. c Soft and strong adhesionterm·key by SACPsterm: images showing the pulling of a plastic box loading with a 0.5 kg weight (left) and the lifting of a 5 kg water bottle (right) via the SACP-based adhesives. d Large-scale application of the SACPsterm: images of a roll of SACP-coated PET tapes (top) and an A4 SACP-coated PET sheet (20 × 25 cm) (bottom).

Results Design and fabrication of SACPsterm for soft electrical interfaces Electrical interfaces for soft electronicsterm·dom require both high bonding stability and a low interface electrical impedance to maintain a good electrical signal transmission. The bonding performance relies on the adhesionterm·key strength and modulus (Fig. S1 ), which may cause delamination, wrinkles, or slippage, resulting in the collapse of interfacial electrical connections (Fig. S1, i–iii) 32 , 33 . Hence, a soft electrical interface of low modulus, strong adhesionterm·key, and high electrical conductivity is capable to address this issue (Fig. S1, iv ).

SACPterm is one of the suitable candidates for soft electrical interfaces, owing to the low modulus, strong adhesionterm·key, and high conductivity. SACPsterm consist of three components, i.e., SMSterm (citric acid and cyclodextrin with a molar ratio of 10:1), elastic polymer networks (chemically crosslinked PVA networksterm with GA), and conductive polymersterm·key (PEDOT:PSSterm) (Fig. 1a ). Briefly, citric acid, cyclodextrin, PVA, and GA were successively added to the aqueous solution of PEDOT: PSS. A homogeneous mixture was obtained and served as a “coffee-ring” free ink (Fig. 1b ). SACPsterm showed various applications in soft electronicsterm·dom owing to the following aspects (Fig. 1c–d ). First, supramolecular interactions occur from hydrogen bond interactions of a large number of carboxyl groups and hydroxyl groups in SMSterm 34 . Meanwhile, SMSterm can form hydrogen bonds and electrostatic interactions with PSSH units and positively charged PEDOT, respectively (Fig. 1a ) 35 . Such interactions significantly inhibit the aggregation of PEDOT chains (Figs. S2 and S3 ), thus increasing the degree of freedom of the polymer chains 36 , and thereby improving their mechanical flexibility. Second, PVA polymer networks, which are formed due to the selectivity of chemical crosslinking of GA with PVA and β-CD (Figs. S2a–b and S4 ), are constructed in PEDOT: PSS composites to obtain large yet reversible stretchabilityterm·key. Notably, the addition of dopantsterm may inevitably cause plastic deformationterm·dom 23 . Weak internal networks may cause greater plastic deformationterm·dom upon stretching 28 . PVA networksterm solve the problem of irreversible deformationterm·dom 37 , resulting in good recovery of SACPsterm (Fig. S5 ). Third, owing to the existence of abundant hydroxyl functional groups and charged molecules, SACPsterm exhibit strong interface adhesionterm·key on substrates due to the synergistic effects of several weak interactions at the interface 38 . Fourth, the components are all soluble in water and capable of forming a homogeneous, stable, and viscosity-tunable aqueous ink (with viscosity on the order of 10 3 –10 4 mPa s, Fig. S6 ), rendering it particularly suitable for solution processing towards the scalable fabrication of flexible devices 39 .

Mechanical and electrical performance of SACPsterm Previous studies indicate that PEDOT:PSSterm composites possess a compromise between low modulus and high conductivity. Here we demonstrated low modulus and high conductivity can be simultaneously obtained in SACPsterm.

We first investigated the doping effect of SMSterm on the mechanical properties of SACPsterm. Stress-strain curves of the SACPterm films with different mass ratios of PEDOT:PSSterm were measured by tensile testing. The mechanical property of SACPterm with a 36.3% mass ratio of PEDOT:PSSterm is hard with fracture strain of 154%, young’s modulusterm·dom of 5.9 MPa, and fracture stress of 14.8 MPa. While, the mechanical property of SACPterm with a 3.6% mass ratio of PEDOT:PSSterm is soft with fracture strain of 736%, Young’s modulusterm·dom of 401.9 kPa, and fracture stress of 1.2 MPa. We observe a significant decrease in Young’s modulusterm·dom and an increase in stretchabilityterm·key after doping. As the mass ratio of PEDOT: PSS decreases from 3.6% to 0.9%, the elastic modulus and fracture stress of SACPsterm gradually decrease to 56.1 ± 13 kPa and 593.2 ± 178.2 kPa. While the fracture strain slightly increases up to 700% at low PEDOT:PSSterm content (mass ratio <3.6%) (Fig. 2a, b and Fig. S7a ). The modulus of SACPterm can be tuned compatibly to that of skin tissue 16 . SMSterm doping provides an effective strategy to soften the PEDOT: PSS composites in a wide range. Fig. 2 Mechanical and electrical performance of SACPterm films. a Stress-strain curves. b Young’s modulusterm·dom, fracture stress, and fracture strain of the free-standing films with different PEDOT:PSSterm loadings. c Images of the films (with PEDOT:PSSterm mass ratio of 3.6%) presenting little residual strain even after being stretched to 400% of strain. d Stress-strain curves of the films (with PEDOT:PSSterm mass ratio of 3.6%) under cyclic loading-unloading tests. e Magnifying stress-strain curves between 0–120% for checking residual strain after stretching. f Conductivity of the films with different PEDOT:PSSterm loadings. g Conductivity of the films (with PEDOT:PSSterm mass ratio of 3.6%) with different thicknesses. h Relationships of Young’s modulusterm·dom and conductivity versus fracture strain of a variety of PEDOT:PSSterm composites reported in references and this work. The error bars show the standard deviations from at least three tests.

Not limited to SMSterm doping, the mechanical properties of SACPsterm can also be further improved with PVA networksterm as well as the water drying process. Residual strain is one key parameter for soft materials when they need to be stretched in practical applications. By introducing a PVA elastic network, we lower the strain residual of PEDOT:PSSterm composites. SACPsterm with a 3.6% mass ratio of PEDOT:PSSterm show a large range of reversible stretchabilityterm·key (400%) (Fig. 2c ), and the residual strain is <50% even at a large tensile strain (Fig. 2 d and e ). Not only strain residual, but we also observed the improvement of elastic resilience as the increase of concentration of the GA crosslinker (Fig. S7b and Supplementary Video 1 ). In addition, the mechanical performance is significantly changed as water evaporation time varies (Fig. S8 ), due to the impacts of water molecules on the hydrogen bond interactions in SMSterm.

Along with the tuning of mechanical properties of SACPsterm, the electrical conductivity also shows remarkable controllability ranging from 1 to 37 S/cm. We investigated the conductivity of the SACPsterm at a different mass ratio of PEDOT:PSSterm (Fig. 2f ). Interestingly, we observed an increase in the conductivity at the small mass ratio and a gradual decrease at the large mass ratio, with a maximum conductivity of 37 S/cm at a 36.3% mass ratio. Given that the SMSterm and PVA networksterm are insulating, the conductivity of SACPsterm compliance percolation theoryterm·mth 22 , 40 . Therefore, there is a trade-off between mechanical flexibility and conductivity. Besides, the as-made composites can be prepared as conductive films. Interestingly, the as-made SACPterm films with different thicknesses (4–340 μm) by spin-coating (for thin-film) and casting (for thick-film) methods showed good uniformity in electrical conductivity (Fig. 2g ). According to the dynamic electrical resistance variations test (Fig. S9 ), the relative electrical resistance changes <1 at a strain of 100%, and the variation values remained constant under repeated strain-releasing cycles. This result indicates that the SACPterm films present good stability of resistance at a tensile strain within 100%.

Figure 2h shows a comparison of Young’s modulusterm·dom and conductivity of PEDOT:PSSterm and their composites. Intrinsic PEDOT:PSSterm has a large young’s modulusterm·dom (>500 MPa), low stretchabilityterm·key (<5%), and low conductivity (<1 S/cm) (Fig. 2h , black square) 41 . And it is difficult to reconcile mechanical and electrical properties. For example, ionic liquid-doped PEDOT: PSS composites have high conductivity (>100 S/cm mostly) (Fig. 2h , purple region) 36 , 42 , 43 . However, such composites exhibit a high young’s modulusterm·dom (>10 MPa), poor stretchabilityterm·key (fracture tensile strain <200%), and serious plastic deformationterm·dom. Recently, to produce elastic PEDOT:PSSterm composites with low young’s modulusterm·dom and high conductivity, ionic liquids plasticizers were used to adjust the microstructure of PEDOT complexes. For example, polyurethane and ionic liquid-doped PEDOT:PSSterm composites have been achieved at a modulus of 7 MPa and electrical conductivity of 140 S/cm 44 . In contrast, PEDOT: PSS hydrogel was developed with some features of low modulus (<1 MPa) and large ductility (200–600%) (Fig. 2h , green region) 26 , 27 , 45 . Although hydrogels are of great significance in the application of bioelectronics, the conductivity of hydrogel materials is much poorer (0.01–23 S/cm) as compared to their counterparts. What is more, the strain residual is typically larger than 100% for hydrogels and there is also serious plastic deformationterm·dom. The present SACPterm exhibits relatively good electrical properties (up to 37 S/cm) and much superior mechanical properties, i.e., lower modulus (up to 56 kPa), low strain residual (<50% at 500% tensile strain), smaller plastic deformationterm·dom, and larger stretchabilityterm·key (up to 700%), rendering it one promising candidate for soft electronicsterm·dom. Taking into account the compromise in mechanical flexibility, conductivity, and interface adhesionterm·key (in following discussions), SACPsterm with PEDOT:PSSterm mass ratio of 3.6% presented suitable mechanical property (modulus of 401.9 kPa) and conductivity (3.79 S/cm) meet the requirements of bioelectrodeterm·key.

Interface adhesionterm·key performance Traditional PEDOT:PSSterm films will break apart during repeated bending cycles, and the poor interface adhesionterm·key makes them easy to peel off from the substrate (Fig. 3a ). The present SACPterm film exhibits high adhesionterm·key performance on PI and elastic Ecoflex substrates (Fig. 3 b, c and Supplementary Video 2 ). The adhesionterm·key force of the SACPterm film on the substrate can tolerate a variety of load conditions (hanging, pulling, and lifting) (Fig. 1c and Fig. S10 ), and even applicable to biological tissues such as the liver surface of pigs (Fig. 3d ). Fig. 3 Interface adhesionterm·key performance of SACPterm films. a – b Images of pure PEDOT:PSSterm ( a ) and SACPterm ( b ) films on PI substrate, insert: magnifications of adhesionterm·key interfaces. c Image of peeling off PI film from the adhesive film. d Image of adhesionterm·key between adhesionterm·key film and liver of a pig (the liver surface was cleaned by absorbent paper). e Illustration (left) and image (right) of 180° peeling tests of SACPterm films. f Measured peeling force per width of SACPterm films with different PEDOT:PSSterm loadings on PI substrates. g Summary of adhesionterm·key strength of SACPterm films on a wide variety of substrates. h Images of the bending state of substrates by releasing pre-stretched SACPterm film-adhered substrates. i Adhesionterm·key stability of SACPterm films after being stored in the constant temperature and humidity (temperature 30 °C, humidity 40%) for different days. j Adhesionterm·key strength of cyclic adhesionterm·key tests. k Schematic illustration (left) and image (right) of lap shear tests. l Lap shear strength of SACPterm films with different thicknesses. m Summary of lap shear strength of SACPterm films on different substrates. The error bars show the standard deviations from at least three tests.

We first investigated the adhesionterm·key strength of SACPterm by applying a 180-degree peeling test (Fig. 3e ). The results show that the adhesionterm·key depends on the concentration of PEDOT: PSS in SACPterm films. The adhesionterm·key force increases as the PEDOT: PSS content decreases, and the interfacial adhesive force is about 400 N/m for the PEDOT:PSSterm content at 0.9% (Fig. 3f ). Importantly, the self-adhesive film shows fast bonding ability (30 s) (Fig. S11 ), owing to multiple interactions such as hydrogen bonds, ionic interaction, and Van der Waals’ interactions 46 . Notably, the SMSterm of citric acid and cyclodextrin has good interfacial adhesionterm·key (several MPa) 34 . More importantly, the SACPterm film shows substantially high performance of adhesionterm·key on diverse substrates (>150 N/m for PI; >120 N/m for PEEK; >100 N/m for Al, Cu, and PET; >80 N/m for PTFE; >30 N/m for PDMS) (Fig. 3g ). The differences in interfacial adhesionterm·key forces of various polymer substrates were mainly dependent on molecular polarization strength of polymer chains and variations in chemical structures 10 , 47 . The high adhesionterm·key strength on PI or PEEK substrate can be ascribed to the strong dipole interactions of –C=O groups of their polymer primary chains. Meanwhile, PE or PDMS substrate has low interface adhesionterm·key strength due to their weak dipole interactions of polymer primary chains. Besides, the interface roughness and interface energy of substrate are also important factors affecting adhesionterm·key strength. The interface adhesionterm·key strength is even better than that of commercial test tapes (Fig. S12 ). The strength of interface adhesionterm·key on the substrate is strong enough to overcome the bending stress of different substrates (TPU, PC, PI, and TPFE) (Fig. 3h and Supplementary Video 3 ). Besides, the SACPterm films show remarkably high stability in storage and repeated usage. We did not observe any obvious decay in adhesionterm·key strength by storing the film (temperature at 30 °C, humidity of 40%) for 30 days (Fig. 3i ). Although we observed the adhesionterm·key strength reduced by 25% for a second time of the adhesionterm·key on PI substrate, the adhesionterm·key performance remains stable in the subsequent 100 cycles (Fig. 3j ). This feature of repeated adhesionterm·key performance may be originated from physical interaction on the interface, avoiding irreversible damage of delamination 47 .

We further investigated the lap shear strength of SACPterm on diverse substrates. The lap shear test was conducted by depositing the SACPterm film (<50 μm in thickness) on diverse substrates (Fig. 3k ). We observe that the shear adhesionterm·key force increases from 0.6 to ~1 MPa as the thickness of the film increases from 7 to 20 μm as shown in Fig. 3l , and finally reaches a stable adhesionterm·key strength (over 1.2 MPa) from 28 to 43 μm, which is larger than that of the PU adhesive layer 30 . Such a strong lap shear strength can also be visualized by the lifting up of a 5 kg weight by the overlapped film (Fig. 1c ). Like the adhesionterm·key strength, the lap shear strength shows a similar trend relating to different substrates (Fig. 3m ). However, lap shear adhesionterm·key strength is much greater than the peeling adhesionterm·key strength, indicating a significant adhesionterm·key anisotropy in the adhesionterm·key performance of the SACPterm thin film. The strong lap shear adhesionterm·key strength is of significance to improve the stability and durability of SACPterm thin-film devices.

Solution processability SACPterm can be processed by various solution-processing techniques owing to its excellent stability, homogeneity, and “coffee ring” free patterning capability. Although the contact angle of the SACPterm solution shows a slight fluctuation on different substrates (Fig. S13 ), the fluid dynamics of the droplets and the resultant patterns on the substrate are consistent (Fig. S14 ). As a proof-of-concept, we demonstrated the fabrication of SACPterm films and patterns by various solution process techniques as well as their applications in transfer printing and flexible ACEL devices.

Figure 4a–c shows the solution-processing fabrication of SACPterm films and patterns, e.g., microfluid molding, drop-casting, and spin-coating. SACPterm films can be deposited in microchannels, leading to the formation of “U” shape films on the walls of microchannels (Fig. 4a and Fig. S15 ). The combination of drop-casting and laser patterning techniques enables the fabrication of SACP-patterned Ecoflex. As shown in Fig. 4b (i)–(iii) , the as-made patterned film showed no delamination from Ecoflex substrate under repeated stretching, indicating high adhesionterm·key stability of SACPsterm on the soft dynamic surface. In addition, a transparent conductive film with a maximum area of over 8 × 8 cm 2 can be obtained by spin coating (Fig. 4c ), which can serve as transparent electrodes in photoelectric devices 48 . We also demonstrated the fabrication of large-size (13 × 19 cm 2 ) transparent yet adhesive thin film on PET substrate by using Meyer rod coating method, and the corresponding SACPsterm conductive film can be used as electrodes of a large-area ACEL device with an area of 9 × 14 cm 2 (Fig. S16 ). The transmittance of the SACPterm film is close to 70–95% at 550 nm (Fig. 4e ). The as-made SACPterm film exhibits square resistance of about 1000 Ω/□ with the light transmittance at 90% (Fig. S17a ) and good bending stability (Fig. S17 b, c ). We further demonstrated the transfer printing of the SACPterm patterns onto PI from PTFE substrates according to their difference in adhesionterm·key (Fig. 4d and Supplementary Videos 4 ). We also demonstrated the fabrication of SACPterm films on diverse substrates (PDMS, PET, PI, TPFE, glass, and copper) by spin coating (Fig. 4f ). The transparent conductive film on the PET substrate was not broken even pasted by commercial test tape several times and no obvious electrical resistance changes were observed (Fig. 4g and Fig. S17d ). The self-adhesive conductive film can be cut and adhered face-to-face to form a mechanically durable and electrically conductive connection (Fig. 4h and Fig. S17e ). Similar to the superiority of self-healing organic conducting film on electrical recovery after mechanical damage 49 , 50 , this SACPterm film may also open a way to electrical recovery. Moreover, the SACPterm film exhibits a lower impedance as compared with the traditional PEDOT:PSSterm (Fig. S18 ). Fig. 4 Solution processing and patterning of SACPterm patterns and thin films for flexible or even transparent electrodes. a Microfluid molding. b Drop casting. c Spin coating. d Transfer-printing of SACPterm patterns for lighting up an LED lamp from Ecoflex to PI substrate enabled by the difference of adhesionterm·key strength. e UV–vis spectrum and images of different transmittance SACPterm films. f Images of SACPterm films on various substrates by spin-coating. g Images of SACPterm films under repeating peeling tests by commercial test tapes. h Images of adhered PET film under bending and twisting. i Image of alternating current electroluminescence (ACEL) devices by applying SACPterm thin film as transparent electrodes. j Scanning electron microscope (SEM) image of the cross section of the ACEL devices. k Images of the ACEL device (i) under folding (ii), bending (iii) and wrinkling (iv). l Images of the ACEL device immersed in water for 24 h under bright (i) and dark (ii) backgrounds.

We fabricated ACEL devices based on the SACPterm transparent thin-film electrodes by spin-coating. In the experimentterm·mth, the bottom SACPterm electrode (7 μm), the light-emitting layer (ZnS:Cu particles and PDMS, 75 μm), the top SACPterm electrode (7 μm) and the PDMS encapsulation layer (35 μm) were spin-coated on the PET substrate (5 μm) layer by layer, and thus ACEL thin-film device was obtained (Fig. 4i ). The thickness of ACEL film does not exceed 150 μm (Fig. 4j ). ACEL devices have exceptionally high performance in flexibility and can withstand various deformations including bending, kneading, folding even multiple folds (Fig. 4k , Fig. S19 a–c and Supplementary Videos 5 ). Not limited to the ultra-high flexibility, the ACEL device is highly stretchable when using an elastic substrate, and the emission intensity increased at low strain then decreased at high strain, because the electrical field is varied by the trade-off between the resistance and space of electrode under strain (Fig. S19 d–h ). Such a thin and light structure allows the working ACEL device to flutter in the wind. In addition, the good interface adhesionterm·key performance renders it a good packaging, and as such the ACEL device can work stably even in water for 24 h (Fig. 4l ).

Physiological electrical potential monitoring The SACPterm film is an ideal candidate for bioelectronics owing to its low modulus, high adhesionterm·key strength, and good conductivity 15 , 51 . The SACPterm film can conformally adhere to human skin (Fig. 5 a–c ). The adhered film deforms synchronously when skin suffers from various mechanical deformations, i.e., stretching, compressing, and distorting (Fig. 5d ). As a proof-of-concept, SACPterm films are applied as conductive interfaces between human skin and electromyograph (Fig. 5e and f ). The biological EMG signals of a human hand are monitored at different gripping forces. The results reveal that the performance of the adhesive and conductive film electrode is comparable to a commercial Ag/AgCl electrode (Fig. 5g ) and the other conductive materials in literatures 11 , 13 , 15 . By applying 5, 10, 15, and 20 kilograms of forces to the grip strength meter, the SACPterm electrode can monitor the steady increase of the EMG signals (Fig. 5h and i ). In addition, the EMG signals do not show significant attenuation even when the SACPterm electrodes had been used ten times (Fig. 5j and k ). Fig. 5 Skin adhesionterm·key and electromyography (EMG) monitoring of human exercises by using SACPterm electrodes. a – b Adhesionterm·key of films on joints of fingers ( a ) and skin of arm ( b ). c Images of skin texture patterned on an SACPterm film after peeling off from the skin. d Images of SACPterm films adhered on the skin under distorting. e Interface connection between skin and flexible printed wires. f Image of an EMG test: SACPterm film electrodes were attached to the skin near the muscle, and gripping forces were displayed by a dynamometer. g Comparison of EMG signals by using commercial Ag/AgCl electrode (red line) and SACPterm film (black line). h – i EMG signals ( h ) and peak-to-peak value ( i ) under different gripping forces. j – k Stability of EMG signals ( j ) and corresponding peak to peak values ( k ) under repeated using test. The error bars show the standard deviations from at least three tests. l Detection of body exercises from EMG signal and the corresponding images (inset), jumping (i), squatting (ii), dumbbell lifting (iii), and push-up (iv).

We can also monitor various human-body activities by mounting the SACPterm electrodes on different muscle positions (Fig. 5l and Supplementary Videos 6 ). For example, by adhering it to leg muscles, we can monitor body motion by recording EMG signals of jumps and squats (Fig. 5l (i) and (ii)). We can record two characteristic EMG signals during a one-time jump or squat motion. For jumps, the EMG signals of jumping up were larger than that of touching down. While, the EMG signals of squatting down and standing up are almost the same, in the squat-up motion. Besides, we also recorded completely different EMG signal characteristics between dumbbell lifting and push-ups by adhering self-adhesive conductive films to arm muscles (Fig. 5l (iii) and (iv)). The EMG signals generated by continuous arm muscle contraction can be recorded.

Integrations of EMG sensor and ACEL arrays The outputs of EMG signals can be further harvested to trigger the ACEL arrays 52 . The physical movement, especially the strength, can be directly visualized and monitored by ACEL devices. To validate this concept, we designed and fabricated an integrated system for the visualization of muscle training by ACEL. The system consists of two units, i.e., EMG monitoring and displayable ACEL array (Fig. 6a ). The ACEL array is fabricated by spin coating (Fig. 6b ). A set of EMG monitoring electrodes (Fig. 6c , (i)) attached to the forearm transmits the biopotential of the hand muscle to the EMG sensor (Fig. 6c , (ii)), and then the EMG signal is converted into the corresponding value through a signal analysis and control unit (Fig. 6c , (iii)). In accordance with EMG signals, the pixels of ACEL arrays are turned on/off depending on the threshold values, realizing the visualization detection of EMG signals of muscle motion (Fig. 6c , (iv)). As shown in Fig. 6d , by applying different levels of grip strength, the corresponding EMG signals are detected. After signal filtering analysis and processing, a set of control signals are generated to control the on/off of ACEL arrays. By setting appropriate switching thresholds, the ACEL arrays display according to the strength of gripping (Supplementary Videos 7 ). SACPsterm serve not only the bioelectrodesterm·key for harvesting the EMG signals but also the adhesive transparent thin-film electrodes for ACEL devices. This study demonstrates a concept of the direct visualization of the physiological electrical signals of the muscles during physical movement and paves the way to develop wearable bioelectronic devices that can visualize the activity strength of our daily life in a real-time fashion. Fig. 6 Demonstration of an integrated EMG visualization system with an array of ACEL devices. a Schematic summary of the system with ACEL display controlled by EMG sensors. b ACEL display with patterned lighting arrays assembled by the transparent conductive film. c Four functional units in the system: (i) conductive film electrode for EMG signals transformation, (ii) commercial EMG sensors for data recording, (iii) commercial single-chip microcomputer for ACEL control, and (iv) flexible ACEL devices for EMG signals display. d Patterns of flexible ACEL display turning on and off based on the level of EMG signals.

Discussions SACPterm is a type of soft, adhesive, and conductive polymer compositeterm·dom that is easily fabricated by doping the SMSterm into the rigid and nonstick PEDOT: PSS composite. Compared with the other kinds of PEDOT: PSS-based conductors, SACPterm simultaneously possesses low modulus, strong interface adhesionterm·key strength, and high conductivity in one material, rendering it an ideal candidate for soft electrical interfaces in soft electronicsterm·dom. By using SACPterm, we have fabricated self-adhesive conductive films/patterns via a variety of solution processing methods such as drop coating, spin coating, and microfluid molding, and even transfer printing of a pre-fabricated circuit between different substrates. The proof-of-concept demonstrations of the SACPterm, as soft and conformal bioelectronic devices, are carried out, including thin film ACEL devices and bioelectrodesterm·key for monitoring EMG signals of muscle during exercising. We finally verify the possibility to visualize muscle training by an SACP-based bioelectronic system that integrates EMG sensors and ACEL flexible arrays. The SACP-based electronics show promising features to further develop wearable and comfortable bioelectronic devices with the physiological electric signals of the human body readable and displayable during daily activities.

Methods Preparation of SACPterm films The SACPterm films were prepared by drying SACPterm precursor solution. The SACPterm precursor solution was prepared by mixing SMSterm solution, polymer solution, and conductive polymerterm·key solution. SMSterm solution comprised of β-cyclodextrin (β-CD, AR, Aladdin) and citric acid (H 3 Cit, AR, Aladdin), which were mixed in water at the molar ratio of 1:10 under agitation 34 . Polymer solution contained poly (vinyl alcohol) (PVA 1799, DP of 1700, 98–99% hydrolyzed, Aladdin) aqueous solution as polymer network and glutaraldehyde (GA, 50% volume percentage, Macklin) as cross-linker. Finally, SACPterm precursor solutions were obtained by adding SMSterm solution and polymer solution into PEDOT: PSS aqueous solution (1.1–1.3% solid content, Clevios™ PH1000, Heraeus Electronic Materials) under vigorous agitation for 10 min. The pH value of SACPsterm precursor solution was about 3–4 due to the presence of citric acid. The crosslinking reaction of GA and PVA was triggered in presence of H 3 Cit as an accelerator. The crosslinking density of the PVA networkterm was controlled by adjusting the added volume of GA. The details of SACPsterm with various mass ratios of PEDOT:PSSterm were presented in Supplementary Table 1 . The mass ration of PEDOT:PSSterm ( X ) in SACPsterm was calculated according to the following equation: 1 X = X 0 m 0 X 0 m 0 + m SMSterm + m PVA + m GA × 100 % Where, X 0 is the weight percentage of PEDOT:PSSterm in original solution (≈1.3%); m 0 is the weight of PEDOT:PSSterm solution; m SMSterm , m PVA, and m GA is the weight of SMSterm, PVA, and GA, respectively.

For thick SACPterm films, we applied a casting method. Briefly, after degassing in a vacuum desiccator for 20 min, the SACPterm dispersion was poured into a polycarbonate (PC) mold and dried at 40 °C for 24 h. Thus, free-standing SACPterm films were obtained. The film thicknesses were controlled by changing the volume of the SACPterm precursor solution. For SACPterm thin films, we applied a spin-coating method. In a typical experimentterm·mth, the substrates were cleaned by DI water and alcohol, respectively. Then surface hydrophilic treatment of substrates was conducted by plasma cleaning for 15 min. Thin SACPterm films were deposited on substrates by spin-coating at speed of 1000 rpm for 60 seconds and drying at 70 o C for 5 min. By changing the conditions of spin-coating, SACPterm films with diverse thicknesses were obtained. For the microfluid molding method, a PDMS microchannel was obtained by solidifying PDMS precursor (10:1, Sylgard 184, Dow Corning) on a homemade silicon wafer template patterned with a photoresist. Then, the resultant PDMS microchannel was covered by a PET film with a hole punched on the end of the channel. The hole was sealed with an SACPterm precursor solution droplet (2–3 g) and then degassed in a vacuum desiccator. Afterward, the SACPterm precursor solution was injected into PDMS microchannel once the air pressure of the vacuum desiccator gradually returns to atmospheric pressure, due to the pressure difference between the inside and outside of the channel, as detailed in our previous work 53 . After removing the covered PET film and drying at 40 °C for 24 h, 3D deposited SACPterm films were obtained.

Measurement of the electrical performance of SACPsterm Conductivity measurements of all samples were carried out using the 4-point probe method (Keithley 2400 digital multimeter, Keithley). A rectangle film of SACPterm composite (about 30–40 mm of length, 10 mm of width) was used to test sample electrical conductivity. The thickness of the sample film was obtained by an optical microscope. Silver paste (conductive silver paint, SPI) was cast on both ends of the sample as electrodes. Electrical conductivity was calculated from the following equation: 2 σ = 1 R L w h where σ is conductivity (S/cm) and R is resistance (Ω). L , w , and h denote for length, width, and thickness of the sample (cm), respectively. Conductivity values of each condition were obtained by averaging over a minimum of three to five measurements. Resistance vs. strain relationship scrutinized by concurrent measurement of mechanical tensile strain and resistance. The sample was clamped in a tensile stage (Tensile testing machine, CMT 6103, MTS) with silver paste cast on each end as the electrode and stretched at a constant tensile speed of 10 mm/min.

Measurement of the mechanical and adhesionterm·key performance of SACPsterm Tensile strain tests of all samples were carried out by a mechanical testing machine with a 50 N load cell. The measurement was conducted at 25 °C with a relative humidity of 40–45%. The reported statistical data were obtained from at least three times tests of the same sample. Standard rectangular test samples (length ≈30–40 mm, width ≈10 mm) were clamped in the tensile stage and stretched at a tensile speed of 50 mm/min, if not stated otherwise. Cyclic loading-unloading tests were conducted by stretching the samples to a strain of 200% for 100 cycles. Step loading-unloading tests were performed at a multi-step strain from 50%, 100%, 200%, 300%, 400% and 500%, respectively.

Adhesionterm·key forces of free-standing SACPterm films adhering to various substrates were measured using a 180° peeling test with the mechanical testing machine. All substrates were treated using 3 M tape to remove surface contaminants. Then, free-standing SACPterm film was attached to the diverse substrates. The dimensions of the samples were 8-10 mm (width) and 60–80 mm (length). Loading-displacement curves were conducted with a tensile mold at a peeling test speed of 50 mm/min. To analyze the stability of adhesionterm·key force, various contact times (0.5, 1, 30, 60, and 120 min) of the substrate in contact with SACPterm film were conducted by measuring the adhesionterm·key force of 180° peeling test. Cyclic adhesionterm·key force tests were performed. The adhesionterm·key force was determined by averaging the peeling force ( F ) and then dividing the force by the width of the sample ( w ).

The interfacial shear strength was measured by a lap-shear test. The lap-shear test samples were prepared by spin-coating SACPterm film on the substrate. A film same to the substrate was covered on the upper surface of the SACPterm film with an overlap area of 10 mm (length) × 10 mm (width). The tap-shear tests were conducted at the tensile speed of 50 mm/min. The interfacial shear strength was determined by dividing the peak measured force by the overlap area.

Preparation of ACEL devices The ACEL devices were prepared by spin-coating. A PET film (~10 μm in thickness) was used as the substrate, SACPterm films were used as top/bottom electrodes, and ZnS:Cu/PDMS was used as the emissive layer. The ZnS:Cu/PDMS composite was prepared by mixing ZnS: Cu microparticles (2 g, Shanghai KPT company) and PDMS (2 g, Sylgard 184, Dow Corning). The PET films were covered on glass and surface treated by a plasma cleaning machine (HARRICK Plasma PDC-002) with 400–500 mtorr in air for 15 min. The SACPterm dispersion was coated on PET film as the bottom electrode at a spin rate of 1000 rpm for 30 s, then dried at 70 °C for 5 min. The ZnS: Cu/PDMS composite was coated onto the SACPterm bottom electrode at a spin rate of 2000 rpm for 120 s. The ZnS:Cu/PDMS was thermally cured at 60 °C for 2 h. To improve deposition of the top electrode, ZnS:Cu/PDMS layer was surface treated by plasma cleaning for 15 min. The top electrode of the SACPterm layer was coated by a spin rate of 1000 rpm for 30 s and dried at 70 °C for 5 min. Finally, a pure PDMS package layer was coated on by a spin rate of 1500 rpm for 30 s. Silver wires were connected to the top and bottom SACPterm layer. The ACEL device was lighted by an input power purchased from the Shanghai KPT company.

Monitoring of EMG signals To evaluate the performance of bioelectric communication of tissue-electrode interfaces, we recorded EMG signals by using the SACPterm films as bioelectrodesterm·key. Two conductive films were pasted on the forearm, and another conductive film was pasted on the upper arm, serving as three sets of electrodes for EMG signals monitoring. Then a certain force was applied to the grip meter by hand to stimulate the generation of electromyographic signals. For comparison between SACPterm films and commercial electrodes, the commercial Ag/AgCl electrode was also used. The EMG signals were recorded by a digital oscilloscope (TBS 1152B, Tektronix).

The integrated system of EMG visualization detection by ACEL devices An integrated system of EMG sensor and ACEL array to realize the visualization detection of EMG signals. The system includes self-adhesive electrodes, EMG sensors, signal analysis controllers, and ACEL arrays. ACEL array was prepared by spin coating with a patterned template. The signal analysis controller is a single-chip microcomputer from Arduino, for processing EMG signals and controlling electronic switches. Each column of the ACEL array was controlled by electronic switches. The threshold of electronic switches was set into four levels in a single-chip microcomputer according to the EMG signals. The strength of EMG signal was determined by the single-chip microcomputer, and then was applied to control the ACEL array through electronic switches. The Arduino code for the integrated system of EMG visualization detection was provided in supplementary files.

Morphological and chemical characterization Scanning electron microscopy (SEM) was executed on APREO S (Thermo scientific). SEM samples were prepared by drying under heat. Atomic force microscopy (AFM) was performed using a XE15 Litho AFM (Park Systems) in tapping mode with NCHR cantilever (spring constant of 42 N/m). Optical microscope images were obtained on an optical microscope (SMZ-18, Nikon) equipped with a CMOS camera (DS Ri-2). Optical images were taken with a digital camera (D7100, Nikon). The impedance spectra were taken on an LCR meter (Keithley) with the dual-electrode method in the ranges of 10 2 –10 6 Hz. UV–vis spectra were collected on a UV-2550 (Shimadzu) spectrophotometer. Apparent viscosity analysis was conducted by using a TA rheometer. The size distribution of PEDOT was measured using dynamic light scattering (DelsaMax CORE). The Surface element component was analyzed by X-ray photoelectron spectroscopy (Thermo Fisher Scientific) using Al Kα radiation. The Raman spectrum was recorded using microscopic laser confocal Raman spectrometer from Renishaw. The characterization of the chemical structure was measured by Fourier transform infrared spectrometer (IR Affinity-1, Shimadzu) and X-ray diffraction (D8ADVANCE, Bruker).

Reporting summary Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Supplementary information Supplementary Information Peer Review File Description of Additional Supplementary Files Supplementary Movie 1 Supplementary Movie 2 Supplementary Movie 3 Supplementary Movie 4 Supplementary Movie 5 Supplementary Movie 6 Supplementary Movie 7 Reporting Summary