An Ultrastretchable and Self-Healable Nanocomposite Conductor

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An Ultra-Stretchable and Self-Healable Nanocomposite Conductor Enabled by Autonomously Percolative Electrical Pathways Sun Hong Kim, Hyunseon Seo, Jiheong Kang, Jaeyoung Hong, Duhwan Seong, HanJin Kim, Jaemin Kim, Jaewan Mun, Inchan Youn, Jinseok Kim, Yu-Chan Kim, HyunKwang Seok, Changhee Lee, Jeffery B.-H. Tok, Zhenan Bao, and Donghee Son ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00160 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019

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An Ultra-Stretchable and Self-Healable Nanocomposite Conductor Enabled by Autonomously Percolative Electrical Pathways Sun Hong Kim1†, Hyunseon Seo2†, Jiheong Kang3†, Jaeyoung Hong4, Duhwan Seong2, Han-Jin Kim5, Jaemin Kim3, Jaewan Mun3, Inchan Youn2, Jinseok Kim2, Yu-Chan Kim2, Hyun-Kwang Seok2, Changhee Lee1, Jeffrey B.-H. Tok3, Zhenan Bao3*, Donghee Son2*

1 Department of Electrical and Computer Engineering, Inter-University Semiconductor Research

Center, Seoul National University, 1-Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea 2

Biomedical Research Institute, Korea Institute of Science and Technology, Seoul, 02792,

Republic of Korea 3

Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025, USA

4

Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, 02792, Republic

of Korea 5

Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic

of Korea

†These

authors contributed equally to this work

*To whom correspondence should be addressed. E-mail: [email protected] & [email protected]

KEYWORDS: nanocomposite conductor, electrical self-boosting, self-healability, ultrastretchability, human-robot interfaces

1 ACS Paragon Plus Environment

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ABSTRACT Both self-healable conductors and stretchable conductors have been previously reported. However, it is still difficult to simultaneously achieve high stretchability, high conductivity and self-healability. Here, we observed an intriguing phenomenon, termed “electrical self-boosting”, which enables reconstructing of electrically percolative pathways in an ultra-stretchable and selfhealable nanocomposite conductor (over 1700% strain). The autonomously reconstructed percolative pathways were directly verified by using micro-computed tomography and in-situ scanning electron microscopy. The encapsulated nanocomposite conductor shows exceptional conductivity (average value: 2578 S cm-1; highest value: 3086 S cm-1) at 3500% tensile strain by virtue of efficient strain energy dissipation of the self-healing polymer, self-alignment and rearrangement of silver flakes surrounded by spontaneously formed silver nanoparticles and their self-assembly in the strained self-healing polymer matrix. In addition, the conductor maintains high conductivity and stretchability even after recovered from a complete cut. Besides, a design of double-layered conductor enabled by the self-bonding assembly allowed a conducting interface to be located on the neutral mechanical plane, showing extremely durable operations in a cyclic stretching test. Finally, we successfully demonstrated electromyogram signals can be monitored by our self-healable interconnects. Such information was transmitted to a prosthetic robot to control various hand motions for robust interactive human-robot interfaces.


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Self-healing is a prevalent phenomenon in nature. It has inspired investigations in interdisciplinary research on biomimetics,1-3 robotics,4,5 and stretchable electronics,6-14 in hoping to achieve electronic skin devices that offer the opportunity to advance healthcare and wearable electronics. Specifically, developments of intrinsically stretchable and self-healable conductors are desirable to interface with active electronic modules that provide reliable and efficient power and data transmission even after mechanical damages.15,16 However, simultaneously possessing high stretchability, high conductivity and autonomous self-healability in conductors has been challenging. Nonetheless, several groups have made significant advances. For example, Kotov et al. reported stretchable composite conductors in which gold nanoparticles can reorganize inside a polymer matrix under tensile stress while showing conductivity of 35 S cm-1 at 485% strain.17 Similarly, Coleman et al. reported that the low crosslinking density of polymer matrix allows the graphene nanosheets to be more mobile, which enabled extremely sensitive electromechanical sensors.18 However, the conductivity reported for high strain remained low. Someya et al. described printable conductors with improved conductivity of 935 S cm-1 at 400% strain through in-situ formation of silver nanoparticles (AgNPs) from silver flakes (AgFs) in a fluorinated elastomer.19 In addition to stretchability and conductivity, another highly desired feature is self-healability. Many research groups have reported self-healing electronic materials based on different approaches.20-26 For example, embedded microcapsules composed of a monomer and a catalyst were utilized to heal cracks.21 However, healing can only take place once in the same location. External stimuli such as ultraviolet (UV) light and heat can also be introduced into self-healing polymers.21,24 Recently, we developed a highly stretchable and tough self-healing polymer that


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has multi-strength hydrogen bond interactions in crosslinking network.27 Furthermore, EGaIn (liquid metal) was used as the conductive layer supported on such a self-healing polymeric substrate. Although this demonstrated the highly stretchable and self-healable electrode, liquidtype electrode is difficult to apply to conventional electronic devices and processes. Onedimensional conducting nanomaterials have also been deposited on self-healing substrates as a semitransparent electrode for a multi-component electronic skin system.28 However, the selfhealing efficiency of electrical conductivity was low because it was hard to align the conducting layer on the surface between two cut electrodes. Furthermore, the conductivity was significantly degraded when stretched to ~250% strain. Here, we report an stretchable and highly conductive nanocomposite with electrical and mechanical self-healability, fabricated by combining the a self-healing polymer (SHP, PDMS4,4’-methylenebis(phenyl urea) (MPU)0.4-isophorone bisurea units (IU)0.6)27 and 2-dimensional AgFs. This nanocomposite conductor can be fabricated by drop-casting the concentrated composite chloroform solution onto octadecyltrimethoxysilane (OTMS)-treated silicon wafer or Teflon substrates without any heat or surfactant. The conductivity of our nanocomposite conductor is as high as 1137 S cm-1 even under 3500% tensile strain. Most interestingly, conductivity gradually increased to 3086 S cm-1 over 60 hrs under 3500% strain. We hypothesized that such stretchability and self-boosted high conductivity result from a synergistic effect: an efficient strain energy dissipation of SHP and self-alignment and rearrangement of AgFs with spontaneously assembled AgNPs in response to dynamic nature of the strained polymer matrix. This rearrangement phenomenon of AgFs-AgNPs under strain is confirmed by using micro-computed tomography (μ-CT) and in-situ scanning electron microscopy (in-situ


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SEM). Furthermore, we observed that a double-layered conducting structure formed using a selfbonding process showed extremely reliable stretching endurance over 1000 cycles at 50% strain, owing to its effective conducting interface located on the neutral mechanical plane. Finally, we successfully demonstrated that electromyogram (EMG) signals recorded by a flexible wireless bio-integrated system or commercial sensing modules were transmitted in real-time to a prosthetic robot arm to control various hand motions after a complete cut and autonomous selfhealing.

RESULTS AND DISCUSSION An ultra-stretchable and self-healable nanocomposite conductor was fabricated by mixing AgFs with the SHP (Figure 1a, Figure S1, and Methods). Such a free-standing structure can be transfer-printed onto the desired target substrates or devices as conductive interconnects allowing highly integrated architectures. The size distribution of the AgFs in the as-prepared composite conductor ranges from ~500 nm to 2 μm, which was confirmed by dynamic light scattering (Figure S2). To obtain a homogeneous conducting film, the viscosity of as-prepared AgF-SHP solution needs to be carefully controlled due to fast evaporation of the chloroform solvent in the ambient condition (Figure S3). Using this methodology, the initial conductivities of nanocomposite conductors with different ratios of AgF to SHP were measured and compared to the data fitted by a 3-dimensional percolation theory (Figure S4 and Note S1).29 The graph showed that the theoretical percolation threshold (Vc) of 12.12% is similar to that of the nanocomposite conductor (AgF:SHP, 1.5:1). The initial conductivity was improved from 19 S cm-1 to 833 S cm-1 when the weight ratio of AgFs to SHP was increased (Figure S5). Trend of


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our results matched well with results previously reported by other groups.19,30 In addition, areal uniformity of the conducting film was measured by testing 15 regions (10 mm  10 mm) over 20.25 cm2 area showing variation of 7.84% (Figure S6). Next, we characterized electrical properties of our nanocomposite conductors with various weight ratios of AgF to SHP (2:1 to 5:1) while stretching the samples up to 1200% strain at a stretching speed of 100% per minute (Figure 1b). The nanocomposite conductor with a ratio of 4:1 showed the best conductivity (741 S cm-1) at 1200% strain. When stretched beyond 1200% strain, the breakdown strain was observed to be dependent on the percentage of SHP in the conductor (Figure 1c). The optimized nanocomposite conductor (a ratio of 4:1) was capable of not only being stretched up to 1700% strain, but also maintaining high conductivity of 599 S cm-1 (Figure 1d and Figure S7). In addition, we tested the dependence of the nanocomposite conductor properties on the annealing temperatures (at both 80 °C and 130 °C). Compared to the nanocomposite conductor fabricated at r.t., the stretchability and conductivity of the conductors at 80 °C or 130 °C were degraded, most likely due to AgF oxidation or aggregation in the polymer matrix (Figure 1d). An interesting observation is an electrical self-boosting phenomenon under tensile strain through rearrangement of AgFs in the SHP matrix. We observed that the final conductivity was 2579 S cm-1 at 1700% strain after 20 hrs (Figure 1d). This drastic improvement of conductivity under strain has not been reported previously. To further investigate this process in detail, real-time monitoring of resistance of the nanocomposite conductor was performed on every strain interval of 400% and followed by a 20 min of hold (Figure 1e,f, and Methods). After 1600% strain, we monitored the electrical self-recovery of the nanocomposite conductor (Figure 1f, blue-line). The maximum resistance values immediately after reaching at each strain of 400%,


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800%, 1200%, and 1600% were 3.19 Ω, 9.49 Ω, 49.8 Ω and 4.79 kΩ, respectively. After being held at 20 min at these strains, the resistance value of each state decreased to 2.62 Ω, 6.61 Ω, 23.91 Ω and 102.68 Ω. In addition, after 11 hrs of holding, the resistance value became 17.8 Ω, indicating that the conductivity improved continuously after the initial state when stretched. To gain further insights, each conductivity at 1000% strain before and after rearrangement sequence (releasing to 800% strain; rest for 2 hrs; stretching to 1000%) is compared (Figure 1g). The conductivity of the initial state is 608 S cm-1, while that of the rearranged state shows 841 S cm-1. Based on this result, we hypothesized that this phenomenon of “electrical self-boosting”, is probably related to the dynamic behaviors of the SHP matrix as further discussed below. Besides, when the nanocomposite conductor was stretched to above 100% strain, it was not fully reversible due to their plastic deformation (Figure 1c and Figure S8). However, if it is encapsulated by the SHP layer, its mechanical reversibility can be regained well (Figure S9,10). Furthermore, both conductivity and stretchability of the encapsulated conductor were significantly improved (Figure 2a,b). The conductivity of the encapsulated conductor was 632 S cm-1 (highest, 1137 S cm-1) at 3500% strain and increased up to 2578 S cm-1 (highest, 3086 S cm-1) after 60 hrs of rearrangement (Figure S11,12). To the best of our knowledge, this conductivity at 3500% strain is a record-high value (Figure S13). Although initial conductivity of the encapsulated conductor was not as high as those of other electrodes,17,19 its low resistance values (0.5 Ω at 0% strain, 15.2 Ω at 3500% strain and decreased to 5.6 Ω after 60 hrs) are enough to support its practicability within a range of ~200% strain. Such electrical and mechanical performances are attributed to efficient strain energy dissipation at homogeneous self-bonding interfaces between the conductor and the encapsulation layer, differentiating our interface from


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previously reported ones (Figure 2c).19 The improved fracture energy (3390 J m-2) of the encapsulated conductor fully supported our assumption (Figure 2d and Figure S14).31,32 Specifically, the electrical self-boosting effect of our conductor would allow conventional electronic devices to be still working well even under extreme strain conditions. As a proof-ofconcept example, we prepared a simple set-up which consists of the encapsulated and standalone conductors connected to each commercial diode while stretching up to 1700% strain (Figure 2e,f). As we expected, the diode interfaced with the encapsulated conductor did not show critical changes at 1700% strain owing to its high conductivity (Figure 2e). In the other case, although the slope of current-voltage curve of the diode changed significantly due to its degraded conductivity at 1700% strain, dynamic rearrangement of AgFs in the conductor fully recovered the diode performances after 60 min (Figure 2f). A schematic diagram of the nanocomposite conductor shows uniformly distributed AgFs surrounded by spontaneously formed AgNPs in the SHP matrix (Figure 3a). AgNPs, formed by reactions of Ag+ ions diffused from AgFs with carbonyl group of the SHP, were directly observed by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) (Figure 3b). The unsaturated carbonyl group in the composite may donate the electrons to the diffused Ag+ ions.13 The AgNPs may contribute to the electrical paths in the nanocomposite conductor (Note S2).19 The contribution of AgNPs formed in the SHP matrix to the electrical performance was also indirectly confirmed by comparing the electrical properties of AgF composites fabricated using other elastomers (Figure S15,16) and by adding AgNPs to our conductor (Figure S17).


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Schematic illustration of the hypothesized mechanism for the electrical self-boosting under tensile strain is shown in Figure 3c. We hypothesized that the effective length of percolation pathway in the nanocomposite conductor became shorter after being stretched, through selfalignment and rearrangement of AgFs-AgNPs in the SHP matrix (Note S3). This hypothesis was directly confirmed by the visualization using µ-CT and in-situ SEM (Figure 4a,b and Figure S1822). As the nanocomposite was stretched, the AgFs-AgNPs were spontaneously self-aligned in the direction of strain and the closer contact between AgFs-AgNPs may thus improve conductivity (Figure 1d, 3c, 4a and Figure S18-20). At a strain above ~500%, conductivity started to drop due to partially broken or disconnected percolation in stretching direction (Figure 1d, 3c, 4a, Figure S19, and Note S3). After 20 hrs of rearrangement at 1000% strain, however, the disconnected AgFs-AgNPs became in contact with or closer to each other and the conductive pathway was recovered (Figure 4b and Figure S22). A major driving factor of such dynamic behaviors of AgFsAgNPs is a stress-relaxation phenomenon of the dynamically crosslinked self-healing polymer matrix (Figure 1c and 3c).18 Specifically, such phenomena can be explained by the increase in the overall entropy through relaxation of the strain aligned polymer chains (Figure 1c). AgFs can be reconnected in the free volume of the relaxed polymer matrix owing to favorable AgF-AgF interaction, compared to AgF-polymer interaction (Figure S23).28,33,34 Schematics and corresponding optical images for self-healing of the nanocomposite conductor were shown in Figure 5a. We confirmed that the line-shaped damaged conductor (a ratio of 3:1) was healed after being held at 60 °C for 1.5 hrs. The efficiency of self-healing depends on the amount of SHP at exposed cross-sectional area.15 So, we investigated the self-healing performances with various weight ratios (2:1 to 4:1) of AgF to SHP (Figure 5b). We found that


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the one with a ratio of 3:1 exhibited the highest stretchability (~1700% strain) and reliable conductivity (maintaining 200 S cm-1 at 100% strain) after being damaged and self-healed. Although the nanocomposite conductor can be self-healed at r.t., heating helped to accelerate the process (Figure 5c,d and Figure S24).27 Furthermore, both the mechanical and electrical selfhealing performances (222 S cm-1 at 100% strain; 119 S cm-1 at 1700% strain) of the nanocomposite conductor were enhanced compared to those (200 S cm-1 at 100% strain; 10 S cm-1 at 1700% strain) without the SHP encapsulation layers (Figure 5e,f). A simple self-healing demonstration of the encapsulated conductor was shown in Figure 5g. A double-layered conductor (DLC) fabricated by transfer-printing and self-bonding processes is a good candidate to fabricate robust interconnect of deformable electronics. Specifically, the homogeneously self-bonded conducting interface that was located on the neutral mechanical plane of the DLC structure may be suitable for alleviating plastic deformation induced electrical degradations (Figure 6a, Figure S25,26 and Note S4). To verify this hypothesis, we first checked the conductivity of single- and double-layered structures (a ratio of 3:1), while being stretched up to 1500% strain (Figure S27). As expected, the DLC had less decrease of conductivity under strain than the single-layered configuration. Based on this observation, subsequent tests for stretching durability of 4 different conductors at 50% strain were performed up to 1000 cycles (Figure 6b). Our obtained results indicate that the encapsulated DLC was the most robust among all. The encapsulated DLC also exhibited stable electrical performance under the cyclic test at 200% strain even after a high deformation was applied (stretching and releasing of 700% strain) (Figure S28). More surprisingly, the DLC without encapsulation was more reliable than the encapsulated single one (Figure 6b). In addition, compared to self-healability of the encapsulated single layer


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(the highest conductivity: 261 S cm-1 at 500% strain; 119 S cm-1 at 1700% strain), the encapsulated DLC shows better performances (the highest conductivity: 346 S cm-1 at 500% strain; 149 S cm-1 at 1700% strain), enabling significant electrical self-boosting (711 S cm-1 over 20 hrs) (Figure 6c,d). To support long-term interactions between human and machine, even after critical damages, we applied our DLC conductors encapsulated with SHP layers to a flexible wireless EMG signal monitoring system. This bio-integrated system was conformally attached onto skin by covering with a transparent skin patch (3M Tegaderm, USA) (Figure 7a and Figure S29,30). The fabricated interconnect has a low impedance (1.5 kΩ) at ~100 Hz that is similar to intrinsic frequency of flexor carpi ulnaris (Figure 7b).35 The interconnects allow real-time EMG monitoring for muscle contraction of forearm flexor even without using the typical conductive wet gels (Figure 7c, Figure S30, and Movie S1,2). In addition to electrophysiological measurements, our interconnect has elastic properties within a range of strain of 50% before and after critical damage (Figure S31). Furthermore, the pristine and self-healed interconnects can be stretched up to >4000% and ~2980%, respectively (Figure S32). To realize self-healable prosthetic skin, each interconnect composed of 3 conductors was assigned to the reference, ground, and active electrodes, and was used to deliver a series of commands, including grabbing, spreading, and pointing, to the robot hand (Figure S33,34). In addition, we confirmed reusability and stretchability of our interconnects after damages (Figure 7d and Figure S35). We put the bisected parts together while applying heat at 60 °C for 1.5 hrs. As expected, the self-healed interconnect was able to reenact previous robot hand motions precisely (Figure 7e and Movie


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S3,4). For long-term safe and reliable measurements, Ag used as the self-healable interconnect can be functionalized or encapsulated by biocompatible materials.36-38

CONCLUSION We report a phenomenon of self-recovery of electrical conductivity due to rearrangement of conducting particles in a SHP matrix. This enabled an electrically and mechanically self-healable nanocomposite conductor that has stretchability of 3500% with high conductivity of 3086 S cm-1. Furthermore, our DLC fabricated by taking advantage of self-bonding assembly of SHP showed superb durability in the cyclic stretching test, since the position of the AgF-rich region is located on the neutral mechanical plane. Furthermore, our fabricated DLC, when encapsulated with SHP layers, has reliable stretchability even after making a complete cut. Last, we demonstrated that the self-healable interconnects based on our nanocomposite conductors and encapsulation layers are highly applicable to the robust interactive human-robot interfaces.

METHODS Materials preparation. Synthesis of the SHP follows our previous report.27 The synthesized SHP and chloroform are mixed for 1 hr (1.5 g/8 mL). Ag flakes (AgFs, Daejoo Electronics, DSF500MWZ-S) are then dispersed in the solution, stirred for 30 min (3 to 7.5 g for 2:1 to 5:1 AgF:SHP weight ratio). Well-mixed solution is poured on the octadecyltrimethoxysilane (OTMS, Sigma-Aldrich, 376213)-treated silicon dioxide wafer and cured at r.t. for 1 hr. The nanocomposite conductor film can be easily detached from OTMS-functionalized substrate after the curing process.


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µ-CT, SEM, and TEM observation. For macro- and micro-scale visualization of dynamic behaviors of AgFs in the strained SHP matrix, µ-CT (SKYSCAN1172, Bruker Co., USA) and FE-SEM (Field emission scanning electron microscopy, Inspect F50, FEI Co., USA) are used. µ-CT scanning condition is as follows: source voltage (89 kV), source current (112 µA), binning mode (4K), pixel size (0.7 µm), rotation step (0.2°/step), frame averaging (6). Sample preparation for cross-sectional SEM imaging is performed by loading conductor films vertically before and after 1700% stretched. In-situ tensile SEM imaging was carried out using the microtensile tester (Gatan-MicroTest300N). 300N standard load cell was used of which resolution is 0.1% of full-scale. Stretching speed was controlled by using Deben Microtest (Ver. 6.1.83) software. For better SEM images, we coated platinum onto each conductor film. For observation of AgNPs in the SHP matrix, samples prepared by ultramicrotome (Ultra Cut S, Leica Co., Germany) are observed by TEM equipped with X-FEG (Talos F200X, FEI Co., USA). Images are taken in STEM mode. The EDS analysis is acquired with Super-XEDS (Bruker Co., USA).

Electrical characterizations of nanocomposite conductor at a stretch mode. Stretching test is conducted by using a manual stretching stage at speed of 100% per 30 s. Electrical recovery test (Figure 1e,f) is performed by using an automatic stretching machine with a stretching speed of 100% per 22.5 s. The components of resistance-probe and motor-fixed parts are designed by Rhinoceros (Robert McNeel & Associates) and printed on a three-dimensional printer (Anartz Engine). The conventional stepper motor is operated by an Arduino Uno, and the customized


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strain program is written using the C programing language. Round-tip test probe pins (P75-D), a digital multimeter (NI USB-4065), and LabVIEW (National Instrument) are used to measure electrical resistance.

Experimental set-up for cyclic durability of nanocomposite conductor. Single layer conductor with and without the SHP encapsulation and the DLC with and without the SHP encapsulation were compared. Repetitive stretching (strain of 50%, over 1000 cycles) and releasing tests were performed. NI-DMM (National instrument-digital multi meter) system was used for real-time recording of resistance of the conductors.

A flexible wireless EMG recording system. A flexible wireless EMG recording circuit is composed of an amplifier for amplifying EMG signal, Bluetooth module for transmitting signals and commands, and a microcontroller for operating circuits (Figure 7a). The circuits are soldered on the flexible printed circuit board (FPCB), thereby facilitating easy installation on curvilinear human body. There are two electrodes to be connected to the human body. One is interconnected to the ground plane of the FPCB board, and the other is interconnected to the high pass filter whose output is directly interconnected to the input terminal of the amplifier. The gain is adjustable by exchanging a resistor interconnected to the gain control terminals of the amplifier. The amplified EMG signal can be measured by analog-digital converter of the microcontroller, and the converted digital value can be transmitted to the external device through the Bluetooth module. Customized LabVIEW software of the external device can acquire the transmitted EMG signal and analyze it (Movie S1,2).


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Interactive human-robot interface. For detection of EMG signals from human, we use a wireless EMG system (Delsys Trigno Wireless EMG system, Delsys Inc., Natick, MA, USA). The sensor has 4 Ag bar contacts (99.9% silver, 5  1 mm, fixed contacts distance: 10 mm parallel to muscle fibers) for detecting the EMG signal at the skin. Our conductors are highly compatible with these Ag bar electrodes. Interactive human-robot interface is demonstrated using a Robot hand (Ottobock Co., Republic of Korea). The three long-line conductor (width: 2 mm, length: 115 mm) encapsulated with the SHP layers is conformally mounted on skin.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Supplementary data (Figure S1-S35) and discussion (Note S1-S4) A video showing a flexible wireless EMG recording system using self-healable conductor as an interconnection laminated on skin (Movie S1) A video showing real-time EMG monitoring using the flexible wireless EMG recording system (Movie S2) A video showing repetitive stretching and releasing of the interconnect after completely cut and self-healed (Movie S3) A video showing demonstration of human-robot interface (grabbing, slightly and fully spreading, and pointing) using the self-healed interconnect (Movie S4)


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The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contributions †S.H.K., H.S., and J.K. contributed equally.

ACKNOWLEDGMENTS This work was supported by KIST intramural grants (2E29340, 2E29680). This research was also partially supported by the convergence technology development program for bionic arm through the National Research Foundation of Korea (NRF) funded by the Ministry of Science & ICT (No. 2017M3C1B2085292). This research was also partially supported by the Ministry of Trade Industry & Energy (MOTIE, Korea), Ministry of Science & ICT (MSIT, Korea), and Ministry of Health & Welfare (MOHW, Korea) under Technology Development Program for AI-Bio-Robot-Medicine Convergence (20001655). Z.B. acknowledges support from Air Force Office of Scientific Research (award no. FA9550-18-1-0143).

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Figures

Figure 1. Electrical and mechanical characterizations of an ultra-stretchable and selfhealable nanocomposite conductor. (a) Composition and photograph of an ultra-stretchable and self-healable nanocomposite conductor. (Inset) SEM image showing the composition of the nanocomposite conductor. (b,c) Conductivity- (b) and stress- (c) strain characteristics of the nanocomposite conductors prepared with different weight ratios of AgF. (d) Effect of annealing temperature on conductivity-strain characteristics of the nanocomposite conductors. (e) Resistance-strain characteristics of the nanocomposite conductor that is held for 20 min at every 400% interval. (Inset) Low-resistance regime. (f) Electrical self-recovery at each strain shown in (e) as arrows. Long-term regime (right). (g) Conductivity-strain characteristics of the nanocomposite conductor showing electrical self-boosting at 800% and 1000% strain.


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Figure 2. Structural evolution of nanocomposite conductor enabled by SHP encapsulation. (a) Photographs of SHP-encapsulated nanocomposite conductor stretched up to 3500% strain. (b) Conductivity- (green) and resistance- (orange) strain characteristics of the encapsulated conductor showing electrical self-boosting at 3500% strain with 3086 S cm-1 and 5.6 ohm after 60 hrs. (c) Cross-sectional SEM image of the SHP-encapsulated nanocomposite conductor. (d) Fracture energies of the nanocomposite conductor with and without SHP encapsulation layers. (e,f) Current-voltage characteristics of commercial diodes interfaced with the encapsulated (e) and the stand-alone (f) conductors showing electrical self-boosting.


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Figure 3. Microstructures and conductive pathway of nanocomposite conductor. (a) A schematic diagram of the nanocomposite conductor. (b) TEM and EDS images of AgNPs spontaneously formed around AgFs in the SHP matrix. (c) A schematic illustration showing dynamic behaviors of AgFs in the strained SHP matrix, which result in changes of percolation pathway.


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Figure 4. Visualization of self-alignment and rearrangement of AgFs-AgNPs in the strained SHP matrix. (a) μ-CT images of the nanocomposite conductor before and after 1700% stretched showing self-alignment of AgFs. (b) In-situ SEM images of the nanocomposite conductor before, just stretched to 1000% strain, and 20 hrs of rearrangement after 1000% stretched. Magnified images in green- and orange-dashed regions show rearrangement of AgFs.


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Figure 5. Self-healability of nanocomposite conductor. (a) Schematic illustrations and optical microscope images showing self-healability of the nanocomposite conductor. (b) Conductivitystrain characteristics of self-healed nanocomposite conductors with different weight ratios of AgF to SHP. (c,d) Stress-strain characteristics (c) and stretchability (d) of 3:1 (AgF:SHP) nanocomposite conductors before damaged (black) and after self-healed at 60 °C for 1.5 hrs (red) and at r.t. for 48 hrs (blue). (e,f) Mechanical (e) and electrical (f) properties of self-healed 3:1 (AgF:SHP) nanocomposite conductors without (black) and with (blue) SHP-encapsulation. (g) Demonstration of self-healing process of the SHP-encapsulated nanocomposite conductor connected to a LED.


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Figure 6. Double-layered structure of nanocomposite conductor. (a) A schematic illustration and SEM images showing the formation of self-bonded homogeneous interface in the doublelayered structure. (b) Effects of double-layered structure with encapsulation on cyclic durability of the conductor. (c) Photographs showing (left) self-healing of the DLC with encapsulation and (right) the self-healed encapsulated DLC at 1700% strain. Inset shows the pristine state. (d) Conductivity-strain plot of the self-healed encapsulated DLC with a ratio of 3:1.


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Figure 7. Robust interactive human-robot interface based on stretchable and self-healable interconnect. (a) A photograph of a flexible wireless EMG recording system using self-healable conductor as an interconnection. (Inset) Components of the flexible wireless EMG recording chip. (b) Impedance- (red-line) and phase- (blue-line) frequency characteristics of self-healable interconnect. (c) Wireless EMG signal recorded by self-healable interconnect. (d) Photographs of the self-healable interconnect that is sequentially cut, self-healed, and stretched. (e) Demonstration of human-robot interface (grabbing, spreading, and pointing) using the selfhealed interconnect after (d) process.


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An Ultrastretchable and Self-Healable Nanocomposite Conductor

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