Ultra-Stretchable Conductive Polymer Complex as Strain Sensor with

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Applications of Polymer, Composite, and Coating Materials

Ultra-Stretchable Conductive Polymer Complex as Strain Sensor with Repeatable Autonomous Self-Healing Ability Yang Lu, Zhongqi Liu, Haoming Yan, Qing Peng, Ruigang Wang, Mark E. Barkey, Ju-Won Jeon, and Evan K. Wujcik ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05464 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Materials & Interfaces

Ultra-Stretchable Conductive Polymer Complex as Strain Sensor with Repeatable Autonomous SelfHealing Ability Yang Lu,† Zhongqi Liu,‡ Haoming Yan,§ Qing Peng,§ Ruigang Wang,‡ Mark E. Barkey,∥ Ju†

Won Jeon*§⊥ and Evan K. Wujcik.* †

Materials Engineering and Nanosensor [MEAN] Laboratory, Department of Chemical and

Biological Engineering, The University of Alabama, AL 35487, USA ‡

Department of Metallurgical and Materials Engineering, The University of Alabama,

Tuscaloosa, AL 35487, USA §

Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa,

AL 35487, USA ∥Department

of Aerospace Engineering and Mechanics, The University of Alabama, Tuscaloosa,

AL 35487, USA ⊥

Department of Applied Chemistry, Kookmin University, Seoul, 02701, Republic of Korea

KEYWORDS: ultra-stretchable, self-healing, linearity, polymer complex, strain sensor

ACS Paragon Plus Environment

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ABSTRACT

Wearable strain sensors are essential for the realization of applications in the broad fields of remote healthcare monitoring, soft robots, immersive gaming, among many others. These flexible sensors should be comfortably adhered to skin and capable of monitoring human motions with high accuracy, as well as exhibiting excellent durability. However, it is challenging to develop electronic materials that possess the properties of skin—compliant, elastic, stretchable, and selfhealable. This work demonstrates a new regenerative polymer complex composed of poly(2acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA), polyaniline (PANI) and phytic acid (PA) as a skin-like electronic material. It exhibits ultrahigh stretchability (1935%), repeatable autonomous self-healing ability (repeating healing efficiency > 98%), and quadratic response to strain (R2 > 0.9998), linear response to flexion bending(R2 > 0.9994) — outperforming current reported wearable strain sensors. The deprotonated polyelectrolyte, multivalent anion, and doped conductive polymer, under ambient conditions, synergistically construct a regenerative dynamic network of polymer complex crosslinked by hydrogen bonds and electrostatic interactions, which enables ultrahigh stretchability and repeatable self-healing. Sensitive strain-responsive geometric and piezoresistive mechanisms of the material owing to the homogenous and viscoelastic nature provide excellent linear responses to omnidirectional tensile strain and bending deformations. Furthermore, this material is scalable and simple to process in an environmentally-friendly manner, paving the way for the next generation flexible electronics.


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INTRODUCTION With recent advances in material science and microelectronic fabrications, great efforts have been made in the area of wearable devices, especially wearable strain sensors for applications in healthcare and medical diagnosis,1-2 e-skin,3-4 soft robotics,5-6 prosthetics,7-8 immersive gaming,9-10 and various fields. They are directly worn on human skin for monitoring physiological signals and body motions.11-14 Due to the soft, compliant, and complex nature of human skin, as well as the natural bending or rotational motion associated with joints, a wearable device should be soft and mechanically robust enough for the wearer to comfortably perform motions including bending, stretching, and twisting.15 Conventional semiconductors, including silicon, gallium arsenide, and metal oxides, possess an intrinsic brittle and rigid nature thus are not suitable for wearable strain sensors.16-17 Generally, there are three strategies to achieve stretchability of electronic materials: i. designing a stretchable architecture as a conductive network in elastomers,18-23 ii. uniformly dispersing nanofillers inside an elastomer matrix to reach the percolation threshold,3, 24-25 or iii. utilizing intrinsic conductive and stretchable polymers.26-28 To date, the majority are based on the first and second approaches, by means of conductive nanofillers including carbon nanotubes (CNTs),20-21, 27, 29-30 graphene,19, 25, 31-33 silver nanowires,24, 34-35 silicon nanowires,18 and metallic nanoparticles.36 Designing stretchable architecture (buckling, spring, coiled, open mesh, etc.) as the backbone of a conductive network typically requires complex fabrication procedures, works only in uni-axial stretching direction, has poor interfacial adhesion, and low cycling stability.9, 18, 22, 37 Dispersing nanofillers inside of elastomer matrices to reach the percolation threshold typically requires high nanofiller loadings, exhibits limited conductivity, and low workable stretchability.25,

32, 35

In addition, both approaches have poor repeatability of

performance and linearity due to uncontrollable fabrication and nanostructure formation via non-


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uniform nanofiller size and poor dispersion.9, 38 The intrinsic conductive and stretchable polymer route is, hence, considered a preferable strategy for stretchable electronics. The homogenous and isotropic nature of amorphous polymers has made them more scalable, consistent, and with predictable responses. In addition to stretchability and predictable response—as heavily used and exposed materials—to prevent long-term performance decline and deterioration, the ability to continually self-heal without external stimuli is extremely desirable. Self-healing materials are usually designed by either encapsulating healing agents into micro-containers within matrix materials (extrinsic self-healing) or involving dynamic bonds such as covalent bonds, hydrogen bonds, and ionic bonds (intrinsic self-healing).39-40 Extrinsic self-healing materials need the assistance of liquid monomers and catalysts, and the healing is unrepeatable. Meanwhile, intrinsic self-healing materials typically require external stimuli such as heat, light, force, etc.41-42 A truly autonomous self-healing material, however, is still limited, especially in regards to electronic materials. Fu et al. reported a tough conductive hydrogel based on interpenetrating polymer network. The sensor shows a stretchability of 300% and a gauge factor of 11 (GF = ΔR / (ε × R0), ΔR as the change in resistance, R0 as the resistance at 0% strain)—a metric representing sensitivity. But this conductive hydrogel lacks the self-healing properties.43 Xu et al. successfully developed a self-healable and stretchable conductive polymer composite, composed of PANI, poly(acrylic acid), and phytic acid. However, the stretchability is limited under 500%, and external mild pressure is required for activating the self-healing process, which exceeds 24 hours.44 Herein, we report a piezoresistive wearable strain sensor based on a new regenerative conductive polymer complex, having a homogenous morphology, featuring ultrahigh omnidirectional stretchability, quadratic response to strain, and linear response to bending. More importantly, this electronic material is capable of self-healing without external stimuli. The ultra-


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stretchable conductive polymer complex is comprised of three components: poly(2-acrylamido-2methyl-1-propanesulfonic acid) (PAAMPSA), polyaniline (PANI) and phytic acid (PA). PAAMPSA is an anionic polyelectrolyte in which sulfonic acid group can be deprotonated and absorb water under ambient conditions (25 °C and 50~60% humidity).45 PANI is an intrinsically conductive polymer. PA is a cyclic protonic acid which easily forms complexes with cations.46 Both PAAPMSA and PA act as dopants and cross-linking agents. The hydrogen bonds and electrostatic interactions between PAAMPSA, PA and PANI synergistically construct a homogeneous regenerative network, which contribute to the elasticity and soft compliant nature of the as-prepared electronic material, along with extremely high omni-directional stretchability (1935 %) and excellent self-healing ability. The autonomous self-healing process is fast and highly repeatable. It is shown that human motions can be accurately monitored using the self-healable strain sensor in real time with high GFs—while exhibiting an extremely high stretchability. Geometric and piezo-resistive effects are the basis of the underlying mechanisms of the material, which contributes to the quadratic changes in resistance change and linear changes in GF with respect to strain. The stress and strain in present work are engineering tensile stress (σt = Fn / A0, Fn as the normal force acting perpendicular to the area, A0 as the original cross-sectional area) and engineering strain (ε = [(l – l0) / l0] × 100%, l as the stretched length, l0 as the original length, illustrated in Figure S1). RESULTS AND DISCUSSION


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Figure 1. a) Schematic illustration showing the synthesis of PAAMPSA/PANI/PA polymer complex. b) Cross-links forming a regenerative dynamic network by hydrogen bonds and electrostatic interactions between PAAMPSA and PA with PANI. c) Photographs of autonomous self-healing process after 3 hours, blue dashed circles show the healed severed-strip. d) Schematic diagram of the self-healing process.


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The PAAMPSA/PANI/PA conductive polymer complex is prepared via a simple two-step method (reaction, casting) as illustrated in Figure 1a. Compared with the typically complicated fabrication processes of flexible electronic materials,3-4,

47

the two-step fabrication of

PAAMPSA/PANI/PA is simple, scalable, and shows an ease of processability. A homogenous aqueous solution of PAAMPSA, PA, and aniline was first prepared (Figure S2a), then the polymerization initiator ammonium persulfate (APS) was introduced into the solution to initiate the oxidative polymerization of aniline (Figure S2b). Here, PAAMPSA acts as a template to guide the PANI polymerization. Additionally, both PAAMPSA and PA are dopants which protonate PANI from its insulating emeraldine base form (EB-PANI) to its electrically conductive emeraldine salt form (ES-PANI). PANI is known for its poor water solubility, which severely limits its applications.48 In the presence of water, both PA and PAAMPSA carry excessive negative charges due to the deprotonated sulfonic acid and phosphate groups, which enables a stable dispersion of PANI.49-50 The partially deprotonated PA and PAAMPSA form the regenerative interactions of both hydrogen bonding and electrostatic interactions with PANI as shown in Figure 1b. The PAAMPSA/PANI/PA colloidal dispersion was observed to be stable for months (Figure S2c). The change of zeta potential versus the concentration of PAAMPSA/PANI/PA mixture solution is given in Figure S3. A high zeta potential (~ −70 mV) over a wide range of concentrations indicates an excellent colloidal dispersion stability, which aligns our observations. The self-healable conductive film is formed by evaporating water through the solvent casting method. The highly stable aqueous solubility is ideal for environment-friendly processing layers to be easily deposited and patterned into different geometries for potential flexible electronics (Figure S4). The ease of patterned processing also allows the material to be efficient and convenient for scalable mass production. Through thermogravimetric analysis (TGA) and


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differential scanning calorimetry (DSC) analysis, the thermal stability and degradation analyses are conducted (Figure S5, S6). The retained water content is determined to be ~ 6.14 Wt.% and the material exhibits high thermal stability without weight loss, other than water, within 100 °C. Under ambient conditions, the film shows elasticity (Movie S1)—owing to its unique PAAMPSA/PANI/PA dynamic network formed by intermolecular interactions, Figure 1b. To demonstrate self-healing capability, the PAAMPSA/PANI/PA film was first cut into two pieces, then two freshly severed surfaces were reconnected, Figure 1c. After three hours, the severed film was completely self-healed. It is noteworthy to reiterate that the self-healing process is completely autonomous, meaning no external stimuli such as heat, light, force, etc. are required. The hypothesized self-healing mechanism is shown in Figure 1d. The interface is partially healed due to instantaneous formation of electrostatic interactions and hydrogen bonds at cross-linking anchor points provided by ES-PANI on severed surfaces. Bond strength continues to increase with time as the polymer chains gradually diffuse and re-entangle due to viscoelastic nature and intermolecular attractions of the material. Ultimately, the polymer chains become fully reentangled with all of ES-PANI anchor points filled, forming an identical homogenous regenerative network.


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Figure 2. a) Cross-sectional SEM micrograph and EDS elemental mapping (carbon, nitrogen, oxygen, sulfur and phosphorus) of PAAMPSA/PANI/PA material. b) FT-IR spectra of PAAMPSA/PANI/PA and PAAMPSA/PA control sample. c-e) High-resolution XPS spectra of PAAMPSA/PANI/PA polymer complex with deconvoluated peaks: c) Nitrogen, N 1s, d) Sulfur, S 2p, e) Phosphorus, P 2p.


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A single smooth phase is observed on cross-sectional scanning electron microscope (SEM) micrograph and all elements are uniformly distributed based on energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 2a). This confirms that the stretchable material is highly homogenous with a uniform single phase demonstrating good miscibility between PAAMPSA and PANI. Fourier transform infrared (FT-IR) provides clues for self-healing capability of the PAAMPSA/PANI/PA material, by confirming the existence of strong hydrogen bonding (Figure 2b). The characteristic peaks at 1112 cm-1, 1496 cm-1, and 1575 cm-1 are attributed to –NH+=, benzenoid ring, and quinonoid ring stretching, respectively. These are absent in the PAAMPSA/PA control sample, confirming the presence of PANI in the experimental sample.51 The peaks at 1036 cm-1 and 1651 cm-1 are assigned to H2PO4– and HPO42– stretching in PA.52-53 Wavenumbers of 1218 cm-1 and 1559 cm-1 are assigned to –SO3H and secondary amides N-H stretching in PAAMPSA.54 The –OH stretching peak is fundamentally sensitive to hydrogen bonds, with characteristic peaks at 3200~3400 cm-1 and is associated with intermolecular hydrogen bonded –OH (polymeric association) stretching.54 By comparing with the control sample, this absorption peak has flattened and redshifted to a lower wavenumber (from 3310 to 3250 cm-1). This can be explained as the strength and stretching of originally free O–H bonds are partially weakened and restricted after forming the O–H···N bonds. This indicates the presence of strong hydrogen bonding in material.55 X-ray photoelectron spectroscopy (XPS) examinations reveal the electrostatic interactions in PAAMPSA/PANI/PA material (Figure 2c-e, survey spectra with labeled elements in Figure S7). The elemental composition and chemical states are in consistent with EDS and FT-IR results. The peaks located at 399.2 eV, 400.0 eV and 401.3 eV in N 1s spectra (Figure 2c) are corresponding to the neutral amine (NH), and the protonated nitrogens (N+ and N2+) of ES-PANI, respectively.48 The peaks at 167.6 eV in S 2p (Figure 2d) and 133.9 eV in P 2p spectra


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(Figure 2e) are assigned to the deprotonated sulfonic acid groups (–SO3–) of PAAMPSA and the dihydrogen phosphate groups (H2PO4–) of PA, respectively. These groups, which are in consistent with FT-IR results, are associated with the protonated nitrogens (polarons) in ES-PANI through electrostatic interactions.48, 56


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Figure 3. a) Conductivity healing efficiency (HE%c) of PAAMPSA/PANI/PA as a function of autonomous self-healing time under ambient conditions. Tests were repeated for five times with error bars indicating sample standard deviations. b) Self-healing cycling tests of the cut-heal process at the same location for 6 cycles. c). Stress-strain curves of PAAMPSA/PANI/PA under three conditions: dried (1), low humidity (2) and ambient conditions (3). Self-healed sample under low humidity conditions (4). PAAMPSA/PA control sample under ambient conditions (5). d) Stress-strain curves of (2-5).


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Figure 4. a) Optical microscope images exhibiting the evolution of a rapid scratch self-healing process. A 32 μm width gap of scratch was cut by a razor blade. The scratched surface was gently pressed (contact pressure: ~ 50 kPa) for several seconds (~5 seconds) to activate the expedited healing. After 10 minutes of autonomous self-healing, the material was completely healed with its surface cosmetically appeared the same as the pristine surface. b) Demonstration of an expedited healing (~5 seconds) of a severed conductive material in series with an LED light showing the restored electrical conductivity and mechanical strength. (Movie S2). c) The comparison of selfhealing performances with recent literature, in terms of healing time and tensile strength healing efficiency (HE%t). Notes on the repeatability and required stimuli for each material are provided. "Gentle press" describes contact pressures within 70 kPa. References: a,57 b,58 c,39 d,42 e,59 f,41 g,60 h,40 i,61 j,62 (k, l , m: present work).


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To better evaluate the self-healing performance of the material, three healing efficiency (HE%) metrics are established with respect to conductivity (subscript "c"), stretchability (subscript "s") and tensile strength (subscript "t"), as follows: 𝜎𝜎

𝜀𝜀

𝜎𝜎

𝐻𝐻𝐻𝐻%𝑐𝑐 = 𝜎𝜎 𝑐𝑐 × 100%, 𝐻𝐻𝐻𝐻%𝑠𝑠 = 𝜀𝜀 × 100%, 𝐻𝐻𝐻𝐻%𝑡𝑡 = 𝜎𝜎 𝑡𝑡 × 100% 𝑐𝑐0

0

𝑡𝑡0

(1)

Where σc0, ε0, σt0 and σc, ε, σt are conductivity, elongation at break, ultimate tensile stress of pristine and self-healed sample, respectively. The conductivity of PAAMPSA/PANI/PA polymer complex is measured to be 2 S m-1. The plot of HE%c versus autonomous healing time under ambient conditions without external stimuli is shown in Figure 3a. The conductivity is rapidly restored to 78.8 ± 5.7% of the initial value within 5 min, while the remaining is completely restored (HE%c up to 98.6 ± 2.2%) within 3 hours. It should be noted that many self-healing materials require additional self-healing agents or external stimuli to realize the self-healing process.58, 63 However, the PAAMPSA/PANI/PA polymer complex requires neither. Self-healing cycling tests (Figure 3b) of the cut-heal process at the same location show an extraordinary repeatability with no reduction in healing efficiency. The average HE%c is found to be 98.2 ± 0.8% for six healing cycles. The Environmental conditions have great influence on mechanical properties of PAAMPSA/PANI/PA (Figure 3c, d). The developed material is compliant and ultrastretchable (εmax = 1935%, σt-ultimate = 504.2 kPa (ultimate tensile stress)) under ambient conditions (blue curve (3) in Figure 3d, photographs shown in Figure S8), but relatively stiff (εmax = 121.6%, σt-ultimate = 12.4 MPa) under dry conditions (~15% humidity, Figure S9). The retained water in the material is essential for the self-healing and ultra-high stretchability, as hydrogen bonds and electrostatic interactions are significantly weakened in anhydrous state. The ultra-soft nature under ambient conditions with a low Young's modulus of 24 kPa is comparable to human skin (20~100


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kPa for dorsal forearm and palm).64 This is a highly desirable feature for wearable electronics (minimized discomfort, no motion restriction). Under low humidity conditions (30~40% humidity), the material has a higher tensile strength (orange curve (2) in Figure 3d, σt-ultimate = 952.5 kPa) with a slightly decreased stretchability (εmax = 1059.5%), but still substantially higher than the stretchability of other previously reported materials (εmax mostly under 400%).10, 25-26, 63 Here, the mechanical properties and material behavior can be readily switched and conditioned by adjusting the humidity or storage temperature. To access the self-healing efficiency in terms of mechanical properties, the stress-strain curve of healed material under low humidity conditions is shown as purple curve (4) in Figure 3d. HE%s (stretchability) and HE%t (tensile strength) are calculated as 93.1% and 96.9%, respectively. For comparison, the stretchability and self-healing ability of the PAAMPSA/PA control sample without PANI are also tested, which shows that the control sample is not self-healable and exhibits a much lower stretchability (red curve (5) in Figure 3d, εmax = 420.9%). This can be attributed to the lack of electrostatic interactions and much weaker hydrogen bonds, which supports our proposed mechanisms in Figure 1b, d. The addition of PANI provides countless anchor points for cross-linking, to construct a highly regenerative network, enabling the polymer complex to be ultra-stretchable and self-healable. All three HE% metrics point to PAAMPSA/PANI/PA having a high self-healing efficiency which could ideally approach 100%. The efficiency slightly falling short of 100% is attributed to the imperfect recombination of the two severed surfaces. It is hypothesized that this can lead to minor defects slightly decreasing the ultimate strength and maximum strain of the material at the self-healed site. Also the tiny discrepancy of strip width at the self-healed site can result in an increase in resistance across the material. However, this can be greatly improved by standardizing the material shape and placing ready to heal materials in a specialized mold for a precise surface recombination. As discussed


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earlier, the average healing time under ambient conditions without external stimuli is 3 hours. It is worth mentioning that a rapid healing, within several seconds, can be achieved with a gentle press (contact pressure: ~ 50 kPa) on the fractured or scratched surface. Without pressure, the reentanglement of polymer chains only occurs at the contacted surfaces where the dynamic bonds are formed. As the severed surfaces gradually healed, more areas are contacted indicating more cross-linking anchor points are exposed to each other and ultimately finishes the re-construction of polymer complex network. By applying a pressure, the better contact of surfaces allows more cross-linking points to be exposed simultaneously. Moreover, it accelerates the polymer-polymer inter-diffusion making network to be constructed in seconds. The evolution of an expedited scratch self-healing process is optically revealed in Figure 4a (details are given in Figure S10). A 32 μm width gap of scratch was cut by razor blade. With a gentle press applied for 5 seconds, the scratch was expeditiously healed leaving a 1.2 μm width. After 10 minutes of autonomous self-healing, the material was completely healed with its surface cosmetically appeared the same as the pristine surface. A battery-powered circuit was constructed to demonstrate the restored conductivity and mechanical strength after an expedited self-healing (from a completely severed sample) for 5 seconds in Figure 4b. It should be noted that PAAMPSA/PANI/PA can self-healed after 3 hours even without applying mild pressure 1: undamaged conductor. 2: completely severed material causing open circuit. 3: with a gently press applied for only 5 seconds, the electrical conductivity and mechanical strength were restored showing an expedited healing performance (Movie S2). With features of rapid and repeatable self-healing, high healing efficiency, and no required stimuli, the PAAMPSA/PANI/PA material is overall superior to the majority of reported self-healable materials (comparisons are shown in Figure 4c). For the present works: "k" shows the autonomous self-healing with a healing-efficiency of 97% after 180 minutes; "l" shows the expedited self-


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healing reaches 90% healing-efficiency after applying gentle press for only 5 seconds; "m" shows the expedited self-healing reaches 97% healing-efficiency after applying gentle press for 60 seconds. This result signifies that when wearable electronic devices are damaged and cracked, they can be healed quickly without any interventions, hence significantly decreasing the malfunction and prolonging the lifetime of the wearable devices. The rapid and repeatable healing without the reduction of healing efficiency indicates that the PAAMPSA/PANI/PA material can act as a promising self-healable electronic material.


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Figure 5. a) Plot of ΔR/R0 versus applied tensile strain. b) Plot of GF versus applied tensile strain. c) The comparison of present work with recent reported flexible strain sensors in terms of stretchability, GF, linear/quadratic (L/Q) response, and self-healing ability. References: a,65 b,10 c,31 d,26 e,66 f,22 g,67 h,36 i,68 j,27 k,29 l,19 m,69 n,70 o,71 p,34 q,72 r,47 s,73 t (present work). d) Cycling stability test of strain sensor: ΔR/R0 responding to a repetition of 70 loading and unloading cycles at 20% strain.


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Figure 6. a) Schematic diagram of the flexion bending test with a bending radius of 20 mm. The sensor is in line with the outer bending curve. b) Plot of ΔR/R0 versus flexion angles from 0 to 125°.

c)

Schematic

diagram

demonstrating

omni-directional

sensing

capability

of

PAAMPSA/PANI/PA strain sensor. A square shaped strain sensor was fabricated and was stretched horizontally and vertically. d) Plot of ΔR/R0 versus time with tensile stretching horizontally and vertically. Incremental strains at 0%, 200%, 330%, and 460% were applied with a time frame of 5s.


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Based on the merits of the PAAMPSA/PANI/PA polymer complex, it has great potential in flexible and stretchable electronics. One of the applications demonstrated here is wearable strain sensor. It can be fabricated by placing this component onto a two-sided stretchable adhesive tape substrate with both sides connected by copper wires, as schematically shown in Figure S11. It can be attached to human skin as a wearable device. To characterize the sensor performance, its electromechanical response to tensile strain was first tested. The plot of relative resistance change ((ΔR / R0) × 100%) versus tensile strain perfectly fits three quadratic regressions (for 0 to 500%, 500 to 1,000%, and 1,000 to 1,500%, respectively) with coefficients of determination (R2) over 0.9998 (Figure 5a). A relative resistance change as large as 21,700% is observed for a corresponding strain at 1,500%. As for GF versus tensile strain, the curve significantly fits three linear regressions (100 to 500%, 500 to 1,000%, and 1,000 to 1,500%, respectively) with coefficients of determination over 0.9955 (Figure 5b). Within 100% strain, however, the GF-strain plot follows a quadratic trend rather than a linear trend. The GF is determined to be 1.7 for strain at 100% and linearly increases with three incremental slopes to 14.52 for an extreme strain at 1500% suggesting a high sensor sensitivity. Plots without overlapped regression curves for ΔR/R0 and GF versus strain are presented in Figure S12. Figure 5c compares the performance of the PAAMPSA/PANI/PA strain sensor with previously reported wearable strain sensors in terms of stretchability, GF, linear/quadratic response, and self-healing ability (a comparison table with same references is given in Table S1).10, 19, 22, 26-27, 29, 31, 34, 36, 44, 47, 65-73 The ultra-high stretchability allows the self-healable strain sensor to remain intact up to 1,935% (Figure S8), the highest stretchability for elastic strain sensor by far to the best of our knowledge. Although some metallic nanofiller embedded strain sensors exhibit higher GFs, issues including poor stretchability, nonlinearity, and lack of self-healing ability restrict their applications.10, 66-68, 70 The excellent fit of


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quadratic and linear regressions over a wide range is highly desirable, as it represents a repeatable and accurate sensor response. Typically, piezoresistive strain sensors exhibit poor linearity, especially at large strains, as they incorporate conductive nanofillers with stretchable architecture that have non-homogenous morphologies—resulting in the observed non-linearity.16, 24, 38 The well fitted quadratic and linear responses of the PAAMPSA/PANI/PA material reveals its underlying strain-responsive mechanisms: geometric and piezoresistive effects. The geometric effect dominates the relative resistance change as the conductor thickness and width decreases, while its length increases under tensile stress. Assuming the material is incompressible and the effect of necking deformation on resistance is negligible (the material shows a relatively uniform width in mid-range of the strip where the two voltage-sensing electrodes are placed to eliminate the necking on the two edges, thus the tested range of strip is considered cuboid), the deformation of the sensor can be expressed by Equation 2. 𝑙𝑙 = (1 + 𝜀𝜀)𝑙𝑙0 , 𝑤𝑤 =

𝑤𝑤0

√1+𝜀𝜀

, 𝑡𝑡 =

𝑡𝑡0

(2)

√1+𝜀𝜀

Here, ε is applied longitudinal strain, l the length, w the width, t the material thickness, and l0, w0, t0 are the initial states of those properties. At a strain of ε, the electrical resistance (R) is given by Equation 3: 𝑙𝑙

𝑅𝑅 = 𝜌𝜌 𝑤𝑤𝑤𝑤 =

𝜌𝜌

𝜌𝜌0

𝑙𝑙

𝜌𝜌

(𝜌𝜌0 𝑤𝑤 0𝑡𝑡 )(1 + 𝜀𝜀)2 = 𝜌𝜌 𝑅𝑅0 (1 + 𝜀𝜀)2 0 0

0

(3)

where ρ is the electrical resistivity. The theoretical relative resistance change ΔR/R0 is given by Equation 4: ∆𝑅𝑅 𝑅𝑅0

𝜌𝜌

𝜌𝜌

= 𝜌𝜌 (1 + 𝜀𝜀)2 − 1 = 𝜌𝜌 𝜀𝜀 2 + 0

0

2𝜌𝜌𝜌𝜌 𝜌𝜌0

𝜌𝜌

+ �𝜌𝜌 − 1� 0

(4)


21


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and gauge factor (GF = ΔR / (ε × R0)) is given by Equation 5:

∆𝑅𝑅

𝐺𝐺𝐺𝐺 = 𝜀𝜀𝑅𝑅 = 0

𝜌𝜌

𝜌𝜌0

2𝜌𝜌

𝜀𝜀 + 𝜌𝜌 + 0

�

𝜌𝜌 −1� 𝜌𝜌0

𝜀𝜀

(5)

These theoretical ΔR/R0 and GF are in accordance with experimental electromechanical responses: ΔR/R0 follows quadratic regressions (Figure 5a), and GF follows linear regressions (when strain is over 100%, Figure 5b). Note that the ρ/ρ0 coefficient presented in Equation 4 and 5 is known to be the fractional resistivity change which is caused by the piezoresistive effect: change in material electrical resistivity when mechanical strain is applied.74 The ρ/ρ0 increases as the applied strain increases, which is in consistent with our observations in the ΔR/R0 regressions (quadratic coefficients are 0.650 for 0 to 500%, 1.092 for 500 to 1,000%, and 1.825 for 1,000 to 1,500%, respectively). The responses of both ΔR/R0 (R2 > 0.9998) and GF (R2 > 0.9955 for ε over 100%) to applied strain show the sensor has highly predictable quadratic or linear responses. As for the GF under 100% strain, we observe that the curve follows a quadratic regression rather than linear (Figure S13). This is due to, in low strain range (< 100%), the hyperbola term (ρ/ρ0)/ε in Equation 5 and the effect of necking deformation are not negligible as they were in assumptions. The good agreement between theoretical and experimental based regression equations indicates the underlying strain responsive mechanisms are geometric and piezoresistive effects, as hypothesized. To examine the cycling stability, a repetition of 70 loading-unloading cycles at 20% strain was performed (Figure 5d). The ΔR/R0 returned to baseline values upon relaxation while the (ΔR/R0)max remained unchanged upon stretching for all cycles—indicating high cycling stability and reversibility. The sensor response to deformations of flexion and twisting were also investigated. Flexion describes a bending movement of a particular joint that decreases the angle between a segment and its proximal segment by pulling the adjacent bones closer. The response


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to flexion bending is crucial for motion monitoring devices as the body movements are supported by joints, especially the synovial joints. A schematic diagram of the flexion test with a bending radius of 20 mm is shown in Figure 6a. The sensor is placed in line with the outer bending curve. Figure 6b shows the plot of ΔR/R0 versus flexion angles from 0 to 125°. Continuous bending at 9 incremental flexion angles were applied and the recorded relative resistance change follows a linear regression indicating the sensor can easily monitor bending movements with a high accuracy. The plot of ΔR/R0 versus time is given in Figure S14. Surprisingly, this strain sensor is flexible enough to be twisted even at 720°, demonstrating its versatility. A plot of ΔR/R0 versus twist angles from 0 to 720° is shown in Figure S15. Unlike flexion angles, the relationship of twisting angles with relative resistance change obeys quadratic or cubic polynomials. For a twist angle of 720°, the ΔR/R0 increases up to more than 640%. Another important advantage of the present PAAMPSA/PANI/PA strain sensor is its omni-directional sensing capability. Generally wearable strain sensors are only capable of detecting strain in a single direction, in which the underlying stretchable network is constructed or conductive nanofillers are aligned (uni-directional).18, 72-73, 75 One way to overcome this challenge is by arranging three independent serpentine-shaped sensors at a fixed angle of 120° to build a rosette-type gauge.37, 75-76 However, this approach is complicated by the inherent restricted stretchability and complex operation—requiring multi-signal reception and processing. Owing to the homogenous nature of the PAAMPSA/PANI/PA material, the strain sensor is responsive in all directions (Figure 6c, d). A square shaped sensor was prepared then to be stretched at incremental strains of 0%, 200%, 330%, and 460%, respectively in both horizontal and vertical directions. The recorded relative resistance change for ultra-high strains is indistinguishable in either direction, with a difference within 2%.


23


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Figure 7. The fabricated strain sensor for human motion detections. (a) ΔR/R0 vs time for wrist bending 75° downwards and 45° upwards. (b) ΔR/R0 vs time for finger knuckle bending at angles of 30, 45, 60, 75, 90°, (c) and knee bending at angles of 20, 40, 60, 80, 100, 120°. The schematic diagrams of the motions are presented, and the average ΔR/R0 with their standard errors for all angles are plotted. (d) ΔR/R0 vs time for complex knee motions including extending and flexing, walking, squatting and jumping from squatting.


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To demonstrate the real-life applicable potential of the PAAMPSA/PANI/PA strain sensor as a wearable for human motion detection, the fabricated strain sensor was directly attached onto the skin of body parts including wrist, finger knuckle, knee, and elbow (Figure 7). The soft nature, due to the elastic modulus (~ 24 kPa) being close to human skin, aids in performing comfortable motions without restriction. A noteworthy advantage of the PAAMPSA/PANI/PA strain sensor is its negligible resistance drift after the completion of motions. The ΔR/R0 versus time for wrist bending 75° downwards and 45° upwards (motion interval: 3 seconds) is shown in Figure 7a. The sensor response is shown to be rapid, stable, and consistent. Observe that our sensor is not only able to detect the human motions, but also able to distinguish the direction and magnitude. Here, downward bending stretches the sensor—returning a higher resistance—while upward bending contracts the sensor—decreasing the conductive pathway thus yielding a lower resistance. Figure 7b shows the finger knuckle bending detection by placing the sensor at the proximal interphalangeal joint of index finger as illustrated. The ΔR/R0 increases linearly with increasing bending angles. The resistance change is sufficiently high to correlate the response with the bending, in a highly reliable manner. Notably, the sensor can detect bending angles of the finger knuckle with a high linearity. The coefficient of determination is shown to be 0.9935 with extremely low standard deviations (±1.1% for all bending angles) indicating that the sensor can accurately monitor the extent and frequency of human finger motion in real time. This linear response is in consistent with the flexion angle test as discussed earlier. The reasons for a slightly lower linearity (R2 = 0.9935 for finger knuckle bending, R2 = 0.9994 for flexion bending) are the imperfect curvilinear finger knuckle, complex skin texture, and human error of motion performance compared to the flexion test. To further demonstrate the extensive usability and applicability to accurately detect both small and large scale human motions, a larger strain sensor


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was prepared to cover an extended area of skin on a knee joint as illustrated in Figure 7c. Similar to the finger knuckle motion detection, the sensor is capable of accurately detecting bending motions of knee joints with a linear response (R2 = 0.9914). The same trend is observed in elbow bending detection (Figure S16). In addition, the present strain sensor can detect and discriminate complex motions, as shown in Figure 7d. The knee motions of extending and flexing (ΔR/R0 = 36~40%), walking (ΔR/R0 = 23~27%), and squatting (ΔR/R0 = 68~70%) each show their own characteristic signal intensity, pattern, and frequency. Relative resistance changes are proportional to the extension and intensity of all motions. For sequential motions such as jumping from squatting, an intensive peak corresponding to squatting was detected followed by a moderate peak caused by slight recoil from landing on the ground. Finally, the relative resistance change returns to the baseline value, due to the recovery of an upright standing position. CONCLUSIONS In summary, an ultra-stretchable omnidirectional strain sensor based on the autonomous self-healable polymer complex is extensively demonstrated. The presented sensor exceeds the performance of previously reported state-of-the-art stretchable strain sensors in terms of stretchability, linear/quadratic responses, and self-healing capability. The self-healable and ultrastretchable sensor can easily be fabricated and processed into various geometries and is conveniently scalable, giving it consistent and reliable sensing performance over multiple sizescales. Furthermore, the PAAMPSA/PANI/PA material exhibits a repeatable autonomous selfhealing capability with high healing efficiencies and tunable healing time. Due to its homogenous nature, the observed ΔR/R0 and GF are well fitted in quadratic and linear regressions in responses to strain, unveiling the sensing mechanisms to be geometric and piezoresistive effects. As a wearable device, it can be attached directly on the skin to accurately detect and discriminate


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complex human motions. Moreover, with ultrahigh stretchability, highly predictable responses, autonomous

self-healing ability,

and

ease

of processing,

the

applications

of the

PAAMPSA/PANI/PA electronic material can be extended to pressure sensing, bio-signal monitoring, soft robotics, and artificial skin, etc. We believe the concept of reported conductive and regenerative polymer complex material should be applicable to other polymer electrolyte/conductive polymer/protonic acid system, and will present a new paradigm for next generation flexible electronics. EXPERIMENTAL SECTION Preparation of PAAMPSA/PANI/PA Polymer Complex: The polymer mixture solution was prepared by mixing 50 g poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA) solution (average molecular weight 800,000, 10 wt.% in water, Acros Organics), 2.5 g phytic acid (PA) solution (50 wt.% in water, Acros Organics) and 0.5 g aniline (Sigma-Aldrich). Later 0.685 g ammonium persulfate (APS) was dissolved in 2.5 g deionized water, then added into polymer mixture solution to initiate the polymerization of aniline. The polymer mixture was magnetically stirred at 0 °C in ice-water bath for first 3 hours and kept stirring at 20 °C for another 20 hours. The conductive polymer film was finally formed through solvent casting method by pouring the mixture solution into a 50 mL PTFE evaporating dish at 30 °C for 24 hours to evaporate the excessive water. Different film shapes can be processed by changing the patterned molds. The PAAMPSA/PA film without PANI was also prepared as the control sample by mixing 50 g PAAMPSA and 3 g PA solution for 3 hours followed by the same solvent casting procedure. Fabrication of Strain Sensor: The strain sensor was fabricated by first cutting PAAMPSA/PANI/PA conductive polymer film into rectangular pieces with a fixed size of 40 mm


27


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× 10 mm × 0.9 mm. A commercial transparent double-sided adhesive tape (3M VHB-4910 tape) was used as the encapsulant and adhesive substrate which can be mounted onto the skin using adhesive medical tape. The rectangular strip was connected with electric wires on both longitudinal sides and carefully placed onto the VHB tape and waited for 24 hours under ambient conditions to achieve the ultimate bonding strength between VHB tape and material strip. A fully encapsulated strain sensor was also fabricated by encapsulating both sides of material strip with the 3M VHB tapes. Characterization of PAAMPSA/PANI/PA Conductive Polymer Material: Zeta potential of the polymer dispersions was collected on a Zetasizer Nano ZS (Malvern Panalytical). Thermal gravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were performed on a Simultaneous Thermal Analyzer 8000 (PerkinElmer Inc.) by heating from 30 °C to 820 °C in N2 atmosphere. The heating rate was set as 10 °C/min. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) mapping were performed on a JEOL 7000 FE SEM. Sample was vacuum dried in a turbo pump for 24 hours prior to imaging. FT-IR spectra with a range between 700 to 4000 cm-1 was collected by a Nicolet iS50 (Thermo Fisher Scientific Inc.) FT-IR Spectrometer in the transmission model with 32 scans and a resolution of 4 cm-1. X-ray photoelectron spectroscopy (XPS) was carried out by a Kratos Axis DLD spectrometer with a monochromatic Al Kα radiation under ultra-high vacuum (operating pressure: 10-10 torr). The photoelectron emission spectra were recorded using an Al-Kα (hν = 1486.6 eV). Prior to the data analysis, the binding energy of the core level C 1s peak was set at 284.4 eV to compensate any surface charging effects, and all elemental spectra were shifted accordingly. Optical images for self-healing process evolution were token by a Zeiss Axio Lab.A1 microscope. The electrical conductivity of the PAAMPSA/PANI/PA electronic film was measured using a standard four-


28

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point probe method under ambient conditions using a Keithley 2450 source meter. Tensile tests were completed using a universal tensile tester (MTS QTest 25). The sample strip was held by pneumatic grips at 20 psi to prevent slipping during the stretching process. The gauge length was set at 20 mm while the strain rate was set at 8 mm/min. Sensor Characterization and Body Motion Detections: All the relative resistance changes were measured using a Keithley 2450 source meter. For stretching test, the sensors were fixed on home-built stretching stages to apply different strains. For flexion angle test, the sensor was attached on top of a digital protractor with a fixed bending radius of 20 mm and monitored the relative resistance change at different flexion angles. For twisting angle test, the strain sensor was held and twisted by hands at four twisted angles of 180°, 360°, 540° and 720°. For body motion detections, the strain sensor was attached on human skin at different body parts (elbow, wrist, knee, finger knuckle, etc.) by using adhesive medical tape. Various body motions were performed. For bending motions of finger, elbow, and knee, four or five evenly-spaced incremental angles are selected. Each bending angle were performed 6 times with bending duration of 2 seconds and bending motion intervals of 8 seconds. In addition to the motion performer, another group member helped in guiding the motion using digital protractor to ensure the targeted bending angles were accurate and motion tests were well controlled. ASSOCIATED CONTENT Supporting Information. Additional data including: TGA&DSC Analysis; Schematic illustration of tensile stress and strain; photos of stable polymer complex solution after 4 months; zeta-potential of polymer complex solution; various geometries of conductive films; TGA thermograms; DSC thermograms; XPS


29


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survey spectra; Photographs showing ultra-stretchability (strain at 1935%); Stress-strain curve under dried condition; Optical microscope images of expedited scratch self-healing process; Schematic diagram of strain sensor assembly; Plots of ΔR/R0 and GF versus tensile strain; Analysis of GF under 100 tensile strain; Plot of ΔR/R0 versus flexion angles; Plot of ΔR/R0 versus twist angles from 0 to 720°; Plot of ΔR/R0 versus time for for elbow bending at angles of 15, 30, 45, 75 and 90°. Movie S1. Demonstration of the developed polymer complex's elasticity. (MP4) Movie S2. Demonstration of the developed polymer complex's instant healing, performing a cutheal cycle while in series with an LED light, showing the restored electrical conductivity and mechanical strength. (MP4) AUTHOR INFORMATION Corresponding Author *Email: [email protected] (E.K. Wujcik) *Email: [email protected] (J.W. Jeon) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by two startup funds from the University of Alabama (E.K.W and J.W.J). The use of facilities at Central Analytical Facility of The University of Alabama is gratefully acknowledged.


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SYNOPSIS For Table of Contents Only:


41

ACS Applied Materials & Interfaces Page 42 of 49 Severed Surface

Polymer Chains

1 2 3 4 5 6

Self-healed

Self-Healing

Hydrogen Bonds

Electrostatic Interactions

: PAAMPSA : PANI : Phytic Acid (PA)

Relative Resistance Change (x 1000 %)

Cut

25

Quadratic Regressions: 0~500%: 20 ΔR/R = 0.6501 ε2 + 0.8374 ε + 0.0749 0 R2 = 0.9999 15 500~1000%: ΔR/R0 = 1.092 ε2 - 4.385 ε + 15.76 R2 = 0.9998 10 1000~1500%: ΔR/R0 = 1.825 ε2 - 18.44 ε + 83.39 5 R2 = 0.9999


: Hydrogen Bonds and Electrostatic Interactions

0

200

400

600 800 1000 Strain (%)

1200

1400

OH

Page 43 of 49

a

ACS Applied Materials & Interfaces Hydrogen Bonds

b

Electrostatic Interactions

HO

Phytic Acid

HO

O P

O HO

P

HO

P O

O

O

HO O

O

O O

P

OH O OH

P

CH3 CH3 1 P OH O O n n OH N N OH O OH PAAMPSA S S O 2 H CH3 H CH3 O O Solvent Add APS 3 N N H H N N H H 0 °C Casting PANI H H 4 N N H H PANI N N 5 CH3 CH3 PAAMPSA/PANI/PA Film PAAMPSA/PA/Aniline PAAMPSA/PANI/PA O O O 6 n n in Water Solution HO P O N N S S O O O O PAAMPSA H CH3 H CH3 HO O 7 HO OH O O O P P O O HO O 8 Phytic Acid O OH O O 9 P P O HO OH OH O O P OH c10 d OH 11 Severed Cut 12 Surface 13 Cut 14 Polymer Chains 15 Self-healed 16 Hydrogen Bonds 17 Self-Healing Bind 18 19 Electrostatic Interactions Heal 20 : PAAMPSA 21 : PANI 22 10ACS mm Paragon Plus Environment : Phytic Acid (PA) 23 24 : Hydrogen Bonds and 25 Electrostatic Interactions OH O

O

OH

b ACS Applied Materials & Interfaces C

40 μm

40 μm

P

O 40 μm

N2+

40 μm

40 μm

NH

Absorbance (a.u.)

40 μm

Intensity (a.u.)

1 2 3 N 4 5 6 7 8 c 9 10 11 12 13 14 15 16 17 410 18

S

d N2 , 401.3 eV N+, 400.0 eV NH, 399.2 eV

N+

3250

PAAMPSA/PANI/PA 3310

PAAMPSA/PA 3500 3000

-SO3− 2p3/2, 167.6 eV -SO3− 2p1/2, 168.7 eV -SO3H 2p3/2, 168.5 eV -SO3H 2p1/2, 169.6 eV

+

Intensity (a.u.)

SEM

Page 44 of 49

1112 1036 1218 1651 1303 1575 1496 1036 1218 1651 1303 1559

2500 2000 1500 1000 Wavenumbers (cm-1)

e H2PO4− 2p3/2, 133.9 eV

Intensity (a.u.)

a

H2PO4− 2p1/2, 134.8 eV

ACS Paragon Plus Environment 405

400 395 Binding Energy (eV)

390

172

170 168 166 Binding Energy (eV)

164

138

136 134 132 130 Binding Energy (eV)

100 80

78.8

81.4

84.7

89.3

95.2

98.6

Conductivity Healing Efficiency (%)

ACS Applied Materials b & Interfaces 100

60 40 20 0

5

10

30

60

120

180 Pristine

Self-Healing Time (min) 12

d

2. PAAMPSA/PANI/PA @ Low Humidity

8

3. PAAMPSA/PANI/PA @ Ambient Condition

6

4. Self Healed PAAMPSA/PANI/PA @ Low Humidity

4

0

500

1000

Strain (%)

98.4

97.4

99.2

98.9

97.4

98.2

1st

2nd

3rd

4th

5th

6th

Mean Pristine

80 60 40 20 0

800 600

5

400


1500

100

4

2

3

200

5. PAAMPSA/PA @ Ambient Condition

2

97.7

1000

1. PAAMPSA/PANI/PA @ Dried

10

100

Self-Healing Cycles

Stress (kPa)

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 c 12 13 14 15 16 17 18 19 20 21 22 23 24

Conductivity Healing Efficiency (%)

Page a 45 of 49

2000

0

500

1000

Strain (%)

1500

2000

a

c & Interfacesc ACS Applied Materials

1 2 3 4 5 6 b7 8 91 10 11 12 13 14 15 16

10 min Autonomous Healing Scratch 32 μm

10 μm

10 μm

Healing Time (min)

1000

10 1 0.1

Initial Shape

2

g

e h k, Present Work

100

50

LED Light On

b

a d

a: b: c: d: e: f: g: h: i: j: k: l: m:

i m, Present Work j Repeatable Healing f One-time Healing l, Present Work 60 70 80 90 100 Tensile Strength Healing Efficiency (%)

LED Light Off

Cut into halves


3

Page 46 of 49

One-time One-time One-time One-time One-time, Gentle Press Repeatable, Light Exposure Repeatable, Redox Stimuli Repeatable, Pressure (1 MPa) Repeatable, Heat Exposure Repeatable, Gentle Press Repeatable Repeatable, Gentle Press Repeatable, Gentle Press

LED Light On

300% Strain

Relative Resistance Change (x 1000 %)

aPage 47 of 49 25

Quadratic Regressions: 0~500%: ΔR/R0 = 0.6501 ε2 + 0.8374 ε + 0.0749 R2 = 0.9999 500~1000%: ΔR/R0 = 1.092 ε2 - 4.385 ε + 15.76 R2 = 0.9998 1000~1500%: ΔR/R0 = 1.825 ε2 - 18.44 ε + 83.39 R2 = 0.9999

b

ACS Applied Materials & Interfaces 14

Linear Regressions: 0~100%: GF= 1.317 ε + 0.4955 R2 = 0.9636 100~500%: GF= 0.6225 ε + 0.9596 R2 = 0.9973 500~1000%: GF = 0.7920 ε + 0.04815 R2 = 0.9955

400

d

1000~1500%: GF = 1.279 ε - 4.843 R2 = 0.9980

600

800 1000 Strain (%)

Relative Resistance Change (%)

Gauge Factor

c

Gauge Factor

12 20 1 2 10 3 15 8 4 6 5 10 6 4 7 5 2 8 9 0 200 400 600 800 1000 1200 1400 0 200 10 Strain (%) 11 12 n Self-Healable 13 1000 i 30 h L/Q Response 14 f g 15 100 No L/Q Response e 20 16 a b t, Present Work 17 o s k c 10 18 10 d l j 19 m p q 20 1 0 r 21 ACS Paragon Plus Environment 22 0.1 500 1000 1500 2000 0 10 23 Maximum Strain(%) 24

1200

1400

(ΔR/R0)max

R0 20

30 40 Cycles

50

60

70

a

b

θ = 120°

c

Page 48 of 49

300 250 200 150

Linear Regression y = 2.444 x - 0.3572 R2 = 0.9994 x: Flexion Angle (°) y: ΔR/R0 (%)

100 50 20

d

40

60 80 Flexion Angles (°)

Horizontally Stretching

1022% 440% 0%

ε = 330% 1746% 1032% 438%

0% 0

120

ε = 460%

ε = 200% Horizontally Stretching


100

1776%

Vertically Stretching Relative Resistance Change (%)

1 2 3 4 θ = 90° 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Vertically Stretching 19 20 21 22 23 Square Shaped Strain Sensor 24 25 26

θ = 45°


ACS Applied Materials & Interfaces θ = 0°

ε = 460%

ε = 330% ε = 200% 5 10 Time (s)

15

20

Page 49 of 49

a

b

Finger Knuckle Bending


60

74.20%


Wrist Bending

61.31%

50.20%

2



Rrlative Resistance Change (%)

Kn

ee

Be

nd in

Relative

gA ng

les

(°)

Resistance

Change (%

)

Fin ge rK

nu

ckl e

Relative

Be nd

ing

An gle s

Resistance

(° )

) Change(%

40 1 42.64% 2 60 20 90 Wrist Upward Bending 45° 3 40 -(26~27)% 29.85% 75 20 4 0 0 Wrist Downward Bending 75° 5 60 0 46~50% -20 6 20 45 Tim 7 40 e (s -400 ) 5 10 15 20 25 30 30 8 60 Time (s) 9 0° 45° 90° c 102.3% Elbow 10 91.22% 11 Knee Bending 79.88% 12 Wrist 13 70 58.65% 14 100 60 44.77% 120 50 15 50 Finger 100 40 30.07% 16 Linear Regression 80 0 Knee Ankle 30 0 17 y = 0.7159 x + 8.686 60 20 R = 0.9935 20 18 x: Knuckle Bending Angle (°) 10 40 Tim y: Resistance Change (%) 19 e (s 40 ) 0 15 30 45 60 75 90 60 20 20 Finger Knuckle Bending Angles (°) 21 22 45° 0° d 23 Complex Knee Motions 90° 24 Extending and Flexing Walking Squatting 68~70% Jumping from Squatting 70 36~40% 23~27% 48~53% 25 Squating 60 100 26 Flexing 50 27 80 Extending 40 28 60 30 Linear Regression 29 20 y = 0.7455 x + 15.63 40 30 R = 0.9914 10 x: Knee Bending Angle (°) 20 31 0 y: Resistance Change (%) ACS Paragon Plus Environment 10 20 30 40 50 32 Recovery Time (s) 0 20 40 60 80 100 120 Slight Recoil From Landing Knee Bending Angles (°) 33 34 2

Ultra-Stretchable Conductive Polymer Complex as Strain Sensor with

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