Iono-Elastomer-Based Wearable Strain Sensor with Real-Time

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An iono-elastomer based wearable strain sensor with real-time thermo-mechano dual response Yunsong Xie, Ru Xie, Hao-Cheng Yang, Zhaowei Chen, Jingwei Hou, Carlos Rene López-Barrón, Norman J. Wagner, and Kaizhong Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10672 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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ACS Applied Materials & Interfaces

An Iono-Elastomer Based Wearable Strain Sensor with Real-Time Thermo-Mechano Dual Response

Yunsong Xie,†* Ru Xie,‡ Hao-Cheng Yang,§ Zhaowei Chen§, Jingwei Hou,∥ Carlos R. LópezBarrón,‡ Norman J. Wagner⊥ and Kai-Zhong Gao†*

†

Energy Systems Division, Argonne National Laboratory, Argonne, IL, 60439 USA

‡

ExxonMobil Chemical Company, Baytown, TX 77522, USA

§

Centre of Nanoscale Materials, Argonne National Laboratory, Argonne, IL, 60439 USA

∥ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK

Department of Chemical and Bimolecular Engineering, University of Delaware, Newark, DE 19716, USA

⊥

KEYWORDS: Ionic liquid, elastomer, wearable electronics, motion sensor, temperature sensor ABSTRACT An ultra-stretchable iono-elastomer with resistance sensitive to both elongation strain and temperature has been developed by hierarchical self-assembly of an end functionalized tri-block copolymer in a protic ionic liquid (ethylammonium nitrate) followed by cross-linking. Small angle X-ray scattering experiments in situ with uniaxial elongation reveal a nanoscale microstructural transition of the hierarchically self-assembled cross-linked micelles that is responsible for the material’s remarkable mechanical and ionic conductivity responses. The results show that the inter-micelle distance extends along the deformation direction while the micelles organize into a long-range ordered face-center-cubic (FCC) 1 ACS Paragon Plus Environment

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structure during the uniaxial elongation. Besides good cyclability and resistance to selected physical damage, the iono-elastomer simultaneously achieves an unprecedented combination of high stretchability (340%), highly linear resistance vs. elongation strain (R2 = 0.998), and large temperature gauge factor (ΔR/R = 3.24 %/°C @ 30 ºC). Human subject testing demonstrates that the iono-elastomer based wearable thermo-mechano sensor is able to effectively and accurately register both body motion and skin temperature simultaneously.

1. INTRODUCTION Wearable physical sensors bridge the physical and cyber worlds by converting physical variables (motion, pressure, temperature, light, and humidity, etc) into electronic signals. Such devices generate considerable interest in applications including sports training,1,2 prostheses,3,4 personable healthcare,5,6 and robotics.7–10 Motion and temperature sensing, in particular, have received much attention as they are two of the most commonly considered physical variables related to human activities.3,4,10–17 In sports training, for example, real-time body motion monitoring could aid in training to achieve better sports performance;18,19 meanwhile, a temperature monitoring function could protect the athletes from over-heating, which can cause illness and injuries.20 To this end, wearable thermo-mechano dual-responsive sensors have been previously developed by embedding rigid sensing materials, such as Pt,21 Cu/Al,21 Si,4 Pt/Cr,14 Ag,3 graphene oxide,12 BaTiO3,11 etc., into a stretchable matrix or forming fractal conductive networks. In most cases, the thermal and mechanical sensing units were positioned and fabricated separately due to the different sensing materials used. Consequently, the fabrication process for such dual-responsive sensors becomes complicated and manufacture cost rises. Moreover, such inhomogeneous material configurations have some inevitable disadvantages: 1. the rigid materials embedded in stretchable matrix compromise the sensor stretchability; 2. tiny local damages on the sensing parts may disable the whole sensor due to the disconnection of microcircuit; 3. the as-prepared composite 2 ACS Paragon Plus Environment

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materials cannot be flexibly tailored, jointed and perforated to fit the body parts for special demands; 4. the dimension of sensing devices is predominantly limited by the resolution of the metal patterning. Therefore, the development of a homogenous, highly-stretchable sensing material with independent response to both strain and temperature variation would be highly valuable. To overcome these challenges, we look to develop a new type of mecho-thermo wearable sensors composed of a single material that can be extensively and repeatedly stretched to produce a resistance change upon both physical deformation and subtle temperature variation. In this regard, iono-elastomers, which are fabricated by crosslinking triblock-copolymer micelles that are self-assembled in ionic liquids, are a promising candidate. The long polymer chains can act as bridges connecting the dispersed micelles to enables a large stretchability of the elastomer, 22 which is also a basic requirement of wearable sensors.23–26 At the same time, the high activation energy for ionic conduction in the ionic liquid ethylammonium nitrate (~ 12 kJ/mol 27) bestows a highly temperature sensitive resistivity of ∆R/R ≤ 1.6 %/°C @ 30 °C, which can be even further increased by self-assembling micelles

in ionic liquid.28–30

Therefore, we demonstrate herein the design and fabrication of a wearable thermo-mechano sensor based on an iono-elastomer. By hiearchical self-assembly of Pluronic F127 diacrylate in ethylammonium nitrate followed by chemical crosslinking, an ultra-stretchable thermomechano sensing material can be readily fabricated. In-situ Small Angle X-ray Scattering (SAXS) characterization during stretching reveals that this iono-elastomer exhibits large stretchability of 340% and a two-stage microstructure transition during uniaxial elongation. These thermo-mechano sensors feature superior strain sensing accuracy, fast strain response, good stability/reliability, and robust functionality even when considering minor physical damages including puncture, scratching, and cutting.

As importantly, because the ionic

conductivity of the iono-elastomer is highly promoted by increasing temperature, the sensor 3 ACS Paragon Plus Environment


performs with a record-high temperature sensitivity for stretchable temperature sensing materials (∆R/R ≥ 3.24 %/ºC @ 30 ºC).3,4,11–16 Human subject testing demonstrated that the sensors were able to accurately capture both body motion and skin temperature in real time using wireless communication.

2. EXPERIMENTAL SECTION Preparation of F127-DA/EAN solution and crosslinking The ionic liquid ethylammonium nitrate (EAN) and Pluronic F127 were used as purchased without any pre-processing. The as purchased EAN has a purity of 97% and water content > 2%. The F127-DA was synthesized and crosslinking using our previously reported methods.22,28 The resulted iono-elastomer is composed of 24 wt% F127-DA in EAN. See Figure S1 for further details. Characterization Infrared spectra were obtained from a Fourier transform infrared spectroscopy (Thermo, Nicolet 6700). In the test, both pure EAN Pluronic F127-DA/EAN sol and IL were held in place by two glass slides through capillary forces. The scan range was from 400 cm-1 to 4000 cm-1 with a resolution of 0.94 cm-1. SAXS measurements were carried out at the GISAXS beam line 8-ID-E at the Advanced Photon Source (APS) at Argonne National Laboratory. The incident X-ray energy was 7.35 keV. The above-described in-house built stretching apparatus was used to apply uniaxial deformation to the sample with a constant rate of 0.1 mm/s and stop at target strain, while taking in-situ SAXS measurements. The conductivity of ionic liquid and iono-elastomer can be calculated using Arrhenius equation: 4 ACS Paragon Plus Environment

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Where σ, A, Ea, R, and T are the conductivity, a constant, conductivity activation energy, universal gas constant and temperature in Kelvin, respectively. The resistivity is calculated by

The material temperature gauge factor of the resistance can be calculated by

Considering the activation energy of neat EAN is 12 kJ/mol,27 the temperature gauge factor for pure EAN at 25 °C can be calculated to be 1.6 %/°C. Tensile and electrical measurements and in-situ SAXS measurements In-house built stretching tool was installed in SAXS beamline to measure the mechanical, electrical, and microstructural properties while the sample is being uniaxially stretched. The setup is shown in Figure S2. The ends of the sample are glued onto the electrodes using UV glue. The electrodes are connected to NI ELVIS II data requisition card to perform the electrical property measurement. A force sensor FC2231 is used to measure the tensile strength of the elastomer. The control of the uniaxial stretching tool and electrical/mechanical data requisition are carried out by NI ELVIS II data requisition card. A graphic user interaction system based on LabVIEW program has been developed to synchronize the uniaxial stretching tool control and electrical/mechanical data requisition. The 5 ACS Paragon Plus Environment


iono-elastomer samples were uniaxially stretched at room temperature, using a constant deformation rate in both loading and unloading directions.

3. RESULTS AND DISCUSSION The iono-elastomer, composed of functionalized Pluronic F127, a commercial triblock polymer and ethylammonium nitrate (EAN), a protic ionic liquid, is synthesized by the procedure depicted in Figure S1. Pluronic F127 was end-functionalized with acrylate groups (abbreviated as F127DA) for post UV crosslinking (Figure 1). Driven by thermodynamics, the F127DA self-assembles in EAN to form micelles, which further self-assemble at higher concentration to exhibit an inverse gel transition with heating.22,28 The iono-elastomer was fabricated by simply casting the F127DA/EAN and photo-initiator mixture into a mold with desired shape followed by UV crosslinking. The interactions between F127DA and EAN were revealed by FTIR spectra (Figure 2a). The peaks at around 2510 cm-1 and 2608 cm-1 are assigned to the molecular vibration of N-H bonds in EAN. These shifts can reflect the intermolecular interactions existed in the system because no absorption from F127DA lies on this region. Blue shifts of these peaks were observed after mixing F127DA and EAN, indicating the formation of hydrogen bonds between N–H (as a H-donor) and –O– in PEO block (as a H-acceptor). The crosslinked iono-elastomer exhibits a combination of high stretchability, achieving elongation strains over 340%, and a monotonically increasing electrical resistance upon stretching up to its breaking point (Figure 2b). The iono-elastomer exhibits an initial elastic regime, followed by yielding at elongation strain ε ∼ 45%, and very pronounced strain hardening with onset at ε ∼ 150%. To elucidate the underlying mechanisms giving rise to the high stretchability, in-situ electrical-mechanical-SAXS measurements were performed to monitor the self-assembled structure evolution during stretching process (Figure 2b). The 6 ACS Paragon Plus Environment

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direction of the X-rays beam is perpendicular to the elongation direction and is focused on the center of the sample. The average nearest neighbor distance (DNN) was extracted by using face-centered cubic (FCC) model with paracrystalline distortion 31,32 to fit the 1D azimuthally averaged intensity vs. scattering vector spectrum (I(q) vs. q). The degree of ordering (DOO) was calculated based on the annularly averaged intensity (I(q) vs. Φ).33 (Detail information of DNN, DOO, I(q) vs. q, and I(q) vs. Φ can be found in Figure S3 and S4) The evolution of DNN and DOO during the uniaxial elongation, illustrated in Figure 2b, show that both parameters increase with elevated elongation strain. More specifically, DNN increases immediately upon the elongation of iono-elastomer, from 269 Å to a maximum of 280 Å at the onset of yielding regime (ε ~ 45%). In contrast, the DOO stays near zero for ε < 30%, whereupon the DOO rapidly increases to a maximum of 0.30 at the onset of strain-hardening (ε ~ 150%). This DOO evolution corresponds to a symmetry breaking evolution of the 2D SAXS pattern as shown in Figure 2c and Movie S1 in Supporting Information. The evolution of the 2D SAXS pattern starts with an isotropic circular pattern, which indicates randomly oriented FCC grains of micelles.

28

When ε > 30%, the pattern becomes elliptical with vertical orientation

(corresponding to a horizontal stretched gain in real space) and eventually evolves into fourfold symmetric lobes, which represent micelles aligned into layered hexagonal close-packed (FCC) structure. This two-staged structural evolution during stretching has been schematically shown in Figure 2d. At low elongation strain and initial elastic regime, the bridges, connecting individual micelles, are elongated as suggested by the increasing DNN values. The elongation of the bridges in turn align the micelles, manifesting by the increasing DOO values. At higher elongation strain in the yielding regime, the increasing DOO and DNN implies the continuation of the micelle rearrangement into a lower energy state. In the elongation strain range from 150% to the breaking point, stress continues to increase while both DOO and DNN reach a plateau at the onset of strain hardening and remain nearly constant until breakage. This can be readily understood by considering that once the bridged polymer chains 7 ACS Paragon Plus Environment


connecting micelles reach their maximum elongation, further stretching requires higher energy processes, such as pulling or rupturing bridged polymer chains out from the micelles. These energy costly processes are manifested as macroscopic strain hardening, without entailing further molecular alignment, i.e., nearly constant values of DOO and DNN. 34 In strain sensor applications the sensitivity and accuracy are two of the most important figures of merit to evaluate performance. Historically, because a linear relation is normally assumed between the elongation strain and resistance,4,21,24,25,35 the sensitivity and accuracy of the strain detection are highly related to the strain/resistance gauge factor and linearity of the strain sensing material. We have made such high linearity a higher priority to the large gauge factor because of two reasons. Firstly, a large gauge factor of the sensing material is not the only route to a high strain detection sensitivity. Many electronics circuits have been developed to enhance the capabilities to detect small signals, such as low noise amplifiers and signal filters.36 Secondly, for a physical sensor, as long as a linearity relation has been preassumed, a higher linearity of the sensing response is the only way to improve detection accuracy. Our iono-elastomer delivers an exceptionally linear resistance/strain response by nature. Unlike some reported materials with high gauge factor, in which the resistance change is produced by microscopic cracks and breakage,7,37–39 the continuous and repeatable resistance change of our iono-elastomer is the result of reversible shape deformation of a continuous and homogenous matrix. The real-time repeated resistance response curve of the iono-elastomer was collected through a consecutive loading-unloading repeating tests, as shown in Figure 3a. Even under a large range of stress (1.25% ~ 100%), this iono-elastomer gives a linear and repeatable resistance output. The change in resistance as a function of strain ratio has been summarized in Figure 3b. Each point is achieved by the average of more than 4 repeated loading-unloading tests, and the error bars (too small to be shown in figure) are about 0.03% 8 ACS Paragon Plus Environment

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(∆R/R). More importantly, analysis reveals that even under strain amplitudes as long as 100%, the relation between the resistance and strain ratio has a linearity R2 (statistically measures how close the distribution follows the linear model) value of 0.998. This value is the highest among all the previously reported stretchable strain sensors with at least 50% stretchability to our knowledge (see Table S1). The reliability and robustness of a strain sensor, which is essential for the service lifetime of the sensor in practical application, is tested in cycling experiments. In these tests, four ionoelastomers were subject to different physical damage processes, including intact, rubbed with 150 grit sandpaper, punctured by a needle, and cut along longitudinal direction by a sharp blade. The evolution of the gauge factor for all four iono-elastomers were monitored while the iono-elastomers were undergoing 1000-consecutive loading-unloading stretching cycles at 55% strain ratio. These measured gauge factors (normalized to the intact value) are shown in Figure 3c. Importantly, all the samples survive the corresponding mechanical damage. Although the punctured and cut samples suffer from a small drop in gain at the start of cycling, all cycling evolutions follow a negative exponential curve and become stable after 400 cycles. This behavior can be rationalized by assuming that the initial cycling (~ 100) causes some local rearrangement and “annealing” of the microstructure, and potentially some inter-micelle linking breakage, to obtain a more stable structure. It is believed that the homogeneous nanostructure and high stretchability of the iono-elastomer prevents structural cracks from further propagating and therefore, provides the high tolerance to the minor physical damage. Along with strain sensing, a temperature detection capability is one of the most valuable properties of the iono-elastomer. Unlike strain sensing, where a linear relation is hypothesized, a simple Arrhenius equation model 1/r = σ = Ae(−Ea/RT) can be used to predict the relation between resistance and temperature, where r, σ, Ea, R, T, and A are resistivity, conductivity, activation energy for conductivity, universal gas constant, absolute temperature and pre9 ACS Paragon Plus Environment


determined constant, respectively.40 The temperature/resistance relation of the iono-elastomer has been characterized between -15 °C and 65 °C and shown in Figure 3d. The fitting result shows that the temperature/resistance behavior of the developed iono-elastomer highly correlates to the Arrhenius equation model (R2 = 0.998), which yields an Ea of 26.7 KJ/mol. The result illustrates a highly sensitive and predictable temperature/resistance response throughout the considered temperature range. The temperature gauge factor at 30 °C reads ΔR/R = 3.24 %/°C, which is, to our current best knowledge, twice as high compared to all the previously reported resistance based stretchable temperature sensing materials.3,4,14,15,21,41 A wireless, wearable strain sensor fabricated from the iono-elastomer has been attached to a human body to monitor motion. The strain displacement created by human activities can be recorded by the strain sensor as a change in resistance. This change in resistance is reported in real time by a simple Bluetooth-based wireless sensing electronic circuit and recorded digitally on a computer. The strain sensor was attached on the finger, neck, and knee of a human subject for common joint motion capturing. The photographs of the sensor and the captured resistance changes in time spectrum at all three locations are shown in Figure 4a-c, respectively. These results evidence that the movements of finger, neck, and knee can be clearly and qualitatively recognized based on the resistance change output from the strain sensor. A quantitative evaluation of strain sensor motion capture accuracy has been further carried out by attaching the sensor on the elbow joint of the human subject. The angle between the upper arm and forearm was measured by both video recording and strain sensor monitoring. The visual measurement was carried out by analyzing selected frames of the recorded video (video footage can be found in the Movie S2). The angle measured by the strain sensor is calculated by the elongation caused resistance change of the iono-elastomer. The real-time elbow angle data produced by both methods are plotted as blue scattered points and black solid line in Figure 4d, respectively. A linear numeral fitting of these two sets of 10 ACS Paragon Plus Environment

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data gives R2 = 0.997, which quantitatively proves the high accuracy of the strain sensor human body motion capturing capability. The temperature sensors built upon both iono-elastomer and a conventional thermocouple were used to measure the human body skin temperature. The temperature sensor based on iono-elastomer is constructed by simply placing a small piece of iono-elastomer on a rigid substrate while each end is connected to an electrode (photograph and detailed sensor architecture can be found in Figure S10). Five locations on the human subject, neck, arm, elbow, hand, and leg, were chosen for this temperature sensor accuracy demonstration. Both sensors were attached on the human subject (shown in Figure 4f) to detect the skin temperatures at all five locations. The measurement output of both sensors, shown in Figure 4g, are consistent with each other at all five locations, indicating the high accuracy and repeatability of the iono-elastomer based temperature sensors. A thermo-mechanical dual responsive sensor was realized by combining both strain and temperature sensing potential of the iono-elastomer into one device. The photograph and the detailed sensor architecture are shown in Figure 5a and Figure S11, respectively. This thermo-mechanical sensor has three electrodes that are connected by one iono-elastomer strip. The iono-elastomer portion connecting temperature sensing and ground electrodes sit on a rigid substrate to isolate it from the strain effect, which makes the temperature sensing electrode only response to temperature variation. The resistance of the iono-elastomer portion that connects strain sensing and ground sensing electrodes response to both temperature and strain. Therefore, the readable strain information was acquired by post-processing the output from both electrodes following a simple decoupling algorithm. Figure 5b shows the crosstalk-free responses of both electrodes on the sensor under a testing circumstance that the sensor was being first stretched and then heated/cooled. To illustrate the significance of the developed sensor in applications including personable healthcare and sports training, the 11 ACS Paragon Plus Environment


thermo-mechanical dual responsive sensor was attached on the arm of a human subject while performing a high-intensity anaerobic exercise, push-ups. Such anaerobic exercise is commonly implemented in sports training process and has been repeatedly reported to induce body temperature increase.42 Figure 4c shows the photograph and real-time captured temperature/strain response during the anaerobic exercise, where the exercising period could be clearly seen. The videos of entire exercising in two visual angles and the real-time strain/temperature spectrum can be found in Movie S3. The high-intensity anaerobic exercise caused a skin temperature elevation slope of ~ 0.2 °C/min, consistent with the previous sports research. The results also showed an approximate 3 seconds time delay between the temperature climb/drop and the start/end of the anaerobic exercise, which can be attributed to a combined consequence of the muscle/skin thermal transfer process and muscle heat capacity.43,44 Together, these results demonstrated the feasibility of ion-elastomer based thermo-mechanical dual response in sports monitoring.

4. CONCLUSION In conclusion, we demonstrated an iono-elastomer by self-assembling of a functionalized triblock copolymer in ionic liquid followed by UV-induced cross-linking. The iono-elastomer is highly stretchable (with a maximum elongation of 340%) and sensitive to small motion. The in situ SAXS revealed the rearrangement of micelles to a longer inter-micelle distance and more ordered structure during stretching, providing a microstructural mechanism for the observed, exceptional mechanical and electrical responses. During the performance test, we found that the iono-elastomer performed with high accuracy and reliability in motion tracking because of its high linear strain-resistance response (R2 = 0.998). The iono-elastomer is shown to be robust and can resist external damages such as rubbing, pinching, and directional cutting while maintaining its functionality over 1000 cycles. It also features a large 12 ACS Paragon Plus Environment

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temperature-dependent conductivity, the value of which, 3.24 %/°C @ 30 °C, is more than twice that of the most sensitive reported materials. Three sensors have been developed based on the iono-elastomer for human activity tracking. The strain sensor is able to capture human motion with sufficient sensitivity and accuracy, while the temperature sensor outputs accurate information on the human skin temperature. The thermo-mechanical sensor achieves a simultaneous/real-time strain and temperature tracking on a human subject during a highintensity anaerobic exercise. All results support the argument that this novel iono-elastomer sensor possesses potential for use in sports training, prosthetic, personable healthcare, and robotics applications.

ASSOCIATED CONTENT Supporting Information Supporting information.doc Movie-S1-SAXS-2D-pattern.avi Movie-S2-BT-real-time-movie.avi Movie-S3-Thermo-mechano.avi

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y. X.) * E-mail: [email protected] (K. G.)

Author Contributions 13 ACS Paragon Plus Environment


Y. Xie, R. Xie and H.-C. Yang contributed equally to this work. ACKNOWLEDGMENT This research was performed at beam line 8-ID-E of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory. The research is sponsored under Contract No. DEAC02-06CH11357. N.J.W and R.X. acknowledge support for the work in this paper under cooperative agreements 70NANB12H239 and 70NANB15H260 from NIST, U.S. Department of Commerce. R.X. also acknowledges the support of this work from the National Science Foundation Graduate Research Fellowship Program (Grant No. 1247394) and Delaware Space Grant College and Fellowship Program (NASA Grant NNX15AI19H).

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Figure 1. Upper panel: Iono-elastomer fabrication process. Lower panel: The chemical formula of the end-functionalized block copolymer, F127DA, and ethylammonium nitrate, EAN.

Figure 2. a) FTIR spectrum of the ionic liquid, EAN, and mixture of the block copolymer and ionic liquid, Pluronic F127-DA/EAN. b) Degree of ordering (DOO) and nearest neighboring distance (DNN) extracted from SAXS profile, as well as stress and electrical resistance are plotted as a function of elongation strain from in-situ SAXS-stretching-electrical measurement. c) SAXS 2D patterns of iono-elastomer at indiciated strain. d) Proposed microscopic structural evolution scheme during stretching.


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Figure 3. Electrical sensitivity analysis of iono-elastomers. a) Change in resistance ratio during four repetitions of consecutive loading-unloading cycles (strain rate = 0.01 s-1) to elongation strains: 1.25%, 10%, 50% and 100%. The inset is a zoom-in image of the 4th cycle at strain of 1.25%. R0 is the resistance of the elastomer at zero elongation strain at 25 °C. b) The sensitivity of strain sensor is plotted as a function of strain at small strain regime up to 100%. c) Normalized gauge factor as a function of repeating cycle at strain of 55% for ionoelastomers at four different post-processing conditions: normal, rubbed by a sandpaper, pinched by a needle, and cut by a knife conditions. d) The ratio of resistance change (in linear scale) as a function of temperature. The inset illustrates the ratio of resistance comparing to R0, with x axis being 1000/T (T is the absolute temperature) and Y axis being resistance ratio in log scale.



Figure 4. Photographs of iono-elastomer based strain capturing resistance change at a) finger, b) neck, and c) knee. d) Change at elbow bending angles measured by both camera recording and strain sensor monitoring are plotted as a function of time. The scattered points and line are measured from the recorded video and strain sensor, respectively. The inserts are the captured snapshots of the range of motion at corresponding time. e) The linear relation between the elbow bending angles measured by video and strain sensor. f) Infrared image of human subject and the notation of each individual location. g) The temperatures measured by both thermocouple and iono-elastomer based sensor at all five locations indicated by f).


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Figure 5. a) Photograph of the developed iono-elastomer based thermo-mechano dual responsive sensor. T and S electrodes outputs the information about temperature and strain, respective. GND electrode provides a ground for last two electrodes. b) Measured resistance response from both S (black line) and T (red line) electrodes, respectively. The sensor was first being stressed and then in touch with a cold and hot object. The strain sensor was decoupled to output the strain signal solely. c) Upper panel: photographs of the human subject doing high intensity anaerobic exercise. Lower panel: real-time strain and temperature information captured by the developed thermo-mechano dual responsive sensor.



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Iono-Elastomer-Based Wearable Strain Sensor with Real-Time

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