Asymmetric Structure Based Flexible Strain Sensor for Simultaneous

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Asymmetric Structure based Flexible Strain Sensor for Simultaneous Detection of various Human Joint Motions Youlin Zhou, Yuanzhao Wu, Waqas Asghar, Jun Ding, Xinran Su, Shengbin Li, Fali Li, Zhe Yu, Jie Shang, Yiwei Liu, and Run-Wei Li ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00386 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Applied Electronic Materials

Asymmetric Structure based Flexible Strain Sensor for Simultaneous Detection of Various Human Joint Motions Youlin Zhoua,c‡; Yuanzhao Wua,b,c‡; Waqas Asghara,c,d; Jun Dinge; Xinran Sue; Shengbin Lia,b,c; Fali Lia,b,c; Zhe Yua,b,c; Jie Shanga,c; Yiwei Liua,c*and Run-Wei Lia,b,c*

aCAS

Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, P. R. China bCenter of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, P.R. China cZhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, P. R. China dUniversity of Engineering and Technology Taxila, 47050, Taxila, Pakistan eDepartment of Materials Science and Engineering National University of Singapore, Singapore 119260, Singapore

‡These

authors contributed equally to this work.

*To whom all correspondence should be addressed: [email protected] (Prof. Dr. Y.W. Liu); [email protected] (Prof. Dr. R.-W. Li)

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ACS Applied Electronic Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Joint motion is a very common activity which involves tensile or compressive bending motions. Distinguishing and monitoring of joint motions are important for interactive human-machine interface or rehabilitation of human joint. Here, we present asymmetric structure based, liquid metal embedded, resistive strain sensor, which is prepared by stereo-lithography based 3D printing process. Electro-mechanical characterization results of sensor confirm that current sensor can monitor angle and direction of joints even if the angle amplitude remains the same. The sensor exhibits good mechanical stability and minimum resolution angle of 1°, ranging from 70° to -70°, and. Sensor performance enhances with the increases of its thickness, which is due to additional deviation produced at the center, this deviation causes the resistance of sensor to change greatly both during compressive and tensile bending. Finally, sensor’s capability is practically demonstrated by the monitoring the motion of index finger, wrist and neck joint under various human activities. Our sensor paves a way for real time continuous monitoring of human or artificial robot joint motions.

KEYWORDS: Flexible Strain Sensor; 3D Printing; Asymmetric Structure; Liquid Metal; Piezoresistive; Joint Motions Monitoring

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INTRODUCTION Nowadays, flexible sensors are widely used in the fields of wearable electronics, artificial skin, human motion monitoring, human–machine interface and soft robots 1,2. These sensors sustain various mechanical deformations (bending, folding, stretching, compressing, twisting) and provide long-term monitoring capabilities3-5. For human beings and artificial robots, joint bending is a very common activity in which joint perceives the change of stress and bending angle6. Therefore, bending angle sensors are considered as the most important components in the field of soft robotics or joint rehabilitation of disable patients7,8. Generally, angle sensors are based on resistive9,10, capacitive11-13, or piezoelectric14-16 sensing mechanisms, but resistive sensing is famous out of all because of its easy fabrication, high sensitivity and simple architecture of device17,18. Various researchers have developed bending angle sensors by using metal nanowires19, 20, carbon nanotubes/graphene17, 21-23 and conducting polymers24,25. Deng et al.26 have developed a self-powered, flexible piezoelectric sensor based on PVDF/ZnO nanofibers for remote control of gestures in human-machine interactive system. Reported sensor exhibits excellent bending sensitivity, ranging from 44° to 122° and minimum resolution angle of 10°. Pu et al.27 have reported a joint motion triboelectric quantization sensor for constructing a robotic hand synchronous control system. Sensor exhibits ultrahigh sensitivity with minimum resolution angle of 3.8°. Some joints involve both unidirectional and bidirectional bending (bending up and down) that’s why few researchers have also tried to monitor bidirectional joint motions with diverse bending angles. For example, Xu et al.22 have used graphene and inverse opal cellulose film to develop multifunctional wearable sensor which simultaneously detects and distinguishes different bending directions of wrist joint ranging widely from 60° to 120°. Yan et al.28 have reported a high-performance flexible strain sensor which consists of sandwich structure of polymer elastomer. Reported sensor effectively monitors human joint/muscle activities and simultaneously distinguish bending angles (θ=0, 30°, 60° and 90°) and bending directions (being up/down). Besides this rapid development, newly built angle sensors still need further improvement in terms of simultaneous joint angle and direction detection, high angle resolution, wide range and manufacturing steps involved. Attempt to manufacture highly sensitive angle sensors, increases time consumption, overall cost and complexity of manufacturing process. Also, there exist a weak compatibility between sensing material and complex structure of sensor, which affects the sensitivity and resolution of sensor. Fortunately, stereo-lithography (SLA) based 3D printing process is proved a cost-effective technique which is the best solution of above-mentioned problems. This technique uses UV laser source to convert liquid resin into solid and manufactures highly precise complex geometries of sensors within short time29. Besides, Galinstan (LM alloy) is unique out of other flexible sensing materials because it exhibits excellent metallic conductivity, high deformability and low toxicity30, 31. Therefore, Galinstan can be used effectively, to prepare 3D printed, complex shaped sensing elements of wearable sensors. Herein, we demonstrate an asymmetric structure based, highly flexible, resistive strain sensor, which is capable of monitoring angle and direction of joints simultaneously. SLA based 3D printing process is used to prepare microchannel in a commercial resin (RS-F2-FLGR-02), followed by microchannel filling with Galinstan, to form asymmetric structure-based sensing 3



element. Two types of sensors (3 mm and 5 mm thick) are prepared for comparison purpose. Afterwards, electromechanical performance of both sensors is evaluated by performing 3-point bend tests. Test results confirm high capability of sensor to detect angle and direction of bending simultaneously even if the angle amplitude remains the same. The sensor exhibits good mechanical stability and minimum resolution angle of 1°, ranging widely from 70° to -70°. Finally, it is demonstrated that the current sensor can effectively identify the motion of different human joints like index finger, wrist and neck bending. These features make current sensor reliable to be used for simultaneous detection, of both angle and direction of human or artificial robot joint motions . RESULTS AND DISCUSSION Preparation of Sensor In this work, asymmetric structure-based sensor is used to identify angle and direction of joint bending. Asymmetric structure is formed by placing the sensing element on top surface of sensor. SLA based 3D printing process is used to prepare microchannel in commercially available resin (RS-F2-FLGR-02). Figure 1 shows all the steps involved in manufacturing of flexible strain sensor. Process starts by curing first layer of resin, at bottom of resin bath by using UV laser, as shown in Figure 1a. Afterwards, the strain gauge shaped micro-channel is formed by using selective curing process of resin, as shown in Figure 1b. The last layer is cured completely to finish the geometry of sensor (Figure 1c). Geometry formed in this way contains uncured liquid resin left in the microchannel (Figure 1d), which is removed and finally microchannel is filled with the Galinstan by using syringe as shown in Figure 1e. After that, Cu electrodes are inserted on both ends of LM embedded micro-channel and finally these ends are sealed with glue. Figure 1f shows the final shape of sensor. Two types of sensors, i.e., one of 3 mm thickness and the other is of 5 mm thickness, are prepared for comparison purpose. Figure 1g shows the dimensions of sensor (60×15×5 mm) and Figure 1h shows the considerable flexibility of sensor.

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Figure 1 (a) Curing first layer of resin at bottom. (b) Selective curing process to form the strain gauge shaped micro-channel. (c) Curing of last layer to finish the geometry of sensor. (d) Removal of uncured liquid resin from microchannel. (e) Injection of Galinstan into microchannel. (f) Final shape of strain sensor. (g) Photograph of strain sensor having dimensions 60×15×5 mm. (h) The flexibility of strain sensor. Strain Sensing Performance Joint bending is a very common activity which may be compressive or tensile in nature, as defined in Figure 2a. Capability of angle sensor to identify both compressive and tensile bending-induced strains simultaneously is considered important from practical application point of view. Therefore, 3-point bend tests are performed on both 3 mm and 5 mm sensors to evaluate their electromechanical performance. In compressive bending, edges of sensor bend outward and length of LM channel at the top is decreased. In tensile bending, edges of sensor bend inward and length of LM channel at the top is increased. Figure 2b and Figure 2c shows that resistance change response to the displacement of sensors during compressive and tensile bending, which means both sensors can successfully identify the compressive and tensile bending. Resistance of both sensors changes in a separate manner (opposite direction) under both compressive and tensile strains. During compressive bending, resistance of both sensors decreases as shown in Figure 2b, but △R/R values of 5 mm thick sensor shows larger decrease than △R/R values of 3 mm sensor. While during tensile bending, the resistance of both the sensors increases with displacement as shown in Figure 2c. But △R/R values of 5 mm thick sensor show larger increase than △R/R values of 3 mm thick sensor. It is attributed to the asymmetric structure of the sensor. Due to large thickness, additional deviation is produced at center of 5 mm thick sensor, that’s why resistance change of 5 mm thick sensor is more than that of 3 mm sensor both in compressive and tensile bending. These results indicate that the different direction of the 5



bending-induced strains can be easily identified even if the strain amplitude remains the same, which is important for the detection of complex multidirectional motions. According to Figure 2b and 2c, the sensitivity of the sensor can be calculated. During tensile and compressive bending, our sensor has shown sensitivity of 2.85 and 1.59 respectively, which can be compared with the sensitivity of strain gauge shaped sensors. Figure 2d and 2e shows the repeatability of the 5 mm thick sensor, when the sensors were subjected to compressive strain and tensile strain of 1.5% and at .5% at 2.5 mm/min strain rate. It is obvious from figure, that sensor exhibits excellent repeatability up to 2000 cycles. Sensor’s outstanding repeatability reveals a good mechanical match between components of sensor (microchannel and LM) and ability of microchannel to shrink and recover efficiently during loading and unloading cycles, which contributes to excellent stability and repeatability.

Figure 2 (a) A schematic illustration of compressive and tensile bending. (b) Relative change in resistance during compression bending. (c) Relative change in resistance during tensile bending. (d) The repeatability of 5mm thick sensor when it is the sensors were subjected to compressive strain of 1.5%.(e) The repeatability of 5mm thick sensor when it is subjected the sensors were subjected to tensile strain of 1.5%. 6


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Angle Sensing Ability 3-point bend tests are performed on sensors to evaluate their performance under tensile and compressive bending. Sensors have shown negative response when they are subjected to compressive bending and positive response when subjected to tensile bending as shown in Figure 3 and Figure 4. Forces of different magnitudes are applied on both sensors, to attain various tensile and compressive bending angles (0°~70°); angles are defined in Figure 3a and Figure 4a. When bending force acts on sensor, it bends, which is defined by angle θ. This θ changes with the change of magnitude of bending force. During compressive bending (Figure 3b and Figure 3c), △R/R values decrease with the increase of bending angle. △R/R values decrease from -0.15% to -0.35% in the angle range of 1°~10°, as shown in Figure 3b. If angle range is increased further from 10°~70°, △R/R values of the sensor decrease from -0.35% to 1.8%, as shown in Figure 3c. Resistance of both sensors changes linearly with bending angles (Figure 3d) and are in good agreement with theoretical predicted values. However, And 5 mmthick sensor exhibits higher sensitivity than 3 mm thick sensor, at larger bending angles. During tensile bending (Figure 4b and Figure 4c), △R/R values of the sensor increases from 0.15% to 0.35% with the increase of bending angles from 1°~10° and △R/R values increases from 0.35% to 2.5% in the angle range of 10°~70°. Resistance of both sensors changes linearly with bending angles (Figure 4d) and are in good agreement with theoretical predicted values. Compared to the theoretical values, the experimental values are also in good agreement with them. But And 5 mm thick sensor exhibits higher sensitivity than 3 mm thick sensor and this trend is similar to compressive bending. This trend is attributed to the asymmetric structure and thickness of sensor. Surface of 5 mm thick sensor deviates greatly, due to its large thickness, which results in large change in resistance. This result indicates that the sensors exhibits high sensitivity and different bending angles can be identified clearly. Above mentioned resistance change response confirms the high sensitivity and outstanding capability of sensor to detect various bending angles. Performance of current sensor and previous reported sensors is compared in Table 1, which shows that current sensor exhibits outstanding performance in terms of high angle resolution and can be successfully applied in practical applications.

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Figure 3 (a) A schematic illustration of angle, induced due to compressive bending. (b) Relative change in resistance of 3 mm thick sensor caused due to variation in compressive bending angles ranging from 1°~10°. (c) Relative change in resistance of 3 mm thick sensor caused due to variation in compressive bending angles ranging from 10°~70°. (d) Experimental and theoretically predicted △R/R values of 3 mm and 5 mm thick sensors at various bending angles during compressive bending.

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Figure 4 (a) A schematic illustration of angle, induced due to tensile bending. (b) Relative change in resistance of 3 mm thick sensor caused due to variation in tensile bending angles ranging from 1°~10°. (c) Relative change in resistance of 3 mm thick sensor caused due to variation in tensile bending angles ranging from 10°~70°. (d) Experimental and theoretically predicted △R/R values of 3 mm and 5 mm thick sensors at various bending angles during tensile bending.

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Table 1. Comparison of Performance Parameters for Different Angle Sensors Active materials

Bidirectional sensing capability

Ref.

Unidirectional bending

Minimum resolution angle 30°

Rubrene single crystals SWCNT/hydrogel


30°

24

PVDF/ZnO nanofibers


10°

26

Graphene‐Based Fiber


2°

32

(PVA–PEDOT:PSS)/ PEDOT:PSS Graphite

Bidirectional bending

30°

28


20°

3

Acrylic sheet /ﬂuorinated ethylene propylene thin film Liquid metal


3.8°

27


1°

This work

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Working Mechanism Asymmetric structure is formed by placing the sensing element on top surface of sensor, as shown in Figure 5a. The detection principle of this asymmetric structure-based sensor is as follows: when the sensor is in original state (no bending condition), the length of top surface is equal to the length of central axis of sensor (AA' = NN') as shown in Figure 5b. When the sensor is subjected to compressive bending, the length of top surface becomes less than the central axis length (AA' < NN') as shown in Figure 5c, which induces the decrease in sensing element length and resistance change in negative direction. While in the case of tensile bending, the length of top surface become greater than the central axis length (AA' > NN') as shown in Figure 5c, due to which sensing element length increases and the resistance changes once again but now in positive direction. Change in resistance of sensing element subjected to angular deformation is given by relation: t ∆θ

∆R = ρ2 S

1

Mathematical proof of Equation (1) is given in supplementary information.

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Figure 5 (a) Asymmetric structure-based strain sensor for the detection of human joint angle and direction. (b) Cross sectional view of strain sensor in which sensor is in normal condition. (c) Working mechanism of strain sensor during compressive and tensile bending. Application To demonstrate the practical application of the sensor in wearable devices, 5 mm thick sensor is attached to various human joints with medical tape. Various subtle human joint motions are successfully monitored with the sensor. Figure 6 shows the sensor’s ability to detect motion of human joints. Sensor attached to forefinger successfully detects its movement, as shown in Figure 6a. At the initial state, the curvature of the finger is approximately 0 º , the resistance keeps stable. The resistance increases slowly with the gradual bending of the finger and is maintained at a steady state while the finger remained at fixed position. Importantly, the resistance comes to the original state when the finger is unbent quickly. This is attributed to the channel with strong resilience under the strain process. As the curvature of the finger increases (bending the finger from 0 to 90º), the resistance of sensor also increases accordingly. Impressively, when the forefinger holds a cup and gently changes a subtle angle of 1°, as seen in response curves, our sensor can distinguish the subtle human motion of 1º (Figure 6a). At the same time, the sensor shows excellent stability and resistance signal of sensor responds effectively to every curvature of bending process. Sensor’s ability of sensing complex human/joint motions (bending upwards and downwards) is evaluated by attaching it to wrist joint (Figure 6b). Sensor reliably detects the rapid movement of the wrist. At the initial state (step 1), when wrist is flat (wrist curvature ≈ 0º), the resistance keeps stable. When wrist is bent in upward direction, sensor comes to compressive state induces resistance change in negative direction (step 2). Afterwards, the wrist comes back to its normal position (0º), the sensor’s resistance also returns to its original position (step 3). When wrist is bent down, sensor comes to tensile state due to which resistance increases in positive direction (step 4). Sensor resistance changes successfully with repeated upward and downward bending of the wrist at different amplitudes, which demonstrates the great potential application of current sensor in man-machine artificial robot motion control.

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When sensor is attached to neck, it responded effectively to interval motions of neck, as shown in Figure 6c. When neck is slightly bent, sensor is in tensile bending condition and it shows positive △R/R values (step A). △R/R values increases further (step B) with the additional bending down of neck. When the neck’s up and down bending motion is repeated (step C), the strain sensor shows a repeatable and distinctive electrical signal pattern instantaneously for this complicated neck joint motion. Finally, △R/R values change instantaneously, when the neck moves back to the down bending state (step D). These results demonstrate that our strain sensor can detect skin stretch, caused by slight motion of human joint. The extraordinary performance of sensor demonstrates that current sensor can be employed as the most important components to monitor joints movements in the applications like human motion and health monitoring, human– machine interface, soft robots.

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Figure 6 Real time monitoring of various human joint motions (a) forefinger (b) wrist joint (c) neck joint motion monitoring of up and down motions (from A to D). 13



CONCLUSIONS Asymmetric structure based, flexible resistive angle sensor is prepared by SLA based 3D printing process. When sensor is subjected to tensile or compressive bending, liquid metal-based sensing element placed at top, faces maximum deflection and length change, as compared to remaining geometry of sensor. This enables sensor to identify the joint angle and distinguish the direction of joint movement (up and down) simultaneously and effectively. When performance of 5 mm and 3 mm thick sensor is compared, 5mm thick sensor has shown 2.8% and 1% additional increase in resistance during tensile and compressive bending respectively. The sensor exhibits good mechanical stability and minimum resolution angle of 1°, ranging from 70° to -70°. It paves a new way for continuous joint motion monitoring of human or man-machine artificial robot. Current sensor can be commercialized in future because of its outstanding performance, low cost raw material usage, simple 3D printing fabrication technique, and large-scale production capability in less time.

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MATERIALS AND METHODS Materials and Instruments Commercially available resin (RS-F2-FLGR-02), having 80A (Durometer hardness), has been 3D printed by using Form2 type 3D printer (Form Labs, Somerville, Massachusetts) equipped with SLA technology. Resin is cured optically with a laser of spot size 140 µm and power of 250 mW. 50 µm layer thickness is adjusted during printing. Galinstan (LM alloy of Ga) has been used as a resistive sensing material, which consists of 67.3 wt% Ga, 19.2 wt% In and 13.5 wt% Sn, with a freezing point of -1.4℃ and excellent conductivity of 3.5×10-7 Ω/m 25. Fabrication of the sensor A simple technique has been developed to fabricate the strain sensor. The technique converts liquid resin into solids, point by point through selective curing with a UV laser source. Initially, the first layer with the length of 60 mm and thickness of 4 mm and 2 mm is completely cured. Then another layer of 100 µm, which resembles the strain gauge, it is selectively cured. Lastly, 1 mm thick final layer is completely cured to form strain gauge shaped micro-channel. Two types of sensors are manufactured for comparison, first is of 3 mm thickness and second is of 5 mm thickness. The LM is directly injected into the micro-channel by using syringe. Afterwards copper electrodes have been inserted at both ends of the LM micro-channel to eliminate contact resistance and to make sensor capable of recording resistance variation when deformed. Measurement of the Strain−Resistance Response 3-point bend tests (at 25 ℃) are performed on both sensors by using INSTRON machine (Instron 5942; Instron Corp., USA). The corresponding force has been measured by the machine load cell, whereas mechanical strain (ε) is calculated as the machine crosshead displacement normalized by the gage length of the test specimen. Electro-mechanical tests have been performed at room temperature by using a two-probe configuration. The electrical current has been provided by a source-measure unit (Keithley 237, Keithley Instruments, USA), while the voltage is measured by an electrometer (Keithley 6517A, Keithley Instruments, USA).

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ASSOCIATED CONTENT Supporting Information Mathematical proof of equation AUTHOR INFORMATION Corresponding Author Prof. Dr. Y.W. Liu: [email protected], Prof. Dr. R.-W. Li: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research is supported by China International Cooperation Project (2016YFE0126700), National Natural Foundation of China (61704177, 51525103, 11474295, 61774161 and 51971233), National Key Technologies R&D Program of China (2016YFA0201102), Public Welfare Technology Applied Research Projects of Zhejiang Province (LGG19F010006), Ningbo major science and technology projects (2017B10018), and Ningbo Science and Technology Innovation Team (2015B11001). ABBREVIATIONS Acronyms LM

Liquid Metal

PDMS

Polydimethylsiloxane

3D-Printed

Three-Dimensional Printing

GF Galinstan

Gauge Factor Alloy of Gallium (67.3 wt.% Ga, 19.2 wt.% In and 13.6 wt.% Sn)

UV

Ultraviolet

SLA

Stereolithography

AA′

Length of line marked at top surface of sensor

NN′

Length of central axis which remain same during compression and tension

List of Symbols R

Resistivity

△R/R Relative change of resistivity 16


θ

Bending angle

mW

Microwatt

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△L

The change of length

N

Newton

Cu

Copper

mm

Millimeter

ε

Mechanical strain

μm

Micrometer

ρ

Density

min

Minute

Pa

Pascal

s

Cross-sectional area

t

Thickness of sensor

∆θ

Change of angle

Ga

Gallium

Ω

Ohm

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Asymmetric Structure Based Flexible Strain Sensor for Simultaneous

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