3D Flexible, Conductive and Wearable Fabric Strain Sensors

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International Journal of Material and Mechanical Engineering (IJMME), Volume 4, 2015 doi: 10.14355/ijmme.2015.04.002

3D Flexible, Conductive and Wearable Fabric Strain Sensors Jianfeng Li1, Bingang Xu*2, Xiaoming Tao3 and Hong Hu4 Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China 1, 2, 3, 4

lijianfeng066@gmail.com; *2tcxubg@polyu.edu.hk; 3xiao-ming.tao@polyu.edu.hk; 4tchuhong@polyu.edu.hk

1

Abstract 3D fabrics are flexible and wearable substrates for making functional apparels and relevant medical equipments with many distinguished qualities. This paper proposes an easily-fabricated method for 3D fabric pressure sensor based on carbon black coated 3D textile fabrics. The as-made sensor is flexible, conductive and stable even after many times of cyclic compressions. It can measure external pressure ranging from 0 to 33 kPa. During the compression process of the sensor, its electrical resistance would decrease due to conductive structure deformation. The sensor has the potential to transfer pressure information into the change of electrical resistance of the sensor, which is more easily acquirable and readable. This property of the sensor can find promising applications in many fields.

Keywords 3D Fabric; Pressure Sensor; Flexible; Conductive; Wearable

Introduction Protective apparels or related medical equipments are widely used for protecting human bodies from impact injuries in accidents of various occasions or assisting recovery process of injured patients. Soft foams, bulky fibers and 3D fabrics are widely used as substrate for making protective apparels or related medical equipments, such as protective garment, kneecap, helmet, protective support and so on. These materials are used because of their comfortable, energy-absorbing and easily-integrated properties.

Among all the above-mentioned materials, 3D fabrics are widely used and showing great promise in different protective applications [1]. Warp-knitted spacer fabric is a typical 3D fabric, which has already been used as cushioning materials for their good compression property, moisture transmission capability and air permeability [2]. This kind of fabric consists of two separated outer fabric layers with spacer fibers inserted between them, which can be made on a double-needle bar Raschel machine [2]. Due to all the advantages of warp-knitted spacer fabrics mentioned above, this specific fabric is an ideal material for protective apparels or related medical equipments.

During the interaction process between human body and surroundings, real time, in-situ measurement and analysis of pressure are very important to facilitate the design and fabrication of related apparels or devices. Researchers had developed several kinds of body-worn pressure sensor to classify sitting postures [3], measure feet pressure [4], or detect specific body movement [5]. But they either need extra electrodes to transduce pressure signal to readable signal, or fabricate delicate structures to assist the sensing process.

In this paper, an easily-fabricated method of pressure sensor based on warp-knitted 3D spacer fabrics is reported.

This sensor is flexible, durable and stable even after many times of cyclic compressions. These advantages came from the intrinsic characteristics of the warp-knitted 3D fabrics and a proposed multi-layer circulating carbon black coating process, which endows this structure with excellent electrical and mechanical properties for strain sensing.

Experimental Material The 3D spacer fabrics were made by a warp knitting machine using Dacron and they were tailored into specimens

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of 2x2 cm for mechanical and electrical measurements under cyclic compression. The thickness of the spacer is around 6.5 mm and the diameter of the spacer fibers is 0.2 mm.

Silicone elastomer (SE), ElastosilⓇ LR 6200 A, B agent were bought from Wacker-Chemie GmbH (Germany). 5g component A was mixed thoroughly with component B of the same amount, and then 20 ml cyclohexane were added into the mixture and stirred vigorously. The diluted silicone elastomer mixture was kept at 50℃ for 1h to make solute evenly dispersed between solvent. 600mg carbon black was dispersed in 80ml cyclohexane by 1h ultrasonication at room temperature. These are two basic solutions for our sensor fabrication process.

Measurement Instron 5944 series mechanical testing machine (Instron Corp., USA) was used for cyclic compression, while mechanical and electrical responses were measured and recorded throughout the whole compression experiment.

Compression speed was 8 mm/ min, while compression displacement was set to be 4mm (the height of 3D fabric sensor is 6.5 mm and compression ratio is 61.5%). Topward Dual-Tracking DC 6303A (Topward Electric Instruments Co., Ltd., Taiwan) was applied as the power supply and Keithley 2010 digital multimeter (Keithley Instruments, Inc., USA) was utilized for electrical resistance measurement.

FIG. 1 (A) FABRICATION PROCESS OF WARP-KNITTED 3D FABRIC PRESSURE SENSOR, SIDE VIEW AND FRONT VIEW OF (B, C) CLEANSED ORIGINAL 3D FABRICS, (D, E) CARBON BLACK COATED 3D FABRICS

Fabrication Process FIG. 1(A) shows the fabrication process of 3D carbon black coated fabric pressure sensor. Warp-knitted spacer fabrics were washed by water and ethanol several times to remove all the dirt (as shown in FIG.1. B, C). Cleansed fabrics were dried at 80℃ for 1h, ready for circulating dipping-coating process. Fabrics were first dipped into SE solution for 15s and then hanged in air to drain extra solution. SE coated fabrics were cured at 80℃ for 10 min to evaporate solvent and pre-solidify the SE coating. The slightly cured fabrics were dipped into carbon black solution for 2s, then dried at 80℃ for 5 min. After 4 cycles of this process, surface of 3D fabric was fully covered with carbon black, including spacer fibers in the middle of the structure (as shown in FIG. 1. D, E). Finally, the 4-time carbon black coated fabrics were final-solidified at 80℃ for 2h. By using this fabrication method, fabric sensors with various sizes can be manufactured. In this study, only 2x2 cm fabric specimens were selected for electrical and mechanical testing under cyclic compression process.

Results and Discussions Influence of Compression Displacement on Electromechanical Responses In order to explore optimal working condition, a series of total compression displacement were adopted while corresponding resistance responses were recorded. When total compression displacements of 2mm and 3mm (as shown in FIG. 2 A, B) were tested, the resistance was not proportional to compression displacement. But when compression displacement of 4mm was chosen, the resistance of our sensor was almost proportional to displacement. Therefore 4mm of compression displacement will used for further testing. The relationship between electrical resistance and displacement can be formulated as Y=0.6091X+6.884 with a high magnitude of coefficient of

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determination (r2) of 0.9894 (as shown in FIG. 3). The electrical resistance ranges from 4.47 to 6.85 KΩ, corresponding to the compression displacement (4-0 mm) of the 3D fabric sensor.

FIG. 2 RESISTANCE VERSUS DIFFERENT COMPRESSION DISPLACEMENT CURVES OF FABRICS SENSOR: COMPRESSION DISPLACEMENT IS (A) 2 MM; (B) 3 MM; (C) 4 MM.

FIG. 3 FITTED CURVE WHEN COMPRESSION DISPLACEMENT WAS SET TO BE 4 MM.

FIG. 4 (A) MECHANICAL RESPONSES OF THE 3D FABRIC PRESSURE SENSOR, (B) RESISTANCE AGAINST TIME FOR 60 CYCLES

Mechanical and Electrical Responses of the Sensor under Cyclic Compression The mechanical responses of the 3D fabric pressure sensor under cyclic compression are periodical and constant, as shown in FIG. 4(A). This demonstrates the interior structure of 3D fabric sensor is suitable for cyclic compression and recovery process, which is very important for commercialized products. The sensor can measure the external pressure with a magnitude of 0-33 kPa. FIG. 4(B) illustrates electrical resistance of the sensor against time for 60 cycles in cyclic compression test. It can be noted the resistance is generally distributed within a range of 4.25-7.05 kΩ. Although we can find some variations of the resistance over time, there is not obvious performance decay with these cycles. Stability of the sensor after the long time compression treatment is still acceptable, which is an important property for wearable sensors. Future works will be conducted to improve the 3D fabric sensor setup for better electrical performance and to calibrate the sensor for various applications.

Conclusions In this paper, an easily-fabricated and flexible pressure sensor was developed based on carbon black coated 12

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warp-knitted 3D fabrics. The sensor is flexible, stable and still workable after long time of cyclic compressions. By using this sensor, pressure signals can be easily transduced into resistance signals, which is more easily acquirable and readable. Based on relevant tests, this sensor shows promise to be applicable for compression measurement.

Works for improvement of sensors stability and performance are still going on and will be reported in future.

ACKNOWLEDGEMENT

The authors acknowlege the Hong Kong Polytechnic University for funding supports of this work (project no: G-UC35).

REFERENCES

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J. Meyer, B. Arnrich, J. Schumm and G. Troster, Design and Modeling of a Textile Pressure Sensor for Sitting Posture Classification. IEEE Sensors Journal, 10(8) (2010) 1391-1398.

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L. Shu, T. Hua, Y. Wang, Q. Li, D. Feng, and X. Tao, In-Shoe Plantar Pressure Measurement and Analysis System Based on Fabric Pressure Sensing Array. IEEE Transactions on Information Technology in Biomedicine, 14(3) (2010) 767-775.

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L.E. Dunne, S. Brady, B. Smyth, and D. Diamond, Initial development and testing of a novel foam-based pressure sensor for wearable sensing. J Neuroeng Rehabil, 2(1) (2005): p. 4.

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