A Highly Sensitive Wearable Fiber-Optic Sensor for Pressure and Shear Force Measurement

Over the years, extensive research has been conducted on wearable devices, smart clothes, and functional compression garments where human-device interaction is critical. Hence, monitoring of the tactile interaction between the skin and the device is necessary to prevent poor device functioning due to unwanted interactions like slipping, and over-fitting[1].<br/>Several research groups have proposed various interface pressure measuring technology using piezoelectric, capacitive, pneumatic sensors, but they present challenges of low accuracy at low pressure levels (&lt;10kPa), signal drift, and measurement noise[2-4]. Optical sensors have the strong advantage of being immune to electro-magnetic interference (EMI) and being biocompatible. Fiber-Optic sensor based on light intensity modulation can be made simple, cost-effective, and wearable with only light source and detector, unlike Fiber Bragg Grating sensors (FBG) which require bulky spectrometer for signal analysis[5].<br/>In this research, we propose the Fiber-Optic sensor which is soft, wearable, that can sense low interface pressure and shear force with high sensitivity. Upon bending of the optical fiber, part of the light which was initially contained within the core, enters the clad and propagates as cladding-mode. Based on the light scattering of cladding-mode under the soft polymer contact, the sensor can be made robust towards measurement noises without the loss of sensitivity. The sensor consists of coiled polymer optical fiber (POF) which is the sensing part, and the stretched polymer diaphragm which deforms under the external force. Under the slight contact of the polymer diaphragm at the exterior part of the POF coil, significant light loss is observed due to cladding-mode being refracted into the polymer instead of propagating under the total internal reflection. Unlike the previous soft wearable tactile sensors which involves bulk polymer deformation, the proposed sensor involves polymer diaphragm deformation and can measure low pressure up to 0.498kPa with high sensitivity(6.38%/kPa). The fabricated sensor was light, thin(&lt;2mm), and small(D&lt;20mm) making it a soft wearable interfacial sensor.<br/>With the usage of three coiled POFs, placed symmetrically under the deforming polymer diaphragm, sensing of multi-directional force is possible. The coiled sensors respond similarly under the normal pressure, whereas under the shear force, the coils exhibit different responses.<br/>The proposed sensor allows real-time monitoring of tactile interaction between the skin and the compressive garment or external device which is critical for the compressive devices to effectively interact with human skin.<br/>[1] Y. Xiong and X. Tao, "Compression Garments for Medical Therapy and Sports", <i>Polymers</i>, vol. 10, no. 6, p. 663, 2018. Available: 10.3390/polym10060663<br/>[2] M. Ferguson-Pell, S. Hagisawa and D. Bain, "Evaluation of a sensor for low interface pressure applications", Medical Engineering & Physics, vol. 22, no. 9, pp. 657-663, 2000. Available: 10.1016/s1350-4533(00)00080-1.<br/>[3] G. Nandasiri, A. Shahidi and T. Dias, "Study of Three Interface Pressure Measurement Systems Used in the Treatment of Venous Disease", Sensors, vol. 20, no. 20, p. 5777, 2020. Available: 10.3390/s20205777.<br/>[4] S. Lao, H. Edher, U. Saini, J. Sixt and A. Salehian, "A Novel Capacitance-Based In-Situ Pressure Sensor for Wearable Compression Garments", Micromachines, vol. 10, no. 11, p. 743, 2019. Available: 10.3390/mi10110743.<br/>[5] K. Hill and G. Meltz, "Fiber Bragg grating technology fundamentals and overview", <i>Journal of Lightwave Technology</i>, vol. 15, no. 8, pp. 1263-1276, 1997. Available: 10.1109/50.618320