Multifunctional Flexible and Wearable Sensing System Based on Optical Microber Interferometry

21 Optical segments based flexible systems are the key for the development of futuristic advanced 22 wearable devices for health monitoring, robotics, and ultraprecision positioning in industrial 23 applications. Here, we have demonstrated an processed optical microfiber based multifunctional 24 sensing system, which overcomes the various limitations of most widely reported electronics and 25 material-based flexible devices. By optimizing the position of the post processed microfiber configuration in optimized Polydimethylsiloxane (PDMS) thickness and controlling the interference between the fundamental mode and higher order modes of microfiber to form and 28 tunable interference pattern, we are able to make an efficient, simple, flexible and economical 29 optical wearable vector bending system with a sensitivity as high as 1.01nm/degree. In addition, 30 this skinmountable sensing sensor shows a remarkable and ultrasensitivity of -3.07 nm/ o C. This 31 ultrahigh sensitivity, mechanical robustness, with the excellent flexible and biocompatible nature 32 also makes this sensing system a dominant candidate for wearable medical devices for elder-care 33 facilities, physioclogical monitoring, athletic training, and rehabilitation program.


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Flexible devices have long drawn the attention of researchers for health monitoring, 1,2 36 environmental detection, 3-5 micro-force recognition, 6 and precise industrial movements 7,8 . In the 37 form of skin-like system, they promote the development of robotics, 9,10 smart clothing 11 and 38 numerous physiological monitoring devices. 12 The affectability and sensitivity of such devices are 39 determined by its detecting mechanism and discovery components. Most previous works on 40 wearable sensor have focused on two measure detecting mechanisms: a) materials response to 41 target changes and b) electrical response such as resistive, 13 capacitive, 14 triboelectric 15 and offer exceptional benefits like distant checking capacity, high affectability, non-electromagnetic 50 disturbances, real-time monitoring, and intrinsic electrical security apart from features like 51 reconfigurablity and scaleability. 52 One such optical segment is optical microfibers which possess strong evanescent fields [24][25][26][27] 53 and are sensitive to microforce or relocation. 28,29 Further, when light propogate through the optical 54 microfiber, it excites the higher order modes which interfers with the fundamental modes of single 55 mode fiber to form interference pattern whose period can be tailored depending upon requirement 56 and the interference pattern shifts when subjected to external perturbations. 57 On the other hand polydimethylsiloxane (PDMS) is an excellent material to embeed optical 58 microfiber due to its biocompatibility and robust mechanical stability 30,31 apart from providing 59 mechanical stability to the microfiber which otherwise is fragile and difficult to use. PDMS 60 incorporated optical microfiber also exhibit low loss, high-temperature steadiness with high 61 flexibility. Such an encapsulation also solves the demerits of conventional thermometers such as 62 glass breaking, mercury poisoning, long-term affectability and biocompatibility. These devices 63 can also be directly affixed to the human body to detect small bending of different body joints like 64 the neck, knee, elbow and arms to name few where most conventional goniometers based system Here, we demonstrate a post-processed microfiber based interferometric system which uses 73 double microfiber configuration as it provides more stable optical spectrum with the 74 reconfigurable features, enhanced performance, an extra degree of freedom to tune the notch 75 wavelength and higher mechanical strength and stability compared to the single microfiber. 34,35

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The reconfigurable interference region of the microfiber is embedded in such a way that it can 77 exhibits a U-shape and the two identically tapered regions remain parallel and termed as U-shaped 78 cascaded microfiber interferometer (UCMI). This UCMI probe is suitably embedded in an 79 optimized thin layer of PDMS for monitoring the body temperature and human body joints 80 movements along with its directions. Owing to these advantages, it can be utilized for several

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Concept and Working principle 86 For the proposed wearable system, the fabricated UCMI with optimized waist diameter (13 m), 87 waist length (1 cm) and interference length (6 cm) is placed along the plane of PDMS film as 88 shown in Fig 1(a). Photographs of the real probe which is guiding the red laser light are also shown 89 in Fig 1(b). Here, the shape of the interference length is chosen as U-type to allow simultaneous 90 deformation of the two microfiber waist regions as the interference length apart from the fact that 91 U-type probe are associated with higher evanescent field which will provide higher performance 92 to the sensing system. The optical microscope image of waist diameter is shown in Fig 1(c). The

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UCMI is embedded in an optimized thin film of PDMS having moisture erosion capability due to 97 its hydrophobic nature 36 for underwater applications as wearable and patch type devices. Fig 1(d)

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shows the optical transmission spectrum for the bare UCMI and PDMS embedded UCMI probe.

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Due to the PDMS embedding, the effective index difference increases due to the mode-coupling 100 effect which not only shifts the interference pattern to higher wavelength but offers marginally 101 broad free spectral range (FSR). However, the FSR can be controlled by controlling the waist diameter of UCMI depending upon user's requirement which in turns allows to tailor the 103 performance together with field distribution around the UCMI.

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To study the electric field mode propagation of the PDMS embedded UCMI, two micro-105 regions M1 and M2 each having taper lengths as 1 cm, waist diameter of 13 μm and waist-length 106 of 1 cm, similar to the experimental probe is considereded and shown in Fig 2(a). As can be clearly 107 seen that there is an enhanced electric field near the M1 and M2 of the PDMS embedded probe 108 which is responsible for enhanced performance of the proposed system as discussed and shown Where, 1 and 2 represent the intensity of fundamental mode (LP01) and higher order mode (LP0m). 120 is the phase difference between these modes which can be evaluated from, Where, 1 and 2 are the effective refractive indices of two higher order modes. and 123 represent the interference length and the wavelength of the input broadband source respectively.   is ready for sensing purposes after peeling it from the glass slide.