Owing to the superior mechanical property and excellent sensitivity, this proposed microfiber coupler sensor would have tremendous potential applications in wearable devices for monitoring ultraweak physiological signals, slight human motions and subtle environmental perturbations. By using PDMS as an encapsulation material, this microfiber coupler strain sensor had been developed to be a biocompatible, flexible and durable device, which could be easily and firmly attached to the human skin with comfortable wearability. As shown in Figure 3 a, this flexible microfiber coupler sensor could be directly attached to the skin of human face, neck, wrist, finger, and ankle etc., for real-timely detecting the breath, pulse, gesture, and speech. Breathing is one of the prime functions fulfilled by human, and it can have considerable effects on the morphology and on the craniofacial and cervical functions, and is also closely related to some respiratory diseases. This flexible microfiber coupler strain sensor was mounted on facial masks to monitor strain induced by respiratory motions, where inhalation and exhalation would bring about air pressure changes inside the mask and deform the mask. Three different respiration modes were detected and discriminated by virtues of the distinguishable response patterns as shown in Figure 3b, where 0.2 ΔCR and 38 times/min for deep breath, 0.07 ΔCR and 23 times/min for normal breath, 0.02 ΔCR and 17 times/min for shallow breath, respectively. By detecting the mask deformation, the respiration frequency, breathing depth and various breathing styles could be sensed and detected, which would be helpful to discover and diagnose some respiratory diseases.
Arterial pulse is a significant physiological signal for the clinical diagnosis of cardiovascular diseases. The arterial pulse is evaluated for the contour of the pulse wave and its volume, rate, and rhythm, the intensity of arterial pulse signal is often too weak to be palpated or detected, especially at the fingertip and ankle sites. This flexible microfiber coupler strain sensor was attached to different body’s sites, such as the neck, wrist, fingertip and ankle. By virtues of high sensitivity and low detection limit of this flexible strain sensor, the pulse waveforms at all body’s sites were precisely detected and recorded as shown in Figure 3c. The pulse waveform details could be captured and recovered without distortion, for example, the pulse signals at neck, wrist, and finger exhibited three peak characteristics, and the ankle only had two peaks, which was exact in agreement with the pulse signal characteristics at body’s different sites\cite{Fang_2021}.
The weak motions of human body could be effectively monitored by this flexible microfiber coupler sensor, which would have broad application prospects in human-machine interaction and phonation rehabilitation training. Firstly, this flexible microfiber coupler sensor was attached on the wrist site for the gesture recognition demonstration, where the bending of fingers would drive the wrist to produce weak movements. As shown in Figure 3d, bending and straightening of different fingers could be clearly recognized and distinguished. Then, this flexible microfiber coupler sensor was attached on the neck to monitor the tiny epidermis and muscle movements during speech for phonation recognition. As shown in Figure 3e. this sensor captured distinguishable and repeatable signal patterns when the volunteer spoke some alphabet letters, S-C-U-T. It could be found that the three enlarged detail view of letter “T” waveforms were nearly the same, which indicated that this flexible sensor would be more powerful for in situ real-time monitoring based on further pattern recognition technique.