Stretchable\nand Highly Sensitive Optical Strain Sensors for Human-Activity Monitoring\nand Healthcare

Abstract

Flexible\nand stretchable strain sensors are essential to developing smart wearable\ndevices for monitoring human activities. Such sensors have been extensively\nexploited with various conductive materials and structures, which,\nhowever, are normally in need of complex manufacturing processes and\nconfronted with the challenge to achieve both large stretchability\nand high sensitivity. Here, we report a simple and low-cost optical\nstrategy for the design of stretchable strain sensors which are capable\nof measuring large strains of 100% with a low detection limit (±0.09%),\na fast responsivity (<12 ms), and high reproducibility (over 6000\ncycles). The optical strain sensor (OS²) is fabricated\nby assembling plasmonic gold nanoparticles (GNPs) in stretchable elastomer-based\noptical fibers, where a core/cladding structure with step-index configuration\nis adopted for light confinement. The stretchable, GNP-incorporated\noptical fiber shows strong localized surface plasmon resonance effects\nthat enable sensitive and reversible detection of strain deformations\nwith high linearity and negligible hysteresis. The unique mechanical\nand sensing properties of the OS² enable its assembling\ninto clothing or mounting on skin surfaces for monitoring various\nhuman activities from physiological signals as subtle as wrist pulses\nto large motions of joint bending and hand gestures. We further apply\nthe OS² for quantitative analysis of motor disorders such\nas Parkinson’s disease and demonstrate its compatibility in\nstrong electromagnetic interference environments during functional\nmagnetic resonance imaging, showing great promises for diagnostics\nand assessments of motor neuron diseases in clinics.