A Novel Stretchable and Flexible Sensor Using Graphite-Added Optical Waveguide for Human Motion Detection

According to the Bill-Lambert law, this article reports a flexible optical waveguide sensor based on the polydimethylsiloxane (PDMS) doped with graphite. We quantitatively study the attenuation spectra induced by adding graphite with different particle sizes and amounts into the optical waveguide of the sensor. Adding the graphite’s particle size with $0.5\mu $ m and its concentration of $5\times 10^{-{3}}$ wt% shows that the relationship between the optical attenuation and the tensile strain in the range of 90% is approximately linear. Our further experiment shows that the radial shrinkage of a waveguide is more than 16.8% when the axial tensile strain is larger than 90%, this increases the density of graphite particles on the optical waveguide’s cross-section, resulting in a non-negligible effect on light transmission, which makes the relationship between light attenuation and the tensile strain nonlinear. Our proposed waveguide strain sensor is applied to the motion detection of human joints and muscles. We obtained a series of stable, reliable and durable response no matter the static and dynamic conditions. The sensitivity of the proposed flexible strain sensor is improved by a factor of 3.9 and 1.2 than that of sensors made of the pure PDMS and the polymethylmethacrylate (PMMA), respectively. Meanwhile, it has a high signal-to-noise ratio (SNR) and is insensitive to installation angle errors. There will be a good application prospect in wearable devices that can adapt to the complex curved surface, due to the proposed waveguide strain sensor’s stretchable, sensitive and low-cost characteristics.