Highly stretchable optical sensors for pressure, strain, and curvature measurement

Recent advances in soft sensors using microfluidic liquid conductors enabled sensing of large deformation of soft structures. However, the use of liquids as conductive media carries a risk of leakage in many cases. Furthermore, it could be harmful when exposed to the human body in certain applications. To address these issues, a different sensing mechanism was proposed: highly stretchable optical sensors that could detect multiple modes of deformation. The method of operation involves a simple waveguide and its housing which are both made of silicone elastomer. The soft waveguide is coated with a thin gold reflective layer to encapsulate light propagating internally, with an light-emitting diode (LED) and a photodiode embedded at each end. When the sensor is stretched, compressed, or bent, micro-cracks within the reflective layer form and allow part of the light to escape, resulting in optical power losses in the light transmission. In this paper, we describe the design and fabrication of the proposed soft sensors. A prototype was created and characterized for pressure, strain, and curvature up to 350 kPa, 90%, and 0.12 mm^-1, respectively, showing promising results of reasonable repeatability and linearity in certain ranges.