Reel-to-reel fabrication of strain sensing threads and realization of smart insole

Integrating sensors into our daily wear such as shoes or clothing will revolutionize healthcare and fitness. In this paper, we demonstrate scalable reel-to-reel fabrication of strain sensing threads, which are then used to realize smart shoe insole for gait monitoring. Fabrication includes coating of bare threads with conductive ink using a 3D printed cartridge that enables simultaneous stretching and dip coating, followed by drying in a heated chamber. Notable pre-stretching of threads before dip coating is proposed as an essential step to ensure conductivity and strain measurement under extreme stretching conditions. In this paper, the strain sensitive threads are made from coating of carbon ink on Gütermann elastic threads. They are shown to have gauge factor of 12. To realize a smart shoe insole, threads were arranged in a two dimensional grid of 16 × 8 threads cast inside a highly elastomeric EcoFlex sole. Custom circuitry enables wireless readout from sensor array to any Bluetooth enabled device. We propose advanced calibration and normalization algorithm to correct for mismatch in sensitivity between threads in an array. Further signal processing of the real-time strain information from the 2D grid enables generation of precise 16 × 8 pixel 2D pressure foot map. These foot maps can be used for gait monitoring and other physical activity. Detailed characterization and validation on individual threads and of the smart insole have been provided.