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Wearable ground reaction force (GRF) measurement systems make it possible to measure the GRF in any environment, unlike a commercial force plate. When performing kinetic analysis with the GRF, measurement of multiaxial GRF is important for evaluating forward and lateral motion during natural gait. In this paper, we propose a bendable GRF measurement system that can measure biaxial (vertical and anterior-posterior) GRF without interrupting the natural gait. Eight custom small biaxial force sensors based on an optical sensing mechanism were installed in the proposed system. The interference between two axes on the custom sensor was minimized by the independent application of a cantilever structure for the two axes, and the hysteresis and repeatability of the custom sensor were investigated. After developing the system by the installation of force sensors, we found that the degree of flexibility of the developed system was comparable to that of regular shoes by investigating the forefoot bending stiffness. Finally, we compared vertical GRF (vGRF) and anterior-posterior GRF (apGRF) measured from the developed system and force plate at the same time when the six subjects walked, ran, and jumped on the force plate to evaluate the performance of the GRF measurement system.

Recently, piezo-resistive nanocomposites have emerged as an important smart material for realizing less obtrusive and more comfortable stretch sensing applications. To manufacture cost-effective and skin-mountable stretch sensor, dispenser printing is advantageous method because piezo-resistive nanocomposites can be directly printed on a woven elastic fabric in various patterns. However, both electrical and mechanical properties of the nanocomposites need to be modulated to achieve favorable sensing performance as well as strong adhesion between the nanocomposite and the fabric to sustain large strains. Moreover, inherent hysteretic behavior of the soft nanocomposite should be compensated to obtain consistent stretch sensing. This paper presents silicone rubber mixed with long multi-walled carbon nanotubes (Long-MWCNTs) composites as a piezo-resistive transducing material for dispenser printing. High aspect ratio of the Long-MWCNTs resulted in low viscosity of a liquid state nanocomposite and high electrical conductivity. Due to the low viscosity, the liquid state nanocomposite could permeate into gaps of the woven elastic fabrics and ensured strong bonding force in large strains up to 35%. In addition, a modified Prandtl-Ishilinskii (MPI) model was adopted to compensate for piezo-resistive hysteresis of the nanocomposite. For validation, the skin-mountable sensor was applied to estimate rotation angle of a wrist. The sensor system estimated the rotation angle of the wrist with an estimation error of 1.93 degrees within 65 degrees range (2.9%) for the step increment and decrement test, and 7.15 degrees within 75 degrees range (9.5%) for the arbitrary movement test. Thus, the experimental results show that the dispenser printing method incorporated with hysteresis compensation can provide a guideline to implement skin-mountable smart fabrics for stretch sensing using various nanocomposites

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