Development and Characterization of a 3D Printed Soft Sensor to Identify Physiological Joint Forces

Evaluating the influence of human-centered devices, such as exoskeletons on internal body forces, is important in order to make safe and effective designs. In-vivo measurements are typically difficult to obtain, however, instrumented artificial limbs that approximate human limbs may be appropriate test-beds. In this paper, a pneumatic sensing chamber (PSC) is designed and characterized for sensing biomechanical joint forces in an artificial tibiofemoral limb. The flexible soft PSC was 3D printed using the soft material Ninjaflex (NinjaTek, USA) and extensive characterization tests (drift, ramp, cyclic) were conducted on a universal testing machine. A specific interface was created to tightly fit the PSC to the non-uniform bone surfaces of the femur and tibia bones, and additional testing conducted with the PSC embedded in the artificial lower leg setup. In general, we found that PSC pressure output followed the UTM forces closely with a linear relationship for the static tests (RMS error of 0.82 kPa/1.68N between experimental data and linear model) with little drift over extended hold tests. Cyclic tests showed the PSC could follow the dynamic force signal closely, although some hysteresis was found between the loading and unloading cycles. Our results show that, overall, a reliable sensor can be designed, characterized and calibrated in the application of identifying biomechanical joint pressures and forces.