The application of hydrogels as articular cartilage (AC) repair or replacement materials is limited by poor tribological behaviour, as it does not match that of native AC. In cartilage, the pressurisation of the interstitial fluid is thought to be crucial for the low friction as the load is shared between the solid and liquid phase of the material. This fluid load support theory is also often applied to hydrogels. However, this theory has not been validated as no experimental evidence directly relates the pressurisation of the interstitial fluid to the frictional response of hydrogels. This lack of understanding about the governing tribological mechanisms in hydrogels limits their optimised design. Therefore, this paper aims to provide a direct measure for fluid load support in hydrogels under physiologically relevant sliding conditions. A photoelastic method was developed to simultaneously measure the load on the solid phase of the hydrogel and its friction coefficient and thus directly relate friction and fluid load support. The results showed a clear distinction in frictional behaviour between the different test conditions, but results from photoelastic images and stressrelaxation experiments indicated that fluid load support is an unlikely explanation for the frictional response of the hydrogels. A more appropriate explanation, we hypothesized, is a non-replenished lubricant mechanism. This work has important implications for the tribology of cartilage and hydrogels as it shows that the existing theories do not adequately describe the tribological behaviour of hydrogels. The developed insights can be used to optimise the tribological performance of hydrogels as articular cartilage implants.
Highly stretchable capacitive sensors are of great interest for soft robotic control due to their ability to measure relatively large strains. These sensors are often multilayered materials, with one or more of the layers made from silicones filled with functional particles. However, the models used to describe the material behavior do not always account for the hyperelastic nature of the silicones, the altered material properties due to fillers, and potential anisotropy due to the layered structure. Large errors arise when predicting capacitance using widespread assumptions of linear elastic mechanics and isotropic material properties. This study demonstrates how these modeling assumptions are inadequate for predicting sensor performance, and compares alternative models based on empirical material mechanics. The Poisson's ratio of multi‐layered hyperelastic capacitors is measured in both the width and thickness directions by imaging the sensor dimensions during strain. The results indicate that the sensors are anisotropic and have a strain‐dependent Poisson's ratio, demonstrating the validity of the proposed model. Considering these properties in capacitance models will lead to an improved ability to predict sensor performance, especially at high strains.
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