Piezoresistive Behavior of MWCNT-UHMWPE Composite for Compressive Stress Sensing Applications

With increasing interest from research laboratories and industry in instrumented knee replacement arthroplasties, we aim to fabricate a novel stress sensor that can be embedded into the tibial knee bearing with the least alteration to the mechanical properties of the bearings. Therefore, the most commonly used material for tibial inserts, ultrahigh molecular weight polyethylene (UHMWPE), was used as the matrix for the sensor. Multiwalled carbon nanotubes were added as nano fillers to transform the insulating UHMWPE into an electrically conducting nano-composite. The electrical percolation threshold of the nanocomposites was calculated to be 0.34 wt%. The samples showed a linear current-voltage response when excited with a sine-wave electrical current source. Under quasi-static compressive loading, samples resistances showed a decreasing trend with increasing stress. The 0.3 wt% sample showed a higher stress coefficient, but also a lower degree of linearity compared to the 0.5 wt% samples, owing to the significant change in the tunneling resistance of carbon nanotube networks under stress. Reasonably repeatable piezoresistive response between loading cycles (with coefficient of variation less than 5%) was also observed during short cyclic compression sequences.