Fabrication of a strain sensor for bone implant failure detection based on piezoresistive doped nanocrystalline silicon

One common complication following total-hip-replacement arthroplasty – one of the most performed elective surgical procedures – is the loosening of the prosthetic stem and cup, responsible for more than 80% of non-successes. A combination of piezoelectric and piezoresistive materials in a sensing device for dynamical and quasi-static analysis of the implant internal environment is envisaged by the authors. In this paper, thin-film piezoresistive millimeter-long strain sensors fabricated on flexible plastic substrates using n- and p-type hydrogenated nanocrystalline silicon films with gauge factor GF ∼ −30 and +20, respectively, are reported. A maximal value of GF = −43 is theoretically predicted for isotropic n-type multicrystalline Si. A sensor consists of an array of piezoresistors, each connected to a Wheatstone bridge-type external circuit, and to the control electronics. A sensitivity of 30 mV/μm was achieved in sensors bearing both longitudinal and transverse orientations of the resistors relative to the strain direction. Used as a shape sensor the device was able to map the contour of the hip implant. Cell growth tests show that osteoblasts grow faster on P-doped nc-Si:H thin films than on the control sample. Genotoxicity tests show that cell DNA is preserved if cultured in contact with n-type nanocrystalline silicon.