Noncontact and noninvasive strain monitoring of osseointegrated prostheses

The objective of this study was to develop a noncontact and noninvasive technique for monitoring the strain-states of osseointegrated prostheses. Knowing the stress-or strain-state is important, especially for ensuring patients comply with rehabilitation exercises post-surgery or for detecting prosthesis failure. The proposed method utilizes electrical capacitance tomography (ECT) coupled with a strain-sensitive thin film deposited onto osseointegrated prostheses. Here, ECT employs a set of noncontact electrodes, which can be used to excite a sensing region using a time-varying electric field, while capacitance measurements are simultaneously obtained. An inverse tomography problem can be solved, using capacitance as inputs, to reconstruct the permittivity distribution of the interrogated region. Since the permittivity of metals alone (i.e., prosthesis) is insensitive to stress and strain, a carbon nanotube-based thin film, which can be deposited onto the prosthesis surface prior to implant, was designed such that its permittivity is sensitive to applied strains. In this work, the ECT algorithm was first implemented and validated. Second, the thin film was deposited onto prosthesis surrogates, and the film-coated prosthesis was subjected to uniaxial tensile tests. ECT was performed at different strain-states to map, without making contact, the change in permittivity of the film-coated prosthesis. The results showed that ECT could identify the rod and detect changes in permittivity in response to applied strains. In general, the proposed tomographic technique coupled with strain-sensitive thin films can serve as a new noninvasive imaging and noncontact strain sensing modality.