Performance of a capacitive monitoring system for instrumented bone implants

• Autor(a) principal: Cachão, João Henrique Gamito Trindade Carvalho
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/31235
• topic: Bioelectronic device; Instrumented implant; Bone-implant interface; Capacitive technology; Osseointegration

Musculoskeletal disorders are becoming an ever-growing societal burden and, as a result, millions of bone replacements surgeries are performed per year worldwide. Although total joint replacements are recognized among the most successful surgeries of the last century, implant failure rates exceeding 10% are still reported. These numbers highlight the necessity of technologies to provide an accurate monitoring of the bone-implant interface state. This work aims to identify the performance of an instrumented implant to monitor implant stability using a planar capacitive technology. A 5x10x0.8 mm printed circuit with two 5x2 mm electrodes was fabricated to be integrated in an implantable device, with the objective of assessing the effect of a fully implantation into a biological specimen. The implant was fabricated with a conic geometry, to achieve a press-fit fixation, with 55 mm of length and a minimum/maximum diameters of 12 and 15 mm, respectively. After implantation, the system was put under compression and decompression cycles, so the bone-implant interface could be altered. In the compression cycle, the observed capacitance values decreased, indicating the sensor was moving away from the bone; and contrarily, in the decompression cycle, the capacitance increased with the progressive unloading. Values were obtained in intervals of [2.2090; 3.0764] pF for the compression and [1.9806; 3.1841] pF for the decompression. The mean percentage of capacitance change for the compression cycle was 3.67% and 5.06% for the decompression, indicating a greater change rate in the decompression cycle. Additional tests were carried where the implant and the sensor were rotated 90 and 180º, to show the influence of different interfaces in the measured capacitance. The latter tests allowed to support the results obtained without rotation, as different sensor positions provided different behaviors of the capacitance change. Further development is still needed related to the experimental setup, more specifically the in vitro specimens fixation and the environment control of the experiment room. In addition, energy harvesting to create self powering systems to avoid exernal links or finite-life alternatives are also a necessity for future instrumented implants. This work further demonstrated the potential of capacitive technologies to monitor the bone-implant fixation. Therefore, it also contributed towards the design of a new era of high-sophisticated implantable medical devices.