A passive biodegradable implant for subcutaneous soft-tissue trauma monitoring

In-body medical devices can play an important role in clinical monitoring and diagnosis of diseases. Wireless devices implanted within a patient have to be physically small, and must overcome the challenges of having a little or no onboard electrical power and the highly attenuating electromagnetic propagation environment which is the human body. In this paper, we investigate the use of biodegradable implant to monitor the healing of soft-tissue trauma and to allow early stage diagnosis of infection. The implantable tag is designed to degrade in a predetermined and controlled method, the stage of which can be measured from outside the body without the need for further surgical intervention. The speed of degradation of the tag depends on the temperature and acidity of the subcutaneous tissue in which the tag is implanted. We show that as the electrical length of the tag pattern increases due to degradation, the resonant frequency changes significantly, and this change in resonant frequency can be detected from outside the patient. Results are presented showing the tag's performance at normal and oblique incidence, and techniques for miniaturizing and enhancing the tag's response sensitivity are given. As the entire tag is biodegradable, there is no need for further postoperative surgery to remove it from the patient at the end of its useful life.