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17 April 2013 A flexible insert for wireless strain sensing
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We fabricated and tested (in a laboratory configuration) a flexible insert for wireless strain sensing in an intramedullary tibial nail. The nail is a thick-walled titanium tube with an anatomic bend, and our flexible insert was designed to be advanced into the hollow of the nail by a surgeon during the late stages of the fracture fixation process. The flexible insert is a stainless steel rod, 170 mm long, with a part-circular section, roughly 4 mm wide and 2 mm deep that is slightly smaller than the hollow. A lithium niobate SAW device developed by our research team, with an operating frequency of 468 MHz, is bonded to the insert and demonstrates wireless strain sensing when the insert is bent. We describe the mechanics of the flexible insert, which can be used for structural monitoring applications where conventional strain gauge installation and wiring would be impractical. Because of its flexibility, the insert can be advanced into an irregular channel, such as the hollow of a tibial nail, or a narrow access path drilled into a solid. The insert will bend elastically into a configuration with at least three points of contact with the host body. The insert will be prestrained (in bending) during installation and need not be anchored or bonded to the host body other than through contact. Bending strain in the insert will vary as the host body deforms. We discuss possible application questions, and we demonstrate strain sensing in a laboratory specimen.
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Irving J. Oppenheim, David W. Greve, and Antonia F. Chen "A flexible insert for wireless strain sensing", Proc. SPIE 8695, Health Monitoring of Structural and Biological Systems 2013, 86951Y (17 April 2013);

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