Hip Implant for Temperature Measurements Figure 1. Cross-section of a model of the modified hip implant with a metal head. The temperature telemetry with thermistor, electronic circuit and power/data coil are placed inside the neck of the implant. doi:10.1371/journal.pone.0043489.g001 Fairchild Semiconductor). C is then discharged over L resulting in magnetic pulses. After each pulse, both FET open and C is charged again. The transmitted pulses have a duration of 3 ms, and this telemetry circuit has a power consumption of 7 mW. All active and passive components are surface mount devices on both sides of an 1865 mm wide substrate (Figure 1). The NTC is positioned near the center of the prosthetic head. To shield the electronic components against the magnetic field, a tube of PERMENORM 5000 H2 with a wall thickness of 0.25 mm is slipped over the whole circuit and fixed with epoxy structural adhesive DP190 (3 M Scotch-Weld). The telemetry (40 mm long, 6.1 mm Ø) is fixed inside the prosthetic neck with DP190. External Measuring System The specially developed unit ‘TELETEMP’ contains a power oscillator, amplifier and microcontroller (AVR-ATmega128, Atmel) with a USB interface and display. Because the transmitted magnetic pulses are low in power, a very sensitive sensor had to be chosen. This giant magnetoresistive (GMR) field sensor (AA002- 02, NVE-Corporation) consists of a Wheatstone bridge and has an on-chip flux concentrator to increase its sensitivity along a specified axis. Because the external powering field is much stronger than the field produced by the signal pulses, care must be taken that the GMR sensor measures as little as possible of the 4-kHz powering field. The spatial positions of GMR sensor and Figure 2. Principle of the temperature telemetry. Energy and temperature data are transferred through the titanium implant by induction. doi:10.1371/journal.pone.0043489.g002 PLOS ONE | www.plosone.org 3 August 2012 | Volume 7 | Issue 8 | e43489