Sensing the micro-motion of an orthopaedic implant: sensor head and antenna development

Theoretical thesis.Bibliography: pages 275-297.Every year millions of people undergo orthopaedic surgeries that help them live a better life. About 10% of them require revision due to implant loosening. In this thesis, a small millimetre sized non-contact electromagnetic sensor is explored that can detect the micro-motion of the implant and therefore predict the impending implant failure. The research focuses on two parts namely, development of an eddy current sensor head implanted inside the bone and the design of a bone implantable antenna to facilitate the data telemetry. This research explores the use of eddy current sensors in the human body through extensive electromagnetic simulations and experiments for monitoring the micro-motion of an orthopaedic (tibial) implant. A statistical curve fitting technique is established to find the sensitivity, range and optimal frequency of operation. The effect of the human body is mitigated by exploiting E-Field confinement due to the dielectric contrast. This helps in increasing the sensitivity and range and robustness of the sensor.The simulations are verified experimentally by using a femur bone from cow. A quantum tunnelling effect based Tunnelling Magneto Resistor is integrated with an eddy current loop for magnetic field detection. A heterodyne detection technique is developed to convert the signal to low frequency. This increases the sensitivity of the displacement sensor by an order of magnitude as compared to the traditional eddy current sensor.The sensor should be immune to the changes in the structure and properties of the human tissue. For this purpose, a detailed analysis of the effect of variation in complex tissue permittivity of bone is carried out and recommendations are made. The effect of positioning error on the sensing characteristics is examined closely and maximum tolerances at different stand-off distances are provided. The optimal geometry of the sensor is also determined for obtaining the high sensitivity and range of operation. A highly miniaturized antenna implanted inside the bone and operating at 2.4 GHz ISM band is developed. Complementary Archimedean spirals printed in two layers and connected with a shorting pin are designed on a high dielectric permittivity substrate to achieve miniaturization. The location of the shorting pin is varied to tune the resonant frequency and impedance matching. A detail parametric analysis with respect to the geometric parameters and the tissue parameters is performed.A new family of fractal geometries, named 'Family of M Segment Quadratic Fractal Curves', is proposed. It offers a design flexibility in terms of miniaturization, form factor and antenna complexity. This new fractal is used to design two miniaturized implantable slot antennas - one narrow band and one wide band. An empirical expression for the design of slot antennas on substrates of varying thickness is also developed. A complete step by step process for designing an implantable CPW fed slot antenna operating at the desired resonant frequency is detailed1 online resource (xxxi, 297 pages : illustrations