Sensing the micro-motion of an orthopaedic implant: sensor head and antenna development
Creators: name: Khokle, Rajas P.
Publisher: name: Macquarie University
Year: 2022
Subjects: Other education not elsewhere classified

description: 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 detailed