Compact battery free implantable wireless medical device - antenna design and system integration
Mugisha, Andrew James Muhumuza
To ensure that a patient does not experience hypothermia, during surgery, the patient’s core body temperature is continuously monitored, when under general anaesthetic. As such, temperature monitoring and perioperative thermoregulation have become routine clinical practices performed on a patient, during and after surgery. Conventional temperature sensors used in monitoring core temperature, during surgery, are tethered. Tethers are susceptible to accidental breaking or dislodging. This raises the potential for post-operative infection. A compact, wireless, implantable device is desirable, to ensure patient comfort and ease of implantation during and post-surgery removal. The goal of this work was to develop a miniature battery free implantable wireless medical device (IWMD), that can monitor the core body temperature of patients under general anaesthetics. Battery operated medical devices add to the size, design complexity and costs associated with operating the device. Hence, a battery free IWMD was realised, using a class 3 commercial of the shelf (COTS) based ultra-high frequency radio frequency identification (UHF RFID) sensory tag chip (SL900A). The SL900A, developed by Austria Microsystems (AMS), can be operated as a battery-free or battery assisted RFID tag. In this work, the SL900A was integrated onto a printed circuit board (PCB) of a custom designed planar antenna, to realise the complete IWMD. The proposed device incorporates biosensing, biotelemetry and energy harvesting functions, in a compact, low-profile module. A custom meanderline patch hybrid dipole antenna (MPHD), was realised on a 22.75×20×1 mm FR4 substrate. At the design frequency, f0 = 915 MHz, the physical dimensions of the MPHD (≈0.0696λ₀x0.0612λ₀x0.0031λ₀), were significantly less than λ₀/10. Hence categorising the custom antenna as an electrically small antenna (ESA). The fabricated MPHD prototype’s measured impedance bandwidth (S₁₁ ≤ -10 dB) was 29 MHz (902.94 – 931.94 MHz). This exceeded the EU’s UHF RFID upper band of interest (902-928 MHz). The MPHD topology presents a diametric (bi-directional) radiation pattern, radiating from its broadside and ground surfaces, simultaneously. The implant was encapsulated in polydimethylsiloxane (PDMS), a ubiquitous, organic biocompatible transparent silicone polymer. Pre-encapsulation and post encapsulation measurements of the implant were performed in free space, in a water environment and biotissue. Detailed design methods, simulations, and prototype measurement results, for the pre and post encapsulation implant, are presented in this work. Characterisation of the pre and post encapsulated implant, was performed using a compact UHF RFID reader, from Betalayout Ltd. The Betalayout has a verified maximum output power level of +27 dBm (0.5 Watts). The Betalayout reader was assembled with a +9 dBi linearly polarised antenna. The Reader system’s radiated power was +36dBm (4 Watts). This power level complies with the European UHF RFID standards. Localised ambient temperature measurements performed by the implant were backscattered to the Betalayout reader. The temperature results reported by the Betalayout reader were compared with measurements performed by a reference temperature data logger, the Elitech RC4. The Betalayout reader also characterised the power, backscattered by the Implant modules, reporting the received power using the metric of receiver strength signal indicator (RSSI). A reliable wireless link between the reader and encapsulated implant was demonstrated at 30 cm in free space. The implant was placed under the skin (subcutaneous) of commercially sourced poultry biotissue - from a supermarket. A reliable wireless link of 3.6 cm was demonstrated. Alternative applications for the proposed device include: (i) post-surgical monitoring of patients’ temperature, as an implantable or wearable application, (ii) non-invasive temperature monitoring of neo natal patients (premature or critically ill infants), (iii) monitoring instantaneous temperature variations of individuals in crowded environments, (iv) a wearable application for the general monitoring of spikes in temperature, where social distancing cannot be practised (Covid-19 containment/prevention), (v), a subcutaneously implanted wireless repeater for deep implant biosensors.