The Limits of Implantable Bluetooth Links in DIY Gel Phantoms: A Channel Gain Evaluation

Despite the extensive research in implantable anten-nas, there are limited reports of practical biotelemetry links or the effects of the channel on in-body communication in ISM-bands. We present a detailed evaluation of an in-body/off-body link using a lossy phantom, showing for the first time the effects of the human body complementing phantom-based studies. A 2.4 GHz patch antenna, with a peak measured gain around -25 dBi, is used in the investigation. A gel phantom is developed using water, sugar, salt, and agar as a binding agent, which can be shaped using 3D-printed moulds. The antenna's reflection coefficient is matched around 2.4 GHz in both the DIY phantom and pork mince, indicating their similarity. Through channel gain measurements, using a VNA and the RSSI of Bluetooth Low Energy (BLE) transceivers, we show that the apparent gain of the implantable antenna can be up to 10 dB lower than its peak gain. A 5.5 m Bluetooth link is demonstrated showing around 15 dB variation in the apparent gain, measured through the RSSI.