Magneto-Inductive Powering and Uplink of In-Body Microsensors: Feasibility and High-Density Effects

TL;DR

It is shown that the frequency- and location-dependent signal fluctuations in such swarms allow for significant performance gains when utilized with adaptive matching, spectrally-aware signaling and node cooperation, and in particular for future medical in-body applications.

Abstract

This paper studies magnetic induction for wireless powering and the data uplink of microsensors, in particular for future medical in-body applications. We consider an external massive coil array as power source (1 W) and data sink. For sensor devices at 12 cm distance from the array, e.g. beneath the human skin, we compute a minimum coil size of 150 um assuming 50 nW required chip activation power and operation at 750 MHz. A 275 um coil at the sensor allows for 1 Mbit/s uplink rate. Moreover, we study resonant sensor nodes in dense swarms, a key aspect of envisioned biomedical applications. In particular, we investigate the occurring passive relaying effect and cooperative transmit beamforming in the uplink. We show that the frequency- and location-dependent signal fluctuations in such swarms allow for significant performance gains when utilized with adaptive matching, spectrally-aware signaling and node cooperation. The work is based on a general magneto-inductive MIMO system model, which is introduced first.

Figures (5)

• Figure 1: Circuit abstraction of $N_{\mathrm{T}}$ transmitting and $N_{\mathrm{R}}$ receiving electrically small loop antennas (coils) for wireless communication or power transfer. The matching networks can be full multiports or individual two-ports per coil.
• Figure 2: Biomedical setup with an in-vivo swarm of micro-scale sensor nodes, each equipped with a multi-turn coil, located $12\,\mathrm{cm}$ beneath the skin. They receive power from and send data to an external array of 21 coils. The sensors and the accompanying resonant passive relay coils have random arrangement.
• Figure 3: Spectrum of the downlink channel to a micro-scale node with a $350\,\text{\textmu}\mathrm{m}$-sized coil. This addresses the channel after maximum-ratio combining at the external array. The solid line graph refers to using the two-port matching networks at the array which are assumed for the uplink. The results beyond $1\,\mathrm{GHz}$ are increasingly unreliable due to increasing electrical size.
• Figure 4: Downlink power transfer efficiency and uplink data rate as a function of the sensor-side coil diameter (which is set equal to the coil length). The sensor is located $12\,\mathrm{cm}$ apart from the external array.
• Figure 5: Uplink data rates from one or multiple (cooperating) in-body sensors, with and without nearby passive resonant relay coils in random arrangement, to an external device at $12\,\mathrm{cm}$ distance. Either case considers 20 in-body coils of $350\,\text{\textmu}\mathrm{m}$ size. The external device uses $1\,\mathrm{W}$ to supply power wirelessly. The results are shown as cumulative distribution function (CDF).