Optimal Radiation of Body-Implanted Capsules

Abstract

Autonomous implantable bioelectronics requires efficient radiating structures for data transfer and wireless powering. The radiation of body-implanted capsules is investigated to obtain the explicit radiation optima for Eand B-coupled sources of arbitrary dimensions and properties. The analysis uses the conservation-of-energy formulation within dispersive homogeneous and stratified canonical body models. The results reveal that the fundamental bounds exceed by far the efficiencies currently obtained by conventional designs. Finally, a practical realization of the optimal source based on a dielectric-loaded cylindrical-patch structure is presented. The radiation efficiency of the structure closely approaches the theoretical bounds and shows a fivefold improvement over existing systems.

Dielectric-Loaded Conformal Microstrip Antennas for Versatile In-Body Applications

“…Robustness to heterogeneous and uncertain electromagnetic (EM) properties of tissues was considered in [18]- [21], and a reconfigurable capsule-conformal antenna without nulls in its radiation pattern was proposed in [22]. Effects of dielectric loading on radiation performance were studied in [23]- [25]. Although high-permittivity ceramics, such as alumina (ε r ≈ 10) or zirconia (ε r ≈ 29), have been applied for miniaturization of inbody antenna superstrates [12], [14], [20], no practical designs have been reported for materials having the higher permittivity than high-water-content tissues (i.e.…”

Influence of Body-Implanted Capsule Dimensions and Materials on Achievable Radiation Efficiency

“…The radiation efficiency η follows a skew-normal distribution with its peak defined as an optimal operating frequency fopt. Both η and fopt strongly depend on the phantom formulation (addressed previously in [21], [17]) and the source parameters. As for the latter, the physical length of the source has the strongest effect on achievable radiation performance.…”

“…Two sources are considered: 1) an "electric" TM10 defined as Js = [0, 0, cos(πz/L)] and 2) a "magnetic" TE10 defined as Js = (0, 1, 0). The radiation efficiency is obtained using the Poynting's theorem [23] as η ≡ Re(Pe)/Re(Ps), where the exiting power Pe and the supplied power Ps are evaluated as in [17].…”

“…A wide range of RF antennas has been proposed for body-implantable applications [4]- [15]. Yet, establishing robust links between an in-body capsule and external equipment remains a major challenge because of too low efficiencies (< 0.1%) of the antennas operating in lossy media with uncertain electromagnetic (EM) properties [16], [17]. Considering typical maximum input power levels ranging from a few to about 50 mW [18] (limited by safety standards) and Rx sensitivities, this efficiency provides an operating range up to only a few meters [19].…”

“…Impact of tissues on radiation performance has been considered in [20] for small inductor sources and in [16], [21] for infinitesimal magnetic and electric sources. Finite-sized TM10 and TE10 sources were considered in [17]. In this study, we evaluate the effects of the antenna dimensions and surrounding capsule materials on achievable radiation efficiencies in the 0.1-4-GHz range.…”