Wireless Power Transfer to Millimeter-Sized Gastrointestinal Electronics Validated in a Swine Model

Electronic devices placed in the gastrointestinal (GI) tract for prolonged periods have the potential to transform clinical evaluation and treatment. One challenge to the deployment of such gastroresident electronics is the difficulty in powering millimeter-sized electronics devices without using batteries, which compromise biocompatibility and long-term residence. We examined the feasibility of leveraging mid-field wireless powering to transfer power from outside of the body to electronics at various locations along the GI tract. Using simulations and ex vivo measurements, we designed mid-field antennas capable of operating efficiently in tissue at 1.2 GHz. These antennas were then characterized in vivo in five anesthetized pigs, by placing one antenna outside the body, and the other antenna inside the body endoscopically, at the esophagus, stomach, and colon. Across the animals tested, mean transmission efficiencies of −41.2, −36.1, and −34.6 dB were achieved in vivo while coupling power from outside the body to the esophagus, stomach, and colon, respectively. This corresponds to power levels of 37.5 μW, 123 μW and 173 μW received by antennas in the respective locations, while keeping radiation exposure levels below safety thresholds. These power levels are sufficient to wirelessly power a range of medical devices from outside of the body.


Theoretical Limits of Resonant Inductive (Near-Field) Coupling
While near-field coupling has been demonstrated in many medical applications, it is only effective when the distance between the transmitting and receiving coils is on the order of the distance between the coils. For separation distances that are much larger than the coil size (as will be the case for ingestible, GI-resident devices powered from outside the body), near-field wireless power transfer is considered inefficient.
Here, this is demonstrated by establishing the theoretical near-field efficiency between two coils that are 6.8 mm in diameter and 6 cm apart. A circuit representation of resonant inductive power is shown below (Supplementary Fig. S1).

Figure S1
The efficiency of this circuit is given by the expression below (see [1] for derivation): where Q is the quality factor of each coil and k is the coupling factor between the two coils. This expression makes use of the assumptions that (1) the coils resonate at the same frequency, (2) the coupling factor is small (k 2 << 1), and (3) the load resistance has been optimized for maximum efficiency. In reality, the efficiency will be lower, but this expression provides an upper bound.
The quality factor for an inductor with inductance L and series resistance R at its resonant frequency, ω 0 , is defined [1] as: The coupling coefficient between the two coils depends on the radius of the coils and the distance between them. For two single-turn coils, it can be approximated [2] as: Substituting the size of and distance between the coils, the efficiency curve can be plotted as a function of distance and quality factor, as in the figure below (Supplementary Fig. S2). The quality factor is assumed optimistically to be as high as 70 [3].
It is apparent from the figure that for even very high-quality coils, the theoretical efficiency at near-field at a distance of 6 cm is -50 dB or less, lower than the simulated (and in vivo measured) efficiencies in the mid-field regime. For more typical values of the quality factor, the near-field efficiencies are even lower. This is the motivation to use mid-field coupling as opposed to conventional near-field coupling.

Reflection (S 11 ) Characteristics of Antennas
The mid-field antennas were fabricated and matched to resonate at about 1.2 GHz. The reflection coefficient or S 11 parameter identifies the frequency at which an antenna resonates, and is shown here (Supplementary Fig. S3) for one of the antennas that were fabricated. The S 11 parameter was measured by a VNA when the encapsulated antenna was in free space, as well as when the encapsulated antenna was placed in chopped porcine stomach tissue.

Figure S3
While there is a small shift in the resonant frequency when the antenna is placed in tissue, the reflection coefficient remains about -30 dB (0.1%). Thus, about 0.1% of the incoming power is reflected back to the VNA -the rest is either radiated away or dissipated in the antenna, cables, and connectors.

Mean and Standard Deviations of In Vivo Study
The mean power levels were calculated by first converting the measured transfer coefficients across the five animals from logarithmic ratios to linear ratios. The linear ratios were all multiplied by 500 µW (equal to 27 dBm, the transmit power level set by the SAR limit) to obtain the power levels that could be received by antennas placed in each of the five animals. For each location, the mean of the five measurements was calculated and is shown in Table 1.
Along with the mean power levels, the standard deviations can also be calculated. The standard deviations in the esophagus, stomach, and colon are 52.1, 130, and 286 µW. The magnitudes of the standard deviations are quite large because the transfer coefficient measurements across the five animals are spread out over an order of magnitude. As discussed in the paper, this is most likely due to anatomical variations among the swine and to variations in the position and orientation of the antenna inside the animal body.