Simulation and Ex Vivo Characterization

Simulations were conducted in COMSOL (Burlington, MA) to estimate the efficiency of wireless power transfer between two small square loop antennas each 6.8 mm in length. The antenna length was chosen to fit on a printed circuit board inside the largest ingestible capsule (000 capsule), which is 9.5 mm in diameter. A 5.8-cm thick, multilayer tissue model consisting of air gaps, skin, fat, muscle, and stomach tissue was constructed (Fig. 1A). The dielectric properties of tissue were obtained from an existing database18.

Figure 1: Simulations were performed to estimate mid-field coupling efficiency and specific absorption rate (SAR) of radiation. (A) The multilayer tissue model (not drawn to scale) used in simulation studies consists of 2 mm of skin, 20 mm of fat, 10 mm of muscle, and 18 mm of stomach tissue arranged as shown. Antennas are placed 4 mm away on either side of the tissue model. (B) The simulated transmission coefficient or S 21 parameter is shown as a function of distance through the multilayer tissue model. The dotted lines show the transitions between different tissue. From left to right, they are: air-skin, skin-fat, fat-muscle, and muscle-stomach tissue. The efficiency across the complete multilayer tissue is about −41 dB. (C) For the SAR calculations, a 10-g mass of tissue (striped blue; a cube of tissue of side length 2.15 cm, not drawn to scale), directly under the center of transmitting antenna, was used. The location of the tissue was based on inspection of the SAR in different planes in the x-axis, as indicated by the labels on the right side of the tissue model. (D) The relative magnitude of SAR is shown in different slices (planes in the x-direction). Each slice is labeled with its distance from the transmitting antenna. Red indicates regions of high absorption, while blue indicates regions of low absorption. In the slices that are outside of the tissue (at x = 1 and x = 58 mm), the magnitude of the mean-squared electric field is plotted instead of the absorption. The color scale is for qualitative comparison within a slice only, as the scale differs among slices. (E) Averaged value of SAR is as a function of the power delivered into the transmitting antenna. IEEE sets a low-tier (blue) and high-tier (red) standard for safety limits. Full size image

Two antennas were placed on either side of the multilayer tissue model, and the power transfer efficiency was recorded as a function of distance at a frequency of 1.2 GHz. The distance was varied by moving the receiving antenna through the multilayer tissue model, while keeping a fixed, 4-mm air gap around it. The power efficiency was quantified by plotting the transmission coefficient, S 21 parameter, which represents the fraction of power that is fed from a port into a transmitting antenna that is picked up by a receiving antenna and absorbed by its associated port.

Simulations were also performed to determine the maximum power that could be radiated into the stomach before specific absorption rate (SAR) limits in tissue were exceeded. In this work, SAR limits identified by IEEE19 were used. These limits apply to SAR averaged over a 10-g tissue sample (a cubical sample 2.15 cm on each side) where the electric field is the strongest. In our case, as shown in Fig. 1C, the sample was centered directly under the antenna as it was assumed (and later verified) that the electric fields would be highest in that volume. The absorbed radiation for a variety of external power levels was simulated, and the maximal amount of power that could be coupled safely into the tissue was identified.

After verifying through simulation that antennas operating at 1.2 GHz could transfer power at a reasonable efficiency through the stomach, a series of small loop antennas were fabricated and impedance matched to resonate at 1.2 GHz (see Fig. 2a). Resonance and impedance matching was achieved by placing 2.2 pF surface-mount capacitor in parallel with the loop antenna, followed by an L-network consisting of a series 33 pF capacitor and a shunt 1 nH inductor. Small MMCX connectors with footprints of 7.4 by 4.5 mm were attached to the boards.

Figure 2: Tests in stomach tissue identified optimal antennas for in vivo work. (a) 1.2 GHz small loop antenna fabricated on FR4 substrate with lumped components soldered onto the board. (b) Antennas were encapsulated and then placed in ground porcine stomach tissue, separated at various distances to characterize the transmission efficiency. (c) The measured transmission coefficient or S 21 parameter is shown as a function of distance. Six measurements were taken for each distance in different parts of the tissue. The error bars shown the maximum and minimum values recorded from the six measurements at each distance. The solid line passes through the mean of the maximum and minimum measurement. Full size image

The antennas were coated in a layer of 5-minute epoxy20 (Devcon) and then encapsulated in a layer of PDMS (Sylgard® 184 Silicone Elastomer) to ensure stability in tissue and the GI environment. The PDMS coating was created by suspending the circuit board into a 3D-printed plastic mold with a 9-mm inner diameter. The PDMS was cured at room temperature for 48 hours. This created a coating of PDMS between 1–4 mm thick around the circuit board.

The encapsulated antennas were placed in porcine stomach tissue ground into roughly 1 cm3 chunks (see Fig. 2b) to characterize ex vivo wireless power transfer. An HP 8753 C vector network analyzer (VNA) was used to record the transmission coefficient between the antennas, which were held apart at different distances within the tissue. Six measurements were taken at each distance; between each measurement, the antennas were removed from the tissue and placed in a different part of the tissue to account for the heterogeneity in stomach tissue. Antennas validated ex vivo were then used for the in vivo study.

In Vivo Measurements

All animal work was approved by the Committee on Animal Care at the Massachusetts Institute of Technology, and all experiments were carried out in accordance with the guidelines and regulations of approved protocol 1013–094–16.

Five healthy female Yorkshire pigs weighing 60–75 kg were used for this study. Animals were fasted overnight prior to the procedure. On the day of the procedure the morning feed was held. The animals were sedated through intramuscular dosing of Telazol (tiletamine/zolazepam) 5 mg/kg, xylazine 2 mg/kg, and atropine 0.04 mg/kg. Esophageal intubation and placement of an esophageal overtube (US Endoscopy) was performed.

One antenna was attached to an endoscope (Pentax) and placed in the esophagus. Once the antenna had been situated in a stable position inside of the esophagus, 30 centimeters from the mouth, it was held still while the other antenna was held outside of the body, pressed against the skin, and adjusted to determine the position and orientation for optimal power transfer efficiency. The device inside the body was not intentionally moved or rotated to mimic actual deployed devices, which could not be adjusted once inside the body. The same procedure was repeated with the antenna placed inside the stomach (75 centimeters from the mouth) and inside the colon. In the latter case, the endoscope was placed through the rectum, 25 cm from the anus.

The power transfer efficiency was measured by observing the transmission coefficient at the resonant frequency. Once the external antenna had been placed next to the skin, a continuous 60-second measurement of the S 21 parameter was made at 4 Hz. Within this measurement, the 6-second window with the highest average reading was identified and the average and standard deviation was recorded. Representative x-rays and optical images were taken to indicate the position of the antenna inside of the animal, as shown in Fig. 3a,b. This was repeated for the stomach and colon.