Prolonged energy harvesting for ingestible devices

Abstract

Ingestible electronics have revolutionized the standard of care for a variety of health conditions. Extending the capacity and safety of these devices, and reducing the costs of powering them, could enable broad deployment of prolonged monitoring systems for patients. Although prior biocompatible power harvesting systems for in vivo use have demonstrated short minute-long bursts of power from the stomach, not much is known about the capacity to power electronics in the longer term and throughout the gastrointestinal tract. Here, we report the design and operation of an energy-harvesting galvanic cell for continuous in vivo temperature sensing and wireless communication. The device delivered an average power of 0.23 μW per mm² of electrode area for an average of 6.1 days of temperature measurements in the gastrointestinal tract of pigs. This power-harvesting cell has the capacity to provide power for prolonged periods of time to the next generation of ingestible electronic devices located in the gastrointestinal tract.

Figure 1. Initial in vivo characterization and anode comparison

(a) Mg-Cu electrodes for probing the available power in vivo in different areas of the stomach and duodenum, with results shown in b, c and d. (b) Example of measured electrode voltage and output power density versus load current density (2 mm × 2 mm electrodes, duodenum). (c) Measured peak power density, taken at the peak indicated in b (N = 3). (d) In vivo voltage at peak power in c (N = 3), (e) Electrode configuration for anode comparison in vitro with measurements given in f and g. (f) Measured peak power density across time for both anode configurations. (g) Summary of the measured performance.

Figure 2. Electrical characterization of the gastric battery in a porcine model

(a) Simplified architecture of the measurement system. (b) Photograph of the front and reverse sides of the system along with encapsulation using epoxy and PDMS. The PCB includes the programmable load resistor (DCP), crystal (XTAL), microcontroller (μP), RF matching network (MATCH), and antenna (ANT) on the front side, and the battery (BATT) and decoupling capacitor (CAP) on the reverse. (c) Diagram of the experimental setup, including photograph of the encapsulated pill in contact with gastric fluid inside the porcine stomach. (d, e, f) In vivo power characterization for a representative device (C4) including d: the voltage at the point of maximum power extraction during each sweep frame, e: the maximum observed extracted power in each frame and f: the measured body temperature. (g) X-rays at two time points showing passage from the stomach to the small intestine and the corresponding drop in observed power. (h) Statistical summary of the source voltage characterization data for 8 deployed devices (window size = 1h). (i) Corresponding peak power measurements for the 8 devices.

Figure 3. Demonstration of the gastric cell powering temperature measurement, wireless transmission, and drug delivery

(a) Architecture of the harvesting system. (b) The fabricated and encapsulated system PCB. (c) Snapshot of storage capacitor during continuous harvesting in SGF. (d) Summary of the in vivo measured performance of three deployed devices in a porcine model. (electrode area: 30 mm × 3 mm, thickness: 2 × 250μm). (e) Example of the full in vivo measurement data for a representative device (D1), including the estimated average power harvested by the board in t = 1 h windows versus time and the overall average power (red line). (f) In vivo measurement of the body temperature performed using the harvested power. (g) Received signal strength indication (RSSI) at the receiver for packets transmitted from the body using the harvested power. (h) Image of a drug release prototype device, placed on a United States dime. (i) Cross-sectional view of the device in h, where methylene blue is contained in a PMMA reservoir sealed with a 300 nm gold membrane and epoxy. (j) Demonstration of self-powered release (blue tail) from the device (yellow box) after activation in a beaker of porcine gastric fluid. Inset shows sequential images where the simulated drug is released in gastric fluid through gold corrosion. The gold membrane is intact in the beginning (t = 5 min) before triggered corrosion weakens the gold membrane causing crack formation on the film at t = 155 min (as shown by blue arrows), and ultimately the release of significant amount of methylene blue as shown at 355 min (blue color dye, shown in the red arrow). (k) Electrical profile during delivery of a pulse of charge to the release electrode. The dark line is the storage capacitor voltage and the lighter line is the voltage on the gold release electrode.