Solar Cells Shown Feasible to Power Medical Implants

One of the solar cell pack used in the study (Lukas Bereuter, Springer.com)

4 January 2017. Biomedical engineers in Switzerland built and field-tested small solar cells that generated enough energy for implanted medical devices like heart pacemakers. A team from University of Bern reports its findings in the 3 January issue of the journal Annals of Biomedical Engineering, published by Springer.

The researchers led by doctoral candidate Lukas Bereuter are seeking better ways to power pacemakers and other implants, such as brain and nerve stimulation devices. The size, shape, and weight of implanted devices are often determined by the batteries providing their power. In addition, batteries have finite, and in many cases limited, lifetimes that require periodically replacing the battery or entire device, procedures usually requiring surgery.

Solar energy offers a potential alternative to traditional batteries, given its abundance and the low cost of solar cells to capture sunlight and convert it to energy. In their paper, Bereuter and colleagues in Bern’s biomedical engineering research lab and the university’s affiliated hospital tested prototype solar cells under conditions simulating an implanted medical device, where the solar cell itself is also placed under the skin. A number of researchers and labs proposed solar energy for powering medical implants, but none so far tested devices in real-life conditions.

The team first experimented with a layer of pig skin placed over solar cells in the lab, since the skin of pigs, as the authors note, is similar anatomically and in optical properties to human skin. Tests with 16 such samples showed solar cells covered by a layer of skin could still capture enough energy from sunlight to power implanted devices. The tests also provided specifications for filters placed over solar cells later on that emulate implanted conditions.

The researchers recruited 32 volunteers in Switzerland to test the solar energy packs consisting of 3 commercial solar cells, each 3.6 square centimeters (1.4 square inches), in a 3-D printed housing and under a filter meeting the specifications derived from earlier lab tests with pig skin coverings. The test participants, which included 8 individuals age 65 and older — the age group most likely to need a pacemaker — wore the solar packs outside their clothing on an upper arm in randomly assigned one-week periods for a little more than 6 months, from 21 June through December 2015. In some cases, participants were instructed to cover the solar packs, when wearing a scarf or high collar to simulate conditions for a heart pacemaker placed near the neck or collarbone.

The results show the solar packs generated more power as expected on sunny rather than cloudy days, and when outdoors rather than indoors. And as expected, the amount of power generated declined month-by-month from July through December as the amount of daylight lessened. Yet, even when indoors, ambient light from windows or room lights contributed to the power supplies.

The results show that one of the 3.6 square centimeter solar cells could provide more than enough power each day for a heart pacemaker, even if implanted under the skin. Pacemakers require from 5 to 10 microwatts of power each day, while the solar cells provided on average at least 12 microwatts per day. “By using energy-harvesting devices such as solar cells to power an implant,” says Bereuter in a Springer statement, “device replacements may be avoided and the device size may be reduced dramatically.”

The authors note that solar-powered implants would also need energy accumulators that store surplus power for use at night and during low-sunlight days. These accumulators, however, would likely be smaller and lighter than batteries, since power would be quickly replenished once the solar cells were exposed to light. Also, implanted brain or nerve stimulation devices would require larger solar cells than for pacemakers since stimulation devices need more power.

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