Up until now, annoying cables or batteries have kept medical implants running. In the future however, humans will provide the power and pacemakers etc. will no longer need a separate power supply.
Whether it be prostheses, pacemakers, artificial organs or nerve stimulators, microelectronic implants are now an essential part of modern therapies. Highly complex medical electronics is regarded as the high-tech response to common illnesses and the specific diseases of an aging population. It is therefore a guaranteed growth area. According to BBC Research, last year’s turnover of US$24.6 billion will increase to US$37.6 billion by 2020, with a compound annual growth rate (CAGR) of 8.8 percent.
Progress in semiconductor technology has seen the “internal” healers shrink to sizes of less than a millimeter. However, the power supply prevents further miniaturization. Miniaturized yet robust power supply concepts are therefore needed for active implants. Another reason is the fact that battery changes and cable solutions are always associated with risks.
The body as a power supply
The desire for an unlimited, independent power supply has long been at the top of the list of priorities in medical electronics. At the end of the 1950s, pacemakers fueled by plutonium were destined to permanently solve the energy problem. In the seventies, Siemens carried out tests on animals with glucose fuel cells. These were then abandoned in spite of the promising results achieved following the launch of high-performance lithium batteries but have since started again at the University of Freiburg and its Department of Microsystems Engineering (IMTEK). With these pacemakers, precious metal catalysts are used to generate electrical energy through the direct electrochemical conversion of the body’s own blood sugar (glucose) from the tissue fluid. However, it will be another 5-10 years before finished products are available.
An independent power supply for medical implants also promises so-called “energy harvesting” – i.e. obtaining small quantities of electrical energy from the devices’ environment through light, temperature change, heat flow, vibration/impact, radio waves or sound. What is needed however is medical electronics that is able to collect, process and pass on data with minimal energy consumption.
Piezo harvesting for self-sufficient pacemakers is by far the most popular research topic. This is hardly surprising. After all, over a million pacemakers are implanted every year – around 70,000 of them in Germany.
Piezo for medical implants
Scientists at the University of Illinois at Urbana-Champaign used the natural movements of the heart, lungs and diaphragms as a power source. To do this, they embedded flexible bands of lead zirconate titanate in silicon strips. If they are stretched or compressed, the piezo effect discovered by the brothers Jacques and Pierre Curie in France in 1880 produces the necessary power. This electricity which is produced throughout the patient’s life is temporarily stored in a microbattery.
The Fraunhofer EMFT is developing a similar system in the EU project entitled “MANpower” which is being led by the Tyndall National Institute, Ireland. In addition to MEMS and IC building blocks, the self-sufficient, ultraminiaturized pacemaker will contain an energy harvesting system that will convert the low heartbeat frequencies of just a few hertz into “usable” energy.
However, non-piezo systems are also possible. With the torpedo-shaped pacemaker from the Inselspital in Bern, a paddlewheel in the bloodstream drives a microgenerator. In order to pump blood into the bloodstream, the heart uses more than a watt – around 200,000 times the average power consumption of a modern pacemaker (around 5 microwatts).
The Inselspital has also produced a prototype based on the automatic mechanism of a Swiss watch. Sewed directly onto the heart muscle, an imbalance which winds up a mechanical spring rotates owing to the heart’s movement. Once the spring reaches its maximum tension, it unwinds and drives an electrical microgenerator in the process.
Creativity seems to know no bounds. However, the need for research in this area of electronics is immense and what is announced as a “breakthrough” may disappear again later on. Many medical implants are still being tested on animals which means that it could take years before products for humans are available. So it remains to be seen when the first implant powered by the human body will roll off the production line.