Glucose Fuel Cells, Bioelectronics Pair Off in Next-Gen Implants

From birds to implantable medical devices, reducing size, weight, and energy consumption represents the Holy Grail of good engineering design, according to Rahul Sarpeshkar, associate professor of electrical engineering and computer science in the Analog Circuits and Biological Systems Group at the Massachusetts Institute of Technology (MIT).

In a quest to find that Holy Grail, Sarpeshkar and his team have spent a few years developing glucose fuel cells for use in a range of medical devices, from cochlear and brain implants to cardiac and diagnostic devices. The trick, however, has been to develop a parallel ultra-low-power electronics technology that can power these glucose fuel cells. Exploring this theme, Sarpeshkar will present "Case Study: Overcoming the Unique Challenges Surrounding Implantable Devices" at BIOMEDevice Boston on April 10.

"We are focusing on glucose fuel cells as an energy-harvesting solution for implantable devices," Sarpeshkar remarks. "Glucose fuel cells oxidize glucose to make electricity--just like your body uses chemical reactions to oxidize glucose. These fuel cells function like a battery, but the chemical reactions are different." However, while glucose fuel cells have high energy density and could obviate the need for batteries in implantable systems, they cannot supply much power. To be practical, they therefore require concomitant innovations in ultra-low-power electronics--a requirement that has led the MIT researchers to develop ultra-low-power bioelectronics.

"Such bioelectronics can be integrated with glucose fuel cells on the same semiconductor wafer and can sense, actuate, process, and communicate with nerves and the nervous system," Sarpeshkar explains. "Consisting of platinum or platinum-alloy wires, these electronic systems stimulate nerves via an electrical current. And because nerves are electrically excitable, the current conveys a signal to the brain." A variety of implantable medical devices contain such metallic wires, which function as electrodes.

In the case of such sensory prosthetics as cochlear implants, for example, the electronics incorporates a microphone that transduces an electrical signal, processes this signal, and sends this information wirelessly across the patient's skin. Then, a wireless transceiver receives the signal and electrically stimulates the auditory nerve. When the auditory nerve is stimulated, the brain interprets this signal as sound.

"Essentially, all implantable neuroelectronic devices leverage the fact that nerves are electrically excitable, and some technologies also leverage the fact that nerves themselves create detectable signals," Sarpeshkar comments. Thus, when paralyzed patients want to move their arms, certain nerves in the brain fire. By electrically recording these firings and decoding whether patients want to move their arms to the left, right, up, or down, the brain implant can send a signal to a robot arm or a native muscle to move the arm in the desired direction. "Thus, you're either stimulating the nervous system electrically by sending it electrical signals, or you're recording from the nerve signal," Sarpeshkar adds.

A major challenge in the medical implant field is ensuring that devices can sense, actuate, process, and communicate while consuming very little power. Capable of meeting these challenges, the MIT team's ultra-low-power bioelectronics is quite advanced. Such advanced technology is necessary for the glucose fuel cell to be practical. "By purposely choosing a fuel-cell technology that is completely abiotic and unlikely to excite the immune system, we compromised efficiency to achieve biocompatibility and longevity," Sarpeshkar says. "Nevertheless, because we have developed ultra-low-power bioelectronics, the whole system is practical. Building a complete glucose-powered medical implantable device is therefore quite feasible." --Bob Michaels

For more articles on medical device power sources, see:

Glucose Fuel Cells Sweeten the Future of Energy-Harvesting Implants

The Six Most Promising Alternative Power Technologies