Neurotech Vendors Pursue Alternative Power Strategies

by Warren Grill, senior technical editor

The implanted stimulators used in cardiac pacing, cardiac defibrillation, and neural stimulation are, in most cases, powered by primary cell batteries, and when these batteries are depleted the devices must be replaced. Surgical replacement, which is expensive and carries associated risks, is required for continued treatment.

Deep brain stimulators have a median lifetime of less than four years and in applications requiring high charge injection may last less than one year. Similarly, spinal cord stimulators for pain fail due to battery depletion at an average of 41 to 48 months with the patient-preferred higher settings, and at two-year follow-up three out of 23 spinal cord stimulators to treat angina pectoris had failed due to battery depletion.

Battery depletion is also the leading cause of pacemaker replacement. Rechargeable batteries have not yet displaced primary cells for implant power. They have a limited number of charge-discharge cycles and may require replacement at similar intervals, and even occasionally depleted batteries are not suitable for devices that require full-time stimulation to function. To date the primary focus of research to prolong implant lifetime has been on battery technology. However, now several teams are pursuing alternative sources of electrical energy as a means to power implanted devices.

Lewandowski and colleagues, from Case Western Reserve University, reported last fall at the NIH Neural Interfaces Workshop on an implanted piezoelectric power generator. The concept is to implant the generator between the tendon of a muscle and a bone, use electrical stimulation to cause muscle contraction, and use the piezo to convert the resulting force into electricity.

Their initial results suggested that the power generated by the piezo was two to three orders of magnitude larger than the power required to stimulate the muscle. Thus, at its root, this is a system that converts ATP, which powers muscular contraction, to electricity to power implanted electronics.

Another approach is to take advantage of the thermoelectric effect—a temperature difference across a thermopile enables the conversion of heat energy into electrical energy. Thermo Life Energy Corp. has developed a low power thermoelectric generator. A 5-degree C temperature difference across the device generates 30 µW of continuous power, and larger temperature differences generate larger amounts of power.

Thermo Life Energy Corp. is headquartered in Riverside, CA, and is a wholly owned subsidiary of Applied Digital (NASDAQ: ADSX).
Several other more speculative approaches to power implanted devices have been conceived based more strongly on biological power.

For example, David LaVan from Yale University and colleagues proposed the use of ATP-powered ionic pumps in electrocytes to produce power for implanted devices. Robert Sclabassi and Mingui Sun from the University of Pittsburgh have developed prototype biofuel cells that generate electricity from the oxidation of glucose and the reduction of oxygen.

To date the power generated by these approaches appears insufficient to supply modern implantable stimulators, but as power requirement are reduced, and the efficiency of power generation increases, these novel approaches may begin to displace both primary cell and rechargeable batteries.