Bionic humans, as the fictional writers portray them, may be something for the distant future. But battery-powered implants, sometimes called smart implants or intelligent implants, have found a significant place in today's market. And as we progress closer to creating a bionic human, batteries become increasingly important.  

The cardiac pacemaker has been around more than 50 years. Meanwhile, advances in nanotechnology, microelectronics and polymers open the door for a wider range of implants to address diverse indications. Implant miniaturization will continue to put pressure on battery manufacturers to develop ever smaller energy sources. In addition, implants are finding applications in younger patients who expect these implants to last a long time before replacement.  And as the human population generally enjoys an increased life span, the need for more implants with longer life increases, creating additional demand for long-lasting power sources. 

Yet, meeting this need requires batteries with longer service life, which would seem to run contrary to the trend toward miniaturization. That is, smaller batteries normally hold less energy, setting another challenge for battery developers.

Market Potential Greater than Ever
The market potential for implantable devices is expected to increase dramatically at nearly 8 percent annually through 2011. Just one portion of the devices, implantable drug delivery, is expected to grow to over $12 billion by 2010. As a result, battery manufacturers are entering a technically difficult and demanding medical market. Fortunately, in applications other than the most stringent (e.g., implants) off-the-shelf battery technology is often quite suitable.
  
So far, battery providers are meeting the challenges. The human body is a sensitive, caustic, dynamic unit operating at elevated temperatures. It has little room or tolerance for invasive devices. Yet a cardiac pacemaker, which is designed to operate continuously for a minimum of five years, typically has actual operations exceeding twice that. The tight tolerance in materials and manufacture require failure rates not to exceed 0.005 percent in a production environment.

The State of Current Technology
Some of the more common battery chemistries used in implants are lithium iodine for cardiac pacemakers, lithium silver vanadium oxide for defibrillators, and lithium carbon monofluoride for drug pumps.

Lithium iodide is considered the gold standard, having a very high chemical stability as defined by the lack of any detrimental side reactions (heat, acid or gas generation). It generates sustainable power for long periods. A generally accepted design criteria is that an implantable device is considered to be at risk when overall performance reduces by 10 percent.

Three battery chemistries are at the forefront for implantation devices:

• Lithium/polycarbon fluoride, which has relatively high energy density. The drawback is that it uses a liquid electrolyte requiring special attention to sealing against gas or liquid leakage.
• Lithium manganese dioxide, which also has relatively high energy density. Like lithium/polycarbon fluoride, its drawback is that it uses a liquid electrolyte requiring special attention to sealing against gas or liquid leakage.
• Lithium thionyl chloride, which has relatively high energy density. It too uses a liquid electrolyte requiring special attention to sealing against gas or liquid leakage. Unlike the other two lithium-based systems, this system may represent some concern as thionyl chloride is highly toxic and very corrosive.

Replace the Implant or the Battery?
Currently, the emphasis is on replacing an implant at its expected end of life. Other technologies have been tried with varying levels of success.  The most popular is recharging the battery through a transcutaneous approach, which can extend the life of the implant.

While this is an attractive approach, it has drawbacks. One is that with each recharge the battery loses some of its total life. Another is that as the battery takes a charge it normally generates heat, so care must be taken not to damage the surrounding tissue. It also requires attention by the patient to remember to do the recharging and remain immobile during the process.  Yet another is that in the course of a 5-10 year life span of a totally implantable device, the technologies progress significantly.  The surgeon may opt to take advantage of these improvements by exchanging the implant at end-of–life of the battery.

One battery technology being considered for rechargeable batteries is silver-zinc, used primarily in implantable hearing aids. Another is the lithium-ion battery used in neurostimulators.

Alternative Battery Technologies
Additionally, technologies are in development that take advantage of the body’s natural chemical/electrical generation. Development of biothermal power sources using thermoelectric materials include nanoscale-based, thin-film technologies that convert the body's natural thermal energy into electrical power. Biophan is developing a thin-film battery that uses the difference between the temperature inside the body and the temperature at the surface of the body to create energy for low-power; implantable medical devices like pacemakers, sensors, or miniature drug pumps that will last up to 30 years.

Research into plastic batteries, batteries as small as a grain of rice, those that bend and some that can be lithographically printed are also being considered. Rensselaer Polytechnic Institute has taken a paper and nanotechnology combination to create extremely thin, flexible batteries capable of using human blood and sweat as a chemical source.

Of the leaders in the implantable batteries industry, Greatbatch, EaglePitcher, Rayovac, and Medtronic, Medtronic is unique in that it is a vertically integrated company producing batteries exclusively for its own products. Newer entries include Biophan Technologies and Micro Power. Currently the competition is not severe. Each developer can find a niche and service the market well, but as newer technologies emerge, this will change. The drive for higher power, longer-lasting energy in a smaller package presents not just a problem to be solved, but also an opportunity for these companies—or any newcomers with better technology.