A Heart Pump without a Cord
A new wireless power scheme could make implanted devices more comfortable and reduce the risk of infection.
When an ailing heart can’t move blood on its own, an implanted pump can help keep it flowing smoothly. But there’s a major drawback: the power supply is large, must be housed outside the body, and is usually connected to the pump via an electric cord that runs through the abdominal wall—a source of constant irritation and potential infection.
Researchers have now demonstrated a prototype wireless heart pump that eliminates the need for the cord altogether. And unlike some wireless implants, it is reliable and efficient over a range of distances, from a few centimeters to a meter or more.
The pump was developed by Josh Smith, associate professor of computer science and electrical engineering at the University of Washington, and Pramod Bonde, a heart surgeon at the University of Pittsburgh Medical Center, and presented at the American Society for Artificial Internal Organs’ annual meeting in Washington, D.C., last month.
Most implanted medical devices, such as pacemakers and defibrillators, can work with internal batteries, but heart pumps and artificial hearts require more power. An artificial heart called the AbioCor is powered wirelessly, but the power transmitter, affixed to the skin, has to stay aligned with the receiver inside the body. “Just a few millimeters of separation and misalignment results in energy loss,” says Bonde.
Smith and Bonde’s new wireless pump gets around the alignment problem by modifying the way power is sent and received. The external power transmitter is a metal coil that emits an oscillating magnetic field around 6.78 megahertz and 13.56 megahertz. The receiving coil inside the body is tuned to resonate with the frequency of the transmitting coil at about 80 percent efficiency. When the distance between the coil changes, however, the efficiency drops, unless it is possible to adjust the frequency over which the power is transferred. Smith has designed a feedback system that maximizes efficiency by automatically adjusting the frequency of the power transfer.
Transferring power via a magnetic field instead of an electric field avoids harmful heating, but there is still a slight rise in temperature from a residual electric field. Smith says that eliminating some of the coil packaging will ultimately reduce heating to a negligible amount.
The new wireless power system could free up heart-pump designers to innovate. A power supply that can operate over a range of distances could be worn in a vest or even installed in a house. Smith and Bonde envision a whole-home system in which a person can move freely without wearing a power supply. A small implantable battery could also offer a half-hour of backup power.
Recent work in wireless power transfer has focused mainly on increasing efficiency, says Matt Reynolds, assistant professor of electrical and computer engineering at Duke University. “Adapting to movement by tuning the operating frequency and maintaining resonance,” he says, “is also critical to achieving widespread acceptance of this technology.”