A tiny injectable implant, smaller than a grain of rice, might one day take the place of large neural stimulators used to treat chronic pain and other neurological disorders. The novel device, under development by MicroTransponder, a Dallas-based startup, owes its small size to the use of RFID (radio-frequency identification) technology like that used to tag clothes to prevent shoplifting.

The device works similarly to spinal-cord stimulators for managing chronic pain. The idea is that the electrical jolts delivered by the device override the neural pain signals being transmitted to the spinal cord. However, the precise mechanism is not yet clear.

Existing devices have a battery and controller implanted beneath the skin, which delivers electrical pulses to a connected set of leads placed near the spinal cord. The MicroTransponder device, in contrast, is wireless and has no batteries. The implanted portion consists of small electrodes and a small coil, which is powered by an external battery-powered coil worn like a cuff on the arm or leg. The stimulation parameters are programmed via laptop or PDA and would be tailored to the individual patient.

Like some cochlear implants and other medical devices, the implant is powered with radio-frequency transmission: radio waves transmitted by the external coil generate a magnetic field in the internal coil, which powers the electrodes. Adopting technologies from the rapidly advancing RFID world has allowed the researchers to further shrink the device. “Instead of trying to transfer energy from two coupled antennas to do telemetry, which is a common approach for medical devices, RFID is geared to have very small transponders, so you don’t need a large coil,” says Joseph Pancrazio, a program director at the National Institute for Neurological Disorders and Stroke, a government funding agency, in Bethesda, MD, that has given the company small business loans.

The research is still in a very early stage. Researchers have developed a prototype device, which they are testing in rats. The device can effectively stimulate peripheral nerves in rats, although it’s not yet clear whether the electrical stimulation alleviates chronic pain. (Scientists assess chronic pain in rats by recording how much the animals eat; a rat in pain won’t eat as much.)

Some scientists are skeptical that the device will be powerful enough to deliver a therapeutic level of stimulation. “The main limitation of any electronic device small enough to be injected into the body is that it must receive enough power to operate its circuitry and provide the required stimulation parameters,” says Gerald Loeb, director of the Medical Device Development Facility at the University of Southern California, in Los Angeles. Loeb has also developed an injectable radio-powered microstimulator, which he says has encountered substantial limitations in range and power.

“We believe we can do it with less power,” says Scott Armstrong, MicroTransponder’s chief technical officer. However, he declined to give further details of the technology for proprietary reasons.

If it does prove successful, the device could have a number of applications. Researchers at MicroTransponder plan to test it as a treatment for tinnitus, a perceived ringing in the ears that is particularly common among veterans with head injuries.