SAN FRANCISCO, Calif. Medtronic has developed prototype implant that uses light to alter the behaviors of nerve cells in the human brain. The work was described in one of a handful of papers at the International Solid State Circuits Conference (ISSCC) seeking more refined electronic techniques to diagnose or apply therapies to neural conditions.
Several researchers have approached Medtronic asking for a new kind of device to stimulate light-sensitive proteins in nerve cells. The emerging field of optogenetic neuro-modulation could address several challenges with implanted electrical nerve stimulators that Medtronic and others make to address Parkinsons disease and many other ailments.
Today's implants use relatively strong amounts of electrical stimulation, effectively creating noise that makes it difficult to track the quieter neural signals researchers are trying to monitor. The devices cannot be used in tandem with magnetic resonance imaging systems, pose a risk of over-stimulating neurons and are generally difficult to direct to target cells.
"That limits therapy and research," said Tim Denison, a Medtronic fellow. "It's also hard to stop cellular activity, it's indirect at best and very energy inefficient," he said.
Denison described a module about the size of a pack of chewing gum that uses an optical fibre to direct light from a blue or green LED at so-called opsins, light-sensitive channel proteins in nerve cells. The module sports a rechargeable battery and 175 KHz ISB band wireless data links as well as a separate optical stimulator IC, power management unit and microcontroller.
"We see this as a building block for research not necessarily tied to optogenetics," said Denison. "There's a long pathway in pre-clinical trials before this is ready for prime time," he added.
Two other researchers described more conventional electronic implants.
Chii-Wann Lin of National Taiwan University described an implant the size of a quarter that stimulates an electrode placed in the spine to relieve lower back pain. The module uses a programmable ASIC that communicates over a 402 MHz wireless link to a handheld device. Patients can use the device to determine when and how much pulsed RF stimulation the implant delivers.
Lin showed the device effective treating lab rats using two 3.3V pulses per second while reaching a temperature of no more than 39 degrees C. Existing implants apply as much as 10.5V stimulation, raising heat in the affected area to as much as 42 degrees C which can cause peripheral nerve damage and sensitivity.
In addition, Lin's prototype uses inductive coupling to power the implant. That eliminates the need for a battery which takes up as much as two-thirds of the size of existing implants.
Separately, Eric Chow of Purdue described an implant to measure eye pressure for patients suffering from glaucoma. Using a MEMS capacitive pressure sensor, the implant collects measurements every five minutes that are stored to on-board FRAM.
The results are downloaded and the implant recharged once a day using a 2.4 GHz link. The device is shaped like a tiny tadpole with a tail acting as an antenna.
The implant's ASIC, fabricated in a 130nm process at Texas Instruments, consumes just 675 picoW in a 24 hour cycle. Chow showed results from tests of the implant in a live rabbit. The paper also described an application for the wireless sensor monitoring pressure in cardiac stents.