Neurostimulators have not yet reached the sophistication of the latest pacemaker-defibrillator devices, whose closed-loop systems sense electrical activity in a patient's heart and respond as needed with appropriate electrical impulses to restore normal function. In contrast, technicians or medical personnel program neurostimulators, adjusting such parameters as frequency and amplitude of pulses based on what they have learned from observing past patients in clinical and research settings.

Even so, the latest neurostimulators, like most other electronic devices, feature microprocessors that offer ever more functionality in tinier packages. Integration of such features as MEMs sensors is also on the horizon.

Some device makers design their own custom integrated circuits to control the implanted pulse generators. Others rely on chip manufacturers, such as X-FAB, STMicroelectronics and AMI Semiconductor. Many IC makers, however, still avoid medical implants because of concerns over product liability issues.

“The quality demands that are put on you as a supplier in medical applications are very significant,” says Todd Schneider, vice president of AMI's Medical Group, “but we are used to that as a major supplier to the auto industry.” Northstar Neuroscience, for example, is working with AMI in the design and manufacture of an application specific integrated circuit for its Stroke Recovery System (see main story).

The ASICs used in implanted pulse generators (IPGs) typically combine analog and digital signals on a single chip and are designed to consume as little energy as possible to extend the life of the rechargeable lithium-ion batteries that power most IPGs. Stimulus output is typically in the 10- to 20-V range for neurostimulation applications.

The latest IPGs also integrate RF communications, featuring ultra-low-power wireless chips designed to operate within the 402-405 MHz band of frequencies that the FCC has assigned to MICS (medical implant communications service). AMI Semiconductor, for instance, offers a standard wireless transceiver for this application, called the 53000, as well as a less complex chip, the 52100, for even lower power use. Wireless communication allows technicians, using an exterior controller, to program the IPG, as well as recharge the implant's batteries. Data collected by the implant can also be wirelessly transmitted to the outside world for review by medical specialists.

Moving forward, Schneider of AMI expects future chips for implants will incorporate A-to-D converters for closed-loop operation, as well as digital signal processing for more accurate and flexible control. Others, such as James Cavuoto, editor/publisher of Neurotech Reports, believe next-generation devices will also integrate sensor channels that will allow stimulus therapy to be modified in real time for maximum patient benefit.