Pain Fighters

Cleveland, OH—In the US alone, about 400,000 stroke victims must live with severe and persistent shoulder pain—the result of paralysis that causes the upper arm to sag and pull out of the shoulder socket. What's more, some 50,000 new cases appear each year.

Engineers at NeuroControl Corp., a startup company backed by Cleveland medical giant Invacare, believe they can bring relief with a new electrical stimulator called the RestoreStIM system. During years of design work, the development team relied heavily on testing equipment that included DMMs, scopes, power supplies, environmental chambers, ESD guns, and other instruments. Result: An effective low-power therapeutic device that has already received Europe's CE mark and is now awaiting FDA clearance. The pager-size external stimulator provides targeted pulse therapy via four electrodes implanted in the shoulder muscles.

Dr. John Chae (right), a physical medicine and rehabilitation specialist, and engineer Zi-Ping Fang, VP of R&D for NeuroControl.

In contrast, RestoreStIM requires minimal involvement of healthcare personnel after the initial implant and programming of the device, and clinical studies show marked reduction in shoulder pain after the typical six-week therapy regimen (see "For patients, a measure of relief ").

Bringing RestoreStIM from idea to reality required close cooperation between NeuroControl engineers, led by VP of R&D Zi-Ping Fang, and clinical professionals, directed by Dr. John Chae, a physical medicine and rehabilitation specialist at Cleveland's MetroHealth Medical Center. Both men have strong ties to Case Western Reserve University, a world-class center for research in functional electrical stimulation (FES) for restoring movement in individuals paralyzed by spinal cord injuries and similar motor impairments.

Fang, who has a BS in electrical engineering, received his PhD in biomedical engineering at Case Western and did post doctoral studies in neuroscience there. Chae, whose undergraduate degree is in bioengineering, came to Cleveland in 1994 after his medical residency to work with Case Western biomedical engineer Hunter Peckham, a leader in FES technology. "Nowhere else in this country do you see the kind of cooperation that you have here in Cleveland between clinicians and engineers," says Dr. Chae. "There's a great desire to learn from one another."

Fang and Chae were both very familiar with the use of electrical stimulation to induce hand movement in patients paralyzed by spinal cord injuries. In fact, NeuroControl earned FDA marketing approval in 1997 for Freehand, an FES system originally developed by Peckham and his team to enable patients to grasp objects, perform personal care, and do other tasks (see "Close-up of an FES innovation lab ").

Figure 1 RestoreStIM features four implanted electrodes linked to an external stimulator. Pulse-width modulation delivers the stimulus needed to contract muscles, bringing relief from pain and improving range of motion. Courtesy of NeuroControl.

Even so, they had to come up with a design dramatically different from early stimulator systems developed at Case Western, which used a brick-size external stimulator weighing 1.1 kg attached to external wires leading through the skin to implanted electrodes. Recalls Fang: "I wanted a stimulator that would fit inside a conventional pocket pager."

Working with Soheyl Pourmehdi, NeuroControl's director of electrical engineering, Fang ultimately designed a 50-g, 6x5x1-cm device that can deliver the same stimulation parameters as the much larger stimulators first developed at Case Western Reserve.

The device incorporates a current driver that delivers charge-balanced, current-regulated, biphasic pulsed stimuli. A high-efficiency DC/DC converter generates the 40 V required to deliver the 20-mA, maximum-amplitude current pulses to a patient's muscle tissue (Figure 2). The current driver operates under the control of a low-power microcontroller. In addition, the microcontroller drives a digital-to-analog converter.

Figure 2 The RestoreStIM device incorporates a current driver that delivers charge-balanced, current-regulated, bipolar pulsed stimuli. A DC/DC converter generates the 40 V required to deliver the 20-mA current pulses.  Courtesy of NeuroControl.

A 3.4-V lithium battery powers the device, delivering about 300 hours of service. The device automatically shuts off when not in use, allowing a single battery to operate throughout the typical treatment regimen of six hours a day for six weeks.

In developing RestoreStIM, the NeuroControl design team relied on conventional bench-level instruments for tasks ranging from analyzing power consumption to verifying pulse amplitude and duration. Among the most commonly used tools: an HP E3611A power supply, a Fluke 87III DMM, and a Tektronix TDS210 digital oscilloscope with Tek's YT5060 probe.

Figure 3 Development tests included frequent oscilloscope analysis of output waveforms from the stimulator to pinpoint problems in the circuitry. The cathodic pulse amplitude should be 20 mA (±2 mA), while the anodic pulse amplitude should be in the 0.4-to-0.7-mA range.

The oscilloscope, DMM, and power supplies also played a major role in verifying a circuit design that needed to boost power to 40 V, while reducing the noise level in the stimulation channels, maximizing battery efficiency, and delivering a low-current stimulus.

Although the device incorporates a transient voltage suppressor, the engineers still had to address electrostatic discharge (ESD), which might occur when a patient touches the stimulator's plastic housing. To address that problem, the engineers used a handheld ESD gun to ensure that the stimulator met the standards outlined under IEC 60601-1-2 (2001-09) for medical devices (Ref. 1). Under the standard, the device must withstand a 4-kV contact discharge and an 8-kV air discharge.

Fang notes that electromagnetic interference and emissions were less of a concern with this device, because it is a basic DC-powered pulse generator with no sensing capability. But to ensure compliance with EMC standards, the team turned to an outside lab, where technicians did spot an ESD problem: Zapping the metal screws on the stimulator housing pulled an arc that interrupted the microcontroller's operation. Solution: Substitute plastic screws.

By contrast, EMC issues played a much bigger role in another device that Fang and his team designed: A universal external controller for patients using the Freehand system. Among other things, this device monitors user inputs from positioning sensors and generates RF signals that power an implanted stimulator.

Outside the scope of electronics test, the device had to undergo prescribed drop tests of 1 m onto a hardwood floor to demonstrate both continued operation and safety. And during the design stage when using Orcad software for the circuit layout, the engineers had to maintain minimal air spacing of 2.4 mm between the plastic enclosure and the electronic components and traces to protect against short circuits from moisture or other contaminants.

To manufacture the RestoreStIM device, NeuroControl relied on a contract manufacturer, but the company's engineers set down a long litany of tests during production for such factors as battery life, insulation impedance, pulse duration, current, and pulse amplitude.

In addition, each device undergoes Hipot tests during manufacturing to meet IEC-60601-1 standards for dielectric strength. The concern here: Protecting the patient from shock that might occur when the stimulator is placed near a lamp or other device powered by AC current. Also related to patient safety are current-leakage tests, performed with DMMs. These tests ensure that, if any component fails within the circuit, it will not threaten patient safety. To protect against such leakage, the stimulator's design uses two capacitors in a series with the current pulse outputs.

Although technicians use microscopes to visually inspect such components as boards and electrode leads, Pourmehdi emphasizes the importance of environmental stress tests to guard against contamination sometimes found in boards supplied by outside vendors. Boards typically undergo 48 hours of testing in a Z-Series test chamber from Cincinnati Sub-Zero, with temperature/humidity cycles ranging from 5°C, 30% RH, to 50°C, 90% RH. "This is a very vital test for our type of operation," says Pourmehdi.

Among the problems detected by the environmental tests were instances of solder solution contamination under surface-mount components, which halted the stimulator's operation. In such instances, NeuroControl sometimes relies on outside labs to perform further failure analysis as a basis for manufacturing changes.

For NeuroControl, the many years of design and testing on RestoreStIM have yielded a device that could not only help thousands of patients annually, but also could establish the firm as an emerging leader in an estimated $100 million annual market for electrical therapy for pain relief and rehabilitation.

"We've talked to several large hospitals, and there is great interest among rehabilitation physicians in this device," says J.B. Richey, president and CEO of NeuroControl. "Current therapies too often fall short in relieving the terrible pain that these patients suffer."

The device is expected to cost patients about $3500, not including the one-hour procedure for the electrode implants, but major medical insurers have agreed on reimbursement.

NeuroControl and the clinicians who worked with the company to develop RestoreStIM also see the potential for using the same basic technology to design similar devices for treatment of other sources of chronic pain.

For the engineers who developed the stimulator, the project has brought tremendous satisfaction. "I was trained in electronics and physiology, and I've also been involved in clinical testing," says Fang, "and it is very rewarding to see engineering design help patients directly."

Maloney, Lawrence D., "A Bridge to Independence," Design News, March 6, 2000. p. 90. www.designnews.com.

Peckham, P. Hunter, et al., "An Advanced Neuroprosthesis for Restoration of Hand and Upper Arm Control Using an Implantable Controller," The Journal of Hand Surgery, March 2002. p. 265. www.jhandsurg.org.

Yu, David T., MD; John Chae, MD, ME; Maria Walker, MSE; and Zi-Ping Fang, PhD, "Percutaneous Intramuscular Electric Stimulation for the Treatment of Shoulder Subluxation and Pain in Patients with Chronic Hemiplegia," Archives of Physical Medicine and Rehabilitation, January 2001. p. 20. www.archives-pmr.org.