Losing an arm is a traumatic, life-changing event. Too many military personnel suffer that loss. Even in peacetime, as many as 20 armed services members endure upper or lower limb amputations annually due to noncombat accidents. To help wounded warriors return to active and healthy lifestyles, the Department of Veterans Affairs not only provides medical assistance, prosthetics, training, and other assistance, it also funds R&D into better prosthetics.
At the Advanced Platform Technology Center (APTC) at the VA Medical Center in Cleveland, one team of biomedical researchers is working to develop technology that would give veterans (and civilians) with upper-limb losses a sense of feeling in their prosthetics. They are installing sensors on the artificial hand portion of the prosthetic, then processing and routing those sensor signals to the user’s brain via the nerves that once served the missing hand.
The goal of the project, according to team member Assistant Professor Dustin Tyler, Director of Engineering, Quality, and Regulatory Affairs at APTC and a faculty member in nearby Case Western Reserve University’s Biomedical Engineering Dept., is determining what kind of sensations the sensors on prosthetics can provide users, how many sites can be given this artificial sense of touch, and what technologies can best do the job.
“The sensors we currently use are thin-film, force sensor resistors (FSRs) from Tekscan Inc.,” says Tyler.
An FSR consists of two electrodes separated by a thin sheet of material. Applying pressure moves the electrodes closer together and changes the sensor’s resistance as a function of pressure. FSRs are simply taped to the artifical hand, a classic three-jaw hand prosthesis from Ottobock with one degree of freedom — the index and middle finger move together toward and away from the thumb. The center’s current prosthetic has four FSRs that measure the finger opening or span between the thumb and fingers, as well as pressure at the tip of the thumb, index finger, and middle finger.
“These let users grasp and release objects and give an indication of how much force they are exerting in their grasp. But in the future, sensors need to be more rugged to withstand daily use without breaking down. And they will be mounted so they are protected. But there are some drawbacks to ‘hardening’ sensors,” notes Tyler.
“One medical-device manufacturer puts sensors on the internal structural metal parts of its prosthetics. This provides a level of protection, but limits the range of forces they can detect,” Tyler says.
“Down the line, as we try to detect forces distributed over artificial hands, we will need to find mounting locations that let sensors detect surface sensations and pressure with better resolution, and not just a gross output of total forces but better spatial resolution,” says Tyler. “We will always want to detect feelings from fingers, so sensors will likely mount on the prosthetic’s fingertips, which means they will have to be protected against wear and liquids.”
In APTC’s current setup, an exterior battery powers the sensors. Actual prosthetics, however, would not tether users to external batteries or power supplies.
“Batteries are getting more capable, and motors and sensors are becoming more efficient, so the ideal would be to have a single-charged battery pack last all day,” says Tyler. “But until then, biomedical engineers will likely design artificial hands and arms with quick-change-out battery packs holding a minimum of 4 hours of use.”
The PC and nerve stimulator
Raw sensor data gets amplified, filtered, digitized, and then sent to the PC at no more than 100 Hz, which is enough bandwidth for an artificial hand’s sense of touch. The PC, a standard desktop model running Windows, records all the sensor data and performs some numerical processing on it using Matlab (from Mathworks).
In general, the PC maps the sensor information to stimulation patterns for nerves that will generate the desired sensations in users. Currently, only researchers can adjust or change settings on the PC and stimulator, letting them update programming and try new combinations of stimulation pulses. That’s one reason the stimulator and PC will likely remain separate even in later versions, says Tyler.
“In the future, users might be able to control some gain parameters so that the hand is more sensitive for doing delicate work and less sensitive for heavier-duty work,” explains Tyler. “But it would likely be nothing more sophisticated than a volume knob.”
The relatively simple stimulator, the product of biomedical engineers at the VA's Functional Electrical Stimulation Center (FESC) in Cleveland, creates three current-controlled pulse trains. The three stimulation signals get sent to the three major nerves that usually carry sensory signals from the hand to the brain: the median, ulnar, and radial nerves. This stimulation is ac in nature and biphasic with balanced current flow in successive negative and positive impulses. Biomedical engineers long ago discovered that long-term monophasic stimulation with all positive or all negative pulses creates chemical and charge imbalances that break down nearby blood vessels and muscle tissues.
The APTC project’s goal is to eventually replace the PC with an embedded processor that would be small enough to mount inside the prosthetic. It would pull in sensor data, do some processing, and match incoming sensor data with the proper nerve stimulation signal (pulse, timing, and amplitude).
“One long-term approach is to use prosthetic-mounted sensors that detect the raw haptic data, which will be processed in a module attached to the artificial hand, perhaps no larger than a wristwatch,” says Tyler. “This would communicate with a stimulator implanted in users like a pacemaker but send sensory signals to the proper nerves. The prosthetic would also record the muscle activity (electromyogram, or EMG) of the user trying to control his missing hand. These EMG signals would get processed and be used to control the prosthetic’s drive motors that move the digits and hand.”