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EETE SEP 2015

Haptics & user interfaces Haptic prosthesis gives back natural feel for missing limb By IJulien Happich n close collaboration with the University Clinic for Reconstructive Plastic and Aesthetic Surgery in Innsbruck / Austria, Professor Hubert Egger from the department of medical engineering of the FH Upper Austria (University of Applied Sciences) developed a sensor-laden artificial limb able to let the wearer feel the surface he or she is walking on. The prosthesis uses six pressure sensors mounted within its foot sole and sends the pressure signals back to the wearer via vibrating elements embedded within the prosthesis shaft, as haptic feedback directly applied to the wearer’s stump skin. This only works after the surface of the stump has been surgically reinnervated, that is, once the residual severed nerves which ought to control the missing limb have been brought back to the stump’s skin surface to reinnervate the surface muscles and provide a sensory interface relevant to the missing limb. From there on, a sensory map of the lost limb can be established on the skin and used both ways, to control a prosthesis using surface electromyography (EMG) and to receive sensory stimuli from the prosthesis (here in the form of small actuators). For the foot prosthesis he developed, Egger worked with patient Wolfgang Rangger from Austrian self-help group “living with amputation” (www.lebenmit amputation.at) who had to have his leg amputated below knee after a stroke and subsequent thrombosis in 2007. Rangger had suffered severe phantom pain after his legamputation, and as a first clinical solution, Rangger underwent surgical operation for a so-called “targeted sensory reinnervation” as suggested by work from Todd Kuiken, MD, PHD from the Rehabilitation Institute of Chicago in the late 1990, explained Egger. Since the patient now received real sensory data from his limb, rather than his brain being left guessing, he no longer felt phantom pain. The stimulators used in the 6-channel vibratory stimulator socket prototype are magnetic resonators. They contain a coil with a ferric-anchor. By powering the coil with rectangular current of 80mA and 160Hz the anchor gets in resonance and produces vibration applied on the surface of the reinnervated skin. “Targeted sensory reinnervation is a method where residual (no more used) sensory nerve endings in the stump of an amputee are redirected towards a small skin area (reinnervation area or target area) where nerve endings grow towards skin receptors”, clarified Egger. “As soon as the reinnervation process is completed (nerve growth is about 1mm per day), skin receptors supply the redirected nerve endings with sensation from the targeted skin. Eight months after surgery, if Wolfgang Rangger is touched on the reinnervation area, i.e. medial shank, he feels the lateral margin of the foot and the heel” says Egger. “The reinnervation area acts as a kind of “interface” to the prosthesis. The patient of our case study, Wolfgang Rangger, can “feel” heel strike and roll off motion of the prosthesis at six spots as fed back by the sole’s sensors. The patient can “feel” conditions of the ground like slopes, kerbside and obstacles up to about 5cm”, continued Egger. In theory, according to further research published by Kuiken, if the prosthesis was equipped with other types of sensors (such as temperature) tied to equivalent forms 22 Electronic Engineering Times Europe September 2015 www.electronics-eetimes.com


EETE SEP 2015
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