Scientists replace amputee's hand ... along with its sense of touch
By Stu Robarts
February 10, 2014
A man who lost his left hand in an accident nine years ago has had his sense of touch restored using a prosthetic hand surgically wired to nerves in his upper arm. During the trial, Dennis Aabo Sørensen was able to grasp objects, detect the strength of grip, distinguish shape and identify objects by touch while blindfolded. The work was carried out by scientists from École Polytechnique Fédérale de Lausanne (EPFL) and Scuola Superiore Sant'Anna (SSSA).
Sørensen's hand was damaged whilst handling fireworks during a family holiday and was immediately amputated on arrival at the hospital. A prosthesis allows him to grasp objects by detecting muscle movements from the remaining portion of his arm, but cannot feed sensory information back to the nervous system to replace the sense of touch.
The sensory-enhanced prosthesis trialed by Sørensen first required four electrodes to be implanted into the ulnar and median nerves of his arm, which are responsible for sensation in the third and fourth fingers and the thumb, first, second and third fingers respectively. The electrodes were developed at Freiburg University and had to be "ultra-thin" and "ultra-precise" in order to successfully relay weak electrical signals into the nervous system. Stanisa Raspopovic, of the EPFL team, explained the implantation procedure to Gizmag.
"The active sites are 80 um (0.08 mm) in diameter, made by platinum and iridium oxide over the substrate of polyamide," he told us. "Each electrode had 14 intraneural active sites and two grounds. The implantation process is done surgically under general anesthetic."
A great deal of research was required beforehand to ensure that the electrodes would continue to work even after any scar tissue developed following the surgery. Before the prosthesis could be connected to the electrodes, a period of 19 days of testing was required to map the stimulation of certain areas of the nerves to the sensations felt by Sørensen in his "phantom hand."
Following the preliminary testing, the prosthesis was then fitted to Sørensen and connected to the electrodes every day for a further week of testing. According to Raspopovic, this included fine force control tests in which Sørensen had to regulate the force of his prosthesis, tests for shape recognition and tests for stiffness recognition. All tests were carried out with Sørensen blindfolded and acoustically insulated, as well as without any prior training.
The team was initially concerned about whether or not Sørensen's nerves would still be sensitive enough to work with the prosthesis, having not been used for so long. They need not have worried, though. "The sensory feedback was incredible," says Sorensen in a press release. "I could feel things that I hadn’t been able to feel in over nine years."
The prosthesis itself is fitted with sensors that detect information about touch. Tension in the artificial tendons that control the movement of the prosthetic fingers is measured and converted into an electrical current. The current is then translated using a series of computer algorithms, into a format that the nerves can interpret. That signal is then sent to the electrodes, to give the sensation of touch.
Raspopovic says that the next steps for developing the technology involve the miniaturization of computers, the creation of long-term implants and further clinical trials with increased sample sizes.
Previous work on restoring the sensation of touch in amputees has included the SmartHand, another project in which SSSA was involved.