Restoring Natural Sensory Feedback in Real-Time Hand Prostheses
Dennis Aabo Sorensen is the first amputee in the world to feel sensory rich information — in real-time — with a prosthetic hand wired to nerves in his upper arm; Sorensen could grasp objects intuitively and identify what he was touching while blindfold. Silvestro Micera and his team at EPFL (Switzerland) and SSSA (Italy) developed the revolutionary sensory feedback that allowed Sørensen to feel again while handling objects. A prototype of this bionic technology was tested in February 2013 during a clinical trial in Rome under the supervision of Paolo Maria Rossini at Gemelli Hospital (Italy). The study is published in the February 5, 2014 edition of Science Translational Medicine, and represents a collaboration called Lifehand 2 between several European universities and hospitals.
Micera and his team enhanced the artificial hand with sensors that detect information about touch. This was done by measuring the tension in artificial tendons that control finger movement and turning this measurement into an electrical current. But this electrical signal is too coarse to be understood by the nervous system. Using computer algorithms, the scientists transformed the electrical signal into an impulse that sensory nerves can interpret. The sense of touch was achieved by sending the digitally refined signal through wires into four electrodes that were surgically implanted into what remains of Sørensen’s upper arm nerves.
A specialized group of surgeons and neurologists, led by Paolo Maria Rossini, implanted so-called transneural electrodes into the ulnar and median nerves of Sørensen’s left arm. The ultra-thin, ultra-precise electrodes, developed by Thomas Stieglitz’s research group at Freiburg University (Germany), made it possible to relay extremely weak electrical signals directly into the nervous system. A tremendous amount of preliminary research was done to ensure that the electrodes would continue to work even after the formation of post-surgery scar tissue. It is also the first time that such electrodes have been transversally implanted into the peripheral nervous system of an amputee. The electrodes were removed from Sørensen’s arm after one month due to safety restrictions imposed on clinical trials, although the scientists are optimistic that they could remain implanted and functional without damage to the nervous system for many years.
The clinical study provides the first step towards a bionic hand, although a sensory-enhanced prosthetic is years away from being commercially available and the bionic hand of science fiction movies is even further away. The next step involves miniaturizing the sensory feedback electronics for a portable prosthetic. In addition, the scientists will fine-tune the sensory technology for better touch resolution and increased awareness about the angular movement of fingers.