Dennis Aabo Sørensen was able to feel smoothness and roughness in real time with a prosthetic fingertip that was surgically connected to nerves in his upper arm. The technology to deliver this tactile information was developed by Silvestro Micera, PhD, and his team at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, and Scuola Superiore Sant’Anna (SSSA), Pisa, Italy, together with Calogero Oddo, PhD, and his team at SSSA. The results, published March 8 in eLife, provide what the researchers said is new and accelerated avenues for developing bionic prostheses with sensory feedback.
Sensors in the fingertip generate an electrical signal by moving across the textured surface. Photograph by Hillary Sanctuary, courtesy of EPFL.
Sørensen, who is from Denmark, lost his left hand in a fireworks-related accident 11 years ago. In 2013, he had electrodes surgically implanted into the ulnar and median nerves of his residual limb. He then was fitted with a bionic prosthetic hand that allowed him to achieve real-time bidirectional sensory feedback, enabling him to distinguish shapes and softness of objects, as well as the strength of his grasp. Micera and his colleagues also developed that technology.
In the latest development, Sørensen is said to be the first person in the world to recognize texture using a bionic fingertip connected to electrodes that were surgically implanted in his residual limb. Nerves in Sørensen’s arm were wired to an artificial fingertip equipped with sensors. A machine controlled the movement of the fingertip over different pieces of plastic engraved with smooth or rough patterns. As the fingertip moved across the textured plastic, the sensors generated an electrical signal. This signal was translated into a series of electrical spikes, imitating the language of the nervous system, then delivered to the nerves. He was able to distinguish between rough and smooth surfaces 96 percent of the time, and the bionic fingertip attained a superior level of touch resolution than he experienced in 2013 experiment.
“The stimulation felt almost like what I would feel with my hand,” said Sørensen. “I still feel my missing hand, it is always clenched in a fist. I felt the texture sensations at the tip of the index finger of my phantom hand.”
This same experiment to test coarseness was performed on a cohort of able-bodied individuals, without the need for surgery. The tactile information was delivered through fine needles that were temporarily attached to the arm’s median nerve through the skin. This cohort was able to distinguish roughness in textures 77 percent of the time. The research demonstrated that the needles relay the information about texture in much the same way as the implanted electrodes, giving scientists new protocols for improving touch resolution in prostheses.
Further, the team sought to determine whether the touch information from the bionic fingertip resembles the feeling of touch from a real finger. To do this, the scientists compared brain-wave activity of the test cohort, once with the artificial fingertip and then with their own fingers. The brain scans collected by an EEG cap on the subject’s head revealed that analogous regions in the brain were activated.
“This study merges fundamental sciences and applied engineering: It provides additional evidence that research in neuroprosthetics can contribute to the neuroscience debate, specifically about the neuronal mechanisms of the human sense of touch,” said Oddo, of The BioRobotics Institute of SSSA.
Editor’s note: This story was adapted from materials provided by École Polytechnique Fédérale de Lausanne.