New prosthetic hand helps amputees to experience touch again

Using neural electrical stimulation amputees were able to feel how much pressure they were exerting with their prosthetic hand when they touched or held something.

The prosthetic hand, described in a study released today in Science Translational Medicine, was tested on two amputees who had lost a hand in traumatic incidences. We talked to two of the study’s authors, Dustin Tyler and Emily Grazcyk from Case Western Reserve University, to find out how this prosthetic is helping amputees to ‘feel’ their hand again for the first time since they lost it.

RG: Could you briefly explain how the prosthetic hand you designed works?

Dustin Tyler and Emily Graczyk: When a prosthetic user grasps an object, touch sensors on the prosthetic hand pick up information about how hard the user is gripping the object. This information is converted into the neural code for the correct amount of pressure, based on the relationships described in this paper. This neural code is delivered directly to the nerves with tiny amounts of electricity via small electrodes surgically implanted around the subject’s nerves. From there, the touch information is carried up to the subject’s brain and the person perceives the touch to be coming from their missing hand. This system works like natural touch in a real hand – except the natural sensors in the skin are replaced with artificial sensors on a prosthetic hand, and the peripheral nerves get the touch information from stimulation through a neural interface.

RG: What is significant about the findings of your research?

Tyler and Graczyk: Our findings demonstrate that artificial sensation in the missing hands of amputees can achieve the same characteristics and sensitivity as natural sensation in a real hand. This is a critical step forward for sensory restoration for amputees because prosthetic users need to know how hard they are grasping things, not just when they make contact with an object. Feeling fine, graded pressure is important for precise manipulations of delicate objects or in communication through touch, such as when holding a loved one’s hand. In addition, we found that the perceived intensity of the artificial sensations can be controlled by the “activation charge rate” of the stimulation. This quantity will greatly simplify the implementation of intensity scaling in neuroprostheses, making it easier for scientists and researchers around the world to understand how sensory restoration algorithms will feel to subjects.

RG: To what degree was the sensation of touch restored in the amputees who took part in your study?

Tyler and Graczyk: That is actually a complex question. Electrical stimulation reproduces tactile sensations on both small areas of the subjects’ missing hands or over larger surfaces, depending on how the stimulation is applied. Stimulation through individual electrode contacts produces sensation on the tip of the index finger or the tip of the thumb, for example. The sensations feel like touch or vibration or any number of sensations that they might experience with a real hand. What is important to note is that users are able to feel as though their entire hand is interacting with objects and that they can perform complex and delicate tasks. We are presently more limited by the mechanics of the prosthesis than the restored sensation.

RG: How did the patients describe the experience of being able to touch again and discern the amount of pressure they were exerting?

Tyler and Graczyk: Adding the sense of touch completely changes the experience of both the prosthesis itself, and more importantly, the person’s ability to interact with objects and others around them. Without sensation, our subjects describe the prosthesis as a tool to assist their remaining hand. However, with sensation, they describe themselves as becoming two-handed again – so they actually used the prosthesis as a hand again. There is also an added sense of confidence with the prosthesis. One subject specifically relayed the story of picking up his grandchild. Without sensation, he would only use his remaining arm out of concern of squeezing the child too hard with the prosthesis. With the sense of touch, especially graded intensity, he would easily and comfortably use both hands to pick up and hold the child.

RG: How long do you estimate it will take until bionic limbs become clinically viable?

Tyler and Graczyk: We are developing a fully implantable sensory and motor restoration system that is planned to be ready for clinical trials in about three years. We are optimistic that it is possible to see a fully available sensory restoration system within about 10 years.

Image courtesy of Brett Sayer.