Most of us don't think twice about the smooth skin of an apple, or the soft fur of a pet, but people who have lost limbs often miss these familiar sensations. Researchers now show that it's possible to activate sensory perception in prosthetic hands, in a report appearing in the journal Science Translational Medicine.
In the study, two men who lost their hands in industrial accidents were fitted with the prosthetics, which were connected to their remaining nerves. Improving a sense of touch appears to be a key part of perceiving the prosthetic hand as a natural extension of the body. To encourage this sense, Dustin Tyler and colleagues at Case Western Reserve University hooked up the participants during lab visits to a machine that sent electrical pulses through the prosthetic hand.
Unlike most electrical stimulation methods, the intensity of the electrical stimulation varied during these sessions, to excite different neurons with different patterns. This patterned approach is most similar to what happens in normal hands when touching a table or grabbing a doorknob. After stimulation, the patients reported feeling like they were grasping objects with their own hand, rather than feeling like they were using an external tool.
Although this sensation was limited to the lab, both individuals were able to perform everyday tasks for over a year. They were also able to perform strenuous outdoor activities such as chopping wood, demonstrating the long-term durability of the prosthetic.
In a separate study focused on recreating natural limb movement, Max Ortiz-Catalan and colleagues at Chalmers University of Technology in Sweden show that an artificial arm directly connected to the bone, nerves, and muscles of one adult male functions more like a real arm. The new prosthetic has an increased range of motion and more precise control over small movements compared to a traditional prosthetic, and doesn't require as many adjustments.
The prosthetic arm is anchored to the bone in the limb stump by a titanium rod that acts like an artificial extension of the skeleton. The patient used the integrated arm for daily life and work activities, and even occasionally slept with it for over a year with no problems. This approach appears to work better than conventional socket prostheses, in which artificial limbs are suspended via a compression socket over the amputee's stump.
The researchers believe the improved performance of the integrated arm is due to better electrode placement. The patient in this study had electrodes woven under the skin for constant sensory feedback to help stimulate nerves. In conventional socket prosthetics, electrodes are often limited to muscles closest to the surface of the skin, and can be disrupted by environmental conditions such as cold or heat.
Both studies bring scientists closer to mimicking the intuitive control, freedom of movement, and sense of touch in natural limbs. In the future, amputees may live in a world with few functional differences between artificial limbs and real limbs.