Published by the Students of Johns Hopkins since 1896
February 21, 2024

New prosthetics incorporate sense of touch

By JOSH SCARALIA | October 26, 2013

You’ve made it to the final round in a million dollar competition! There is one last challenge you must complete, and it seems elementary; they present two cups to you, and, while blindfolded, you must determine which cup has hot water and which cup has cold water. They blindfold you, and you put your hands out. You grab both cups and feel that one is hotter than the other. You have made your decision! Congratulations! You won!

Now imagine taking the same challenge, but this time you have lost your sense of touch. Sorry, it’s a 50/50 chance and you choose incorrectly. Better luck next time.

Many people in need of prosthetics run across this problem all the time. Although they have prosthetics that allow for function, these prosthetics do not allow for any sense of touch. However, the days of numbness are almost over.

Through a series of experiments using monkeys, researchers at the University of Chicago and the Hopkins Applied Physics Laboratory (APL) have developed a method for the implementation of touch sensory into prosthetic hands.

The first set of experiments aimed to develop an algorithm to determine the location of a touch. First, researchers taught monkeys to respond in a certain way when they felt something touch them on each finger. The researchers then mapped the brain activity that arose when the monkeys’ fingers were touched and placed electrodes at the locations that were activated. An electrical stimuli was applied to the electrodes and the monkeys responded in the same way as they did when they were physically touched.

After showing that they could successfully map touch location through an electric stimulus, researchers determined how to implement the sense of pressure. They successfully developed an algorithm that created a specified magnitude of electric current that brought about a sensation of pressure. As with the first set of experiments, the monkeys responded the same with and without the physical touch.

The researchers were successfully able to generate a sense of touch and pressure through electrical signals but were still missing one key characteristic of touch; timing. Since contact with an object is not an instantaneous event, the electric signal must be maintained until the object is released. To combat this problem, the researchers analyzed brain activity upon the grasp and release of an object. They were then able to show that the activity could be efficiently translated into an electrical stimulation.

The sense of touch comes through the sensory pathway. Touching another surface activates nerves underneath the skin. These nerves transmit a signal to the spinal cord, which then relays the signal to the brain. This process occurs almost instantaneously.

The sensory pathway is established before birth.

There are roughly 20 different types of nerves in the body. Pain, heat, cold and pressure constitute the most common nerves. And although no one likes “pain”, the pain nerves are argued to be the most important as they relay important information as to when you (or parts of you) are in danger.

One disease termed Congenital insensitivity to Pain (CIP) results in the loss of pain. This disease is either the result of an increased production of endorphins in the brain or a mutation in the voltage-gated sodium ion channel NAv1.7 which is typically overexpressed in pain neurons.  People with this disease may repeatedly mutilate or cause harm to themselves without knowledge of doing so.

There are two basic kinds of touch; fine touch and crude touch. Fine touch allows for localization of the touch whereas crude touch allows for recognition of touch but inability to decipher where exactly the touch came from. Through these experiments, the researchers were successfully able to implement fine touch into electric signals.

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