One of the differences that has long separated the realm of science fiction and reality, at least where prosthetics and artificial human augmentation are concerned, is our ability to smoothly knit synthetic or cybernetic components into the human body. In Star Wars, Star Trek, the Marvel Cinematic Universe, or games like Deus Ex, these are treated as solved problems to one extent or another. In real life, building artificial limbs with sophisticated gripping or balancing capabilities is still very much a work in progress. But a prosthetic arm we’ve covered before, now officially known as the Deka LUKE arm (and named after Luke Skywalker), has now gone a step further and partially restored an amputee’s ability to feel sensation again.
The paper abstract, in Science Robotics, states:
Electromyographic recordings from residual arm muscles were decoded to provide independent and proportional control of a six-DOF prosthetic hand and wrist—the DEKA LUKE arm. Activation of contact sensors on the prosthesis resulted in intraneural microstimulation of residual sensory nerve fibers through chronically implanted Utah Slanted Electrode Arrays, thereby evoking tactile percepts on the phantom hand. With sensory feedback enabled, the participant exhibited greater precision in grip force and was better able to handle fragile objects. With active exploration, the participant was also able to distinguish between small and large objects and between soft and hard ones. When the sensory feedback was biomimetic—designed to mimic natural sensory signals—the participant was able to identify the objects significantly faster than with the use of traditional encoding algorithms that depended on only the present stimulus intensity. Thus, artificial touch can be sculpted by patterning the sensory feedback, and biologically inspired patterns elicit more interpretable and useful percepts.
The Utah Slanted Electrode Array was developed for implantation in the peripheral nervous system. As the name implies, an array of 100 1.5mm long silicon microneedles projects outwards from a tiny substrate, with electrode lengths that range from 0.5mm to 1mm. The electrode array has been used along with a non-slanted version (the Utah Electrode Array) to study parallel information processing and how muscles are controlled.
The LUKE arm has been modified to relay information to the human brain, allowing an amputee to sense information about objects they are holding. Previous research has indicated that the ability to feel things is key to knowing how hard to grip them — remove it, and it’s much harder to avoid crushing objects.
“We changed the way we are sending that information to the brain so that it matches the human body. And by matching the human body, we were able to see improved benefits,” said Jacob George, study author and biomedical engineering doctoral student at the University of Utah. “We’re making more biologically realistic signals.”
One of the amputees who received the arm, Keven Walgamott, was able to successfully remove grapes from their stems without crushing them, pick up an egg without crushing or cracking it, and even hold his wife’s hand. He reported similar sensation in his “fingers” to that of a human hand. “It almost put me to tears,” Walgamott said after using the LUKE Arm for the first time in 2017. “It was really amazing. I never thought I would be able to feel in that hand again.”
Making the arm work was a complex process. Research on helping amputees feel by connecting prosthetics to remaining nerves in the forearm has been underway for years. But actually transmitting sensation requires more than just hooking the hand up to a nerve that can be made to transmit a “move” command. In order to interface with nerves, the hand had to have sensors in it that could carry the data in a manner that nerves could understand as sensation to begin with. Transmitting nerve impulse data in what seems to be a spiking neuron model based on the description was key to making the arm actually work. (The site notes: “Upon first contact of an object, a burst of impulses runs up the nerves to the brain and then tapers off. Recreating this was a big step.”)
Researchers apparently modeled nerve transmission in primates to understand how to build an equivalent model in humans. The team is now working on a version of the Deka LUKE arm that can be fully mobile, rather than partially wired to a computer outside the body. The Utah Slanted Electrode Array is capable of sending signals that transmit more than just touch — pain and temperature can also be signaled as well, though this research focused on touch, not the other senses. In the future, the team wants to expand to address the needs of amputees above the elbow as well as its existing work with patients who lost limbs below the elbow. It’s hoped that patients could be fitted for a LUKE arm they could take home and use by 2020 or 2021. The arm has been in development for some 15 years.
Feature Image Credit: Dan Hixson/University of Utah College of Engineering.