Neural Prostheses Step Up: New Technology Allows Users to Feel

Image Credit: U. S. Food & Drug Administration

When sweat started to pool between his prosthesis and the remnant of his left leg, an idea sparked in the mind of Stephen Lander. Long-distance running was one of the few sports he could still indulge in after an accident with a forklift had left him an amputee. Later, while an undergraduate student at Calvin College, Michigan he and a few of his classmates made a prosthetic device that cools the area around the socket preventing perspiration. The TEC-Pro is not available commercially but is one device among many prosthetic options that aims to make life a little easier for amputees and paralyzed individuals.

Peg-legged Long John Silver from Robert Louis Stevenson’s novel Treasure Island may come to mind when you think of prosthetics. He tap-tapped his way through the book striking fear in the hearts of readers. From wooden stumps to mind-controlled prosthetics, the field has come a long way. The day may come when prostheses users will even feel warmth and touch through their prosthetics.

The first evidence of a prosthetic limb dates back to 950-710 BC in Ancient Egypt. It was not an arm or a peg leg but a carefully constructed bit of wood and leather that resembled a big toe. During the following centuries, people created a wide range of metal and wooden prostheses, including forelimbs with in-built spoons, brushes, hooks or welding arms. Modern-era warfare and specially World Wars I & II caused so many amputees that the demand for prosthetics skyrocketed. New approaches were needed to improve quality and functionality – hinged units to mimic joints and suction attachments to improve the point of connection between the stump and the prosthetic were introduced; ideas that are used in prosthetics even today. Over time, the need to replace a missing limb or structure has been revised. More and more, prosthetics must not only look, but act and feel the way human limbs do.

Nowadays, several companies manufacture prosthetics that can help disabled individuals regain some level of original function. A company in California called SecondSight is working to grant partial vision with a retinal implant or bionic eye. Other companies EksoBionics (California, USA) produce exoskeletons that can help people with paralysis “walk” again. BionX Medical technologies (Massachusetts, USA) creates a bionic leg with ankles that incorporate a range of functionalities – from mountain climbing to dancing. The UK-based Open Bionics uses 3D printing to create prostheses for arm amputees. It even offers three special models for children that at least visually look like they are right out of Star Wars, Iron Man or Disney’s Frozen. Using an open source code that is available online, users can even print their own arms.

According to Dr. Levi Hargrove, Director at the Rehabilitation Institute of Chicago, a prosthesis for either a paralyzed person or an amputee must be lightweight, safe, not overheat, immune to electromagnetic waves from other devices and above all, well-designed. Besides these similarities, crafting a neural prosthetic device for a paralyzed individual or an amputee varies in one critical aspect. In paralysis, there is little to almost no nervous control, so the prosthesis must have direct contact with the brain. This usually involves surgically implanting multiple, thin wires (the diameter of a strand of hair) in the brain that connect with the prosthetic. In amputees, however, “electrical signals from surviving muscles can be used to control the movements of a prosthesis”, said Dr. Andrew Jackson, at Newcastle University, United Kingdom. For such prostheses, the electrodes must line up with the muscles in a non-invasive fashion which requires a tightly fitted socket.

With mind-controlled or neural prostheses, thinking about an action gets it done. Just as you or I can lift our hand up or reach out for a book, the wearer of a neural prosthesis can use it intuitively. However, these devices can require the implantation of electrodes into the brain or spinal cord or sometimes, may need other machinery that keeps the person chained to a lab. Such caveats prevent these proof-of-concept devices from being used in the real world. In 2016, a group of researchers from the Swiss Federal Institute of technology in Lausanne helped a paralyzed monkey walk. Grégoire Courtine’s group implanted microelectrodes in the monkey’s brain that could communicate wirelessly with an electrical pulse generator inserted into the spine. These electrical signals stimulated movement in the monkey’s legs allowing it to walk. Being wireless, this prosthetic device actually has a shot at being used by patients outside the lab. Dr. Tomislav Milekovic, a co-author of the paper, believes achieving the ability to feel through the prosthesis is the next step and “certainly necessary for good control”.

Yet, a prosthesis would truly replace the original limb only when a two-way or closed loop interaction takes place. This would mean not only mind control but also detecting sensation through the prosthesis. Can the prosthesis feel heat, pressure or pain? In October 2016, researchers at University of Chicago and University of Pittsburgh took the first steps towards prostheses for paralyzed individuals that can receive sensory input. The scientists used imaging techniques to scan the brain of the quadriplegic patient, Nathan Copeland, to determine which neurons were excited when he imagined being touched in specific regions on his hand or bending his fingers. Then, the group surgically implanted four microelectrodes in his brain. When the scientists pulled on the fingers of the robotic arm, Copeland’s brain was stimulated and he “felt” the touch in his own, paralyzed fingers. “It’s our brain that actually feels,” said Dr. Robert Gaunt at the University of Pittsburgh.  The neurons responsible for feeling the touch were triggered in his brain and he experienced a pressure sensation. “They were natural sensations”, said Dr. Sliman Bensmaia at the University of Chicago. Grasping a piece of cake too hard can crush it, while holding a mug with not enough pressure can have it slip from your fingers. These, less obvious functions of human hands, are often taken for granted. This project, funded by the Pentagon’s Defense Advanced Research Project Agency, may someday help war amputees go back to their old jobs.

The problem gets more complicated when dealing with arms than with legs. The upper limbs are more demanding with regard to dexterity, flexing, extension and rotation. Lower limb prosthetic must be stable and support weight. “Penalty for failure of a lower limb could be much, much higher”, said Gaunt.

The main reason for developing prostheses that can sense is for the patient to have better control over the device. A more subliminal motivation is the feeling of connection that we experience with the world through touch. While being able to touch a person or object might not drastically improve the function of a prosthetic limb, this emotional aspect adds valence to it. “As much as we need to control things, we need to feel things”, said Dr. Steven Prawer, Co-founder and CTO of iBionics, a Canadian company that is developing a bionic eye.

While researchers like Gaunt are working on neural prostheses that sense for paralyzed or amputated persons, the road ahead is long and bumpy. It is a lot simpler for the brain to send information than to receive it. For instance, the brain may receive up to 1000 signals from receptors all over the skin but send out only 10 instructional signals. It is easy to see why the task of sensitizing prostheses would be a complex challenge. Nevertheless, Prawer believes, “Closed loop is definitely the way of the future.”

Neural prostheses that interface with the brain or muscle do not come cheap. With insurance companies unwilling to reimburse more than one prosthesis per patient, it will be difficult for them to buy these devices, which can cost upwards of $100,000. Prosthetic research aims to restore not only movement but the sense of touch. “Because we did the basic science, because we understood how the brain controlled movement that then enabled us to build technologies to restore motor functions in patients” said Jackson. In the future, individuals who have lost sensation through loss of limb or paralysis may be able to feel again. With the intersection of 3D printing, robotics and the development of brain-computer interfaces, the field of prosthetics is rapidly progressing.

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