Many modern prosthetic limbs are so intricate that they seem like something from the sci-fi cyborg realm. Unfortunately, to the wearer these marvels still feel like lumps of dead metal and plastic. DARPA's recently announced Hand Proprioception and Touch Interfaces (HAPTIX) program aims to change this. Using implantable sensors linked wirelessly to external modules, the goal is to provide lifelike prosthetic limbs with such a high degree of sensory feedback that they bring a sense of being part of the the wearer’s body, not something just strapped on.
The problem of prosthetics comes down to one word: proprioception. That is, the ability to move your limbs without looking at them, yet still being confident that you know exactly where they are. In other words, it’s the sense that your limbs are yours and not stuck-on tools. This is so common that most of us never think about it until a limb goes to sleep and feels like someone has attached a log to your body.
This is one of the major hurdles facing designers of upper limb prosthetics. Such prosthetics can be strong, articulate, beautiful to look at, and even respond to nerve impulses to provide a sense of touch like real limbs, but if they don’t feel like real limbs, they’re not much more than elaborate tools.
In addition, there’s the matter of functionality. Aside from having an arm that doesn't feel like an arm or a hand that doesn't feel like a hand, we rely on sensory feedback to control our limbs. It’s what allows you to rummage in a drawer without looking or fetch a tennis ball from under a sofa. It’s also what allows a skilled typist to work a keyboard without looking at keyboard or fingers.
Part of DARPA’s Biological Technologies Office (BTO), the HAPTIX program’s goal is to give amputees artificial limbs with a sense of feeling that will make them more natural to wear and using them more intuitive, as well as reducing incidence of “phantom pain,” where amputees have feelings in a limb that is no longer there.
The key to HAPTIX is the peripheral nerves in the limb stump. According to Doug Weber, the DARPA program manager, these are the best way to provide the needed sensory feedback.
“Peripheral nerves are information-rich and readily accessible targets for interfacing with the human nervous system,” says Weber. “Research performed under DARPA’s [Reliable Neural-Interface Technology (RE-NET)] program and elsewhere showed that these nerves maintain motor and sensory fibers that previously innervated the amputated limb, and that these fibers remain functional for decades after limb loss. HAPTIX will try to tap into these biological communication pathways so that users can control and sense the prosthesis via the same neural signaling pathways used for intact hands and arms.”
The idea is that the HAPTIX program will investigate ways to measure and decode motor signals passing to and from the peripheral limbs and the muscles. Using technology for advanced prosthetics created for DARPA’s Revolutionizing Prosthetics program, HAPTIX aims to develop implantable sensors that provide tactile and proprioceptive feedback in such a way that it not only provides a sense of touch, but also of the prosthetic limb’s movement and position.
Because trying to restore a sense of embodiment may have a powerful psychological effect on patients, the HAPTIX program will include psychologists as well as engineers and medical specialists. This is especially important because the planned final phase of the project includes a year of in-home testing of the new technology on amputee patients.
“We have the opportunity to not only significantly improve an amputee’s ability to control a prosthetic limb, but to make a profound, positive psychological impact,” says Weber. “Amputees view existing prostheses as if they were tools, like a wrench, used only to perform a specific job, so many people abandon their prostheses unless absolutely needed. We believe that HAPTIX will create a sensory experience so rich and vibrant that the user will want to wear his or her prosthesis full-time and accept it as a natural extension of the body. If we can achieve that, DARPA is even closer to fulfilling its commitment to help restore full and natural functionality to wounded service members.”
If the program goes according to plan, DARPA will eventually seek US Food and Drug Administration permission for human trials with the ultimate goal of developing fully-implantable, modular, and reconfigurable neural-interface microsystems that communicate wirelessly with external modules to provide a link between prosthetic and wearer.