What the word covers
A neuroprosthetic is a device that connects with the nervous system, either to send signals into it, to read signals from it, or both. This is a wide category. A cochlear implant that stimulates the hearing nerve counts, and so does an experimental artificial hand wired to nerves in the arm. The focus here is on nerve-connected devices and what research has shown, with the caution that most advanced examples remain experimental. Devices that read activity directly from the brain are a related but separate branch of research.
Electricity and nerves
The basic idea that electricity affects living tissue goes back to the eighteenth century. Luigi Galvani’s experiments with frog legs, published in the 1790s, suggested that electrical stimulation could make muscle twitch, and Alessandro Volta’s work, including the voltaic pile, soon followed. Through the nineteenth century, physicians tried electrical treatments for many conditions, with mixed and often unproven results. More solid knowledge came in the early twentieth century, when physiologists mapped how nerve impulses travel and how muscles respond.
Stimulation devices
Several devices that act on the nervous system became clinical tools in the second half of the twentieth century.
- Cochlear implants. In 1957, the French researchers André Djourno and Charles Eyriès placed an electrode on the hearing nerve of a deaf patient, who reported hearing sounds, though not speech. Work in the United States and Australia led to devices with several electrode channels in the 1970s and 1980s, and later to widely used systems that give many people some understanding of speech. They show that an artificial signal can give a useful, if imperfect, sense.
- Functional electrical stimulation. In 1961, the Slovenian engineer Wladimir Liberson described stimulating a nerve in the leg to lift a paralyzed foot during walking, an early use of stimulation to restore movement. Later systems used implanted or surface electrodes to stimulate muscles for people with spinal injury, with limited but real functional gains in some users.
- Deep brain stimulation. From the late 1980s, electrodes implanted in deep brain regions were used to reduce tremor and other symptoms of movement disorders, and the treatment has since been studied for other conditions.
- Pacemakers and implantable devices. Electrical pacing of the heart belongs to the same broad family, though the target is heart muscle.
Reading signals from nerves
For artificial limbs, a nerve connection could provide control signals for a prosthesis and a path to feel it. Researchers developed several interfaces: cuff electrodes that wrap around a nerve, needle-like electrodes that penetrate it, and flexible arrays that sit between its bundles. Cuffs are more stable and less damaging but record less detail, while penetrating electrodes get finer signals but can irritate tissue.
An important surgical approach was targeted muscle reinnervation, developed in the early 2000s. Nerves that used to control the missing hand are redirected into spare muscles, which then serve as biological amplifiers for the signals. It showed that intuitive control of a powered limb was feasible without electrodes on the nerve itself.
Giving a sense of touch
Another goal is to restore feeling. Because a nerve carries information from the hand, stimulating it can create a sensation in a missing hand. A study published in 2014 described a man with an arm amputation who, with electrodes on nerves in his upper arm, could feel sensations from sensors in a hand and use the information to judge the stiffness and shape of objects for a short time. Later studies at American universities reported that participants could use feedback from implanted nerve cuffs for periods of months or more, and that it could improve performance, for example by reducing how hard people looked at their hand while grasping. Many of these devices used external cables and laboratory equipment, so they were not suitable for use at home.
Sensors on the hand for this purpose can be built from artificial skin and similar materials, a field explored in other research. Combining that skin with nerve stimulation is an active area, but evidence is limited to small studies.
What has and has not been shown
By the 2020s, researchers had shown that:
- stimulation of the auditory nerve can give hearing, and this is used clinically
- stimulation of nerves can produce touch-like sensations in an amputated hand, in small studies
- nerve or muscle signals can give a prosthetic hand intuitive control in laboratory settings
What has not been established is that these systems work reliably for years in everyday life for large groups of people. Implanted parts can fail, scar tissue can alter signals, and wires that pass through the skin raise the risk of infection. Wireless, fully implanted systems are being developed. Costs and the need for specialist teams are also barriers. A cautious reading of the history is that neuroprosthetics has delivered a few clear clinical successes, especially in hearing, and a growing set of promising experiments elsewhere.