Has the potential to one day allow nerves to be connected directly to artificial limbs
Scientists at Sandia National Laboratories have announced a breakthrough in prosthetics that may one day allow artificial limbs to be controlled by their wearers as naturally as organic ones, as well as providing sensations of touch and feeling. The scientists have developed a new interface consisting of a porous, flexible, conductive, biocompatible material through which nerve fibers can grow and act as a sort of junction through which nerve impulses can pass to the prosthesis and data from the prosthesis back to the nerve. If this new interface is successful, it has the potential to one day allow nerves to be connected directly to artificial limbs.
It all sounds very simple as an idea, but attaching nerves to a mechanical limb isn’t like securing a wire to a terminal with a spot of solder. For one thing, you need a very special type of “solder” and that’s what organic materials chemist Shawn Dirk and robotics engineer Steve Buerger, working in collaboration with teams at the University of New Mexico and MD Anderson Cancer Center in Houston, are trying to create.
The interface that they are working on must be biocompatible. In other words, it mustn’t harm the nerves, which are notoriously delicate. The interface must also be able to interact with the nerves and that’s very difficult to engineer because, unlike in electronics, the nerves’ specs cannot be in any way changed, so the interface material has to carry the burden. The interaction has to be very subtle and has to carry thousands of nerve impulses of all kinds every second and it must do so accurately. While it is doing this, it also has to be very flexible, very fluid and very conductive.
This is a very tall order.
Creating the interface
The interface came about through Buerger’s original attempt to produce implantable neural electronic interfaces as part of a robotics approach to the problem. It soon became apparent that the heart of the problem was how to form an interface with the nerves themselves, so Dirk and his team were brought in. They took this problem down to the level of the material itself and turned to a technique called projection microstereolithography. This involves projecting a pattern of ultraviolet light on to a wafer coated with Polydimethylsiloxane (PDMS). This is a silicon-based organic polymer more commonly known as dimethicone, which is used in contact lenses, medical devices, shampoos, play putty and other products.
When subjected to microstereolithography, the PDMS forms a thin, porous membrane with holes only 79 microns in diameter. This provides a mechanically compatible scaffolding through which nerve fibers can grow. The addition of carbon nanotubes to the PDMS makes it conductive in a way that is highly controllable, so the basic interface could be formed.
All this talk about putty and nanotubes may seem a long way from anything practical, but it’s the final link in a very important story. The number of amputees in the world is unknown, but in the United States alone there are some two million people living with the loss of one or more limbs. Of these, 1,400 were US soldiers fighting in the recent wars in Iraq and Afghanistan. It’s a curious paradox that as advances in medicine and surgery save more lives, they leave behind more amputees who would previously have died of their diseases or injuries, especially among military and civilian casualties in wartime. However, thanks to advances in prosthetics, the loss of a limb does not automatically mean a life of confinement and dependence.