"Interface scaffolds" could wire prosthetics directly into amputees' nervous systems

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Test structure inserted into lab rats as part of the program to develop neural interfaces for prostheses (Image: Sandia Labs)

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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.

Recent developments

Modern prostheses have come a long way since the days when the best that could be hoped for was a carved wooden leg. Modern artificial limbs benefit from a wide variety of lightweight plastics and composite materials that make them lighter, stronger and more comfortable, with custom-fitted attachments that fit the prosthetic to the body so closely that they're often held by suction alone. Some have sophisticated joints that, for example, mimic the movements of the
human knee. Others use electronics, hydraulics, pneumatics or the wearer's own muscles to power motors and gears that can make limbs flex and turn or
fingers grasp with such precision that they can pick up an egg without cracking it.

Some artificial arms can be covered with cosmetic skin so lifelike that it's almost unnerving to realize that it's made of metal and plastic rather than flesh and bone. There are even specialized limbs, such as curved springs made out of composite materials that can turn a man with no legs into an Olympic-level sprinter.

All of this is remarkable, but at the end of the day, these incredible bits of engineering aren't much more advanced than Captain Hook's hook or Long John Silver's peg leg. That's because no matter how exotic the materials or clever the design, they are still worn by the amputee while the arm and the leg they replace was an integral part of that person. While the prosthetic is worn by the amputee, it is ultimately still separate from them.

The difference between a real limb and a prosthesis is that a living arm or leg does what the person wants. We could say that it "responds to commands," but that isn't quite true. Because our limbs are connected directly to our nervous systems, we don't need conscious thought to make them go. The nervous system is like a series of extremely sophisticated triggers that set off complex actions with little more than intent on our part. Sometimes, when we touch a hot stove, for instance, intention isn't even needed as the nervous system yanks the burned hand away before we're even aware. It allows us to do things without thinking about it. I'm typing these words, but I'm thinking about what I'm writing, not what my fingers are doing at the keyboard. The nervous system does the hard work of moving the fingers to the right keys in the right order.

Artificial limbs don't have that sort of direct command. Worse, they have no sense of feeling. There's no touch. If a prosthesis is to be truly successful, it needs these attributes. That's why scientists an engineers have been working on the problem for decades. There have been a lot of advances and, from an engineering point of view, the artificial limbs of today are very good, but the tricky bit is providing them with the necessary degree of natural control and sensation. And it isn't a question of having one or the other. Both are necessary for the artificial limb to work as it should.

A way of understanding what's involved is to look at one of the early attempts at direct control, the Boston Arm, which was developed in 1969 by Dr. Melvin J. Glimcher, professor of orthopedic surgery at Harvard University and Prof. Robert W. Mann of the Massachusetts Institute of Technology. This was an artificial arm for above the elbow amputees with a motorized elbow joint and an articulated hook for grasping things. What made the Boston Arm novel was that it had sensors that attached to the stump of the wearer's upper arm. When the wearer flexed his bicep, the arm would pick up the electrical impulse the muscle made as it flexed and the arm would bend and rise. If the wearer flexed his triceps, the arm would straighten and drop.

We can rebuild him ...

So far so good, but the problem is, it's one thing to get the arm moving, it's another thing to get it moving the right way. Too little power and the arm won't move, too much and you'll smack yourself in the face picking up a spoon. Glimcher and Mann solved this by introducing a forced feedback system into the arm. There were strain gauges installed that told the arm how much effort was involved in lifting something. If the object being lifted was heavy, the voltage to the motors would drop and the wearer would have to flex his muscles harder to compensate by ordering the arm to increase the voltage. In this way, the wearer could learn how to lift anything from a cup to at ten-pound weight with confidence.

This early arm showed the importance of sensation. The simple feedback system of the Boston Arm was very successful and it even inspired the television series The Six Million Dollar Man, but it also paved the way for the artificial limbs of tomorrow. Already DARPA and scientists in U.S. and the U.K. are working on developing nanosensors that can provide sensations far beyond earlier, cruder attempts using micro-switches and the like. Eventually, it's hoped that these will lead to remote presence devices and artificial limbs, but the key to the entire problem is how to connect the nerves to the machines. Once that is licked, the rest follows. It will then be possible to build limbs that have a much more natural means of control and the necessary senses to allow the user to experience touch and the feedback needed to accurately control the device.

The current work at Sandia Laboratories is still in the proof of concept stage, but the stakes are very high. If they pan out and the gap between man and machine can be bridged, we could see the first true cyborg produced. Or, at the very least, the liberation of hundreds of thousands of people from physical limitations.

Source: Sandia Labs

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