Prosthetics

When a person loses a limb, such as hand, foot, leg, or arm, these days it is possible to replace that limb with a very life-like functional prosthetic.

Up until recently, even the simplest of these fake limbs have been incredibly expensive, but when 3D printers became inexpensive, it was suddenly possible to create your own 3D-printed replacement for the price of the plastic and wires it was made of.

In South Africa, a carpenter, Richard van As, accidentally sliced four fingers off his right hand. A myoelectric prosthetic (which senses muscle impulses and moves the prosthetic’s fingers) would have cost him tens of thousands of dollars, which was well out of his budget. After seeing a video about 3D printers, he got in contact with Ivan Owen, who worked in mechanical special effects in the US. Together, they designed a low-cost (about $500 at the time) 3D-printable hand that can open and close the fingers based on the movements of the wrist.

He then shared the design on the Internet for others to print. Within months, the simple design had been printed in hundreds of places all around the world, and the design itself was modified in many ways by other 3D print designers.. There are videos online showing children holding onto objects for the first time ever in those hands.

These home-made prosthetics are very advanced, compared to even the best that technology could supply only twenty years ago. But, technology has not been idle.

The most complex prosthetic hand available at the moment has about 30 possible movements, and with gloves on, you can’t tell which hand is real and which is not.

It is even possible to control prosthetics with the mind. The simplest method is to connect a signal sensor to a muscle that you would normally not use. For example, to control an arm, you might put a sensor on a muscle in the chest. Or to control the hand, you might put sensors in the forearm. The user then flexes those muscles, and the electronics pick up the signal and translate that as an instruction to move the prosthetic’s motors.

More advanced control can be made by connecting directly to the brain itself. In some experiments, scientists taught a monkey to control a robot arm using impulses from the monkey’s brain, and in others, human test subjects were able to control the arms of other humans.

The most significant advances recently came when technologists discovered how to translate sensations from sensors on the prosthetics into signals that the brain can interpret as touch, etc.

We can easily extrapolate forward only a few years to a time when it will be possible to download the design for a new hand, input some custom measurements to fit your own case, and have a personalised hand printed out, ready to replace the one you just lost somehow. You would just need to supply the motors and other sundries yourself, which would use off-the-shelf parts with common sizes, so there should be no difficulty.

We are also learning to build prosthetic eyes. The human eye typically contains four types of light detector – red, green, blue, and light level. To replace an eye, you need to emulate the output of these cones in the prosthetic and connect the prosthetic to the optic nerve.

The best we have managed so far is to give a person enough sight that they could vaguely make out shapes. Enough that they might be able to recognise a letter H, for example, but not enough that they could read a sentence. This modicum is a large advance over complete blindness, though, and is just the beginning.

With further work, it should be possible to connect to 100% of the optic nerves, possibly by using a device which would automatically recognise nerve endings and hook them up to the prosthetic, reducing the need for a human surgeon to manually connect each nerve.

A very interesting thing can happen then – the patient could be given back their sight as normal, or they could opt for enhanced sight, capable of seeing much more than is possible with the original human eye.

The human eye focuses by distorting a lens, which changes the focal length of the eye. However, the range of focus is limited. A typical person can see clearly between 10cm near and 100m far. Everything outside that is blurred or too small to discern.

A prosthetic, though, could be created that can focus microscopically or telescopically as the user desired.

A user that signals for the optic prosthetic to focus near, could trigger a microscopic view, for example, along with automatic stabuilisation so unconscious movements (such as those caused by blood moving through the veins of the head) don’t make it difficult to see.

Or, the user might focus far, triggering a telescopic view, where the eye focuses into the far distance, but also reduces peripheral sight and expands the central sight so that those distant objects that would normally look clear but small in normal good eyesight, are clear and large.