For people who have suffered an amputation of a hand, the current generation of prostheses can only go so far to replacing what they've lost. They can restore some of the aesthetic and motor properties of the absent body part, but what's still missing is a sense of fine touch that can tell, for instance, whether something is rough or smooth. And this is critical for being able to manipulate objects correctly. Now we're a step closer to being able to restore this missing modality thanks to a bionic finger that uses piezoresistive sensors to detect surface textures and turn them into nerve signals that an amputee's brain can understand. Calogero Oddo is its creator, and she spoke about it to Chris Smith.....
Calogero - The natural skin has the receptors internally that are the transducers. The physical interaction of our hand with the external world, they transfer mutual sequence of neural signals that are then conveyed to the nerves and then arrive through the nerves up to the brain, and then represented tactile information to our brain so that we can have a percept.
Chris - What is the material that you're working with that is capable of discerning textural features or how rough the surface is? What are you making these things from?
Calogero - The system is multilayer so we have this rubber that covers the sensors. The sensors are made of silicon which is like glass which has, inside, piezoresistors that transform mechanical information so the external information into the electrical signal. Then this is acquired by embedded electronics that converts this information into digital and then there is an artificial neuron model that keeps the continuous waveforms that are acquired from the artificial sensors. And that transforms into the spikes that are those on/off events to stimulate the nerves.
Chris - How do you teach your computer programme what each surface feels like and then how do you marry its recognition of that surface, and its output of spikes into the nervous system so that it’s putting them into the right pattern of spikes that correspond to a surface, which is what would happen in an intact individual, someone with a normal hand?
Calogero - This is done by microneurography – the technique that is minimally invasive to record from the nerves so the intact subjects when they touch naturally the surfaces. By means of this technique, we are learning how the natural code is arranged in correspondence to particular experiences and then we try to emulate.
Chris - So in essence, what you do is you're learning what the surface feels like to your particular system by recording what sorts of patterns of nerve activity a person gets when they run their natural skin over the same surface. And then when your new detectors experience that same surface contour, you just generate the same nerve output of spikes that a normal person would and that’s what you're going to put into the nervous system.
Calogero - Yes. This is exactly what we are doing. We try to capture the secrets of the natural sense of touch and to engineer it in an artificial manner so we developed a bionic finger. The bionic finger is able to reproduce to some extent the natural patterns so that are generated by the natural finger. The nice thing is that our finger is not specialised to the surfaces that we tested in this experiment. But in principle, if we test other surfaces, it is able to generalise. So, it is able to reconvert the other surface into a different sense of the spikes that the brain can interpret and recognise.
Chris - If you actually do the experiment and you take the output from your system and you apply it to the nervous system of an individual, does it really feel to a normal human that that surface is the one that they should be feeling?
Calogero - Of course, not 100 per cent but it makes sense to the brain of the subject. This was declared by Dennis, one of the amputee soldier that tested the technology. He said exactly this, not 100 per cent but it makes sense to his brain, meaning that he was able to interpret the stimulus and to imagine the stimulus in pictures.
Chris - How do you get the signals from your bionic finger back into the nervous system of the amputee in the first place?
Calogero - There was an implanted interphase that allowed the stimulus to be delivered to the nerve. What we do is insert into the nerves electrodes like wires to electrically connect the nerve and to be delivered at the stimuli.