British companies take the lead in advanced prosthetics

The field of advanced prosthetics moves fast. When <i>Eureka</i> reported on Scottish company Touch Bionics' i-Limb technology in 2010 (<a href="">see here</a>), the product seemed extraordinary enough. However, less than three years on, the technology of advanced prosthetic hands has moved on a long, long way. What is more, two of the world leaders in this field are British.

The i-Limb Ultra and i-Limb Ultra Revolution from Livingston-based Touch Bionics are the latest generations of active prosthetic hands from the company. Although based on the i-Limb Pulse product featured in Eureka's 2010 article, they now offer a range of additional functionality. In the case of the Revolution, this includes a powered rotating thumb and individually articulating fingers offering unparalleled dexterity and reliable access to precision grip patterns. New remote electrodes offer a higher level of sensitivity giving the wearer enhanced control, while a variety of flexible wrist options enable more natural positioning of hand when gripping or picking up objects. The powered rotation of the thumb can be either controlled directly by the wearer's muscle signals, or can move automatically into position as part of a pre-set grip pattern or gesture. Probably the most notable feature of the Ultra Revolution, however, is the biosim mobile control application. Compatible with the latest Apple products, this gives the wearer greatly expanded control capability via 24 Quick Grips – each from a single screen tap. This collection of grips is editable and can be customised by the users for daily needs. For example, wearers can select the 'work' favourite, which triggers the i-limb into preferred grips such as typing, holding papers, or using a mouse. In addition to its value as a means of selecting grips, this app also offers a Hand Health Check, which activates the prosthesis' diagnostics to ensure that the i-limb is functioning properly. It also allows users to access training modes to help the wearer learn how to access all of the functionality available. Bertolt Meyer, who recently presented the Channel 4 programme 'How to Build a Bionic Man' and is a wearer of the i-Limb Ultra, describes the Ultra Revolution as "The most advanced, and easy-to-use prosthesis that I have ever worn. Powered thumb rotation, combined with the mobile app and quick access to all these new grips, gives me natural hand function that I never imagined would be possible." Another British company that also lays claim to offering the most advanced active prosthetic hand is Leeds-based RSL Steeper. In fact, its bebionic3 hand recently won a da Vinci award. Like the i-Limb, it is myoelectric, meaning it is controlled by electrical impulses from the patient's remaining forearm muscles. Each finger is controlled by an individual motor controlled by microprocessors that allow the wearer to operate 14 grip patterns and hand positions. The hand can automatically adjust its grip to match the task or if it senses that an object is slipping, while the fingers fold away realistically when brushing against people or objects. The wrist joint also comes in a variety of forms to suit individual patient needs. Ted Varley, RSL Steeper's director of development and operations, describes the limitations of mopre basic myoelectric arms. He says: "There is a sensor on the inside and on the outside. The battery and wrist interface are mounted in the hard outer casing and, with training, a user is able to give an open signal, a close signal and what is known as a co-contracted signal, which is basically a fist that fires both electrodes... It's a simple three-jaw chuck design with a fixed linkage between fingers one and two and the thumb and it just goes through a standard pinch system. You can get a fantastic grip with something like that. It's very powerful, but it's not compliant. So if you're trying to hold a wine glass or a plastic cup, it's very difficult to get control. Also, it's not able to grip thinner objects like umbrellas because there's a big gap, meaning that people have to add foam to everyday objects in order to hold them." The aim from RSL Steeper's point of view was to create something very much more sophisticated. Says Varley: "The design idea we came up with was to take that system from the wrist and push the boundaries in terms of the capability of the hand. So we wanted to make it as natural as possible so that it looked like a hand. We also wanted to make it compliant so that you could get a nice, stable grip on objects. It has more advantages than just looking good." Clearly, the human hand is an incredibly complex thing and this creates immense design challenges. This meant that the development team had to dramatically simplify the motions of the hand. Says Varley: "There are a limited number of grips that will give you 50-60% of functionality. The fingers on this move in a simple trajectory. They can get a pinch grip, but they can also roll into a fist. We rotated all the trajectories inwards so that, rather than finger adduction [the movement of the fingers apart and together] they roll together as it creates a fist, allowing the user to get a partial grip on a pen or a piece of cutlery." Another key issue in the design process was ensuring that the bebionic3 was extremely robust. Here, though, the team initially made what Varley describes as "a fatal error". He explains: "We looked at our own hands and tried to decide what it would take to break a human hand. This is a mistake for a prosthetic. The issue is that the hand is compliant. So, if you're falling over, your hand automatically opens to form the shape of the thing you're going to hit. This spreads the load over the full palm, reducing the force. The problem with the prosthetic is that it's hard, so the first thing that hits the surface is going to break, which is no good." At the same time, of course, a prosthetic hand does not offer feedback (ie pain) when it suffers a negative impact. The only feedback offered by a prosthetic is when it breaks. Having not initially appreciated these factors, Varley's team had designed a product with all the expensive high-precision motors, PCBs, etc. safely locked away in the palm and as close to the body as possible. Meanwhile, the fingers were made out of lightweight plastic, with the idea being that if one broke, it could be replaced easily. Says Varley: "We also made fuses that broke at a certain level, so if you broke a finger, you could put a fuse in with a pair of pliers and your finger's back up again. That's fine if you're technically-minded. If you're not, though, and you've only got one arm and you're a plane ride away from the clinic where it's available, then it's clearly unacceptable." Instead, then, the design shifted to a 'belt and braces' solution, changing the chassis structure to CNC machined aluminium, with two knuckles that fully integrated within it, making them rock solid. The proximals are also made out of stainless steel, with stainless crosslinks and sealed cartridge bearings on all fingers. The end result, says Varley, is an incredibly strong and robust product able to operate far beyond normal human capability. "We went from 16 kilos, which is what we think would break the human hand, to 40 kilos to break one finger," he says. "So with two fingers, you can pick up someone's bodyweight. It's beyond normal usage. We've also got a full metallic structure on the thumb, so it's beyond normal capability… It's so strong that if you break, you have to be doing something wrong." Another key design issue lay in how to enable users to access the full range of grip patterns. The decision, therefore, was to use the two manually-operable thumb positions to set the grip. "You can knock it to opposed, give it a close signal, open it again and get a secondary grip," says Varley. "You can then move the thumb back to non-opposed, give a close signal and again you have a different grip. So from that you can get four grips. There's a button on the back of the hand and if you knock that, you get another four." In addition, the hand has a 'push and hold function that makes it go into glove mode. This was initially developed to allow the silicone 'skin' for the hand to be put on easily. However, it soon became apparent that the function was also useful for things like putting jackets on easily. The hand also offers users the ability to offer 'threshold' and 'proportional' grips. In other words, if the user has not got particularly good control of their muscles, the threshold system enables it to go at maximum speed once a signal has reached a certain level. Equally, the proportional function operates by allowing a user to give a weak signal to make the fingers go slowly or a strong one to make it go faster. Clearly, the control system behind this is complex. Says Varley: "The way we did it was with an encoder system whereby every time a motor rotates it counts. With that, we could build up a matrix so that for every grip there is an encoder value for each fingertip position. So if you imagine tripod, which is the three finger grip, those independent motors have got to converge at the right time again and again and again." For all that it is highly advanced technology, Varley is still very much aware of the limitations of the Bebionic3 compared to the human hand. He says: "We've only gone a part of the way along this road. This still has only 10 or 15% of the functionality of a human hand, to be honest." With this in mind, there are a number of improvements in the pipeline. These include ongoing work with a team that makes RF chips that react to proximity of a transceiver. This will allow users to attach these tiny stickers everywhere so that, for instance, in the proximity of the keyboard, the hand will immediately go into finger-point mode or, with a sticker in the lapel of the jacket, the hand will automatically go into 'glove mode' so that it can go into the jacket smoothly.