Bionic exoskeleton could transform lives of paraplegics

5 min read

It is a truism to point out that the ability to walk is one that most of us are fortunate enough to be able to take for granted. However, if anything is likely to cure one of complacency about this ability, it is an appreciation of the sheer engineering difficulties posed by any attempt to replicate this seemingly mundane activity mechanically.

This fact is starkly brought home by the ReWalk, a lightweight, powered exoskeleton that allows paraplegics (those paralysed from the chest or waist down who are more usually obliged to use a wheelchair) to stand, walk, and take stairs themselves. Worn around the legs and torso, the device works using a combination of motion sensors, electric actuation motors, sophisticated control algorithms and real-time software running on on-board computers – all controlled by a wristband. The wireless control unit allows users to choose modes for walking, sitting, climbing and descending stairs. There are four servomotor units in the knees and thighs. These power gears and levers to drive additional motors that bend the joint and feed back information on the angle through which it moves. Each servomotor is controlled by a microprocessor governed by a small central computer that is carried with batteries in a backpack. While it is not new, the device has recently been given particular prominence by Claire Lomas, who used it this year to complete the course of the London Marathon, a feat which also saw her given the honour of lighting the celebration cauldron in Trafalgar Square before the 2012 Paralympics. A former event rider and chiropractor, Lomas suffered a riding accident in 2007, which left her paralysed from the chest down. Refusing to accept that a wheelchair was her only option, she saw a video of the ReWalk system on the internet. From there, she viewed the system at an exhibition and went on to use it at the East Yorkshire-based Cyclone clinic, where she went on to train for the Marathon. She has now become the first person in the UK to take one of the devices home (it having previously only been available in an institutional model). Lomas is clear that the impact the device has had on her life, saying: "I am able to stand, walk, talk to my friends and family eye-to-eye…one of the greatest experiences is to be able to stand at a bar – I'm no longer excluded from any social event." The ReWalk story began in Israel in 1997, when mechanical engineer Dr Amit Goffer was left tetraplegic by a road accident in 1998. He says: "I'm not a great believer in wheelchairs. Confinement to a wheelchair causes lots of problems, with the biggest one being the loss of dignity…so, after my accident, when I stopped all my medicines and the IQ came back to an acceptable level, it was natural for me to go looking for a solution. This solution eventually became (with the help of industrial design consultancy Taga) the ReWalk system. Dr Goffer makes it clear that, despite media references to 'robot legs', this is not a robotic device, but instead one that offers the user independence and autonomy. "The person must be in charge of inititating, stopping and setting the device," he says. The natural place to start, he felt, was with powered exoskeletons. This was an area that had seen considerable investment and research from the US military for the Augmented Performance Human Being (Supersoldier) programme to allow soldiers to perform beyond normal human limits or abilities. One of the outcomes of this was the HULC (Human Universal Load Carrier) developed at Berkeley for Lockheed Martin, which in turn gave birth to a rehabilitative device for healthcare called eLegs (exoskeleton lower extremity gait system). However, ReWalk is the first system of its type to be available for individual use. At the heart of the ReWalk system is a tilt sensor. This serves to sense shifts in the wearer's balance by, for instance, showing their desire to take a step forward. This then triggers the suit to mimic the response that the joints would have if they were not paralysed. Says Dr Goffer: "The user shifts the crutches forward and the tilt sensor 'feels' the tilt of the device and then triggers the forward step – but only one step. The user then has to move again to trigger the next step. The user is therefore in control of every movement, but shouldn't have to think too much about the walking itself – it should come naturally. This also solves any safety issue. If there is no tilt, the device reverts to a standing position." Although it requires the use of crutches, Dr Goffer is at pains to make it clear that they are very much a part of the overall design, offering not only support, stability and safety, but also a very important facet of the walking process. He says: "The crutches are necessary because they give so much for nothing. Giving up the crutches is a very complicated problem because it means replacing so many muscles that are simply not there." Safety, of course, is a major consideration in the overall design of the ReWalk. Says Dr Goffer: "Your users are in a wheelchair or handicapped and you cannot therefore offer a device that is not 100% safe. If ithat means getting to market a year later, then so be it." A number of features exist to ensure this level of safety. For instance, when climbing stairs, the user must press 'Acknowledge' on the control unit to ensure that their feet are in the right place or adjust their position using the crutches. Only then can they press for the next step. Another consideration is the need to avoid any damage to the user's legs when reverting to a standard position when there is no tilt sensed. "The software is aware of the centre of gravity of the individual and the situation of the legs," says Dr Goffer. There is also a 'soft mechanical' obstacle in the gearing system designed to activate when it encounters undue resistance. In the event of a catastrophic failure in the system such as a wire getting cut, the device is designed to return the user to a sitting position. However, it is clearly important that it does not do this too precipitously, so the device defaults to what Dr Goffer describes as "a graceful collapse" or "baby fall" that slowly brings the user to a sitting position, giving them ample time to seek assistance or make other necessary adjustments. Also key to the system's value is its long battery life. This was a key element in the design as far as Dr Goffer was concerned, as it is his goal that the system becomes a replacement for wheelchairs as opposed to merely being used in addition to them. For this to be the case, the system obviously needs to be able to function for a full day. "My drive was to create a device that works from morning to night and can then be charged overnight," he says. The original device used a heavy gel/lead battery. Subsequently, lithium ion was used for the main battery and lithium polymeric for the auxiliary battery. The main battery was originally 2.4 Amps/hr, while the more up-to-date system is 2.8 and the next, according to Dr Goffer, will be 3.1 or 3.2. Eventually, he believes, fuel cells will be used to power the device. A bitter irony of the ReWalk in its current incarnation is that its inventor cannot use it. This is because he is tetraplegic and lacks the requisite upper body strength. However, he makes it clear that he does not intend that this should be the case in the future, saying: "This is only the beginning of the device's development. What about tetraplegia? What about quadriplegia? Hemiplegia? What about Multiple Sclerosis? Children? The list goes on and on." Dr Goffer is also refreshingly frank about what he perceives to be the shortcomings of the device at the moment. "My nature is that I'm never satisfied in terms of engineering," he says. "For instance, the wrist-based wireless remote control was bulky and I changed it to this watch-type device. I don't like that either because it's awkward, so I'm looking at introducing more intuitive control systems such as buttons on the side. I'm also looking at speech control. "The more advanced one we're selling for personal use is still in my opinion too bulky," he continues. "It should be possible to wear it easily and it should be faster – and that's where we're going. Ours is currently the fastest one around at 2.5km per hour, but we want to get that up to 4.5km/hr. It's also a very bulky device. The footprint of the device is huge at the moment. But that is going to change." For all these criticisms, however, the ReWalk nonetheless remains a remarkable device that is genuinely capable of transforming wearers' lives. On this, Dr Goffer has the final word, saying: "Limited mobility is a daily struggle involving loss of self-esteem and dignity… Our mission is, by changing their status from a wheelchair user to a crutch user, to change the life of the user dramatically."