British bionics play a winning hand

Touch Bionics has employed world beating innovation to offer hope to those who have lost hands, arms or fingers. Paul Fanning reports.

A Scottish company is leading the world in the field of bionic prosthetic technology with an award-winning combination of advanced mechatronic design and leading edge control software.

Based in Livingston, Touch Bionics is a leading developer of advanced upper-limb prosthetics (ULP). Its history goes back to a programme of work conducted at the Princess Margaret Rose Hospital in Edinburgh from 1963, starting with comprehensive research into developing prosthetic solutions for children affected by Thalidomide.

The myoelectric principles on which electrical prosthetics are based have been well understood in the prosthetics industry for many years and, put simply, depend on the sensation described by many amputees that their hand still exists – referred to as 'phantom' feelings. When encouraged to generate a strong signal, the patient is often asked to move and flex the missing hand to generate a strong control signal to a prosthetic.

Such devices have, until recently, been fairly limited in terms of function. This changed following the 2007 launch of the i-LIMB Hand, the world's first microprocessor-controlled, myoelectric hand with five articulating digits (including a rotatable thumb) to offer users a range of grip patterns previously unavailable to them.

At the heart of this, according to Touch Bionics' Danny Sullivan, was the company's development of the articulating finger. "That was the true revolution," he says. "Prior to 2007, the industry standard for an electronic hand was a claw-like, pincer device. The fingers were completely rigid and essentially just supplied a pinch-type grip. There are several limitiations to such a design. It's not a human-like grip and these devices also offered only one strength of grip and made it impossible to grip any delicate objects without smashing them."

Touch Bionics' development of the articulating finger allowed a range of grip and dexterity functions to be made available to patients. These grip options enhance dexterity and support almost all daily living activities. For example, patients are now able to point the index finger to operate a PC keyboard, or to rotate the thumb to meet the side of the index finger to hold a plate or turn a key in a lock. None of these functions had been possible before.

The inclusion of a thumb that can be rotated into different positions means the grasp of the hand is much more like that of a human hand with the articulating fingers able to close tightly around objects. Built-in detection tells each individual finger when it has sufficient grip on an object and, therefore, when to stop powering. Individual fingers lock into position until the patient triggers an open signal via a simple muscle flex.

The key design challenge to the development of the articulating finger lay in making motors and gearboxes small enough to fit within the constraints of the human hand. Because each finger has its own motor and gearbox driving it, the technology had to evolve.

Developing systems for gearing the digits proved difficult as with most types of gearing, when the drive force is removed, the gears become free to move in the reverse direction unless a ratchet or braking system is introduced. The company therefore configured a worm and wheel gearing system in such a way that 'backdrive' problem would not arise. By avoiding backdrive, if the user closes the hand around an object and switches off the hand, ideal grip will be maintained.

A small-barrelled DC motor contained in each finger drives the digits. At first, the motor proved to have too much speed and not enough torque, but by experimenting with the ratios of the worm to wormwheel, a suitable compromise was found that allowed the fingers to move both quickly and powerfully.

In designing the body of the gearbox, it was necessary to minimise the size of its housing as thoroughly as possible. In practice, the plastic fingers fit onto or around the gearbox, which does all the hard work and forms the hub of each completed digit.

Touch Bionics also overcame the problem of holding delicate objects by devising a 'stall' function. Signals from the patient's muscles are processed by software and released to each motor in the hand, controlling their movement. As the motor moves, it draws current from a printed circuit board and a detection system causes the motor to stall when, during the cycle of its motion, it comes across an obstruction. Normally, motors that require such high-precision handling are fitted with a servo or encoder, but the hand is too small to contain such things.

Instead, the processor monitors the current drawn by each motor and if one or more motors hits the stall current value, the processor allows the motor to continue to stall and then cuts its power, maintaining the grip.

The launch of the i-Limb changed everything for the company. Since the launch of the i-LIMB Hand, more than 1,400 patients worldwide have benefitted from the technology. Quite how groundbreaking it is can be inferred from the fact that, since its launch three years ago, no competitive technology has emerged. A host of awards has also come the company's way, including Most Innovative Company of the Year in Europe in the 2010 International Business Awards, an IET Innovation Award in 2009 and Time Magazine naming the hand as one of the top inventions of 2008.

More importantly, perhaps, this success has allowed the company to develop other solutions. One of these has been ProDigits, which are designed specifically for users with partial hand loss, an affliction for which there was previously had no powered options available to patients. Touch Bionics has also acquired a US company Living Skin, which specialises in the development of high-definition silicone passive hand coverings.

The company's latest release is the i-LIMB Pulse, which has taken the technology several steps further. Constructed on a chassis of aluminium, rather than high-density plastic, the Pulse is stronger than its predecessor, whilst remaining as light. The new device also responds 14% more quickly than its predecessor, thanks to a new battery assembly that allows it to draw more power to the motors. The improvement that gives the Pulse its name, however, is its improved grip strength. Says Sullivan: "If users have the pulsing technology activated, it will force the motors in the finger to drive forward and provide additional strength and grip." This was not possible with the previous design.

Perhaps the most significant advance for the Pulse, however, is its use of software to refine and improve the use of different finger positions on the hand. Prior to the Pulse, if a different grip pattern such as an index point for using a keyboard was required, users would hold the finger while it was open and send a close signal to the hand and – because of the stall function – that finger would remain extended.

For the iLimb Pulse, however, the key innovation has been the invention of BioSim, a Bluetooth-enabled piece of software that allows both users and clinicians to connect to the Pulse and effect changes. This gives users the ability to automate many of the grip patterns used in day-to-day life. Says Sullivan: "These can now be activated in the software so that you could connect to the hand when you visit your prosthetist and from that you may determine that a pinch action would be useful to activate through the day. These features are activated by a series of triggers based on open/close signals. You would then need to send a range of signals that would be recognised by the software."

These signals come in several recognised forms; a common one being the 'hold open' signal that, after a set length of time, will automatically trigger the hand to go into a particular grip pattern. There is also a 'double impulse' or 'triple impulse' to quickly open one's hand two or three times to perform another action. Another is the 'Co-contract' signal to open or close at the same time. These four different triggers mean that users can have up to four different grip patterns available at any one time from a selection of eight.

Naturally, the software opens up a range of possibilities for the future, says Sullivan: "Rather than having to invent a whole new version of the hand, it could be possible to simply send out a software update to users to allow them to access different grip patterns. At the moment, the ability to effect different changes to the hand is limited by the number of inputs, so there is work going into looking at increasing the number of ways in which you can activate the different features."

Author
Paul Fanning

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