Tying shoelaces, folding bed linen, tearing open a bag of potato chips - the list of activities that are difficult or impossible to do with one hand is long. People who have lost a hand due to an amputation or an accident are confronted with such hurdles on a daily basis. Better and better prostheses are coming onto the market to make their everyday lives easier.

Most of us are only familiar with functional prostheses from science fiction films, in which artificial limbs are given superhuman powers. In real life, however, bionic hand prostheses do not turn their wearers into superheroes, but they can enable them to perform many activities that other people take for granted. The British company Steeper has developed the small myoelectronic hand prosthesis Bebionic for this purpose. It is controlled by electrical signals. These are generated by muscle contractions and can be measured with electrodes on the skin, similar to an ECG in cardiac diagnostics.

Two electrodes integrated into the prosthesis shaft detect the myoelectronic signals - i.e. the electrical voltage generated in the muscle cells in the microvolt range - and transmit them to the control electronics. The latter amplifies these signals and uses them to activate five small electric motors, which then move the fingers and thumbs; the hand opens or closes. The strength of the muscle contraction determines the speed and gripping force: a weak signal produces a slow movement, a strong signal a fast one.

The muscles whose signals are used to open and close the prosthetic hand are normally responsible for moving the wrist. The wearer of the prosthetic hand must therefore learn that they now have a different function. "The human brain is incredibly adaptable. After just a short time, people perform the movement just as intuitively as drivers slam on the brakes when they want to stop," says Ted Varley, Technical Director at Steeper.

More motors for more control

The first myoelectronic hand came onto the market in the early 1980s. It was powered by a single motor and only had a simple gripping mechanism: the thumb, index and middle fingers could be closed to form a pincer grip. The ring finger and little finger were only there for cosmetic reasons and had no gripping power. This concept was fundamentally changed around ten years ago for the Bebionic hand. "We found that people accept less gripping force per finger in exchange for more flexibility," explains Ted Varley. In order to be able to control the fingers individually, each finger of the Bebionic hand is therefore equipped with its own electric motor. Four of the finger motors are located in the palm area, the fifth in the thumb itself. Encoders are integrated into the motors in order to precisely record the position of the fingers at all times.

... are taken for granted by other people.

© Steeper

The fingers can be arranged into a total of 14 different grip patterns using the individual controls. The key grip, which moves the thumb up and down when the fingers are bent, can be used to hold flat objects such as plates, keys or cheque cards. The hook grip can be used to carry heavy loads of up to 25 kg, while the outstretched index finger allows the use of keyboards and remote controls. If the thumb is in the opposition position and all fingers are closed until they encounter resistance, this results in the power grip. This is used to hold irregularly shaped objects such as wine glasses. "This position looks much more natural than a pincer grip. The grip is also more stable when all fingers are used," emphasizes Varley.

The Bebionic wearers also use their arm muscles to switch between the individual grip patterns. If they give another open signal when the hand is already open, the prosthesis switches to the next mode. An additional signal is provided by the thumb, which can either be moved sideways to the fingers with the biological hand or brought into the opposite position. Depending on which thumb position is selected, different grip patterns are available. Hand owners can decide for themselves which of the 14 possible gripping patterns they want to use and in which order they are called up. They can also program the prosthesis individually themselves using software.

"It's often the little things that are made easier by the prosthesis. But all in all, they lead to a significantly improved quality of life," says Ted Varley. The artificial hand also has a major psychological effect: "Many prosthesis users report that their self-esteem has increased with the Bebionic, as they are interested and fascinated by their new high-tech prosthesis." The attractive design of the prosthesis also plays an important role in this context; the use of aluminum and stainless steel ensures an appealing appearance. In addition, the external shape was modeled as closely as possible on the natural model.

Tailor-made drive for the thumb

The artificial hand weighs between 400 and 600 g and is therefore about as heavy as the natural hand. The design of the prosthesis plays an important role. The external shape is adapted as closely as possible to the natural model.

© Steeper

"Our approach to developing the third generation of Bebionic was rather unusual in prosthetics: we first developed the housing and then looked for solutions for accommodating the individual components in it," emphasizes Varley. "Five years ago, this approach would not have been possible for such a small hand - the technology was not yet ready for it." The DC micromotor from the 1024 SR series, which was predestined for this application, was also still in the development phase when Steeper approached Faulhaber with his project in 2013. The project teams on both sides then pushed ahead with the development of the motor series and hand prosthesis at the same time. This intensive collaboration ultimately resulted in a motor with an exceptional power-to-volume ratio and a customized drive for the thumb.

The DC micromotor in question delivers a holding torque of 4.6 mNm with a diameter of 10 mm and a length of 24 mm. Thanks to its flat speed/torque curve, it also has a consistently high torque over the entire speed range. This strong performance has been made possible, among other things, by the development of a new coil design that contains 60% more copper than its predecessor and is combined with powerful rare earth magnets. In order to ensure the quietest possible movement, the motors are equipped with tailor-made 10/1 series planetary gearboxes.

"Another particular challenge was the development of the linear drive system, which had to be integrated into the thumb," adds Tiziano Bordonzotti, Sales Engineer at Faulhaber Minimotor. By using a high-precision four-point bearing from Faulhaber subsidiary Micro Precision Systems (MPS), it was possible to make the drive significantly shorter than usual at 49 mm in length. The special properties of the four-point bearing enable it to withstand the high axial forces of up to 300 N required for the application despite its small dimensions.

Author:
Ellen-Christine Reiff is an employee of the Stutensee editorial office.