Tech Briefs

The polymer’s light weight and self-lubrication play important role in the hand’s design.

Researchers at the University of Southampton, UK, have chosen low-density thermopolymer to create various parts of the next generation of its innovative Southampton-Remedi prosthetic hand, which has been developed over several decades. Continued development and improvements on the hand have been the subject of several PhD programs over the years. The newest version is a low-mass, sensor-rich artificial hand designed to be a highly functional, adaptive prosthesis.

The Southampton-Remedi Hand uses a low-density thermopolymer to achieve low mass.

For this iteration, the researchers used Vesconite Hilube thermopolymer, manufactured by Vesconite, Johannesburg, South Africa. The thermoploymer offers a lower density compared with other alloys. This was an important factor in the decision to integrate the thermopolymer into the next iteration of the prosthetic hand.

“A main constraint in the design of a hand for the replacement of a lost natural hand is that its mass should be kept as low as possible,” says primary investigator Paul Chappell, an associate professor in the electronics and computer science department at the university.

To address the need for light weight, the Southampton-Remedi hand is constructed of carbon fiber sheets and Vesconite Hilube, with metals being used only on the actuators of the electric drives, he says.

In addition to using the thermopolymer for the prosthetic hand’s thumb, the researchers also used it as the bearing material for the ends of the worm and for the wheel shafts at the base of the fingers and thumb. According to Chappell, the polymer’s self-lubricating properties are critical for this application because they ensure that the gearbox does not require additional bearings at the end of the shafts.

Vesconite Hilube incorporates an internal lubricant that translates into an exceptionally low friction coefficient. Combined with excellent dimensional stability, low wear rates, and a high load-bearing capacity, Hilube has a PV rating (pressure × velocity) four times that of nylon bushings.

The Southampton-Remedi hand has four motors that move the fingers and two motors that allow for flexion (movement toward the palm) and extension (movement away from the palm) as well as rotation of the thumb. The hand can grip and grasp objects securely and, when electrical power is turned off from the batteries, a stable grip is maintained using worm-wheel gearboxes. In addition, the current generation of the hand also incorporates touch, position, slip, texture, and temperature sensors.

“Battles have resulted in hand loss, and this trauma has led to the development of artificial replacements,” says Chappell. “In the sixteenth century, Götz Von Berlichingen, who was a German warrior, and Ambriose Paré, a French surgeon, made hands from metal components,” he says of the early origins of prosthetic hands.

Various hand prosthetic developments followed, including the split hook — a device that attached to the shoulders with leather straps that used the shoulder muscles to open the hand against a spring. World War I and World War II, as well as current conflicts, have seen advancements in the design of prosthetic hands, adds Chappell. Southampton University has been at the forefront of significant work on artificial limbs and is well known for the Southampton Hand Assessment Procedure, which assesses hand function.

For more information, visit here and here.