Shape-memory alloy 'muscle fibres' give a helping hand to prosthetics

Bundles of hair-thin shape-memory alloy (SMA) wires performing duty as muscle analogues could for the basis for a new generation of powerful, lightweight artificial hands both for industrial robots and prosthetics, according to engineers at Saarland University in Germany. The alloy wires can contract and relax, and also act as position sensors within the hand.

Artificial hands often depend on other external systems for their movement, such as electric motors or pneumatics.  This makes them complex, bulky, heavy and inflexible. ”In contrast, tools fabricated with artificial muscles from SMA wire can do without additional equipment, making them light, flexible and highly adaptable,” said research leader Prof Stefan Seelecke of the Centre for Mechatronics and Automation Technology (ZeMA) at Saarland.

The artificial muscles are composed of bundles of wire made from a nickel-titanium alloy; each wire being around 30µm in diameter. ”These wires have the largest energy density of all known drive mechanisms, which enables them to perform powerful movements in restricted spaces,” Seelecke said in a statement. The wires are arranged in bundles around the thickness of cotton sewing thread, with one bundle on the connecting the joints on the front side of each finger and another on the back. Each thread has the tensile strength of a thick steel wire.