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'Artificial muscles' help bones heal better

Professional Engineering

(Credit: Oliver Dietze)
(Credit: Oliver Dietze)

Smart implants that massage broken bones can promote healing, according to engineers and scientists at Saarland University.

"We're developing a smart implant that does not require any additional surgical intervention or additional equipment. An orthopaedic implant is typically a passive fixation plate that is used to set and stabilize the fractured bone. But we can now give it completely new capabilities," explains Professor Stefan Seelecke, who heads research groups at Saarland University and at the Center for Mechatronics and Automation Technology.

Take a broken leg, for instance. If all the fragments of the bone can be aligned the leg should heal properly—but about 14 per cent of the time this doesn't happen. And once the leg has been operated on, doctors can't look inside the leg to see how things are going until weeks later, when the first x-ray is taken. 

The new smart implant provides continuous information on how the fracture is healing. If a patient puts too much pressure on the fracture, it will give a warning, for instance. It can be more or less rigid as required, and undergo tiny motions that deliver 'micro-massages' to the fracture. This helps stimulate growth. And, it can all be managed using a smartphone. 

To work, the implant relies on 'shape memory' wires no thicker than a human hair. "We use these shape memory wires as mechanical actuators that can alter the local rigidity of the implant and can make it move or exert a force. But we also use them as sensors to monitor processes taking place at the fracture site," explains Paul Motzki who holds a cross-institutional professorship in smart material systems for innovative production at Saarland University and at ZeMA.

These wires can contract and relax like real muscle fibres depending on whether an electric current is flowing or not. "Nickel-titanium alloy is what is known as a shape memory material. At the level of the crystal lattice, the alloy can exist in two phases that can transform into each other," explains Paul Motzki. "If electric current flows through the wire, the material heats up, causing it to adopt a different crystal structure with the result that the wire becomes shorter. When the current is switched off, the wire cools down and returns to its original length. The engineers fabricate bundles of these fine wires, just as muscle fibres in nature are grouped into fibre bundles. By alternately tensing and relaxing the wires, the engineers can simulate the movement of flexor or extensor muscles." 

"The more wires we have, the greater the surface area and the faster we can dissipate heat, which means faster contractions," says Seelecke. The wires are able to exert a substantial force over a very short distance. "These wires have the highest energy density of all known drive mechanisms and they can deliver a substantial tensile force," explained Seelecke further.

But theses artificial muscles also have their own intrinsic sensor properties. "When the wires change shape, so too does their electrical resistance. We can assign precise resistance values to even the smallest of deformations, which allows us to extract sensory data," said Susanne-Marie Kirsch, who is doing doctoral research in the Saarbrücken group. The measurement data enable the team to monitor minute changes occurring in the gap between the bone fragments. 

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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