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Researchers at Saarland University in Germany have developed a smart fracture plate and “artificial muscles” made from shape memory wires that allow healthcare providers to manage the fracture healing process through smartphone technology.
“We’re developing a smart implant that does not require an additional surgical intervention or additional equipment,” said Professor Stefan Seelecke, Ph.D., head of the Saarland University research group. “An orthopedic implant is typically a passive fixation plate that sets and stabilizes the fractured bone. But we can now give it completely new capabilities.”
Bones are dynamic tissue that grow and withstand a great amount of force. When a bone breaks, it can regenerate and heal if the fractured pieces are properly aligned. However, breaks that occur in the lower leg are at increased risk of improper healing, even after surgeons perform surgery to fix and set the fractured bone.
Fractures that fail to heal properly are often first identified when x-rays are taken several weeks after surgery. The team of engineers, computer scientists and medical specialists at Saarland University wanted to give surgeons an earlier indication of potential complications by creating a smart fixation plate that monitors the healing process and an artificial muscle that promotes bone growth at the fracture site.
The artificial muscle comprises ultrafine wires made of nitinol shape memory material. The wires contract like real muscle fibers when electric current flows through them and relax when the current stops. Tensing and relaxing fabricated bundles of the wires in alternating fashion simulate the movement of flexor or extensor muscles and exert a substantial force over a very short distance.
“The more wires that are used, the greater the surface area and the faster heat is dissipated when energy is applied, which creates faster contractions,” Dr. Seelecke said. “These wires have a high energy density and can deliver a substantial tensile force.”
The fixation plate can be made more rigid or less rigid when bone fragments need to be realigned to fill the fracture gap throughout the healing process. It can also be activated to undergo micro-manipulations that promote healing and bone growth.
Electrical signals are applied remotely to cause the wires to lengthen, contract or remain unchanged, which lets providers adjust the rigidity of the fixation plate as fractures heal. When the artificial muscle at the fracture gap becomes more difficult to contract, surgeons know that the bone tissue in that area is hardening and the healing process is progressing.
“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,” said Paul Motzki, Ph.D., who holds a professorship in smart material systems for innovative production at Saarland University. “But we also use them as sensors to monitor healing that’s taking place at the fracture site.”
When the wires change shape, their electrical resistance also changes. The researchers can extract sensory data by assigning resistance values to the various wire deformations to monitor slight changes that occur in the gap between bone fragments.
The fixation plate’s battery can be recharged wirelessly, allowing the device to provide a constant stream of information about how the healing process is progressing. For example, an alert will sound if the patient puts too much pressure on the fracture.
In the future, this data will be wirelessly transmitted to a smartphone, allowing providers to determine when mechanical stimulation at the fracture site is needed to promote healing. A smartphone could be used to activate a highly precise motion sequence for the fixation plate to perform that speeds the bone mending process.
The Saarland research team is interested in applying their research to commercial and industrial applications.
