If a fracture doesn't heal as it should, these novel implants can respond mechanically at the fracture line by adjusting their rigidity or applying controlled micromovements to mechanically stimulate the tissue and promote bone regeneration. 

Currently, it often takes several weeks before doctors can see the first X-ray that will indicate whether a fractured bone is healing properly. Until then, the healing process is not monitored. This is precisely the gap that researchers at Saarland University aim to bridge. Professor Bergita Ganse, a medical scientist and expert in fracture healing, coordinates the "Smart Implants" project at Saarland University, funded by the Werner Siemens Foundation with 8 million euros. The goal is to develop customized implants that can continuously monitor and visualize, in vivo, the degree of success or failure of fracture healing. The implants are also designed to dynamically adapt to the healing process, becoming more rigid or more flexible as needed. In addition, they can actively promote bone regeneration by providing micromechanical stimulation at the fracture site.

Actuators with integrated sensor functionality at the fracture site

Professor Paul Motzki and his team are responsible for the technology incorporated into the implants. The engineers in Saarbrücken specialize in smart material systems—materials that offer inherent self-sensing and actuation capabilities. In collaboration with Professor Ganse's group, the engineers have already developed several prototypes of smart fracture implants, resulting in multiple patents.

At this year's Hannover Fair, Motzki's team is presenting a prototype implant capable of precisely monitoring and actively supporting the fracture healing process. First, it can measure whether new bone tissue is forming at the fracture site; the implant detects minute movements at the fracture edges that indicate healthy healing. "As new tissue grows, stiffness at the fracture site increases, and this progression can be read from the measurement data," explains Paul Motzki. The data can also indicate when movement is hindering the healing process—for example, if a patient is putting too much weight on the injured leg. The system allows for individually setting the permissible load limit.
Second, the smart implant uses a patented mechanism that allows it to mechanically adapt to the changing situation within the fracture space. Initially, when the bone fragments require firm support, the implant can stiffen and stabilize the fracture. As the healing process progresses, the implant can switch to a more flexible mode. Furthermore, by coordinating robotic microactuators, the smart implant can execute tiny movements—from gentle contractions to rapid vibrations—specifically designed to trigger growth processes. “Healing is faster when the fracture line is subjected to minute, highly controlled movements and when the tissue at the fracture edges is mechanically stimulated. These miniature oscillating movements, with a stroke length of between 100 and 500 micrometers, are usually sufficient to initiate tissue growth processes,” explains Professor Bergita Ganse.

How the Technology Works:

At the heart of the smart implant technology are bundles of ultra-thin nickel-titanium (nitinol) wires, which serve as both microactuators and integrated position sensors.
These wire bundles can contract, bringing the two sides of the fracture gap closer together, or—when necessary—they can relax to allow the gap to open slightly. This is made possible by the special properties of nickel-titanium, an alloy known for its shape-memory properties. Nitinol has two phases whose crystal lattices differ in length. When the nickel-titanium wires are heated, the alloy transitions from one phase to the other. If an electric current flows through the wires, the material heats up, causing it to adopt a different crystal structure, resulting in the wires contracting. Upon cooling, the wires return to their original position. Motzki's team harnesses this phase transformation to generate movement, using the wires as actuators.

“The nickel-titanium alloy has the highest energy density of all known drive mechanisms, so by using nitinol, we are able to exert considerable pulling force in very confined spaces,” explains Paul Motzki. The researchers use bundles of these ultrafine wires, since a greater number of wires means a larger surface area and, therefore, faster cooling rates. These shorter cycle times enable rapid movements at high frequencies. The movement of the wires can be controlled with great precision because the material is also self-sensing. This inherent sensing capability is due to the fact that the electrical resistance of the nitinol wires changes as soon as they are deformed. Each mechanical deformation of the wire can be associated with a specific resistance value. “We use this measurement data to train neural networks,” explains Motzki. Through large amounts of training data, the artificial neural networks learn to recognize characteristic signal patterns. “Our data-trained neural networks are able to calculate positional information efficiently and accurately, even under disruptive influences,” explains Paul Motzki, who, in addition to holding a professorship at Saarland University, is also the Scientific Director and Managing Director of the Center for Mechatronics and Automation Technology in Saarbrücken.

Thanks to the expertise gained through close collaboration with the Bergita Ganse team, the combination of smart implant technology and AI-assisted monitoring of the fracture gap allows medical teams to assess whether the stiffness of the fracture site increases over time, all without the need for X-ray imaging. The measurement data also enables highly precise control and movement of the wire bundles. Engineers can model and program specific movement sequences or simply hold the wires in any chosen position. In future clinical applications, the implant data will be wirelessly transmitted to a smartphone and controlled via the same device.
Research teams in Saarbrücken are determined to further miniaturize their technology, and this work is now supported by EU funding under the Horizon Europe program, as part of the €21 million SmILE (Smart Implants for Life Enrichment) research project.

Background

The research team based in Saarbrücken uses shape memory technology for a wide range of applications, from novel cooling and heating systems to robotic grippers. The technology continues to be developed in numerous doctoral and graduate research projects at Saarland University. The research is funded through several large-scale research projects, and the results have been widely disseminated in high-impact scientific journals, with numerous articles receiving international recognition.