Banebrydende mikroaktuator kan forbedre knogleheling
I forbindelse med Hannover Messe demonstrerer Saarland Universitet en mikroaktuator implantat med indbyggede sensorer, der kan forbedre knoglehelingsprocessen.
At present, it usually takes several weeks before clinicians can view the first X-ray image that will tell them whether a fractured bone is healing as it should. Until that time, the healing process remains unmonitored. This is precisely the interval that Saarland University researchers aim to bridge.
Medical scientist Professor Bergita Ganse is an expert in fracture healing and coordinates the ‘Smart Implants’ project at Saarland University, which is funded by the Werner Siemens Foundation with €8 million.
The goal is to develop customized implants that can continuously monitor and visualize in vivo how well, or how poorly, a fracture is healing. The implants are also designed to dynamically adapt to the healing process by becoming stiffer or more compliant as required. They can also actively promote bone regeneration by providing micromechanical stimulation of the fracture site.
Actuators with built-in sensor functionality at the fracture site
Professor Paul Motzki and his team are responsible for the technology embedded in the implants. The Saarbrücken engineers are specialists in smart material systems, i.e. materials that offer inherent self-sensing and actuation functionalities.
Working together with Professor Ganse’s group, the engineers have already developed several prototypes of intelligent fracture implants, resulting in multiple patents.
At this year’s Hannover Messe, Motzki’s team is presenting a prototype implant that can precisely monitor the fracture healing process as well as actively support it. First, it can measure whether new bone tissue is forming at the fracture site; the implant detects the minute movements at the fracture edges that are indicative of healthy healing.
- As new tissue grows, the stiffness at the fracture site increases - and that progression can be read from the measurement data, explains Paul Motzki. The data can also show when movement is harming the healing process, for example if a patient places too much weight on the injured leg. The system enables the permitted load limit to be set individually.
Second, the smart implant makes use of a patented mechanism that enables it to mechanically adapt to the changing situation at the fracture gap. Early on, when the bone fragments need firm support, the implant can stiffen and stabilize the fracture.
As the healing process advances, the implant can switch to a more compliant mode. In addition, by coordinating robotic micro-actuators, the smart implant can execute small movements - from gentle contractions to rapid vibrations – designed to specifically trigger growth processes.
- Healing is faster when the fracture gap is subjected to tiny, highly controlled motions and when the tissue at the fracture edges is mechanically stimulated. These miniature oscillating movements with a stroke length of around 100 to 500 micrometres are often enough to initiate tissue growth processes, explains Professor Bergita Ganse.
How the technology works
At the heart of the smart implant technology are bundles of ultrafine nickel-titanium (nitinol) wires, which serve as both micro-actuators and as built-in position sensors. These wire bundles can contract and thus pull the two sides of the fracture gap closer together, or - when necessary - they can relax to allow the gap to open up slightly.
This is made possible by the special properties of nickel-titanium, an alloy renowned for its shape-memory properties. Nitinol has two phases whose crystal lattices differ in length.
When the nickel-titanium wires are heated, the alloy switches 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 with the result that the wires contract. After cooling, the wires are returned to their starting position. Motzki’s team exploits this phase transformation to generate motion - using the wires as actuators.
- Nickel-titanium alloy has the highest energy density of all known drive mechanisms, so by using nitinol, we’re able to exert a substantial tensile force in very small spaces, explains Paul Motzki.
The researchers use bundles of these ultrafine wires, as more wires mean a greater 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 very precisely because the material is also self-sensing. This inherent sensing capability arises from the fact that the electrical resistance of the nitinol wires changes as soon as they deform.
- Each mechanical deformation of the wire can be matched to a specific resistance value. We use this measurement data to train neural networks. Using 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 in the face of disruptive influences, explains Paul Motzki.
In addition to holding a professorial position at Saarland University, Paul Motzki also Scientific Director/CEO at the Center for Mechatronics and Automation Technology in Saarbrücken.
Thanks to the insights that have come from close collaboration with Bergita Ganse’s team, the combination of smart implant technology and AI-assisted monitoring of the fracture gap is enabling medical teams to assess whether fracture site stiffness is increasing over time - and all without the need for X-ray imaging.
The measurement data also allows the wire bundles to be controlled and moved with high precision. Engineers can model and program specific movement sequences or simply hold the wires in any chosen position. In future clinical use, data from the implant will be transmitted wirelessly to a smartphone and be controlled via the same device.
The Saarbrücken research teams are keen to miniaturize their technology even further - and this work is now being funded by the EU under the Horizon Europe programme as part of the €21 million research project SmILE (Smart Implants for Life Enrichment).
