As part of an EU-funded project, researchers are miniaturizing the technology for use in thin intramedullary nails (IMNs), which provide internal support and stability during the healing process.
If you break one of your body's long bones, such as the tibia, your surgeon will likely use a stabilizing rod, known as an intramedullary nail (IMN), to support the bone while it heals. The long "nail" or rod is carefully inserted into the bone through the central, marrow-filled medullary cavity, stabilizing the bone and allowing it to heal. Compared to fitting a brace, which is attached to the bone from the outside, the INM insertion process is generally less invasive, as it doesn't require opening the fracture site in surgery. Usually, all that's needed is a small incision at the end of the affected bone. "The tissue and blood circulation around the fracture site are not compromised, which is beneficial for healing. Our patients can immediately put weight on their leg after surgery, which facilitates mobility and reduces postoperative complications," explains Bergita Ganse, a fracture healing expert and professor of Innovative Implant Development at Saarland University.
Together with a team of engineers led by Professors Paul Motzki and Stefan Seelecke at the Center for Mechatronics and Automation Technology (ZeMA), trauma surgeon Ganse is working to give these intramedullary nails new properties. By continuously transmitting fracture gap measurement data, this new type of intramedullary nail (IMN) allows doctors to monitor from the outset whether the bone is healing correctly. Until now, doctors could only obtain occasional snapshots from X-ray images of the fracture site. Another feature of this new technology is the INM's ability to harden or relax at the fracture site. If the patient wants to walk, they have all the stability provided by the rigid intramedullary nail; if they are resting, however, they can soften it via an app on their phone. "Our goal is to create an intramedullary nail that actively promotes healing." "We are developing an INM that will stimulate the growth of new bone tissue by micromassaging the fracture gap," says Bergita Ganse.
This is something the research team has already achieved with orthopedic fixation plates. The plates are able to measure the forces acting on the fracture gap and can flex independently of each other, thus optimizing the applied load and improving the healing process. The team has been developing these smart implants for more than five years as part of an €8 million project funded by the Werner Siemens Foundation.
Researchers are now miniaturizing the technology so it can also be used in intramedullary nails. “The results and experience we’ve gained with fixation plates are now being incorporated into the new implants,” explains Paul Motzki, Professor of Smart Materials Systems for Innovative Production at Saarland University and CEO of ZeMA. The current project is funded by the EU under the Horizon Europe program as part of the €21 million SmILE (Smart Implants for Life Enrichment) research project, in which 25 partner institutions from twelve European countries are investigating how to protect older people from musculoskeletal disorders.
The engineering team led by Paul Motzki and Stefan Seelecke had to devise a series of innovative ideas to adapt the technology to the interior of the intramedullary nail, which is only a few millimeters wide. Paul Motzki explains one of the challenges: “We have to ensure that the mechanism that hardens the implant precisely at the fracture site doesn’t cause any thickening at that point, as this could severely damage the already fragile fracture site.” The solution to this problem led to the development of a patented movement mechanism. Two miniature actuators working in opposite directions are configured so that one pulls a rod with a conical head toward the opening of a soft, elastically flexible sleeve. Once in position, the conical head is held firmly in place, and then the second actuator pulls it out of the sleeve. When the conical head is inserted into the elastomeric sleeve, the IMN hardens at that point, but does not increase in size. When the rod retracts from the sleeve, the IMN softens again.
The actuators in this case are bundles of ultrathin wires made of shape-memory nickel-titanium alloy. “We use these wire bundles as actuators capable of exerting a directed force within a restricted space, which in this case allows us to manipulate the rod within the intramedullary cavity. These actuators are capable of exerting extraordinarily large forces for their size. In fact, nickel-titanium wire has the highest energy density of all known actuation mechanisms,” explains Professor Motzki. The wire can be contracted by applying brief pulses of electrical current; when the current is switched off, the wire returns to its original length.
The reason for this behavior lies in the alloy's crystalline structure. "Nickel-titanium has two crystal lattice structures that can transform into one another," explains Paul Motzki. One of the lattices is shorter than the other. If an electric current flows through the wire, the material heats up, causing it to adopt the crystal structure with the shorter lattice and thus shorten the wire. When the current is switched off, the wire cools, the material switches to the other lattice structure, and the wire returns to its original length. The researchers use bundles of these ultrathin wires as artificial muscles to control small mechanical components. "The advantage of using bundles of ultrathin wires is that a bundle has a large total surface area and can therefore dissipate heat more quickly, allowing us to contract the wires at high frequencies," says Paul Motzki. After years of research on shape memory alloys, the Saarbrücken team knows how to customize these wire bundles for different technical applications by carefully configuring them in terms of thickness and number of wires used.
The sensor technology needed to control the movement of the wire as it is inserted into or withdrawn from the intramedullary cavity is supplied by the actuators themselves. “When the wires change shape, so does their electrical resistance. We can assign precise resistance values to even the smallest deformations, and we use this data to train a neural network. The resulting AI is now so good that it can efficiently and accurately generate positional information even in the face of disruptive influences,” says Motzki. “Our approach allows us to extract all the sensory data needed to control the movement of the wire bundle,” explains Susanne-Marie Kirsch, a PhD student in the Saarbrücken group. And because this technology is self-sensing, the researchers have a way to monitor the fracture healing process. Even the slightest change in the fracture gap produces a change in the wires’ resistance, allowing medical staff to see whether new bone tissue is growing at the fracture site.
The clinical aspects of the research project are managed by a research team led by Bergita Ganse. Her team specializes in extracting biomechanical information from measurement data. They perform gait analysis, conduct computer simulations, and use artificial intelligence systems to gain the necessary medical insights. By monitoring the increase in stiffness at the fracture site and using blood flow measurements, the medical team can assess the healing process. They are also interested in understanding which factors promote bone healing. The goal is to ensure that the actuators used in the IMNs perform precisely coordinated movements that promote bone tissue growth. “We need to build the implant in such a way that it can perform the micromovements and pressure changes necessary to encourage healing,” explains Professor Ganse.
“The goal is for everything to be controlled by an app on the patient’s phone so that, after receiving medical instructions, they can adjust the mechanism themselves,” says Paul Motzki. The electrical impulses that control the movement of the actuators will be supplied by a battery in the body that can be charged wirelessly via induction.
The engineers in Saarbrücken want to further miniaturize their technology so that it can be used in much smaller bones. “Our technology is scalable. The next goal is to develop smart implants for use in maxillofacial surgery, for example, in the treatment of jaw fractures,” explains Paul Motzki.
Paul Motzki and Stefan Seelecke’s team will present their technology and demonstrate prototypes of these smart medical implants at this year’s Hannover Fair.
Background:
The research team led by Stefan Seelecke and Paul Motzki uses shape memory technology for a wide range of applications, from innovative cooling and heating systems to robotic grippers, valves, and pumps. At the Hannover Fair, the Saarbrücken-based experts in smart materials systems will demonstrate miniature intelligent drives, energy-efficient robotic gripper systems, and a novel elastocaloric cooling and heating system. The technology is continuously being developed by PhD students conducting research as part of their doctoral dissertation projects. The research is funded through several large-scale research projects, and the results have been widely reported in high-impact scientific journals, with numerous articles receiving international recognition.
To facilitate the transfer of their application-based research to commercial and industrial applications, the researchers established the company mateligent GmbH.
ZeMA, the Saarbrücken Center for Mechatronics and Automation Technology, is a research center for collaborative projects involving researchers from Saarland University, Saarland University of Applied Sciences (htw saar), and industrial partners. ZeMA focuses on industrially relevant development work aimed at transferring ideas and technology from academic research to industry.
