<p style="text-align: justify">Smart actuators, especially shape memory alloys (SMAs), are nowadays being explored in a wide range of technical areas. Thermal SMA wires made of a nickel-titanium (often referred to as Nitinol) have the ability to contract under a defined temperature increase. This process is based on a reversible phase transformation from the low-temperature phase martensite to the high-temperature phase austenite and vice versa. Another advantage of SMA wires is their sensory capability, in addition to their function as actuators. In fact, by monitoring the electrical resistance of the SMA, conclusions can be drawn about the degree of phase transformation and thus about the degree of wire contraction. In the medical field, SMA technology has been mostly established in stent and guide wire applications, due to the biocompatible properties of the material. Nitinol SMAs are particularly suitable for this purpose. Aside from these applications, which mostly rely on the “passive” behaviour of SMA, the exploitation of the actuation properties of the material is becoming more relevant in this field of research. SMA actuators shaped as thin wires, with diameters ranging from 25 to 100 µm, can be used in even the smallest medical devices. Thanks to their high energy and power densities, SMA wires permit to achieve better performance than conventional drive elements.</p><p style="text-align: justify">The goal of this work is to develop an SMA wire-based actuator prototype for assisting rehabilitation of patients with bone fractures. In particular, the developed prototype extends the classical stabilizing function of a medical implant for bone fracture healing by implementing an actively controllable stiffness feature. The reversible stiffness change allows individual stimulation patterns to be imprinted by the implant into the fracture site, thus influencing and accelerating the healing process of the patient. Despite it is known that targeted loading patterns represent a major influencing factor on bone healing, until now these effects could neither be individually detected nor influenced by means of conventional actuators. The presented approach addresses this problem by transferring smart materials-based sensor/actuator technology onto this specific medical problem. By evaluating the movement data, possible incorrect loading by the patient can be detected, and corrections can be actively made during the healing process. The presented implant prototype makes fracture treatment internally modifiable, even postoperatively. Rapid prototyping methods allow this implant to be adapted to each patient individually, quickly and cost-effectively. In this way, revision surgeries and significantly prolonged treatment times can be reduced. The first prototype is planned for direct application to a fracture site, for example, the tibia or fibula. In this paper, a first design of such a smart implant prototype is presented. The prototype demonstrates the capability of reversible stiffness change, and the possibility of stimulating the fracture site with SMA actuation. Finally, the functionality of the actuator system is experimentally validated in terms of force, stroke, and power consumption.</p>

An SMA-Based Multifunctional Implant for Improved Bone Fracture Healing

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2021/volume/85499

ASME 2021 Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2021)

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