Effects of ultrasound settings on temperature changes in NiTi implants

Background: Shape memory alloys (SMAs) are well-known for their unique ability to undergo a shape change in response to a thermal stimulus. A frequently-used SMA for biomedical devices is NiTi, although its superelastic features tend to be emphasized more than the ability to change shape. Minimally invasive NiTi implants which can reconfigure or adjust their shape across several temperature points could provide desirable surgical outcomes. For decades, therapeutic ultrasound has been used medically as a non-invasive method for tissue thermal therapy. Ultrasound's ability to quickly raise temperatures, and transcutaneously activate shape changes in NiTi implants is a novel approach for eliciting the martensitic thermoelastic transformation. Methods: The purpose of this study was to investigate the features of therapeutic ultrasound that correspond with temperature changes in different NiTi specimens. For this purpose, ultrasound was applied to two NiTi specimens for two minutes each at varying low- and high-frequency and power settings using a Sonicator 740 and a Dynatron 150. Findings: The baseline temperature for all 32 trials was room temperature (23.0 ± 1.7°C). This study successfully increased the specimen temperature with the application of Sonicator 740 and Dynatron 150 therapeutic ultrasound machines (2.2 ± 2.4°C and 1.5 ± 1.15°C, respectively). From the statistical analyses of the experimental data, it was clear that there is a significant difference between low- and high-power settings on mean temperature change using the Dynatron 150 (ANCOVA; p = 0.013). Interpretation Of clinical relevance, NiTi implants can quickly and easily increase in temperature when applying therapeutic ultrasound. Ultrasound power causes temperature changes and should be accounted for when designing orthopedic implants for applications where dimensional changes are desirable.