ADVANCED MATERIALS & PROCESSES | MARCH 2026 36 ADVANCEMENTS IN SHAPE MEMORY IMPLANTABLE MEDICAL DEVICES Recent developments in bioelectronics and power supply miniaturization are providing new design opportunities for in vivo shape memory devices. Scott Robertson* Resonetics, Nashua, New Hampshire Is the medical device community on the verge of a new paradigm of in vivo shape memory devices? Much like self-expanding superelastic implantable medical devices revolutionized the industry more than 30 years ago, a new stage is being set. The emergence of miniature leadless pacemakers with sealed power supplies that last multiple years, pulsed field ablation for controlled in vivo heating, and brain computer interfaces with complex bioelectronics all demonstrate the rapidly evolving landscape where implantable power/heat sources are on the verge of becoming commonplace. These advancements in implantable power management may very well usher in a wave of shape memory medical devices that require heat activation. EARLY MEDICAL-DEVICE DESIGN Some early medical devices such as the Memotherm Stent and Simon Nitinol IVC Filter were marketed as having shape memory. These devices were tuned with an austenitic finish temperature, Af, above room temperature and below body temperature (e.g., Af = 30 ± 5°C). While these devices feel malleable when handled at room temperature and take on a predefined shape at body temperature, thus the perception by users as having “shape memory,” they more accurately fall under the category of self-expanding superelastic implants. Instead, a true shape memory implant is one in which a predefined shape is achieved through the application of heat while the device is in its target location in the human body (i.e., austenitic start temperature, As, or R-phase start temperature, RS, is > 37°C and the device takes on a new shape when heated above body temperature). With the exception of a few benign prostatic hyperplasia implants that rely upon a warm 55-60°C saline flush to achieve full expansion during implantation (e.g., Memokath, Endocare Horizon Stent), no commercial devices exist that truly utilize the shape memory properties of Nitinol. A primary obstacle to the use of the shape memory effect in medical devices is the sensitivity of human tissue to damage at relatively low temperatures and short durations. Indeed, tissue damage occurs after just 1 second of exposure to 70°C (Fig. 1). Common binary Nitinol alloy formulations that would seem desirable for in vivo FEATURE actuation (e.g., Fort Wayne Metals Niti#6 with an As ≥ 35°C) require temperatures as high as 90°C to achieve complete actuation above the Af temperature. Therefore, exceedingly short-duration heating of conventional binary nickel- titanium, or the development of narrower hysteresis ternary/ quaternary shape memory alloys may be required for the advancement of shape memory implants. NEW PARADIGM While few commercial implants have used shape memory actuation to date, the aforementioned advancements in implantable power and bioelectronics have inspired a new paradigm. Are these advancements capable of driving the heat-actuation times down to a realm that is safe for surrounding tissue? Three examples of devices currently under development suggest that the answer is “yes.” Glaucoma is caused by elevated pressures due to fluid buildup in the eye. When pharmaceutical treatments fail, Fig. 1 — Tissue necrosis time versus temperature curve; adapted from Moritz & Henriques with the red portion of the graph extrapolated to 90°C consistent with the Af temperature of common binary Nitinol in vivo shape memory material[1]. *Member of ASM International (continued on page 38)
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