Known for their elasticity, bendability, and biocompatiblity, shape-memory alloys (SMAs) are used to fabricate a variety of medical devices, including implants, guidewires, and stents. However, despite their utility, SMAs have traditionally been able to remember only a single shape--that is, until now. Determined to enhance SMA's functionality, researchers at the University of Waterloo (Ontario, Canada) have developed a 'smart material' that they say can remember a range of shapes, not just one.

"The memory in traditional materials is based on a certain transformation temperature," explains Ibraheem Khan, a research engineer and PhD candidate at Waterloo. "Below that temperature, the alloy can be deformed and shaped, and then once it is heated up, it returns to its original shape." This type of material is known as a single-memory alloy.

In contrast, the new technology--dubbed multiple memory material--allows virtually any memory material to be quickly and easily embedded with additional local memories. "Our smart material process enables you to invent multiple memories for an alloy," Khan says. "Thus, we can take a single sheet, single wire, or any shape-memory alloy and then form and train it to have different transformation temperatures. Effectively, we program it." The addition of multiple transformation temperatures results in multiple memories, Khan adds. "It's more of a multiple-memory material than just a memory material."

Shape-memory alloys typically have two states: a low-temperature martensite state and a high-temperature austenite state. In the low-temperature state, the material is typically very malleable; it can be bent every which way and retain its shape. But once it has been heated up and brought to the austenite state, it returns to its original shape. The point at which the low-temperature martensite state becomes the high-temperature austenite state is the material's transformation temperature.

"Multiple transformation temperatures result in multiple transition zones," Khan comments. "While one local zone has one transformation temperature, another local zone on the same piece of material has another. Those two zones have different memories." A transition zone can be as small as a few microns in width, whereby each zone has a discrete transition temperature. As the processed shape-memory material is subjected to changing temperatures, each treated zone changes shape at its respective transition temperature. Created side by side, the transition zones facilitate unique and smooth shape changes.

While the Waterloo team has received government funding to develop the new alloy for automotive applications, Khan notes that materials with multiple memories can benefit the medical device realm as well. "For example, we're hoping to control the deployment of stents using our technology," he says. "By controlling the shape of the stent's valve, our aim is to enable doctors to control how open or closed the stent is at any given time." The material could also contribute to increased microgripper control in such applications as neurosurgery and endoscopic procedures, Khan adds. "And in the area of orthopedics, we hope to enhance the pseudoelastic and shape-memory properties of bone staples to improve the treatment of bone fractures."