Experimental Characterization of Shape Memory Alloy Nitinol Springs for Medical Devices

Background: The unique effect of a thermo-responsive Shape Memory Alloy (SMA) is the ability to recover its original shape from a significant and seemingly plastic deformation when heated. The recovered large strain and associated force developing during this transformation, make SMAs very attractive as thermally stimulated actuators. Nitinol, an equiatomic NiTi alloy SMA, is biocompatible which makes it suitable for bio-medical applications. SMA Spring-based actuators are superior to the wire-based ones for applications that require large strokes. At the present, the number of works concerning SMA springs is limited, and over-simplified models that neglect important aspects of the SMA spring behavior are being used. Consequently, there is a pressing need for a design framework of SMA spring actuators that are based on experimentally verified constitutive models. Methods: To tackle this need we constructed an experimental apparatus that measures and controls the three parameters that define the SMA spring state: temperature, elongation and force. The tested spring is located inside a water filled container and is attached to an industrial motorized tension-compression stand. The water in the container together with the spring are heated\cooled by a thermoelectric Peltier device. The whole thermo-mechanical system is controlled by a dedicated computer running a LabVIEW program. Results: Preliminary tests of the Nitinol springs included: a) fixed temperature tests while the force-elongation curve was measured and b) temperature cycling between 80°C and 30°C while the force/elongation was kept fixed and the elongation/force was measured. Conclusion: Our experiments demonstrated that Nitinol springs undergo large and reversible deformations under fixed loads or, alternatively, apply a range of loads at fixed elongation. The outcome of these experiments demonstrate that currently available spring models do not predict the spring behavior correctly. A set of experiments for accurate characterization of Nitinol springs behavior is being conducted in our lab at the present.