Shape memory alloy micro-actuators for medical applications

Most of the current ’applications’ of microelectromechanical systems can be found in the sensor field. The development of micro-actuators is severely hindered by the low power-to-volume ratio of most actuator technologies. Most of the presented micromotors also require reduction gearing and suffer from wear or fatigue problems. Shape Memory Alloy (SMA) actuators offer the advantage of extremely high work-to-volume ratios that are comparable to those for hydraulic actuation [1]. Moreover they enable a very simple direct drive actuator design and they can be directly driven by an electrical current using simple resistive heating. Shape memory based microactuators were presented by Honma [2] and Kuribayashi [3]. In the medical field, “real" actuators are mostly used in minimal invasive surgery tools like endoscopes. Some examples are the active catheter tip developed by Dario [4], the active endoscope presented by Ikuta [5], or the miniature active catheter developed by Guo [6]. This paper describes the design of micromechanisms actuated by Shape Memory Alloys. They are to be used for different medical applications and illustrate a very simple microactuation technology conceived around a simple wire-shaped SMA element. Four different systems are described: The first one is a very simple microvalve to be used in drug delivery systems. The three other examples are situated in the minimal invasive surgery area: a mirror actuation system, a zooming system, and a bending actuator for colonoscopy.