Development of a Shape Memory Alloy Actuator for a Micromechanical Sterilisation Cycle Counter

The steam sterilization of reusable medical instruments is a critical process. Standardized treatments with hot, saturated steam at maximum temperatures of up to 138 °C often represent a significant thermal load, which is repeated with varying number of cycles depending on the medical device. Until now, there is no possibility for medical device manufacturers to monitor how often a product has been sterilized. However, this is necessary for both safety and warranty issues, since according to the European Medical Device Regulation (EU-MDR), the manufacturer must specify how often a product can be sterilized.In this paper the actuator approach for a micromechanical ""sterilization cycle counter"" is presented. It is designed to autonomously record, count and store steam sterilizations directly on the instrument by combining silicon micromechanics with shape memory alloy (SMA) actuators. This enables an autonomous operation without additional energy sources such as batteries. During the steam sterilization cycle, a certain temperature limit is exceeded once and detected by the SMA. The system development aims at the heterogeneous integration of standard SMA wires into a silicon microstructure. The transformation temperatures of the SMA is thereby increased to the relevant range by prestressing.In detail, the paper first describes the approach of the counting mechanism and the possibilities and limitations of implementing and pretensioning of SMA wires in silicon microstructures. Based on that, the development of the SMA actuator geometry using an SMA Finite Element Analysis (FEA) model according to the approach of Aurichio is described. The model is validated using an up-scaled test bench of the system, in which various geometric parameters can be varied. Finally, the results will be discussed in particular regarding the MEMS process chain to be carried out in the next step.