Design and development of shape memory alloy based novel actuators for minimally invasive surgeries

Abstract

This work relates to the design and development of Shape Memory Alloy (SMA) based novel actuators for Minimally Invasive Surgeries (MIS). Compared to conventional open surgery, MIS procedures are favorable and developments are possible towards improving the effectiveness of MIS. Handheld slender instruments used in MIS are with limited degrees of freedom achieved using conventional actuation mechanisms which possess disadvantages in handling, durability, and cost. In this study, an SMA-based approach was considered to improve the operation of an MIS tool effectively. The complex behaviour of SMA led to the investigation of material behaviour before the application-oriented study. SMAs are smart alloys that are capable of remembering a parent shape according to the heat treatment (HT) temperature and aging time providing unique activation temperatures. Commercially available binary SMA material that is known as NiTiNOL was used for the study. NiTiNOL was subjected to different HT conditions and Differential Scanning Calorimetric (DSC) analysis was performed on the resulting material to obtain transformation temperatures. Test results demonstrated the ability to alter activation temperature by varying the HT conditions. Also, the samples were observed under an Optical Microscope (OM) and Scanning Electron Microscope (SEM) to identify morphology and elemental composition by Energy Dispersive X-Ray Spectroscopy (EDX) respectively. Furthermore, an SMA spring actuator element was fabricated using a NiTiNOL wire through a novel fixture to obtain desired spring parameters and geometry. NiTiNOL wire was held in the fixture undergoing a HT at experimented temperatures and aging times. Then, spring actuators were characterized based on maximum attainable force using a specially developed apparatus. A customizable hardware controller and a software interface were developed to set values, monitor temperature, and force output. Using the mentioned apparatus, the controller was validated in both temperature and force feedback controlling modes based on a Proportional-Integral-Derivative (PID) type controller. Two linear actuators were designed and developed using the characterized spring element. Firstly, an actuator was developed based on external heating using a heated fluid and cooled fluid to heat and cool the spring element, respectively. A novel actuator structure was developed to facilitate the spring element with leak-proof assembly and was used as the drive source of a gripper mechanism. Strain gauge-based force sensing and PID-based force feedback controlling methods were introduced to the gripper assembly. The second approach was utilizing a Joule heating-based method for activation which the passing current generates heat due to the inherent resistance of NiTiNOL resulting in an increment in temperature. The actuator was characterized in terms of stroke and then introduced to a laparoscopic retractor application to control the flexion-extension motion. A specially developed apparatus and a software interface are used to control parameters and acquire data. Finally, the retractor tool was characterized in terms of stroke.