Cryo-Processed Electrode-Assisted Micro-EDM of Nitinol Towards Improved Machining Performance

Abstract The superelastic Nitinol shape memory alloy (SMA) is known for several essential medical applications such as orthodontic archwires, cardiovascular stents, orthopedic implants, neurosurgical clips, surgical instruments, and prosthetic devices. However, the processing of Nitinol faces various challenges mainly as a result of its intense temperature sensitivity, high hardness, and excessive cost. Therefore, to meet strict regulatory and stringent requirements of modern healthcare, emphasizing miniaturization, micro-electric discharge machining (EDM) has been proven as an effective micro-fabrication technique to produce advanced micro-features, including dimensional precision and surface modification. Further, the growing urgency for micro-fabricated devices in tissue engineering and drug delivery systems demands faster production with high precision and enhanced properties. Therefore, this study aimed to improve the surface of biomedical Nitinol while minimizing cutting time (CT) and diametrical variation (DV) using a 0.98 mm diameter solid carbide micro-tool electrode in unmodified and deep cryogenically (cryo) processed conditions. Subsequently, various trial runs were executed on a Hyper-15/Sinergy NanoSystems micro-EDM equipment, following the one factor at a time (OFAT) method, with 48 mA current, 10 μs pulse-on time, 200 V voltage, 46 μs pulse-interval, 100 μF capacitance, and 300 rpm micro-tool electrode speed as the influential micro-EDM variables. Further, a Zeiss/GeminiSEM/500-field emission scanning electron microscope (SEM) and a Pro stopwatch (Model: EISCO-Digital) were assigned to calculate the CT and DV during each micro-EDM trial run. Furthermore, the processed surfaces were characterized using SEM, energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) facilities to ascertain the alterations in the micro-EDMed attributes. The findings reported that the cryo-processed micro-tool electrode outperformed the unmodified one and acquired considerable reductions in CT and DV by 33.24% and 36.49%, respectively. The cryo-processed one also achieved superior surface morphology and chemistry, suggesting that the Nitinol surfaces processed with the cryo-processed micro-tool electrode might exhibit enhanced biocompatibility characteristics.