Full-field strain measurements on medical devices using digital image correlation: Considerations and practical...

Access to direct stress or strain measurements on implantable medical devices could transform device design, durability assessment, and computational model development and validation activities. However, examples of acquiring such measurements are lacking. Here, we consider the use of optical digital image correlation (DIC) for characterizing full-field strains on medical devices and medical device surrogate specimens. Several common and unique speckle pattern application approaches are investigated including aerosol painting, application of transfer or decal papers, low- and high-pressure airbrushing using traditional and ultra-low reflectance paints, deposition of monodisperse graphite powder, and custom microstamping. Among the methods considered, comparative speckling studies show high-pressure airbrushing generates the best patterns at the millimeter-scale and monodisperse graphite powder or microstamping generate the best patterns at the sub-millimeter scale. Full-field DIC strain measurements additionally reveal Lüders-like phase transformation bands on nitinol wire, nitinol Z-specimens, and nitinol stent rings under bending loads. Overall, the study demonstrates the feasibility of direct strain measurement on real medical devices at fields of view ranging from millimeters to hundreds of microns and can be used to inform the planning and execution of future DIC studies on medical devices.