Computational modeling and simulation are commonly used during the development of cardiovascular implants to predict peak strains and strain amplitudes and to estimate the associated durability and fatigue life of these devices. However, simulation validation has historically relied on comparison with surrogate quantities like force and displacement due to barriers to direct strain measurement–most notably, the small spatial scale of these devices. We demonstrate the use of microscale two-dimensional digital image correlation (2D-DIC) to directly characterize full-field surface strains on a nitinol device coupon under emulated physiological loading. Experiments are performed using a digital optical microscope and a custom, temperature-controlled load frame. Following applicable recommendations from the International DIC Society, hardware and environmental heating studies, noise floor analyses, and in- and out-of-plane rigid body translation studies are first performed to characterize the microscale DIC setup. Uniaxial tension experiments are also performed using a polymeric test specimen up to nominal stains of 5%. Sub-millimeter fields of view and sub-micron displacement accuracies (9 nm mean error) are achieved, and systematic (mean) and random (standard deviation) errors in strain are each estimated to be approximately 1,000 μϵ. The system is then demonstrated by acquiring measurements at the root of a 300 μm-wide nitinol device strut undergoing fixed-free cantilever bending motion. Lüders-like transformation bands are observed originating from the tensile side of the strut that spread toward the neutral axis at an angle of approximately 55°. Optical microscale 2D-DIC setups like that demonstrated herein will be useful in future studies for characterizing cardiovascular implant micromechanics, validating computational models, and guiding the development of next-generation material models for simulating superelastic nitinol.

Superelastic Nitinol is a widely used material to manufacture implantable medical devices. Simulations are extensively used in the design, optimization, and durability assessment of Nitinol implants. The constitutive response of superelastic materials is non-linear, anisotropic, and asymmetric in tension vs. compression. The existing methods to determine the superelastic material properties used in finite element simulations typically do not account for some of these complexities. The goal of this work is to introduce a method to determine the superelastic material constitutive properties using full-field surface strain measurements. We propose using digital image correlation to obtain surface strain fields during tensile testing of specially designed diamond specimens. The material properties are determined by minimizing a difference metric defined in terms of the local strain difference and the global load difference between the experiment and trial simulations. We also propose a metric in terms of the local strain difference between experimental measurement and simulation prediction to report the credibility of the material properties fitted using this method. The philosophy behind this method and the proposed metric to quantify the accuracy of material property fitting reflects the growing consensus in the implantable device community that the simulation results must be accompanied by a quantification of their credibility.