This paper aims to review the State of the Art in metal self-expanding stents made from nitinol (NiTi), showing shape memory and superelastic behaviors, to identify the challenges and the opportunities for improving patient outcomes. A significant contribution of this paper is its extensive coverage of multidisciplinary aspects, including design, simulation, materials development, manufacturing, bio/hemocompatibility, biomechanics, biomimicry, patency, and testing methodologies. Additionally, the paper offers in-depth insights into the latest practices and emerging trends, with a special emphasis on the transformative potential of additive manufacturing techniques in the development of metal stents. By consolidating existing knowledge and highlighting areas for future innovation, this review provides a valuable roadmap for advancing nitinol stents.

Despite the application of the Additive Manufacturing process and the ability of parts’ construction directly from a 3D model, particular attention should be taken into account to improve their mechanical characteristics. In this paper, we present the effect of individual process variables and the strain-rate sensitivity of Onyx (Nylon mixed with chopped carbon fiber) manufactured by Fused Filament Fabrication (FFF), using both experimental and simulation manners. The main objective of this paper is to present the effect of the selected printing parameters (print speed and platform temperature) and the sensitivity of the 3D-printed specimen to the strain rate during tensile behavior. A strong variation of tensile behavior for each set of conditions has been observed during the quasi-static tensile test. The variation of 40 °C in the platform temperature results in a 10% and 11% increase in Young’s modulus and tensile strength, and 8% decrease in the failure strain, respectively. The variation of 20 mm·s⁻¹ in print speed results in a 14% increase in the tensile strength and 11% decrease in the failure strain. The individual effect of process variables is inevitable and affects the mechanical behavior of the 3D-printed composite, as observed from the SEM micrographs (ductile to brittle fracture). The best condition according to their tensile behavior was chosen to investigate the strain rate sensitivity of the printed specimens both experimentally and using Finite Element (FE) simulations. As observed, the strain rate clearly affects the failure mechanism and the predicted behavior using the FE simulation. Increase in the elongation speed from 1 mm·min⁻¹ to 100 mm·min⁻¹, results in a considerable increase in Young’s modulus. SEM micrographs demonstrated that although the mechanical behavior of the material varied by increasing the strain rate, the failure mechanism altered from ductile to brittle failure.

In Additive Manufacturing, wherein the construction of parts directly from 3D models is facilitated, a meticulous focus on enhancing the mechanical characteristics of these components becomes imperative. This study delves into the nuanced impact of the orientation of deposited layers on the mechanical properties of 3D printed Polylactic Acid (PLA) parts. Experimental testing, coupled with predictive modeling using Tsai–Hill and Tsai–Wu criteria, forms the crux of our investigation. The predicted ultimate strength from both criteria exhibits commendable agreement with the 3D printed specimens across a spectrum of orientation angles. Concurrently, Finite Element Simulations are meticulously executed to forecast mechanical behavior, taking into account the observed elasticity and plasticity in various orientations. Our observations reveal a significant augmentation in Young’s modulus and ductility/elongation—40% and 70%, respectively—when transitioning from θ = 0° to θ = 90°. Furthermore, the ultimate strength experiences a notable increase, leading to varied failure modes contingent upon θ. These findings underscore the pivotal role played by the orientation of printed layers in shaping the anisotropic behavior of 3D printed PLA parts, thereby integrating key process variables for optimization objectives. This study contributes valuable insights for professionals in the engineering, design, and manufacturing domains who seek to harness the advantages of 3D printing technology while ensuring that the mechanical integrity of 3D printed parts aligns with their functional requisites. It emphasizes the critical consideration of orientation as a design parameter in the pursuit of optimization objectives.

This study investigates the thermomechanical performance of additively manufactured NiTi tubes as a foundational step toward designing and fabricating NiTi tubes for harnessing elastocaloric effect (eCE) using additive manufacturing. By leveraging the design flexibility of additive manufacturing, particularly producing thin-wall tubes and intricate structures, the eCE can be enhanced. The mechanical properties of these tubes must meet the necessary load bearing requirements for use in eC-based devices. To this end, NiTi tubes were fabricated using laser powder bed fusion process, and heat treatment was applied to ensure eCE requirements, mainly transformation temperatures. The thermomechanical performance of as-fabricated and heat-treated tubes under compressive load were evaluated. The results demonstrate that heat treatment significantly improves the material's transformation temperature and stress hysteresis for practical eCE applications. Differential Scanning Calorimetry (DSC) analyses confirmed that the applied heat treatment effectively decreased the austenite finish temperature (Af) from 15.65 °C to 0.84 °C, enabling suitability for near-room-temperature cooling applications. Compression testing showed that heat-treated samples initiated martensitic transformation at ~400 MPa and exhibited significantly reduced stress hysteresis compared to their as-built counterparts, which enhances energy efficiency in cyclic applications. To the authors’ best knowledge, this work is the first to report on the compressive behavior of LPBF-fabricated Nitinol tubes, including their buckling performance. While full device demonstrations are beyond the present scope, our findings establish key design benchmarks and provide essential groundwork for integrating additively manufactured NiTi tubes into future elastocaloric cooling systems.

Elastocaloric (eC) cooling technology, leveraging the latent heat associated with phase transformations in shape memory alloys, presents a promising alternative to traditional refrigeration systems. In this study, a three-dimensional phenomenological constitutive model based on Gibbs free energy is employed and coupled with an energy balance equation in cylindrical coordinates as a necessary step toward system-level simulation of eC tubes. After calibration against experimental data from the literature, the model is used to investigate an innovative functionally graded (FG) superelastic tube in which the transformation temperature (TT) is intentionally varied along the length. The primary objective is to determine whether a spatially tailored transformation-temperature profile can counteract the strong temperature gradients established during regenerative cycling. The simulations demonstrate that such grading promotes a more uniform phase transformation under constant loading and directly improves the material-level coefficient of performance. Notably, the graded profile reduces hysteresis losses by 7.85% and 10.88% at 650 MPa and 750 MPa, respectively through a gradual, distributed transformation that lowers the local critical stresses for forward and reverse transformation. The improvement in hysteresis in the FG design arises from the spatial variation in TTs across the tube, which leads to a gradual and distributed phase transformation rather than a sharp transition. In the model, this grading reduces the critical stresses required for both forward and reverse transformations in different tube segments. While the model does not directly alter the hysteresis loop size, the lowered transformation stresses effectively narrow the hysteresis loop by reducing the stress gap between the forward and reverse paths. This mechanism is embedded in the model influencing martensitic volume fraction, and subsequently through the stress–strain response. The FG design, therefore, enables a smoother transformation response with reduced internal energy dissipation, which manifests as improved (reduced) hysteresis. This work demonstrates the transformative potential of FG structures in creating practical eC systems.