Achieving biocompatible stiffness in NiTi through additive manufacturing

Abstract

This article seeks to reduce the stiffness of NiTi parts from a nonporous state to that of human bone by introducing porosity. Compact bone stiffness is between 12 and 20 GPa while the currently used bone implant materials are several times stiffer. While very stiff implants and/or fixation hardware can temporarily immobilize healing bone, it also causes stress shielding of the surrounding bone and commonly results in stress concentrations at the implant or immobilization hardware’s fixation site(s). Together these processes can lead to implant or fixation hardware and/or the surrounding bone’s failure. Porous NiTi can be used to reduce the stiffness of metallic implants while also providing necessary stabilization or immobilization of the patient’s reconstructed anatomy. In this work, mechanical behavior of porous NiTi with different levels of porosity is simulated to show the relation between the stiffness and porosity level. Then porous structures are fabricated through additive manufacturing to validate the simulation results. The results indicate that stiffness can be reduced from the bulk value of 69 GPa to as low as 20.5 GPa for 58% porosity. The simulation shows that it is possible to achieve a wide range of desired stiffness by adjusting the level of porosity.

Fabrication of NiTi through additive manufacturing: A review

“…Porous NiTi scaffolds are often fabricated with the purpose of altering mechanical properties such as stiffness (i.e., modulus of elasticity) in order to match to the bone stiffness [6,98,103]. However, one should note that introducing porosity also affect other mechanical properties.…”

Laser Powder Bed Fusion of Ti-rich TiNi lattice structures: Process optimisation, geometrical integrity, and phase transformations

“…There is a growing interest in additive manufacturing (AM) of TiNi shape memory alloys (SMAs), as they provide an interesting combination of functional performance due to the shape memory effect (SME), as well as unique mechanical and physical properties (e.g. low elastic modulus, biocompatibility [1,2], mechanical strength, and excellent corrosion and wear resistance [3,4]). They have been employed in biomedical implants and devices [5], aerospace components [6], and functional devices [7].…”

“…Vacuum casting is essential to avoid increasing the impurity levels (O, C, and N), which could generate TiC, TiO 2 , and Ti 4 Ni 2 O X secondary phase particles, which degrade the functional and mechanical properties of TiNi SMAs [4,8]. PM will also result in impurity pick-up, and is limited in its geometrical complexity [2]. Machining of TiNi encounters significant tool wear owing to the intrinsic mechanical behaviour of SMAs, making it essential to use special machining processes (e.g.…”

Design, Modeling, Additive Manufacturing, and Polishing of Stiffness-Modulated Porous Nitinol Bone Fixation Plates Followed by Thermomechanical and Composition Analysis

“…An Abaqus user-defined material subroutine (UMAT) developed based on the micro-plane theory [37][38][39] was used for capturing the mechanical behavior of the NiTi bone fixation plates. The UMAT is capable of simulating both superelastic and shape memory effects of shape memory alloys under multiaxial loading conditions [15] and requires the thermomechanical properties of the standard coupons (e.g., Young's modulus for austenite and martensite, transformation temperatures, critical stresses, etc.) as the input.…”

“…Although NiTi has a relatively lower level of stiffness, it still provides a higher level of stiffness in comparison to the bone tissue. We have shown that by introducing an engineered level and type of porosity to a bone fixation plate, one is able to further reduce the stiffness of NiTi bone fixation plates and reach the level of bone tissue [15,16]. We have also shown that the additive manufacturing method, in the form of selective laser melting (SLM), can be used for the fabrication of porous NiTi bone fixation plates [17,18].…”