Highly porous Ni-free Ti-based scaffolds with large recoverable strain for biomedical applications

Highlights

• Highly porous Ni-free Ti–Zr–Nb–Sn alloy scaffolds with porosities of 70%, 75% and 80% were prepared by fiber metallurgy.

• The Ti–Zr–Nb–Sn scaffolds exhibited three-dimensional structure with fiber-fiber sintering joints.

• The compressive yield stress and elastic modulus of 673 K aged scaffolds were similar to those for cancellous bone.

• Recovery strain of 3.7% was obtained in the 673 K aged scaffold with porosity of 80%.

• Recovery strain was improved in 673 K aged scaffolds due to the ageing-hardening of α phase.

Abstract

Highly porous Ti–18Zr-12.5Nb–2Sn (at.%) scaffolds with porosities of 70%, 75% and 80% were prepared by sintering rapidly solidified alloy fibers. Superelastic behavior of alloy fiber was investigated by the dynamic mechanical analyzer (DMA). Phase identifications of alloy fiber and scaffold were studied by X-ray diffraction (XRD). Precipitates in alloy fiber were investigated by transmission electron microscope (TEM). Porous structure of alloy scaffold was observed by scanning electron microscope (SEM). Mechanical properties and the superelasticity of alloy scaffold were investigated using compressive test. The superelasticity of alloy fibers was improved after aged at 673 K for 3.6 ks due to α precipitates. The sintered scaffolds showed three-dimensional networks with fiber-fiber sintering joints. The compressive yield stress and elastic modulus of the alloy scaffolds aged at 673 K for 3.6 ks were in the range of 5.0–16.7 MPa and 0.33–1.05 GPa, respectively. These values were a close match to those for cancellous bone. The recoverable strain increased from 3.2% to 3.7% with the increase in porosity from 70% to 80% in aged Ti–Zr–Nb–Sn scaffolds when tested at human body temperature (310 K).

Introduction

Porous metallic biomaterials have a lot of applications in medicine, one major aspect of which is to replace damaged bones or to provide support for healing bone defects [1]. For bone replacement, implantable materials are required to show superelastic behavior similar to that of human bones exhibiting the superelasticity with a recoverable strain of near 3% [2], which can mimic the natural biomechanical properties of bone and provide good biofunctionality. Ni-free Ti–Nb-based shape memory alloys (SMAs) are recognized as attractive biomaterials due to their superior mechanical properties, biocompatibility and non-toxicity [3]. Additionally, Ti–Nb-based bulk SMAs exhibit a stable recoverable strain of over 4% which is adequate for the requirement of bone replacement [4]. In addition to the superelasticity, a high porosity is another important aspect for cancellous bone substitute biomaterials because implants with porosities close to that of human cancellous bone (70%–90%) had the best bone ingrowth and highest cell viability [5].

Although numerous studies on porous Ti–Nb-based alloys have been carried out to access mechanical properties similar to that of human cancellous bone, porous Ti–Nb-based alloys prepared by powder metallurgy (PM) have some apparent problems because they cannot satisfy the requirements of both high porosity (70%–90%) and large recoverable strain (~3%). For example, porous Ti-20.8Nb-5.5Zr (at.%) alloys with 46% porosity prepared by sintering atomized alloy powders presented superelastic recovery strain less than 1% [6]. Porous Ni-free Ti-based alloy bulk metallic glass with porosities of ~60% were developed by spark plasma sintering (SPS) [7] and porous Ti-35.4Zr–28Nb (wt.%) alloys with porosities of ~65% [8,9] were fabricated by space holder method, but authors did not show the superelastic stress-strain curves. Although much highly porous Ti–Nb–Ta alloy with a porosity of 80% [10], Ti–20Nb–15Zr (wt.%) alloy with a porosity of 75% [11] and Ti–35Zr–28Nb (wt.%) alloy with a porosity of 83% [12] were fabricated either from element powders by specific sintering methods or selective laser melting (SLM), authors also did not investigate the superelasticity. Therefore, it is crucial to develop practical methods to produce novel Ni-free porous materials that have both high porosity and large recoverable strain comparable to that of natural cancellous bone.

For porous metallic biomaterials prepared by conventional PM, the high porosity makes the recoverable strain greatly deteriorated [13]. Thus, it is difficult to obtain large recoverable strain at a high porosity level with this method. In this study, an alternative and promising method for highly porous scaffolds was developed using the sintering of Ni-free Ti-based short alloy fiber segments. It has been reported that Ti–18Zr-12.5Nb–2Sn (at.%) alloy bulk exhibited a recovery strain of 6.0% at room temperature [14]. However, this alloy bulk presented higher Young's modulus (about 45 GPa) than that of human cancellous bone (~2 GPa) [15], which would cause the subsequent bone resorption due to stress shielding. By preparing a Ti–18Zr-12.5Nb–2Sn (at.%) alloy with a highly porous structure, it is expected to obtain mechanical properties similar to that of human bone. Therefore, we produced Ti–18Zr-12.5Nb–2Sn (at.%) alloy fibers by using a melt overflow process [16] and then fabricated scaffolds with a three-dimensional network and high porosity (70%~80%). Based on our recent work which investigated the effect of annealing treatment on the microstructure and superelastic properties of this scaffold [17], in the present work, a possible optimum heat treatment condition for the scaffolds to obtain more proper mechanical and superelastic properties was deduced. We also investigated the influence of porosity in scaffolds on the mechanical performances by using compressive tests at human body temperature (310 K).