Biological response of novel high strength-low elastic modulus β-Ti2448 alloy was evaluated.
The formation of fine apatite-like crystals in simulated body fluid confirmed the superior bioactivity of the alloy.
The unique surface properties of the Ti2448 alloy provided favorable osteogenic microenvironment for cell–material interaction.
The β-Ti2448 alloy with a modulus of ∼49 GPa may avoid the undesirable stress-shielding effect.
Low modulus β-titanium alloys with non-toxic alloying elements are envisaged to provide good biocompatibility and alleviate the undesired stress shielding effect. The objective of this study is to fundamentally elucidate the biological response of novel high strength-low elastic modulus Ti2448 alloy through the study of bioactivity and osteoblast cell functions.
Characterization techniques such as SEM, EDX, XRD, and fluorescence microscopy were utilized to analyze the microstructure, morphology, chemical composition, and cell adhesion. The cellular activity was explored in terms of cell-to-cell communication involving proliferation, spreading, synthesis of extracellular and intracellular proteins, differentiation, and mineralization.
The formation of fine apatite-like crystals on the surface during immersion test in simulated body fluid confirmed the bioactivity of the surface, which provided the favorable osteogenic microenvironment for cell-material interaction. The proliferation and differentiation of pre-osteoblasts and their ability to form a well mineralized bone-like extracellular matrix (ECM) by secreting bone markers (ALP, calcium, etc.) over the surface point toward the determining role of unique surface chemistry and surface properties of the Ti–24Nb–4Zr–8Sn (Ti2448) alloy in modulating osteoblasts functions.
These results demonstrated that the low modulus (∼49 GPa) Ti2448 alloy with non-toxic alloying elements can be used as a potential dental or orthopedic load-bearing implant material.
Titanium and its alloys have been in extensive use for several decades as a solid implant material for fabrication of biomedical devices. Titanium alloys such as Ti–6Al–4V has α + β-type microstructure and meet some of the most important requirements including high specific strength, good corrosion and fatigue resistance in physiological media and are used as implant materials for dental, orthopedic, and maxillofacial applications . However, the elastic modulus of these titanium alloys is high in the range of ∼90–115 GPa, while the modulus of cortical hard bone (outer region of femoral and tibial bones) is low at 16–20 GPa, and that of cancellous or trabecular soft bone is 1–4 GPa . This large elastic modulus mismatch between the implant and surrounding bone tissue is expected to develop unbalanced stress distribution around the implant, leading to stress shielding-bone resorption and implant loosening . Hence, the ability to reduce modulus mismatch is envisaged to be an advancement in the biomedical arena for the treatment of load-bearing dental, orthopedic, and segmental bone defects .
A promising approach to alleviate biomechanical mismatch is through alloy design or by fabricating porous structures that are characterized by significantly reduced modulus . Even though the porous structures have low elastic modulus, they were limited in load-bearing implant applications due to poor mechanical properties. In this regard, high strength (600–700 MPa)-low elastic modulus (49 GPa) body-centered cubic (bcc) β-type Ti–24Nb–4Zr–8Sn (TNZS or Ti2448) alloy has been recently developed by the authors associated with the Institute of Metals Research, Chinese Academy of Sciences . In relation to other titanium alloys, Ti2448 alloy does not contain any toxic elements such as V and Al, and has low elastic modulus. Previous studies have shown that alloying elements such as Nb, Ta, Zr, and Sn improve the corrosion resistance of the alloy by forming stable oxides such as Nb 2 O 5 , Ta 2 O 5 , ZrO 2 , and SnO 2 . Other β-type and Ti–Nb alloys such as Ti–Nb–Zr and Ti–Nb–Ta–Zr are also being considered as potential implantable materials because of their high strength, high fatigue resistance, good formability and hardenability, low modulus, and non-toxic nature . In comparison to other alloying elements in Ti–Nb based alloys, Zr and Sn are effective in suppressing the formation of both ω-phase and α″-martensite phase, which may potentially deteriorate the mechanical properties of the alloy . Furthermore, alloying titanium with more thermodynamically stable elements such as Nb and Zr makes the implant more corrosion resistant, biocompatible, and promotes cell adhesion .
The objective of the study described here is to explore the in vitro biological activity of the novel low elastic modulus Ti2448 alloy in terms of bioactivity, osteoblast cell attachment, proliferation, differentiation, and mineralization. The Ti2448 alloy with low elastic modulus is a potential new biomaterial to replace current high modulus titanium alloys consisting of toxic elements.
An ingot of the Ti–24Nb–4Zr–8Sn alloy with diameter of 280 mm was fabricated by vacuum arc melting using a Ti–Sn master alloy and pure Ti, Nb and Zr as raw materials. The ingot was hot forged at 1123 K to round bar 55 mm in diameter. The chemical composition of the ingot obtained by wet chemical and gas analysis is presented in Table 1 . The experimental material was metallographically polished to mirror finish using standard metallography procedure. All experiments were carried out on mirror polished Ti2448 alloy specimens (1 cm 2 ) as obtained by metallographic polishing. Polished samples were first subjected to a cleaning and sterilization procedure involving 1 h sonication in an ultrasonic bath containing distilled water, followed by washing with acetone, isopropanol, ethanol, and deionized water. The samples were then sterilized in an autoclave and dried overnight.
The microstructure of the Ti2448 alloy was analyzed through metallography and optical microscopy, and the mechanical properties are presented in Table 2 . Tensile tests were conducted in air at room temperature (∼295 K) using specimens with a gage diameter 3 mm and a gage length 15 mm at a strain rate of 1.3 × 10 −4 s −1 . Young’s moduli (E) were measured by the free resonant vibration method using cylinders 6 mm in diameter and 60 mm in length. Vickers hardness was measured using a hardness tester with a load of 100 g held for 15 s. The mechanical properties of the Ti2448 alloy presented in Table 2 reveal the high strength (700 MPa)-low elastic modulus (49 GPa) property of the alloy.