Biological and Mechanical Properties of MAX Phases and MXene/PLA Nanocomposites

Abstract

Dental implant surgery has been popularly trusted by the worldwide patients for oral rehabilitation. However, the poor resistance to physical abrasion and fatigue fracture, and the detrimentally released ions of the traditional Ti-based materials (such as pure Ti and Ti-6Al-4V alloy) are crucial factors from the aspect of materials influencing on the survival rates. MAX phase is a terminology for a family of layered ternary compounds, which possess high flexural strength, fracture toughness, and damage tolerance, good tribological properties, excellent machinability, and good oxidation and corrosion resistance because of their unique structure, in which alternate near-close-packed layers of covalent M6X octahedrons are interleafed with metal-like A atom layers. Therefore, MAX phases have the potential to be used in the biomedical application, in particularly dental and orthopedic fields. Nevertheless, the results of previous works which merely focus on the Ti3SiC2 are inconsistent. The reason for this phenomenon is still blurry. Since the biological responses to host tissue cells must be considered in the development of synthetic biomaterials, it is essential to systematically evaluate the biocompatibility of MAX phase and establish the general mechanism for such a numerous family (more than 70 members). This thesis first explored the biocompatibility of the selected MAX phases (Ti3AlC2, Ti3SiC2, and Ti2AlN) on basis of different A-site and X-site atoms. The results of cell tests showed that these phases were innocuous to preosteoblasts and fibroblasts. Compared with the strong viable fibroblasts, the different cellular responses of these materials were clearly distinguishable for the preosteoblasts. Under an osteoblastic environment, Ti2AlN exhibited better cell proliferation and osteogenic differentiation performance than Ti3AlC2 and Ti3SiC2. Moreover, the performance was superior to that of a commercial Ti-6Al-4V alloy and comparable to that of pure Ti. A general mechanism was suggested based on the different surficial oxidation products, which were determined from the binding energy of the adsorbed Ca2+ ions using first-principles calculations. Compared with the partially oxidized TiCxOy layer on Ti3AlC2 and Ti3SiC2, the partially oxidized TiNxOy layer on the Ti2AlN had a stronger affinity to the Ca2+ ions, which indicated the good biocompatibility of Ti2AlN. In addition, the (001) surfaces were proved to possess the strongest binding energy with the Ca2+ ions in a certain MAX phase. This phenomenon disclosed the good biocompatibility of the 2-dimensional transition metal carbides or nitrides (MXenes), which mainly were composed of these (001) surfaces. Guided bone regeneration (GBR) technique is a reliable and validated therapy to augment the bone for the surgery of dental implantation. Although the resorbable GBR membrane is fascinating, which protects the bone healing from the interference of non-osteogenic tissue, and exempts the secondary surgery for the removal of remaining membrane, the poor mechanical properties and cellular responses are critical issues, which usually result in insufficient amount of regenerated bone. In the second work of this thesis, Ti3C2Tz nanosheets (delaminated Ti3C2Tz, d-Ti3C2Tz), the representative of MXenes, were used to enhance the mechanical properties and biocompatibility of Poly (lactic acid) (PLA) membrane. As a prerequisite, the intrinsic biocompatibility of d-Ti3C2Tz was first authenticated using that of widely-discussed graphene oxide (GO) for reference. The strong and biocompatible d-Ti3C2Tz enhanced PLA nanocomposite membranes were fabricated using the interfacial mediation with n-octyltriethoxysilane (OTES). To the best of our knowledge, this was the first effort to introduce MXene into polymer matrix for biomedical applications. The optimized ultimate tensile strength (UTS) of OTES-d-Ti3C2Tz/PLA nanocomposite membrane was 72 MPa (obtained at 0.5 wt.%), which increased by 33%. This enhancement was almost the highest compared with the graphene-enhanced works, where the solvent casting was adopted. The reason could be ascribed to the strong interaction between OTES-Ti3C2Tz and PLA, which ensured the efficient load transferring between these two components. The d-Ti3C2Tz was also certified to significantly promote the adhesion, proliferation, and osteogenic differentiation of preosteoblasts on the nanocomposite membranes, which exhibited ascending trend along with the increasing filler content (0 to 1 wt.%).