Production and evaluation of porous titanium scaffolds with 3-dimensional periodic macrochannels coated with microporous TiO₂ layer

Abstract

This study examined the utility of a combination of the thermoplastic green machining (TGM) and micro-arc oxidation (MAO) for the production of porous Ti scaffolds with 3-dimensional (3-D) periodic macrochannels coated with a microporous TiO₂ layer, which would provide high mechanical properties and excellent biocompatibility simultaneously. The TGM technique allowed for the creation of tightly controlled 3-D periodic macrochannels with a diameter of ∼828–837 μm by machining a thermoplastic compound consisting of 70 vol% titanium hydride (TiH₂) powder and 30 vol% thermoplastic binders, followed by heat-treatment in a vacuum. The overall porosity and mechanical properties of the porous Ti scaffolds were controlled by creating various periodic arrays of 6 × 6, 7 × 7, or 8 × 8 macrochannels in each face of a cube. The compressive strength and modulus was decreased from 358 ± 7 to 100 ± 8 MPa and from 5.2 ± 0.66 to 3.5 ± 0.32 GPa, respectively, with increasing porosity from 48 vol% to 64 vol%. The biocompatibility and bioactivity, which was assessed by in vitro cellular assays, were improved remarkably by creating a microporous TiO₂ coating layer using the MAO treatment.

Introduction

Titanium (Ti)-based metals (pure Ti and its alloys) have been used extensively in orthopedic and dental implants on account of their outstanding mechanical properties, chemical stability, and good biocompatibility [1], [2]. However, their relatively high stiffness compared to that of the surrounding bone is likely to cause bone resorption, eventually leading to a loosening of the Ti implant. One of the promising approaches for overcoming this limitation is to create pores in the bulk Ti implants, in that the elastic modulus of the porous materials can decrease with increasing porosity [3]. Furthermore, these pores can provide a favorable environment for bone ingrowth when used as a scaffold [1].

Thus far, a variety of manufacturing methods have been developed for the production of porous Ti scaffolds, including the sintering of metal powders, space holder method, rapid prototyping (RP) method, replication of sponge method, and freeze casting method [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. However, there is still a need for novel methods that can create 3-dimensionally interconnected pores in a controlled manner to improve the mechanical properties of porous Ti scaffolds without sacrificing their high porosity, as well as improving their biocompatibility [3], [8], [14].

Therefore, we herein demonstrate the utility of a combination of the thermoplastic green machining (TGM) and micro-arc oxidation (MAO) for the production of porous Ti scaffolds with high mechanical properties and excellent biocompatibility. In particular, for the first time, the TGM was employed for creating 3-dimensional (3-D) periodic macrochannels in Ti bulks, which can directly machine a thermoplastic compound consisting of ceramic powders and thermoplastic binders through the assistance of a computer-numeric-controlled (CNC) system [15], [16], [17]. More specifically, thermoplastic blocks consisting of TiH₂ powders as the starting source for Ti metal [10], [11], [12], [14] and thermoplastic binders with dimensions of 12 × 12 × 12 mm were machined to create a variety of 3-D periodic arrays of 6 × 6, 7 × 7, or 8 × 8 macrochannels in each face of a cube. This would determine the overall porosity and mechanical properties of the resulting porous Ti scaffolds produced after heat-treatment at 1300 °C for 2 h in a vacuum. After which, the surfaces of Ti walls were coated successfully with a microporous TiO₂ layer using the micro-arc oxidation (MAO) for improving their biocompatibility [18], [19], [20], [21], [22], [23]. The porous structure of the porous Ti scaffolds produced, such as the porosity, pore size, and densification of the Ti walls, was examined to demonstrate the utility of the TGM process. The compressive strength tests were conducted to evaluate the structural integrity of the samples. The preliminary osteoblastic activity of the sample with the TiO₂ coating layer was also evaluated using in vitro tests and compared to that of the sample without the TiO₂ coating layer.