Production and evaluation of porous titanium scaffolds with 3-dimensional periodic macrochannels coated with microporous TiO2 layer
Highlights
► The thermoplastic green machining (TGM) was used for creating 3-dimensional periodic macrochannels. ► The porous Ti scaffolds showed high compressive strength and elastic modulus. ► The micro-arc oxidation (MAO) was used to create a microporous TiO2 coating layer. ► The sample with the TiO2 coating layer showed improved biocompatibility.
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.
Section snippets
Materials and methods
Commercially available titanium hydride (TiH2) powder (325 mesh, Alfa Aesar, Ward Hill, MA, USA) was used as the source for Ti metal. A thermoplastic compound was prepared by mixing 70 vol% TiH2 powders with 30 vol% thermoplastic binders consisting of ethylene ethyl acrylate (EEA 6182; Union Carbide, Danbury, CT, USA) and Acryloid B67 (IBMA; Rohm and Haas, Philadelphia, PA, USA) resins using a heated shear mixer (Jeong-Sung Inc., Seoul, Korea) at 110 °C.
Once compounded, the thermoplastic
Results and discussion
Thermoplastic green machining (TGM) technique was used to produce porous titanium (Ti) scaffolds with tightly controlled 3-D periodic macrochannels. The thermoplastic compound could be machined in a controlled manner without any noticeable machining defects, such as chipping or cracking, due to the use of a high TiH2 content (70 vol%). In addition, the thermoplastic binder comprised of 50 vol% EEA and 50 vol% IBMA was used to improve the machinability of the thermoplastic compound [15]. All of
Conclusions
The thermoplastic green machining (TGM) technique was demonstrated as a promising method for the production of porous Ti scaffolds with tightly controlled 3-D periodic macrochannels, particularly owing to a high green strength of a thermoplastic compound, comprised of 70 vol% TiH2 powder and 30 vol% thermoplastic binders. All of the produced samples showed 3-D periodic macrochannels with a diameter of ∼828–837 μm, while various periodic arrays of 6 × 6, 7 × 7, or 8 × 8 macrochannels were formed
Acknowledgments
This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. (contract number : K00041294_15097).
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