Porous Titanium Scaffolds with Various Pore Structure by Modified Camphene-Based Freeze Casting for Orthopedic Application

Abstract

this was followed by freeze drying and sintering. As the casting time increased from 24 to 48 h, the initial columnar structures turned into lamellar structures, with the porosity decreasing from 69 to 51%. This reduction in porosity caused the compressive yield strength to increase from 121 to 302 MPa, with an elastic modulus of the samples being in the range of 2–5 GPa. In addition, it was demonstrated that reverse freeze casting can also be successfully applied to various other raw powders, suggesting that the method developed in this work opens up new avenues for the production of a range of porous metallic and ceramic scaffolds with highly aligned pores. To accomplish the enhanced biocompatibility, silica xerogel / chitosan hybrid material which containing bone morphogenetic protein-2 (BMP-2) was coated on the surface of porous Ti scaffolds. The release behavior of BMP-2 was monitored and the in-vivo study in a rabbit calvarial model was performed. These included scaffolds with random pores, aligned pores, aligned pores with hybrid coating, and aligned pores with BMP-2 containing hybrid coating. The average bone formation ratio of porous Ti scaffolds with aligned pores was slightly higher (about 16%) than that for the control group (about 13%). However, hybrid coating layer and BMP-2 loaded hybrid coating layer showed significantly enhanced bone formation ratio, as expected (about 26% and 34%, respectively). These results showed that the freeze casting (including reverse freeze casting) combined with BMP-2 loaded hybrid coating is a very promising method for producing semi-permanent porous bone replacements.

The present study reports novel techniques for producing highly porous titanium (Ti) scaffolds by camphene-based freeze casting. To accomplish our goals (large interconnected pore channels, high porosity, aligned pores, suitable mechanical properties), many conditions of freeze casting technique - casting time, casting temperature, freezing direction, etc - were modified. In the first study, porous Ti scaffolds were fabricated by freezing titanium hydride (TiH2)/camphene slurries at 33 °C for 24 h. Titanium hydride (TiH2) powder was selected as the source for the formation of Ti metal via hydride decomposition. Titanium hydride/camphene slurries with various TiH2 contents of 15, 20, and 25 vol.% prepared via ball-milling at 60 °C were frozen at 33 °C for 24 h, followed by freeze-drying and heat-treatment in vacuum to produce porous Ti scaffolds. All of the fabricated samples revealed highly porous structures having large pores up to 100 μm in size surrounded by Ti metal walls without any secondary phases. When the initial TiH2 content was increased from 15 to 25 vol.%, the porosity was decreased from 63 to 49%, while the compressive strength was significantly improved from 81 to 253 MPa. It was also verified that how the freezing time affects the development of the pore structure and compressive strength of porous Ti scaffolds using the modified camphene-based freeze casting. To accomplish this, a titanium hydride (TiH2) /camphene slurry with an initial TiH2 content of 10 vol.% was frozen at 42 °C for various times (1, 4, and 7 days). As the freezing time was increased from 1 to 7 days, the pore size obtained was increased significantly from 143 to 271 μm due to the continual overgrowth of camphene dendrites. However, interestingly, the formation of the micro-pores inside the Ti walls was suppressed at longer freezing time. This resulted in a significant increase in compressive strength up to 110±17 MPa with a porosity of 64%. It is believed that this unusually high compressive strength with large interconnected pores makes this material suitable for applications as load bearing parts. Furthermore, to enhance the mechanical properties (at the same porosity level), aligned pore structure was necessary. Therefore, highly porous Ti scaffolds with aligned pores were developed by unidirectional freeze casting. TiH2 /camphene slurry was frozen from the bottom to make aligned pores. Two kinds of experiment (various TiH2 contents, various casting time) were performed to control the pore structure. As the initial TiH2 content was increased from 10 to 20 vol.%, the compressive strength was significantly improved from 174 to 249 MPa. And as the casting time was increased, average pore size increased, but the compressive strength was decreased after more than 4 days of casting. Furthermore, the essential casting time to make large pores was reduced significantly by adjusting casting temperature. The camphene dendrites grew much faster at 45 °C, due to higher diffusion rate. After 1 day of casting, average pore size was almost 300 μm. In the second study, highly porous titanium with aligned large pores up to 500 lm in size, which is suitable for scaffold applications, was successfully fabricated using the reverse freeze casting method. In this process we have newly developed, the Ti powders migrated spontaneously along the pre-aligned camphene boundaries at a temperature of 45.5 °C and formed a titanium–camphene mixture with an aligned structure