Authors of an new Early View story on the website of the Journal of the American Ceramic Society report about a solution they have found to some of the problem of shrinkage and deformation that occurs during sintering of large and complex parts composed of one type of bioactive glass.
The investigators, who are from the Department of Materials Science and Engineering, University of Erlangen-Nuremberg (Germany) and the BAM Federal Institute for Materials Research and Testing (Berlin), have been looking at how to improve the performance and production of 3D-printed “13-93” bioactive glass and they say the addition of hydroxyapatite powder, creating a glass–ceramic composite for 3D printing, creates a finished product that retains more of the critical shape and dimensions during sintering than pure powders of the glass.
13-93, a silicate-based glass, isn’t new and several groups of researchers (such as Rahaman et al.) have generally documented that 13-93 is a good candidate material for non-load bearing uses in joint replacement and tissue engineering. The active interest in bioactive glasses, such as 13-93, is in large part due to the apparent ability of the material to accelerate the body’s natural healing process.
Different groups had experimented with using different processes to create green body structures using 13-93 powders and filaments, including fairly precise 3D fabrication and finishing methods, such as selective laser sintering. However, generally speaking, the larger and more complex the green body is, the more problematic sintering becomes. The authors of the JACerS paper report these types of parts “may deform significantly as a result of gravity, surface tension, intrinsic strain or temperature and density gradients. This complicates congruent or net-shape processing.”
The attractiveness of 3D processing is the promise of high-quality and easily reproducible shapes, pore size and distribution, etc.
The payoff in the German group’s work is that they found that a 13-93/HAp powder mix using 40 wt% of crystalline material provided the best combination of geometric stability and viscous sintering. They tested this formulation using the complex cellular cubic structure pictured above, and they were quite happy with the results. They note
“In this way, an overall axial shrinkage of about 20.5 ± 0.5% was obtained in all three dimensions. The diameter of cells was reproduced with an accuracy of 15 ± 5%, whereby the deviation is most probably related to surface effects induced by the printing process and manual powder removal. The ratio between individual cell diameters—the fingerprint of the specific structure—was reproduced with an accuracy of about 2%. … these data demonstrate very good reproduction of the 3D-printed part after sintering.”
Also, the addition of the HAp powder seems to not increase the propensity for crystallization of the bioglass, another problem that may change the properties that made the material desirable in the first place.
The authors suggest that other glass–ceramic composite candidates should be suitable for similar production methods.
More information can be found in “Sintering of 3D-Printed Glass/HAp Composites (doi: 10.1111/j.1551-2916.2012.05368.x).