Keywords: ceramics, hydroxyapatite, alumina, 3D printing, grafts
Summary:Materic is developing a technique to manufacture custom bone grafts made of hydroxyapatite (the primary material in bone) that are extremely strong, can be produced in complex shapes, and have variable densities to match a patient’s own bone. Compared to autograft or allograft procedures, ceramic graft materials such as hydroxyapatite (HAp) do not face quantity limitations, carry limited risk of morbidity or infection of the donor site, and are easily sterilized and stored. Unlike most ceramics which are generally fragile, have poor mechanical strength, and can be difficult to mold into a desired shape; the Direct Coagulation Printing (DCP) process allows for 3D printing complex geometries without the need for “pre-scaffold” structures. Additionally, the inherent isotropic sintering of the material allows for increased mechanical properties, to control sintered density (to match the local bone density), and reduce the failure rate of manufactured parts. Direct Coagulation Printing (DCP) occurs similarly to the FFF/FDM process, though a slurry paste mixture is extruded at room temperature. The paste coagulates in air leaving a green part which can be isotopically sintered to achieve the resultant part. The DCP process also allows for other ceramic printing such as alumina for on-demand production of parts. The slurry is prepared by combining the desired ceramic material, such as alumina or HAp, with Triammonium Citrate (TAC) and DI water in a ball mill. The pH of the resulting slurry is then optimized to produce the desired coagulation. The method of coagulation and extrusion has demonstrated how overhangs can be printed with a hollow model rocket design. A closed-cell hollow part and fully dense turbine propeller prototype were also attained with this method. All parts were printed without the use of support material. The sintered parts are translucent white, transmitting light through walls as thick as 2 mm. This is further evidence of the high relative density and purity of the sample because of the sensitivity of optical properties of alumina to defects such as pores and impurities. It was found that, among 15 samples, there was an average isotropic shrinkage of 19% with a standard deviation of 3%. Our next steps are to modify an open FFF 3D printer for paste printing using an auger-based setup. In the initial tests, a syringe-based extruder was used but the extrusion rate was inversely proportional to the level of paste in the syringe. The paste is compressible and would experience elevated levels of shear thickening as the build progresses. With an auger-based extrusion we hope to get more consistent values of shear thickening and therefore more consistent extrusion rates at different levels of paste. We then plan to develop a scalable SOP for HAp and alumina printing. Market share for synthetic grafts is expected to grow significantly in the coming years because they are generally more biocompatible, with lower risk of disease transmission and better patient acceptance.