Resorbable dicalcium phosphate bone substitutes prepared by 3D powder printing

Bioceramic bone substitutes with programmed architecture were manufactured at room temperature in this study using a novel 3D printing process that combined 3D powder printing with calcium phosphate cement chemistry. During printing, biphasic alpha/beta-tricalcium phosphate (Ca-3(PO4)(2), TCP) powder reacted with a liquid component consisting of phosphoric acid solution to form a matrix of dicalcium phosphate dihydrate (CaHPO4 center dot H2O, DCPD, brushite) and unreacted TCP. Printed samples showed compressive strengths between 0.9-8.7 MPa after printing depending on the acid concentration. A further strength improvement to a maximum of 22 MPa could be obtained by additional hardening of the samples in phosphoric acid for three one minute washes. After this treatment, the samples mainly consisted of brushite with minor phases of unreacted TCP and a lesser amount of dicalcium phosphate anhydrate (CaHPO4, DCPA, monetite). Hydrothermal conversion of brushite to DCPA resulted in an increase of porosity of approximately 13% and a decrease of strength to 15 MPa, however the resorption rate in vivo was increased as demonstrated after intramuscular implantation over 56 weeks. Major advantages compared with commonly used sintering techniques are the low processing temperature, which enables the fabrication of thermally instable and degradable matrices of secondary calcium phosphates.