Cell seeding into calcium phosphate cement

To improve the effectiveness of calcium phosphate cement (CPC), we have developed a method to seed osteoblasts into the cement. CPC powder is mixed with water to form a paste that can be shaped to fit a bone defect in situ. The paste hardens in 30 min, reacts to form hydroxyapatite, and is replaced with new bone. Reacted CPC is biocompatible but unreacted CPC paste was found to have toxic effects when placed on cell monolayers (MC3T3-E1 cells). In contrast, when cells were indirectly exposed to CPC paste using a porous membrane or by placing a coverslip containing adherent cells onto a bed of CPC paste, the unreacted CPC was nontoxic. These results suggested that gel encapsulation of the cells might protect them from the CPC paste. Thus, cells were encapsulated in alginate beads (3.6-mm diameter), mixed with CPC paste, and incubated overnight. Both vital staining (calcein-AM and ethidium homodimer-1) and the Wst-1 assay (measures dehydrogenase activity) showed that cell survival in alginate beads that were mixed with CPC was similar to survival in untreated control beads. These results suggest that gel encapsulation could be used as a mechanism to protect cells for seeding into CPC.