Enhanced injectability of aqueous β-tricalcium phosphate suspensions through PAA incorporation, gelling and preshearing

Abstract

The major shortcoming of aqueous calcium phosphate suspensions used in biomedical applications is their unstable flow during delivery by mechanical means. In this study, microstructural changes and the resulting flow instabilities of aqueous β-TCP suspensions are demonstrated under both pressure-induced and drag-induced flow regimes and then remedied with the incorporation and subsequent gelling and preshearing of Carbopol 940, a biocompatible hydrogel. Mixing and dispersion of calcium phosphate particles into the hydrogel matrix was not efficient under simple agitation conditions. Swelling of the polymer chains was induced at approximately pH = 9.0 by water and particle intrusion within the opened-up coil structure due to deprotonation of the carboxylic acid groups by NaOH. As a result the composite material underwent a rapid viscoplastic transition into a doughy state which was not amenable to further processing without preshearing. Manual kneading converted the material into viscous state and enhanced the flow behavior significantly. Preshearing and probing the microstructure by mechanical spectrometer revealed multiple microstructural mechanisms responsible for the observed stable flow behavior, including improved dispersion of the particles, attrition of the polymeric network into microgel domains, enhanced adhesion and lubrication between the solid and liquid phase, crosslinking of the polymeric network. The net effect of these probable mechanisms was stiffening of the composite matrix, mobilization of solid particles and a marked enhancement in the stability of pressure-induced flow. The resistance of the material to liquid phase migration and its ability to undergo wall-slip and relax under stress were confirmed by simultaneous capillary rheometry and thermogravimetric analyses. The processing method enables improvements in the delivery of this composite material for injection and direct ink writing of scaffolds.