Hydroxyapatite-βTri-Calcium Phosphate Scaffold Assembly and Characterization

Objectives:

Directed assembly and characterization of composite Hydroxyapatite-bTri-Calcium Phosphate (HA-bTCP) scaffolds with designed pore distribution for controlled solubility.

Methods:

Concentrated (45 v/o), aqueous colloidal gel ink for extrusion-based direct writing was formulated with commercial HA and bTCP powders blended with, ammonium poly(acrylate) (dispersant), hydroxypropylmethylcellulose (viscosifier), and Polyethyleneimine (flocculant). Prior to ink formulation, HA and bTCP powders were calcined, milled, dried and characterized by X-ray diffraction. Mixtures of HA:bTCP in volumetric ratios of 15:85 and 60:40 formulated as inks. Disk samples (11mm diameter x 3mm thick) with an internal lattice structure of 250µm rods, spaced by 250µm or 500µm in a periodic, open pore network were printed and sintered at 1100^oC for 4 hours. The control set for each composition and pore structure had empty pores while the test groups were filled with 2.4% Chitosan gel or Calcium Sulfate (CaS). The scaffolds were immersed in simulated body fluid (SBF) and weight loss was measured by gravimetric analysis on a three day frequency, with SBF change.

Results:

Empty, HA rich (60:40) scaffolds decreased in weight for 14-16 days due to bTCP dissolution, and stabilized at approximately 10% weight loss. Empty, bTCP rich (15:85) scaffolds decreased monotonically in weight 45 days showing approximately 20% total weight loss, but a slowing of weigh loss rate after 7 days. CaS-filled scaffolds decreased in weight, with the largest weight loss due to CaS dissolution. In contrast, chitosan filled samples showed minimal weight loss, attributed to shielding of the bTCP from SBF by the relatively stable chitosan hydrogel.

Conclusion:

Designer porous HA-bTCP scaffolds may also include controlled solubility behavior by controlling HA:bTCP ratio and selecting soluble pore filler. CaS may be used as a sacrificial pore filler to moderately retard scaffold dissolution, whereas chitosan hydrogel may persist as filler for longer periods.