Biologically Analogous Calcium Phosphate Tubes from a Chemical Garden.

Calcium phosphate (CaPO) tubes with features comparable to mineralized biological microstructures, such as Haversian canals, were grown from a calcium gel/phosphate solution chemical garden system. A significant difference in gel mass in response to high and low solute phosphate equivalent environments existed within 30 min of solution layering upon gel (p = 0.0067), suggesting that the nature of advective movement between gel and solution is dependent on the solution concentration. The transport of calcium cations (Ca) and phosphate anions (PO) was quantified and changes in pH were monitored to explain the preferential formation of tubes within a PO concentration range of 0.5-1.25 M. Ingress from the anionic solution phase into the gel followed by the liberation of Ca ions from the gel was found to be essential for acquiring self-assembled tubular CaPO structures. Tube analysis by scanning electron microscopy (SEM), X-ray diffraction (XRD), and micro X-ray florescence (μ-XRF) revealed hydroxyapatite (HA, Ca(PO)(OH)) and dicalcium phosphate dihydrate (DCPD, CaHPO·2HO) phases organized in a hierarchical manner. Notably, the tubule diameters ranged from 100 to 150 μm, an ideal size for the permeation of vasculature in biological hard tissue.