Biologically Analogous Calcium Phosphate Tubes from a Chemical Garden

Calcium phosphate (CaPO₄) 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 (Ca²⁺) and phosphate anions (PO₄^3-) was quantified and changes in pH were monitored to explain the preferential formation of tubes within a PO₄^3- concentration range of 0.5-1.25 M. Ingress from the anionic solution phase into the gel followed by the liberation of Ca²⁺ ions from the gel was found to be essential for acquiring self-assembled tubular CaPO₄ structures. Tube analysis by scanning electron microscopy (SEM), X-ray diffraction (XRD), and micro X-ray florescence (μ-XRF) revealed hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) and dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O) 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.