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
(PO43–) was quantified and changes in
pH were monitored to explain the preferential formation of tubes within
a PO43– 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.