Controlled mineralisation and recrystallisation of brushite within alginate hydrogels

Abstract

Abstract. Due to high solubility and resorption behaviour under physiological conditions, brushite (CaHPO 4 ·2H 2 O, calcium monohydrogen phosphate dihydrate, dicalcium phosphate dihydrate) has great potential in bone regeneration applications, both in combination with scaffolds or as component of calcium phosphate cements. The use of brushite in combination with hydrogels opens possibilities for new cell based tissue engineering applications of this promising material. However, published preparation methods of brushite composites, in which the mineral phase is precipitated within the hydrogel network, fail to offer the necessary degree of control over mineral phase, content and distribution within the hydrogel matrix. The main focus of this study was to address these shortcomings by determining precise fabrication parameters needed to prepare composites with controlled composition and properties. Composite alginate microbeads were prepared using a counter-diffusion technique which allows for simultaneous crosslinking of the hydrogel and precipitation of an inorganic mineral phase. Reliable nucleation of a desired mineral phase within the alginate network proved more challenging than simple aqueous precipitation. This was largely due to ion transport within the hydrogel producing concentration gradients that modified levels of supersaturation and favoured the nucleation of other phases such as hydroxyapatite and octacalcium phosphate which would otherwise not form. To overcome this, incorporation of brushite seed crystals resulted in good control over the mineral phase and by adjusting the amount of seeds and precursor concentration, the amount of mineral could be tuned. The material has been characterized with a range of physical techniques, including scanning electron microscopy, powder X-ray diffraction and Rietveld refinement, Fourier transform infrared spectroscopy and thermogravimetric analysis, in order to assess mineral morphology, phase and amount within the organic matrix. The mineral content of the composite material converted from brushite into hydroxyapatite when submerged in simulated body fluid, indicating possible bioactivity. Additionally, initial cell culture studies revealed that both the material and the synthesis procedure is compatible with cells relevant to bone tissue engineering.

A correlative spatiotemporal microscale study of calcium phosphate formation and transformation within an alginate hydrogel matrix

“…We have previously shown the potent effect of brushite seed crystals on phase selection within alginate hydrogels and with the added benefit of our spatiotemporal characterization toolbox we further investigate this effect here. [15] It has previously been shown that alginate, and especially the guluronate residues (Gblocks) which have a high affinity for calcium, have an inhibitory effect on the nucleation (and growth) of calcium phosphate crystals. [16].…”

“…This approach allows control over the resulting CaP phase and has recently been investigated in particular for the formation of HAp and brushite. [14,15] In order to produce phase pure alginate-brushite composites, seed crystals were used to initiate nucleation, since conditions which normally produce brushite when precipitated in solution, resulted in HAp inside the gel network, irrespective of the precursor concentrations and initial pH. [15] Further investigation into this phenomena revealed an inhibitory effect of alginate on the growth and nucleation of brushite in the presence of small amounts of alginate.…”

“…[14,15] In order to produce phase pure alginate-brushite composites, seed crystals were used to initiate nucleation, since conditions which normally produce brushite when precipitated in solution, resulted in HAp inside the gel network, irrespective of the precursor concentrations and initial pH. [15] Further investigation into this phenomena revealed an inhibitory effect of alginate on the growth and nucleation of brushite in the presence of small amounts of alginate. [16] A thorough understanding of CaP formation and transformation processes is essential for both fundamental studies of biomineralization and for the development of synthetic hard tissue engineering scaffold biomaterials.…”

Mapping the Distribution of Particles in Hydrogel in Use for Nanoplasmonic Biosensors: A Comparison of Preparation Methods

“…Some of these stimuli-responsive features of the hydrogel have shown to be very useful in controlling drug release [1] or to be useful for label-free and specific sensing of biomolecules [19,22]. Other properties of the gel-polymer network may also be of importance for the crystallization of minerals [4,23] or for the nucleation and growth of metal nanoparticles made by chemical reduction in the nano-fillers of the hydrogel matrix [24,25]. These hydrogels embedded with noble metal nanoparticles (NMNP) are fascinating materials often utilized for wound healing [26,27] but also impressive materials as nanoplasmonic systems [28,29].…”

Nucleation and Growth of Brushite in the Presence of Alginate

“…15 A recent study conducted in our group showed alginate hydrogels mineralized with brushite were potential candidates for bone tissue engineering applications. 16 Incorporation of bioceramics in alginate hydrogels enhances both the mechanical properties and the bioactivity of the polymer network. Brushite was chosen as the mineral phase due to its high solubility among calcium phosphates under physiological conditions, which enables for ready transformation and bone remodeling by providing necessary space and ion source for bone-like mineral formation in vivo.…”