title: A correlative spatiotemporal microscale study of calcium phosphate formation and transformation within an alginate hydrogel matrix.
abstract: The modification of soft hydrogels with hard inorganic components is a method used to form composite materials with application in non-load-bearing bone tissue engineering. The inclusion of an inorganic component may provide mechanical enhancement, introduce osteoconductive or osteoinductive properties, or change other aspects of interactions between native or implanted cells and the material. A thorough understanding of the interactions between such components is needed to improve the rational design of such biomaterials. To achieve this goal, model systems which could allow study of the formation and transformation of mineral phases within a hydrogel network with a range of experimental methods and high spatial and time resolution are needed. Here, we report a detailed investigation of the formation and transformation process of calcium phosphate mineral within an alginate hydrogel matrix. A combination of optical microscopy, confocal Raman microspectroscopy and electron microscopy was used to investigate the spatial distribution, morphology and crystal phase of the calcium phosphate mineral, as well as to study transformation of the mineral phases during the hydrogel mineralization process and upon incubation in a simulated body fluid. It was found, that under the conditions used in this work, mineral initially formed as a metastable amorphous calcium phosphate phase (ACP). The ACP particles had a distinctive spherical morphology and transformed within minutes into brushite in the presence of brushite seed crystals or into octacalcium phosphate, when no seeds were present in the hydrogel matrix. Incubation of brushite-alginate composites in simulated body fluid resulted in formation of hydroxyapatite. The characterization strategy presented here allows for non-destructive, in situ observation of mineralization processes in optically transparent hydrogels with little to no sample preparation.
  
  
  
  The precipitation and transformations of calcium phosphates (CaP) is a complex process, where both formation kinetics and the stability of different mineral phases control the outcome. This situation is even more complex if CaP is precipitated in a hydrogel matrix, where one can expect the organic matrix to modulate crystallization by introducing supersaturation gradients or changing the nucleation and growth kinetics of crystals. In this study we apply a range of characterization techniques to study the mineral formation and transformations of CaP within an alginate matrix with spatiotemporal resolution. It demonstrates how a detailed investigation of the mineral precipitation and transformations can aid in the future rational design of hydrogel-based materials for bone tissue engineering and studies of biomineralization processes.
authors: Department of Physics, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Electronic address: sindre.bjornoy@ntnu.no.
authors: Sindre H
authors: SH
authors: Bjørnøy
authors: Department of Physics, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Electronic address: david.bassett@ntnu.no.
authors: David C
authors: DC
authors: Bassett
authors: Department of Chemical Engineering, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Electronic address: seniz.ucar@ntnu.no.
authors: Seniz
authors: S
authors: Ucar
authors: Department of Biotechnology, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Electronic address: berit.l.strand@ntnu.no.
authors: Berit L
authors: BL
authors: Strand
authors: Department of Chemical Engineering, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Electronic address: jens-petter.andreassen@ntnu.no.
authors: Jens-Petter
authors: JP
authors: Andreassen
authors: Department of Physics, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Electronic address: pawel.sikorski@ntnu.no.
authors: Pawel
authors: P
authors: Sikorski
mesh_terms: D000464:Alginates / Q000737:chemistry*; D002130:Calcium Phosphates / Q000737:chemistry*; D020723:Glucuronic Acid / Q000737:chemistry; D006603:Hexuronic Acids / Q000737:chemistry; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008858:Microscopy, Phase-Contrast; D061848:Optical Imaging; D062211:Spatio-Temporal Analysis; D013059:Spectrum Analysis, Raman
chemical_list: D000464:Alginates; D002130:Calcium Phosphates; D006603:Hexuronic Acids; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D020723:Glucuronic Acid
keywords: Alginate; Calcium phosphate; Hydrogel; Raman spectroscopy