Amorphous Calcium Phosphate and Amorphous Calcium Phosphate Carboxylate: Synthesis and Characterization

The authors acknowledge financial support from the European Union’s Horizon 2020 Research and Innovation Program under grant agreement no. 857287 and Baltic Research Programme Project No. EEA-RESEARCH-85 “Waste-to-resource: eggshells as a source for next generation biomaterials for bone regeneration (EGGSHELL)” under the EEA Grant of Iceland, Liechtenstein and Norway No. EEZ/BPP/VIAA/2021/1. Institute of Solid State Physics, University of Latvia, received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART. 2Amorphous calcium phosphate (ACP) is the first solid phase precipitated from a supersaturated calcium phosphate solution. Naturally, ACP is formed during the initial stages of biomineralization and stabilized by an organic compound. Carboxylic groups containing organic compounds are known to regulate the nucleation and crystallization of hydroxyapatite. Therefore, from a biomimetic point of view, the synthesis of carboxylate ions containing ACP (ACPC) is valuable. Usually, ACP is synthesized with fewer steps than ACPC. The precipitation reaction of ACP is rapid and influenced by pH, temperature, precursor concentration, stirring conditions, and reaction time. Due to phosphates triprotic nature, controlling pH in a multistep approach becomes tedious. Here, we developed a new ACP and ACPC synthesis approach and thoroughly characterized the obtained materials. Results from vibration spectroscopy, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), true density, specific surface area, and ion release studies have shown a difference in the physiochemical properties of the ACP and ACPC. Additionally, the effect of a carboxylic ion type on the physiochemical properties of ACPC was characterized. All of the ACPs and ACPCs were synthesized in sterile conditions, and in vitro analysis was performed using MC-3T3E1 cells, revealing the cytocompatibility of the synthesized ACPs and ACPCs, of which the ACPC synthesized with citrate showed the highest cell viability. © 2023 The Authors. Published by American Chemical Society --//-- https://pubs.acs.org/doi/10.1021/acsomega.3c00796. Published under the CC BY 4.0 licence.EEA Grant of Iceland EEZ/BPP/VIAA/2021/1; Horizon 2020 Framework Programme 857287, EEA-RESEARCH-85; institute of Solid State Physics, University of Latvia, received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART.

Development of nanocomposite hydrogel using citrate-containing amorphous calcium phosphate and gelatin methacrylate

Nanocomposite hydrogels are suitable in bone tissue engineering due to their resemblance with the extracellular matrix, ability to match complex geometries, and ability to provide a framework for cell attachment and proliferation. The nanocomposite hydrogel comprises organic and inorganic counterparts. Gelatin methacrylate (GELMA) is an extensively used organic biomaterial in tissue engineering due to its excellent biocompatibility, biodegradability, and bioactivity. The photo-crosslinking of GELMA presents a challenge when aiming to create thicker nanocomposite hydrogels due to opacity induced by fillers, which obstructs the penetration of ultraviolet (UV) light. Therefore, using a chemical crosslinking approach, we have developed nanocomposite GELMA hydrogel in this study by incorporating citrate-containing amorphous calcium phosphate (ACP_CIT). Ammonium persulfate (APS) and Tetramethylethylenediamine (TEMED) were deployed to crosslink the methacrylate group of GELMA. The oscillatory shear tests have confirmed that crosslinking enhances both storage (G′) and loss modulus (G″) of GELMA. Subsequently, incorporation of ACP_CIT in GELMA hydrogel shows further enhancement in G′ and G″ values. In vitro analysis of the developed hydrogels revealed that chemical crosslinking and incorporation of ACP_CIT do not compromise the cytocompatibility of the GELMA. Hence, for developing nanocomposite GELMA hydrogels employing APS/TEMED crosslinking emerges as a promising alternative to photo-crosslinking