At the UKON, 4-Nitrophenylphosphate (4NPP) was used as a photolabile molecule for phosphate release. By adjusting the pH and the concentrations of the calcium source and the photolabile molecule, as well as the accessibility of CO2 from the air, we were able to find the perfect conditions to precipitate calcium phosphate. Also, we were switching to the synthesis of larger ring structures as photoactive agents, with more red-shifted absorbance than 4NPP. We performed full characterization of the molecules and the splitting mechanism (Absorption, NMR and titration experiments). We calculated that 4NPP has a photochemical quantum yield of 2 %, whereas the larger ring structures according to the NMR studies split off almost completely. Subsequently, a calcium source was inserted and in the case of 4NPP, a solid precipitate was obtained. Characterization (IR, pXRD, SEM/ EDX) doubtlessly shows that the precipitate consists of amorphous carbonate-apatite Type B. For the hard matrix in the case of 4NPP, an agarose-sucrose mixture was used, transparent to visible light; we also established other matrices: polyacrylamide and organic solvent-based matrices for the larger organic ring structures. We were able to cover wavelength ranges 390-460 nm, establishing a huge toolbox of photoinitiators, matrix precursors, and photolabile molecules to adapt the system for user needs. Nonetheless, the characterization of calcium phosphate inside the matrix, hard to achieve, is still under ongoing research. To sum up, we were able to precipitate a calcium phosphate locally inside a matrix and characterize it, which can be of great interest in the future for research on implant materials. At the University of Bologna, two novel routes were explored to reach the precipitation of calcium carbonate through the irradiation of a precursor in the liquid-state. (1) As a first approach, the source of calcium ions was modified from the original formula [Menichetti A. et al, 2021]: an aqueous suspension based on calcium ions and ketoprofen in the form of a fluid paste was investigated. The main work concerned the optimization of formulations. Upon LED irradiation the paste was quickly converted into amorphous calcium carbonate, as proven by IR-spectroscopy. Nonetheless, significant amount of the photo-reaction sub-products and water still remain in the final materials, affecting its mechanical properties and not making it actually suitable for 3D-printing. Indeed, further studies will be necessary to tackle these drawbacks. (2) As a second approach, the light-induced aggregation was performed by mixing calcium carbonate powder and a photocurable-polymeric resin. New formulas were studied by mixing calcium carbonate powder and a photo-polymer resin with different methods. The stability of the blend over time was checked, and it was confirmed that mixtures were stable in a time range suitable for 3D-printing processes. Simple 3D-models were printed by loading the 3D-printer with the novel formulas. The models appeared compact and regular in shape, and the chemical-homogeneity all over the objects was confirmed by IR spectroscopy. These sounded like great background for the next investigations.
