Bioglass/carbon nanocomposites from additive manufacturing of silicone-based mixtures

Contributors : : Dr Enrico Bernardo (1), Dr Hamada Elsayed (1), Dr Franco Matias Stabile (2)

Contact : enrico.bernardo@unipd.it, hamada.elsayed@unipd.it, mstabile@cetmic.unlp.edu.ar  

Institution : (1) Department of Industrial Engineering, University of Padova, Italy ; (2) CETMIC (Technology Center of Mineral resources and Ceramic), CIC-CONICET-UNLP, Manuel B. Gonnet, Argentina

Topic : Additive manufacturing for bioceramics

Silicones have been considered for the manufacturing of bioceramics for more than a decade. The feasibility of plastic shaping processes, to obtain products with complex shapes (to be later ceramized by heat treatment), is highly attractive, especially considering modern additive manufacturing technologies. Polycrystalline Ca-based silicates as well as Biosilicate glass-ceramics (see, as an example https://lnkd.in/e55HjN9k and https://lnkd.in/dfy_wWZc) are easily obtained, according to the introduction of suitable oxide fillers. Bioglasses, on the contrary, remain quite challenging.

A recently investigated methodology, provides a first solution for a new generation of 'polymer-derived bioglasses'. Calcium nitrate tetrahydrate, already used for sol-gel processed CaO-SiO2 bioglasses, constitutes a new CaO precursor. This specific additive is introduced in a blend of commercial silicone polymer and photocurable acrylates, by emulsification. Due to the low melting point, the nitrate salt forms liquid droplets, within the silicone-based blend, just by simple mixing. The droplets are kept nano-sized and homogeneously dispersed according to the use of a surfactant. The result is a uniform suspension of nano-sized salt particles in photocurable liquid matrix. Homogeneous mixtures lead to homogeneous vat stereolithography printed (highly porous) scaffolds. These scaffolds are finally converted into crack-free, fully amorphous ceramic composites, by pyrolysis at only 700 °C, in flowing N2. Emulsions support both printing, by limiting the scattering of light, which could be observed with coarse powder fillers, and synthesis of amorphous material, by impeding local oxide concentrations.

The final samples feature pyrolytic carbon nanoclusters embedded in a matrix resembling 70S30C bioglass (70 mol% SiO2, 30% CaO). The dispersed phase is interesting, since it enables a marked photothermal effect (https://doi.org/10.1016/j.addma.2023.103731).

Calcium nitrate tetrahydrate may be used also in form of aqueous solutions. Droplets, with the support of surfactants, may be emulsified again in silicone/acrylate blends and subjected to additive manufacturing. Liquid matrices with high silicone content, added with calcium nitrate tetrahydrate solution, are printable also by direct ink writing. The geometrical stability of reticulated scaffolds is achieved by a subsequent UV curing step, active on acrylate component. The use of a solution, instead of salt, enhances the gas release upon pyrolysis, resulting in highly porous scaffolds with ‘spongy’ struts, as illustrated by the image reported below.

The current research is intended at updates both in the chemical formulation (by introduction of sodium, magnesium and phosphorous salts), aimed at the development of further bioglass or glass-ceramic matrices, and in the topology, mainly to maximize the strength-to-density ratio.

Microstructural details of the printed scaffolds, using silicone-based emulsions, after ceramization: a,b) Top view; c,d) Side view