Rheological characterisation of ceramic inks for 3D direct ink writing: A review

3D printing is a competitive manufacturing technology, which has opened up new possibilities for the fabrication of complex ceramic structures and customised parts. Extrusion-based technologies, also known as direct ink writing (DIW) or robocasting, are amongst the most used for ceramic materials. In them, the rheological properties of the ink play a crucial role, determining both the extrudability of the paste and the shape fidelity of the printed parts. However, comprehensive rheological studies of printable ceramic inks are scarce and may be difficult to understand for non-specialists. The aim of this review is to provide an overview of the main types of ceramic ink formulations developed for DIW and a detailed description of the more relevant rheological tests for assessing the printability of ceramic pastes. Moreover, the key rheological parameters are identified and linked to printability aspects, including the values reported in the literature for different ink compositions.Peer ReviewedPostprint (published version

Correlating rheological properties with direct ink writing printability in hydrogel – calcium phosphate slurries: effect of polymeric and ceramic content

Direct Ink Writing enables the fabrication of personalized scaffolds for bone tissue engineering. The ink must be smoothly extruded through a narrow nozzle without clogging to form continuous filaments, which must retain the nozzle shape and be capable of supporting its own weight during the assembly. This is linked to the viscoelastic properties of the ink, which should have a shear-thinning viscous behaviour at high shear rates, a high storage modulus at rest and a fast elastic recovery when flow stops [1]. The aim of this study was to develop a reliable method that allows linking the rheological properties of a calcium phosphate/hydrogel paste with its printability, analysing the effect of polymeric and ceramic contents. CaP pastes were obtained by mixing Pluronic hydrogels (polymeric content: 20-35 wt.%) with β-TCP powder (ceramic concentration: 50-70 wt.%). The rheological characterisation was carried out in a rotational rheometer, using a 20 mm rough parallel plate geometry and a 500 μm gap. All slurries showed a viscoelastic behaviour with a strong shear- thinning and a fast elastic recovery, both provided by the hydrogel network. The ceramic content affected significantly the properties at low shear rates: the elastic stiffness at rest, the percentage of elastic strength recovery and, above a 60 wt.%, the yield stress. Regarding printability, the filament shape-retention of extrudable pastes was assessed by image analysis of the sagging of a single filament printed over a row of pillars with increasing separation, whereas shape fidelity was evaluated by comparing 3D-printed scaffolds with the virtual model

Microsphere incorporation as a strategy to tune the biological performance of bioinks

Although alginate is widely used as a matrix in the formulation of cell-laden inks, this polymer often requires laborious processing strategies due to its lack of cell adhesion moieties. The main objective of the present work was to explore the incorporation of microspheres into alginate-based bioinks as a simple and tuneable way to solve the cell adhesion problems, while adding extra biological functionality and improving their mechanical properties. To this end, three types of microspheres with different mineral contents (i.e. gelatine with 0% of hydroxyapatite, gelatine with 25 wt% of hydroxyapatite nanoparticles and 100 wt% of calcium -deficient hydroxyapatite) were synthesised and incorporated into the formulation of cell-laden inks. The results showed that the addition of microspheres generally improved the rheological properties of the ink, favoured cell proliferation and positively affected osteogenic cell differentiation. Furthermore, this differentiation was found to be influenced by the type of microsphere and the ability of the cells to migrate towards them, which was highly dependent on the stiffness of the bioink. In this regard, Ca2+ supplementation in the cell culture medium had a pronounced effect on the relaxation of the stiffness of these cell-loaded inks, influencing the overall cell performance. In conclusion, we have developed a powerful and tuneable strategy for the fabrication of alginate-based bioinks with enhanced biological characteristics by incorporating microspheres into the initial ink formulation.Peer ReviewedPostprint (published version

Numerical simulation of the micro-extrusion process of printable biomaterials

This work aims to gain a better understanding of how the rheological properties of printable materials affect their processability, as well as the quality of the final product, which at the end can lead to reducing time and costs of the process and increase product development. As the first step, the proper rheological non-Newtonian models are extracted from experimental studies. Later, three-dimensional numerical simulation of extrusion process is performed in the context of Direct Numerical Simulation (DNS) of governing equations, where the whole physics of fluid motion is taken into account. A finite-volume fractional step approach is used to solve the Navier-Stocks equations on collocated arbitrary meshes. Geometrical volume-of-fluid (GVOF) interface capturing approach is used to resolve the topological changes of the moving interface. The governing equations are solved using High-Performance Computing (HPC) parallel approaches. Besides the contribution of this work to the advancement of numerical techniques applied to multiphase complex flows, obtained results will shed light on the nature of non-Newtonian extrusion process with vast applications in the 3D printer industrial sectors.This work was developed in the context of a research project (BASE3D 001-P-001646) co-financed by the European Union Regional Development Fund within the framework of the ERDF Operational Program of Catalonia 2014-2020 with a grant of 50% of total cost eligible.Postprint (published version