Design of composite hydrogels for extrusion bioprinting: molecular interactions, rheological behavior and printability

Bioinks for extrusion bioprinting consist of a cell population suspended in a biopolymer solution or a weakly crosslinked hydrogel. In order to be properly extruded in a well-defined 3D objects, bioinks must possess an adequate rheological behavior. More precisely, the polymer matrix of the ink should display (i) shear thinning properties, (ii) a yielding behavior and (iii) a rapid recovery of their viscosity post printing. Such properties are seldom achieved with simple biopolymer solutions.To address this issue, nanoparticles (NPs) have been recently introduced in bioink formulations. By interacting with the polymer chains in the ink, NPs have been found to induce shear-thinning and yielding behaviors in polymer-based bioinks thereby improving their printability. However, there is no systematic understanding of the link between the NP/polymer interactions, the resulting rheological properties and the ultimate printability of the system. Herein, we address this question in two distinct parts. Firstly, by characterizing the rheology of synthetic polymer solutions (Pluronic) and submitting them to printing experiments, we precisely quantified, for the first time, the rheological parameters (shear thinning coefficients and yield stress values) to be targeted in order to ensure an adequate printability. Secondly, by performing rheology and printing experiments on composite systems in which biopolymers and NPs interact in different ways, we described the effect of specific molecular interactions on the rheology and printability of composite systems. For that, hyaluronic acid (HA) was used as biopolymers in combination with silica nanoparticles (SiNPs). HA was selected for its biocompatibility, biodegradability and its previous use in bioink formulations4 while SiNPs were chosen because their surface chemistry can be modified so as to tune their interaction with HA.5 Different formulations of the composite systems were prepared upon varying structural (SiNP size, chain length, concentration) and chemical (negatively and positively charged SiNP surface) parameters. The result of rheological analysis showed that the nature and the extent of the polymer/NP interaction greatly influence the rheology of composite systems. Drawing a parallel with the first part of this work, we demonstrated that weak NP/Polymer interactions (electrostatic, hydrogen bond) were not sufficient to reach the targeted rheological parameters for adequate printing.