Abstract
Hydrogels are an interesting class of materials used in extrusion-based 3D printing, e.g., for drug delivery or tissue engineering. However, new hydrogel formulations for 3D printing as well as a detailed understanding of crucial formulation properties for 3D printing are needed. In this contribution, hydrogels based on poly(ethylene glycol) diacrylate (PEG-DA) and the charged monomers 3-sulfopropyl acrylate and [2-(acryloyloxy)ethyl]trimethylammonium chloride are formulated for 3D printing, together with Poloxamer 407 (P407). Chemical curing of formulations with PEG-DA and up to 5% (w/w) of the charged monomers was possible without difficulty. Through careful examination of the rheological properties of the non-cured formulations, it was found that flow properties of formulations with a high P407 concentration of 22.5% (w/w) possessed yield stresses well above 100 Pa together with pronounced shear thinning behavior. Thus, those formulations could be processed by 3D printing, as demonstrated by the generation of pyramidal objects. Modelling of the flow profile during 3D printing suggests that a plug-like laminar flow is prevalent inside the printer capillary. Under such circumstances, fast recovery of a high vicosity after material deposition might not be necessary to guarantee shape fidelity because the majority of the 3D printed volume does not face any relevant shear stress during printing.
Highlights
The formulation of hydrogels for extrusion-based 3D printing has attracted increasing attention in recent publications [1,2,3]
All hydrogel formulations tested could be transferred from a sol state into a gel state above a certain temperature and showed high shear thinning above this temperature
Usability of the formulations, e.g., for direct cell encapsulation, is probably limited due to cytotoxicity of Poloxamer 407 (P407), the cured and washed hydrogels could be used as scaffolds in tissue engineering or as mineralization templates
Summary
The formulation of hydrogels for extrusion-based 3D printing has attracted increasing attention in recent publications [1,2,3]. PEG-based hydrogels through different cross-linking reactions, such as radical polymerization [27] or the thiol–Michael reaction [28] involving acrylate end groups, thiol-ene reactions involving unsaturated end groups [29], or enzymatic cross-linking using lysine and glutamine end groups [30] In all of these cases, it is possible to introduce functional building blocks into the otherwise inert PEG hydrogel, such as oligopeptides [31,32], charged moieties [33], or degradable groups [34]. Many functional acrylate monomers are commercially available and can be integrated into the hydrogel network by mixing the monomers into the hydrogel formulation before curing [35] In this way, charged monomers were integrated into PEG-DA hydrogels, resulting in tuned properties concerning, e.g., the equilibrium degree of swelling (EDS), stiffness, mineralization of inorganic salts inside the hydrogels, and the response of biological cells [33,36,37]. It can be expected that for formulations possessing significant yield stress in combination with shear thinning behavior, an interaction between yield stress and shear thinning concerning the flow profile becomes effective, as already shown for injectable hydrogels [44,45,46]
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