Abstract
Direct 3D bioprinting of bioreactors containing microorganisms embedded inside hydrogel structures is a promising strategy for biotechnological applications. Nevertheless, microporous hydrogel networks hinder the supply of nutrients and oxygen to the cell and limit cell migration and proliferation. To overcome this drawback, we developed a feedstock for 3D bioprinting structures with hierarchical porosity. The feedstock is based on a highly particle-filled alumina/alginate nanocomposite gel with immobilized E. coli bacteria with the protein ovalbumin acting as foaming agent. The foamed nanocomposite is shaped into a porous mesh structure by 3D printing. The pore radius diameters inside the non-printed, non-foamed nanocomposite structure are below 10 µm, between 10 and 500 µm in the albumin-stabilized foam and with additional pores in the range of 0.5 and 1 mm in the printed mesh structure. The influence of albumin on the bubbles and hence pore formation was analyzed by means of interfacial shear rheology and porosity measurements with X-ray microtomography (µCT). Furthermore, averaged diffusion coefficients of water in printed and non-printed samples with different albumin concentrations were recorded using nuclear magnetic resonance (NMR) tomography to assess the water content in the porous structure. Moreover, the effective viability and accessibility of embedded E. coli cells were analyzed for various material compositions. Here, the addition of albumin induced bacterial growth and the porosity increased the effective viability of the embedded bacteria, most likely because of enhanced accessibility of the cells. The experimental results demonstrate the potential of this approach for producing macroscopic bioactive materials with complex 3D geometries as a platform for novel applications in bioprocessing.
Highlights
The development of new hydrogel compositions and biocompatible cross-linking strategies opened the door to customize cell-containing materials into complex shapes by 3D bioprinting [1,2,3,4]
All compositions show similar flow behavior which indicates that the addition of albumin concentrations did not significantly influence viscosity and flow behavior of the suspensions which is instead dominated by the interactions between alginate and alumina particles
The results show that mostly albumin was responsible for film formation while alginate or the particles did not significantly stabilize the foam
Summary
The development of new hydrogel compositions and biocompatible cross-linking strategies opened the door to customize cell-containing materials into complex shapes by 3D bioprinting [1,2,3,4]. Hydrogel-based bioinks [11] are usually printed via extrusion of a viscoelastic feedstock followed by the deposition of the gel-like filaments and optionally a chemical cross-linking procedure. Both the printing process and the feedstock formulation must ensure high cell compatibility to avoid cell death during printing and cross-linking [12,13,14,15] while the viscoelastic properties of the feedstock need to be carefully tuned for optimal printing performance [16]. One of the most used natural hydrogels is alginate, which is a gelling polysaccharide with high biocompatibility that undergoes biocompatible ionotropic gelation with Ca2? ions [17]
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