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A crucial property for the evaluation of bioinks, besides biocompatibility, is printability, which is determined by resolution and shape fidelity. Recently, fiber reinforcement was used to overcome rheological limitations and introduce biomimetic structuring. This study provides a systematic approach to evaluate the printability of fiber reinforced hydrogels. Alginate and Pluronic hydrogels were blended with cellulose nanofibers (CeNF) and polycaprolactone (PCL) microfibers. SEM imaging revealed fiber-induced structural changes. Oscillatory rheological experiments showed that the addition of fiber fragments significantly altered the complex viscosity. A customized setup was utilized to determine strut spreading behavior in a real extrusion printing process. Strikingly, the data displayed excellent correlation with viscoelastic model-based predictions. CeNF increased the shape fidelity of both hydrogels, while PCL microfibers increased the viscosity but resulted in a time dependent loss of structural integrity in Pluronic. The results emphasize the need to complement shear-rheological analysis of bioinks by print-related customized analytical tools. Graphic abstract

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When characterizing the viscoelastic properties of polymers, shear rheological measurements are commonly the method of choice. These properties are known to affect extrusion and nozzle-based processes such as fiber melt spinning, cast film extrusion and 3D-printing. However, an adequate characterization of shear thinning polymers can be challenging and still insufficient to not only describe but predict process relevant influences. Furthermore, the evaluation of rheological model systems in literature is mostly based on stress–relaxation experiments, which are rarely available for various polymeric materials. Therefore, a simple approach is presented, that can be used to evaluate and benchmark a wide range of rheological model systems based on commonly accessible frequency sweep data. The approach is validated by analyzing alginate PH176 solutions of various concentrations, a thermoplastic poly-urethane (TPU) Elastollan 1180A melt, the liquid silicon rubber Elastosil 7670 and a polycaprolactone (PCL) fiber-alginate composite system. The used rheological model systems, consisting of simple springs and dashpots, are suitable for the description of complex, viscoelastic material properties that can be observed for polymer solutions and gel-like systems. After revealing a suitable model system for describing those material properties, the determination and evaluation of relevant model parameters can take place. We present a detailed guideline for the systematic parameter revelation using alginate solutions of different concentrations as example. Furthermore, a starting point for future correlations of strut spreading in 3D-bioprinting and model parameters is revealed. This work establishes the basis for a better understanding and potential predictability of key parameters for various fabrication techniques.

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