Systematic analysis of the extrusion process in 3D bioprinting is mandatory for process optimization concerning production speed, shape fidelity of the 3D construct and cell viability. In this study, we applied numerical and analytical modeling to describe the fluid flow inside the printing head based on a Herschel–Bulkley model. The presented analytical calculation method nicely reproduces the results of Computational Fluid Dynamics simulation concerning pressure drop over the printing head and maximal shear parameters at the outlet. An approach with dimensionless flow parameter enables the user to adapt rheological characteristics of a bioink, the printing pressure and needle diameter with regard to processing time, shear sensitivity of the integrated cells, shape fidelity and strand dimension. Bioinks consist of a blend of polymers and cells, which lead to a complex fluid behavior. In the present study, a bioink containing alginate, methylcellulose and agarose (AMA) was used as experimental model to compare the calculated with the experimental pressure gradient. With cultures of an immortalized human mesenchymal stem cell line and plant cells (basil) it was tested how cells influence the flow and how mechanical forces inside the printing needle affect cell viability. Influences on both sides increased with cell (aggregation) size as well as a less spherical shape. This study contributes to a systematic description of the extrusion-based bioprinting process and introduces a general strategy for process design, transferable to other bioinks.
The formulation of high quality emulsions is a key challenge in many industrial applications. The premix emulsification process in porous membranes enables the generation of tailored emulsions with fine and narrow droplet size distributions under low shear and energy input. However, the droplet deformation and breakup process within porous structures is a complex mechanism and single breakup events are hard to relate to the local stress conditions and the pore geometry. This relation however is required for the proper design of membrane structures with specific emulsification behavior (i.e., avoidance of stress peaks). Thus, in this contribution, the stress residence time behavior of single droplets during deformation and breakup in idealized micro-pores is investigated for different Capillary numbers and droplet sizes. The interface stress induced droplet deformation and breakup process is to be analyzed in a generic flow configuration. The results show that interface stresses are applied by the wall interface (wall-droplet interface) and by the liquid-liquid (continuous-droplet interface) interface and that both stress contributions have to be considered separately in order to understand the droplet deformation and breakup process. Only at the liquid-liquid interface, stress induced deformation is possible. The analysis of the stress conditions delivers a correlation between the stress residence time behavior and the interface deformation, which can be directly related to the pore geometry. As a result, main deformation and breakup trends are derived. This enables better opportunities for proper membrane design and handling of shear sensitive media in the premix emulsification process.
Photocatalytic fuel cells (PFCs) are constructed from anodized photoanodes with the aim of effectively converting organic materials into solar electricity. The syntheses of the photoanodes (TiO2 , WO3 , and Nb2 O5 ) were optimized using the statistical 2(k) factorial design. A systematic study was carried out to catalog the influence of eleven types of organic substrate on the photocurrent responses of the photoanodes, showing dependence on the adsorption of the organic substrates and on the associated photocatalytic degradation mechanisms. Strong adsorbates, such as carboxylic acids, generated high photocurrent enhancements. Simple and short-chained molecules, such as formic acid and methanol, are the most efficient in the corresponding carboxylic acid and alcohol groups as a result of their fast degradation kinetics. The TiO2 -based PFC yielded the highest photocurrent and obtainable power, whereas the Nb2 O5 -based PFC achieved the highest open-circuit voltage, which is consistent with its most negative Fermi level.
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