Figure 5.

Coaxial and microfluidic methods are versatile tools to induce rapid crosslinking of extruded filaments. Additionally, using such extrusion devices permit to print multiple materials in a single process, as well as to control the composition and physicochemical properties of the forming hydrogel strands. A) Coaxial bioprinting of an alginate-based bioink (inner compartment) and its crosslinking agent CaCl2 (outer compartment). The rapid crosslinking via calcium ions enabled in situ crosslinking ensuring high shape fidelity printing. Reproduced with permission.^[149] Copyright 2016, IOPScience. B) Microfluidic approaches like the utilization of y-junctions in combination with a coaxial nozzle allowed the deposition of strands with parallel aligned multiple cell types, but has also been used to extrude different types of materials. Reproduced with permission.^[18] Copyright 2017, Elsevier. C) By coaxially extruding an alginate-based ink through the outer and the crosslinking agent through the inner nozzle, perfusable inks could be produced. Reproduced with permission.^[148] Copyright 2015, Elsevier. D) Microfluidic approaches can be used to generate gradients which play crucial roles in biology. Methods like flow focusing (images in the middle) can be also applied to 3D printing and help reducing filament diameters. Reproduced under the terms of the Creative Commons CC-BY 4.0 License (https://creativecommons.org/licenses/by/4.0/).^[163] Copyright 2018, The Authors, Published by MDPI.