Creating More Complex Constructs with Coaxial Bioprinting
Introduction to coaxial bioprinting
Multimaterial bioprinting is used to achieve complex structures composed of two or more hydrogels that are able to modulate mechanical, chemical or biological properties. One such method is coaxial extrusion, which simultaneously dispenses two or more bioinks or other biomaterials arranged concentrically in a single filament. The approach permits the extrusion of a sacrificial sheath and a core material in a single filament, or it enables the extrusion of cell-laden layers for co-culture models. Most commonly, coaxial bioprinting involves using a needle nozzle with two or more concentric orifices, but it is also possible to print designs that require many more orifices.
Requirements for coaxial bioprinting
To enable the extrusion of biomaterials in configurations, an equal number of cartridges and extrusion sources to orifices is needed; and this often limits the use of coaxial bioprinting with advanced bioprinters that can independently control many printheads.
The biomaterials that can be used in a coaxial bioprinter include hydrogels, cells, sacrificial materials and crosslinking solutions. Depending on the technique selected, the technology allows for the creation of extruded filaments with a variety of structures, including solid fibers, hollow tubes, composite 3D structures and complex layered structures. Whether a biomaterial is used as the core or as the sheath component in coaxial extrusion is important for the resulting characteristic of the filament. For example, cellular co-culture is common in coaxial bioprinting, as the coaxial configuration allows two different cell-laden materials to be co-deposited in defined layers.
Advantages of coaxial bioprinting
The scalability of bioprinted constructs toward clinically relevant sizes is often limited by the accessibility of nutrients throughout the construct because nutrient access and waste removal depend solely on diffusion-mediated transport. As a result, perfusable networks within bioprinted constructs are imperative to create tissues of clinically relevant size since they allow adequate nutrient availability and will prevent waste accumulation in the innermost regions of the construct. The transfer of nutrients and waste will, in turn, facilitate the maturation of multilayered constructs and shorten the gap between in vivo and in vitro functionality. Additionally, hollow tubular structures can also be beneficial for generating multilayered large- and medium-diameter vascular grafts for use in either transplantation or disease modeling.
Applications for coaxial bioprinting
Applications enabled in the broader bioprinting field that require filaments that are multimaterial and multilayered in composition include many tissues and 3D models—from vasculature networks or tissue structures (such as blood-brain barrier) to tubular tissues (such as peripheral nerve or renal nephron units) to components of the small intestine. Additionally, coaxial extrusion can be used to perform in situ crosslinking in order to enable the extrusion of otherwise unprintable biomaterials such as alginates. One way to create tubular structures is by printing a single or multilayer hydrogel sheath out of an alginate-based material with a CaCl2 crosslinking solution core. In a simpler application, the technique can be used for the fabrication of smaller models such as spheroids and organoids by depositing droplets comprised of two or more layered biomaterials or cell types.
Best practices when doing coaxial bioprinting
Retest your bioinks before printing
When using coaxial bioprinting, one must consider the narrower range of bioinks that are compatible with the confined geometry of a coaxial needle. It is often advantageous and necessary to retest bioinks for coaxial bioprinting, although the bioinks are already being used for traditional bioprinting. This can often be done by using needle lengths and gauges that match the coaxial needle setup that one intends to use (e.g, 0.5-inch length and pressures below 200 kPa). For the extrusion of biomaterials requiring over 200 kPa, an external pump can be connected to the bioprinter to reach a pressure up to 700 kPa.
Minimize the use of thickeners
Bioinks and biomaterials that have a better chance of success in coaxial bioprinting include lower viscosity or homogenous biomaterials because of their better compatibility with needles. Heterogeneous biomaterials that contain microparticle thickeners may have a higher rate of clogging in the coaxial needle given the geometrical restrictions. It may be necessary to minimize or eliminate the use of thickeners in coaxial bioprinting. One can compensate for the loss of shear thinning and other favorable mechanical properties imparted by thickeners by using a crosslinking solution, such as CaCl CaCl2, or photocuring techniques, during or after extrusion to allow in situ crosslinking and reach desired mechanical properties.
Consider the added complexity
When analyzing the printability of a multimaterial filament, one must consider not only the target diameter of the filament but also the respective ratio of the core to sheath components. The multiple material streams involved in the generation of coaxial filaments result in added complexity to the filament structure. Optimizing flow rates and considering material-material interactions are critical for the consistent generation of uniform filaments. For example, the overextrusion of the core component of a bioprinted coaxial filament can result in filament bulging, while underextrusion of the core component can result in structural failure and discontinuity in the bioprinted filament.