Fabrication of Extremely Soft Cellular Hydrogels for Vascular Modelling
In the past three decades, microtissue models grown inside miniaturized fluidic systems fabricated using soft lithography, were probably the most widespread form of engineered cellular structures. This conventional methodology has now been challenged by biofabrication techniques that offer shorter lead times and greater three-dimensional design freedom, while circumventing the manual alignment and inter-layer bonding challenges of soft lithography. As a result, attention has moved towards additive fabrication solutions.
Fused deposition modelling (FDM), inkjet, and stereolithographic projection-based 3D-printing solutions have demonstrated the possibility of printing master molds and sacrificial structures as well as encapsulated fluidic networks directly. As an example, we will briefly explore one such indirect fabrication strategy that we have developed for large area fabrication of multi-material soft structures (Multi-layered Micro-casting). Despite the simplicity and cost-efficiency of such indirect methods, an alternative direct bioprinting technique would overcome the geometrical limitations and enable better automation and process yield. However, direct printing in extremely soft hydrogels has its own diversity of challenges. These techniques typically require the use of support structures when printing overhanging features such as encapsulated fluidic networks. This support material is, in some cases, entirely impractical to remove from small-scale channels. Additionally, due to the material deformation during printing, most existing printing techniques are limited to materials that are orders of magnitude higher in elastic modulus than biological tissue.
In contrast, we introduce a new additive technique, computed axial lithography (CAL), which enables volumetric 3D-printing by illuminating a rotating volume of photosensitive material with a 3D light intensity map constructed from the angular superposition of many 2D projections. Oxygen inhibition-induced thresholding of the materials’ dose response enhances patterning contrast. We report the application of CAL to fabricate structures in methacrylated gelatin with elastic moduli as low as 100 Pa, as well as stiffer acrylates. Uncured resin provides mechanical support during printing, eliminating the need for solid support. Among other advantages are print durations as low as a few minutes and the possibility to overprint gels on/in other structures.