Computed axial lithography (CAL) is a volumetric additive manufacturing method in which a three-dimensional light dose distribution is constructed in a photopolymer from the superposition of illumination patterns from many different angles. The technique’s advantages over layer-by-layer light printing methods stem from the fact that in CAL hydrodynamic stresses are effectively eliminated from the resin precursor material during printing. This key difference allows a wider range of materials to be processed, including high-viscosity or thermally gelled precursors, and allows polymeric objects to be printed around pre-existing solid objects (‘overprinting’). In this talk we describe some of the current limitations on spatial resolution, printing speed, and mechanical properties in CAL. We also describe a computationally efficient approach to modeling the occlusion of light by objects suspended in the printing volume, which supports the optimization of overprinting processes.
Soft lithography provides a convenient technique for prototyping miniaturized fluidic systems. However, 3D-printing techniques offer shorter lead times and greater three-dimensional design freedom, as well as 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 as well as encapsulated fluidic networks directly. However, all of these techniques typically require the use of solid support structures when printing overhanging features as are required for encapsulated fluidic channels. This support material is time-consuming or, in some cases, entirely impractical to remove from small-scale, encapsulated channels. Additionally, most existing printing techniques are limited to materials that are orders of magnitude higher in elastic modulus than biological tissue. Finally, process-induced surface roughness makes microscopy challenging.
In contrast, we have introduced 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. The projections are computed via the exponential Radon transform followed by iterative optimization. Oxygen inhibition-induced thresholding of the materials’ dose response enhances patterning contrast. Here, we report the application of CAL to fabricate transparent 3D fluidic networks in highly compliant and resilient methacrylated gelatin hydrogels, as well as in stiffer acrylates. Uncured resin provides mechanical support during printing, so the need for solid support structures is eliminated.
A predominant unsolved challenge in tissue engineering is the need of a robust technique for producing vascular networks, particularly when modeling human brain tissue. The availability of reliable in vitro human brain microvasculature models would advance our understanding of its function and would provide a platform for highthroughput drug screening. Current strategies for modeling vascularized brain tissue suffer from limitations such as (1) culturing non-human cell lines, (2) limited multi-cell co-culture, and (3) the effects of neighboring physiologically unrealistic rigid polymeric surfaces, such as solid membranes. We demonstrate a new micro-engineered platform that can address these shortcomings. Specifically, we have designed and prototyped a molding system to enable the precise casting of ~100μm-diameter coaxial hydrogel structures laden with the requisite cells to mimic a vascular lumen. Here we demonstrate that a fine wire with diameter ~130 μm or a needle with outer diameter ~300 μm can be used as a temporary mold insert, and agarose–collagen composite matrix can be cast around these inserts and thermally gelled. When the wire or needle is retracted under the precise positional control afforded by our system, a microchannel is formed which is then seeded with human microvascular endothelial cells. After seven days of culture these cells produce an apparently confluent monolayer on the channel walls. In principle, this platform could be used to create multilayered cellular structures. By arranging a fine wire and a hollow needle coaxially, three distinct zones could be defined in the model: first, the bulk gel surrounding the needle; then, after needle retraction, a cylindrical shell of matrix; and finally, after retraction of the wire, a lumen. Each zone could be independently cell-seeded. To this end, we have also successfully 3D cultured human astrocytes and SY5Y glial cells in our agarose–collagen matrix. Our approach ultimately promises scalable and repeatable production of vascular structures with physiologically realistic mechanical properties.
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