Improving 3D Printing of Fine Structures in Artificial Tissue
The biggest challenge in tissue printing is the achievement of sufficient control over small scale structure to produce a vasculature that can supply the tissue as it develops. Without that capacity, tissue growth is limited to thin sheets and tiny organoids. Advances have been made in recent years, such as the work of Volumetric, but there is still a way to go before large tissue sections are regularly generated for use in medicine. This is in large part why work on decellularization continues to proceed apace, taking donor tissue and stripping the cells from it to leave the extracellular matrix structure, with all of its fine-scale detail and chemical cues to guide new, patient-matched cells into the correct locations and development activities.
Bioprinting is based on 3D-printing technology, using cells and biopolymer to create biological structures and tissues. One of the most promising types of 3D-bioprinting is called digital light processing (DLP) bioprinting. Within this branch of 3D-bioprinting, progress has been impeded by practical and technical impediments. It has proven difficult to print tissues with high cell densities and finely resolved structures.
DLP-based 3D bioprinting uses a digital micromirror device (DMD) to project a 2D cross-section of the 3D model to the photo-crosslinkable bioink. When exposed to light, the photocrosslinkable bioink, which can be either synthetic or natural, solidifies. Then, a motorized stage lifts up the bioink by a few tens microns to 200 microns, which allows uncured bioink to refill the gap. When the next cross-section is projected to the bioink, a new layer solidifies and the process repeats.
When all goes well, a newly formed layer precisely matches the shape of the projected cross-section. However, with existing methods, the incorporation of cells in the bioink can cause severe light scattering, which blurs the projected light in the bioink. As a result, the newly formed layers cannot replicate the fine details of the projected cross-sections. The researchers reduced this light-scattering effect by tenfold, allowing them to print with high cell densities and high resolution thanks to the contrast agent iodixanol, a new ingredient in the bioink.
Tuning the refractive index of the bioink minimizes the scattering effect and significantly improves the fabrication. The new research shows that a ~50 µm feature size can be achieved in a refractive-index-matched gelatin methacrylate (GelMA) bioink with a cell density as high as 0.1 billion/mL. This approach introduces a few novel technical innovations, including a hollow organic vascular network embedded in a cell-laden thick tissue, enabling it for perfused and long-term culture, and a snow-flake and spoke shape to showcase the high resolution for both positive and negative features.