Bioengineers have cleared a major hurdle on the path to 3D-printing replacement organs with a new technique for bioprinting tissues. It allows scientists to create entangled vascular networks that mimic the body's natural passageways for blood, air, lymph, and other vital fluids.
A road block to generating functional tissue replacements has been the inability to print the complex vasculature that can supply nutrients to densely populated tissues. Human organs contain independent vascular networks — like the airways and blood vessels of the lung or the bile ducts and blood vessels in the liver. These networks are physically and biochemically entangled, and the architecture is intimately related to tissue function. The new bioprinting technology addresses the challenge of multi-vascularization.
The goal of bioprinting healthy, functional organs is driven by the need for organ transplants. More than 100,000 people are on transplant waiting lists in the United States alone, and those who do eventually receive donor organs still face a lifetime of immune-suppressing drugs to prevent organ rejection. Bio-printing has attracted intense interest because it could theoretically address both problems by allowing doctors to print replacement organs from a patient's own cells.
The open-source bioprinting technology — called stereolithography apparatus for tissue engineering or SLATE — uses additive manufacturing to make soft hydrogels one layer at a time.
Layers are printed from a liquid pre-hydrogel solution that becomes a solid when exposed to blue light. A digital light-processing projector shines light from below, displaying sequential 2D slices of the structure at high resolution, with pixel sizes ranging from 10-50 microns. With each layer solidified in turn, an overhead arm raises the growing 3D gel just enough to expose liquid to the next image from the projector. Food dyes are added that absorb blue light. These photoabsorbers confine the solidification to a very fine layer. In this way, the system can produce soft, water-based, biocompatible gels with intricate internal architecture in a matter of minutes.
Tests of a lung-mimicking structure showed that the tissues were sturdy enough to avoid bursting during blood flow and pulsatile “breathing,” a rhythmic intake and outflow of air that simulated the pressures and frequencies of human breathing. Tests found that red blood cells could take up oxygen as they flowed through a network of blood vessels surrounding the “breathing” air sac. This movement of oxygen is similar to the gas exchange that occurs in the lung's alveolar air sacs.
All source data from the experiments is freely available. In addition, all 3D-printable files needed to build the printing apparatus are available, as are the design files for printing each of the hydrogels used in the study.