Every day, 20 people in need of an organ transplant die because there are no viable organs available for them. One idea to mitigate this shortage is to manufacture organs in the laboratory. In an exciting advancement, a research team at Carnegie Mellon University printed 3D beating heart tissues.

3D bioprinters allow scientists to control the exact placement of multiple cells and proteins in a defined architecture which could be ideal for copying the complicated structures of organs. In practice, bioprinting organs has been limited, partially because creating holes in a tissue (such as a channel for a blood vessel or the chambers of the heart) requires a removable scaffold to hold the cells in a particular shape during printing, but that can also be removed without disrupting the tissue structure. 

Published in Science, the researchers developed a method that uses gelatin microparticles as a scaffold during bioprinting. The gelatin bath is a semi-solid jelly that melts away at body temperature. Using this scaffold, the scientists precisely print pre-defined shapes with collagen, a structural protein that surrounds cells within body, then melt the gelatin, leaving a collagen structure behind. The precision of this technique allowed the researchers to print a full-sized replica of an infant heart with the collagen, demonstrating the accuracy of the technique to print delicate, complicated structures.

While this scaffolding method may solve structural limitations of bioprinting, the organs must also be functional. Heart muscle cells must contract rhythmically to pump blood. In living hearts, an electrical pulse moves through the heart cells, causing the entire heart to contract and making blood flow through the body. 

In this study, the scientists printed a millimeter wide cup-shaped heart ventricle. Heart muscle cells added within the walls of the printed ventricle began to contract within a few days, and by pulsing the tissue with an electrical signal, the scientists got it to beat like a real heart. 

To print a heart, you need blood vessels, heart shaped structures, and a beating tissue. Now we just need to bring these pieces together into functional organs that integrate heart muscle and the other cells of the heart. This advancement in printing collagen into physiological structures serves as an exciting tool for the field moving forward, bringing us one step closer to making organs in the lab.