Engineered tissue goes bigger, lives longer

Engineered, living tissue could help scientists and doctors test for drug safety or even repair injured tissue. Researchers have already used cells and biocompatible polymers to build materials that look and behave like living tissue in many ways. But they have struggled to create tissue that is thicker than about 1 mm and can survive for more than a couple weeks, says Jennifer A. Lewis of Harvard University.

Lewis and her team have now turned to three-dimensional printing and an assortment of innovative “bioinks” to create tissue thicker than 1 cm that can live longer than six weeks (Proc. Natl. Acad. USA 2016, DOI: 10.1073/pnas.1521342113). The team can direct these 3-D printed constructs to behave like bone-generating tissue found in people.

In the past, researchers including Lewis have worked with gelatinous polymers that cross-link under ultraviolet light to mimic the biological environment in which cells live in the body. But UV light also scatters from the polymer it cures, limiting how thick researchers can make their synthetic matrices, Lewis says.

To get around this problem, Lewis’ team relied on enzymes—thrombin and transglutaminase—and multiple bioinks. One ink contains gelatin, fibrinogen, and stem cells from bone marrow and another is a so-called fugitive ink, which researchers can remove from the tissue after it sets to create empty channels for vasculature.

The researchers print a 3-D lattice, made up of layers of cell ink and fugitive ink. They then fill the lattice with more gelatin containing fibrinogen proteins and the enzymes. These biomaterials diffuse and mingle with the printed features.

The thrombin converts fibrinogen to insoluble fibrin, which the slower-acting transglutaminase crosslinks with the gelatin to lock in the tissue’s structure.

After removing the fugitive ink, the researchers can then coat the vasculature with endothelial cells and perfuse nutrients through the tissue to sustain it for many weeks.

To further demonstrate the potential of this approach, Lewis and her team flowed growth factors through the tissue to direct the printed stem cells to behave like bone-growing cells and make their own collagen and calcium phosphate.

These are “very cool, exciting results,” says Lisa E. Freed, a tissue engineering researcher with Draper, a not-for-profit company, and an affiliate research scientist with Massachusetts Institute of Technology. She adds that the researchers brought together novel materials in an exciting way.

Lewis’s team feels the same. “Our team is excited by this latest advance and its implications for tissue engineering,” Lewis says.