Engineering heart tissue presents particularly tough problems for researchers, since the heart is an active organ, contracting rhythmically to pump blood at high forces throughout the body. Scientists at MIT have found a new material on which to grow heart cells that better mimics the properties of heart muscle. The material, reported in Nature Materials this week, could be used to grow patches of tissue to repair heart injuries and defects, or to screen heart drugs in the lab.

Several labs have been working on ways to grow heart tissue by seeding living heart cells or stem cells onto artificial scaffolds. These scaffolds are designed to support the cells initially and then degrade over time as the cells form their own external support structures, leaving functioning tissue behind. But, says George Engelmayr, a postdoctoral fellow in the Harvard-MIT Division of Health Sciences and Technology who led the study, the scaffolds designed for other kinds of tissues did not have the right mechanical properties for heart tissue. Heart tissue must be flexible enough to change shape as the heart contracts, but also strong enough to withstand the intense forces generated by these contractions.

So, the researchers used a polymer originally developed in the lab of Robert Langer at MIT. “It’s elastic like a rubber band,” Engelmayr says, so it can withstand repeated stretching while only gradually losing strength as it degrades. Furthermore, the polymer can vary in stiffness, depending on how long it has been cured with heat, allowing the team to vary its mechanical properties with precision.

Cells of the heart are arranged in specific directions, which allows the heart chambers to be stiffer and stronger around their circumference than in the longitudinal direction. The researchers designed the scaffold to encourage cells to align themselves in the same direction to better mimic this property of natural heart muscle tissue. Using a laser cutting technique, they created a pattern of oblong holes in the polymer; the result is a flexible, honeycomb-like structure that is stiffer in one direction than another.

The researchers seeded small patches of the scaffold with heart cells from newborn rats and grew them for one week. They found that the mechanical and electrical properties of the engineered tissue varied in different directions. For instance, when the cells were lined up parallel to an electric field, they beat in sync more readily.