Heart disease—the leading cause of death in the U.S.—is so deadly in part because the heart, unlike other organs, cannot repair itself after injury. This is why tissue engineering, including the wholesale production of whole human hearts for transplantation, is so important to the future of cardiac medicine.
To build a human heart from scratch, researchers need to replicate the unique structures that make up the heart. This involves restoring the helical geometry that creates the twisting motion when the heart beats. This twisting motion has long been theorized to be critical for pumping blood at high volumes, but proving it has been difficult, in part because creating hearts with different geometries and alignments has been a challenge.
Now, bioengineers at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed the first biohybrid model of human ventricles with spirally arranged heart cells and have shown that straightening the muscles does dramatically increase the amount of blood the ventricle can pump with each contraction.
This progress was made possible by a new textile additive manufacturing method, focused spin jet spinning (FRJS), which has enabled the production of high-performance helically aligned fibers with diameters ranging from a few micrometers to hundreds of nanometers. Developed at SEAS by Keith Parker’s disease biophysics group, FRJS fibers direct cell alignment, enabling the formation of controlled tissue-engineered structures.
The study was published in Science.
“This work is an important step forward for organ biofabrication and brings us closer to our ultimate goal of creating a human heart for transplantation,” said Parker, the Tarr Family Professor of Bioengineering and Applied Physics at SEAS and senior author of the paper.
This work has its roots in a centuries-old mystery. In 1669, English physician Richard Lower—a man who counted John Locke among his colleagues and King Charles II among his patients—first noticed the spiral arrangement of the heart muscles in his seminal work Treatise de Cordet.
Over the next three centuries, doctors and scientists gained a more complete understanding of the structure of the heart, but the purpose of these spiral muscles was still very difficult to study.
In 1969, Edward Salin, former head of the Department of Biomathematics at the University of Alabama at Birmingham School of Medicine, argued that spiral alignment of the heart is critical to achieving high ejection fractions—the percentage of how much blood the ventricles pump with each contraction.
“Our goal was to build a model in which we could test Salin’s hypothesis and examine the relative importance of the helical structure of the heart,” said John Zimmerman, a SEAS postdoctoral fellow and one of the paper’s authors.
To test Salin’s theory, the SEAS researchers used the FRJS system to control the alignment of twisted fibers on which they could grow heart cells.
The first stage of the FRJS works like a cotton candy machine – a liquid polymer solution is loaded into a reservoir and pushed through a tiny hole by centrifugal force as the device spins. As the solution leaves the tank, the solvent evaporates and the polymers solidify to form fibers. The focused airflow then controls the orientation of the fibers as they descend onto the collector. The team found that by tilting and rotating the collector, the fibers in the flow align and twist around the collector as it rotates, mimicking the spiral structure of heart muscle.
Fiber alignment can be adjusted by changing the angle of the collector.
“The human heart actually has several layers of spirally arranged muscles with different alignment angles,” said Huibin Chang, a SEAS postdoctoral fellow and one of the authors of the paper. “With FRJS, we can recreate these complex structures in a very precise way, forming single and even four-chambered ventricular structures.”
Unlike 3D printing, which gets slower as features get smaller, FRJS can quickly spin fibers as small as one micron – or about fifty times smaller than a single human hair. This is important when it comes to creating a heart from scratch. Take, for example, collagen, the extracellular matrix protein in the heart, which is also one micron in diameter. It would take over 100 years to 3D print every bit of collagen in a human heart at this resolution. FRJS can do it in one day.
After spinning, the ventricles were seeded with rat cardiomyocyte cells or human stem cells. After about a week, several thin layers of beating fabric covered the frame, and the cells matched the alignment of the fibers beneath.
The beating of the ventricles mimicked the same twisting or twisting motions that are present in human hearts.
The researchers compared ventricular deformation, electrical conduction velocity, and ejection fraction between ventricles made of spirally arranged fibers and circumferentially arranged fibers. They found that on every front, the helically aligned fabric outperformed the circumferentially aligned fabric.
“Since 2003, our group has worked to understand the structure-function relationships of the heart and how disease pathologically disrupts these relationships,” Parker said. “In this case, we returned to a never-proven observation about the spiral structure of the laminar architecture of the heart. Fortunately, Professor Salin published a theoretical prediction more than half a century ago, and we were able to build a new production platform that allowed us to test his hypothesis and solve this age-old question.” .
The team also demonstrated that this process can be scaled up to the size of a real human heart and even larger, to the size of a minke whale heart (they did not seed the larger models with cells, as this would require billions of cardiomyocyte cells).
In addition to biofabrication, the team is also exploring other applications of its FRJS platform, such as food packaging.
Harvard’s Office of Technology Development has secured intellectual property related to this project and is exploring commercialization opportunities.
It was supported in part by the Harvard Materials Science and Technology Center (DMR-1420570, DMR-2011754), the National Institutes of Health with the Center for Nanoscale Systems (S10OD023519), and the National Center for the Advancement of Translational Sciences (UH3TR000522, 1- UG3-HL-141798-01).