A significant step forward in the creation of a human-sized artificial heart.

 Bioengineers created the first biohybrid model of human ventricles with helically aligned beating cardiac cells, demonstrating that muscle alignment significantly increases the amount of blood the ventricle can pump with each contraction.

A significant step forward in the creation of a human-sized artificial heart.
Illustration of a human heart cross-section (stock image). Credit: matis75 / stock.adobe.com

Heart disease is the leading cause of death in the United States in part because, unlike other organs, the heart cannot repair itself after injury. That is why tissue engineering, which eventually includes the wholesale fabrication of an entire human heart for transplant, is critical for the future of cardiac medicine.

Researchers must replicate the unique structures that make up the heart in order to build a human heart from the ground up. This includes recreating helical geometries, which cause the heart to twist as it beats. It's long been assumed that this twisting motion is essential for pumping blood at high volumes, but proving it has been difficult, thanks in part to the difficulty of creating hearts with different geometries and alignments.

Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) bioengineers have created the first biohybrid model of human ventricles with helically aligned beating cardiac cells, demonstrating that muscle alignment does, in fact, dramatically increase the amount of blood the ventricle can pump with each contraction.

This advancement was made possible by Focused Rotary Jet Spinning (FRJS), a new method of additive textile manufacturing that enabled the high-throughput fabrication of helically aligned fibers with diameters ranging from several micrometers to hundreds of nanometers. FRJS fibers, which were developed at SEAS by Kit Parker's Disease Biophysics Group, direct cell alignment, allowing the formation of controlled tissue engineered structures.

The study was published in the journal Science.

"This work is a significant step forward for organ biofabrication, bringing us closer to our ultimate goal of building a human heart for transplant," said Parker, the Tarr Family Professor of Bioengineering and Applied Physics at SEAS and the paper's senior author.

This work is based on a centuries-old mystery. In his seminal work Tractatus de Corde, English physician Richard Lower, who counted John Locke among his colleagues and King Charles II among his patients, first noted the spiral-like arrangement of heart muscles in 1669.

Over the next three centuries, physicians and scientists gained a more complete understanding of the heart's structure, but the function of those spiraling muscles remained frustratingly difficult to investigate.

Edward Sallin, former chair of the Department of Biomathematics at the University of Alabama Birmingham Medical School, argued in 1969 that the helical alignment of the heart is critical to achieving high ejection fractions (the percentage of blood the ventricle pumps with each contraction).

"Our goal was to build a model where we could test Sallin's hypothesis and study the relative importance of the heart's helical structure," said co-first author and SEAS postdoctoral fellow John Zimmerman.

To put Sallin's theory to the test, the SEAS researchers used the FRJS system to control the alignment of spun fibers on which cardiac cells could be grown.

The first step of FRJS works like a cotton candy machine: a liquid polymer solution is loaded into a reservoir and centrifugal force pushes it out through a tiny opening as the device spins. The solvent evaporates as the solution exits the reservoir, and the polymers solidify to form fibers. The orientation of the fiber is then controlled by a focused airstream.

The fiber alignment can be adjusted by adjusting the collector angle.

"The human heart actually has multiple layers of helically aligned muscles with different angles of alignment," explained Huibin Chang, a postdoctoral fellow at SEAS and the paper's co-first author. "With FRJS, we can precisely recreate those complex structures, forming single and even four chambered ventricle structures."

Unlike 3D printing, which slows down as features become smaller, FRJS can rapidly spin fibers at the single micron scale – roughly fifty times smaller than a single human hair. This is critical when constructing a heart from the ground up. Consider collagen, an extracellular matrix protein found in the heart that is also a single micron in diameter. At this resolution, 3D printing every bit of collagen in the human heart would take more than 100 years. FRJS can complete the task in a single day.

The ventricles were seeded with rat cardiomyocyte or human stem cell derived cardiomyocyte cells after spinning. Within a week, several thin layers of beating tissue covered the scaffold, with cells aligning themselves.

The beating ventricles resembled the twisting or wringing motion of human hearts.

The researchers compared the deformation of the ventricle, the speed of electrical signaling, and the ejection fraction of ventricles made of helical aligned fibers and those made of circumferentially aligned fibers. They discovered that helically aligned tissue outperformed circumferentially aligned tissue on all fronts.

"Our group has been working since 2003 to understand the structure-function relationships of the heart and how disease pathologically compromises these relationships," Parker explained. "In this case, we returned to address an untested observation about the helical structure of the heart's laminar architecture. Professor Sallin, fortunately, published a theoretical prediction more than a half-century ago, and we were able to build a new manufacturing platform that allowed us to test his hypothesis and answer this centuries-old question."

The researchers also demonstrated that the process can be scaled up to the size of a human heart and even larger, to the size of a Minke whale heart (the larger models were not seeded with cells because it would require billions of cardiomyocyte cells).

Aside from biofabrication, the team is looking into other uses for their FRJS platform, such as food packaging.

The intellectual property associated with this project has been protected by the Harvard Office of Technology Development, and commercialization opportunities are being explored.

Source: Materials provided by Harvard John A. Paulson School of Engineering and Applied Sciences.

DOI: 10.1126/science.abl6395


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