Bionic Muscles Toughen Up

Hybrid materials made of cardiac cells and carbon nanotubes might patch damaged hearts and provide muscle for robots made of living tissues.

The strength and physical properties of natural tissue are often hard to reproduce in synthetic materials.

The tissues of the heart are mechanically tough and electrically conductive, and they keep a strong, rhythmic beat—properties that are tough to mimic in the lab. But a new hybrid material that combines cell-friendly gel, strong, conductive carbon nanotubes, and living cardiac cells mimics natural heart tissue more successfully than previous attempts. Eventually the new material could be useful in both medical and robotic applications.

The bionic tissues, made by Ali Khademhosseini, a professor at the Harvard-MIT Division of Health Sciences and Technology in Cambridge, Massachusetts, could serve as muscles for biological machines—moving, programmable living tissues that take synthetic biology beyond single cells. A lot of the things that natural tissues and biological cells can do, such as sense and respond to their environment, are hard for engineers to achieve with the synthetic materials used in conventional robotics. Researchers hope that building machines from biological materials like heart tissue will expand what’s possible. The new tissues can swim untethered in water, swing back and forth, and perform other moves programmed by controlling their shape and thickness.

If these materials turn out to be safe for use in the human body, they might also be used to patch tissue damaged by heart attacks. Researchers engineering heart tissues in the lab often use polymers and gels to provide cardiac cells with an environment in which they will grow and behave as they do in the body. The resulting materials have two critical flaws, says Khademhosseini. They don’t match the electrical conductivity of heart tissue, nor are they as mechanically strong.

“When the heart beats, cells respond to that mechanical force and release chemicals that encourage growth,” says Thomas Webster, a chemical engineer at Northeastern University in Boston, who was not involved with the work. And if the patch is less conductive than the rest of the heart, electrical signals might experience delays. If a patch without just the right properties is placed on a patient’s heart, it might not grow properly, and it might not be able to beat in time with the rest of the heart, says Webster.

The Cambridge group solves this problem by adding carbon nanotubes to tissue-engineering gels. The result is a squishy gel with a tangle of strong, conductive carbon fibers embedded in it. Khademhosseini seeded cardiac cells on these gels and studied their properties. The bionic tissues were similar in elasticity to rat heart—much more elastic than previous lab-made materials. They also had much better conductivity. And the tissues were better at heart tissue’s main job, beating in synchrony. Khademhosseini exposed the bionic tissue to various chemicals and found that it was it was relatively resistant to damage—perhaps because the carbon nanotubes provide electrical links between cells that can maintain communication even when under stress. This work is described online in the journal ACS Nano.

Webster says before any medical applications can be considered, researchers will have to demonstrate that carbon nanotubes are not toxic—especially since they are not biodegradable and would be likely to stay in the body for a long time. He notes that even if the carbon materials themselves are safe, the manufacturing process for nanotubes might leave traces of toxic metal catalysts.

Khademhosseini says the first use for the materials may be in biological machines used to assess and restore toxic environments or repair buildings. Last year, researchers demonstrated free-swimming jellyfish-like robots and walking biological machines built from heart tissues and polymers. But without conductive materials, their applications are limited, says Rashid Bashir, a bioengineer at the University of Illinois at Urbana-Champaign, who made the walking robot. “If you can pattern the base material, you could make circuits inside,” he says.

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