Collagen is the workhorse structural protein of the heart valve leaflets, providing the tensile strength that allows them to withstand the relentless mechanical demands of pumping blood. The fluid surrounding the heart contains collagenases that would, under ordinary circumstances, dismantle this critical protein scaffold. Yet healthy valve leaflets remain sturdy and functional for decades, opening and closing millions of times each year without losing their integrity. Understanding why has been a longstanding puzzle in cardiovascular biomechanics.

Lead author Hsiao-Ying Shadow Huang, an associate professor of mechanical and aerospace engineering at North Carolina State University, explained the motivation behind the research. The team knew that collagen was critical for keeping heart valves strong, and that the leaflets open and close millions of times every year. They also knew that the fluid surrounding the heart contains collagenases designed to break that collagen down, yet the leaflets remain sturdy for decades. When people develop heart disease, however, the structural integrity of those leaflets can degrade very quickly — and the researchers wanted to explore what role, if any, biomechanics might play in these contrasting outcomes.

To investigate, the researchers conducted experiments using pig hearts, whose valve anatomy closely resembles that of humans. They tested different levels of strain on heart valve leaflets, pulled the tissue in different directions, and varied the levels of collagenase in the fluid surrounding the hearts. The experimental design was carefully calibrated to replicate the natural mechanical environment of a beating heart while systematically altering key variables to isolate the protective mechanism.

The key insight came from understanding the precise geometry of leaflet deformation during each heartbeat. When a healthy heart pumps and the aortic valve leaflet is in the open position, the leaflet is stretched to 70 percent of its maximum capacity lengthwise but to only 20 percent of its maximum capacity laterally. The researchers tested the leaflets under those conditions but also examined other strain levels that retained that same 70-to-20 ratio, such as 35 percent to 10 percent. They also tried changing the ratio so that the leaflets experienced the same amount of strain in both directions. All of these experiments were conducted with both normal levels of collagenase and elevated levels of the enzyme.

After each experiment, the team evaluated the microstructure of the leaflet tissue to determine whether the collagen had remained intact or had been damaged. The results were striking. Increasing the strain did not damage the collagen, even with higher levels of collagenases, as long as the ratio of lengthwise strain to lateral strain stayed the same. But when the researchers changed the strain ratio, the collagen broke down — regardless of the collagenase levels present in the surrounding fluid.

This finding reveals that stretching the leaflet in the same direction as the collagen-rich fibers serves as a biomechanical protective mechanism, provided the naturally occurring strain ratio is preserved. The directional alignment of mechanical tension appears to physically shield the collagen molecules from enzymatic attack, effectively locking the protein into a configuration that collagenases cannot easily access or degrade. It is a remarkable example of how the body harnesses mechanical forces not just to move and pump, but to actively defend its own structural components from chemical breakdown.

Previous studies had identified this protective mechanism in collagen-rich tissues such as tendons, but that research only pulled the tissue in one direction. Heart valve leaflets, by contrast, experience strain in more than one direction every time they open and close, making the current finding a significant step forward in understanding how multiaxial mechanical loading protects soft tissues. The study, published in the Journal of Biomechanical Engineering under the title

The experimental system used in the study offers a useful window into how biomechanics research is actually conducted. Pig hearts are a common stand-in for human cardiac tissue because the dimensions, layered architecture, and collagen organization of porous valve leaflets closely mirror those found in people, which makes the findings plausibly transferable to human physiology. By isolating leaflets and subjecting them to controlled stretch in two perpendicular directions while varying the collagenase concentration in the surrounding fluid, the researchers could decouple mechanical loading from enzymatic exposure — two factors that are inseparable in a living heart.

The concept underlying the discovery is sometimes referred to in the literature as mechanical stabilization of collagen against proteolysis, meaning the breakdown of proteins by enzymes. Collagenases and related matrix-degrading enzymes normally access the triple-helical collagen molecule at specific sites along its length. When the molecule is held under tension along its fiber axis, those vulnerable regions can become less accessible, effectively hiding the degradation targets from the enzymes. The new work extends this idea to a more realistic loading scenario, because valve leaflets are stretched along their length and across their width simultaneously with every heartbeat, not in a single direction as in earlier tendon experiments.

The 70-to-20 strain ratio observed in the open aortic valve is not arbitrary. It reflects the natural architecture of the leaflet, in which collagen fibers are predominantly aligned in the circumferential direction, the direction that bears the greatest load when the valve is pressurized by blood. Stretching preferentially along that fiber direction appears to be what stabilizes the collagen, whereas distorting the ratio — for example, by stretching the tissue equally in both directions — disrupts the protective configuration and leaves the protein exposed to enzymatic attack regardless of how much collagenase is present.

This mechanism may help explain a broader clinical observation: structural valve deterioration tends to accelerate in settings where valve mechanics are altered. When heart disease changes the pressures and geometries inside the heart, the leaflets are deformed differently than they would be in a healthy beat, and the study suggests that this loss of the natural strain ratio strips away a biomechanical defense the tissue normally relies on. In such a state, even ordinary collagenase activity could produce damage far more rapidly than it would in a healthy valve.

The findings also carry implications for the engineering of replacement heart valves. Bioprosthetic valves, which are typically built from animal-derived tissue, eventually fail in part because their collagen degrades over time inside the body. If the protective effect of physiological strain ratios can be reproduced in engineered biomaterials — either by designing scaffolds that align collagen fibers appropriately or by conditioning implants to experience the same multiaxial loading patterns as native leaflets — it could extend the functional lifespan of valve replacements and reduce the need for repeat surgeries.

There are also broader lessons for connective tissue biology. Tendons, ligaments, skin, and blood vessel walls are all rich in collagen and all experience mechanical loading in daily life. The idea that tension along the fiber direction shields collagen from enzymatic breakdown suggests that mechanical environment is not merely a passive background condition but an active participant in tissue maintenance. Loss of appropriate loading — through immobilization, disease, or altered mechanics — may therefore expose collagen-rich tissues to faster degradation than their enzyme levels alone would predict.

The study, published in the Journal of Biomechanical Engineering, was co-authored by Siyao Huang, a former Ph.D. student at NC State, and Kaitlyn Barbour, a former master’s student at NC State. The work was an experimental study conducted in animal tissue, and the authors reported no competing interests. Because the experiments were performed on excised leaflets under controlled conditions rather than in living hearts, further work will be needed to confirm that the same strain-ratio-dependent protection operates continuously in the beating heart and to determine how the mechanism might be harnessed therapeutically. Even so, the results provide a concrete biomechanical account of a puzzle that has long puzzled cardiovascular scientists: how a tissue subjected to both relentless motion and a hostile enzymatic environment can remain intact for a lifetime.

Subject of Research: Biomechanical strain-stabilization of collagen in heart valve leaflets

Article Title: Study offers insights on how heart valves last so long

Article References: Study offers insights on how heart valves last so long. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: heart valves, collagen, biomechanics, collagenase, strain-stabilization, proteolysis, cardiovascular research, tissue engineering, Journal of Biomechanical Engineering, NC State University, offers, insights