Quantifying the role of hyaluronic acid on collagen fiber mechanics via femtosecond laser ablation
This study demonstrates that femtosecond laser ablation combined with second harmonic generation imaging reveals hyaluronic acid's critical role in facilitating collagen fiber sliding and viscoelastic behavior in tendons, showing that its depletion impairs micromechanics while supplementation with methacrylated hyaluronic acid effectively restores these properties.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Tendons are the tough, rope-like cords that connect muscle to bone, allowing us to run, jump, and lift. They are not made of living cells like muscle tissue; instead, they are primarily a dense, fibrous scaffold of collagen proteins, held together by a gel-like substance. This gel contains molecules called glycosaminoglycans, which act like sponges, holding water and giving the tendon its ability to stretch and snap back. One specific molecule in this gel, hyaluronic acid, has long been used in medicine to treat joint pain and tendon injuries. Doctors inject it into sore areas, hoping it will soothe the tissue and help it heal. Yet, despite its widespread use, scientists have struggled to understand exactly how this molecule works inside the tendon. Does it simply lubricate the fibers, or does it actively change how the tiny collagen strands slide against one another to absorb shock? Without knowing the answer, it is difficult to design better treatments or understand why tendons become stiff and prone to injury as we age.
To solve this puzzle, a team of researchers at the University of Colorado Boulder developed a new way to look inside a tendon without taking it apart. They focused on the microscopic level, where individual collagen fibers are bundled together. In a healthy tendon under tension, these fibers are not rigidly locked in place; they can slide and shift slightly against their neighbors, a movement that helps the tissue absorb energy and prevents it from snapping. The researchers wanted to see if hyaluronic acid was the key that allowed this sliding to happen. They used a highly precise tool, a femtosecond laser, which acts like a microscopic scalpel. By aiming this laser at a single collagen fiber inside a living tendon that was being gently stretched, they could cut a tiny segment of that fiber. Once the fiber was severed, the tension holding it taut was released, causing the cut ends to recoil, or snap back, away from the cut. By measuring how far and how fast these ends moved, the team could calculate exactly how easily the fibers were sliding past each other.
The researchers first tested this method on tendons from young adult mice and compared them to tendons from older mice. They found that the aged tendons, which naturally contain less hyaluronic acid, showed significantly less fiber recoil. The fibers in the older tendons barely moved after being cut, suggesting they were stuck together and unable to slide. To confirm that hyaluronic acid was the missing ingredient, the team took healthy young tendons and treated them with an enzyme that specifically eats away hyaluronic acid. The result was the same: without the molecule, the fibers lost their ability to slide, and the recoil disappeared. This proved that the presence of hyaluronic acid is directly linked to the ability of collagen fibers to move and absorb stress at the microscopic level.
Having identified the problem, the team asked if they could fix it. They developed a method to introduce a new form of hyaluronic acid back into the tissue. They used a low-molecular-weight version of the molecule, which is small enough to slip deep into the dense tendon structure. Once inside, they used a light-activated chemical reaction to link these small molecules together, effectively trapping them in place so they would not wash out. When they applied this treatment to the tendons that had been stripped of their natural hyaluronic acid, the fibers regained their ability to slide. The recoil returned to normal levels, mimicking the behavior of healthy, young tissue. Even more promising, when they treated the naturally aged tendons with this same method, the fiber sliding improved, bringing the mechanical behavior of the old tissue closer to that of a young adult.
The study also revealed a surprising limitation in how we usually test tendon health. When the researchers measured the tendon as a whole, using standard stretching tests, they found no difference between the healthy, the aged, and the treated samples. The big, macroscopic numbers looked the same. It was only by zooming in to the microscopic scale and watching the individual fibers move that the true differences became visible. This suggests that the damage caused by aging or the loss of hyaluronic acid happens at a level too small to be seen by traditional methods, yet it is significant enough to change how the tissue functions. The findings suggest that the role of hyaluronic acid in tendons is not just about lubrication, but about maintaining the fluid environment that allows collagen fibers to slide freely. By restoring this molecule, it may be possible to reverse some of the mechanical stiffening that comes with age, offering a new path for treating tendon injuries that goes beyond simply reducing pain.
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