A recent study shows that ultraviolet radiation changes the microstructure of human skin, particularly its effect on collagen, making the skin stronger. This study provides insight into the biological reasons why people's skin appears "leathery" after long-term exposure to the sun.
It is generally believed that long-term exposure to the sun will eventually harden the skin. Think of the "leathery" skin of farmers, road crews and others who work outdoors for long periods of time, or who frequent the sunroom or beach in the summer.
However, despite these observations, few studies have biologically explained why this occurs—until now.
Researchers at Binghamton University recently published a study in the Journal of Mechanical Behavior of Biomedical Materials that explored how ultraviolet radiation changes the microstructure of human skin. Particularly affected is collagen, the fibrous protein that holds together tissues, tendons, cartilage and bones throughout our bodies.
"We don't want to add a 'don't go out in the sun' fear factor here," said Guy German, an associate professor in the Department of Biomedical Engineering at Binghamton University. "But prolonged exposure to UV rays does harden the skin and lead to an increased risk of cancer."
Abraham Ittycheri, a doctoral student in the Thomas J. Watson School of Engineering and Applied Science's Department of Biomedical Engineering, Zachary Lipsky '21, Ph.D., and Assistant Professor Tracy Hookway led the research with German.
The new study builds on previous research by German and Lipsky, which focused on the stratum corneum, the top layer of the skin. This time, the Binghamton University team compared full-thickness skin samples before and after varying levels of UV exposure.
"One way to characterize the material properties of skin is to perform mechanical stretch tests on skin. If the skin stretches easily, then it is relatively compliant, but if it is much harder to stretch, you can say it is much stiffer," Ittycheri said. "My experiment was to see what the separate effects of UV rays are and compare them to a situation where the skin is not exposed to UV rays."
The researchers found that as the skin absorbs more UV radiation, the collagen fibers in the skin become more tightly packed together, resulting in increased stiffness and making the tissue harder to break. The Germans believe this has to do with the cross-linking aging theory, which holds that the accumulation of bad molecular bonds over time leads to cellular dysfunction.
Hookway, who received a National Science Foundation CAREER Award earlier this year for her work on heart cells, believes there are similarities in the way heart cells and skin cells deal with damage, even though their functions are very different.
"Our bodies have this natural reaction when any tissue suffers some kind of damage, and that damage is likely to occur in the stratum corneum," she said. "First, wherever there is some kind of weakening, it has to be compensated by the rest of the tissue, or else you're going to have catastrophic failure. The same thing happens to the heart when you have a myocardial infarction -- you get scarring, and your heart won't work the same way anymore."
She added that sometimes the body's response is to keep you alive, but it's not necessarily a good outcome and can lead to other medical problems down the road. Understanding how this happens could allow future doctors to steer responses in healthier directions.
Following this study, further collaborations between Iticelli, Gelman and Hukovi are already in the works. Our skin is the body's largest organ and the first line of defense against microorganisms and other external aggressions, so it's obviously beneficial to maintain and even strengthen it.
"Any disruption to the skin's normal processes can be extremely dangerous and damaging to our overall lifestyle," says Ittycheri. "And that's not even getting into the beauty aspect. When someone's skin doesn't look good, it challenges their perception of themselves."