USC Viterbi research has uncovered a surprising fact about the whites of our eyes, Image/Wikimedia Commons
Biology textbooks often assume that living tissues are stiffest along the direction of their collagen fibers — like wood along its grain. Engineers at the USC Viterbi School of Engineering have now mapped a clear exception to that rule in the eye: a hidden “reinforcing belt” around the eye’s equator that is stiffest across the grain. The finding, published in Science Advances, reframes how researchers model the eye, offering new pathways for earlier detection of glaucoma risk, better understanding of myopia, and smarter design of ocular treatments. The work was conducted in collaboration with researchers at the University of California, Irvine, and the University of Pittsburgh.
The big idea, shown in the eye
The white of the eye, known as the sclera, is the tough outer layer surrounding the eyeball, providing protection and maintaining its shape. Prior research has focused on understanding the front and back regions of the sclera, leaving the mid-eye “equator” poorly understood. In the new study, researchers discovered that the equatorial sclera is mechanically circumferentially stiff (around the eye) and relatively soft meridionally (front-to-back), even though the local collagen fibers predominantly run front-to-back. In other words, stiffness can run against the grain — across the collagen fibers, not just with them.
“This is a textbook exception,” said Runze Li, the study’s lead author and a former Ph.D. student in the Biomedical Ultrasound Lab in the Alfred E. Mann Department of Biomedical Engineering. “The eye’s equator breaks the grain rule: it’s stiffer across the fibers. That reshapes the biomechanical map of the eye and hints that other tissues may hide similar rule-bending zones.”
Why this matters beyond the eye
Lead author on the study, Runze Li, is a former BME Ph.D. student at USC Viterbi.
Li said that biologists and engineers often use fiber orientation as a shortcut for predicting how tissues deform under load. The new map shows that architecture and layering can create strength that doesn’t simply follow fiber direction, cautioning against over-reliance on a single metric.
“Directional biomechanics at the eye’s equator could sharpen how we stratify glaucoma risk and how we design implants — this is the kind of mechanistic detail that is very much needed,” said Mark S. Humayun, University Professor and co-director of the USC Roski Eye Institute.
“While the present result is specific to the eye, the principle — a structure that is strongest across its local fibers — may apply wherever tissues are arranged to resist circumferential stretch, shear, or pressure spikes,” Li said.
The latest discovery holds a range of implications for the detection and treatment of many eye conditions, including:
- Glaucoma: A circumferentially stiff equatorial belt may help buffer sudden intraocular-pressure changes and protect the retina. Measuring this directional stiffness could complement today’s structural imaging and help flag higher-risk eyes before visible damage occurs.
- Myopia (near-sightedness): The softer front-to-back axis offers a mechanical rationale for why near-sighted eyes tend to elongate axially rather than bulge outward — refining disease models and informing interventions that target growth forces.
- Trauma and treatment design: A mid-eye stiffness map is a blueprint for devices and therapies — from sealants and implants to drug-delivery systems — that must integrate with the eye’s natural mechanics, especially around the equator.
How the research team saw the eye’s “belt”
Rather than tugging on tissue directly, the researchers used wave-based elastography, a noninvasive approach akin to “biomechanical sonar.” They launched tiny mechanical waves in pig eyes — a close stand-in for human tissue — and measured how fast the waves traveled. Faster waves indicate stiffer tissue. Two complementary tools, ultrasonic elastography and optical coherence elastography, produced matching stiffness maps.
“With optical coherence elastography we can noninvasively map stiffness at high resolution. Seeing stiffness run across the local fiber direction challenges a common assumption and opens new directions for ocular diagnostics,” said Zhongping Chen, professor of biomedical engineering at UC Irvine and the Beckman Laser Institute.
To visualize the underlying structure, polarized light microscopy revealed that collagen fibers near the equator mostly align front-to-back — highlighting the surprising mismatch between fiber direction and mechanical stiffness.
“Polarized light microscopy shows the fibers mostly run meridionally at the equator, yet the mechanics are strongest circumferentially — this mismatch is exactly what makes the result exciting for glaucoma biomechanics,” said Ian A. Sigal, associate professor of ophthalmology and bioengineering at the University of Pittsburgh.
“What thrills me is that we’ve uncovered a built-in ‘reinforcing belt’ around the eye that no one had measured before,” Li said. “It overturns the grain-equals-strength shortcut and gives us a clear, measurable target. If we can translate this wave-based scan to the clinic, doctors could spot eyes whose sclera are too soft — long before vision is lost — and engineers can design smarter sealants and implants that match the eye’s natural stiffness pattern.”
The research team has discovered that tissue is mechanically stiffest against the grain at the eye’s equator. Image/Runze Li
Towards new clinical approaches
The team is now adapting the wave-based scan for noninvasive clinical use in patients to test whether equatorial stiffness metrics improve early risk stratification in glaucoma and progressive myopia. The team will explore whether similar rule-breaking regions exist in other load-bearing tissues, and other ocular tissues as well, such as the cornea. In parallel, the new measurements provide ground-truth numbers for finite-element modelers and tissue engineers — replacing assumptions with validated material parameters for the equatorial sclera.
The research was led by Li under the supervision of Qifa Zhou, Zohrab A. Kaprielian Fellow in Engineering and professor of biomedical engineering and ophthalmology. Co-authors include collaborators from the Roski Eye Institute at USC’s Department of Ophthalmology, Allen and Charlotte Ginsburg Institute for Biomedical Therapeutics at USC, the University of Pittsburgh, the University of California, Irvine, and the University of California, Los Angeles. Additional contributions came from Yi Hua, professor at the University of Mississippi and a former postdoctoral fellow in Sigal’s lab, who led the polarized light microscopy analysis. Further contributions came from Fengyi Zhang, a Ph.D student in Chen’s lab at UC Irvine who led the optical coherence elastography experiments and analysis. The work was supported by the National Institutes of Health, including the National Eye Institute, and by an unrestricted grant to the Department of Ophthalmology from Research to Prevent Blindness. The work was also supported by a Stein Innovation Award from Research to Prevent Blindness.
Published on August 28th, 2025
Last updated on August 28th, 2025
