The eye-on-a-chip model developed by the research team.
By Nora Laberee
Scheie Vision Annual Report 2019
In August 2019, researchers from the University of Pennsylvania, School of Medicine; Bioengineering and Material Science; and Engineering published a groundbreaking research development in Nature Medicine. Led in part by Mina Massaro-Giordano, MD, Vatinee Y. Bunya, MD, MSCE, and Vivian Lee, MD in the Department of Ophthalmology, this study demonstrates the successful creation and testing of an artificial human eye model that can be used to replicate and study diseases affecting the surface of the eyes.
The device is a blinking, in-vitro model of the human ocular surface, designed and constructed by Dan Huh, PhD, Associate Professor of Bioengineering in the Department of Bioengineering and Jeongyun Seo, a graduate student in Dr. Huh's lab. His lab specializes in creating organs-on-a-chip that simulate their counterparts within the body. This allows for in depth scrutiny of the functions and malfunctions of these organs that would not be feasible otherwise.
The ocular surface in humans consists of mainly two structures:
- the cornea, the transparent cover over the iris and pupil and the surface where a contact lens would sit; and
- the conjunctiva, the thin layer that covers the white part of the eye and the inner surface of the eyelids.
The team first started by 3D-printing a small scaffold, similar in shape to the human cornea. Human corneal and conjunctival cells were then grown on the scaffold, utilizing a unique cell culturing technique developed by the Huh lab. This technique allowed for the two types of cells to be positioned in relation to one another as they are in humans. They also created an eyelid out of gelatin that mimicked the function of a human eyelid, engineered to blink and spread tears across the surface at the same rate as the human eye.
This blinking device serves many purposes, and researchers in the Department of Ophthalmology are most interested in the ways the eye-on-a-chip can be used to study eye diseases and drug treatment testing. Dry Eye Disease (DED) is one of the most common ocular surface diseases worldwide. It is a complex disease, involving many pathways including tear deficiency, inflammation, and meibomian gland dysfunction. This can cause extreme discomfort and visual deficits for those affected by the condition, and can make day-to-day tasks difficult or impossible. Dr. Mina Massaro, Professor of Clinical Ophthalmology at Penn Medicine, and Dr. Vatinee Bunya, Assistant Professor of Ophthalmology at Penn Medicine, are Co-Directors of the Penn Dry Eye and Ocular Surface Center at the Scheie Eye Institute. The pair has been interested in DED for much of their careers, and was joined by Dr. Vivian Lee, Assistant Professor of Ophthalmology at Penn Medicine who studies epithelial cell biology, as members of the research team for the development of this new technology.
"Although dry eye is a very common, debilitating disease, there are currently only two FDA-approved treatments, which do not work for all patients. There is a significant need for better methods for understanding dry eye in order to develop new and more effective therapies," said Dr. Bunya.
The team first set out to determine if they could induce DED in the eye model. Their results suggest that simulating DED in the model was more complicated than the team previously thought. "Initially, we thought modeling DED would be as simple as just keeping the culture environment dry. But as it turns out, it's an incredibly complex multifactorial disease with a variety of sub-types," Dr. Huh said. "Regardless of type, however, there are two core mechanisms that underlie the development and progression of DED. First, as water evaporates from the tear film, salt concentration increases dramatically, resulting in hyperosmolarity of tears. And second, with increased tear evaporation, the tear film becomes thinner more rapidly and often ruptures prematurely, which is referred to as tear film instability. The question was: Is our model capable of modeling these core mechanisms of dry eye?"
The end results suggest that it is, and Drs. Massaro, Bunya, and Lee put the model to the test by examining its performance against human eyes, in patients both with DED and without. Through multiple tests, they found that the eye-on-a-chip successfully mimicked what occurs in human eyes. Once they established this, the research team was ready to investigate the effects of potential treatments for real human eyes on this artificial model. This granted the researchers the valuable ability to avoid possible damage to real human eyes through risky new treatments.
The team began their investigation by testing the effect of lubricin, a mucinous glycoprotein that is secreted in joints, on the DED model of the eye-on-a-chip. Previous research suggested that the production of lubricin is impaired in individuals with DED, and the team hoped that testing this drug on their model would counteract some of this effect. The results were impressive. "While we knew patients with dry eyes were deficient in lubricin, its role in the pathophysiology of dry eyes was unknown. Testing lubricin with our device showed that it not only helps maintain the ocular surface by decreasing frictional forces exerted on the eye, but more importantly also suppresses inflammation. This suggests lubricin may play a key role in modulating multiple pathways," said Dr. Lee.
The team also found that the artificial 'eyelid' in their device plays an important role in cell differentiation. Corneal cells matured faster and more efficiently when the gelatin eyelid was blinking on top of them, suggesting a mechanical role in cell differentiation. This discovery could have important implications for understanding cell function, as well as uncovering how DED, or a change in blinking rates, impacts cell function in the eye.
For Drs. Massaro, Bunya, and Lee, the study represents a huge step forward in understanding DED mechanisms. Looking forward, they hope to further study the effectiveness of various drug treatments in their eye model, and gain a better understanding of the mechanisms involved in DED. According to Dr. Massaro, "This model has the potential to revolutionize the way we understand fluid dynamics on the surface of the eye and their effects on human cells, and could accelerate the discovery of new treatments." They credit much of the study's success to the collaborative work fostered at the University of Pennsylvania, which brought together multiple schools across Penn's campus, each specializing in an important aspect of the study.
The team of researchers (from left to right): Mina Massaro-Giordano, MD, Vivian Lee, MD, Vatinee Y. Bunya, MD, MSCE, Jeongyun Seo, and Dan Huh, PhD.