People who spend eight or more hours a day staring at a computer screen may notice their eyes becoming tired or dry and if those conditions are severe enough, they may eventually develop dry eye disease (DED). DED is a common disease with few FDA-approved drug options, partially because of the difficulties of modeling the complex pathophysiology in human eyes.
A blinking eye-on-a-chip was developed that is used as an artificial human eye replica to help scientists and drug developers improve their understanding and treatment of DED, among other potential uses. The eye model can imitate a healthy eye and an eye with DED, allowing them to test an experimental drug without risk of human harm.
To construct the eye-on-a-chip, researchers started with a porous scaffold engineered with 3D printing, about the size of a dime and the shape of a contact lens, on which they grew human eye cells. The cells of the cornea grow on the inner circle of scaffolding, dyed yellow, and the cells of the conjunctiva, the specialized tissue covering the white part of human eyes, grow on the surrounding red circle. A slab of gelatin acts as the eyelid, mechanically sliding over the eye at the same rate as human blinking. Fed by a tear duct, dyed blue, the eyelid spreads artificial tear secretions over the eye to form what is called a tear film.
Blinking serves to spread tears and generate a thin film that keeps the ocular surface hydrated. It also helps form a smooth refractive surface for light transmission. This was a key feature of the ocular surface that researchers wanted to reproduce in the device. For people with DED, that tear film evaporates faster than it is replenished, resulting in inflammation and irritation.
The team evoked DED conditions in the eye-on-a-chip by cutting the device’s artificial blinking in half and carefully creating an enclosed environment that simulated the humidity of real-life conditions. When put to the test against real human eyes, both healthy and with DED, the corresponding eye-on-a-chip models proved their similarity to the actual organ on multiple clinical measures. The eyes-on-a-chip mimicked actual eyes’ performance in a Schirmer strip, which tests liquid production; in an osmolarity test, which looks at tear film salt content; and in a keratography test, which evaluates the time it takes for a tear film to break up.