New device to reproduce tear film on contact lens

A device may help reproduce a tear film on the surface of a contact lens, to make it more closely mimic the eye.

The chemical engineers at Stanford University in Northern California on the U.S. west coast devised the machine based new insights on the mechanical properties of the tear film on the eye's surface.

In the United States, more than 30 million people currently wear contacts, but roughly half of them switch back to glasses because of contact lens-induced symptoms such as dry eye, an irritation that outweighs the convenience of contacts.

"As a student, I had to stop wearing lenses due to the increased discomfort," said Saad Bhamla, a Stanford postdoctoral scholar in bioengineering who conducted the work as a graduate student and authored with his colleagues a study published in the March issue of the journal Investigative Ophthalmology & Visual Science (IOVS).

Bhamla and his co-authors suspected that most of the discomfort of wearing lens arises from the breakup of the tear film, a wet coating on the surface of the eye, during a process called dewetting.

In the study, the researchers outline two functions of the lipid layer, an oily coating on the surface of the tear film. One is to provide mechanical strength to the tear film. Lipids have viscoelastic properties that allow them to stretch and support the watery layer beneath them.

The lipid layer also prevents the tear film from evaporating away. Eyes are roughly 95 degrees Fahrenheit, or 35 degrees Celsius, which is usually warmer than the ambient air. Like any liquid on a hot surface, the eye is constantly heating its liquid coating and losing moisture to the air.

"We recognized early-on that the fluid mechanical responses of the lipid layer were just as important as the conventional view that its role was to control evaporative loss," said Gerald Fuller, a professor of chemical engineering at Stanford. "And it's been gratifying to realize that the combined role of these two forces is now accepted."

To solve this, Bhamla and Fuller built a device, known as the Interfacial Dewetting and Drainage Optical Platform, or i-DDrOP, to reproduce a tear film on the surface of a contact lens, and allow manufacturers to handle the array of variables that affect the tear film, including temperature, a variety of substances, humidity and the way gravity acts along a curved surface.

With the ability to accurately recreate a tear film on the contact lens surface and test how quickly it breaks up, manufacturers are now armed with the tools to make a more comfortable lens. Endit