In a recent study published in ACS Applied Materials & Interfaces, a group of scientists reported successfully developing a contact lens that changes color as drugs are released. The lens acts as a visual indicator to both doctor and patients that medications used to treat eye diseases have been properly applied.
From the American Chemical Society article "Eyes are adept at keeping things out. When something ventures into or toward an eye, the lids blink and tears start rapidly flowing to avoid infection and damage from foreign objects. These processes are usually helpful, but they can hinder the uptake of much-needed medications. Studies suggest that less than 5 percent of drugs in eye drops and ointments are absorbed, and much of the absorbed medication ends up in the bloodstream instead of the eye, causing side effects. Contact lenses may be a more effective way to deliver drugs directly to the eye, but real-time monitoring of drug release is still a challenge. So Dawei Deng and Zhouying Xie sought to create a drug-delivering contact lens that would change color as the medication is released into the eye."
The lens uses a technique known as molecular imprinting, which creates molecular cavities in a polymer structure to match the size and shape of the target medication. In laboratory experiments, the contact lens was loaded with a drug used to treat glaucoma and immersed in a solution of artificial tears, as a stand-in for a real eye. As the drug released from the contact lens, it changed the architecture of the molecules around it, changing the color of the lens's iris area. An added benefit of the contact is the color change uses no dye and can be seen by the naked eye.
Read more about the color-changing contacts that track drug delivery at the American Chemical Society.