The researchers decided to use caffeine to induce citric acid to form a polymer network along with polyethylene glycol (PEG). When mixed with citric acid and PEG, and slightly heated, caffeine opens up an oxygen-containing ring in the PEG, allowing it to react with citric acid to form chains that consist of alternating molecules of PEG and citric acid. If drug molecules are present in the mixture, they also become incorporated into the chains.
The researchers can vary the chemical and mechanical properties of the gel by altering its composition. They created gels that contain either PEG or polypropylene glycol, as well as some that combine the two polymers in different ratios. This allows them to control properties such as the material’s strength, surface structure, and the drug-release rate.
“Depending on what the application may be, or what drugs are being incorporated, you could mix and match to find an optimal mixture,” Traverso said.
The gels can also be imprinted with patterns such as the microscale architecture found on the surface of lotus leaves, which allows them to repel water. Altering the surface traits of the material could help researchers control the rate at which the gels move through the digestive tract.
The resulting gels contain a small amount of caffeine, roughly the same as that found in a cup of tea. In preliminary safety tests, the researchers found no harmful effects in four types of human cells, or in rats.