 In order to load the hydrogel matrix with a certain drug, contact lenses are simply placed in a solution of the drug and the drug diffuses into the matrix.Since this loading technique is driven solely by the gradient of the drug concentration surrounding the lens relative to the hydrogel matrix, the diffusion rate and amount of drug that is loaded can be controlled solely by the concentration of the drug solution.Since this process allows for specific amounts of a certain drug to be loaded to the hydrogel matrix, this method of loading has become important for patient-specific (personalized) medicine and treatments.