In a 16-year-old girl, a dentist detected patches of white enamel that has lost its natural luster in the paracervical regions of her teeth 11 and 21. The enamel surface is smooth and dense to probing. Thermal stimulation results are negative. The lesions can be stained with a 2%. aqueous solution of methylene blue. In this case, the infiltration technique was chosen for the treatment of initial caries. What material is used in this technique?