Rauen, Charles Alex, et al. “Effect of Bleaching Agents Containing Fluoride or Calcium on Enamel Microhardness, Roughness and Permeability”. Brazilian Journal of Oral Sciences, vol. 14, no. 4, Sept. 2016, pp. 262-6, https://periodicos.sbu.unicamp.br/ojs/index.php/bjos/article/view/8646125.