text: While multiple classification standards for tissue clearing exist, the most common classifications use the chemical principle and mechanism of clearing to group tissue clearing methods. These include hydrophobic clearing methods, which may also be known as organic, solvent-based, organic solvent-based, or dehydration clearing methods; hydrophilic clearing methods, which may also be known as aqueous-based or water-based methods, and may be further sub-categorized into simple immersion and hyperhydration (also called delipidation/ hydration); and hydrogel-based clearing methods, which may also be known as detergent or hydrogel embedding methods. Tissue-expansion clearing methods use hydrogel, and may be included under hydrogel-based clearing or as their own category.