Freeze Etching: A replica technique in which cells are frozen to a very low temperature and cracked with a knife blade to expose the interior surfaces of the cells or cell membranes. The cracked cell surfaces are then freeze-dried to expose their constituents. The surfaces are now ready for shadowing to be viewed using an electron microscope. This method differs from freeze-fracturing in that no cryoprotectant is used and, thus, allows for the sublimation of water during the freeze-drying process to etch the surfaces.Freeze Drying: Method of tissue preparation in which the tissue specimen is frozen and then dehydrated at low temperature in a high vacuum. This method is also used for dehydrating pharmaceutical and food products.Acid Etching, Dental: Preparation of TOOTH surfaces and DENTAL MATERIALS with etching agents, usually phosphoric acid, to roughen the surface to increase adhesion or osteointegration.Freeze Fracturing: Preparation for electron microscopy of minute replicas of exposed surfaces of the cell which have been ruptured in the frozen state. The specimen is frozen, then cleaved under high vacuum at the same temperature. The exposed surface is shadowed with carbon and platinum and coated with carbon to obtain a carbon replica.Dental Etching: Preparation of TOOTH surfaces, and of materials bonded to teeth or DENTAL IMPLANTS, with agents and methods which roughen the surface to facilitate adhesion. Agents include phosphoric or other acids (ACID ETCHING, DENTAL) and methods include LASERS.Freeze Substitution: A modification of the freeze-drying method in which the ice within the frozen tissue is replaced by alcohol or other solvent at a very low temperature.Freezing: Liquids transforming into solids by the removal of heat.