Lastly, if the solvent-exchange approach did not facilitate the activation process, then complex activation processes, such as freeze-drying or supercritical CO2 activation, come to the rescue. Typically, the supercritical CO2 technique is more popular than the freeze-drying method. Unlike the conventional thermal activation wherein the entrapped liquid solvent is evaporated, in freeze-drying and supercritical CO2 method, the direct transition from liquid phase to vapor phase is bypassed. As results, the effect of the liquid surface tension that would arises capillary forces, is not a concern any more. In freeze-drying, the solvent is cooled to form a solid phase, and then through sublimation process the solid is transformed directly into vapor.