A template (a molecule preferably of molecular size >500 Da, or a larger entity such as a cell, virus or tissue sample) is dissolved or suspended in a fluid. The fluid is frozen, and the template is removed (e.g. by dissolution or electrophoresis, or mechanically) to leave an "imprinted" frozen fluid. This is capable of selectively adsorbing the template substance. It is usable as a separation medium, a recognition element in sensors and assays, and as a catalyst.