The invention features a method for producing replicas of a desired structure, a master and a mold obtained from it for use in such a method. The master includes a desired structure (200) to be reproduced and an additional structure (206, 208) that assists in removal of the master from a mold, e.g. by permitting portions of the mold on opposite sides of the additional structure to flex away from one another when the master is removed from the mold. The method includes immersing the master comprising the desired structure (200) and the additional structure (206, 208) in a molding material (142), solidifying the molding material (142) to make a mold, removing the master from the mold and using the mold to produce replicas of the desired structure (200). For example, the desired structure of the master may include a loop that defines a fluid flow pathway, and the additional structure includes a membrane configured to prevent the molding material from completely passing through the loop when the mold is made from the master.