Abstract: It is described a sealed cassette for molding a composition used for biological analysis having a first opening for introducing the composition and a second opening for contacting the composition. The sealed cassette further comprises a stopper removably mounted onto the cassette to seal the second opening. Another cassette comprises a first shell and a second shell. The first shell and the second shell are adapted for mating together in a first respective position keeping a first distance between the first shell and the second shell and for mating together in a second respective position keeping a second distance between the first shell and the second shell. The first and second shells, when mated together, are adapted to receive the composition. Methods for making such cassettes are also described.