Abstract: A target substance (8) is subject to a freeze drying process in which the substance is first passed into a freeze chamber (31) to reduce the temperature of the substance, and then passed into a separate vacuum chamber (32) in which a vacuum is applied to promote drying of the target substance. Because the carrier (10) and target substance (8) is moved from a freeze chamber (31) to a separate vacuum chamber (32) the environment can be more closely controlled and the cycling of freezing to drying can be more rapid. In this way the carrier (10) and target substance (8) is subject to the full temperature cycling without the surrounding chamber and apparatus being subject to the full cycle. This reduces time and energy costs. The technique is particularly suited to manufacturing biosensors using freeze dried reagents.