A flexographic press of conventional design is used to print on a container, with the container to be printed upon replacing the web and the impression roll of the conventional press. The source and drain regions are formed underneath the trench and along the substrate surface, respectively, with a non-linear channel region therebetween. A secondary tubular pole telescopically receives the primary pole. The cover plate fits over the base plate, and the spring clips are situated between an upper portion of the base plate and the underneath side of an upper portion of the cover plate. The invention makes a continuous measurement of the oxygen partial pressure possible, that almost shows no time delay. This invention provides methods of obtaining novel enzymes that have optimized physical or biological properties.