Hydrogels, particularly the tough, low water content materials, have potential advantages in the field of clinical biosensors because of their established use as medical polymers. The factors that control transport behaviour in these polymers are discussed with particular reference to ion selectivity. The nature of the transport behaviour in relation to coated wire electrode performance is presented and an extension of these permselectivity studies to the fabrication of miniaturized devices, such as ISFETs, is described. Linear soluble hydrogel polymers, coated on to sensor substrates, may be converted to insoluble membranes using solid photosensitive aromatic monomers, such as N-vinyl carbazole. Photolithographic patterning is achieved using a UV source together with appropriate masking, followed by an oxygen plasma etch process. Gas plasma etching, which selectively removes uncross-linked (masked) areas forms the basis of an all dry, low-temperature patterning process capable of giving micrometre-scale resolution. This novel photographic process, which does not damage or extract enzymes or ionophores, can advantageously be extended to the fabrication of poly(vinylchloride)-based membranes.