The nature of hydrogel polymers is described together with the range of biomedical applications in which their use has been suggested or described in the literature. The field of contact lens materials has provided the greatest variety of synthetic hydrogels and the material requirements for extended wear lenses present problems that are typical of those encountered in biomedical applications in general. The patient literature relating to contact lenses is reviewed and the development of the composition of hydrogel materials is thereby traced and compared with the range of lens materials currently available. In contrast the most commonly encountered, in fact almost the sole, hydrogel material in the literature relating to those areas more conventionally regarded as biomedical, is poly(2‐hydroxeythyl methacrylate) or polyHEMA. The use of this and related hydrogels in various applications including prostheses, ocular surgery, sature coatings, artificial internal organs and drug delivery systems is reviewed.

Differential scanning calorimetry (DSC) has been used in the study of a series of hydrogels in an attempt to correlate water binding and transport properties. DSC and oxygen transport studies were carried out on a series of styrene‐2‐hydroxyethyl methacrylate copoly‐mers. The transport of dissolved oxygen through those copolymers that contained no freezing water was found to be negligible in comparison to those in which both freezing and non‐freezing water was present. This correlates well with the observation that the dense layer of asymmetric cellulose acetate reverse osmosis membranes was found to contain little or no free water. On this basis, the use of DSC in the design of a hydrogel that contains little or no freezing water (to promote salt rejection) with a reasonably high total water content (to maximise water flux) for use in reverse osmosis, is described. The resultant copolymer of acrylamide, methacrylic acid and styrene has a total equilibrium water content (30 per cent) that is twice that of a typical dense cellulose acetate used in reverse osmosis and a very low (< 1 per cent) freezing water content. Examination of the fine structure of the melting endotherms and freezing exotherms associated with various hydrogels shows several interesting features which are interpreted in terms of the existence of a continuum of water states. These range from water that is unaffected by its polymeric environment to water (on average two or less molecules per monomer repeat unit) that is hydrogen bonded to functional groups in the polymer.

Some of the problems and advantages in the use of non‐hydrogel polymers in contact lenses are discussed together with studies on a series of such polymers which have potential advantages over the established material, poly(methyl methacrylate), in that they are both more flexible and more oxygen‐permeable. Of the polymers examined which are all too hydrophobic for direct use, poly(4‐methylpent‐l‐ene) proved to be the most readily modified in such a way that its surface became sufficiently wettable to sustain a coherent tear film without reducing its optical qualities to an unacceptable level. The ‘dissolved’ and ‘gaseous’ oxygen permeability coefficients of this polymer were studied as a function of film thickness, surface hydrophilicity and temperature. A pronounced boundary layer effect was observed in ‘dissolved’ oxygen permeability studies, although this decreased as the surface was treated to make it more wettable (as indicated by the equilibrium advancing water contact angle). The ‘gaseous’ permeability coefficients of oxygen were found to be some 4‐6 times greater than those for nitrogen. A discontinuity corresponding to the glass transition temperature was observed at 28°C with both permeants and apparent activation energies for permeation were determined both above and below this temperature.

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Abstract5‐Methyl‐5‐propyl‐1,3,2‐dioxathiolan‐4‐one 2‐oxide (MPAS) and 5‐methyl‐5‐isopropyl‐1,3,2‐dioxathiolan‐4‐one 2‐oxide (MiPAS), which are isomers of the previously studied 5,5‐diethyl‐1,3,2‐dioxathiolan‐4‐one 2‐oxide (DEAS), have been synthesized and their polymerizability compared with that of the last compound. The two unsymmetrically substituted monomers polymerize by a mechanism which is substantially identical to that of their symmetrically substituted counterpart. In dry nonhydroxylic solvents the rate‐determining process is the primary scission of the ring, which takes place with elimination of sulfur dioxide and concurrent ring contraction to form an α‐lactone intermediate. In this reaction, the parent acid, produced by reaction of the monomer with adventitious traces of moisture, acts as the initiating species. The resultant polymers are all hydroxyl/carboxyl‐terminated, but, whereas those derived from the two unsymmetrically substituted monomers are amorphous and readily soluble in a variety of organic solvents, those derived from the diethyl‐substituted ring have been shown to be highly crystalline materials which dissolve in very few solvents. The relative polymerization rates are illustrated by the first‐order rate constants for decomposition in nitrobenzene at 90°C: DEAS, 20.1 × 10−5 sec−1; MiPAS, 11.0 × 10−5 sec−1; MPAS, 9.7 × 10−5 sec−1. The role of the substituents in determining the magnitude of these constants is discussed in terms of both the Thorpe‐Ingold effect and electron donation at C‐5.