Hydrogels permeability

SH Gehrke, M Palasis, M Lund, J Fisher. Factors determining hydrogel permeability. Ann NY Acad Sci 831 179-207, 1997. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Cross-linked versions of water-soluble polymers swollen in aqueous media are broadly referred to as hydrogels (52) and have a growing commercial utility in such apphcations as oxygen-permeable soft contact lenses (qv) (53) (Table 4) and controUed-release pharmaceutical dmg deflvery devices (54). Cross-linked PVP and selected copolymers fit this definition and are of interest because of the following stmcture/performance characteristics ... 

The number of contact lens wearers has grown to an estimated 24 million in the United States and 50 million worldwide. Concurrendy, there has been a proliferation of contact lens manufacturers and products. The 1980s saw the widespread introduction of lens products made of more oxygen-permeable materials, ie, rigid gas-permeable (RGP) materials that made PMMA lenses virtually obsolete and high water content hydrogels that competed with HEMA-based lenses. 

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

Polymer composites of mbber-hydrogel are permeable for water-soluble low-molecular weight compounds in a swollen state. Therefore, it is suitable for dmg delivery carrier. Many papers have researched the basic role of composite morphology in low-molecular weight component transport through two-phase composites where only one phase played the main role. 

SEM. They conhrmed the suggestion that the probability of mutual contact of particles, and thus the permeability, grows with growing hydrogel concentration. The existence of aggregates decreased the permeability. 

The hydrophilic/hydrophobic SIN composition of PDMS with poly(HEMA) and poly(AAC) were proposed as a potential apphcation for high-permeability soft contact lenses. Other sUicone-containing IPNs for contact lenses include polymerization of MMA in the presence of polymerized methacryloxypropyl trimethoxysilane, the cross-linking of a polymeric hydrogel of a copolymer of NVP during the final compression or injection-moulding process. 

The equilibrium swelling degree is the most important property of a hydrogel it directly influences the rate of water sorption, the permeability to drugs, and the mechanical strength of the gel. It also affects the biocompatibility of the... 

The key property of a hydrogel is its swelling degree, since this directly influences other properties such as permeability. The theoretical description of hydrogel... 

The significance of the permeability depends upon the particular system at hand. If the hydrogel forms a barrier between a drug reservoir and a solution, the flux across the hydrogel membrane at steady state is simply... 

M Palasis, SH Gehrke. Controlling permeability in responsive hydrogels. Proc Int Symp Controlled Release Bioact Mater 17 385-386, 1990. 

The effect of hydrophobicity of the polymer on the permeability of poly(2-hydroxyethyl methacrylate (HEMA)-co-methacrylic acid (MAAc) hydrogels was studied [12], The hydrophobicity was controlled by copolymerization with butyl methacrylate (BMA). The dependence of permeability on pH increased as the hydrophobicity increased even though the rate of diffusion decreased. Cross-link density of the hydrogel also contributed to pH-dependent permeability. 

Permeability control of solutes through thermosensitive hydrogel membranes may be thought of in terms of the degree of hydration and kinetics of... 

Prokop, A., Kozlov, E., Carlesso, G. and Davidsen, ]. M. Hydrogel-Based Colloidal Polymeric System for Protein and Drug Delivery Physical and Chemical Characterization, Permeability Control and Applications. Vol. 160, pp. 119-174. 

Examples of silicones in ophthalmic application have been described.5 1 However, some problems have been reported with the use of silicones in retinal tamponade.542 Silicones are widely used in contact lenses because of their unusually good oxygen permeability and bioinertness, but the need for novel materials continues. Low-modulus fluorosiloxane-based hydrogel lenses with a wide range of water content and high-oxygen permeability have been described in a number of papers.543-... 

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