Hydrogel membranes, permeation

SW Kim, JR Cardinal, S Wisniewski, GM Zenter. Solute permeation through hydrogel membranes, hydrophilic vs. hydrophobic solutes. ACS Symp Ser 127 347-359, 1980. 

GM Zentner, JR Cardinal, SW Kim. Progestin permeation through polymer membranes, II Diffusion studies on hydrogel membranes. J Pharm Sci 67(10) 1352-1355, 1978. 

Recently, we have examined solute permeation through hydrogel membranes in an effort to develop models which describe in detail the transport phenomena with particular emphasis on the role of water in this process. These studies have utilized p-HEMA and its copolymers, and both hydrophobic and hydrophilic solutes (7., i). It was determined that p-HEMA and its copolymers are permeable to both hydrophobic and hydrophilic solutes. 

Membrane permeation Easy to use Pol5Tner in hydrogel state UV detection possible Long times required to reach steady state Potential to mpture membrane... 

The absorption/desorption method is more involved experimentally compared to the membrane permeation technique, since the protein must be incorporated into the hydrogel, but not adsorbed, and subsequently dried. The dissolution study is typically carried out in USP no. 2 apparatus, with media continuously being monitored by UV analysis. Penetration of the dissolution media into the dehydrated polymer complicates the diffusion process, commonly producing a lag time prior to protein release. 

Zentner, G. M., Cardinal, J. R., and Gregonis, D. E., 1979, Progestin permeation throirgh polymer membranes III Polymerization solvent effect on progesterone permeation through hydrogel membranes, J. Pharm. Sci 68(6) 794-797. 

Before the introduction of brush-decorated porous membranes, the best candidates for signal-responsive permeation were hydrogel membranes that contract or expand in response to a change of an external parameter [44]. 

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. 

In conclusion, 1) Hydrophilic solutes permeate p-HEMA and p-HEMA crosslinked with lower mole % EGDMA via the "pore" mechanism. The diffusion coefficients of the solutes depend on the molecular size and may utilize the "bulk-like" water in the hydrogels. As the water content of hydrogel increases, the solute permeability increases. 2) Hydrophobic solutes permeate p-HEMA and p-HEMA crosslinked with EGDMA via either the "pore" or "partition" mechanisms. Diffusion coefficients are lower than those of hydrophilic solutes however, steroids can permeate even in p-HEMA with 5.25 mole % EGDMA due to the predominant "partition" mechanism for hydrophobic solute permeation in this membrane. Hydrophilic solutes fail to permeate the high crosslinked hydrogels. 

In contrast, neither acrylamide nor N-aUcylacrylamides could be anionically polymerized, due to proton abstraction from their acidic amide protons. Among such monomers, N-isopropylacrylamide (NIPAM) is the most often used, with recent interest in its polymer - poly(N-isopropylacrylamide) (PNIPAM) - having increased in exponential fashion due to its possible use as hydrogels, in drug-delivery devices, in biomedicine, and in permeation membranes - all of which reflect the polymer s water-solubiUty and thermoresponsive nature (Tc = 32 °C). 

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