Bioresorbable and Nonresorbable Macroporous Thermosensitive Hydrogels Prepared by Cryopolymerization. Role of the Cross-Linking Agent

Abstract

Macroporous poly( N-isopropylacrylamide) (pNIPA) gels (so-called cryogels), cross-linked with different bis-acrylic compounds, N,N'-methylenebisacrylamide (MBAAm) and dimethacrylate-tyrosine-lysine-tyrosine (DMTLT), were prepared through free-radical polymerization at subzero temperature in dioxane/water media. DMTLT is a hydrolytically degradable cross-linker with relatively hydrophobic character. The effects of different synthesis conditions, namely the concentration of monomers, the cross-linker, and the initiator in the reaction mixture, on the structure of the pNIPA-cryogels have been studied. The equilibrium swelling ratio of the DMTLT cross-linked pNIPA cryogels at temperatures below lower critical solution temperature (LCST) of pNIPA, was over ten times higher than that of the gels synthesized at room temperature from the same feed composition. The MBAAm cross-linked pNIPA cryogels synthesized in water exhibited the highest equilibrium swelling and the fastest response. The critical transition temperature, T c, was lower ( T c approximately 31 degrees C) for pNIPA-cryogels synthesized in dioxane/water media or cross-linked with DMTLT as compared to MBAAm cross-linked pNIPA cryogels synthesized in water (T c approximately 33 degrees C). Scanning electron microscopy (SEM) revealed different porous structure and pore surface morphology depending on the cross-linker (MBAAm or DMTLT) and the solvent (water or dioxane/water) used. Gels and cryogels were also characterized by SAXS, showing that the nanostructure of the samples is related to swelling.

Porous polymer monoliths: Amazingly wide variety of techniques enabling their preparation

“…In both cases the solution must be frozen prior to the reaction. Using the latter, a vast variety of cryogels has been prepared from monomers including 2-hydroxyethyl methacrylate [177], acrylamide [181-185], dimethylacrylamide [186], N-isopropylacrylamide [187,188], and N-vinylcaprolactam [189]. Crosslinkers such as methylenebisacrylamide and poly(ethylene glycol) diacrylate are used in the polymerization mixture to reinforce the macroporous structure and prevent dissolution of the monolith in the aqueous mobile phases during their application.…”

Cryotemplation for the rapid fabrication of porous, patternable photopolymerized hydrogels

“…Three fabrication parameters for cryotemplated photopolymerization were investigated for their ability to modulate pore structure: freezing temperature 37–39 , gelation time 38, 39 , and ratio of polymer to photoinitiator 21, 39, 40 (Figure 2). Pore geometry was typically spherical and randomly oriented whether the solution was frozen at −20 °C (Supp.…”

Thermoresponsive cryogels reinforced with cellulose nanocrystals

“…Upon freezing, the major part of the solvent forms crystals leaving only a liquid microphase in which the reactants are concentrated. PNIPAAm is a well-known thermoresponsive polymer, with a lower critical solution temperature (LCST) of around 32-33 C. 2,30 Typically, hydrogels of PNIPAAm are swollen and hydrated below the LCST, while they shrink above the LCST where they become hydrophobic and expel water. Depending on the reaction conditions and choice of monomer and other reactants, the size of the pores (within a cryogel) can be varied in a large range, from 0.1-200 mm.…”