Hydrogel Foams
“Porous hydrogels are usually prepared by a solution polymerization technique, which entails polymerizing monomers in a suitable solvent. The nature of a synthesized hydrogel, whether a compact gel or a loose polymer network, depends on the type of monomer, the amount of diluent in the monomer mixture, and the amount of crosslinking agent. As the amount of diluent (usually water) in the monomer mixture increases, the pore size also increases up to the micron range. Hydrogels with effective pore sizes in the 10-100 nm range and in the 100 nm-10 .mu.m range are termed "microporous" and "macroporous" hydrogels, respectively. In practice, the terms "microporous" and "macroporous" are used interchangeably simply due to the fact that there is no unified definition of micro- and macro-pores in hydrogels. Accordingly, hydrogels having pores up to about 10 .mu.m can be called either microporous or macroporous. “ (Park, Chen and Park, US Patent 6,271,278; 8/7/2001)
"Superporous hydrogels (SPHs) are a class of macroporous hydrogels developed for fast-swelling applications (Gemeinhart, et al. (2000) Polymers Adv. Technol. 11:617). SPHs can be prepared from addition monomers, such as PEGDA, by a gas foaming technique wherein the foaming and gelation processes are simultaneous to yield hydrogels with a macroporous network. The gas foaming technique is carried out by adding macromer, initiator, and foam stabilizer to a tube; acidifying this solution to retard the polymerization process; and subsequently adding sodium bicarbonate to generate carbon dioxide bubbles, which makes the foam rise. The addition of sodium bicarbonate increases the pH, resulting in faster polymerization of macromers. Due to this methodology, SPHs have a highly porous interconnected structure and large surface-to-volume ratio throughout the scaffold (Gemeinhart, et al. (2000) supra)." (Gemeinhart, US Patent Application #20090291115, 11/26/2009)
Recent Journal Articles
Polymeric hydrogels and supercritical fluids: The mechanism of hydrogel foaming
(2819-2826) Polymer 52 #13 (2011)
Tsioptsias et al of the Aristotle University of Thessaloniki,, Greece, used hydrogel foaming to produce foams with supercritical carbon dioxide (CO2). This method is applied to crystalline hydrophilic polymers that, practically, exhibit no phase transition (melting or glass transition) below thermal decomposition temperature and, due to their crystallinity, do not absorb CO2. Such polymers are mainly natural (semi)-crystalline polymers (e.g. chitosan, cellulose, etc.) for which the classical polymer foaming method with supercritical carbon dioxide is not applicable. The hydrogel foaming process (similar to classical polymer foaming) is applied to gelatin, chitosan, and gelatin/chitosan blend hydrogels that are physically crosslinked and may also be chemically crosslinked with glutaraldehyde vapour. After the foaming process, water is removed from the gels by mild freeze-drying leading to porous materials. Pore size control can be achieved by controlling different process parameters. Gelatin exhibits solubility in water up to high concentrations and forms thermoreversible hydrogels, rendering it a suitable choice for the investigation of the process mechanism. The sorption and Raman spectroscopy measurements suggest that, besides dissolution in water (of the hydrogel), extensive CO2 sorption by the polymer also occurs. (RDC 6/2/2011)