A Pinch Of Salt Softens Hydrogel

A new type of hydrogel made from a polymer electrolyte complex can be softened and processed into a biomaterial with mechanical properties suitable for medical implants simply by adding table salt to disrupt the ionic cross-links (Biomacromolecules, DOI: 10.1021/bm900373c). Claudine H. Porcel and Joseph B. Schlenoff of Florida State University developed combinations of cationic poly(diallyldimethyl ammonium) and anionic poly(styrene sulfonate) and found that ultracentrifuging the PDADMA/PSS materials with a concentrated sodium chloride solution forces salt into the complex where it breaks up some of the ion pairs that hold the polymer chains together. The partially liberated chains become much more mobile, similar to using heat to soften up a thermoplastic, Schlenoff says. This novel “saloplasticity” is reversible by adding water to dilute the salt. The hydrogel has micropores filled with extra polyanion, similar to the porous structure of cartilage that contains negatively charged proteoglycans. The extra polymer generates osmotic pressure to swell the hydrogel, making it elastic like a springy shock absorber. The team found one polymer combination that has properties similar to intervertebral discs—a type of cartilage made from a natural cross-linked hydrogel—and potentially could be used to make artificial replacements.