PNIPAm-based microgels with a UCST response

Abstract

Thermoresponsive microgels with an upper critical solution temperature (UCST) response (i.e., showing volume expansion at higher temperature) are highly desirable for a variety of applications, but quite uncommon, in particular if they are to work in physiological conditions. Here we present a novel UCST-type microgel showing a remarkable transition in physiological conditions, with a diameter increase larger than 150% from 34 °C to 40 °C. This novel material is based in poly(N-isopropyl acrylamide), pNIPAm, a polymer well known for its opposite temperature response (featuring a lower critical solution temperature, LCST). The unusual properties of our material are due to a small fraction of copolymerized methacrylic acid and nitrocatechol monomers. The collapsed conformation in PBS buffer below 34 °C is stabilized by hydrogen bonding, with repulsion by partially ionized nitrocatechol and methacrylic units being screened by the ionic strength and the formation of hydrophobic clusters by the methyl groups in methacrylic acid. The increase in temperature leads to the collapse of pNIPAm segments, which disrupts the balance of supramolecular interactions, leading to the large volume expansion of the gel.