A novel multiresponsive hydrogel has been synthesized by initiated chemical vapor deposition (iCVD). Hydrogels are known for their dynamic swelling response to aqueous environments. A chemical functionalization of the hydrogel surface was performed to add other stimuli-responsive functionalities and obtain a smart material that responds to two stimuli: light irradiation and exposure to aqueous environment. Modifying the hydrogel surface with solution-based methods is often problematic because of the damages caused by the permeation of solvents in the hydrogel. This issue is completely bypassed by the use of solvent-free techniques. Cross-linked polymers of 2-hydroxyethyl methacrylate (HEMA) were functionalized with azobenzene groups, as confirmed by IR spectroscopy and X-ray photoelectron spectroscopy (XPS). Through photoisomerization of the azobenzene, the polarity within the hydrogel is modified and as a consequence the affinity to water. Light irradiation modifies the degree of swelling within thin hydrogel films from 13% before exposure to UV light to 25% after exposure. The possibility of controlling the degree and rate of swelling by light irradiation was never reported before on these time scales and can have exceptional implications for light-induced drug delivery or light-controlled microfluidic systems. The light-responsive hydrogels showed also biocompatibility, which makes them suitable for a great variety of applications as biomaterials.
Thermoresponsive polymers undergo a reversible phase transition at their lower critical solution temperature (LCST) from a hydrated hydrophilic state at temperatures below the LCST to a collapsed hydrophobic state at higher temperatures. This results in a strong response to temperature when in aqueous environment. This study shows that hydrogel thin films synthesized by initiated chemical vapor deposition show fast and strong response to temperature also in water vapor environment. Thin films of cross-linked poly(N-isopropylacrylamide), p(NIPAAm), were found to have a sharp change in thickness by 200% in water vapor at temperatures above and below the LCST. Additionally, the stimuli-responsive poly(N,N-diethylacrylamide) was investigated and compared to results found for p(NIPAAm). Analysis of the swelling kinetics performed with in situ spectroscopic ellipsometry with variable stage temperature shows differences for swelling and deswelling processes, and a hysteresis in the thickness profile was found as a function of temperature and of temperature change rate.
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