Rapid plasmonic actuation of thermoresponsive hydrogel structures

Abstract

Poly (N-isopropylacrylamide) (pNIPAAm) is a thermoresponsive polymer that is often used as functional material in soft robotics, as well as biocompatible, adaptable, and external tunable compound in many biomedical applications. Crosslinked and exposed to water, pNIPAAm can form soft hydrogels with a lower critical solution temperature (LCST) of 32 °C above which they undergo a phase transition from a swollen, hydrophilic to a collapsed, hydrophobic state. In this work, the kinetics of the swelling and collapsing process of such pNIPAAm based polymer networks are investigated under various external conditions in the millisecond time range. For this purpose, a thin layer (~ 800 nm in the swollen state) of pNIPAAm based hydrogel is attached to a substrate with arrays of plasmonic nanoparticles (NPs). Their localized surface plasmon resonances (LSPRs) are used for rapid and local plasmonic heating and simultaneous time-resolved monitoring of the gel with an inhouse developed optical system. The state transition is studied for increasing heating strength, varying ambient temperature before the actuation of the transition and for different heating durations. The results reveal that the dependence of the transition kinetics on the external parameters for the swelling and the collapse are different. While the collapse time is minimized to 3.2 and 1.5 ms with increasing heating intensity and ambient temperature respectively, the swelling shows to be more independent of the previously applied temperature difference. However, two consecutive phases evolve for the swelling process, when the network is preconditioned close to the LCST by elevated ambient temperature, with an increasing first phase of slow swelling, which delays the second actual process of exponential swelling by up to 20 ms. Furthermore, with longer heating durations, another superimposed transition process with a time constant of τ ~ 20 ms is found for the collapse, which was not observed before in similar experiments with pNIPAAM polymer brushes.These observations could contribute to a better understanding of the widely used material and thereby benefit the design and development of new devices in the field of biomedical engineering and soft micro-machines and -robotics.