Phase transition of pNIPAM grafted on plasma-activated PEO monitored in-situ by quartz crystal microbalance

Heinz, Paul; Mannelli, Ilaria; Valsesia, Andrea; Ceccone, Giacomo; Gilliland, Douglas; Rauscher, Hubert; Rossi, Francois; Bretagnol, Frederic; Sirghi, Lucel; Ceriotti, Laura Italy

Thermorsponsive polymer layers offer the possibility to prepare smart surfaces with properties that are switchable due to a phase transition, usually close to the lower critical solution temperature of the polymer. In particular, poly(N-isopropyl acrylamide) (pNIPAM) has gained a lot of attention, since it has such a phase transition in a physiologically interesting temperature regime. We have prepared an ultrathin thermoresponsive coating by grafting pNIPAM on a plasma-CVD deposited, poly(ethylene oxide)-like polymer substrate which was activated in an Ar plasma discharge to initiate the grafting. The presence and integrity of pNIPAM has been verified by XPS and TOF-SIMS, and a drastic change in the wettability during the phase transition can be identified in temperature-dependent contact angle measurements. The transition from the hydrated to the random coil conformation has been analysed by temperature-dependent QCM measurements and by AFM. An unexpected, reversible behaviour of the viscoelastic properties is seen directly at the phase transition from the swollen to the collapsed state. The phase transition leads to a switching from protein repulsion to a state which allows adsorption of proteins. Switching from cell attachment to a cell detachment regime is also observed, but this depends on the previous adsorption of specific proteins. The pNIPAM layer can also be micropatterned by lithography to yield alternating regions with temperature-dependent and temperature-independent properties which expose thermoswitchable pNIPAM and antifouling PEO regions, respectively. The former show the same protein repelling and attracting properties as non-structured pNIPAM films.

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