Simultaneous Measurementof Mechanical and SurfaceProperties in Thermoresponsive, Anchored Hydrogel Films.

Hydrogel films have been used extensively in the preparationofbiosensors and biomedical devices. The characteristics of the aqueousinterface of the polymer layer are significant for the biosensor ordevice function; likewise, the changing mechanical properties of thermoresponsivepolymers are an important feature that affects the polymer behavior.Atomic force microscopy was used here to characterize both the surfaceand the mechanical properties of polymeric hydrogel films preparedfrom a thermoresponsive terpolymer of N-isopropylacrylamideand acrylic acid with benzophenonemethacrylate as a photoreactivecross-linker comonomer. The force–distance curves thus obtainedwere analyzed to assess both the surface forces and the mechanicalresponse that were associated with the hydrogel. These propertieswere investigated as a function of temperature, in water and in Trisbuffer, for different degrees of polymer cross-linking. For samplesin water, the distance over which the surface forces were effectivewas found to remain constant as the temperature was increased from26 to 42 °C, even though the mechanical response indicated thatthe samples had been heated past the lower critical solution temperature,or LCST. The bulk of the polymer becomes less soluble above the LCST,although this does not seem to affect the surface properties. Thismay be due to the segregation of the acrylic acid-rich polymer segmentsnear the gel surface, which is in agreement with reports for relatedsystems. [ABSTRACT FROM AUTHOR]