Active Surface Topographies in Constrained Hydrogel Films
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Active Surface Topographies in Constrained Hydrogel Films Ophir Ortiz1, Ajay Vidyasagar2 and Ryan Toomey2 1 University of South Florida, Dept. of Electrical Engineering, 2Dept. of Chemical and Biomedical Engineering, 4202 East Fowler Ave., Tampa, FL 33620, U.S.A. ABSTRACT Silane-anchored Poly(N-isopropylacrylamide)-co-methacroyloxy-benzophenone (MaBP) thin films have been produced which exhibit formation of surface features which manifests in cusps or folds when exposed to solvents. This response has been quantified by measuring the characteristics of these cusps or folds produced on the thin films as a result of biaxial strain. The strain is a result of constrained swelling during the solvent exposure steps, as the film is anchored to a rigid surface and is therefore unable to swell in 3 dimensions. This anchoring causes a residual stress in the form of surface corrugations to form in the thin film. The results indicate that cusp characteristics are highly dependent on the film thickness, which ranged from 30 to 1200 nm. INTRODUCTION Periodic wrinkles or corrugations can appear on the free surfaces of constrained hydrogel films [1-3]. The constraint generates a residual compressive stress that is partially relieved through deviation of free surfaces from a planar configuration. The characteristics of the surface instability, which include morphology and wavelength, are generally well-defined and depend on the strength of the constraint. If swelling of the hydrogel overcomes the constraint, the film can slip along its substrate, altering the resultant compressive stress and surface topography. By tuning this slippage, we aim to direct the surface features presented by thin hydrogel films. Active control of surface morphology could have important implications in several applications ranging from sensors to controlled tissue harvesting [4,5]. In this work, we discuss a method of tuning the amount of slippage and surface topography of patterned poly(N-isopropylacrylamide) (poly-NIPAAm) films. The films were fabricated from photo-crosslinkable polymers comprising NIPAAm and photo-active methacroyloxy-benzophenone (MaBP) monomers. The films were anchored to a substrate through a hydrogen-bonding self-assembled monolayer prior to the photo-crosslinking step.. The relationship between pattern dimensions and solvents were investigated with respect to the mechanical instability generated at the free surface. Two different polymer patterns were fabricated via lithography, and the surface instabilities that were produced after a developing step were measured. We observed that the wavelength, width, and amplitude of the instability always increased with the thickness of the polymer pattern. The change in geometry yielded similar structures.
EXPERIMENTAL DETAILS Poly(N-isopropylacrylamide)-co-MaBP was spin-cast onto silanated silicon to produce uniform thin films. The single side polished, 500 µm-thick silicon substrates were first silanated for polymer anchoring purposes; the conditions for this process were a d
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