Smart surfaces: reversible switching of a polymeric hydrogel topography.

M.A. MOLINA; C.R. RIVAROLA; BROGLIA, M.F.; ACEVEDO D.F.; BARBERO C.A.

Simposio; SIMPOSIO ARGENTINO DE POLIMEROS; 2011

Smart hydrogels are stimuli responsive materials which suffer a phase transition, with volume change, in response to changing environmental conditions such as temperature, pH, solvent composition or electrical stimuli (Y. Qiu and K. Park, 2001). The decrease of thickness (e.g., in thin layers) is an appropriate way to create structures with reasonable response time (Kuckling D., 2009). Direct patterning of hydrogel structures would allow applications of those materials in fields such as adhesion and growth of immobilized cells. Direct Laser Interference Pattering (DLIP), permits rapid fabrication with high design flexibility (A. Lasagni and et al, 2007). The shape and dimension of the interference patterns can be adjusted by controlling the number of laser beams as well as their intensity. The structures are produced without any secondary step (e.g. etching) by the direct interaction of the laser light with the material (e.g. photothermal, photochemical or photophysical ablation). This communication describes a new method of creating patterns on polymeric poly(N-isopropylacrylamide), PNIPAM hydrogel films by DLIP of a dyed polymer. The pattern is imprinted only on the film, not reaching the glass substrate below. When the dry film is wetted, the pattern features disappears and the surface became flat because the hydrogel swelling closes the grooves. However, the original pattern reappears when the film is dried again. A similar behavior is observed when a patterned PNIPAM film, which has became flat by swelling in water, is heated up above the phase transition temperature of PNIPAM. The hydrogel volume collapses, revealing the pattern again. Methods Glass Substrate Preparation. Methacryloxypropyltrimethoxysilane (MPTES) functionalized glass surface cover slips were used as substrates to polymerization of vinyl monomers. We used a procedure base on the Aminosilane-Glass Method (Anseth and et al, 2002). Synthesis of hydrogels films. Monomer N-isopropylacrylamide (NIPAM), 0.5 M) and crosslinker (N,N-methylenebisacrylamide BIS, 10 mM) were dissolved in distilled water. The free radical polymerization was carried out over the MPTES-functionalized glass cover slips at room temperature for 30 min, using ammonium persulphate (APS, 0.001 g/ml) and N,N,N?,N?-tetramethylethylenediamine (TEMED, 10μl/ml) as redox initiator and activator, respectively. Film doping with a dye. The hydrogel surfaces are colorless so they have to be dyed to ensure that pulse laser is absorbed. We used tris(2,2`-bipyridine)ruthenium (II) (RBPY) due to its high absorbance and good retention inside acrylamides (Molina et al, 2010). Direct laser interference patterning (DLIP) experiments. A high-power pulsed Nd: YAG laser (Quanta-Ray PRO 290, Spectra Physics) with a wavelength of 355 nm was used for the laser interference experiments. The fundamental laser beam was split into two sub-beams and guided by mirrors to interfere on the sample surface. The samples were irradiated with laser fluencies up to 600 mJ cm-2. Parallels lines were obtained. Atomic Force Microscopy (AFM) measurements were made with an Agilent 5420 AFM/STM microscope. A commercial Point Probe® Plus Non-Contact /Tapping Mode - Long Cantilever (PPP NCL) with a force constant 6N/m and resonance frequency 156 Hz, was used in the Acoustic AC (AAC) mode. RESULTS AND DISCUSSION The evolution of the functionalization was monitored by contact angle (q) and FTIR Spectroscopy measurements. The presence of vinylic groups in MPTES-functionalized glass cover slips were tested by Baeyer´ test characteristic reaction for alkenes, which is cold oxidation reaction with alkaline MnO4K. Table 1 show the q changes of clean glass, silane monolayer and attached PNIPAM on the glass. q increases in agreement to the hydrophobic characteristic of the isopropyl group anchored to the surface. After to dye the film, only one laser pulse of duration 10 ns was applied in each experiment. Table 1: Contact angle (CA) on functionalized dry surface measurement at room temperature. Dq=±3º. Surface Contact angle (q) Clean glass 31º Vinyl silane covered glass 56º Glass covered with PNIPAM 62º In Fig. 1, it can be seen the optical micrographs of a dry hydrogel film ablated using DLIP. Fig. 1. Microscopic Photograph of micro-pattering of PNIPAM, dyed with RBPY solution. Scale Bar: 10 mm. A patterned film showing a flat topography (Fig. 2) upon swelling in water at 25 oC (below the phase transition temperature of PNIPAM), after is heated up to 40 oC (above the phase transition temperature of PNIPAM). (A) (B) Fig. 2. Atomic Force Microscopy images of patterned PNIPAM hydrogel. (A) wet PNIPAM hydrogel at 25 oC. (B) wet PNIPAM hydrogel stabilized at 40 oC. The dry hydrogel present clear patterns of parallel lines with a period of 4.5 mm and a depth of 98 nm. As it can be seen, the pattern reappears on the surface when the hydrogel collapses. Upon cooling the wet film, the surface becomes flat again. Similarly, upon swelling in water before phase transition, the surface becomes flat and the pattern disappears. However, upon film drying, the pattern reappears. In addition, result of AFM indicates that the cantilever is probing only one kind of material, therefore only the polymer is patterned and the glass substrate is not reached by the ablation process. This is reasonable since the depth of the grooves is of 98 nm while the film is 2.6 mm thick. CONCLUSIONS Periodic topographic patterns are imprinted onto polymeric hydrogel films by a single step using direct interference laser patterning of the hydrogel films doped with suitable dyes. The patterned surface of the dry gel becomes flat upon swelling of the hydrogel but the original pattern reappears upon drying of the gel. The pattern on the surface, which became flat upon swelling in water, is also restored by heating up the film above the phase transition temperature of PNIPAM. Patterned of thermosensitive hydrogel film can disappear and reappears reversibly.