Stimuli-responsive polymer gels - the new buzz in polymer research. What is it anyway? Simply put, stimulate plastics externally by changing temperature, light or chemical conditions to suit your application needs. Under external stimuli, for example, fluid inflated plastics or polymer gels can reversibly swell or shrink. Some of these intelligent materials have potential in sensors and display devices because these polymer gels can modulate the intensity of light for transmission or reflection. In fact, juggling colorant concentrations, Octopuses do change their body colour. They live alone under rocks and cracks on the ocean bottom. They do not have a hard shell for protection and are under continuous threat from predators such as moray eels or sharks. But they are masters of disguise and know how to change their body colour to their surroundings and moods. How do they do it? Elastic colour cells (chromatophores or sacs) all over their body contains pigments to which muscle fibers are attached. As muscle fibers contract, the sacks (mantle) become large, the pigments spread giving the colour. If the cells relax, the opposite happens. Pigments are squeezed to a pinpoint and the colour disappears. Basically, Octopuses use the reversible mechanism of diffusion and aggregation of pigments.
Ryojiro Akashi1 and his colleagues at Fuji Xerox Company term these colour cells as micro-machines in the natural world. They have developed a light-modulation material based on stimuli-responsive gel (NIPAM) containing a high concentration of pigments. NIPAM is N-ispropylacrylamide, a molecule when cross-linked creates a soft gel. Light modulation is caused by a reversible volume change of coloured gel particles. In other words, it is caused by a synergetic effect between the change of area of light absorption and the absorption efficiency of pigments in the gels. However, the difference, in this material is that the colour change is induced by the absorption of the pigments themselves. Pigment particles were homogenously dispersed to get good optical properties but were not soluble at molecular level. Dr. Akashi's group made the coloured NIPAM gel particles by inverse-phase suspension polymerization. To show how the pigmented plastic gel can be used for display devices, researchers dispersed the coloured gel particles in an aqueous solution of polyvinyl alcohol containing a dispersant and then sandwiched between two glass plates through polystyrene spacer beads. At swollen state (ambient temperature) the colour is deep but fades when heated above 34°C and the gel shrinks. Exactly what Octopus does in the natural world. Since pigments in the NIPAM matrix influence the volume change (a key factor to observe the colour change), Dr. Akashi is working on the optimization of polymerization conditions. What's ahead - optical device such as smart glass and information displays - the group concludes.
Another important piece of work came out from University of Tokushima in Japan. Hiroaki Misawa and colleagues2 used a laser beam to shrink NIPAM gel (volume phase transitions). This work showed that the radiation from the laser and not the temperature transform the gel reversibly. Possibly sooner than later, laser light could be used for actuating or sensing gel-based systems.
Use of NIPAM can be expanded to make smart suit that can keep one warm in icy water. In fact, Midé Technology of Medford, MA is developing a hydro-gel impregnated suit made out of neoprene. The hydro-gel (NIPAM) swells when it is warmed and shrinks as it gets cooled. It is reported that a suit with hydro-gel containing neoprene provided 70% more thermal protection than with neoprene alone3.
Very recently, Prof. Ian Manners and G.A. Ozin's group at University of Toronto developed a redox-active silica-polymer composite material. The metalliferous electroactive polymer swells and shrinks reversibly in response to a solvent. Since the swelling gels conducts electricity, an applied current (stimuli) is able to control how much solvent the polymer can suck up4. The optical response is pretty fast. The composite gel switches from the dry shrunken state to a fully swollen state in less than half a second. Dubbed as P-ink or 'photonic ink', this material can change it's colour just like a switch. It can turn to any colour across the entire visible spectrum. Applications could range from filters, sensors to switches and displays.
The colourful strategy of swell and shrink of an Octopus can actually bring the stimuli-responsive polymer gels to the forefront of modern technology.
Further Reading
1. Advanced Materials, 14 (24), pp.1808-1811 (2002)
2. Nature, 408, pp.178-181 (2000)
3. Technology Review, May 2003
4. Advanced Materials, 15 (6), pp. 503-507 (2003)
2. Nature, 408, pp.178-181 (2000)
3. Technology Review, May 2003
4. Advanced Materials, 15 (6), pp. 503-507 (2003)
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