Responsive hydrogels – structurally and dimensionally optimized smart frameworks for applications in catalysis, micro-system technology and material science

Abstract

Although the technological and scientific importance of functional polymers has been well established over the last few decades, the most recent focus that has attracted much attention has been on stimuli-responsive polymers. This group of materials is of particular interest due to its ability to respond to internal and/or external chemico-physical stimuli, which is often manifested as large macroscopic responses. Aside from scientific challenges of designing stimuli-responsive polymers, the main technological interest lies in their numerous applications ranging from catalysis through microsystem technology and chemomechanical actuators to sensors that have been extensively explored. Since the phase transition phenomenon of hydrogels is theoretically well understood advanced materials based on the predictions can be prepared. Since the volume phase transition of hydrogels is a diffusion-limited process the size of the synthesized hydrogels is an important factor. Consistent downscaling of the gel size will result in fast smart gels with sufficient response times. In order to apply smart gels in microsystems and sensors, new preparation techniques for hydrogels have to be developed. For the up-coming nanotechnology, nano-sized gels as actuating materials would be of great interest.

Modular microfluidic valve structures based on reversible thermoresponsive ionogel actuators

“…In particular, patterned hydrogel valves have the advantage of combining miniaturised scale with small dead volume, it is a simple set-up and they can be integrated into Microsystems easily. [8] In particular, poly(N-isopropylacrylamide) (NIPAAm) hydrogels shrink and expand due to dramatic structural changes that occur at the so-called lower critical solution temperature (LCST), which is around 32 ºC in water. In an aqueous environment, individual NIPAAm chains show a hydrophilic behaviour with an expanded coil structure below the LCST, but as the temperature increases above 32 ºC, hydrophobic isopropyl groups are exposed undergoing a reversible conformational change to form compact globule shapes.…”

Recognition‐Mediated Hydrogel Swelling Controlled by Interaction with a Negative Thermoresponsive LCST Polymer

“…[3] Similarly,synthetic smart polymer gels,capable of swelling or deswelling in response to various physical (e.g. temperature,light, electric and magnetic fields) and chemical stimuli (e.g.p H, ions,m olecules), [4][5][6] have been developed during the last decades. [7][8][9][10][11][12][13][14][15][16] Reversible supramolecular binding of guest molecules or metal ions has also been reported to promote isothermal polymer phase transitions leading to the contraction or expansion of hydrogels.…”

Organic Polymers as Porogenic Structure Matrices for Mesoporous Alumina and Magnesia

“…Likewise, organic hydrogels have also been shown to be versatile porogenic matrices for porous oxidic materials [27][28][29][30][31]. We have recently described the utilization of photo cross-linked poly(dimethylacrylamide)-based hydrogels [32,33] as matrices for mesoporous alumina [34,35]. Here we report on the utilization of non-cross-linked water-soluble polymers as porogenic species for mesoporous Al 2 O 3 and MgO; the synthesis process is thus simplified.…”