Abstract
Changes in the chemical environment can trigger large motions in chemomechanical polymers. The unique feature of such intelligent materials, mostly in the form of hydrogels, is therefore, that they serve as sensors and actuators at the same time, and do not require any measuring devices, transducers or power supplies. Until recently the most often used of these materials responded to changes in pH. Chemists are now increasingly using supramolecular recognition sites in materials, which are covalently bound to the polymer backbone. This allows one to use a nearly unlimited variety of guest (or effector) compounds in the environment for a selective response by automatically triggered size changes. This is illustrated with non-covalent interactions of effectors comprising of metal ions, isomeric organic compounds, including enantiomers, nucleotides, aminoacids, and peptides. Two different effector molecules can induce motions as functions of their concentration, thus representing a logical AND gate. This concept is particularly fruitful with effector compounds such as peptides, which only trigger size changes if, e.g. copper ions are present in the surroundings. Another principle relies on the fast formation of covalent bonds between an effector and the chemomechanical polymer. The most promising application is the selective interaction of covalently fixed boronic acid residues with glucose, which renders itself not only for sensing, but eventually also for delivery of drugs such as insulin. The speed of the responses can significantly increase by increasing the surface to volume ratio of the polymer particles. Of particular interest is the sensitivity increase which can be reached by downsizing the particle volume.
Highlights
Sensors are increasingly being used for monitoring changes in environmental conditions in a fully automated manner
The motions of flexible polymers can be activated by voltage (Electrically Activated Polymers, EAP’s) [5], in the form of ionic polymer-metal composites (IPMC’s) [6], to furnish efficient artificial muscles
In the present review, which tries to cover the literature until the end of 2006, we focus on how chemomechanical polymers can respond to specific external chemical signals, mainly by size changes
Summary
Sensors are increasingly being used for monitoring changes in environmental conditions in a fully automated manner. A unique feature of chemomechanical polymers is that they function simultaneously as both sensors and actuators These intelligent materials can be designed to respond to changes in external conditions via macroscopic or microscopic motions, which may be used to control electric circuits or to open or close, e.g., vessels for the release or uptake of different agents. The major advantage in comparison to traditional control systems is that chemomechanical polymers can operate without any additional devices such as transducers or transmitters, and without any external power supply These new materials can be fabricated in a variety of ways, down to the micro- or nano scale, or as thin films, to afford the added advantages of enhanced sensitivity and speed of response. Expansion of hydrogels by uptake of guest molecules G and of water for solvation
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