Chapter 2: Chemomechanical Materials: Sensors and Actuators

This chapter on chemomechanical materials provides an overview of smart materials, which by action of external chemical signals change their size. Their unique property is that they offer sensing and actuating features within the same macro- or nanosized system, and that they can also serve independently as a sensor and/or as e.g. artificial muscles. Most chemomechanical materials are hydrogels, which are equipped with suitable sensing sites or molecular recognition units. The oldest applications are size changes triggered by pH, solvent and salt variation; for these the presence of ionic sites is usually sufficient. Organic and biological solutes trigger size changes if units are incorporated into the hydrogel which act by e.g. dispersive or hydrophobic forces. Cooperativity between several effectors is, with gels, more easily achieved than in solution; of particular interest is the simultaneous action of metal ions and e.g. peptides, which in the sense of logical gates allows the use of otherwise undetectable compounds as effectors. Macroscopic differentiation between enantiomers can by realized if gels contain chiral units. Particularly attractive is the use of special hydrogels which by covalent interactions allow insulin release as a function of glucose level in blood. Biological applications use for example antigen–antibody complexes, or intercalating units for interaction with nucleic acids.