Polymers move smartly

Hans-Jörg Schneider and Kazuaki Kato, University of Saarlandes, Germany, introduce polymers that respond to chemical and biological stimuli through movement
Chemomechanical polymers are a new type of smart polymer with specific recognition sites that can respond selectively to chemical and biological stimuli through large movements.

They have the unique feature of combining a sensor and an actuator (a mechanical device for moving or controlling a mechanism or system) within one single unit, without the need of external devices such as a transducer or a power supply.

When exposed to chemical or biological stimuli - such as nucleotides, amino acids or peptides - in the local environment these polymers produce large and reversible expansions and contractions. They can also be downsized to thin films or microparticles, with enhanced velocity and sensitivity of response.

Until recently, chemomechanical polymers were only known for responding to rather unspecific changes in pH, salts and solvents. In the last few years the Schneider group has applied known principles of supramolecular chemistry to make hydrogels with suitable recognition sites, leading to materials that respond selectively to external organic stimuli by non-covalent interactions.

A large variety of flexible hydrogels have now been made containing these supramolecular binding sites that can selectively identify specific organic molecule signals being transmitted from the surrounding aqueous environment.

A particular highlight of these polymers is an ability to distinguish between optical isomers. Recently the interaction of chitosan gels and tartaric acid derivatives was for the first time shown to directly translate chiral recognition into large shrinkages of the hydrogel particle.

It was shown that exposing a chitosan hydrogel particle to D- dibenzoyl-tartraric acid caused it to shrink by 94 per cent, but when the same hydrogel was exposed to the L-enantiomer only 20 per cent shrinkage was seen.

The motions of these smart hydrogels are also strongly dependent on the pH of the surrounding environment. And scientists have now taken advantage of this to make hydrogels that can act as simple logic gates, where the motion depends on both the presence of a trigger (such as a nucleotide) and the correct pH.

A related logic gate effect is also seen in polymers with both ester and amine functional groups. A rather spectacular example is a polymethyl(methyl)acrylate-based gel containing ethylenediamine-type binding sites - where the motion induced by aminoacids and peptides is only triggered when copper or zinc ions are also present.

It is hoped that these smart materials will find multiple future uses including in systems for controlled drug delivery, in the uptake of toxic compounds, in controlling flow in medical devices and even in microfluidic machineries. Also of interest is putting these smart hydrogels into tubes or onto flexible sheets to make artificial muscles that can translate the energy produced by non-covalent binding of a trigger molecule into mechanical motion.

The future for this research is bright, and it is hoped that the implementation of more sophisticated recognition elements into chemomechanical polymers and a better understanding of the underlying mechanism will lead to even smarter materials.