LETTERS Surprising shrinkage of expanding gels under an external load

H ydrogels are fascinating and useful in that they can showlarge volume changes in response to various stimuli, suchas temperature or chemical environment1. Here we report the peculiar observation that chemically crosslinked hydrogels that normally expand owing to a change in electrolyte pH can be made to shrink in certain circumstances. Specifically, these hydrogels contract when tested at a constant compressive force and subjected to a pH change that causes expansion in the absence of the applied load. When tested under tension, the gels always expand. Although the effects of external stress on the swelling of gels is known2–4, the concomitant change in gel mechanical properties during pH switching was found to be a more dominant effect in our studies. However, existing mechanical models5,6 used to predict dimensional changes in actuator materials could not explain both the tensile and compression results. In addition, we show that the friction between metal plates of the apparatus and the gel is a key facto