What materials reversibly swell up slightly when in a moist environment? I am aware that some "Super Absorbent Polymers" can swell up enormously when they absorb water. However, I am looking for a material which swells up (maybe about 30% to 100%) when in a moist environment, and which will contract again when moved to a less moist environment.

I.E. by measuring the expansion of the material, one could determine the moisture content of the environment the material was in. It would act like a moisture measurement device. 

Are there any such materials? Perhaps there are some "Fairly Absorbent Polymers"
A classic hygrometer consists of bundle of a dozen or so human hairs (blonde are supposedly best, because they're typically thinnest, i.e. react fastest to humidity changes), stretched between a metal spring with a pointing device and a fixed point. 

A uniaxial effect is much easier to record precisely, and hair is thin (=fast) and far more mechanically reliable than a piece of swollen hydrogel.