Responds to Relative Humidity

I need a material that responds mechanically as an indicator of relative humidity. I have looked inside commercial humidistats used
in home heating systems. They use some sort of plastic strip that expands and contracts with changes in humidity and trips a microswitch. Is this plastic strip some exotic material, or is it something that is generally available? Another possibility I was wondering about was bonding wood veneer to an inert sheet so that the variations in wood expansion would translate to different degrees of warpage.
-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
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I don't know what the strip IS, but back in the dark ages that preceded polymer science, the strip WAS hair. Your hair. Horsehair. Whatevhair.
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David Kiewit, Reg. Patent Agent
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