A new smart and responsive material can stiffen up like a worked-out muscle, say the Iowa State University engineers who developed it. Stress a muscle and it gets stronger. Mechanically stress the rubbery material -- say with a twist or a bend -- and the material automatically stiffens by up to 300 percent, the engineers said. In lab tests, mechanical stresses transformed a flexible strip of the material into a hard composite that can support 50 times its own weight.