As promising a technology as artificial muscles have been, most of the time they're still a bit too artificial, often made of plastics, nylon, rubber, waxy carbon nanotubes and the like. That might make them fine for robots, but natural proteins could make them more compatible for use inside the human body.For the new study, researchers at the University of Freiburg created artificial muscles that are entirely "bio-based." They're made of elastin, a natural protein that gives tissues like skin and blood vessels their elasticity. From that starting point, the team made two variations of the protein that respond to different stimuli – fluctuations in temperature and acidity. These were then combined in layers to create a muscle that would flex in one direction in response to one stimulus, and in a different direction when the other stimulus is applied.