Robotic Fish Uses only One Servo
At the Center for Biorobotics in Estonia, Eszter Ozsvald built a mechanical fish named A.riel that can model the movements of actual fish surprisingly well, and using only one servo inside a carefully made silicon-based mold. It took many iterations before the final product, but found that in the end she could develop the same vortex patterns as actual fish. Her site has extensive documentation on the build process and is definitely worth a look for the mold-making processes alone.
