The thesis consists of designing a fin inspired by the cownose ray, entirely made of silicone rubber, without any rigid mechanism inside, and actuated by SMA (Shape Memory Alloy) coils. The lack of rigid elements inside the fin helps reproduce better the natural movement of these fishes, and arranging the coils in pairs, they work as antagonistic muscles. The work includes optimizing the SMA actuator arrangement inside the fin, defining the activation sequence to obtain the desired movement, measuring the forces generated for different configurations, performing experimental tests on the fin, and build the robot. The experiments will be conducted at the Bioinspired Robotics Lab @ PoliMi Lecco.