  The optimal stiffness for soft swimming robots depends on swimming speed,
which means no single stiffness can maximise efficiency in all swimming
conditions. Tunable stiffness would produce an increased range of
high-efficiency swimming speeds for robots with flexible propulsors and enable
soft control surfaces for steering underwater vehicles. We propose and
demonstrate a method for tunable soft robotic stiffness using inflatable rubber
tubes to stiffen a silicone foil through pressure and second moment of area
change. We achieved double the effective stiffness of the system for an input
pressure change from 0 to 0.8 bar and 2 J energy input. We achieved a resonant
amplitude gain of 5 to 7 times the input amplitude and tripled the high-gain
frequency range comparedto a foil with fixed stiffness. These results show that
changing second moment of area is an energy effective approach tot
unable-stiffness robots.
