Figure 5. Sonically camouflaged hydrogel actuators and robots in water.

(a–c) Speed of sound measurements for pure water, PAAm-alginate hydrogel, Ecoflex, Elastosil and Sylgard 184. The curves indicate the ultrasound signals travel through the samples from the transducer to the hydrophone with the source frequency of 40 kHz (a), 200 kHz (b) and 1 MHz (c). The t=0 corresponded to the time at which the ultrasound signal was sent from the transducer and the signal amplitudes were measured by the hydrophone upon the arrival of the transmitted ultrasound signals through the samples. The ultrasound signals sent at each frequency had the same amplitude while the attenuation varied among sample materials due to different acoustic impedance and viscous effect of each material. (d,e) Ultrasound image of the hydraulic fish-like actuators made of Ecoflex (d) and PAAm-alginate hydrogel (e) in the water tank. Dotted lines are introduced in e to indicate the boundaries of transparent hydrogel structures in water.