Fig. 1: Overview of ray robot topics covered in this review.

a (i) Undulatory rays are generally more rounded and swim with greater than 1 wave present on the fin, (ii) while oscillatory rays are often more triangular and swim with less than 0.5 waves present along the fin. Disc width and length correspond to the width and length of the entire ray, excluding the tail, and span width and chord length correspond to the width and length of the fin. b (i) Many ray robots use pitching for propulsion, generally achieved using a flapping actuator and flexible fin material. (ii) Others use a bending actuator to achieve spanwise bending, and (iii) some robots use multiple actuators or harness material properties to achieve chordwise bending; some implement all three bending and pitching modalities. (i) The pitching diagram shows a side fin profile, (ii) spanwise bending shows a front fin profile, (iii) and chordwise bending shows a side profile. c (i) Ray robots require depth, velocity, and heading control accomplished by varying properties of wavelike actuation, such as frequency, amplitude, and phase offset. (ii) Sensing uses limited internal and external sensor types, including depth sensors, inertial measurement units (IMUs), gyroscopes, and optical sensors. d Various actuators include: (i) tissue engineered actuators (example in Fig. 4b); (ii) dielectric elastomer actuators (DEAs) (Fig. 5a and b); (iii) ionic polymer metal composites (IPMCs) (Fig. 5c); (iv) servos (Fig. 2); (v) hydraulically amplified, self-healing, electrostatic (HASEL) actuators (Fig. 5d); (vi) shape memory alloy (SMA) actuators (Fig. 4d); (vii) pneumatic and fluidic actuators (Fig. 4a, c); and (viii) tensegrity actuators (Fig. 3).