considered included polymer films and polymer fabrics. Polymer films tend to be vulnerable to puncture. Polymer fabrics are often used in air beams (inflatable beams used in temporary buildings). These beams are commonly constructed of a braided or woven polymer fabric outer layer and an inflatable bladder inner layer. Neither polymer films nor polymer fabrics precisely fit our needs because we wanted the ability to vary the membrane properties. We decided to use a printable elastomer to create the geometries, then bond on a thin fabric to create a high-elastic modulus composite. The fabric was omitted in places where we wanted concentrated deformation, such as the robot's " knees. " It was important to consider the transitions between extensible and inextensible parts of the membrane as they easily become stress concentrations. Undulating borders provide natural buckling points and reduce unnecessary stress. Refer to the supplementary material for more details the fabric patterns we used (available at https://doi. org/10.1109/MRA.2022.3204234). An alternate way to concentrate deformation at desired points would be to make the appendage thinner at the joints and use tendons to stabilize these joints when they are not supposed to bend. This would be more complex to assemble but result in energy savings. Optimizing so that routine motions involve almost pure buckling rather than stretching would also be best for the robot's durability. EVALUATION OF DEMONSTRATION AMPHIBIAN ROBOT We timed the robot's metamorphosis, walking, and swimming in lab conditions. We also operated the robot in a small lake. Swimming Morphing Air Tail Motors Walking Controls Leg Motors Inflated Tail USB Port Retracted Legs (a) Float Tail-Swinging Tendons USB Ready Batteries Push Airtight Seal Motor Lift Foot Retraction Tendon Swing Back (b) FIGURE 2. Design and layout of the demonstration amphibian robot. (a) An exterior view of the morphing process and side view of the internal layout. (b) Left to right: a schematic of the swimming motion, top view of the internal layout, and schematic of the walking motion. Tendons, which are active in the positions shown, are drawn in red; momentarily slack tendons are drawn in black. 24 IEEE ROBOTICS & AUTOMATION MAGAZINE SEPTEMBER 2023https://doi