Harnessing bistability for directional propulsion of soft, untethered robots 
Chen, T., Bilal, O.R., Shea, K.,& Darario, C., (2018), Proc. Natl. Acad. Sci., doi.org/10.1073/pnas.1800386115
Table of Content : Significant Statement | Images | Video
A major challenge in soft robotics is the integration of sensing, actuation, control and propulsion. Here we propose a material-based approach for designing soft robots. We demonstrate the first untethered, soft swimming robot, which can complete pre-programmed tasks without the need for electronics, controllers or power sources on board. To achieve propulsion, we utilize bistable shape memory polymer muscles connected to paddles that amplify actuation forces. As a proof of principle, we show that these robots can be preprogrammed to follow specific routes, or deliver a cargo and navigate back to their deployment point. The proposed design principle can have a broad impact in soft robotics based on programmed materials.
Design principle of the autonomous swimming robot. The Shape Memory Polymer (SMP) muscle triggers the bistable element which then drives the fins back, and the robot forward. The hollow floaters ensure that the robot stays afloat.
Two energy wells of the bistable element. Exploiting the asymmetry of the mechanical behavior of the bistable element. When triggered, the system moves from a higher to a lower energy state, causing a release of the strain energy.
Propulsion through bistability: (a) schematic of a 3D printed, soft robot (parts are false colored for visualization) (b) Energy potential of the bistable element with two stable states, I and III. The asymmetry in the curve indicates the need for larger amount of energy to move backward than forward. The SMP “muscle” shown in the inset are rotated 90 degree with respect to the bistable element for visualization. (c) The SMP muscle in the deployed (I), transitioning (II) and activated phases (III). (d) Screen captures of the deployed robot in temperature (
T ≥ T) at the different phases of activation.
Demonstration of the internal unstable triggering mechanism.
Actuator design: (a) FEM simulations of the constrained recovery of the bistable muscle pair. The vertical dashed black line separates thicknesses unable to activate the bistable element (left) from functional thicknesses (right). (b) Experimental and numerical correlation between the thickness of the SMP muscle and its recovery forces as well as the time it takes to heat to its original shape. The inset shows the different muscles tested. The error bars in the force readings represent the standard divination. The error bars for the activation times represent the error in reading the times from video recordings of the experiments
Sequential and directional propulsion: (a) A schematic of a robot with two bistable element-muscle pairs. A thinner muscle with faster actuation time is placed at the rear, i.e.,
t. b. Three distinct phases of the actuation. (b) Activation sequence: (I) initial state with both muscles programmed. (II) the thinner muscle activates, triggering the first bistable element and pushing the second muscle to touch the second bistable element. (III) The second (thicker) muscle triggers the second bistable element. (c-e) Snapshots from deploying three configurations of the multi-stroke swimmer showing three different directional motion at each of the three phases. 1 < t 2
Reverse navigation: A schematic of a robot with two muscles and a single bistable element. The first muscle,
M, is fabricated with a material that activates at 1 T. The second muscle, L M, activates at 2 T, which is higher than H T. (b) Sequence of activation, (I) at room temperature, L T, both muscles are programmed. (II) As water temperature increases to R.T. T, the first muscle triggers the bistable element and propels the swimmer forward. (III) As water heats to L T, the grippers relax and release the cargo. (iv) When the water temperature reaches H T, the second muscle reverses both the bistable element and re-programs the first muscle. (c) Snapshots from the deployment of the robot demonstrating the four different phases. H