[Paper Review] Harnessing bistability for directional propulsion of untethered, soft robots

Tian Chen, Osama R. Bilal+2 more|arXiv (Cornell University)|Oct 12, 2017

Advanced Materials and Mechanics18 citations

TL;DR

This paper presents a fully soft, untethered swimming robot that achieves preprogrammed directional propulsion using bistable elements actuated by shape memory polymer (SMP) muscles responding to environmental temperature changes. The robot autonomously navigates forward, delivers a cargo upon heating, and reverses to its starting point using temperature-triggered, sequential actuation of SMP muscles with different glass transition temperatures.

ABSTRACT

In most macro-scale robotics systems , propulsion and controls are enabled through a physical tether or complex on-board electronics and batteries. A tether simplifies the design process but limits the range of motion of the robot, while on-board controls and power supplies are heavy and complicate the design process. Here we present a simple design principle for an untethered, entirely soft, swimming robot with the ability to achieve preprogrammed, directional propulsion without a battery or on-board electronics. Locomotion is achieved by employing actuators that harness the large displacements of bistable elements, triggered by surrounding temperature changes. Powered by shape memory polymer (SMP) muscles, the bistable elements in turn actuates the robot's fins. Our robots are fabricated entirely using a commercially available 3D printer with no post-processing. As a proof-of-concept, we demonstrate the ability to program a vessel, which can autonomously deliver a cargo and navigate back to the deployment point.

Motivation & Objective

• To develop an untethered, entirely soft robot capable of directional propulsion without on-board power or electronics.
• To enable autonomous navigation and cargo delivery using environmental temperature as the sole actuation trigger.
• To demonstrate reversible locomotion through sequential activation of SMP muscles with distinct glass transition temperatures.
• To create a monolithic, 3D-printed robot with no post-processing or complex assembly.
• To establish a design framework for programmable soft robots using bistability and shape memory materials.

Proposed method

• The robot uses a bistable element as a mechanical 'engine' that undergoes snap-through instability to generate large displacements.
• Shape memory polymer (SMP) muscles with tunable glass transition temperatures (Tg ≈ 35°C and 60°C) are 3D printed using multi-material Stratasys Connex printers.
• The SMP muscles are attached to the bistable element such that heating above their Tg triggers snap-through, causing directional fin motion and propulsion.
• A second SMP muscle with a higher Tg is used to reverse the bistable element's state, enabling backward motion.
• A shape memory gripper at the robot's front holds a 3D-printed cargo and releases it when heated above its Tg.
• Finite element simulations using Abaqus with a thermal-viscoelastic material model validate the design and predict actuation behavior.

Experimental results

Research questions

• RQ1 Can a fully soft, untethered robot achieve directional propulsion without batteries or on-board electronics?
• RQ2 How can environmental temperature changes be used to program sequential actuation and locomotion in soft robots?
• RQ3 Can a single robot autonomously deliver a cargo and return to its starting point using only temperature-responsive materials?
• RQ4 What design principles enable reversible locomotion through bistable mechanics and SMP actuation?
• RQ5 Can complex, preprogrammed motion sequences be achieved using only material and geometric tuning in a monolithic 3D-printed structure?

Key findings

• The robot achieves autonomous forward propulsion when the first SMP muscle (Tg ≈ 60°C) is heated above its transition temperature, triggering the bistable element to snap forward.
• At a higher temperature (Tg ≈ 60°C), the second SMP muscle activates and reverses the bistable element, causing the robot to move backward.
• The shape memory gripper releases the cargo when heated to its Tg ≈ 35°C, enabling autonomous delivery.
• The robot successfully demonstrated a complete mission: forward motion, cargo release, and return to the starting point in a single 3D-printed, monolithic structure.
• Finite element simulations confirmed the predicted actuation behavior and force generation of the bistable system under thermal loading.
• The entire robot, including SMP muscles, bistable structure, and gripper, was fabricated in one print using multi-material 3D printing with no post-processing.