Harnessing bistability for directional propulsion of soft, untethered robots

Harnessing bistability for directional propulsion of

soft, untethered robots

Tian Chen

a,1

, Osama R. Bilal

b,1

, Kristina Shea

a,2

, and Chiara Daraio

b,2

Engineering Design and Computing Laboratory, Department of Mechanical and Process Engineering, Eidgen

ossische Technische Hochschule (ETH) Zurich,

8092 Zurich, Switzerland; and

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125

Edited by John A. Rogers, Northwestern University, Evanston, IL, and approved April 23, 2018 (received for review January 9, 2018)

In most macroscale robotic systems, propulsion and controls are

enabled through a physical tether or complex onboard electronics

and batteries. A tether simplifies the design process but limits the

range of motion of the robot, while onboard controls and power

supplies are heavy and complicate the design process. Here, we

present a simple design principle for an untethered, soft swim-

ming robot with preprogrammed, directional propulsion without

a battery or onboard electronics. Locomotion is achieved by using

actuators that harness the large displacements of bistable ele-

ments triggered by surrounding temperature changes. Powered

by shape memory polymer (SMP) muscles, the bistable elements

in turn actuate the robot’s fins. Our robots are fabricated using

a commercially available 3D printer in a single print. As a proof

of concept, we show the ability to program a vessel, which

can autonomously deliver a cargo and navigate back to the

deployment point.

soft robots

programmable materials

mechanical bistability

shape memory polymer

autonomous propulsion

oft robotics (1–5) and robotic materials (also known as pro-

grammable matter) (6–8) are blurring the boundary between

materials and machines while promising a better, simpler, safer,

and more adaptive interface with humans (9, 10). Propulsion

and navigation are core to both soft and rigid robotic systems.

Autonomous (or preprogrammed) propulsion is a central ele-

ment in the road map for future autonomous systems, enabling,

for example, unguided traversal of open waters [e.g., studying

marine biology (11) or ocean dynamics (12)]. Power supply to

enable propulsion remains one of the major obstacles in all forms

of locomotion. One of the easiest solutions for supplying power

to soft robots is the use of a tether (2). Tethered pneumatics, for

example, enabled active agonistic and antagonistic motion (13)

and an undulating serpentine (14). Dielectric elastomers were

used to create tethered soft crawlers (15) and to simulate the up

and down motion of a jellyfish (16). Electromagnetics were used

to create a tethered earthworm-like robot (17) and untethered

microswimmers under a rotating magnetic field (18). A pres-

sure deforming elastomer was used to design an artificial fish

tail that can perform maneuvers (19). Untethered robots, on the

contrary, sacrifice simplicity in design for moving freedom with-

out restriction. An untethered robot needs to encapsulate pro-

graming, sensing, actuation, and more importantly, an onboard

power source.

The demonstration of an entirely soft (composed of materials

with elasticity moduli on the order of

−

Pa) untethered

robot, “the Octobot” (4), opened the door for a new generation

of robots (5). The Octobot is powered through regulated pres-

sure generating a chemical reaction. Fabrication of the Octobot

requires a combination of lithography, molding, and 3D print-

ing. However, it does not exhibit locomotion. A common feature

of all current demonstrations of soft robots is the presence of a

complex internal architecture as a result of a multistep fabrica-

tion and assembly process. Here, we present a methodology for

designing an untethered, soft robot, which can propel itself and

can be preprogrammed to follow selected trajectories. Further-

more, the robot can be preprogrammed to reach a destination,

deliver a cargo, and then reverse its propulsion direction to

return back to its initial deployment point. The robot can be

fabricated using a commercially available 3D printer in a single

print. However, the presented prototypes are partitioned to high-

light the different components and speed up the printing process.

We focus on the actuation, design, and fabrication of a robot

that exploits bistable actuation for propulsion and responds to

temperature changes in the environment to control its direc-

tional locomotion. We use shape memory polymers (SMPs) to

create bistable “muscles” that respond to temperature changes

in the environment. Bistable actuation is often found in biolog-

ical systems, like the Venus fly trap (20) and the Mantis shrimp

(21). When working near instabilities, bistability can amplify dis-

placements with the application of a small incremental force

(22). Engineers started to integrate instabilities in design (10)

[for example, in space structures (23), energy absorption mecha-

nisms (24, 25), and fly-trapping robots (26)]. By amplifying the

response of soft SMP muscles, snap-through instabilities can

instantaneously exert high force and trigger large geometrical

changes (27). Bistability has also been used to sustain a propagat-

ing solitary wave in a soft medium (22). More recently, bistability

enabled the realization of the first purely acoustic transistor and

mechanical calculator (28). A typical bistability is found in the

Von Mises truss design, which allows a simple 1D system to

have two stable states (29). Combining this principle with mul-

timaterial 3D printing, it is possible to realize bistable actuators

with a tunable activation force through material and geometry

changes (30). These actuators can be used to create load-bearing,

Significance

A major challenge in soft robotics is the integration of sens-

ing, actuation, control, and propulsion. Here, we propose a

material-based approach for designing soft robots. We show

an untethered, soft swimming robot, which can complete

preprogrammed tasks without the need for electronics, con-

trollers, or power sources on board. To achieve propulsion,

we use 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 programed materials.

Author contributions: O.R.B. and C.D. designed research; T.C. and O.R.B. performed

research; T.C., O.R.B., K.S., and C.D. analyzed data; and T.C., O.R.B., K.S., and C.D. wrote

the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the

PNAS license

T.C. and O.R.B. contributed equally to this work.

To whom correspondence may be addressed. Email: kshea@ethz.ch or daraio@caltech.

edu.

This article contains supporting information online at

www.pnas.org/lookup/suppl/doi:10.

1073/pnas.1800386115/-/DCSupplemental

Published online May 15, 2018.

5698–5702

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PNAS

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May 29, 2018

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vol. 115

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no. 22