Robotic Materials

58 results (View BibTeX file of all listed publications)

2024

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Hexagonal electrohydraulic modules for rapidly reconfigurable high-speed robots

Yoder, Z., Rumley, E., Schmidt, I., Rothemund, P., Keplinger, C.

Science Robotics, 9, September 2024 (article)

Abstract
Robots made from reconfigurable modular units feature versatility, cost efficiency, and improved sustainability compared with fixed designs. Reconfigurable modules driven by soft actuators provide adaptable actuation, safe interaction, and wide design freedom, but existing soft modules would benefit from high-speed and high-strain actuation, as well as driving methods well-suited to untethered operation. Here, we introduce a class of electrically actuated robotic modules that provide high-speed (a peak contractile strain rate of 4618% per second, 15.8-hertz bandwidth, and a peak specific power of 122 watts per kilogram), high-strain (49% contraction) actuation and that use magnets for reversible mechanical and electrical connections between neighboring modules, thereby serving as building blocks for rapidly reconfigurable and highly agile robotic systems. The actuation performance of each hexagonal electrohydraulic (HEXEL) module is enabled by a synergistic combination of soft and rigid components; a hexagonal exoskeleton of rigid plates amplifies the motion produced by soft electrohydraulic actuators and provides a mechanical structure and connection platform for reconfigurable robots composed of many modules. We characterize the actuation performance of individual HEXEL modules, present a model that captures their quasi-static force-stroke behavior, and demonstrate both a high-jumping and a fast pipe-crawling robot. Using embedded magnetic connections, we arranged multiple modules into reconfigurable robots with diverse functionality, including a high-stroke muscle, a multimodal active array, a table-top active platform, and a fast-rolling robot. We further leveraged the magnetic connections for hosting untethered, snap-on driving electronics, together highlighting the promise of HEXEL modules for creating rapidly reconfigurable high-speed robots.

link (url) DOI [BibTex]

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Cutaneous Electrohydraulic (CUTE) Wearable Devices for Pleasant Broad-Bandwidth Haptic Cues

Sanchez-Tamayo, N., Yoder, Z., Rothemund, P., Ballardini, G., Keplinger, C., Kuchenbecker, K. J.

Advanced Science, (2402461):1-14, September 2024 (article)

Abstract
By focusing on vibrations, current wearable haptic devices underutilize the skin's perceptual capabilities. Devices that provide richer haptic stimuli, including contact feedback and/or variable pressure, are typically heavy and bulky due to the underlying actuator technology and the low sensitivity of hairy skin, which covers most of the body. This paper presents a system architecture for compact wearable devices that deliver salient and pleasant broad-bandwidth haptic cues: Cutaneous Electrohydraulic (CUTE) devices combine a custom materials design for soft haptic electrohydraulic actuators that feature high stroke, high force, and electrical safety with a comfortable mounting strategy that places the actuator in a non-contact resting position. A prototypical wrist-wearable CUTE device produces rich tactile sensations by making and breaking contact with the skin (2.44 mm actuation stroke), applying high controllable forces (exceeding 2.3 N), and delivering vibrations at a wide range of amplitudes and frequencies (0-200 Hz). A perceptual study with fourteen participants achieved 97.9% recognition accuracy across six diverse cues and verified their pleasant and expressive feel. This system architecture for wearable devices gives unprecedented control over the haptic cues delivered to the skin, providing an elegant and discreet way to activate the user's sense of touch.

DOI [BibTex]

Electrohydraulic Musculoskeletal Robotic Leg for Agile, Adaptive, yet Energy-Efficient Locomotion
Electrohydraulic Musculoskeletal Robotic Leg for Agile, Adaptive, yet Energy-Efficient Locomotion

Buchner, T. J. K., Fukushima, T., Kazemipour, A., Gravert, S., Prairie, M., Romanescu, P., Arm, P., Zhang, Y., Wang, X., Zhang, S. L., Walter, J., Keplinger, C., Katzschmann, R. K.

Nature Communications, 15(1), September 2024 (article)

Abstract
Robotic locomotion in unstructured terrain demands an agile, adaptive, and energy-efficient architecture. To traverse such terrains, legged robots use rigid electromagnetic motors and sensorized drivetrains to adapt to the environment actively. These systems struggle to compete with animals that excel through their agile and effortless motion in natural environments. We propose a bio-inspired musculoskeletal leg architecture driven by antagonistic pairs of electrohydraulic artificial muscles. Our leg is mounted on a boom arm and can adaptively hop on varying terrain in an energy-efficient yet agile manner. It can also detect obstacles through capacitive self-sensing. The leg performs powerful and agile gait motions beyond 5 Hz and high jumps up to 40 % of the leg height. Our leg’s tunable stiffness and inherent adaptability allow it to hop over grass, sand, gravel, pebbles, and large rocks using only open-loop force control. The electrohydraulic leg features a low cost of transport (0.73), and while squatting, it consumes only a fraction of the energy (1.2 %) compared to its conventional electromagnetic counterpart. Its agile, adaptive, and energy-efficient properties would open a roadmap toward a new class of musculoskeletal robots for versatile locomotion and operation in unstructured natural environments.

Press release Video (overview) Video (technical description) Article in pdf link (url) DOI [BibTex]

Buchner, T. J. K., Fukushima, T., Kazemipour, A., Gravert, S., Prairie, M., Romanescu, P., Arm, P., Zhang, Y., Wang, X., Zhang, S. L., Walter, J., Keplinger, C., Katzschmann, R. K. Electrohydraulic Musculoskeletal Robotic Leg for Agile, Adaptive, yet Energy-Efficient Locomotion Nature Communications, 15(1), September 2024 (article)

Press release Video (overview) Video (technical description) Article in pdf link (url) DOI [BibTex]

2023

Electrochemically Controlled Hydrogels with Electrotunable Permeability and Uniaxial Actuation
Electrochemically Controlled Hydrogels with Electrotunable Permeability and Uniaxial Actuation

Benselfelt, T., Shakya, J., Rothemund, P., Lindström, S. B., Piper, A., Winkler, T. E., Hajian, A., Wågberg, L., Keplinger, C., Hamedi, M. M.

Advanced Materials, 35(45):2303255, Wiley-VCH GmbH, November 2023 (article)

Abstract
The unique properties of hydrogels enable the design of life-like soft intelligent systems. However, stimuli-responsive hydrogels still suffer from limited actuation control. Direct electronic control of electronically conductive hydrogels can solve this challenge and allow direct integration with modern electronic systems. An electrochemically controlled nanowire composite hydrogel with high in-plane conductivity that stimulates a uniaxial electrochemical osmotic expansion is demonstrated. This materials system allows precisely controlled shape-morphing at only −1 V, where capacitive charging of the hydrogel bulk leads to a large uniaxial expansion of up to 300%, caused by the ingress of ≈700 water molecules per electron–ion pair. The material retains its state when turned off, which is ideal for electrotunable membranes as the inherent coupling between the expansion and mesoporosity enables electronic control of permeability for adaptive separation, fractionation, and distribution. Used as electrochemical osmotic hydrogel actuators, they achieve an electroactive pressure of up to 0.7 MPa (1.4 MPa vs dry) and a work density of ≈150 kJ m−3 (2 MJ m−3 vs dry). This new materials system paves the way to integrate actuation, sensing, and controlled permeation into advanced soft intelligent systems.

link (url) DOI [BibTex]

2023

Benselfelt, T., Shakya, J., Rothemund, P., Lindström, S. B., Piper, A., Winkler, T. E., Hajian, A., Wågberg, L., Keplinger, C., Hamedi, M. M. Electrochemically Controlled Hydrogels with Electrotunable Permeability and Uniaxial Actuation Advanced Materials, 35(45):2303255, Wiley-VCH GmbH, November 2023 (article)

link (url) DOI [BibTex]

A Multifunctional Soft Robotic Shape Display with High-speed Actuation, Sensing, and Control
A Multifunctional Soft Robotic Shape Display with High-speed Actuation, Sensing, and Control

Johnson, B. K., Naris, M., Sundaram, V., Volchko, A., Ly, K., Mitchell, S. K., Acome, E., Kellaris, N., Keplinger, C., Correll, N., Humbert, J. S., Rentschler, M. E.

Nature Communications, 14(1), July 2023 (article)

Abstract
Shape displays which actively manipulate surface geometry are an expanding robotics domain with applications to haptics, manufacturing, aerodynamics, and more. However, existing displays often lack high-fidelity shape morphing, high-speed deformation, and embedded state sensing, limiting their potential uses. Here, we demonstrate a multifunctional soft shape display driven by a 10 × 10 array of scalable cellular units which combine high-speed electrohydraulic soft actuation, magnetic-based sensing, and control circuitry. We report high-performance reversible shape morphing up to 50 Hz, sensing of surface deformations with 0.1 mm sensitivity and external forces with 50 mN sensitivity in each cell, which we demonstrate across a multitude of applications including user interaction, image display, sensing of object mass, and dynamic manipulation of solids and liquids. This work showcases the rich multifunctionality and high-performance capabilities that arise from tightly-integrating large numbers of electrohydraulic actuators, soft sensors, and controllers at a previously undemonstrated scale in soft robotics.

YouTube video link (url) DOI [BibTex]

Johnson, B. K., Naris, M., Sundaram, V., Volchko, A., Ly, K., Mitchell, S. K., Acome, E., Kellaris, N., Keplinger, C., Correll, N., Humbert, J. S., Rentschler, M. E. A Multifunctional Soft Robotic Shape Display with High-speed Actuation, Sensing, and Control Nature Communications, 14(1), July 2023 (article)

YouTube video link (url) DOI [BibTex]

A Versatile Jellyfish-Like Robotic Platform for Effective Underwater Propulsion and Manipulation
A Versatile Jellyfish-Like Robotic Platform for Effective Underwater Propulsion and Manipulation

Wang, T., Joo, H., Song, S., Hu, W., Keplinger, C., Sitti, M.

Science Advances, 9(15), American Association for the Advancement of Science, April 2023, Tianlu Wang and Hyeong-Joon Joo contributed equally to this work. (article)

Abstract
Underwater devices are critical for environmental applications. However, existing prototypes typically use bulky, noisy actuators and limited configurations. Consequently, they struggle to ensure noise-free and gentle interactions with underwater species when realizing practical functions. Therefore, we developed a jellyfish-like robotic platform enabled by a synergy of electrohydraulic actuators and a hybrid structure of rigid and soft components. Our 16-cm-diameter noise-free prototype could control the fluid flow to propel while manipulating objects to be kept beneath its body without physical contact, thereby enabling safer interactions. Its against-gravity speed was up to 6.1 cm/s, substantially quicker than other examples in literature, while only requiring a low input power of around 100 mW. Moreover, using the platform, we demonstrated contact-based object manipulation, fluidic mixing, shape adaptation, steering, wireless swimming, and cooperation of two to three robots. This study introduces a versatile jellyfish-like robotic platform with a wide range of functions for diverse applications.

YouTube video link (url) DOI [BibTex]

Wang, T., Joo, H., Song, S., Hu, W., Keplinger, C., Sitti, M. A Versatile Jellyfish-Like Robotic Platform for Effective Underwater Propulsion and Manipulation Science Advances, 9(15), American Association for the Advancement of Science, April 2023, Tianlu Wang and Hyeong-Joon Joo contributed equally to this work. (article)

YouTube video link (url) DOI [BibTex]

Biodegradable Electrohydraulic Actuators for Sustainable Soft Robots
Biodegradable Electrohydraulic Actuators for Sustainable Soft Robots

Rumley, E. H., Preninger, D., Shagan-Shomron, A., Rothemund, P., Hartmann, F., Baumgartner, M., Kellaris, N., Stojanovic, A., Yoder, Z., Karrer, B., Keplinger, C., Kaltenbrunner, M.

Science Advances, 9(12), 2023, Ellen H. Rumley and David Preninger were co-first authors, and Christoph Keplinger and Martin Kaltenbrunner were shared corresponding authors. (article)

Abstract
Combating environmental pollution demands a focus on sustainability, in particular from rapidly advancing technologies that are poised to be ubiquitous in modern societies. Among these, soft robotics promises to replace conventional rigid machines for applications requiring adaptability and dexterity. For key components of soft robots, such as soft actuators, it is thus important to explore sustainable options like bioderived and biodegradable materials. We introduce systematically determined compatible materials systems for the creation of fully biodegradable, high-performance electrohydraulic soft actuators, based on various biodegradable polymer films, ester-based liquid dielectric, and NaCl-infused gelatin hydrogel. We demonstrate that these biodegradable actuators reliably operate up to high electric fields of 200 V/μm, show performance comparable to nonbiodegradable counterparts, and survive more than 100,000 actuation cycles. Furthermore, we build a robotic gripper based on biodegradable soft actuators that is readily compatible with commercial robot arms, encouraging wider use of biodegradable materials systems in soft robotics.

YouTube video DOI [BibTex]

Rumley, E. H., Preninger, D., Shagan-Shomron, A., Rothemund, P., Hartmann, F., Baumgartner, M., Kellaris, N., Stojanovic, A., Yoder, Z., Karrer, B., Keplinger, C., Kaltenbrunner, M. Biodegradable Electrohydraulic Actuators for Sustainable Soft Robots Science Advances, 9(12), 2023, Ellen H. Rumley and David Preninger were co-first authors, and Christoph Keplinger and Martin Kaltenbrunner were shared corresponding authors. (article)

YouTube video DOI [BibTex]

A Soft, Fast and Versatile Electrohydraulic Gripper with Capacitive Object Size Detection
A Soft, Fast and Versatile Electrohydraulic Gripper with Capacitive Object Size Detection

Yoder, Z., Macari, D., Kleinwaks, G., Schmidt, I., Acome, E., Keplinger, C.

Advanced Functional Materials, 23(3):2209080, 2023 (article)

Abstract
Soft robotic grippers achieve increased versatility and reduced complexity through intelligence embodied in their flexible and conformal structures. The most widely used soft grippers are pneumatically driven; they are simple and effective but require bulky air compressors that limit their application space and external sensors or computationally expensive vision systems for pick verification. In this study, a multi-material architecture for self-sensing electrohydraulic bending actuators is presented that enables a new class of highly versatile and reconfigurable soft grippers that are electrically driven and feature capacitive pick verification and object size detection. These elec-trohydraulic grippers are fast (step input results in finger closure in 50 ms), draw low power (6.5 mW per finger to hold grasp), and can pick a wide variety of objects with simple binary electrical control. Integrated high-voltage driving electronics are presented that greatly increase the application space of the grippers and make them readily compatible with commercially available robotic arms.

A Soft, Fast and Versatile Electrohydraulic Gripper with Capacitive Object Size Detection YouTube video DOI [BibTex]

2022

A Pocket‐Sized Ten‐Channel High Voltage Power Supply for Soft Electrostatic Actuators
A Pocket‐Sized Ten‐Channel High Voltage Power Supply for Soft Electrostatic Actuators

Mitchell, S. K., Martin, T., Keplinger, C.

Advanced Materials Technologies, 7(8), August 2022 (article)

Abstract
As soft electrostatic actuators find applications in bio-inspired robotics, compact and lightweight high voltage electronics that independently address many actuators are required. Here, a pocket-sized, battery-powered, 10-channel high voltage power supply (HVPS) is presented, which independently addresses each channel up to 10 kV. The HVPS uses one HV amplifier to create a HV rail and each output connects to the rail via custom optocouplers that are pulse-width modulated to vary their conductance. These optocouplers distribute charges to and from electrostatic devices at each output, creating a charge-controlled driving scheme that can generate independent and nearly arbitrary actuation waveforms for each channel. The HVPS weighs 250 g and measures 8.4 cm × 13.3 cm × 2 cm, about the size of a smartphone. The HVPS is characterized when driving hydraulically amplified self-healing electrostatic (HASEL) actuators. While powering a 5 nF actuator, the output of the HVPS reaches 8 kV in 100 ms and drives a 1.5 nF actuator at 100 Hz (0 to 5.4 kV). The HVPS powers an active surface consisting of an array of HASELs and generates undulatory locomotion of a soft robotic inchworm, highlighting the potential for compact HV electronics that power multi-degree-of-freedom robotic systems based on electrostatic devices.

link (url) DOI [BibTex]

2022

Mitchell, S. K., Martin, T., Keplinger, C. A Pocket‐Sized Ten‐Channel High Voltage Power Supply for Soft Electrostatic Actuators Advanced Materials Technologies, 7(8), August 2022 (article)

link (url) DOI [BibTex]

Electro-hydraulic Rolling Soft Wheel: Design, Hybrid Dynamic Modeling, and Model Predictive Control
Electro-hydraulic Rolling Soft Wheel: Design, Hybrid Dynamic Modeling, and Model Predictive Control

Ly, K., Mayekar, J. V., Aguasvivas, S., Keplinger, C., Rentschler, M. E., Correll, N.

IEEE Transactions on Robotics, 38(5):3044-3063, IEEE, May 2022 (article)

Abstract
Locomotion through rolling is attractive compared to other forms of locomotion thanks to uniform designs, high degree of mobility, dynamic stability, and self-recovery from collision. Despite previous efforts to design rolling soft systems, pneumatic and other soft actuators are often limited in terms of high-speed dynamics, system integration, and/or functionalities. Furthermore, mathematical description of the rolling dynamics for this type of robot and how the models can be used for speed control are often not mentioned. This article introduces a cylindrical-shaped shell-bulging rolling soft wheel that employs an array of 16 folded-HASEL actuators as a mean for improved rolling performance. The actuators represent the soft components with discrete forces that propel the wheel, whereas the wheel's frame is rigid but allows for smooth, continuous change in position and speed. We discuss the interplay between the electrical and mechanical design choices, the modeling of the wheel's hybrid (continuous and discrete) dynamic behavior, and the implementation of a model predictive controller (MPC) for the robot's speed. With the balance of several design factors, we show the wheel's ability to carry integrated hardware with a maximum rolling speed at 0.7 m/s (or 2.2 body lengths per second), despite its total weight of 979 g, allowing the wheel to outperform the existing rolling soft wheels with comparable weights and sizes. We also show that the MPC enables the wheel to accelerate and leverage its inherent braking capability to reach desired speeds—a critical function that did not exist in previous rolling soft systems.

link (url) DOI [BibTex]

Ly, K., Mayekar, J. V., Aguasvivas, S., Keplinger, C., Rentschler, M. E., Correll, N. Electro-hydraulic Rolling Soft Wheel: Design, Hybrid Dynamic Modeling, and Model Predictive Control IEEE Transactions on Robotics, 38(5):3044-3063, IEEE, May 2022 (article)

link (url) DOI [BibTex]

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Towards Enduring Autonomous Robots via Embodied Energy

Aubin, C. A., Gorissen, B., Milana, E., Buskohl, P. R., Lazarus, N., Slipher, G. A., Keplinger, C., Bongard, J., Iida, F., Lewis, J. A., Shepherd, R. F.

Nature, 602(7897):393-402, 2022 (article)

Abstract
Autonomous robots are comprised of actuation, energy, sensory, and control systems built from materials and structures that are not necessarily designed and integrated for multifunctionality. Yet, animals and other organisms that robots strive to emulate contain highly sophisticated and interconnected systems at all organizational levels, which allow multiple functions to be performed simultaneously. Herein, we examine how system integration and multifunctionality in nature inspires a new paradigm for autonomous robots that we call Embodied Energy. Currently, most untethered robots use batteries to store energy and power their operation. To extend operating times, additional battery blocks and supporting structures must be added, which increases weight and reduces efficiency. Recent advancements in energy storage techniques enable chemical or electrical energy sources to be embodied directly within the structures, materials, and mechanical systems used to create robots. This perspective highlights emerging examples of Embodied Energy, focusing on the design and fabrication principles of enduring autonomous robots.

DOI [BibTex]

Aubin, C. A., Gorissen, B., Milana, E., Buskohl, P. R., Lazarus, N., Slipher, G. A., Keplinger, C., Bongard, J., Iida, F., Lewis, J. A., Shepherd, R. F. Towards Enduring Autonomous Robots via Embodied Energy Nature, 602(7897):393-402, 2022 (article)

DOI [BibTex]

2021

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Miniaturized Circuitry for Capacitive Self-sensing and Closed-loop Control of Soft Electrostatic Transducers

Ly, K., Kellaris, N., McMorris, D., Johnson, B. K., Acome, E., Sundaram, V., Naris, M., Humbert, J. S., Rentschler, M. E., Keplinger, C., Correll, N.

Soft Robotics, 8(6):673-686, Mary Ann Liebert, Inc., publishers, December 2021 (article)

Abstract
Soft robotics is a field of robotic system design characterized by materials and structures that exhibit large-scale deformation, high compliance, and rich multifunctionality. The incorporation of soft and deformable structures endows soft robotic systems with the compliance and resiliency that makes them well adapted for unstructured and dynamic environments. Although actuation mechanisms for soft robots vary widely, soft electrostatic transducers such as dielectric elastomer actuators (DEAs) and hydraulically amplified self-healing electrostatic (HASEL) actuators have demonstrated promise due to their muscle-like performance and capacitive self-sensing capabilities. Despite previous efforts to implement self-sensing in electrostatic transducers by overlaying sinusoidal low-voltage signals, these designs still require sensing high-voltage signals, requiring bulky components that prevent integration with miniature untethered soft robots. We present a circuit design that eliminates the need for any high-voltage sensing components, thereby facilitating the design of simple low cost circuits using off-the-shelf components. Using this circuit, we perform simultaneous sensing and actuation for a range of electrostatic transducers including circular DEAs and HASEL actuators and demonstrate accurate estimated displacements with errors <4%. We further develop this circuit into a compact and portable system that couples high voltage actuation, sensing, and computation as a prototype toward untethered multifunctional soft robotic systems. Finally, we demonstrate the capabilities of our self-sensing design through feedback control of a robotic arm powered by Peano-HASEL actuators.

link (url) DOI [BibTex]

2021

Ly, K., Kellaris, N., McMorris, D., Johnson, B. K., Acome, E., Sundaram, V., Naris, M., Humbert, J. S., Rentschler, M. E., Keplinger, C., Correll, N. Miniaturized Circuitry for Capacitive Self-sensing and Closed-loop Control of Soft Electrostatic Transducers Soft Robotics, 8(6):673-686, Mary Ann Liebert, Inc., publishers, December 2021 (article)

link (url) DOI [BibTex]

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Flexible Seaweed-Like Triboelectric Nanogenerator as a Wave Energy Harvester Powering Marine Internet of Things

Wang, Y., Liu, X., Wang, Y., Wang, H., Wang, H., Zhang, S. L., Zhao, T., Xu, M., Wang, Z. L.

ACS Nano, 15(10):15700-15709, October 2021 (article)

DOI [BibTex]

Wang, Y., Liu, X., Wang, Y., Wang, H., Wang, H., Zhang, S. L., Zhao, T., Xu, M., Wang, Z. L. Flexible Seaweed-Like Triboelectric Nanogenerator as a Wave Energy Harvester Powering Marine Internet of Things ACS Nano, 15(10):15700-15709, October 2021 (article)

DOI [BibTex]

HASEL Artificial Muscles for a New Generation of Lifelike Robots—Recent Progress and Future Opportunities
HASEL Artificial Muscles for a New Generation of Lifelike Robots—Recent Progress and Future Opportunities

Rothemund, P., Kellaris, N., Mitchell, S. K., Acome, E., Keplinger, C.

Advanced Materials, 33(19):2003375, 2021, Nicholas Kellaris, Shane K. Mitchell, and Eric Acome contributed equally to this publication. (article)

Abstract
Future robots and intelligent systems will autonomously navigate in unstructured environments and closely collaborate with humans; integrated with our bodies and minds, they will allow us to surpass our physical limitations. Traditional robots are mostly built from rigid, metallic components and electromagnetic motors, which make them heavy, expensive, unsafe near people, and ill-suited for unpredictable environments. By contrast, biological organisms make extensive use of soft materials and radically outperform robots in terms of dexterity, agility, and adaptability. Particularly, natural muscle—a masterpiece of evolution—has long inspired researchers to create “artificial muscles” in an attempt to replicate its versatility, seamless integration with sensing, and ability to self-heal. To date, natural muscle remains unmatched in all-round performance, but rapid advancements in soft robotics have brought viable alternatives closer than ever. Herein, the recent development of hydraulically amplified self-healing electrostatic (HASEL) actuators, a new class of high-performance, self-sensing artificial muscles that couple electrostatic and hydraulic forces to achieve diverse modes of actuation, is discussed; current designs match or exceed natural muscle in many metrics. Research on materials, designs, fabrication, modeling, and control systems for HASEL actuators is detailed. In each area, research opportunities are identified, which together lays out a roadmap for actuators with drastically improved performance. With their unique versatility and wide potential for further improvement, HASEL actuators are poised to play an important role in a paradigm shift that fundamentally challenges the current limitations of robotic hardware toward future intelligent systems that replicate the vast capabilities of biological organisms.

DOI [BibTex]

Rothemund, P., Kellaris, N., Mitchell, S. K., Acome, E., Keplinger, C. HASEL Artificial Muscles for a New Generation of Lifelike Robots—Recent Progress and Future Opportunities Advanced Materials, 33(19):2003375, 2021, Nicholas Kellaris, Shane K. Mitchell, and Eric Acome contributed equally to this publication. (article)

DOI [BibTex]

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A Triboelectric-Nanogenerator-Based Gas–Solid Two-Phase Flow Sensor for Pneumatic Conveying System Detecting

Wang, Y., Liu, D., Hu, Z., Chen, T., Zhang, Z., Wang, H., Du, T., Zhang, S. L., Zhao, Z., Zhou, T., Xu, M.

Advanced Materials Technologies, 6(5):2001270, 2021 (article)

DOI [BibTex]

Wang, Y., Liu, D., Hu, Z., Chen, T., Zhang, Z., Wang, H., Du, T., Zhang, S. L., Zhao, Z., Zhou, T., Xu, M. A Triboelectric-Nanogenerator-Based Gas–Solid Two-Phase Flow Sensor for Pneumatic Conveying System Detecting Advanced Materials Technologies, 6(5):2001270, 2021 (article)

DOI [BibTex]

Shaping the future of robotics through materials innovation
Shaping the future of robotics through materials innovation

Rothemund, P., Kim, Y., Heisser, R. H., Zhao, X., Shepherd, R. F., Keplinger, C.

Nature Materials, 20(12):1582-1587, 2021 (article)

Abstract
New classes of functional soft materials show promise to revolutionize robotics. Now materials scientists must focus on realizing the predicted performance of these materials and developing effective and robust interfaces to integrate them into highly functional robotic systems that have a positive impact on human life.

link (url) DOI [BibTex]

Rothemund, P., Kim, Y., Heisser, R. H., Zhao, X., Shepherd, R. F., Keplinger, C. Shaping the future of robotics through materials innovation Nature Materials, 20(12):1582-1587, 2021 (article)

link (url) DOI [BibTex]

Spider-Inspired Electrohydraulic Actuators for Fast, Soft-Actuated Joints
Spider-Inspired Electrohydraulic Actuators for Fast, Soft-Actuated Joints

Kellaris, N., Rothemund, P., Zeng, Y., Mitchell, S. K., Smith, G. M., Jayaram, K., Keplinger, C.

Advanced Science, 8(14):2100916, 2021 (article)

Abstract
The impressive locomotion and manipulation capabilities of spiders have led to a host of bioinspired robotic designs aiming to reproduce their functionalities; however, current actuation mechanisms are deficient in either speed, force output, displacement, or efficiency. Here—using inspiration from the hydraulic mechanism used in spider legs—soft-actuated joints are developed that use electrostatic forces to locally pressurize a hydraulic fluid, and cause flexion of a segmented structure. The result is a lightweight, low-profile articulating mechanism capable of fast operation, high forces, and large displacement; these devices are termed spider-inspired electrohydraulic soft-actuated (SES) joints. SES joints with rotation angles up to 70°, blocked torques up to 70 mN m, and specific torques up to 21 N m / kg are demonstrated. SES joints demonstrate high speed operation, with measured roll-off frequencies up to 24 Hz and specific power as high as 230 W/kg—similar to human muscle. The versatility of these devices is illustrated by combining SES joints to create a bidirectional joint, an artificial limb with independently addressable joints, and a compliant gripper. The lightweight, low-profile design, and high performance of these devices, makes them well-suited toward the development of articulating robotic systems that can rapidly maneuver.

DOI Project Page [BibTex]

Kellaris, N., Rothemund, P., Zeng, Y., Mitchell, S. K., Smith, G. M., Jayaram, K., Keplinger, C. Spider-Inspired Electrohydraulic Actuators for Fast, Soft-Actuated Joints Advanced Science, 8(14):2100916, 2021 (article)

DOI Project Page [BibTex]

Electromechanics of Planar {HASEL} Actuators
Electromechanics of Planar HASEL Actuators

Kirkman, S., Rothemund, P., Acome, E., Keplinger, C.

Extreme Mechanics Letters, 48, pages: 101408, 2021 (article)

DOI Project Page [BibTex]

Kirkman, S., Rothemund, P., Acome, E., Keplinger, C. Electromechanics of Planar HASEL Actuators Extreme Mechanics Letters, 48, pages: 101408, 2021 (article)

DOI Project Page [BibTex]

Liquid Crystal Elastomers with Enhanced Directional Actuation to Electric Fields
Liquid Crystal Elastomers with Enhanced Directional Actuation to Electric Fields

Fowler, H. E., Rothemund, P., Keplinger, C., White, T. J.

Advanced Materials, 33(43):2103806, 2021 (article)

Abstract
The integration of soft, stimuli-responsive materials in robotic systems is a promising approach to introduce dexterous and delicate manipulation of objects. Electrical control of mechanical response offers many benefits in robotic systems including the availability of this energy input, the associated response time, magnitude of actuation, and opportunity for self-regulation. Here, a materials chemistry is detailed to prepare liquid crystal elastomers (LCEs) with a 14:1 modulus contrast and increase in dielectric constant to enhance electromechanical deformation. The inherent modulus contrast of these LCEs (when coated with compliant electrodes) directly convert an electric field to a directional expansion of 20%. The electromechanical response of LCE actuators is observed upon application of voltage ranging from 0.5 to 6 kV. The deformation of these materials is rapid, reaching strain rates of 18% s−1. Upon removal of the electric field, little hysteresis is observed. Patterning the spatial orientation of the nematic director of the LCEs results in a 2D–3D shape transformation to a cone 8 mm in height. Individual and sequential addressing of an array of LCE actuators is demonstrated as a haptic surface.

DOI Project Page [BibTex]

Fowler, H. E., Rothemund, P., Keplinger, C., White, T. J. Liquid Crystal Elastomers with Enhanced Directional Actuation to Electric Fields Advanced Materials, 33(43):2103806, 2021 (article)

DOI Project Page [BibTex]

2020

Design of a High-Speed Prosthetic Finger Driven by Peano-HASEL Actuators
Design of a High-Speed Prosthetic Finger Driven by Peano-HASEL Actuators

Yoder, Z., Kellaris, N., Chase-Markopoulou, C., Ricken, D., Mitchell, S. K., Emmett, M. B., Segil, J., Keplinger, C.

Frontiers in Robotics and AI, 7, pages: 586216, Frontiers, November 2020 (article)

Abstract
Current designs of powered prosthetic limbs are limited by the nearly exclusive use of DC motor technology. Soft actuators promise new design freedom to create prosthetic limbs which more closely mimic intact neuromuscular systems and improve the capabilities of prosthetic users. This work evaluates the performance of a hydraulically amplified self-healing electrostatic (HASEL) soft actuator for use in a prosthetic hand. We compare a linearly-contracting HASEL actuator, termed a Peano-HASEL, to an existing actuator (DC motor) when driving a prosthetic finger like those utilized in multi-functional prosthetic hands. A kinematic model of the prosthetic finger is developed and validated, and is used to customize a prosthetic finger that is tuned to complement the force-strain characteristics of the Peano-HASEL actuators. An analytical model is used to inform the design of an improved Peano-HASEL actuator with the goal of increasing the fingertip pinch force of the prosthetic finger. When compared to a weight-matched DC motor actuator, the Peano-HASEL and custom finger is 10.6 times faster, has 11.1 times higher bandwidth, and consumes 8.7 times less electrical energy to grasp. It reaches 91% of the maximum range of motion of the original finger. However, the DC motor actuator produces 10 times the fingertip force at a relevant grip position. In this body of work, we present ways to further increase the force output of the Peano-HASEL driven prosthetic finger system, and discuss the significance of the unique properties of Peano-HASELs when applied to the field of upper-limb prosthetic design. This approach toward clinically-relevant actuator performance paired with a substantially different form-factor compared to DC motors presents new opportunities to advance the field of prosthetic limb design.

link (url) DOI [BibTex]

2020

Yoder, Z., Kellaris, N., Chase-Markopoulou, C., Ricken, D., Mitchell, S. K., Emmett, M. B., Segil, J., Keplinger, C. Design of a High-Speed Prosthetic Finger Driven by Peano-HASEL Actuators Frontiers in Robotics and AI, 7, pages: 586216, Frontiers, November 2020 (article)

link (url) DOI [BibTex]

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Miniaturized circuitry for capacitive self-sensing and closed-loop control of soft electrostatic transducers

Ly, K., Kellaris, N., McMorris, D., Johnson, B. K., Acome, E., Sundaram, V., Naris, M., Humbert, J. S., Rentschler, M. E., Keplinger, C., Correll, N.

Soft Robotics, Mary Ann Liebert, Inc., publishers, October 2020 (article)

Abstract
Soft robotics is a field of robotic system design characterized by materials and structures that exhibit large-scale deformation, high compliance, and rich multifunctionality. The incorporation of soft and deformable structures endows soft robotic systems with the compliance and resiliency that makes them well adapted for unstructured and dynamic environments. Although actuation mechanisms for soft robots vary widely, soft electrostatic transducers such as dielectric elastomer actuators (DEAs) and hydraulically amplified self-healing electrostatic (HASEL) actuators have demonstrated promise due to their muscle-like performance and capacitive self-sensing capabilities. Despite previous efforts to implement self-sensing in electrostatic transducers by overlaying sinusoidal low-voltage signals, these designs still require sensing high-voltage signals, requiring bulky components that prevent integration with miniature untethered soft robots. We present a circuit design that eliminates the need for any high-voltage sensing components, thereby facilitating the design of simple low cost circuits using off-the-shelf components. Using this circuit, we perform simultaneous sensing and actuation for a range of electrostatic transducers including circular DEAs and HASEL actuators and demonstrate accurate estimated displacements with errors <4%. We further develop this circuit into a compact and portable system that couples high voltage actuation, sensing, and computation as a prototype toward untethered multifunctional soft robotic systems. Finally, we demonstrate the capabilities of our self-sensing design through feedback control of a robotic arm powered by Peano-HASEL actuators.

link (url) DOI [BibTex]

Ly, K., Kellaris, N., McMorris, D., Johnson, B. K., Acome, E., Sundaram, V., Naris, M., Humbert, J. S., Rentschler, M. E., Keplinger, C., Correll, N. Miniaturized circuitry for capacitive self-sensing and closed-loop control of soft electrostatic transducers Soft Robotics, Mary Ann Liebert, Inc., publishers, October 2020 (article)

link (url) DOI [BibTex]

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Rapid 3D Printing of Electrohydraulic (HASEL) Tentacle Actuators

O’Neill, M. R., Acome, E., Bakarich, S., Mitchell, S. K., Timko, J., Keplinger, C., Shepherd, R. F.

Advanced Functional Materials, 30(40):2005244, August 2020 (article)

Abstract
A comprehensive material system is introduced for the additive manufacturing of electrohydraulic (HASEL) tentacle actuators. This material system consists of a photo-curable, elastomeric silicone-urethane with relatively strong dielectric properties (εr ≈ 8.8 at 1 kHz) in combination with ionically-conductive hydrogel and silver paint electrodes that displace a vegetable-based liquid dielectric under the application of an electric field. The electronic properties of the silicone material as well as the mechanical properties of the constitutive silicone and hydrogel materials are investigated. The hydraulic pressure exerted on the dielectric working fluid in these capacitive actuators is measured in order to characterize their quasi-static behavior. Various design features enabled by 3D printing influence this behavior—decreasing the voltage at which actuation begins or increasing the force density in the system. Using a capacitance change of >35% across the actuators while powered, a demonstration of self-sensing inherent to HASELs is shown. Antagonistic pairs of the 3D printed actuators are shown to exert a blocked force of over 400 mN. An electrohydraulic tentacle actuator is then fabricated to demonstrate the use of this material and actuation system in a synthetic hydrostat. This tentacle actuator is shown to achieve motion in a multi-dimensional space.

link (url) DOI [BibTex]

O’Neill, M. R., Acome, E., Bakarich, S., Mitchell, S. K., Timko, J., Keplinger, C., Shepherd, R. F. Rapid 3D Printing of Electrohydraulic (HASEL) Tentacle Actuators Advanced Functional Materials, 30(40):2005244, August 2020 (article)

link (url) DOI [BibTex]

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A Lesson from Plants: High‐Speed Soft Robotic Actuators

Baumgartner, R., Kogler, A., Stadlbauer, J. M., Foo, C. C., Kaltseis, R., Baumgartner, M., Mao, G., Keplinger, C., Koh, S. J. A., Arnold, N., Suo, Z., Kaltenbrunner, M., Bauer, S.

Advanced Materials, 7(5):1903391, March 2020 (article)

Abstract
Rapid energy-efficient movements are one of nature's greatest developments. Mechanisms like snap-buckling allow plants like the Venus flytrap to close the terminal lobes of their leaves at barely perceptible speed. Here, a soft balloon actuator is presented, which is inspired by such mechanical instabilities and creates safe, giant, and fast deformations. The basic design comprises two inflated elastomer membranes pneumatically coupled by a pressurized chamber of suitable volume. The high-speed actuation of a rubber balloon in a state close to the verge of mechanical instability is remotely triggered by a voltage-controlled dielectric elastomer membrane. This method spatially separates electrically active and passive parts, and thereby averts electrical breakdown resulting from the drastic thinning of an electroactive membrane during large expansion. Bistable operation with small and large volumes of the rubber balloon is demonstrated, achieving large volume changes of 1398% and a high-speed area change rate of 2600 cm2 s−1. The presented combination of fast response time with large deformation and safe handling are central aspects for a new generation of soft bio-inspired robots and can help pave the way for applications ranging from haptic displays to soft grippers and high-speed sorting machines.

link (url) DOI [BibTex]

Baumgartner, R., Kogler, A., Stadlbauer, J. M., Foo, C. C., Kaltseis, R., Baumgartner, M., Mao, G., Keplinger, C., Koh, S. J. A., Arnold, N., Suo, Z., Kaltenbrunner, M., Bauer, S. A Lesson from Plants: High‐Speed Soft Robotic Actuators Advanced Materials, 7(5):1903391, March 2020 (article)

link (url) DOI [BibTex]

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Identification and Control of a Nonlinear Soft Actuator and Sensor System

Johnson, B. K., Sundaram, V., Naris, M., Acome, E., Ly, K., Correll, N., Keplinger, C., Humbert, J. S., Rentschler, M. E.

IEEE Robotics and Automation Letters, 5(3):3783-3790, March 2020 (article)

Abstract
Soft robots are becoming increasingly prevalent, with unique applications to medical devices and wearable technology. Understanding the dynamics of nonlinear soft actuators is crucial to creating controllable soft robots. this letter presents a system identification process and closed-loop control of foldable HASEL (hydraulically amplified self-healing electrostatic) soft actuators. We characterized foldable HASELs with linear frequency response tests and modeled them using a linear superposition of static and dynamic terms. We also identified two responses of the system: an activation and relaxation response. Based on these two responses, we developed a dual-mode controller which was validated through closed-loop control using a capacitive elastomeric strain sensor wrapped around the actuator. Using this integrated sensor, we achieved step response rise times as fast as 0.025 s and settling times as fast as 0.17 s while under load. These system identification and control techniques can be applied to any HASEL-driven soft robot and could be applied to other soft actuators to enable controllable soft robots.

link (url) DOI [BibTex]

Johnson, B. K., Sundaram, V., Naris, M., Acome, E., Ly, K., Correll, N., Keplinger, C., Humbert, J. S., Rentschler, M. E. Identification and Control of a Nonlinear Soft Actuator and Sensor System IEEE Robotics and Automation Letters, 5(3):3783-3790, March 2020 (article)

link (url) DOI [BibTex]

High-Strain Peano-HASEL Actuators
High-Strain Peano-HASEL Actuators

Wang, X., Mitchell, S. K., Rumley, E. H., Rothemund, P., Keplinger, C.

Advanced Functional Materials, 30(7):1908821, 2020 (article)

Abstract
Soft robots are intrinsically safe for use near humans and adaptable when operated in unstructured environments, thereby offering capabilities beyond traditional robots based on rigid components. Soft actuators are key components of soft robots; recently developed hydraulically amplified self-healing electrostatic (HASEL) actuators provide a versatile framework to create high-speed actuators with excellent all-around performance. Peano-HASEL actuators linearly contract upon application of voltage, closely mimicking the behavior of muscle. Peano-HASEL actuators, however, produce a maximum strain of ≈15%, while skeletal muscles achieve ≈20% on average. Here, a new type of HASEL is introduced, termed high-strain Peano-HASEL (HS-Peano-HASEL) actuator, that achieves linear contraction up to ≈24%. A wide range of performance metrics are investigated, and the maximum strain of multiunit HS-Peano-HASEL actuators is optimized by varying materials and geometry. Furthermore, an artificial circular muscle (ACM) based on the HS-Peano-HASEL acts as a tubular pump, resembling the primordial heart of an ascidian. Additionally, a strain-amplifying pulley system is introduced to increase the maximum strain of an HS-Peano-HASEL to 42%. The muscle-like maximum actuation strain and excellent demonstrated all-around performance of HS-Peano-HASEL actuators make them promising candidates for use in artificial organs, life-like robotic faces, and a variety of other robotic systems.

DOI [BibTex]

Wang, X., Mitchell, S. K., Rumley, E. H., Rothemund, P., Keplinger, C. High-Strain Peano-HASEL Actuators Advanced Functional Materials, 30(7):1908821, 2020 (article)

DOI [BibTex]

Dynamics of Electrohydraulic Soft Actuators
Dynamics of Electrohydraulic Soft Actuators

Rothemund, P., Kirkman, S., Keplinger, C.

Proceedings of the National Academy of Sciences of the United States of America, 117(28):16207-16213, 2020 (article)

Abstract
Nature has inspired the design of robots in which soft actuators enable tasks such as handling of fragile objects and adapting to unstructured environments. Those tasks are difficult for traditional robots, which predominantly consist of hard components. Electrohydraulic soft actuators are liquid-filled shells that deform upon the application of electric fields; they excel among soft actuators with muscle-like force outputs and actuation strains, and with actuation frequencies above 100 Hz. However, the fundamental physics that governs the dynamics of electrohydraulic soft actuators is unexplored. Here, we study the dynamics of electrohydraulic soft actuators using the Peano-HASEL (hydraulically amplified self-healing electrostatic) actuator as a model system. Using experiments and a scaling analysis, we discover two dynamic regimes: a regime in which viscous dissipation reduces the actuation speed and a regime governed by inertial effects in which high-speed actuation is possible. For each regime, we derive a timescale that describes the influence of geometry, materials system, and applied external loads on the actuation speed. We also derive a model to study the dynamic behavior of Peano-HASEL actuators in both regimes. Although this analysis focuses on the Peano-HASEL actuator, the presented results may readily be generalized to other electrohydraulic actuators. When designed to operate in the inertial regime, electrohydraulic actuators will enable bio-inspired robots with unprecedented speeds of motion.

DOI [BibTex]

Rothemund, P., Kirkman, S., Keplinger, C. Dynamics of Electrohydraulic Soft Actuators Proceedings of the National Academy of Sciences of the United States of America, 117(28):16207-16213, 2020 (article)

DOI [BibTex]

2019

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An Easy‐to‐Implement Toolkit to Create Versatile and High‐Performance HASEL Actuators for Untethered Soft Robots

Mitchell, S. K., Wang, X., Acome, E., Martin, T., Ly, K., Kellaris, N., Venkata, V. G., Keplinger, C.

Advanced Science, 6(14):1900178, June 2019 (article)

Abstract
For soft robots to have ubiquitous adoption in practical applications they require soft actuators that provide well-rounded actuation performance that parallels natural muscle while being inexpensive and easily fabricated. This manuscript introduces a toolkit to rapidly prototype, manufacture, test, and power various designs of hydraulically amplified self-healing electrostatic (HASEL) actuators with muscle-like performance that achieve all three basic modes of actuation (expansion, contraction, and rotation). This toolkit utilizes easy-to-implement methods, inexpensive fabrication tools, commodity materials, and off-the-shelf high-voltage electronics thereby enabling a wide audience to explore HASEL technology. Remarkably, the actuators created from this easy-to-implement toolkit achieve linear strains exceeding 100%, a specific power greater than 150 W kg−1 , and ≈20% strain at frequencies above 100 Hz. This combination of large strain, extreme speed, and high specific power yields soft actuators that jump without power-amplifying mechanisms. Additionally, an efficient fabrication technique is introduced for modular designs of HASEL actuators, which is used to develop soft robotic devices driven by portable electronics. Inspired by the versatility of elephant trunks, the above capabilities are combined to create an untethered continuum robot for grasping and manipulating delicate objects, highlighting the wide potential of the introduced methods for soft robots with increasing sophistication.

link (url) DOI [BibTex]

2019

Mitchell, S. K., Wang, X., Acome, E., Martin, T., Ly, K., Kellaris, N., Venkata, V. G., Keplinger, C. An Easy‐to‐Implement Toolkit to Create Versatile and High‐Performance HASEL Actuators for Untethered Soft Robots Advanced Science, 6(14):1900178, June 2019 (article)

link (url) DOI [BibTex]

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Mechanical‐to‐Electrical Energy Conversion with Variable Electric Double Layers

Morrissey, T. G., Mitchell, S. K., Jaros, A. T., Ambos, E., Keplinger, C.

Energy Technology, 7(4):1801007, WILEY-VCH, March 2019 (article)

Abstract
The electromagnetic generator requires sophisticated power take-off systems to effectively capture low-frequency mechanical motion such as ocean waves and human gait. Conversely, electrostatic generators are variable capacitors, which harness charge separation processes that can directly capture low-frequency mechanical energy. The fundamentals of mechanical-to-electrical energy conversion with an electrostatic generator that exploits the high energy density of electric double layer (EDL) capacitors and varies capacitance by physical manipulation of the electrode–electrolyte interfacial area are elucidated. A model system is designed where all experimental parameters are easy to access in order to gain a detailed understanding of the energy flow in this conversion process. The system, termed a variable EDL generator, is based on titanium electrodes in NaCl aqueous electrolyte and operated as a charge pump. The net positive electrical energy generation of the system in the voltage–charge work-conjugate plane is analyzed, and it shows that the variable EDL generator can be scaled-up to spin an electric motor and increase the output power of the generator through the use of waste heat. The detailed analysis of this energy conversion principle suggests multiple avenues to enhance the performance of the variable EDL generator.

link (url) DOI [BibTex]

Morrissey, T. G., Mitchell, S. K., Jaros, A. T., Ambos, E., Keplinger, C. Mechanical‐to‐Electrical Energy Conversion with Variable Electric Double Layers Energy Technology, 7(4):1801007, WILEY-VCH, March 2019 (article)

link (url) DOI [BibTex]

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Simulation-driven design to reduce pull-in voltage of donut HASEL actuators

Panwar, S., Gandhi, U., Acome, E., Keplinger, C., Rowe, M.

Proceedings of SPIE, 10966, pages: 1096622, SPIE, March 2019 (article)

Abstract
Soft robotics research has been motivated in part by the versatility and functionality of human muscle. Researchers have tried to mimic the speed and performance of human muscle by using soft fluid actuators; however, these actuators are often slow and bulky. Research conducted in the use of dielectric elastomers has proven to be promising. These dielectric elastomers can produce large strains using high voltage electrical input. However, the development of these dielectric elastomer actuators has been inhibited due to their susceptibility to dielectric breakdown and electrical aging. One recent technology that can solve these issues and advance the field of soft actuators, is that of the hydraulically amplified self-healing electrostatic (HASEL) actuator. Such actuators are comprised of a liquid dielectric enclosed in an elastomer shell with electrodes on either side of the shell. Incorporating a liquid dielectric dramatically reduces the impact of dielectric breakdown on the performance of HASEL actuators and allows for hydraulically-coupled modes of actuation. However, the voltages that are required to operate these actuators are still challenging for commercial applications. Our work uses a simulation-driven approach to determine design parameters for donut HASEL actuators that provide a high actuation strain at a reduced pull-in voltage. We outline a modeling approach that is comprised of calibrating the properties of a multiphysics finite element model using actual HASEL actuator experimental data. The model is validated using a donut-shape HASEL actuator from literature. The model is then applied to determine the optimal electrode size and fluid dielectric permittivity for achieving a low operating voltage. This simulation-driven design assists in the fabrication of soft actuators with potential application to a variety of industries. Keywords: Electroactive polymer, soft actuator, artificial muscles, simulation, finite element method, HASEL

link (url) DOI [BibTex]

Panwar, S., Gandhi, U., Acome, E., Keplinger, C., Rowe, M. Simulation-driven design to reduce pull-in voltage of donut HASEL actuators Proceedings of SPIE, 10966, pages: 1096622, SPIE, March 2019 (article)

link (url) DOI [BibTex]

An Analytical Model for the Design of Peano-HASEL Actuators with Drastically Improved Performance
An Analytical Model for the Design of Peano-HASEL Actuators with Drastically Improved Performance

Kellaris, N., Venkata, V. G., Rothemund, P., Keplinger, C.

Extreme Mechanics Letters, 29, pages: 100449, 2019 (article)

Abstract
The emerging field of soft robotics promises applications in areas such as human–machine interaction, industrial automation, and biomedical devices. Electrohydraulic Peano-HASEL (hydraulically amplified self-healing electrostatic) actuators feature muscle-like linear contraction on activation, fast operation, and direct electrical control, which makes them a versatile actuator for soft robotics. To better understand the impact of geometry and materials on actuator performance, we develop an analytical model – based on minimizing the total energy of the actuator system – that accurately predicts the quasi-static behavior of the actuators without relying on fitting parameters. We present extensive experimental validation of this model for actuators with varying geometries, as well as actuators made from shell materials with different electrical and mechanical properties. Using these results, we identify design rules for the development of actuators with tunable force-strain characteristics. As a key result of this paper, we lay out a roadmap for creating Peano-HASELs with drastically improved specific energies. Specifically, we identify a combination of pouch geometry and an existing high-performance shell material for which the model predicts actuators that achieve a specific energy of over 10,000 J/kg, far exceeding maximum values reported for natural muscle (∼ 40 J/kg).

DOI [BibTex]

Kellaris, N., Venkata, V. G., Rothemund, P., Keplinger, C. An Analytical Model for the Design of Peano-HASEL Actuators with Drastically Improved Performance Extreme Mechanics Letters, 29, pages: 100449, 2019 (article)

DOI [BibTex]

How Inhomogeneous Zipping Increases the Force Output of Peano-HASEL Actuators
How Inhomogeneous Zipping Increases the Force Output of Peano-HASEL Actuators

Rothemund, P., Kellaris, N., Keplinger, C.

Extreme Mechanics Letters, 31, pages: 100542, 2019 (article)

Abstract
Research in soft robotics has yielded numerous types of soft actuators with widely differing mechanisms of operation that enable functionality that is difficult or impossible to reproduce with hard actuators such as electromagnetic motors. The Peano-HASEL (hydraulically amplified self-healing electrostatic) actuator is a new type of electrostatic, linearly contracting, soft actuator that features large strains, fast actuation, and high energy densities. Peano-HASEL actuators are comprised of pouches, which are made of flexible dielectric polymer films, filled with a liquid dielectric, and covered with flexible electrodes. When a voltage is applied to the electrodes, they ‘‘zip’’ together due to the Maxwell stress, which displaces the liquid inside the pouch, and causes the actuator to contract. Zipping can occur homogeneously or inhomogeneously. In this letter we analyze inhomogeneous zipping and its influence on the performance of Peano-HASEL actuators. We develop a theoretical model that describes inhomogeneous as well as homogeneous zipping of the electrodes and characterize the behavior of actuators experimentally. Inhomogeneous zipping occurs (depending on the size of the electrodes) predominantly at large loads, because it allows for larger areas of the electrodes to be zipped. Inhomogeneous zipping increases the blocking force of the actuators and leads to larger actuation strains near the blocking force. Exploiting inhomogeneous zipping by increasing the electrode size enables an increase in the blocking force of the actuators by up to 47%.

DOI [BibTex]

Rothemund, P., Kellaris, N., Keplinger, C. How Inhomogeneous Zipping Increases the Force Output of Peano-HASEL Actuators Extreme Mechanics Letters, 31, pages: 100542, 2019 (article)

DOI [BibTex]

2018

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Dynamically actuated liquid‐infused poroelastic film with precise control over droplet dynamics

Oh, I., Keplinger, C., Cui, J., Chen, J., Whitesides, G. M., Aizenberg, J., Hu, Y.

Advanced Functional Materials, 28(39):1802632, September 2018 (article)

Abstract
Traditional dynamic adaptive materials rely on an atomic/molecular mechanism of phase transition to induce macroscopic switch of properties, but only a small number of these materials and a limited responsive repertoire are available. Here, liquid as the adaptive component is utilized to realize responsive functions. Paired with a porous matrix that can be put in motion by an actuated dielectric elastomer film, the uncontrolled global flow of liquid is broken down to well-defined reconfigurable localized flow within the pores and conforms to the network deformation. A detailed theoretical and experimental study of such a dynamically actuated liquid-infused poroelastic film is discussed. This system demonstrates its ability to generate tunable surface wettability that can precisely control droplet dynamics from complete pinning, to fast sliding, and even more complex motions such as droplet oscillation, jetting, and mixing. This system also allows for repeated and seamless switch among these different droplet manipulations. These are desired properties in many applications such as reflective display, lab-on-a-chip, optical device, dynamic measurements, energy harvesting, and others.

link (url) DOI [BibTex]

2018

Oh, I., Keplinger, C., Cui, J., Chen, J., Whitesides, G. M., Aizenberg, J., Hu, Y. Dynamically actuated liquid‐infused poroelastic film with precise control over droplet dynamics Advanced Functional Materials, 28(39):1802632, September 2018 (article)

link (url) DOI [BibTex]

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Peano-HASEL actuators: Muscle-mimetic, electrohydraulic transducers that linearly contract on activation

Kellaris, N., Venkata, V. G., Smith, G. M., Mitchell, S. K., Keplinger, C.

Science Robotics, 3(14):eaar3276, 2018 (article)

Abstract
Soft robotic systems are well suited to unstructured, dynamic tasks and environments, owing to their ability to adapt and conform without damaging themselves or their surroundings. These abilities are crucial in areas such as human-robot interaction. Soft robotic systems are currently limited by the soft actuators that power them. To date, most soft actuators are based on pneumatics or shape-memory alloys, which have issues with efficiency, response speed, and portability. Dielectric elastomer actuators (DEAs) are controlled and powered electrically and excel with muscle-like actuation, but they typically require a rigid frame and prestretch to perform effectively. In addition, DEAs require complex stacks or structures to achieve linear contraction modes. We present a class of soft electrohydraulic transducers, termed Peano-HASEL (hydraulically amplified self-healing electrostatic) actuators, that combine the strengths of fluidic actuators and electrostatic actuators, while addressing many of their issues. These actuators use both electrostatic and hydraulic principles to linearly contract on application of voltage in a muscle-like fashion, without rigid frames, prestretch, or stacked configurations. We fabricated these actuators using a facile heat-sealing method with inexpensive commercially available materials. These prototypical devices demonstrated controllable linear contraction up to 10%, a strain rate of 900% per second, actuation at 50 hertz, and the ability to lift more than 200 times their weight. In addition, these actuators featured characteristics such as high optical transparency and the ability to self-sense their deformation state. Hence, this class of actuators demonstrates promise for applications such as active prostheses, medical and industrial automation, and autonomous robotic devices.

DOI [BibTex]

Kellaris, N., Venkata, V. G., Smith, G. M., Mitchell, S. K., Keplinger, C. Peano-HASEL actuators: Muscle-mimetic, electrohydraulic transducers that linearly contract on activation Science Robotics, 3(14):eaar3276, 2018 (article)

DOI [BibTex]

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Hydraulically amplified self-healing electrostatic actuators with muscle-like performance

Acome, E., Mitchell, S. K., Morrissey, T., Emmett, M., Benjamin, C., King, M., Radakovitz, M., Keplinger, C.

Science, 359(6371):61-65, 2018 (article)

Abstract
Existing soft actuators have persistent challenges that restrain the potential of soft robotics, highlighting a need for soft transducers that are powerful, high-speed, efficient, and robust. We describe a class of soft actuators, termed hydraulically amplified self-healing electrostatic (HASEL) actuators, which harness a mechanism that couples electrostatic and hydraulic forces to achieve a variety of actuation modes. We introduce prototypical designs of HASEL actuators and demonstrate their robust, muscle-like performance as well as their ability to repeatedly self-heal after dielectric breakdown—all using widely available materials and common fabrication techniques. A soft gripper handling delicate objects and a self-sensing artificial muscle powering a robotic arm illustrate the wide potential of HASEL actuators for next-generation soft robotic devices.

DOI [BibTex]

Acome, E., Mitchell, S. K., Morrissey, T., Emmett, M., Benjamin, C., King, M., Radakovitz, M., Keplinger, C. Hydraulically amplified self-healing electrostatic actuators with muscle-like performance Science, 359(6371):61-65, 2018 (article)

DOI [BibTex]

2017

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High-performance electromechanical transduction using laterally-constrained dielectric elastomers part I: Actuation processes

Koh, S. J. A., Keplinger, C., Kaltseis, R., Foo, C., Baumgartner, R., Bauer, S., Suo, Z.

Journal of the Mechanics and Physics of Solids, 105(105):81-94, Pergamon, August 2017 (article)

Abstract
A dielectric elastomer transducer is a deformable capacitor, and is under development as a sensor, actuator, or generator. Among various geometric configurations, laterally-constrained transducer, also known as pure-shear transducer, is easy to implement and effective to couple mechanical force and electrical voltage. This analytical study reveals that lateral pre-stretch enhances actuation, far exceeding previously reported actuation strokes. Laterally-constrained transducers exhibit complex electromechanical behavior. As voltage increases, an actuator may undergo electromechanical instability, or form wrinkles, or suffer electrical breakdown. We survey the behavior of actuators under all possible states of pre-stretches, and identify five modes of actuation. Our analysis predicts that laterally-constrained actuators can achieve actuation stroke of 1000% for an acrylic elastomer, and 230% for natural rubber. This …

link (url) DOI [BibTex]

2017

Koh, S. J. A., Keplinger, C., Kaltseis, R., Foo, C., Baumgartner, R., Bauer, S., Suo, Z. High-performance electromechanical transduction using laterally-constrained dielectric elastomers part I: Actuation processes Journal of the Mechanics and Physics of Solids, 105(105):81-94, Pergamon, August 2017 (article)

link (url) DOI [BibTex]

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A transparent, self‐healing, highly stretchable ionic conductor

Cao, Y., Morrissey, T. G., Acome, E., Allec, S. I., Wong, B. M., Keplinger, C., Wang, C.

Advanced Materials, 29(10):1605099, 2017, Y.C. and T.G.M. contributed equally to this work. (article)

Abstract
Self-healing materials can repair damage caused by mechanical wear, thereby extending lifetime of devices. A transparent, self-healing, highly stretchable ionic conductor is presented that autonomously heals after experiencing severe mechanical damage. The design of this self-healing polymer uses ion–dipole interactions as the dynamic motif. The unique properties of this material when used to electrically activate transparent artificial muscles are demonstrated.

DOI [BibTex]

Cao, Y., Morrissey, T. G., Acome, E., Allec, S. I., Wong, B. M., Keplinger, C., Wang, C. A transparent, self‐healing, highly stretchable ionic conductor Advanced Materials, 29(10):1605099, 2017, Y.C. and T.G.M. contributed equally to this work. (article)

DOI [BibTex]

2016

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Buckling Pneumatic Linear Actuators Inspired by Muscle

Yang, D., Verma, M. S., So, J., Mosadegh, B., Keplinger, C., Lee, B., Khashai, F., Lossner, E., Suo, Z., Whitesides, G. M.

Advanced Materials Technologies, 1(3):1600055, June 2016 (article)

Abstract
The mechanical features of biological muscles are difficult to reproduce completely in synthetic systems. A new class of soft pneumatic structures (vacuum-actuated muscle-inspired pneumatic structures) is described that combines actuation by negative pressure (vacuum), with cooperative buckling of beams fabricated in a slab of elastomer, to achieve motion and demonstrate many features that are similar to that of mammalian muscle.

DOI [BibTex]

2016

Yang, D., Verma, M. S., So, J., Mosadegh, B., Keplinger, C., Lee, B., Khashai, F., Lossner, E., Suo, Z., Whitesides, G. M. Buckling Pneumatic Linear Actuators Inspired by Muscle Advanced Materials Technologies, 1(3):1600055, June 2016 (article)

DOI [BibTex]

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A Highly Stretchable Autonomous Self-Healing Elastomer

Li, C., Wang, C., Keplinger, C., Zuo, J., Jin, L., Sun, Y., Zheng, P., Cao, Y., Lissel, F., Linder, C., You, X., Bao, Z.

Nature Chemistry, 8, pages: 618-624 , April 2016 (article)

Abstract
It is a challenge to synthesize materials that possess the properties of biological muscles—strong, elastic and capable of self-healing. Herein we report a network of poly(dimethylsiloxane) polymer chains crosslinked by coordination complexes that combines high stretchability, high dielectric strength, autonomous self-healing and mechanical actuation. The healing process can take place at a temperature as low as −20 °C and is not significantly affected by surface ageing and moisture. The crosslinking complexes used consist of 2,6-pyridinedicarboxamide ligands that coordinate to Fe(III) centres through three different interactions: a strong pyridyl–iron one, and two weaker carboxamido–iron ones through both the nitrogen and oxygen atoms of the carboxamide groups. As a result, the iron–ligand bonds can readily break and re-form while the iron centres still remain attached to the ligands through the stronger interaction with the pyridyl ring, which enables reversible unfolding and refolding of the chains. We hypothesize that this behaviour supports the high stretchability and self-healing capability of the material.

DOI [BibTex]

Li, C., Wang, C., Keplinger, C., Zuo, J., Jin, L., Sun, Y., Zheng, P., Cao, Y., Lissel, F., Linder, C., You, X., Bao, Z. A Highly Stretchable Autonomous Self-Healing Elastomer Nature Chemistry, 8, pages: 618-624 , April 2016 (article)

DOI [BibTex]

2015

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Stretchable Conductive Composites Based on Metal Wools for Use as Electrical Vias in Soft Devices

Lessing, J., Morin, S. A., Keplinger, C., Tayi, A. S., Whitesides, G. M.

Advanced Functional Materials, 25(9):1418-1425, March 2015 (article)

Abstract
Soft devices can be bent, stretched, and compressed reversibly, but conventional wires are rigid. This work describes stretchable composites that are easily fabricated with simple tools and commodity materials, and that can provide a strategy for electrical wiring that meets certain needs of soft devices. These composites are made by combining metal wool and elastomeric polymers. Embedding fine (average fiber width ≈25 μm) steel wool (or other metal wools) in a silicone polymer creates an electrically conductive path through the nonconductive elastomer. This composite is flexible, stretchable, compressible, inexpensive, and simple to incorporate into the bodies of soft devices. It is also electrically anisotropic, and shows maximum conductivity along the majority axis of the fibers, but maximum extension perpendicular to this axis. The utility of this composite for creating an electrically conductive path through an elastomer was demonstrated in several devices, including: a soft, solderless breadboard, a soft touch sensor, and a soft strain gauge.

DOI [BibTex]

2015

Lessing, J., Morin, S. A., Keplinger, C., Tayi, A. S., Whitesides, G. M. Stretchable Conductive Composites Based on Metal Wools for Use as Electrical Vias in Soft Devices Advanced Functional Materials, 25(9):1418-1425, March 2015 (article)

DOI [BibTex]

2014

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Ionic Skin

Sun, J., Keplinger, C., Whitesides, G. M., Suo, Z.

Advanced Materials, 26(45):7608-7614, December 2014 (article)

Abstract
Electronic skins (i.e., stretchable sheets of distributed sensors) report signals using electrons, whereas natural skins report signals using ions. Here, ionic conductors are used to create a new type of sensory sheet, called “ionic skin”. Ionic skins are highly stretchable, transparent, and biocompatible. They readily measure strains from 1% to 500%, and pressures as low as 1 kPa.

DOI [BibTex]

2014

Sun, J., Keplinger, C., Whitesides, G. M., Suo, Z. Ionic Skin Advanced Materials, 26(45):7608-7614, December 2014 (article)

DOI [BibTex]

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Inkjet Printing of Conductive Inks with High Lateral Resolution on Omniphobic RF Paper for Paper-Based Electronics and MEMS

Lessing, J., Glavan, A. C., Walker, S. B., Keplinger, C., Lewis, J. A., Whitesides, G. M.

Advanced Materials, 26(27):4677-4682, July 2014 (article)

Abstract
The use of omniphobic “fluoroalkylated paper” as a substrate for inkjet printing of aqueous inks that are the precursors of electrically conductive patterns is described. By controlling the surface chemistry of the paper, it is possible to print high resolution, conductive patterns that remain conductive after folding and exposure to common solvents.

DOI [BibTex]

Lessing, J., Glavan, A. C., Walker, S. B., Keplinger, C., Lewis, J. A., Whitesides, G. M. Inkjet Printing of Conductive Inks with High Lateral Resolution on Omniphobic RF Paper for Paper-Based Electronics and MEMS Advanced Materials, 26(27):4677-4682, July 2014 (article)

DOI [BibTex]

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Natural Rubber for Sustainable High-Power Electrical Energy Generation

Kaltseis, R., Keplinger, C., Koh, S. J. A., Baumgartner, R., Goh, Y. F., Ng, W. H., Kogler, A., Tröls, A., Foo, C. C., Suo, Z., Bauer, S.

RSC Advances, 4(53):27905-27913, June 2014 (article)

Abstract
Clean, renewable and abundant sources of energy, such as the vast energy of ocean waves, are untapped today, because no technology exists to convert such mechanical motions to electricity economically. Other sources of mechanical energy, such as motions of people and vibrations of buildings and bridges, can potentially power portable electronics and distributed sensors. Here we show that natural rubber can be used to construct generators of high performance and low cost. Natural rubber has higher elastic modulus, fracture energy and dielectric strength than a commonly studied acrylic elastomer. We demonstrate high energy densities (369 mJ g−1) and high power densities (200 mW g−1), and estimate low levelized cost of electricity (5–11 ct kW−1 h−1). Soft generators based on natural rubber enable clean, low-cost, large-scale generation of electricity.

DOI [BibTex]

Kaltseis, R., Keplinger, C., Koh, S. J. A., Baumgartner, R., Goh, Y. F., Ng, W. H., Kogler, A., Tröls, A., Foo, C. C., Suo, Z., Bauer, S. Natural Rubber for Sustainable High-Power Electrical Energy Generation RSC Advances, 4(53):27905-27913, June 2014 (article)

DOI [BibTex]

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Soft Actuators and Robots that Are Resistant to Mechanical Damage

Martinez, R. V., Glavan, A. C., Keplinger, C., Oyetibo, A. I., Whitesides, G. M.

Advanced Functional Materials, 24(20):3003-3010, May 2014 (article)

Abstract
This paper characterizes the ability of soft pneumatic actuators and robots to resist mechanical insults that would irreversibly damage or destroy hard robotic systems—systems fabricated in metals and structural polymers, and actuated mechanically—of comparable sizes. The pneumatic networks that actuate these soft machines are formed by bonding two layers of elastomeric or polymeric materials that have different moduli on application of strain by pneumatic inflation; this difference in strain between an extensible top layer and an inextensible, strain-limiting, bottom layer causes the pneumatic network to expand anisotropically. While all the soft machines described here are, to some extent, more resistant to damage by compressive forces, blunt impacts, and severe bending than most corresponding hard systems, the composition of the strain-limiting layers confers on them very different tensile and compressive strengths.

DOI [BibTex]

Martinez, R. V., Glavan, A. C., Keplinger, C., Oyetibo, A. I., Whitesides, G. M. Soft Actuators and Robots that Are Resistant to Mechanical Damage Advanced Functional Materials, 24(20):3003-3010, May 2014 (article)

DOI [BibTex]

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Pneumatic Networks for Soft Robotics that Actuate Rapidly

Mosadegh, B., Polygerinos, P., Keplinger, C., Wennstedt, S., Shepherd, R. F., Gupta, U., Shim, J., Bertoldi, K., Walsh, C. J., Whitesides, G. M.

Advanced Functional Materials, 24(15):2163-2170, April 2014 (article)

Abstract
Soft robots actuated by inflation of a pneumatic network (a “pneu-net”) of small channels in elastomeric materials are appealing for producing sophisticated motions with simple controls. Although current designs of pneu-nets achieve motion with large amplitudes, they do so relatively slowly (over seconds). This paper describes a new design for pneu-nets that reduces the amount of gas needed for inflation of the pneu-net, and thus increases its speed of actuation. A simple actuator can bend from a linear to a quasi-circular shape in 50 ms when pressurized at ΔP = 345 kPa. At high rates of pressurization, the path along which the actuator bends depends on this rate. When inflated fully, the chambers of this new design experience only one-tenth the change in volume of that required for the previous design. This small change in volume requires comparably low levels of strain in the material at maximum amplitudes of actuation, and commensurately low rates of fatigue and failure. This actuator can operate over a million cycles without significant degradation of performance. This design for soft robotic actuators combines high rates of actuation with high reliability of the actuator, and opens new areas of application for them.

DOI [BibTex]

Mosadegh, B., Polygerinos, P., Keplinger, C., Wennstedt, S., Shepherd, R. F., Gupta, U., Shim, J., Bertoldi, K., Walsh, C. J., Whitesides, G. M. Pneumatic Networks for Soft Robotics that Actuate Rapidly Advanced Functional Materials, 24(15):2163-2170, April 2014 (article)

DOI [BibTex]

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Charge Localization Instability in a Highly Deformable Dielectric Elastomer

Lu, T., Keplinger, C., Arnold, N., Bauer, S., Suo, Z.

Applied Physics Letters, 104(2):022905, January 2014 (article)

Abstract
This paper shows that a highly deformable capacitor made of a soft dielectric and two conformal electrodes can switch between two states discontinuously, by a first-order transition, as the total charge varies gradually. When the total charge is small, it spreads evenly over the area of the capacitor, and the capacitor deforms homogeneously. When the total charge is large, it localizes in a small region of the capacitor, and this region thins down preferentially. The capacitor will survive the localization without electrical breakdown if the area of the electrode is small. Such a bistable system may lead to useful devices.

DOI [BibTex]

Lu, T., Keplinger, C., Arnold, N., Bauer, S., Suo, Z. Charge Localization Instability in a Highly Deformable Dielectric Elastomer Applied Physics Letters, 104(2):022905, January 2014 (article)

DOI [BibTex]

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25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters

Bauer, S., Bauer‐Gogonea, S., Graz, I., Kaltenbrunner, M., Keplinger, C., Schwödiauer, R.

Advanced Materials, 26(1):149-162, January 2014 (article)

Abstract
Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self-cleaning and healing and on-skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.

DOI [BibTex]

Bauer, S., Bauer‐Gogonea, S., Graz, I., Kaltenbrunner, M., Keplinger, C., Schwödiauer, R. 25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters Advanced Materials, 26(1):149-162, January 2014 (article)

DOI [BibTex]

2013

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Giant Voltage-Induced Deformation in Dielectric Elastomers Near the Verge of Snap-Through Instability

Li, T., Keplinger, C., Baumgartner, R., Bauer, S., Yang, W., Suo, Z.

Journal of the Mechanics and Physics of Solids, 61(2):611-628, February 2013 (article)

Abstract
Dielectric elastomers are capable of large voltage-induced deformation, but achieving such large deformation in practice has been a major challenge due to electromechanical instability and electric breakdown. The complex nonlinear behavior suggests an important opportunity: electromechanical instability can be harnessed to achieve giant voltage-induced deformation. We introduce the following principle of operation: place a dielectric elastomer near the verge of snap-through instability, trigger the instability with voltage, and bend the snap-through path to avert electric breakdown. We demonstrate this principle of operation with a commonly used experimental setup—a dielectric membrane mounted on a chamber of air. The behavior of the membrane can be changed dramatically by varying parameters such as the initial pressure in the chamber, the volume of the chamber, and the prestretch of the membrane. We use a computational model to analyze inhomogeneous deformation and map out bifurcation diagrams to guide the experiment. With suitable values of the parameters, we obtain giant voltage-induced expansion of area by 1692%, far beyond the largest value reported in the literature.

DOI [BibTex]

2013

Li, T., Keplinger, C., Baumgartner, R., Bauer, S., Yang, W., Suo, Z. Giant Voltage-Induced Deformation in Dielectric Elastomers Near the Verge of Snap-Through Instability Journal of the Mechanics and Physics of Solids, 61(2):611-628, February 2013 (article)

DOI [BibTex]

Stretchable, Transparent, Ionic Conductors
Stretchable, Transparent, Ionic Conductors

Keplinger, C., Sun, J., Foo, C. C., Rothemund, P., Whitesides, G. M., Suo, Z.

Science, 341(6149):984-987, 2013, Christoph Keplinger and Jeong-Yun Sun contributed equally to this publication (article)

Abstract
Existing stretchable, transparent conductors are mostly electronic conductors. They limit the performance of interconnects, sensors, and actuators as components of stretchable electronics and soft machines. We describe a class of devices enabled by ionic conductors that are highly stretchable, fully transparent to light of all colors, and capable of operation at frequencies beyond 10 kilohertz and voltages above 10 kilovolts. We demonstrate a transparent actuator that can generate large strains and a transparent loudspeaker that produces sound over the entire audible range. The electromechanical transduction is achieved without electrochemical reaction. The ionic conductors have higher resistivity than many electronic conductors; however, when large stretchability and high transmittance are required, the ionic conductors have lower sheet resistance than all existing electronic conductors.

DOI [BibTex]

Keplinger, C., Sun, J., Foo, C. C., Rothemund, P., Whitesides, G. M., Suo, Z. Stretchable, Transparent, Ionic Conductors Science, 341(6149):984-987, 2013, Christoph Keplinger and Jeong-Yun Sun contributed equally to this publication (article)

DOI [BibTex]

2012

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Harnessing Snap-Through Instability in Soft Dielectrics to Achieve Giant Voltage-Triggered Deformation

Keplinger, C., Li, T., Baumgartner, R., Suo, Z., Bauer, S.

Soft Matter, 8(2):285-288, October 2012 (article)

Abstract
A soft dielectric membrane is prone to snap-through instability. We present theory and experiment to show that the instability can be harnessed to achieve giant voltage-triggered deformation. We mount a membrane on a chamber of a suitable volume, pressurize the membrane into a state near the verge of the instability, and apply a voltage to trigger the snap without causing electrical breakdown. For an acrylic membrane we demonstrate voltage-triggered expansion of area by 1692%, far beyond the largest value reported in the literature. The large expansion can even be retained after the voltage is switched off.

DOI [BibTex]

2012

Keplinger, C., Li, T., Baumgartner, R., Suo, Z., Bauer, S. Harnessing Snap-Through Instability in Soft Dielectrics to Achieve Giant Voltage-Triggered Deformation Soft Matter, 8(2):285-288, October 2012 (article)

DOI [BibTex]

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Performance of Dissipative Dielectric Elastomer Generators

Foo, C. C., Koh, S. J. A., Keplinger, C., Kaltseis, R., Bauer, S., Suo, Z.

Applied Physics Letters, 111(9):094107, May 2012 (article)

Abstract
Dielectric elastomer generators are high-energy-density electromechanical transducers. Their performance is affected by dissipative losses. This paper presents a theoretical analysis of a dielectric elastomer generator with two dissipative processes: viscoelasticity and current leakage. Conversion cycles are shown to attain steady-state after several cycles. Performance parameters such as electrical energy generated per cycle, average power, and mechanical to electrical energy conversion efficiency are introduced. Trade-offs between large electrical energy and power output and poor conversion efficiency are discussed. Excessive current leakage results in negative efficiency—the dielectric elastomer generator wastes energy instead of generating it. The general framework developed in this paper helps in the design and assessment of conversion cycles for dissipative dielectric elastomer generators.

DOI [BibTex]

Foo, C. C., Koh, S. J. A., Keplinger, C., Kaltseis, R., Bauer, S., Suo, Z. Performance of Dissipative Dielectric Elastomer Generators Applied Physics Letters, 111(9):094107, May 2012 (article)

DOI [BibTex]