- Apr 2020
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S. Hunt, T. G. McKay, I. A. Anderson, Appl. Phys. Lett. 104, 113701 (2014).
This paper describes the development of a self healing dielectric elastomer. The actuator contains a layer of oil which allows the actuator to maintain dimensional stability even after being punctured multiple times.
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S. J. Dünki, Y. S. Ko, F. A. Nüesch, D. M. Opris, Adv. Funct. Mater. 25, 2467–2475 (2015).
This paper describes self‐repairable, high permittivity dielectric elastomers with large actuation strains at low electric fields. The actuators can be operated repeatedly and reversibly at voltages below the first breakdown.
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W. Yuan et al., Adv. Mater. 20, 621–625 (2008).
This paper demonstrates the fault-tolerant dielectric elastomer electrodes consisting of elastomers spray coated with carbon nanotubes. The electrodes allows for isolation of the fault after a dielectric breakdown increasing the reliability of the actuator.
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R. Pelrine, R. Kornbluh, Q. Pei, J. Joseph, Science 287, 836–839 (2000).
This paper describes the change in characteristics of elastomers when pre-strained. The resulting elastomers feature double the strength and speed of previous actuators.
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R. F. Shepherd et al., Proc. Natl. Acad. Sci. U.S.A. 108, 20400–20403 (2011).
This paper describes the development of a quadrupedal soft robot that moves through fluid pressure.
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P. Polygerinos et al., Adv. Eng. Mater. 19, 1700016 (2017).
This paper reviews a specific type of soft robot, elastomeric robots, that are powered by fluid pressure.
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F. Ilievski, A. D. Mazzeo, R. F. Shepherd, X. Chen, G. M. Whitesides, Angew. Chem. Int. Ed. 50, 1890–1895 (2011).
This paper defines the characteristics of many types of soft robots. The authors compile a benchmark of these characteristics for future research reference.
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S. Kim, C. Laschi, B. Trimmer, Trends Biotechnol. 31, 287–294 (2013).
This article is a review of the current common designs and applications of "soft-robots". Soft-robots are robots modeled after things that occur in nature. For example, robotics would traditionally use gears and motors to move. .Soft-robotics use designs that mimic human or even insect muscle.
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Thermally activated coiled polymer fiber actuators (49.9 kW/kg) (11) and shape-memory alloys (50 kW/kg) (11, 26) have higher peak specific power; however, their efficiency is low (<2%) (11, 26) and thermomechanical actuators are more difficult to control than electromechanical actuators.
HASEL has a peak specific power 10 times lower than thermomechanical muscles. However, thermomechanical muscles are much less efficient (<2%) compared to HASEL (21%). HASEL is driven through electrostatic attraction with relatively less energy loss. Thermomechanical muscles expand or contract in the presence of heat to generate force. However, this also means a large portion of the energy used to power them is lost through heat.
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The use of liquid dielectrics enables HASEL actuators to self-heal from dielectric breakdown. In contrast to solid dielectrics, which are permanently damaged from breakdown, liquid dielectrics immediately return to an insulating state (fig. S5 and movie S1). This characteristic allowed donut HASEL actuators to self-heal from 50 dielectric breakdown events
HASEL uses a liquid dielectric instead of a solid dielectric. Dielectric breakdown can be considered like a lightening strike. The sky and the ground are the opposite sides of the dielectric. When the power is too high there is a breakdown and the opposite sides connect and conduct electricity, a lightening strike.
The connection is extremely powerful, just like a lightening bolt, and will often burn holes through the dielectric. HASEL actuators, using a liquid dielectric, are protected from these breakdowns. When a hole is burnt through the liquid dielectric, it is filled in with the surrounding liquid, effectively healing it.
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The actuator with larger electrodes displaced more liquid dielectric, generating a larger strain but a smaller force, because the resulting hydraulic pressure acts over a smaller area (Fig. 1D and fig. S2). Conversely, the actuator with smaller electrodes displaced less liquid dielectric, generating less strain but more force, because the resulting hydraulic pressure acts across a larger area (Fig. 1E).
The relative size of the electrode determines the mount of force it can generate.
Smaller electrodes can squish less liquid dielectric material between the electrodes. However, they generate a larger force by causing a smaller strain (rise) over a larger area in the donut.
Larger electrodes can squish more liquid dielectric material between the electrodes. However, they generate a smaller force by causing a larger strain (rise) over a smaller area in the donut.
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Because Maxwell pressure is independent of the electrode area, actuation force and strain can be scaled by adjusting the ratio of electrode area to total area of the elastomeric shell.
You can control the force generated by the actuator by changing the area of the electrode with respect to the overall area of the actuator.
Think of what the water balloon looks like when you push it down with one finger compared to two. The more fingers you used, the larger the area of the balloon you touch, and the higher the sides of the balloon would rise.
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After the pull-in transition (Fig. 1A), actuation strain further increases with voltage (Fig. 1B). For this design, hydraulic pressure causes the soft structure to deform into a toroidal or donut shape (Fig. 1C).
Once the snap-in voltage is passed, the electrodes move further towards each other with increased voltage. The more voltage applied to the electrodes, the more HASEL actuator is squished.
HASEL actuator is initially shaped like a disk, but when voltage (exceeding the snap-in voltage) is applied, it deforms into a donut shape.
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As voltage increases from V1 to V2, there is a small increase in actuation strain s. When voltage surpasses a threshold V2, the increase in electrostatic force starts to exceed the increase in mechanical restoring force, causing the electrodes to abruptly pull together (Fig. 1B)
As you increase the voltage to the HASEL electrodes the dielectric become more polarized. The electrostatic attraction is fighting against the pressure of the liquid dielectric in HASEL. The electrostatic attraction is equivalent to your finger pushing down on the water balloon and the liquid dielectric pressure is equivalent to the balloon resisting your push.
As voltage is increased, the electrostatic (attractive) force becomes greater than the liquid pressure and the electrodes begin to move towards another. Similar to: when you finally push hard enough, the balloon begins to squish.
The voltage high enough to cause this change is called the "snap-in voltage".
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electrostatic Maxwell stress (20) pressurizes and displaces the liquid dielectric from between the electrodes to the surrounding volume.
When the dielectric is polarized it, it generates a force which pulls the two electrodes together. Electrostatic Maxwell Stress is a mathematical model that explains the strain caused by these two electrodes being electrostatically attracted to each other. This electrostatic force is like pushing the water balloon in the middle.
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HASEL actuators, where an elastomeric shell is partially covered by a pair of opposing electrodes and filled with a liquid dielectric (Fig. 1A).
Researchers chose HASEL's structure because they wanted stable uniform force to be distributed from the actuator.
When you push in the middle of the water balloon you raise all the sides around your finger equally. If you push off-center, or squish balloon on the side, you have a less equal distribution of pressure. Therefore, the researchers placed the electrodes in the middle of the actuator (on top and bottom surfaces).
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The use of liquid dielectrics in HASEL actuators enables self-healing with immediate recovery of functionality after numerous dielectric breakdown events.
Self-healing is key aspect of living things. However, engineers develop technologies which mimic self-healing for a variety of applications. To read more about self-healing robotic technologies:
https://nypost.com/2018/01/08/scientists-develop-self-healing-robot-muscles/
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HASEL actuators generate hydraulic pressure locally via electrostatic forces acting on liquid dielectrics distributed throughout a soft structure.
A HASEL actuator is like a half-filled water balloon. If you press in the middle of the balloon with your finger, you force the water from the middle of the balloon to the outer edges.
HASEL actuator is filled with a liquid dielectric that does not conduct electricity. Electrical power is supplied to the electrodes placed in the middle of the top and bottom surfaces of a HASEL actuator.
The electrical power polarizes the dielectric, and generates electrostatic force in the middle of the actuator, similar to pressing your finger in the middle of the water balloon. This force help inflate the sides of the actuator, generating a hydraulic force in the vertical direction.
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Here, we develop a class of high-performance, versatile, muscle-mimetic soft transducers, termed HASEL (hydraulically amplified self-healing electrostatic) actuators.
The researchers named their actuator "HASEL". They explain that HASEL is a higher-performance, versatile, muscle mimetic soft transducer, combining the advantages of fluidic and electrostatic actuators.
Higher-performance: compared to other soft-robot actuators HASEL uses less energy to generate larger movement.
Versatile: HASEL can lift weights or used to grab delicate objects (soft gripper).
Muscle Mimetic: HASEL mimics the movement of human muscles based on extension and contraction of muscle fibers.
Soft transducer: HASEL transfers electric energy into mechanical energy.
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Dielectric materials made of silicone sponges swollen with silicone oil (18) continued operating after dielectric failure but demonstrated actuation strains only below 5%.
Experiments have been performed to test the ability of liquid dielectrics versus solid dielectrics. Liquid dielectrics are able to repair themselves after dielectric failure or damage. Using liquid dielectrics allow for longer lasting actuators, resistant to breakdown, puncture and other damage. However, they do not generate too much movement (<5% strain).
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Fault-tolerant DE actuators
Fault tolerant dielectrics are able to recover from breakdown. There are multiple ways to accomplish this recovery. The device in the paper uses a liquid dielectric. When the liquid dielectric breaks down, only a portion of the overall dielectric is effected. Then when the breakdown event stops, the electricity stops, the remaining dielectric liquid fills in the damaged areas, effectively healing the actuator.
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However, DE actuators are driven by high electric fields, making them prone to failure from dielectric breakdown and electrical aging (15).
When voltage is applied across dielectric materials, they do not conduct electricity but rather get polarized, by accumulating positive and negative charges within the material. Dielectric breakdown happens when the voltage is so high that the dielectric begins conducting electricity. A breakdown event briefly makes the dielectric material conductive, allowing for high amounts of current to flow through, typically causing permanent damage to the material
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Electrically powered muscle-mimetic actuators, such as dielectric elastomer (DE) actuators, offer high actuation strain (>100%) and potentially high efficiency (80%) and are self-sensing (12–14).
Experiments have been conducted to optimize the abilities of dielectric elastomers. It has been discovered that pre-straining the elastomers, like stretching a balloon, allows the elastomers to achieve a higher strain at a faster rate. Pre-straining is the process of applying a force to a material before actual use. When you stretch a balloon before blowing air into it, you are "pre-straining" the balloon. This allows the material to "get use to" the forces it will experience. Also, computational models helped determine the best ways to pre-strain elastomers such that a specific 3D shape is achieved.
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Thermally activated artificial muscle actuators made from inexpensive polymer fibers can provide large actuation forces and work density, but these are difficult to control and have low efficiency (1.32%) (11).
Similar to the fibers described in cited article, artificial fibers, capable of lifting above 12,000 times its own weight, were created. These fibers are tightly coiled and when heated they expand in diameter, causing dramatic contractions. Read more about artificial muscles at: https://www.newsweek.com/artificial-muscle-can-lift-12600-times-its-own-weight-893237
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Currently, soft robots predominantly rely on fluidic actuators (7), which can be designed to suit a variety of applications (8–10). However, fluidic actuators require a supply of pressurized gas or liquid, and fluid transport must occur through systems of channels and tubes, limiting speed and efficiency.
Fluid actuators, which converts fluid pressure into movement, were previously used in soft robots. This would make actuators bulky, slow and less efficient because of tanks and channel systems needed to generate the pressure.
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This discrepancy in mechanics has inspired the field of soft robotics (1–4), which promises to transform the way we interact with machines and to enable new technologies for biomedical devices, industrial automation, and other applications (2, 5, 6).
Traditional robots require rigid structures. This has caused design considerations to work around the machines rather than the problem being solved. Soft robotics seeks to eliminate the rigid structure problem.
Most importantly, for biomedical problems, soft robots provide flexibility and adaptability for accomplishing tasks in a way similar to humans. They also provide a platform for safer interaction with humans.
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- Mar 2020
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www.scienceintheclassroom.org www.scienceintheclassroom.org
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(D) HASEL actuators can be readily scaled up to exert large forces.
Image showing HASEL actuators ability to generate large forces lifting a full gallon (3.8L) bottle while undergoing large strains.
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X. Zhao, Q. Wang, Appl. Phys. Rev. 1, 021304 (2014).
This paper focuses on classifying the deformations of dielectric elastomers. The classification aims to better characterize properties of elastomers under deformation.
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Z. Suo, Acta Mech. Solida Sin. 23, 549–578 (2010).
This paper is a review of dielectric elastomers, their characteristics and use.
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P. Brochu, Q. Pei, Macromol. Rapid Commun. 31, 10–36 (2010).
This paper reviews the recent advancements in dielectric elastomers which allow for longer use before dielectric breakdown renders them unusable.
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C. Keplinger, M. Kaltenbrunner, N. Arnold, S. Bauer, Appl. Phys. Lett. 92, 192903 (2008).
This paper describes a computational model that attempts to demonstrate of an actuator can be programmed to deform in a specific form.
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G. Kovacs, L. Düring, S. Michel, G. Terrasi, Sens. Actuators Phys. 155, 299–307 (2009).
This paper describes the characteristics of polymer actuators when serially stacked. Stacking actuators allows for significant unidirectional contraction.
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C. S. Haines et al., Science 343, 868–872 (2014).
This paper describes the development of a new type of artificial muscle constructed from high-strength polymer fibers, like fishing line or sewing thread. This new muscle is much more efficient than natural human muscle.
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B. Trimmer, Soft Robot. 4, 1–2 (2017).
This paper discusses the development of a composite material which has characteristics that are highly desirable for soft robotic actuators.
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S. Bauer et al., Adv. Mater. 26, 149–161 (2014).
This paper is based on a 25-year review on the field of soft robotics. Actuators, sensors, and flexible energy harvesters are all reviewed.
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B. Mazzolai, L. Margheri, M. Cianchetti, P. Dario, C. Laschi, Bioinspir. Biomim. 7, 025005 (2012).
This paper is the second of a two part series of papers studying and attempting to replicate an octopus tentacle in a soft robot model.
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D. Rus, M. T. Tolley, Nature 521, 467–475 (2015).
This article reviews the current findings and knowledge in the field of soft robotics.
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D. Trivedi, C. D. Rahn, W. M. Kier, I. D. Walker, Appl. Bionics Biomech. 5, 99–117 (2008).
This paper analyzes possible inspiration for soft-robot designs, such as elephant trunks or octopus tentacles. It also analyzes the benefits of soft-robot designs compared to more rigid traditional robotic designs.
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When a DC voltage was applied to the stacked HASEL actuators, the device grasped delicate objects such as a raspberry (Fig. 2, C to E, and movie S4) and a raw egg (Fig. 2, F and G, and movie S4).
News and Policy:
Watch this video demonstrating HASEL's ability to pick up fragile objects.
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Weibull distribution for dielectric breakdown (19).
A Weibull distribution works similar to a normal distribution or a grading "bell-curve". In previous experiments, it was shown that the likeliness of a dielectric breakdown is based on a specific Weibull distribution.
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