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Updated: 46 min 11 sec ago

Instabilities in HASEL actuators

Mon, 2018-02-05 12:20

In reply to Electromechanical Instability

Hi Qin Lei,

Thank you for your kind comment and questions!

Planar HASEL actuators operate in a way that is similar to dielectric elastomer actuators (DEAs). Electrodes cover the entire liquid dielectric region and when electric field is applied through the layers of dielectric, the device decreases in thickness and expands in area. In contrast to donut HASEL actuators which experience a safe electromechanical instability, planar HASEL actuators experience dielectric breakdown from similar electromechanical instabilities as DEAs. However, the liquid dielectric layer of HASEL actuators enables self-healing from dielectric breakdown.

Just as prestretch helps eliminate electromechanical instabilities in DEAs, prestretch heps prevent electromechanical instabilities in planar HASEL actuators and improves performance. A recent paper by Koh et al. presents a detailed theoretical analysis of performance for laterally constrained DEAs and we found that some of these principles also applied to planar HASEL actuators.

As Christoph alluded to above, we like to figure out how instabilities can be used advantageously. Many papers have taken this approach to achieve interesting and remarkable performance with fluidic actuators (Overvelde et al.) and DEAs (Keplinger et al.). We think that electrohydraulic coupling in HASEL actuators could enable a number of useful instabilities and nonlinear performance. The pull-in instability of donut HASEL actuators is just one simple example and we are looking forward to investigating more in the future.

We would love to hear the ideas and suggestions of the mechanics community on how we could take advantage of the structure and materials of HASEL to create some interesting soft actuators.

Thanks again for your interest!


Eric Acome

Fabulous work! Congratulation!

Mon, 2018-02-05 09:19

In reply to Journal Club for February 2018: HASEL artificial muscles for high-speed, electrically powered, self-healing soft robots




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Dear Christoph:

Thank you very much for posting this fabulous and inspiring work in IMechanica, which enables us to learn more about HASEL.

Soft actuators can contribute significantly to soft robotics. It is great to learn HASEL can couple dielectric elastomers and fluidic actuators, and exhibit unique attributes (for example, self-healing). I may have the following questions, regarding the output force and the applied voltage.

1)   I1) In Fig. 1E, it seems that smaller electrodes can lead to a higher force. This force is induced by the Maxwell stress? Output forces can play an important role in robotic applications. Any suggestions for enhancing these forces (for example, use fluid of high dielectric constant)?


Wh2) What is the density of liquid (which may affect the energy density of the actuator)?

3)      3) Can HASEL be driven by lower voltage, with specific fluid? If the required voltage is lower than 1kV, the robot can be driven by onboard low-voltage battery, associated with a voltage amplifier of a very small volume. Consequently, the robot can achieve untethered design (which may significantly improve the robot’s movement and functionalities).  

Thanks again for the great work! Congratulations!



Congress-Wide Symposium on Additive Manufacturing, IMECE 2018

Mon, 2018-02-05 00:23

In reply to Congress-Wide Symposium on Additive Manufacturing (ASME-IMECE 2018): abstract deadline Feb 26

Please consider submtting your abstracts to the Congress-Wide Symposium on Additive Manufacturing (Topic 2-2) topic at the IMECE 2018.


Best regards,

Mehran Tehrani, PhD
Assistant Professor of Mechanical Engineering
University of New Mexico


Sun, 2018-02-04 22:27

In reply to Hi Xiaoyan, 

Hi Eric,

Thank you very much for your detailed replies.

The full lithiation in silicon leads to a huge volume expansion of 300%, which mechanically degrades, fractures and even pulverizes the anodes. After full lithiation, the lithium concertation x of LixSi reaches up to 4.4. In our work, we just partially inserted the lithium till the maximum lithium concentration is about 0.8. At that time, the volume expansion is only about 30%, which avoids the mechanical degradation due to large volume change from high lithium concertation. We tested the cyclic performance of our lithium-battery-based actuator by repeating the lithiation and de-lithiation with maximum lithium concentration of 0.8, and found that this actuator worked very well after a 100,000 cycles.

You work about HASEL actuator is very impressive and inspiring, and would have a significant impact on development of artificial muscles and soft robotics. It would be greatly expected that your novel HASEL actuator will be widely applied for the robotics in near future. Thank you very much again.

Electromechanical Instability

Sun, 2018-02-04 22:14

In reply to Journal Club for February 2018: HASEL artificial muscles for high-speed, electrically powered, self-healing soft robots

Dear Christoph and all authors,

Thanks for the inspireing work. Heartiest congratulations to you all.

As you mentioned in the post "Donut HASELs undergo a safe electromechanical instability to reach large deformations", my question is does the planar HASEL actuator also survive the electromechanical instability to obtain large deformation? Also, prestretch helps to eliminate electromechanical instability for dielectric elastomer actuator. I was wondering the reason you prestretch the planar HASEL actuator?

Thanks and regards

Qin Lei

Dear Christoph:  Great work!

Sun, 2018-02-04 21:49

In reply to Journal Club for February 2018: HASEL artificial muscles for high-speed, electrically powered, self-healing soft robots

Dear Christoph:  Great work! Congratulations ! 

As many group members from Prof. Zhigang Suo, we have also work in soft robotics in Zhejiang University. We have tried quite alot in robots driven by dielectric elastomer with onboard power source (such as the Fast-moving soft electronic fish, 3, 4, Science Advances, 2017 The fully soft robotic fish can move and turns quickly. However, the electric breakdown problem remains as a great challenge on this type of robots for practical application. The idea that self healing in your paper will truly inspire the design for artificial muscles.  

technique questions:

1. the liquid is enveloped  in between of two elastomric membranes, when the high voltage is applied, will the elastomeric membranes also suffer breakdown? 

2. For our experience in dielectric drieven soft robots, the required high voltage is still quite a challenge for the life time and efficiency, for this novel desigh of actuator, do you have any idea to lower the actuating votlage in future?

thank you again for these  nice papers and ideas. recalls me the great time that we work together in Harvard,already 9 years ago,~ time flies.


Sun, 2018-02-04 21:38

In reply to Bonding electrodes

Thank you, Eric! I realize the significance of adhesion in the actuators, and the adhesion will be vital for either effectve actuation or long time use. There will be many oppotunities to develop diverse adhesion methods for different materials in different applications.

Adhesion of hydrogel and other materials is relatively new, and the methods are pretty much you mentioned. one more paper is by Cha, et al.

For other material systems, there are many commercial products, and also a large body of literature available. Please see some provided in this list.

Looking forward to seeing the next generation of HASEL.




Bonding electrodes

Sun, 2018-02-04 15:56

In reply to Bonding hydrogel electrode and elastomeric shell

Hi Jiawei

Thanks for another great question!

Creating strong adhesion between the elastomer shell and hydrogels was one of the first challenges we encountered for HASEL actuators. This was particularly important for the planar HASEL actuators (Fig. 2 in this blog post). We were able to utilize the method presented by Yuk, et al. to covalently bond the elastomer and hydrogel. This technique worked very well for the planar HASEL actuators and we didn’t have issues with hydrogel de-bonding. However, there are other instances where adhesion between materials of HASEL actuators could be improved.

First, after bonding hydrogels to the elastomer shell, we encapsulate the hydrogel with a thin layer of Ecoflex (silicone elastomer). This was achieved by spin-coating uncured Ecoflex over the hydrogel electrodes. This layer was not very robust and in some cases it would peel away to expose the hydrogel. One way to improve this would be to incorporate methods like the one mentioned above by Paul Le Floch (Wearable and washable conductors for active textiles)

In the case of the donut (Fig. 1 of this post) and Peano HASEL (Fig. 3) actuators, we simply rely on the ‘stickiness’ of the hydrogels to keep the electrodes on the elastomer or polymer shell. This is sufficient because the electrodes only need to be flexible. However, strong adhesion between the electrode and shell materials would be beneficial for improving fabrication and long term reliability.

Quick methods for bonding hydrogels to surfaces of elastomers and polymers would be very useful for constructing HASEL actuators. The method recently presented by Wirthl et al. for instant bonding of hydrogels could reduce fabrication time. I am less aware of work focusing on bonding hydrogels and polymers. Since the substrate (polymer) is flexible yet not stretchable, there may not be much need for research on cyclic deformation of these material systems. Regardless, a method for obtaining strong adhesion between hydrogels and polymers would be useful for Peano HASEL actuators (Fig. 3) which are made from biaxially oriented polypropylene.

In the future we are excited to see developments in adhesion between different soft and flexible materials. For example, a recent paper by Taylor et al. investigates a simple method for bonding elastomers and thermoplastics. Research in this area broadens the range of useful materials and fabrication methods for HASEL actuators and other soft robotic technologies.

Are there other methods for adhering different materials which you think would be useful to look into? Maybe different materials systems besides hydrogel conductors and elastomers would be more convenient and have other advantages for HASEL actuators?


Eric Acome

Fatigue of stretchable materials under prolonged loads

Sun, 2018-02-04 12:13

In reply to Journal Club for February 2018: HASEL artificial muscles for high-speed, electrically powered, self-healing soft robots

Dear Christoph and all authors,

Thank you for sharing this facile and strong work. As creative and productive as you have always been. In addition, it is really enjoyable to read your little summary of progresses in soft materials, and future challenges.

My colleages and I have recently been quite interested in the mechanical properties and behaviors of soft materials under prolonged loads, i.e. fatigue. Here are several examples of different fatigue behaviors we encountered.

1. Under a static load, a pre-cut hydrogel can sustain the load for a long time, but then suddenly fracture completely. [1]

2. under cyclic loads, a hydrogel is very susceptible to pre-existing flaws, and can fracture gradually by a mechanical load much smaller compared to the critical load to cause catastrophic fracture [1, 2, 3].

3. Under cyclic loads, the material property, such as stress-stretch behavior, is not affected in some soft materials (e.g. single covalently crosslinked PAAm hydrogel), but is dramatically different with loading cycles in some other soft materials (e.g. double-network hydrogels). [2]

4. Also under cyclic loads or prolonged large monotonic stretch, a plastic liquid (such as the carbon crease for DEA) on an elastomer (such as VHB) can form various types of instability patterns. [4, 5]

I can imagine all the above examples can take place somehow in specific kinds of soft robot designs. For example, fatigue under cyclic loads must be an important consideration for soft robots used as grips. However, we have seldom place our scientific studies on fatigue into practical engineering applications, like HASEL. I am wondering, what kinds of such fatigue behaviors have you encountered during your design and application of the HASEL devices? If you have, how did you resolve these issues? What are the still remaining challenges regarding fatigue in these devices, if HASEL is to be employed in broad, industrial-level applications?


Thank you again and best regards,


Bonding hydrogel electrode and elastomeric shell

Sun, 2018-02-04 11:28

In reply to Journal Club for February 2018: HASEL artificial muscles for high-speed, electrically powered, self-healing soft robots

Hello Christpoh and Shane,

Thank you for your previous explanation. I would like to bring up one more question about the adhesion between hydrogel electrode and the elastomeric shell. Have you measured the adhesion energy? Do you see any debonding during actuation due to the large deformation of the shell? In particular, during the repetitive actuation, this bonding interface undergoes cyclic deformation, fatigue of bonding may be potentially an issue. Do you have idea how to overcome this and ensure a long-term reliability? Thank you again!

Interested in adhesion of thin coatings?

Sun, 2018-02-04 05:33

In reply to Adhesion between a power-law indenter and a thin layer coated on a rigid substrate

I see this post is gathering a lot of unexpected attention... Are adhesive properties of micro/nanolayers a hot topic for many of you? Discussions are welcome as we are also thinking at some further developments...

Thanks, I have to say I had a

Sat, 2018-02-03 07:45

In reply to This is your first paper as a single author, congratulations!

Thanks, I have to say I had a good advisor!!! 

This is your first paper as a single author, congratulations!

Sat, 2018-02-03 05:45

In reply to Adhesion between a power-law indenter and a thin layer coated on a rigid substrate

It is always nice to see your former students to develop their own career path, and become independent.

And also, with this very nice and impressibly clean analysis.

I think if you dedicate more attention to this problem, you will find application in nano-thin-films testing.

Notice that the "contact challenge" paper I find so useless...

Fri, 2018-02-02 18:23

In reply to another issue that academic papers often are redundant!!

Has 1700 downloads from Tribology Letters web site.   Not bad!  So here we are, with academics spending so large effort.

We are back to the conclusion that the real business is the publishers' one.

another issue that academic papers often are redundant!!

Fri, 2018-02-02 18:20

In reply to perhaps it is better to enlarge the debate,

See an example here

This describes in summary the attempt to use roughness to interpret friction. 

I argue that we haven't made any progress since the times of Leonardo da Vinci.

Does your new system stop this proliferation of papers, or makes it even worse?

permeation barriers

Fri, 2018-02-02 14:28

In reply to Permeation barriers for hydrogels

Hello Paul,

Thank you for your comment! Your work is fascinating and will surely push wearable ionics to the next level of feasibility. I really like the idea of coating the electrodes in a butyl rubber for protection from various environmental factors. As Tim and Eric have mentioned, the air is extremely dry here in CO, so our hydrogels don’t stay hydrated for long in ambient conditions. When our actuators are not in use, we store them in a makeshift humidity chamber made from an old fish tank sealed with a garbage bag and filled with some cups of water. Frugal science at its best.

On a slightly different but related note, we have noticed some interesting phenomena pertaining to the swelling and permeability of the silicone shells. Initially we tried to use a silicone-based transformer oil as the liquid dielectric, but noticed that the oil would swell and warp the elastomer rather quickly (over the course of a few hours). Eventually we settled on a vegetable based transformer oil (Envirotemp FR3) which works quite well. However, over time frames between weeks and months we noticed that this liquid dielectric begins to permeate through the silicone membrane, causing the actuators to appear to ‘sweat’. This sweating can affect performance if a significant amount of liquid seeps out of the actuator.

One of the most fascinating facts about HASEL actuators is that their structure lends itself to a wide range of materials. It would be very interesting to make the shell from a butyl rubber; then the entire actuator could function while submerged in water. I don’t know much about the dielectric properties of butyl rubbers, nor how they interact with liquid dielectrics; however, the liquid dielectric can be tailored to fit the requirements imposed by the shell material.

Thanks again for your comment!

Hello Paul,

Fri, 2018-02-02 14:15

In reply to Permeation barriers for hydrogels

permeation barriers

Fri, 2018-02-02 14:09

In reply to Permeation barriers for hydrogels

permeation barriers

Fri, 2018-02-02 14:07

In reply to Permeation barriers for hydrogels

permeation barriers

Fri, 2018-02-02 14:07

In reply to Permeation barriers for hydrogels


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