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Updated: 12 hours 35 min ago

Congratulations, Christoph!

Wed, 2018-02-07 16:17

In reply to Great job, Christoph!

Hi Christoph,

Congratulations on these two stellar papers! I am glad to see the field of dielectric elastomer actuators maintains so vibrant after almost two decades, thanks to innovative ideas/works from colleagues like you, Siegfried and Zhigang. Two quick questions, in addition to the on-going discussion.

1. Technical one. What is the energy efficiency of the HASEL acutator? While fluid is involved in the actuation process, I guess the energy efficiency of the actuator may be still quite high, comparable to viscoelastic DEAs.

2. Practical one. High-voltage can be a concern, as you make the power density of the actuator higher. What will be the field of targeted applications of HASEL acutators? 

BTW, thanks for referring to our works on harnessing instabilities in soft materials and tough bonding of hydrogels. 

Contact mechanics challenge results: a follow up discussion

Wed, 2018-02-07 16:16

In reply to a "contact sport" between academics

Featured in this nice tribology web site


Thank you

Wed, 2018-02-07 04:56

In reply to Dear Omkar,

Dear Lihua,

Thanks alot for your comment. I really appreciate your inputs on this subject.

Best regards,


Response to your questions

Tue, 2018-02-06 01:28

In reply to Fabulous work! Congratulation!

Dear Jian,

Thank you for your kind words about our recent work. We are beyond grateful for the overwhelming support and encouragement we have been experiencing from the academic community. To answer your questions:

1.) The donut HASEL actuators seen in Fig. 1 follow Pascal’s hydraulic principles ( The Maxwell stress induces a hydraulic pressure within the shell, which is independent of the electrode size. This pressure acts on the shell, with the output force dictated by the surface area of the shell which is in contact with the load. Thus, more surface area in contact with the load during actuation means more force. We demonstrated that an actuator with a smaller electrode yields a larger force than an identical actuator with a larger electrode. However, the smaller electrodes move less fluid during actuation, and therefore this actuator achieved less strain. The actuator with the larger electrode is the exact opposite (lower force but greater strain).  By simply varying the size of the electrode, one can tune the performance of the actuator to suit a specific application.

2.) The density of the liquid dielectric we used in our papers is 0.96 g/cm3.

3.) HASELs are driven by Maxwell stress which is proportional to the electric field squared and the dielectric constant of the composite dielectric. Therefore, one could utilize a composite dielectric structure with very high permittivity to achieve high Maxwell stresses at lower voltages. An easier approach to lower the operating voltages would be to use thin shell membranes with high dielectric strength, since the Maxwell stress has a squared dependence on the field. Peano HASEL actuators do exactly that, and utilize a thin but flexible polymer shell.

Lowering the operating voltage below 1kV is certainly a goal of ours. However, I would argue that with the material system utilized in the Peano HASEL paper, untethered operation is already possible. There are a few companies (EMCO and Pico Electronics) which specialize in miniature DC-DC high voltage amplifiers which can be powered by a cell phone battery and output 10kV. We currently have multiple untethered demonstrations which we will be showcasing at SPIE in Denver CO in the beginning of March.


With that being said, lowering the voltages even more is a necessity to further decrease the complexity of the electronics. This decrease in complexity will lead to even smaller electronic packages that can switch at very high speeds and are extremely lightweight.


New materials and smart fabrication techniques

Tue, 2018-02-06 01:23

In reply to Addendum

Hello Kevin,


Thank you for your insightful comments and suggestions.


The motivation behind this work was to present a new idea that can push the limits to soft actuator technology and open up more possibilities of what we can do. Hence, we resorted to using readily available materials to demonstrate the concepts.


  1. Yes, the PDMS should be permeable to gases. However, it is a relatively slow process. Given enough time, the gas generated during a breakdown should be able to permeate outside.
    New materials that allow fast permeation of gases will improve the performance and reliability following dielectric breakdowns, since the idea is to have the actuator up and running immediately following any breakdown event, without temporarily compromising on the dielectric strength until the gas diffuses out of the system.

  2. That is a great suggestion on hygroscopic salts! For this paper, we used materials that we had prior knowledge about and immediate access to. Hence we used LiCl as it was shown to be more hygroscopic and a good fit for use in conductive hydrogels, as opposed to NaCl or other common salts.

    For future work, we will take cues from your suggested reference and experiment with other salts that can help to prevent drying of hydrogels in low-humidity environments.

  3. In the Peano-HASEL design, we use strips of copper tape as an intermediary to connect the power source to the hydrogel electrodes. At the interface of the copper and hydrogel, we did notice some chemical reaction forming a tiny bit of salt deposit. (green colored deposits, mostly Copper Chloride formed by Cu reacting with the LiCl). However, it did not affect the performance or conductivity of the electrodes. Using a less reactive material system might be able to prevent this from happening.

The Peano-HASEL actuators benefit from the use of an inextensible material as the shell, giving us a wide material selection, most of which are heat-sealable thermoplastics. This makes it simple to heat-seal the pouches, similar to hot embossing, followed by filling with a liquid dielectric. This should be a very simple process to achieve in an industry. That was the motivation behind using heat-sealable BOPP sheets and demonstrating the ease of fabrication.

The planar HASEL actuators on the other hand need an elastomeric system. They could be fabricated from thermoplastic elastomers such as Thermoplastic Polyurethanes. This would potentially make the fabrication process as simple as of Peano-HASELs. Developing thermoplastic elastomers with performance comparable to silicone rubbers would be great for making HASELs.

3D printing is a wonderful method to fabricate complex geometries of HASELs. Like you have noted, we need to use a filler material that can be purged out to create the cavity needed for the liquid dielectric. The paper from Jennifer Lewis’ group on using fugitive inks presents a very simple and scalable method of doing this. Thank you for sharing that. Most 3D systems cannot print multiple materials, and have to use support structures made of the same material while printing with cavities, which remain inside permanently. This could compromise on performance especially in the case of HASELs which take advantage of a completely liquid dielectric. However, with multi-material printers and such interesting solids that are water or alcohol-soluble, we could have support material that can be purged out completely and have clean cavities to fill with dielectric.

Are you aware of any commercially available printable materials that are water soluble? This group for example has presented an ingenious idea using melted sugar as a soluble support material. (

In the last 1-2yrs, there has been a rise in 3D printers capable of working with silicones and gels. So we can soon expect to see developments in rapid prototyping of these actuators, especially with interesting geometries.


Fatigue in HASEL actuators

Mon, 2018-02-05 21:12

In reply to Fatigue of stretchable materials under prolonged loads

Hi Ruobing,

Thanks for the resources on fatigue behavior in these soft systems – particularly hydrogel! It would certainly be beneficial to investigate fatigue behavior in these actuators in more detail.

From what we have noticed, the sort of fatigue and crack propagation that is investigated in [1-3] does not occur in our actuators. In the highly-elastic planar HASEL actuator demonstrated in the first HASEL paper published in Science, we use the benzophenone treatment from Yuk et al. (ref 4) to bond the hydrogel to our elastomeric actuator shell. This reinforcement prevents any sort of crack formation or propagation. In the Peano-HASEL paper, we are using flexible but inextensible materials as the actuator shell, so the hydrogel does not experience any significant strain. The only mechanical failures we have observed occurred in hydrogels that had dried out and become brittle.

As you may have seen in the two HASEL papers, we conducted initial lifetime tests for the three types of actuators presented. The results underscore the fatigue present in these actuators. The donut HASEL actuators actuated for > 1 million cycles without degraded performance or failure (we stopped testing for the sake of time before actuator failure). The soft compliant material used and low material strain likely contributed to the very high lifetime. The planar HASEL actuators lasted for ~ 160,000 cycles before mechanical rupture. While these use a very elastic silicone polymer, the high loads (1 kg) used during testing exert large stresses on the material which lead to mechanical fatigue and failure. Finally, the Peano-HASEL actuators tested actuated ~ 20,000 cycles before failure. Here we use an inelastic system that depends on bending and buckling in a polypropylene shell for actuation. As you can imagine, this system is very susceptible to fatigue. In fact, during many of the lifetime tests, failure occurred through the heat seal in an area subject to repeated bending/buckling during actuation.

The work shown here was more about introducing HASELs as a new platform for soft actuator technology. Moving forward, there are many materials/geometries we could explore that would minimize the fatigue experienced. Speaking for Peano-HASELs, I think that using a more compliant material in areas that undergo significant deformation would limit this fatigue. That, and improved actuator geometries that spread deformation over a larger area rather than localized creasing and buckling.

Hopefully that gives you a little more context for the fatigue in our actuators. It’s certainly an important subject of research – I’m sure the soft robotics community will come up with some creative solutions!

Let us know if you have any more questions!

Self-healing and lower voltage

Mon, 2018-02-05 19:56

In reply to Dear Christoph:  Great work!

Hi Tiefeng.

Thank you very much for the congratulations. We are indeed familiar with your electronic fish work which we find very fascinating. I particularly enjoy the use of the liquid as an electrode in a DEA! To answer your two 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? 

That is correct. The liquid is enveloped by an elastomeric shell which suffers damage during breakdown. The shell is shown as the white material in Fig 1A above as well as the grey material in Fig1F above. Please note in Fig 1F that after breakdown a small area of the elastomer shell is damaged. We found that the elastic behavior of the ecoflex and PDMS material allowed the damaged area to essentially reseal and keep the liquid inside the device even though there was now a small hole present. The use of elastic hydrogels on the outside also helps to ensure the liquid stays encapsulated even after breakdown.  Figure S14 of the Science paper discusses breakdown a bit more.

 Additionally, it is important we point out that the dielectric strengths of the liquid and shell material should be similar for full self-healing from electrical damage. In the case of Peano-HASEL which uses non-eleastic BOPP, we can apply high electric fields before any breakdown, since the BOPP has very high dielectric strength. After the first breakdown however, not only does the shell leak because the BOPP does not reseal, but there is a path of purely liquid dielectric between the electrodes. Since the liquid has a much lower breakdown strength than the BOPP, you can't apply the same high field as you once could before the initial breakdown. We think a use of a mechanically self-healing material such as we discussed in [19,20] and in our initial post above, might be very interesting for future work. 

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?

Yes, we completely agree the use of high voltage is still a challenge for these devices. As you are also aware, it is the electric field that dictates DEA and HASEL performance, rather than the total voltage. Using thinner materials will significantly reduce required voltage; we have used think and easy to fabricate layers for our proof of concept work in Science. We also hope to explore new materials that have different physical characteristics such as permittivity. We are also interested in exploring different classes of dielectric liquids which might help this cause. 

We are confident to have some of this work requiring much lower voltages ready for display at SPIE next month. If you are joining that conference you should certainly plan on checking out the EAP in action session.

[19] C.H. Li, C. Wang, C. Keplinger, J.L. Zuo, L. Jin, Y. Sun, P. Zheng, Y. Cao, F. Lissel, C. Linder, X.Z. You, A highly stretchable autonomous self-healing elastomer. Nature chemistry, 8(6), (2016), p.618.

[20] Y. Cao, T. G. Morrissey, E. Acome, S. I. Allec, B. M. Wong, C. Keplinger, C. Wang, A transparent, self-healing, highly stretchable ionic conductor. Advanced Materials 29, 1605099 (2017).


Mon, 2018-02-05 13:13

In reply to Fascinating work!

I cannot seem to edit my above posting, so I apologize as it was somewhat incomplete.  On the matter of the air-bubbles, having re-read the paper it's a bit more apparent why the issue arises in your design, so do disregard that comment.  Though having an elastomeric sealing composed of just PDMS should allow for diffusion of gas through the membrane (although as noted previous, it also allows for quite a few others to diffuse through it too).  

On the matter of fabrication I had intended to mention that in extrusion printing, I could foresee the usage of something like Jennifer Lewis' fugitive inks to print cavities in an elastomeric material that could then be filled post-print.  The capability to freely selectively bond materials (beyond using a filler material to avoid it, like the injection of air as you have actively done so) doesn't seem present in the field just yet.



Fascinating work!

Mon, 2018-02-05 13:03

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

Greetings Christoph (and Co.),

Great work! This certainly advances many possibilities with soft-robotics, so I'm excited to see where this work leads.  A couple of notes/questions I had:

1. Regarding the air-bubbles: since you're using PDMS, I'm surprised that the gas doesn't simply permeate through the elastomer

2. Regarding Water-retention: Paul already discussed this above, but I wanted to add that LiCl itself is not the most hygroscopic salt out there, so if water-retention is all that you're gunning for you can opt to try other salts with even lower equilibrium RH (see Greenspan).  

3. A few folks have asked about breakdown, but I'm curious (especially since it's a square-wave/high-voltage signal) whether you've noticed any chemical reactions related to the hydrogel electrodes?

4. The fabrication process you've described seems closest to a hot-embossing of polymer films (as least for the peano-HASEL), which certainly would make it very amenable to scaled up fabrication.  I was curious about which advanced fabrication methods you thought were most applicable/promising to your system, as the need for a cavity to fill with the liquid dielectric does complicate the process.  Implementing 



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!


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