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Updated: 12 hours 36 min ago

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

Tim great job AP

Fri, 2018-02-02 12:26

In reply to Hydrogel discussion response


great job


10 000 signatures! This is going very very fast...

Fri, 2018-02-02 12:11

In reply to Corriere della Sera: recruitment in Italian Academia

Thanks to all the 10 000 scientists signing out petition to change the recruitment system in Italian Academia.


10 mila firme raggiunte! Vorrei ringraziare a nome di tutti gli amministratori e moderatori del gruppo ASN, i tanti firmatari della petizione, che sta andando a una velocità che MAI ci saremmo immaginati. Continuate a seguire la vicenda, e a discutere su come meglio continuare a spingerla e indirizzarla.

Permeation barriers for hydrogels

Fri, 2018-02-02 08:53

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

Hello Christpoh,

The work of your group is very inspiring, and open up new possibilties for soft actuators. This is a breath of fresh air for the field!

I would like to come back on the recent comments of Timothy and Eric about the durability of the device in ambient atmosphere, and in water environment. Our group recently published a paper about wearable and washable hydrogels, which could be useful to design HASEL actuators with enhanced durability: Wearable and Washable Conductors for Active Textiles

We notably show that combining the effect of an hydroscopic salt and an elastomer coating can be a solution to create hydrogel devices that doesn't dry out in ambient air. Also, I would like to draw your attention to the fact that silicone elastomers have extremely high permeabilities toward water and oxygen. Ecoflex is convenient for proof-of-concept devices, but there are other rubbers, such as butyl rubber (a crosslinked polyisobutylene), which have a permeability that is about a hundred times lower than Ecoflex. This material is used to make air-tight chamber, and innerliner of tires. It is actually known to be the elastomer with the lowest permeabilities.

In our paper, we also show that the diffusion of salt (NaCl) from a hydrogel to DI water can be slowed down quantitatively with the presence of a thin butyl rubber coating. Using this material as a coating, HASEL actuators could probably work in water environment for some time.

Thank you for your amazing work! 

perhaps it is better to enlarge the debate,

Fri, 2018-02-02 08:28

In reply to I did not say review

It is not very meaningful if only you and I discuss.  Let us see opinion of others.  At present, me and you are at a dead end, not worth going on.

I did not say review

Fri, 2018-02-02 08:19

In reply to your idea is only apparently innovative

I did not say review accurately reflects the quality of papers. But review scores should somehow reflect the quality of the paper if we still have confidence in peer review. Yes, there are journals publishing reviews, see

I hate to repeat, but I have to say that your test is not related with my theory. What your test can find out is whether somebody would like to give opinions on a paper uploaded on arXiv in a public forum setting, my theory is whether the proposed ideas (localized arXiv, public but anonymous reviews/responses, and better metrics measuring paper impacts) work better for journal publication. In fact, your own comment already disclose that you have seen practical examples: you proposed epi-journals using a global arXiv (local arXiv is a refinement); you already know some journals publish reviews. Then, you have to provide other reasons for dismissing these ideas for a meaningful discourse. 

Hi Xiaoyan, 

Fri, 2018-02-02 03:40

In reply to Amazing work

Hi Xiaoyan, 

Thank you for your kind comments and thoughtful questions about our work.

Your research on electrochemical actuators is very interesting. That is a clever way to turn what is usually a problem – volume change from lithium insertion and extraction – into something beneficial! The ability to drive >10 MPa at 4 V is very impressive, especially with 1 second response time. I am curious to know how these perform after a high number of cycles.

We appreciate your questions about operation in different environments and fatigue life. Below are my answers to your questions.

1)    Whether/how environmental factors (such as temperature and humidity) affects the performance and behavior of HASEL actuators?

In our case, humidity can be an important environmental factor. We have used ionic conductors, namely PAM hydrogel swollen with LiCl, (based on reference 3 above) as our electrodes. While LiCl is hygroscopic, below a certain threshold (~10% relative humidity) water loss of the hydrogel becomes significant. Colorado is a dry climate, yet we only have issues with ionic conductors drying out during the winter months. At higher humidity levels we have not observed any change in performance of the actuators. Some HASEL actuators do not require stretchable conductors and as a result flexible electronic conductors can be used.

We have not investigated the effect of temperature on our devices, but this could be very important to consider for certain applications. We know that properties such as viscosity of the liquid dielectric will depend on temperature. The liquid dielectric we used (Envirotemp FR3) has a pour point of -20 deg C, which means below this temperature the liquid does not flow, which would be detrimental for actuation performance. Understanding the limitations of current HASEL actuator materials and determining materials for extreme conditions will be important to consider in some applications.

2)    How about the fatigue behaviors (especially reliability of actuators under high-cycle fatigue) of HASEL actuator?

We have performed some initial tests of life cycle for HASEL actuators. Donut HASEL actuators were tested for more than a million cycles under a load of 150 g. Under these conditions actuation strain was 15% and we noticed no change in performance after a million cycles. We ended the test for sake of time and not because the device failed. It would be interesting to perform longer term tests for this type of HASEL actuator to determine the maximum number for life cycle.

We also tested fatigue behavior for two other designs of HASEL actuators. A planar HASEL actuator, which is made of silicone elastomer and pre-stretched onto a rigid frame, failed after 158,000 cycles from mechanical rupture. Peano HASEL actuators, made from heat-sealed biaxially-oriented polypropylene failed after 20,000 cycles due to electrical breakdown through the heat-seal. Additional experimental testing and a better fundamental understanding of fatigue of soft and flexible materials would be useful for improving the long-term performance of HASEL actuators.

Thank you again for your comment and questions!

Eric Acome

Amazing work

Thu, 2018-02-01 23:08

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

Hi Christoph, it is an amazing work!

Two years ago, I and my collaborators (Prof. Hujian Gao at Brown and Prof. Hui Wu at Tsinghua) built an electrochemical actuator based on a rechargeable battery of LiFePO4 cathode and Si anode, by taking advantage of the giant volume expansion in Si anode microparticles after full lithiation. Our electrochemical actuator of the LiFePO4||Si battery can drive a high load greater than 10 MPa with a device response time less than 1 second. The driven voltage of the device is less than 4 V, which is two order-of-magnitude lower than that of piezoelectric materials. Such actuator might be used for the robotics by the clever design. The relevant paper is titled by Cycling of a Lithium-Ion Battery with a Silicon Anode Drives Large Mechanical Actuation.

Your HASEL actuator couples the hydraulic and electrostatic force, and exhibits the muscle-lie performance. I am curious on two aspects of your HASEL actuators: (1) whether/how the environmental factors (such as temperature and humidity) affects the performance and behavior of HEASEL actuator? (2) how about the fatigue behaviors (especially reliability of actuators under high-cycle fatigue) of HASEL actuator? Thank you very much in advance!

Hydrogel discussion response

Thu, 2018-02-01 18:55

In reply to Wonderful work!

Hello Canhui,

We are thrilled you appreciate the "simple yet effective" solutions we have presented with HASEL. We also hope our opportunities and challenges section leads to some other expert community members of iMechanica in joining us in pushing the boundaries of HASEL. 

Your questions about the hydrogel are very important to us. I answer them directly below

1. Whereas hydrogels can retain water with dissolved hygroscopic salts, the water content fluctuates with the ambient humidity. Does the fluctuation of water content (i.e. resistance) of hydrogels affect the device performance? 

The short answer, yes ambient humidity can influence the properties of the hydrogel. Ambient atmosphere is especially important in climates such as ours, in Boulder, Colorado where we have very dry air, especially in the winter months. We found that in most atmospheric conditions the hydrogel would remain hydrated for hours and there was no significant influence on the performance of the device. However, in the drier winter months, it is true our hydrogels may dry out which may increase the resistance of the electrode. However, since our HASEL devices currently use high voltage, the changes in resistance has minimal influence on actuation performance. We have not done a comprehensive study on atmospheric conditions, hydrogel resistance, and actuator performance. That may be very useful for advanced lifetime test.

Also, we are very fortunate to "stand on the shoulders of giants" and follow the hydrogel recipe developed by Y. Bai. et. al. in Professor Suo's research lab.  These PAM LiCl hydrogels have very good water retention properties. Do you know of other hydrogel compositions that we should consider for enhanced water retention? 

Lastly, in some cases, we encapsulated the hydrogel in a thin layer of ecoflex. The ecoflex was applied to the hydrogel via a spin coating process. This thin layer of ecoflex helps keep water in the hydrogel and does not significantly influence mechanical properties of the actuator. Still, ecoflex is somewhat permeable to water vapor and while improving water retention of the hydrogel electrode, it does not completely solve all hydrogel hydration issues.  The supplemental material section of the Science paper clearly discusses which systems are encapsulated and how.

2. The hydrogel electrodes may contact with other materials, and short circuit might happen if the materials are conductive. 

This is true, if the electrodes came in contact with conductive material this would cause a short circuit. In real-world application we feel this is not a common situation. Additionally, when the device is encapsulated with a thin layer of ecoflex or Kapton film, this insulating layer will make shorting the electrodes even less likely. 

Along the lines of electrodes contacting other materials, it may be possible that the HASEL device is in an environment where the electrodes are physically damaged (cut, scratched, etc.) We think it may be very interesting to incorporate self-healing conductive materials such as work we published in Advanced Materials last year (ref 20 above, 

3. Using hydrogel makes the overall device transparent. But the device cannot work in water environment.  Have you found potential solution for this issue?

Again, this is true, that this device may not work in a water environment as the hydrogels would be influenced by the surrounding water. For an application such as this we can speculate a few possible solutions. 

The first possible solution may be to look for other transparent ionically conductive electrodes such as ionogels. Again, the self-healing material we collaborated with Chao Wang on (ref 20 above) might be one possible material, plus this material has the added benefit of physical self-healing. This material used ion-dipole interaction to enable both self-healing and to keep the ionic liquid in the polymer matrix. The transparency of this material was influenced by atmospheric water vapor so this self-healing material may not be the perfect electrode for a water environment, but might point you in the right direction.

A second possible solution could again be the encapsulation route. Simply sealing the hydrogel inside a polymer that is not permeable to water might allow the device to operate in a water environment. 

Lastly, it is highly possible instead of using ionic conductors, to operate HASELs with electronic conductors. Peano-HASEL has already been demonstrated with electrical conductors. It is, of course, unlikely this device would still be transparent using metallic layers

Do you have any other ideas for this? What specific application did you have in mind, sounds interesting!

Thanks again for your comments. Please let us know what you think of our ideas to address them and reach out with any new questions.

your idea is only apparently innovative

Thu, 2018-02-01 18:10

In reply to Mike, as I said previously, I

Reviews are hardly the quality of journal papers. There is a service already to publish reviews, I forget the name, but there is.

Yes, you don't see the connection with reviewing my paper.  The point is theory to practice.  At the moment, your ideas are theories. Maybe good one, but I don't buy them, until I see them at work in practical example.  And I gave you an example, which you refuse to consider.

This is for me good enough.  You don't prove your ideas, then we are talking of commercial inventions.

Mike, as I said previously, I

Thu, 2018-02-01 18:06

In reply to Wenbin, I remain disappointed you don't want to try

Mike, as I said previously, I have difficulty to figure out the connection between reviewing your paper and my proposed journal ideas. I am not seeing that getting good reviews is easy, but my proposed ideas should help getting better reviews because these reviews will be seen by the community and benefit the community. Comparing to the epi-journal what you have proposed in another thread (epi-journals just cut down storage cost by piggybacking arXiv, which is not much and cdmHUB provided it for free anyway), I thought that these ideas offer further improvements. 


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