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Updated: 5 hours 28 min ago


Wed, 2018-12-05 04:22

In reply to good point, Kiriakos

Certainly Mike, I will have a look at your work. Seems very interesting.

We've done some similar work on rail steel ratcheting simulation, which can be found here (employed the Multi-AF model with an alteration to take into account yield stress variation in depth):

Multiscale mechanics of tough adhesion of soft materials

Wed, 2018-12-05 00:03

In reply to Journal Club for December 2018: Bonding hydrophilic and hydrophobic soft materials for functional soft devices

Hi Qihan,

Thanks a lot for your nice and timely review! I really enjoy reading it and the following discussion. I would like to add a few more thoughts of the important roles that mechanician can play in this important field, following the same line of your discussion with Hyunwoo and Shaoting.

At the very high level, the mechanics of adhesion requires models of interface, bulk, and their coupling. That's what we did in the coupled cohesive-zone and Mullins effect model for the tough adhesion of hydrogel as Hyunwoo discussed. Similar work has also been done for adhesion of plastic materials [1,2] (I only choose two representative work, and defintiely we can find much more). As for the tough adhesion of soft materials, more complicated scenario can emerge due to the nonlinear deformation in bulk and materials near the interface. For example, Hyunwoo showed the intefacial toughness of tough hydrogel depends on the peeling velocity, and possible cavitation can happen [3]. More discussion about the nonlinear mechanical behaviors of soft material adhesion can be found in the review by Dr. Creton and Dr. Ciccotti [4]. I want to briefly highlight the nature of multiscale mechanics of this problem. 

1. At the interface, we need to deal with interactions at atomic and polymer chain level, which are the origin of the response to external stimuli, such as PH value and temperature. 

2. In the bulk, we may have local damage (e.g., Mullins effect), viscoelasticity, fracture, and cavitation. These can vary from mesoscale (\micrometer) to macroscale (mm).

3. The coupling, the interfacial strength will influnce the size of the nonlinear deformation zone, which will also in-turn affect the crack tip stress field [5,6]. This strong coupling makes the modeling and experiments of tough soft material adhesion pretty challenging. Even more challenging, various instabilites, such as fingering, fringe, and fibrillation can happen at the interface or in the bulk materials near the interface [4]. This may sometime make it difficult to clearly distingwish the interface and bulk, and thus requires new multiscale models [7,8]. 

So, I think we do have a universal mechanism of designing tough soft material adhesion: Strong bonding + Energy dissipation. But we may not have a universial material/structure that can form tough bonding under various conditions, at least to my knowledge. This may also indicate the important roles of mechanics in this highly interdisciplinary field, which is to bridge different fields, such as connect bio-engineering with chemestry and polymer physics. Mechanics can quantify the effect of the bulk and interface properties and identify the important factors in certain devices and applications, which can then guide the choice of chemistry and polymer synthesis. In other words, mechanics is the interface in the research community/team of interfaces in soft materials.



1.Kim, Kyung-Suk, and Junglhl Kim. "Elasto-plastic analysis of the peel test for thin film adhesion." Journal of Engineering Materials and Technology 110, no. 3 (1988): 266-273.

2. Wei, Yueguang, and John W. Hutchinson. "Interface strength, work of adhesion and plasticity in the peel test." In Recent Advances in Fracture Mechanics, pp. 315-333. Springer, Dordrecht, 1998.

3. Yuk, Hyunwoo, Teng Zhang, Shaoting Lin, German Alberto Parada, and Xuanhe Zhao. "Tough bonding of hydrogels to diverse non-porous surfaces." Nature materials 15, no. 2 (2016): 190.3.

4. Creton, Costantino, and Matteo Ciccotti. "Fracture and adhesion of soft materials: a review." Reports on Progress in Physics 79, no. 4 (2016): 046601.

5. Long, Rong, and Chung-Yuen Hui. "Crack tip fields in soft elastic solids subjected to large quasi-static deformation—a review." Extreme Mechanics Letters 4 (2015): 131-155.

6.Qi, Yuan, Julien Caillard, and Rong Long. "Fracture toughness of soft materials with rate-independent hysteresis." Journal of the Mechanics and Physics of Solids (2018).

7.Villey, Richard, Pierre-Philippe Cortet, Costantino Creton, and Matteo Ciccotti. "In-situ measurement of the large strain response of the fibrillar debonding region during the steady peeling of pressure sensitive adhesives." International Journal of Fracture 204, no. 2 (2017): 175-190.

8.van der Sluis, Olaf, Tijmen Vermeij, Jan Neggers, Bart Vossen, Marc van Maris, Jan Vanfleteren, Marc Geers, and Johan Hoefnagels. "From fibrils to toughness: Multi-scale mechanics of fibrillating interfaces in stretchable electronics." Materials11, no. 2 (2018): 231.

p.s. more explanation of the stress-life approach

Tue, 2018-12-04 15:53

In reply to good point, Kiriakos

Notice that in the stress-life approach, the notch and surface finish (as well as the load-type factor) are all treated similarly by modifying the SN curve slope b.  This has the effect of changing the slope of the material's SN curve and leaving the intercept unchanged. If we include the modifying factors this new slope, bnotch, can be computed as

and the new notched SN curve will be given by

In the strain-life, as I summarized below, only surface finish remains to modify b-notch, because the other three are not included.  However, even the surface finish one doesn't seem to work properly.  It is perhaps that the entire SN curve is shifted?   I would need to make more numerical tedious experiments to find out.

p.s. another apparent bug

Tue, 2018-12-04 15:50

In reply to Stress-life vs strain life approaches in ?

Another bug in the software seems to be that if I forget to specify the notch radius, the software doesn't stop me, and I am not sure what is the Kf factor computed, given Kt.   Since they use classical equations similar to Peterson', one needs the ultimate strength of the material, but also the notch radius, to obtain Kf, the true effect in fatigue.

good point, Kiriakos

Tue, 2018-12-04 15:48

In reply to What types of cyclic


  the technical background they use is explained here.   The main calculation is using Neuber's rule, with the cyclic curve of the material (which indeed uses RO equation only). The surface finish coefficient is "included in the analysis by altering the slope of the elastic portion of the strain-life curve". The surface finish corrected slope is given by

It is important to note that this correction should be done after the cyclic strength properties are determined.

Now, this means that at static failure, this correction doesn't work, which is fine because one assumes there are no surface finish effects at static failure.  But is this done also in stress-life approach?  It seems so, if one reads the technical background for stress-life approach here.   So it is a mistery what they do, maybe it is a bug in the software?    I notice that even ALTAIR supports this software.   Prof. Socie would not comment further.


By the way, I notice you are an expert of ratchetting.  Long time ago I tried to understand Rolling Contact Fatigue by ratchetting, following the indication of KL Johnson from Cambridge.  However, ratchetting in RCF occurs over millions of cycles and it is extremely delicate to model.  See my two papers here and here I would be interested to know your opinion.





What types of cyclic

Tue, 2018-12-04 14:02

In reply to Stress-life vs strain life approaches in ?

What types of cyclic plasticity models they use? I have checked the website but it is not clear (unless they use the Ramberg-Osgood equation only).

Dear Qihan,

Mon, 2018-12-03 23:40

In reply to Hi Hyunwoo,

Dear Qihan,

I agree that if bulk dissipation is gigantic, weak interfacial interaction can give good interfacial toughness. Actually, we sometimes observe very strong bonding of tough hydrogel on clean substrate due to stickiness of gel although such bonding goes away upon swelling. This can be relevant example we experience daily! Maybe good balance between interface and bulk would be good way to describe the strategy to achieve tough strong bonding I guess.

Your point on weak-interface based preservation of device is really interesting. It is so true that people will not like to see cohesive failures of their hard-made devices! Probably discussing with device experts can give more guidance for future development for people like us.

Hi Zhijian,

Mon, 2018-12-03 21:54

In reply to Dear Qihan,

Hi Zhijian,

Thank you for the valuable input on the chemistry side. For your questions:

(1) There are tons of applications of reversible adhesion in non-stretchable materials (e.g. velcro). Just imaging generalizing these applications to stretchable mateirals can lead to a lot of interesting ideas.

(2) Intuitively yes. But it is quantitatively unclear what do we mean by "strong". More modeling work down to the molecular level must be done before we can connect this intuitive picture to bond strength and do bottom up design from basic chemistry.

(3) If the hydrogel is brittle than you can simply fracture the hydrogel instead of the interface. I would doubt that one can achieve tough adhesion in this case.

Very nice works Shaoting!

Mon, 2018-12-03 21:37

In reply to Mechanics in designing soft and tough adhesion

Very nice works Shaoting! Actually I'm most interested in your ref [6] unpublished work: how these instability influences adhesion. Looking forward to read your paper soon!

Hi Hyunwoo,

Mon, 2018-12-03 21:33

In reply to Dear Chenghai and Qihan,

Hi Hyunwoo,

Thanks for the thoughtful comments. Actually your model shows that if χ·hmax is big, even weak interfacial adhesion can result in good overall adhesion. And such kind of weak yet tough adhesion can be useful as well. A bonding as such can have a decent toughness yet failed on the interface, leaving the materials on the two sides intact. This is actually a better situation for device reparation than cohesive failure, isn't it?

Dear Qihan,

Mon, 2018-12-03 19:10

In reply to Journal Club for December 2018: Bonding hydrophilic and hydrophobic soft materials for functional soft devices

Dear Qihan,

Thanks for your timely summary on the tough adhesion between hydrogels and elastomers. Very interesting topic and impressive work! In chemistry community, reversible bonding, including supramolecular chemistry, ionic interactions and dynamic covalent bonds, is also a very hot topic and lots of works have been reported. These developments in chemistry have also been widely applied in making tough hydrogels and elastomers, and may be helpful in designing reversible tough adhesions. I would like to add a few examples for the discussion:

(1)  Host-guest interactions between cucurbituril/cyclodextrin and ferrocene /adamantane/ azobenzene (Angew. Chem. Int. Ed. 2013, 52, 3140; Chem. Sci. 2014, 5, 3261-3266);


(2)  Quadruple/triple hydrogen bonding (JACS, 2014, 136,19,6969-6977);


(3)  Dynamic covalent bonds, like Schiff base (Polym. Chem. 2012, 3, 3045-3055); 


(4)  Ionic interactions.

As discussed above, the tough adhesion comes from the dissipation in the bulk. It may be necessary to design the interfacial interaction and the weak interaction in the bulk materials carefully. Some insightful theoretical understanding may be helpful. We may also use the surface treatment method to modify the elastomer with these functional groups as anchors.


I also have several questions:

(1)  What are the potential applications of reversible adhesion?


(2)  The tough adhesion comes from the dissipation in the bulk material, usually the hydrogel part. Does it mean that it is good enough if the bonding between the interfaces is stronger than the weak interaction in the hydrogels?

(3)  In some scenarios, the hydrogels are not tough hydrogels. Is it possible to achieve tough adhesion in these situations?

Dear prof. Suo

Mon, 2018-12-03 09:26

In reply to Excellent review of a rapidly emerging field

Dear prof. Suo

Thank you so much for kind comment! We hope this review is enjoyable and informative to read!

Excellent review of a rapidly emerging field

Mon, 2018-12-03 05:22

In reply to Our new review on "Hydrogel Bioelectronics"

Congratulations on this great work.  I have already sent the review to the students here yesterday.

Dear Chenghai and Qihan,

Mon, 2018-12-03 00:29

In reply to Dear Chenghai,

Dear Chenghai and Qihan,


Very insightful perspectives indeed. I hope to add few comments.

(1) From our model [1], the total interfacial toughness actually linearly scales with intrinsic adhesion energy as 

Γ = Γ0 / (1 - χ·hmax) 

where Γ0 is the intrinsic interfacial energy (accounting interfacial interaction), hmax is the maximum hysterisis ratio (accounting bulk dissipation). 

So weak interracial adhesion energy probably not result in high total interfacial toughness, unless new mechanisms can be discovered. Maybe, this is interesting area that need further discoveres and exploration. For example, the new mode of instability [2] Shaoting discovered can be extremely important, as it may give new relations beween intrinsic and total interfacial toughness, independent of chemistry.

(2) More tailored chemistry is indeed one of the biggest remaining challenge in the field. As Qihan mentioned, fast chemistries are generally more toxic (as EDC is fast but also toxic in high concentration). Cyanoacrylates are very toxic but its fast reaction and polymerization are genrally regarded as limiting factor for the toxicity (as higher molecular weight cyanoacrylate is actually got FDA approval for on-skin usage). One interesting question to ask might be whether these optional considerations such as reaction-rate, toxicity et al are important considerations for the target applicaitons. The dilute Cyanoacrylates by Wirthl [3] seems to be a good option for adhering hydrogels by interpenetrating into the networks of both gels and forming strong linkages, which is fast, simple and commercially available. We achieved similar strong linkages with the interpenerating method but did not publish the results.


[1] Teng Zhang#, Hyunwoo Yuk#, Shaoting Lin, Xuanhe Zhao*, Tough and tunable adhesion of hydrogels: experiments and models, ​Acta Mechanica Sinica​ 33​, 543-554 (2017)

[2] Shaoting Lin, Tal Cohen, Teng Zhang, Hyunwoo Yuk, Rohan Abeyaratne, Xuanhe Zhao*, Fringe instability in constrained soft elastic layers, Soft Matter 12, 8899-8906 (2016)

[3] Wirthl, D., et al., Instant tough bonding of hydrogels for soft machines and electronics. Science Advances, 2017. 3(6).

Design, modifying and applications of adhesion chemistry

Sun, 2018-12-02 22:36

In reply to Dear Chenghai,

Dear Qihan,


Thanks for insightful perspectives. Some ideas as follows:


1 The tough adhesion is obtained when the strong interfacial interaction activates the bulk dissipation. And as you said, it may still lead to tough adhesion if a weak interfacial interaction activates a weak dissipation mechanism. Thus, theoretical mechanical understanding may be useful to design and determine the limits of interfacial interactions and bulk toughness. People could design further desired adhesions based on deeply mechanical understandings.


2 Just as Canhui said, every adhesion method has its own advantages and limitations. Everybody could modify the method to fulfill their own needs. Based on certain modifications, we (I and other coauthors) have shown some results that may solve urgent issues in other fields. The paper will come soon to demonstrate this.


3 Actually, Jiawei demonstrated the pioneering detachable tough adhesion in his Advanced Materials paper using pH. I think it’s also important to figure out the possible applications of detachable adhesions.




Mechanics in designing soft and tough adhesion

Sun, 2018-12-02 21:04

In reply to Journal Club for December 2018: Bonding hydrophilic and hydrophobic soft materials for functional soft devices

Dear Qihan,


Thank you for leading the discussion on this extremely important and timely topic. As Hyunwoo mentioned, the mechanism for strong adhesion of soft materials is the synergy between strong linkages at interfaces and bulk dissipation [for example in hydrogels, see 1].


While the linkages at interfaces have been developed by chemists over decades [2], as a mechanician, I am interested in understanding how large deformation and instabilities of bulk soft materials affect the adhesion. In particular, I discovered the fringe instability when detaching thin soft materials (such as a rubber band) from rigid substrates [3]



I further developed a phase diagram to categorize various types of instabilities on soft-hard material interfaces based on their geometries and material properties [4].



In addition, I discovered that stiffening of the soft materials can prevent various instabilities [5]. 



Based on these understanding and discoveries in mechanics, I am designing soft, tough and strong adhesion that only uses simple and common chemistry and broadly applicable and reproducible [6]. 


I believe that the understanding of mechanisms and mechanics, qualitatively and especially quantitatively, is our mechanicians' powerful tool and potential contribution to this interdisciplinary field.


I would like to learn your opinions. Thank you very much!



[1] Hyunwoo Yuk, Teng Zhang, Shaoting Lin, German Alberto Parada, Xuanhe Zhao*, Tough bonding of hydrogels to diverse non-porous surfaces, Nature Materials 15, 190-196 (2016)

[2] Handbook of Adhesion Technology, by Lucas F. M. da SilvaAndreas ÖchsnerRobert D. Adams

[3] Shaoting Lin, Tal Cohen, Teng Zhang, Hyunwoo Yuk, Rohan Abeyaratne, Xuanhe Zhao*, Fringe instability in constrained soft elastic layers, Soft Matter 12, 8899-8906 (2016)

[4] Shaoting Lin, Yunwei Mao, Raul Radovitzky, Xuanhe Zhao*, Instabilities in confined elastic layers under tension: fringe, fingering and cavitation, Journal of the Mechanics and Physics of Solids 106, 229-256 (2017)

[5] Shaoting Lin, Yunwei Mao, Hyunwoo Yuk, Xuanhe Zhao*, Material-stiffening suppresses elastic fingering and fringe instabilities, International Journal of Solids and Structures 139-140, 96-104 (2018)

[6] Unpublished

Very insightful! Thanks!

Sun, 2018-12-02 20:34

In reply to Bonding dissimilar materials

Very insightful! Thanks!

Dear Chenghai,

Sun, 2018-12-02 20:32

In reply to Adhesion between hydrogels and diverse substrates

Dear Chenghai,

Here are my perspectives on these questions:

(1) As Hyunwoo has shown, toughness of adhesion come from the dissipation in the bulk. As long as the interfacial interaction is strong enough to activate the bulk dissipation, tough adhesion is possible. I would guess if we can design a weak dissipation mechanism, then a weak interfacial interaction can still lead to tough adhesion.

(2) This is a very good point. There are still many manufacturing needs unmet by the existing methods. Your specific question would need someone with chemistry expertise to answer. But generally speaking, quick reaction requires reactive reagent and the more reactive the reagent the more toxic it would be. In fact, the EDC chemistry used is still toxic as it modifies native protein, maybe just not as bad as cyanoacrylate.

Bonding dissimilar materials

Sun, 2018-12-02 20:15

In reply to Journal Club for December 2018: Bonding hydrophilic and hydrophobic soft materials for functional soft devices

Dear qihan: Thank you for this timely summary of a nascent yet fast-evolving field. We can envision tremendous opportunities in this new field.

In terms of bonding dissimilar materials, specifically hydrogels and elastomers, all existing strategies (surface treatment, gluing, bulk modification) have found usages. But their limitations have also been noted: the requirement of in-situ polymerization, biocompatibility, and the alteration of bulk properties.  So current status is that, so long as the bonding mechanism is clear, one will chose the most suitable method according to their own applications.

Whereas a more general bonding method might be created, one can use a combination of current methods or use the methods in a modified way to meet new requirements.  For example, based on the bulk modification method, we have developed new technique that are mold-free and oxygen tolerant to make hydrogel coatings on medical devices of complex geometries with strong interficial bonding and tunable thicknesses.

In addition to your questions, strong bonding that can be formed on demand while can also be removed on demand is still missing. In particular, the bonding-debonding process should be able to be repeated for a number of cycles.

Hi Jiawei,

Sun, 2018-12-02 20:14

In reply to Topology of wet adhesion

Hi Jiawei,

Nice point! Being able to form bonding without chemically modifying the substrate is certainly important. And this is required if a glue wants to be generaly applicable to different substrates. A few questions:

1. Why did you only mention wet-adhesion? Wouldn't topological entanglement be the same for whatever network, hydrated or not?

2. The bond-bond topology is actually chemistry specific, thus still suffers from the disadvantage you mentioned. Is there other topology for chemistry non-specific adhesion?

3. How would different topology affect bonding properties? 


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