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Journal Club for December 2018: Bonding hydrophilic and hydrophobic soft materials for functional soft devices
Bonding hydrophilic and hydrophobic soft materials for functional soft devices
Qihan Liu, Postdoctoral Fellow, Harvard John A Paulson School of Engineering and Applied Science
Introduction
Combining hydrophilic and hydrophobic components is the most basic strategy nature uses to produce complex biological functions. For example, at the cellular level, hydrophobic membranes separate a cell into multiple compartments with different functions, Fig.1a. At the tissue level, the myelin sheath forms a hydrophobic wrap that insulates the axon for efficient propagation of nerve impulses, Fig.1b. At the organism level, the stratum corneum is the hydrophobic layer of skin that prevents the unwanted mass exchange between the body and the environment, Fig.1c.
Figure.1. Nature combines hydrophilic and hydrophobic materials to realize complex biological functions. a. Hydrophobic endomembranes separate hydrophilic cytoplasm into compartments of different functions. b. Hydrophobic myelin sheath insulates the hydrophilic cytoplasm of axon for efficient propagation of electric signals. c. Hydrophobic stratum corneum insulates the more hydrophilic internal body from dehydration and contamination from the surrounding environment.
Recently, researchers begin to use the same strategy to engineer soft devices by combining hydrophilic hydrogels and hydrophobic elastomers. Some of the notable works includes the transparent loudspeakers [1], Fig.2a, the stretchable luminescent display [2, 3], Fig.2b, and the soft touchpads [4, 5], Fig.2c. Numerous other exciting works can be found, for example, in the recent review paper on hydrogel ionotronics [6]. Despite the great success of these inspiring works, further development of soft devices using hydrophilic/hydrophobic components has been greatly limited by the bonding between hydrogels and elastomers. Without any treatment, native hydrogels and elastomers have low adhesion energy (typically below 1 J/m2) [7], far below the fracture energy of common hydrogels (typically around 100J/m2), and tough hydrogels or elastomers (typically above 1000 J/m2) [8, 9]. To some extent, one would better describe the native contact between a hydrogel and an elastomer with the word lubrication instead of adhesion. While different strategies have been developed to improve the adhesion between various hydrogels and elastomers, it is often overlooked how bonding procedure is deeply intertwined with manufacturing process and that a bonding method can often impose strong limitation on what structures can be conveniently manufactured.
Figure.2. Engineers combine hydrophilic and hydrophobic materials to make exciting soft devices. a. A transparent loudspeaker developed by J.Y. Sun et al. [1]. b. A stretchable electroluminescent display developed by C. Larson et al. [3]. c. A soft touchpad developed by C.C. Kim et al. [4]. Figures are adapted from the original publications.
In this journal club, I will give an overview of the bonding strategies between hydrogels and elastomers from the perspective of manufacturing. Some open questions are listed at the end as potential topics for discussion.
Curing and bonding
Manufacturing of the soft materials is distinct from the hard materials. Due to their great softness, it is hard to cut or drill soft materials with precision. Consequently, traditional subtractive manufacturing is not suitable for the manufacturing of soft materials. Instead, soft materials are almost always produced by casting, extrusion, or 3D printing. As a result, curing the liquid state precursor (resins/solutions) into solid state becomes the central step in the manufacturing of soft materials. How the bonding procedure interacts with the curing procedure has deep implication in the design of manufacturing process. Depending on whether the bonding procedure happens before or after curing, we can category the bonding methods into three strategies: surface treatment, gluing, and bulk modification.
Surface treatment
Surface treatment activates the surface of a cured materials so that an uncured material can react and bond with the cured surface. Traditionally, surface treatment is widely used to prepare hard materials for coating or painting. The surface treatment of soft materials faces two fundamental challenges not presented in the hard materials. First, both elastomers and hydrogels are crosslinked polymer networks. Below the length scale of one mesh size, the polymer chains do not feel the constraint from the crosslinkers and fluctuate as if in a liquid state. Consequently, surface treatment of soft material is intrinsically temporary as the polymer chains from the bulk keeps replenish the surface. This phenomenon has been well studied in the gradual loss of hydrophilicity of plasma treated PDMS [10]. Second, the hydrogel is usually highly swollen (>90% of water). Consequently, whatever chemical modification applied to the hydrogel polymer network is unlikely to substantially alter the surface chemistry of the hydrogel.
Among the two challenges, the second challenge remains unsolved today, and there is no report of surface treatment that activates a hydrogel surface to bond an elastomer on top. On the other hand, the first challenge has been overcomed by H.Yuk et al. in 2016, which used surface treatment to activate various elastomers and bond a partially cured hydrogel on top [11]. This is also the first method ever reported to bond various hydrogels and elastomers with high interfacial toughness (~1000J/m2). In this method, a photoinitiator called benzophenone is applied to the surface of a cured elastomer and allowed to diffuse into the bulk for a short period. The treated elastomer is then brought into contact with a hydrogel precursor that can be cured by radical polymerization. Upon UV irradiation, the benzophenone can generate radical on the polymer backbone of the elastomer network. Such radicals then initiate the polymerization of the hydrogel that are grafted to the elastomer network. As the photoinitiator diffuses into a surface layer thicker than the mesh size, this method overcomes the replenish issue of the elastomer surface.
Figure 3. Use surface treatment to bond hydrogels on elastomers. The surface of the elastomer is first treated with benzophenone solution and then bought into contact with the hydrogel precursor. Under UV irradiation, surface absorbed benzophenone is excited and abstracts a hydrogen from surrounding unreactive C-H bonds on an elastomer chain. The radical on the elastomer chain leads to the grafting of the polymer networks of the hydrogel onto the elastomer surface. Image and caption adapted from Ref. [11].
Gluing
Glue uses a third material to couple two cured polymer networks together. Traditionally, glues for hard materials often works by interlocking into surface asperities. For soft materials, however, the glue has to directly couple the two materials at the level of polymer chains as the asperities can lose the interlocking feature under large deformation. In addition, the glue must have good affinity to both the highly hydrophobic elastomer and the highly hydrated hydrogel.
The first general method to glue various hydrogels and elastomers were reported in 2017. Withyl et al. showed that cyanoacrylate dispersion in liquid hydrocarbons readily bonds hydrogels and elastomers in less than a minute and achieve high interfacial toughness [12]. The method works for two reasons. First, cyanoacrylate polymerizes in the presence of hydroxyl ions into a glassy polymer. Dispersing cyanoacrylate into tiny droplets forms discrete stiff islands that locks the polymer chains together from both the hydrogel and the elastomer. When deformed, the stiff island moves apart like crosslinks and allows stretchability although the glassy islands themselves are not stretchable (The exact mechanism is still unclear, this is my personal interpretation that seems reasonable). Second, the cyanoacrylate is very reactive towards hydrogels while hydrocarbons are readily absorbed by elastomers. Encapsulating cyanoacrylate in hydrophobic hydrocarbons allows the glue to interact well with both the hydrogel and the elastomer.
Figure. 4. Use glue to bond hydrogels with elastomers. (A) A glue is applied between a cured hydrogel and a cured elastomer to form bonding. (B) The bonding agent links hydrogel network with the elastomer network. The formed interface is instant and tough yet remains stretchable. Image and caption adapted from Ref. [12].
Bulk modification
Bulk modification adds coupling agents into the precursors of elastomers and hydrogels so that the coupling agent can form bonding across the interface independent of the curing process. Unlike surface treatment and gluing, there is no counterpart of bulk modification in the manufacturing of hard materials. Bulk modification is deeply rooted in the network structure of soft materials where polymer chains fluctuate like liquids between crosslinks so that the coupling agent attached on the polymer chains can diffuse around and react with each other. Bulk modification also comes convenient as the manufacturing of soft materials almost always starts from precursors so that adding coupling agent to the precursor can conveniently fit into the manufacturing process.
Using bulk modification to bond various hydrogels and elastomers is first proposed by me and my coworkers in early 2018 using silane coupling agent [13]. Silane coupling agent is a broad category of commercially available chemicals that attaches a silane group to different functional groups. The functional groups can be chosen to react with the precursor of elastomers and hydrogels thus incorporate the silane group into the polymer networks, Fig.5. The silane groups can then undergo hydrolysis and condensation independent of the curing of the hydrogel and elastomers. The condensation of the silane groups across the interface of elastomers and hydrogels results in covalent bonding. The condensation rate can be tuned by catalyst, temperature, and pH independent of the curing kinetics so that the time that bonding forms can be tuned to happen after manufacturing is finished. And it is found that adding surfactant can greatly improve the coupling efficiency, possibly by promoting the partial solvation of the elastomer chains on the interface.
Figure. 5 Bonding hydrogels and elastomers by bulk modification. a. Silane coupling agents are mixed into the precursors of a hydrogel and an elastomer separately. b. During the formation of the two networks, the coupling agents are covalently incorporated into the networks, but do not condensate. c. After a manufacturing process, the coupling agents condensate, add crosslinks in the individual networks, and form bonds between the networks. A surfactant may further promote adhesion. d. Silane coupling agents hydrolyze and form e. Silanol groups, which condensate to form f. Siloxane bond. . Image and caption adapted from Ref. [13].
Manufacturing implications
Among the three bonding strategies, bulk modification offers the most flexibility in manufacturing. Once the precursor is modified, later manufacturing can proceed without any constraint from the bonding procedure and bonding forms automatically after the manufacturing. In fact, bulk modification can bond elastomers and hydrogels in all the manufacturing scenario that other two strategies can. In addition, it is currently the only method to coat elastomers on hydrogels or 3D printing elastomers and hydrogels together.
Gluing is important to control the adhesion post-manufacturing when the coupling agent of bulk modification have reacted. For example, if the device cracks during use, there would be no free coupling agent to heal the crack. Gluing is the only feasible solution in such scenario. Besides, certain applications may require the soft parts to be temporarily fixed together, glues with controllable weak or temporary adhesion can be useful.
Surface treatments may appear less convenient in bonding hydrogels with elastomers comparing to the other two strategies. On the other hand, surface treatment could be the best solution to combine soft materials with hard materials. The surface treatment of hard materials can function like the bulk modification of soft materials. That is, once the hard material is prepared with the surface treatment, the later manufacturing can proceed freely and bonding will form automatically after the manufacturing process. In fact, H Yuk et al. has already developed a promising surface treatment to bond hydrogels with various hard materials using silane coupling agent [14]. In nature, combining soft materials with hard materials is another important strategy in constructing biological functions, e.g. the locomotor system of most animals consists of soft muscles and hard bones.
Look forward
Combining hydrophilic and hydrophobic materials to design soft devices is still a very young field. All the results in Fig.2 were reported in the last five years and all of the three bonding methods in Fig.3-5 were developed in the last three years. The earlier works have shown the great potential that can be realized by soft devices. Yet the quality and the performance of these proof-of-concept works are not good enough for any real world application. With the recent development of various bonding methods, we are building the tool sets that would lead to the design and manufacturing of soft devices that can bring real impact to people’s life. Still, there are many open questions related to the bonding and manufacturing of soft devices. Please give your perspective on the questions listed below or add new questions. I hope this could be a beneficial discussion.
- What are the manufacturing needs that are still not met by the existing bonding strategies?
- Is there other bonding strategies different from the three strategies reviewed here?
- What kind of new methods can be developed for each strategy to enable more convenient manufacturing that is currently impossible?
- What kind of mechanistic understanding of the current bonding strategies are still missing?
References
1. Keplinger, C., et al., Stretchable, transparent, ionic conductors. Science, 2013. 341(6149): p. 984-987.
2. Yang, C.H., et al., Electroluminescence of giant stretchability. Adv. Mater., 2015. 28(22): p. 4480-4484.
3. Larson, C., et al., Highly stretchable electroluminescent skin for optical signaling and tactile sensing. Science, 2016. 351(6277): p. 1071-1074.
4. Kim, C.-C., et al., Highly stretchable, transparent ionic touch pannel. Science, 2016. 353(6300): p. 682-687.
5. Sarwar, M.S., et al., Bend, stretch, and touch, Locating a finger on an actively deformed transparent sensor array. Sci. Adv., 2017. 3: p. e1602200.
6. Yang, C. and Z. Suo, Hydrogel ionotronics. Nature Reviews Materials, 2018: p. 1.
7. Tang, J., et al., Adhesion between highly stretchable materials. Soft Matter, 2016. 12(4): p. 1093-1099.
8. Sun, J.Y., et al., Highly stretchable and tough hydrogels. Nature, 2012. 489: p. 133-136.
9. Gong, J.P., Why are double network hydrogels so tough? Soft Matter, 2010. 6: p. 2583-2590.
10. Bodas, D. and C. Khan-Malek, Hydrophilization and hydrophobic recovery of PDMS by oxygen plasma and chemical treatment—An SEM investigation. Sensors and Actuators B: Chemical, 2007. 123(1): p. 368-373.
11. Yuk, H., et al., Skin-inspired hydrogel-elastomer hybrids with robust interfaces and functional microstructures. Nat. Commun., 2016. 7: p. 12028.
12. Wirthl, D., et al., Instant tough bonding of hydrogels for soft machines and electronics. Science Advances, 2017. 3(6).
13. Liu, Q., et al., Bonding dissimilar polymer networks in various manufacturing processes. Nature communications, 2018. 9(1): p. 846.
14. Yuk, H., et al., Tough bonding of hydrogels to diverse non-porous surfaces. Nature materials, 2016. 15(2): p. 190-196.
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Comments
Dear Qihan,
Dear Qihan,
Thank you for initiating the discussion on this nascent topic in soft materials. It is truly inspiring and well-summarizing the recent developments on this important topic. I would like to add a few points to facilitate the discussion.
The mechanism for tough adhesion of hydrogels was reported in 2015 by Yuk et al [1]. It requires a synergy between:
a. Tough and dissipative hydrogel matrix to prevent cohesive failure and provide dissipation to enhance interfacial toughness.
b. Strong linkages between hydrogels and bonded materials to provide high intrinsic interfacial toughness.
Figure 1. Mechanism for tough bonding of hydrogels by a synergy between interfacial linkages and bulk dissipation [1].
We also developed a cohesive-zone and Mullins-effect model [1, 2] to quantitatively illustrate the mechanism and reported a method for tunable adhesion from 1Jm-2 to 1,000 Jm-2.
Figure 2. Tunable adhesion ranging from 1 to 1,000 Jm-2 by controlling interfacial linkage density via surface modification (silanization time here) [2].
Following the above-mentioned mechanism, we proposed the concept of robust hydrogel-elastomer hybrid in 2016 [3]. Since then, many applications that harnessed the merits of hydrogels and elastomers are rapidly emerging. A few examples from our group:
a. Anti-dehydration hydrogels, where an elastomer coating on hydrogels can prevent their dehydration [3].
b. Living materials and devices, where the hydrogel provides a matrix for living cells and the elastomer skin to prevent dehydration of the material and devices [3,4].
c. Robust hydrogel coatings, where tough hydrogels provide soft, wet and slippery coatings on existing elastomer devices such as medical gloves, tubings and even condoms [5,6].
d. Hydraulic hydrogel robots that are acoustically and optically transparent in water yet fast and forceful enough to catch live fishes [7].
References
[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] Teng Zhang#, Hyunwoo Yuk#, Shaoting Lin, Xuanhe Zhao*, Tough and tunable adhesion of hydrogels: experiments and models, Acta Mechanica Sinica 33, 543-554 (2017)
[3] Hyunwoo Yuk, Teng Zhang, German Alberto Parada, Xinyue Liu, Xuanhe Zhao*, Skin-inspired hydrogel-elastomer hybrids with robust interfaces and functional microstructures, Nature Communications 7, 12028 (2016)
[4] Xinyue Liu#, Tzu-Chieh Tang#, Eléonore Tham#, Hyunwoo Yuk#, Shaoting Lin, Timothy K. Lu*, Xuanhe Zhao*, Stretchable living materials and devices with hydrogel-elastomer hybrids hosting programmed cells, PNAS 114, 2200-2205 (2017)
[5] German A. Parada, Hyunwoo Yuk, Xinyue Liu, Alex J. Hsieh, Xuanhe Zhao*, Impermeable robust hydrogels via hybrid lamination, Advanced Healthcare Materials 6, 1700520 (2017)
[6] Yan Yu#, Hyunwoo Yuk#, German A. Parada#, You Wu, Xinyue Liu, Kamal Youcef-Toumi, Xuanhe Zhao*, Multifunctional hydrogel skins on diverse polymers with arbitrary shapes, Advanced Materials, in press (2018)
[7] Hyunwoo Yuk, Shaoting Lin, Chu Ma, Mahdi Takaffoli, Nicholas X. Fang, Xuanhe Zhao*, Hydraulic hydrogel actuators and robots optically and sonically camouflaged in water, Nature Communications 8, 14230 (2017)
Thanks Hyunwoo! These are
Thanks Hyunwoo! These are truly the pioneering works that inspired all the following works on the bonding between hydrogels and elastomers!
Glass Phase Works for Adhesion
Dear Qihan,
Nice organization of adhesion topics. As your opinion for glue, we already proved that the glass phase plays an important role in the adhesion of polymer. Potentially a widely useful technology. We show some surprising results for hydrogels. The paper will come soon.
Topology of wet adhesion
Dear Qihan,
Thank you for bringing up this interesting yet extremely important research topic. In your brief review of the recent efforts, the promising and fruitful future of this field is foreseen. Here, I would like to add my personal opinions to the discussion.
One clear disadvantage of thses bonding methods is that they (except cyanoacrylate whose mechanism is unclear) rely on the design of the chemistry of bonds to achieve strong adhesion. The hydrogel and the elastomer need to have matching functional groups to form bonds. However, design chemistry of bonds either on the surface or in the bulk may cause many complications: 1, surface modification is inconvenient; 2, bulk modification changes the origical mechanical properties of materials; 3, in some bonding situations, such as inside body, surface and bulk modification is impossible; 4, beyond elastomer, the chemistry of bonds limits the choice of materials.
Our group recently discover a new aspect in wet adhesion--the topolgy of connecting materials. By a topology we mean a type of connectivity through bonds, chains, particles, networks, or their combinations. Significantly, numrous topologies are possible, but not explored. Creating bonds is just one of the topology. Two examples of topologies are following:
1. A stitch-stitch topology [1]
This topology does not require fucntional groups from both materials. We spread polymer chains at the interface of two polymer networks, and trigger them to form a new polymer network in situ, localzied at the interface, in topological entanglement with both preexisting polymer networks, stiching them together like a suture.
2. A bond-bond topology [2]
We use polymer chains to form covalent bonds with both polymer networks.
Reference
[1]. Yang, Jiawei, Ruobing Bai, and Zhigang Suo. "Topological adhesion of wet materials." Advanced Materials 30.25 (2018): 1800671.
[2]. Li, J., Celiz, A. D., Yang, J., Yang, Q., Wamala, I., Whyte, W., ... & Mooney, D. J. (2017). Tough adhesives for diverse wet surfaces. Science, 357(6349), 378-381.
Hi Jiawei,
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?
Hi Qihan,
Hi Qihan,
Yes. The topological adhesion can be applicable for any polymer network, so long as the chain can go into both networks and make stitch.
Molecular stitching is one version, we also have another version, called molecular staple. Can you think anything macroscopic version of fastening two objects that can be created in the molecular level?
The last comment is quite interesting, currently I am also exploring some properties.
Adhesion between hydrogels and diverse substrates
Dear qihan and hyunwoo,
Thanks for the nice organization and pioneering works. I have tried all the adhesion methods above. And I want to discuss some issues with you.
(1) To achieve tough bonding between hydrogels and diverse substrates, previous methods usually rely on two strong interactions at the interface. The first is covalent bonding at the interface including silane method, EDC-NHS chemistry (Li, Science, 2017) and benzophenone chemistry. The second is the topological entanglement including glue method and topohesion (Yang, Advanced Materials, 2018) Can there be another kind of interfacial interaction to achieve tough adhesion?
(2) For cured polymer networks, topological entanglement is a better method without requiring functional groups of the polymer networks. However, the present method requires a long working time (Yang, Advanced Materials, 2018) or is toxic (Wirthl, Science Advances, 2017). What’s a better solution?
Thank you very much!
Dear Chenghai,
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.
Design, modifying and applications of adhesion chemistry
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.
Thanks.
Dear Chenghai and Qihan,
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).
Hi Hyunwoo,
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,
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.
This is interesting thought.
This is interesting thought. But the equation itself does not tell you how strong the weak bonds are that can elicit the bulk hysteresis. It is intriguing to study how the strength of interfacial bonds influence the bulk dissiaption.
Strong adhesion between soft materials
Dear Qihan and Hyunwoo: Thank you both for discussing this topic of great depth and immediacy. Adhering soft materials has been hard. The newly discovered methods achieve strong adhesion between soft materials. These methods have opened an enormous field of opportunity for invention and discovery. I look forward to seeing many more researchers in this field participate in this discussion.
Bonding dissimilar materials
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.
Very insightful! Thanks!
Very insightful! Thanks!
Mechanics in designing soft and tough adhesion
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!
Reference
[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 nice works Shaoting!
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!
Very nice and deep work! I am
Very nice and deep work! I am wondering how two figures interact with each other if you stretch the material really large.
Jiawei, thanks. For
Jiawei, thanks. For neohookean solid, the neighboring fingers suppress with each other and eventually stabilize when the stretch is large. If you further stretch the material, hierachical instabilities initiate and develop in the original fingers. I attached one image showing hierachical instabilities under extremely large deformation. For stiffening materials, the fingerings tend to more stabilize.
Dear Qihan,
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?
Hi Zhijian,
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.
For the last comment, my
For the last comment, my topological adhesion paper has one data shows that the measured adhesion energy is even higher than the fracture energy of the hydrogel.
That is really interesting!
That is really interesting! Where does the fracture happen? At the interface or in the bulk hydrogel?
You will see a very smooth
You will see a very smooth crack goes through the entire hydrogel sample.
Hi, Zhijian. Considering the
Hi, Zhijian. Considering the cyanoacrylate glue, it do not need special chemical group neither and can combine two bulks together. Especially for hydrogel, we can directly see the infiltration and entanglement when the cyanoacrylate work. That’s similar with Jiawei’s method. I think it’s a general method to realize adhesion (reversible or not) for hydrogels by choosing proper bridge polymers. And there are many kinds of reversible or dynamic bond you have mentioned.
Stimuli-responsive adhesion
Dear Zhijian,
Thank you very much for bringing your enormous expertise in chemistry to this discussion. This discussion reminds me of a previous iMechanica discussion hosted by you and Shengqiang, where you discussed extensively on dynamic covalent bonds. These reversible covalent bonds, along with so many noncovalent bonds, have transformed the development of polymers and polymer gels. A particularly lively field is self-healing materials.
It is conceivable that the same dynamic bonds will transform the development of adhesion. Here we can use dynamic bonds to add functions to adhesion. An example is topological adhesion. Here a trigger (a change in pH) can cause both strong adhesion and on-demand detachment. The strong adhesion requires no functional groups in the two adherends. A third species of polymers stitches the preexisting networks of the two adherends. That is, the stitching polymers act like a molecular suture. As noted in the paper, one can imagine other triggers (ions, molecules, temperature, light). The possibilities are wide open.
May the new development of adhesion adhere mechanics and chemistry in new ways.
Dear Zhigang,
Dear Zhigang,
Thanks for your comments. The topological adhesion is really a nice design without the need for treatment of raw materials. This strategy can also be easily combined with reversible polymer networks.
The dynamic covalent bond is a newly emerging field. Up to now, most of the dynamic covalent bonds are triggered by high temperature in the presence of catalysts, which limits their applications in certain fields. Only those which can be triggered by light, including the disulfide bonds and reversible addition fragmentation transfer reagents, have been reported in designing self-healing hydrogels.
More and more dynamic covalent bonds, which can be triggered in mild conditions, may be developed in the future and applied in tough adhesion.
Dynamic covalent bonds vs. noncovalent bonds
Thank you Zhijian. So far as the mechanical behavior of hydrogels is concerned, dynamic covalent bonds and noncovalent bonds serve the same attributes: they can break and reform. These attributes lead to, respectively, toughness and heal.
The similarity between dynamic covalent bonds and noncovalent bonds is clearly articulated in a review of self-healing hydrogels. The devil is in details. Table 1 of the review lists the condition of heal. In our Ca-alginate-polyacrylamide hydrogel, heal happens at elevated temperatures (e.g., 80C). People are cooked by then. Many other gels now can heal at room temperature, in minutes or faster.
From your perspective, do dynamic covalent bonds bring any advantage over noncovalent bonds? Perhaps they are just different chemistries, and have different attributes unrelated to mechanical behavior. I’d love to hear your view.
Dynamic covalent bonds vs. noncovalent bonds
Thanks, Zhigang. Yes. The roles of dynamic covalent bonds and noncovalent bonds are very similar, breaking and reforming. In my opinion, the difference may be bond strength. The covalent bonds are stronger than the non-covalent bonds, which seems important for the tough adhesion in bulk and surface treatment approach. For the topological design, I am not sure whether strong bond strength is critically important or not. Some understanding in these approaches can help the design of tough adhesives.
The disadvantage of dynamic covalent bond may be the side reaction. Sometimes they can not be totally reversible.
I am very interested on
I am very interested on dynamic covalent bond, and I realize that more and more such dynamic bonds are used in hydrogel. In your opinion, is design and synthesis of dynamic bonds in hydrogel more challenging in practice compared to noncovalent bonds?
If we want huge bulk dissipation, is dynamic bonds better than noncovalent bonds?
Dynamic covalent bonds in hydrogels
It depends on what kind of dynamic covalent bonds you want to use. The synthesis of some dynamica covalent bonds can be very simple. While the non-covalent bonds like multiple hydrogen bonding interaction can be a little difficult.
In fact, the newly reported dynamic covalent bonds are those traditionally recognized as stable and themoset covalent bonds. Recently, it is found that, with the addition of catalysts, the covalent bonds can be dynamic at certain conditions. However, in many dynamic covalent bonds (ester, carbamate, Diels-Alder reaction) (Adv. Mater. 2017,29, 1606100), the triggering condition needs high temperature, which limits their applications in hydrogels. Among them, two kinds of dynamic covalent bonds, disulfide bonds and Schiff base may be useful in the tough adhesions. Both of them are summaried in the review mentioned by Zhigang. The disulfide bonds can be triggered by UV light and the Schiff base bonds are sensitive to pH conditions.
The bulk dissipation depends on the time scale of the reaction of dynamic covalent bonds. If the strain rate in the measurement is slow enough, I think the bulk disspation in dynamic bonds and noncovalent bonds may be similar. However, in the measurment, the time scale is much shorter than the time scale of dynamic covalent bonds. So the dynamic covalent bonds act as the conventional covalent bonds. We can not expect huge bulk dissipation in them.
In our lab, we are also interested in the dissipation of the materials with dynamic covalent bonds. We may have some results in the near future.
dynamic covalent bond vs. noncovalent bonds
Hi Zhigang, there are some subtle differences between dynamic covalent bond and noncovalent bond. For a polymer composed of dynamic covalent bond, its viscosity decreases relatively slowly with the increase of temperature, following Arrhenius equation. For a polymer composed of noncovalent bonds (e.g. van der waals interaction for most thermoplastic), its viscosity decreases rapidly with the increase of temperature. In general, the binding dynamics/strength of noncovalent bonds are more temperature-sensitive than dynamic covalent binds. I learned those from Zhijian when he first joined our group.
Another possible advantage for dynamic covalent bonds may be that it can also work equally well in dry elastomer. For the ionic interaction, water is essential for enabling the ion migration. For hydrogen-bond enabled self-healing, dry enviroment may be fine as well.
Recently, people try to use polymer with dynamic convalent bond to replace theromplastic for making fiber reinforced polymer composites. The performance the new composite decays less severely with the increase of temperature.
It will be interesting to compare the differences of the self-healing performance of the gel composed of (different) noncovalent bonds or dynamic covalent bond. I don't think it is clear.
Topological adhesion
Hi Jiawei and Zhigang,
I am very interested in the topological ahesion done by you. In the lab, we actually often use the mechaism implicitly. When we try to glue two elastomer, one way we often adopt is to partially cure both elastomers, put them in contact with each other and let the curring process finish to enable the bonding. For this case, reversible bonding and detachment cannot be achieved, as demonstrated in your work.
Topological adhesion for wood
In ancient China (in particular Ming and Qing Dynasty), people made wood furniture by using glue. The glue is a bio-protein (from for example skin of pigs), which is a liquid at high temperature and gelled at low temperature. The wood furniture can be very study at room temperature. Nowadays, when an experienced craftsman tries to repair those anique, they simply spray boiled water onto the furniture and then can easily dissemble them (the bio-gel turns to liquid state)- (on-demand detachment).
Thermoreversible adhesion, thermodetachable topological adhesion
Excellent point. We've playing with animal glues.
Pig skin protein as stitching polymer
Dear Shengqiao,
Thank you for raising up such interesting glue! I have two small question. 1, Is the pig skin glue you mentioned a gelatin? Because I remember when I cook pig feet, the soup becomes jelly when it is cold, but when I put it on hot rice, it quickly melt. 2, You say it is topological adhesion for wood, does it mean the pig skin proten can interpenetrate with the cellulose of the wood?
Pig skin protein as stitching polymer
Dear Shengqiang,
Thank you for raising up such interesting glue! I have two small question. 1, Is the pig skin glue you mentioned a gelatin? Because I remember when I cook pig feet, the soup becomes jelly when it is cold, but when I put it on hot rice, it quickly melt. 2, You say it is topological adhesion for wood, does it mean the pig skin proten can interpenetrate with the cellulose of the wood?
I was working on a deadline,
I was working on a deadline, but I just couldn't help but join this exciting conversation when a group of mechanicians began to discuss dynamic bonds, topology, self-healing, proteins, furnitures and cocking with significant contents and implications. Soft materials is arguably one of a few topics in mechanics that can cause such excitement, diversity, depth, profoundness, fun and eventually benefits to the society.
A specific reply to Jiawei's post. To our knowledge, the phase seperation and aggregation of proteins can give tough, strong and reversible adhesion. One example is elastin-like polypeptides, whose phase seperation and adhesion can be triggered by ionic strength of the solution. There are other previous examples including gelatin and fibrin that we cited in the paper.
Hi Jiawei
Hi Jiawei, I cannot answer either of the quesiton with confidence. For your first question, I suspect the effective component contains geltain, elastin and other proteins. For the second question, I guess the protein may diffuse into the cellulose of wood to form strong bond, considering the porous structure of wood. I don't have evidence though. It may be fun to take a look under microscope, which should be fairly easy.
About delicious glues
Actually I have played with this kind of glue a lot. We use dankey skin gelatin (阿胶) to glue reed membrane (笛膜) on bamboo flute to get the unique Chinese flute sound. We will use the humidity of our breath to soften the glue and adjust the tension of the membrane before we start playing. And Chinese people certainly have talent with delicious glues. As another example, Chinese caligraphy and painting are framed (装裱) by starch solution(浆糊). I have a vague memory that the Great Wall is built by sticky-rice based glue since we didn't have cement. These glues function in a similar fashion to the modern resin glues e.g. epoxy or acrylic glue. Essentialy, the surface must be porous for the polymer to penetrate. As the glue solidifies, it forms interlocking structure that locks to parts together. However, such macroscopic interlocking mechanism won't work for soft materials, as I mentioned in the main post, because the interlocking structure can readily detach by deformation. For interlocking mechanism to work for soft materials, we need to go down to the mesh level. And that is topological adhesion.
Self-healing mechanics
Dear Zhigang,
I would like to echo your comments regarding self-healing materials with dynamic bonds. Though MD simulations for self-healing materials have been carried out by some researchers, surprisingly, mechanics models to explain the interfacial self-healing remain largely unexplored. In recent years, my group makes several attempts. Hope they can be some references for this journal club:
1. General dynamic bonds:
Kunhao Yu, An Xin, Qiming Wang, Mechanics of self-healing polymer networks crosslinked by dynamic bonds, Journal of the Mechanics and Physics of Solids, 121, 409-431, 2018.
2. Nanoparticle crosslinkers:
Qiming Wang, Zheming Gao, Kunhao Yu, Interfacial Self-healing of Nanocomposite Hydrogels: Theory and Experiment, Journal of the Mechanics and Physics of Solids, 109, 288-306, 2017.
3. Light-induced self-healing:
Kunhao Yu, An Xin, Qiming Wang, Mechanics of Light-Activated Self-Healing Polymer Networks, Journal of the Mechanics and Physics of Solids, 124, 643-662, 2019.
4. Electrically-induced bonding:
An Xin, Runrun Zhang, Kunhao Yu, Qiming Wang, Mechanics of Electrophoresis-Induced Reversible Hydrogel Adhesion, Journal of the Mechanics and Physics of Solids, accepted, 2018.
Best regards,
Qiming
Multiscale mechanics of tough adhesion of soft materials
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.
Reference
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.
Excellent perspective on the mechanics of adhesion
Teng: Thank you for these valuable comments. The papers cited are very helpful. Rong Long's August 2017 iMech jClub discussion gave additional discussion of fracture mechanics of soft materials. The development of new methods of adhesion goes hand in hand with the development of deeper understanding of mechanics. Soft materials bring hard problems. Life in mechanics and materials has not been dull.
An exciting field
Dear Qihan,
Thanks for the timely review and insightful elaborations. Most of us have witnessed the rapid advancement in the field of chemo-mechanics of adhesion in recent years. Though I have noticed most of work cited here, your summary and the incurred discussions indeed make the development of the field much clearer to me.
In chemistry field, recently, there have also been some signficant progress in the adhesion field. For example, the work done by Messersmith group on Mussel inpired adhesion, which must be known to many of you.
I imagine deeper interactions between mechanics and chemistry may lead to new opportunities in the field.
Hydrogel adhesive for medical use
Dear Qihan,
Thank you for your clear classification of the adhesion between hydrophilic and hydrophobic soft materials. When we introduced the progress in this field to the hospital doctors in Xi'an, they were impressed and very interested in using it for hemostasis. We notice that there are several issues not solved in this area:
(1) Biodegradability. Once the hydrogel adhesive is applied, it is better to be dissovlved in vivo to prevent the second surgery. To solve this problem, we proposed a strategy to bond tough biodegradable hydrogels to tissues. The adhesion energy can reach the order of 1000J/m2 and the degradation speed of the hydrogel can be tuned on demand [1].
(2) Poor adhesion of fully swollen hydrogels. It is a common practice to fully swell the as-prepared hydrogel to remove the unreacted chemicals. However, we surprisely found that the fully swollen hydrogels cannot be bonded to other materials, such as the two-step synthesized alginate/PAAm. Not only gels, even fully swollen VHB (in acetone) is not sticky any more. What are the possible reasons? Will the surface properties of hydrogels influence their adhesion properties?
(3) Instant bonding. For the surgical hemostasis, the time window for doctor's operation is only several minutes. It is important to realize good adhesion in this short time. Jianyu and Jiawei showed a succesful strategy for bonding between gels and tissues based on topological adhesion. Is there any other approach to achieve this?
New questions call for new solutions. I wish to hear your opinion.
[1] Hang Yang, Chenghai Li, Jingda Tang and Zhigang Suo. Biodegradable adhesive hydrogel. Unpublished.
Hi Jingda,
Hi Jingda,
Nice questions. For your questions (2) swelling is a problem because you are changing the surface chemistry by diluting the polymer chain density to an extremely low level. There's not much to grab on the surface to form a bonding anymore. In theory, the bulk modification strategy should be insensitive to swelling because the diffusion of the coupling agent is unaffected by the swelling. For the current version with silane coupling agent, you have to swell the hydrogel in controled pH buffer to control the kinetics. I would look for other bulk modification chemistry if fully swollen hydrogel is concerned.
For your questions (3), Chenghai was asking the same question. We can ask Zhijian see if he has some insight in Chemistry.
Hi Qihan,
Hi Qihan,
Thank you for your quick response. The explanation for the poor adhesion of swollen hydrogels is quite reasonable. Bulk modification is a good suggestion for the bonding of hydrogels and elastomers. But for hydrogels and tissues, is it still applicable, considering tissue can hardly be modified?
If we target existing
If we target existing functional group on the tissue surface then we don't need to modify the tissue.
Hydrophilic-hydrophobic materials
Though the hydrogel is most used at present, the strategy for the combination of hydrophilic and hydrophobic structures can also be applied for other materials like ionogel.
[1]Hang Yang, Chenghai Li, Jingda Tang and Zhigang Suo. Soft coating for ionogel. Unpublished.
Adhesion method applied for coating
Dear Qihan,
There're solutions for hydrogels to get a strong adhesion with hydrophobic materials[1-2], and effective coatings were realized[3]. And topological adhesion without modification has also achieved[4]. Based on these adhesion methods, hydrogel coating for medical steel, ceramic, catheter have developed. One problem may get attention. The silane method and benzophenone method all need a special group to get the combination for hydrophilic and hydrophobic coating. However, there is no vinyl for some hydrogel or hydrophilic materials. Is it possible to get a hydrophobic coating for hydrogel bulk without modification?
[1] Hyunwoo Yuk, Teng Zhang, German Alberto Parada, Xinyue Liu, Xuanhe Zhao*, Skin-inspired hydrogel-elastomer hybrids with robust interfaces and functional microstructures, Nature Communications 7, 12028 (2016)
[2] Liu, Q., et al., Bonding dissimilar polymer networks in various manufacturing processes. Nature communications, 2018. 9(1): p. 846.
[3]Paul Le Floch, Xi Yao, Qihan Liu, Zhengjing Wang, Guodong Nian, Yu Sun, Li Jia, and Zhigang Suo. 2017. “Wearable and Washable Conductors for Active Textiles.” ACS Applied Materials and Interfaces.
[4] Yang, Jiawei, Ruobing Bai, and Zhigang Suo. "Topological adhesion of wet materials." Advanced Materials 30.25 (2018): 1800671.
Hi Hang,
Hi Hang,
There must be some modification of the surface for bonding to happen. Even topological adhesion modifies the properties near the interface, although it does not require any modification before hand. So I guess the question is not how to achive bonding without modification. Rather, we should ask what is the suitable modification for a particular situation.
Photodetachable adhesion
Dear Qihan,
Thank you very much for initiating the discussion on this appealing topic. I really learned a lot from your review and the comments posted by all the people. Here, I would like to share our newly published work and learn from your feedback.
Derived from Jiawei’s topological adhesion [1], we proposed an approach to achieve both strong adhesion and easy detachment between hydrogels and other kinds of substrates (e.g. tissue, elastomer, inorganic solid) [2]. The stitching network was functionalized to undergo a gel-sol transition under UV radiation. Therefore, the strong adhesion can be easily removed by exposure to UV light.
We found the adhesion performance was strongly affected by the gelling and dissolving behavior (by rheology study) of the stitching polymer.
The stimuli responsive bonds would enable stimuli responsive adhesion. We are wondering the application of these stimuli responsive adhesions? And what kinds of chemistries are suitable for functionalizing the stitching polymer (it need to be sample, work under mild condition, and provide proper gelation time and dissolution time)?
[1] J. W. Yang, R. B. Bai, Z. G. Suo, "Topological adhesion of wet materials", Advanced Materials. 2018, 30, 1800671.
[2] Y. Gao, K. Wu, Z. G. Suo, “Photodetachable adhesion”, Advanced Materials. 2018, 1806948
Hi Yang,
Hi Yang,
Nice work! I think this is the first controllable adhesion ever reported. I would have included it in my review if it were out earlier.
As for the applications, doctors needs to clamp tissues around when they are doing operation. A controllable detachment would be necessary in this situation. And just like building a tower requires temporty structure, building a complex soft device may very well require tempory structure that disassembles afterwards. I'm really looking forward to more exciting works in this direction!
Hi Qihan,
Hi Qihan,
I have learned a lot form your interesting and inspiring work. My question is would any residual stress exist after the adhension. Some students here found that after adhension of two layers, the bilayer bent a lot. Is the stress good or bad for the adhension?
Tongqing
Hi Tongqing,
Hi Tongqing,
Very good question. The bending you mentioned should come from the residual stress in the bulk material. The interfacial layer is probably too thin to cause anything noticable for the size of a common device. I guess the bending should be gone if you can control the drying of the gel during the bonding process?