In a recent report in Matter, Zhu, Wang, He and co-workers report a straightforward and effective strategy for the design of icephobic hydrogel coatings on the basis of polydimethylsiloxane (PDMS)-grafted polyelectrolyte hydrogels. These passive anti-icing and de-icing coatings were demonstrated to synergistically suppress ice nucleation, ice propagation, and ice adhesion

Hoogenboom, Richard

Jerca, Valentin-Victor

Xu, Xiaowen

English

In a recent report in Matter, Zhu, Wang, He and co-workers report a straightforward and effective strategy for the design of icephobic hydrogel coatings on the basis of polydimethylsiloxane (PDMS)-grafted polyelectrolyte hydrogels. These passive anti-icing and de-icing coatings were demonstrated to synergistically suppress ice nucleation, ice propagation, and ice adhesion.In a recent report in Matter, Zhu, Wang, He and co-workers report a straightforward and effective strategy for the design of icephobic hydrogel coatings on the basis of polydimethylsiloxane (PDMS)-grafted polyelectrolyte hydrogels. These passive anti-icing and de-icing coatings were demonstrated to synergistically suppress ice nucleation, ice propagation, and ice adhesion.

Xu, Xiaowen0001513101958020022613489786432465330000-0003-1404-661948E2B6EC-9BA4-11E5-AA1D-E826B5D1D7B1

Jerca, Valentin-VictorUGent80200229600797370628865988FDC734-E001-11E5-A52B-C053B5D1D7B1

Hoogenboom, RichardWE078020006204320000-0001-7398-20582A45E056-F0EE-11E1-A9DE-61C894A0A6B4

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Bio-inspired hydrogels as multi-task anti-icing hydrogel coatings

Ghent University Academic Bibliography

PreviewBio-inspired Hydrogelsas Multi-task Anti-icingHydrogel CoatingsXiaowen Xu,1 Valentin Victor Jerca,1,2 and Richard Hoogenboom1,*In a recent report inMatter, Zhu, Wang, He and co-workers report a straightfor-ward and effective strategy for the design of icephobic hydrogel coatings on thebasis of polydimethylsiloxane (PDMS)-grafted polyelectrolyte hydrogels. Thesepassive anti-icing and de-icing coatings were demonstrated to synergisticallysuppress ice nucleation, ice propagation, and ice adhesion.1Supramolecular Chemistry Group, Centre ofMacromolecular Chemistry (CMaC), Departmentof Organic and Macromolecular Chemistry,Ghent University, Krijgslaan 281-S4, 9000 Ghent,Belgium2Centre of Organic Chemistry ‘‘Costin D.Nenitzescu’’ Romanian Academy, Spl.Independentei 202B, Bucharest, Romania*Correspondence:richard.hoogenboom@ugent.behttps://doi.org/10.1016/j.chempr.2020.03.015Water is essential to life on Earth and italso plays a diverse and structural role incell biology.1 Water can exist as solid,liquid, and gas within the range of tem-peratures found on Earth, has a uniquestructure and mediates all life’s chemicalreactions. Although water is central toour lives and survival, covering morethan 70% of the Earth’s surface, it canalso have undesirable effects on the soci-ety because of ice formation. Icing is aphenomenon causing numerous and sig-nificant problems in many areas such asbuildings and power supplies, transpor-tation, and telecommunication. Besidesthe aspects concerning human safety,icing is also causing high economic lossesdue to energy waste and reduced oper-ating efficiency.2 Ice formation generallyoccurs through a nucleation and crystalgrowth mechanism.3 The nucleation pro-cess is complicated because it can takeplace in bulk water or at interfaces, andit is highly influenced by the temperatureand other environmental conditions.Consequently, ice formation processescan widely vary, and the different ice-for-mationmechanisms have to be taken intoaccount in order to design efficient anti-icing materials that can successfully beused in real-life applications.Two different approaches have beendeveloped to reduce ice formation,namely anti-icing that aims to prevent820 Chem 6, 808–831, April 9, 2020 ª 2020 Elsevieice growth from a surface and de-icingthat focuses on the removal of the iceafter it grows on a surface. Both ap-proaches can be active when an input ofenergy is used (electrical or human po-wer) or passive when no input of energyis required from the user. Various activestrategies have been employed to inhibitice accretion and remove ice, includingvapor heating, mechanical means, andthe use of ice-melting agents.4 However,active methods are generally costly,inefficient, design-complicated, and envi-ronmentally harmful. Therefore, passiveanti-icing materials, i.e., icephobic mate-rials, that have the capability to preventaccumulation of ice with ‘‘zero-energy’’consumption have attracted intensiveinterests in the last decade.5 Successfulstrategies used for passive anti-icing are(1) timely removal of water dropletsand (2) control of the ice formation,whereas passive de-icing aims to reduceice adhesion. Best anti-icing behavior iscommonly achieved using either micro-scopically rough superhydrophobic sur-faces (SHSs) or slippery liquid-infusedporous surfaces (SLIPSs).6,7 It has been re-ported that SHSs can effectively preventice accretion through sliding or bunchingoff water droplets before they canfreeze.6 The construction of rough sur-faces via the creation of micro- or nano-structures on hydrophobic substrates isof vital importance for the preparationr Inc.of artificial anti-icing SHSs.8 However,the ice adhesion strength values of suchrough SHSs are several times higherthan that of the corresponding smoothSHSs because of interlocking betweenthe ice crystals and the micropatternedSHS. In contrast, the SLIPSs exhibit lowadhesion strength, superior anti-icingand frost-resisting characteristics.7 How-ever, the costly infused lubricant andweak resistance to mechanical abrasionstill limit their practical applications.Natural anti-freeze proteins (AFPs) cancontrol ice formation by selectivelybinding to ice crystal surfaces. Wanget al. have examined both depressionand promotion effects of AFPs on icenucleation by selectively linking theice-binding face (IBF) and non-ice-bind-ing face (NIBF) of the AFPs to solid sub-strates.9 The ice nucleation was foundto be inhibited by the NIBF because ofthe lack of hydrophobic or hydrophilicpatterns as well as the presence ofcharged groups, whereas the IBF facili-tated the ice nucleation. The synergybetween hydrogen bonding and hydro-phobic interactions was found to beresponsible for the regulation of interfa-cial water and the outstanding anti-icingproperties of the NIBF. This exciting dis-covery was developed toward NIBFcoatings, which could prevent ice for-mation down to temperatures of28C. It is, however, still a great chal-lenge to design more robust andeconomically viable synthetic materialsthat can function in a similar way bysimultaneously suppressing ice nucle-ation, ice propagation, and adhesion.Figure 1. Design of Icephobic Hydrogels Inspired by Natural Anti-freeze Proteins(A and B) Shown in (A), polyelectrolyte hydrogel coating functions that suppress all ice formation mechanisms; in (B) are the chemical structure of thehydrogel and precursors that were investigated by He and coworkers. Adapted and reprinted from He et al.10 with permission from Elsevier.Recently, Zhu, Wang, He and co-workers addressed this challenge bydeveloping synthetic AFP-inspiredanti-icing materials that synergisticallytarget all ice formation processes, un-like previous reports that commonlyonly focused on a single process (Fig-ure 1).10 They designed a synthetic hy-drogel platform with unprecedentedwater structuring ability to obtain ice-phobic hydrogel coatings with superioranti-icing properties. More specifically,the hydrogel design combines hydro-philic acrylamide (AAm) units as wellas negatively charged acrylic acid(AAc) and positively charged [2-(met-hacryloyloxy)ethyl]trimethylammonium(META) units with hydrophobic polydi-methylsiloxane (PDMS) grafts. The hy-drophilic and charged groups are hy-pothesized to suppress ice nucleationand ice propagation, whereas the hy-drophobic grafts should reduce iceadhesion strength.To validate this hypothesis, a hydrogelplatformwas developed that allows accu-rate tuning of the hydrophilic-hydropho-bic balance, which is required to suppressthe different aspects of ice formation.Therefore, a poly(acrylamide-co-acrylicacid-co-N-allyacrylamide)-(poly(AAm-co-AAc-co-AAene))-based hydrogel car-rying multiple hydrophobic and chargedfunctional groups was prepared byin situ photocrosslinking with cationicPMETA and monomethacrylate-func-tionalized PDMS monomers havingdifferent PDMS chain length. The hydro-phobicity of this PDMS containing poly-electrolyte hydrogel coating could beeasily tuned by incorporation of PDMSmacromonomers with different PDMSchain length, whereas potential ion-spe-cific effects could be regulated by ionexchange reactions.To evaluate the anti-icing performanceof these hydrogels, the authors investi-gated the heterogeneous ice nucle-ation (HIN) temperature (TH), which isthe temperature at which ice nucleationis observed. The presence of ions in thehydrogels that induced a more ice-likewater structure show a higher TH (e.g.,17.7C for iodine), whereas ions thatpromote a liquid-like water structureinduced a lower TH (e.g., 29.9C forperfluorooctanoate). This latter hydro-gel surface even remained unfrozen upto 4,800 s at 28C, demonstrating itshigh ice nucleation inhibition perfor-mance. Increasing the thickness of thehydrogels in the range from 50 nm to1,000 nm raised the TH significantly,regardless of the nature of the coun-terion, indicating that thinner coatingsare more efficient for suppressing icenucleation. Furthermore, the incorpo-ration of longer PDMS macromono-mers overruled the ion-specificityleading to a TH of around 24C, inde-pendent of the counter anions.Chem 6, 808–831, April 9, 2020 821Even if a hydrogel coating fully sup-presses ice nucleation, ice-crystals canbe induced by contaminants, e.g., dustor defects. Therefore, it is also importantto prevent propagation of formed icecrystals for efficient anti-icing perfor-mance. The effect of the anti-icing hydro-gel composition on the ice propagationrate was investigated in the highlightedstudy by monitoring the area that iscovered by ice in time, revealing thatit also depends on ion-specific andhydrophobic interactions. Similar to theice nucleation, the propagation ratedecreased when the counterions werechanged from iodine to perfluoroocta-noate as the water in the hydrogel has amore liquid-like structure with perfluor-ooctanoate counter anions. Furthermore,the incorporation of longer PDMS chainsdecreased the ice propagation timebecause water can no longer penetratein the hydrogel obstructing the ion-spe-cific interactions that suppress ice nucle-ationandpropagation. Ingeneral, it couldbe concluded that the hydrophobicityand ion specificity of the hydrogel actin a synergistic manner leading to theeffective control of ice nucleation and icepropagation.If both the suppression of ice nucleationand suppression of ice propagationare ineffective, anti-icing can only beachieved by de-icing, that is removalof the formed ice crystals. Therefore,the authors also studied the de-icingproperties of the PDMS containingpolyelectrolyte hydrogels by deter-mining the ice adhesion strength. Theice adhesion strength of the hydrogelswas found to be reduced by morethan one order of magnitude uponincorporation of the PDMS chains thatenable the retention of a viscous lubri-cating liquid-like water layer at thehydrogel-ice interface, whereas thedifferent counterions had no significanteffect on the ice adhesion strength.822 Chem 6, 808–831, April 9, 2020The PDMS containing polyelectrolytehydrogel with short PDMS grafts of900 g/mol and perfluorooctanoatecounterions was found to be the mostefficient anti-icing hydrogel coating,which was shown to maintain low iceadhesion values after 50 icing/de-icingcycles. Furthermore, the authorsdemonstrated that the coating couldbe applied to a wide range of sub-strates (metal, ceramic, glass, and poly-mer) after oxygen plasma activation fol-lowed by surface functionalization withdopamine methacrylamide. A real-lifetest of the coating at 25C revealedthat ice nucleation could be suppressedfor 1,900 s, whereas full coverage of thesurface with ice took around 4,000 s.Nonetheless, the authors claim thatdue to the low ice adhesion strength,the ice layer could be removed by amild wind breeze of 2 m/s.Altogether, the highlighted study offersvaluable insights into the mechanism aswell as applications of hydrogels in anti-icing, especially the importance of coun-terions for the polyelectrolytes to controlwater structuring as well as the hydrophil-ic-hydrophobic balance. Nonetheless,the mechanical strength and robustnessof the reported hydrogels are still limitedas the authors reported that the surface ofthe hydrogels showed cracks after me-chanical abrasion tests. The authors pro-pose that this limitation could potentiallybe overcome by making use of dualcrosslinked hydrogels, double networkhydrogels, or composite hydrogels. It isanticipated that the versatility and func-tionality of the reported strategy to syner-gistically suppress ice formation, icepropagation, and ice adhesion opensup a new era for the development ofanti-icing materials. As such, it mightopen up real-life applications for icingprevention, and it will inspire new direc-tions for the development of next-gener-ation anti-icing materials.ACKNOWLEDGMENTSX.X. acknowledges the Chinese Schol-arship Council (CSC) (201506780014)and Ghent University Special ResearchFund (BOF) (1SC1717) for the financialsupport for his PhD program; R.H. isgrateful to Research Foundation–Flan-ders (FWO), Belgium and Ghent Univer-sity for continuous financial support.R.H. is a founder of AVROXA BVBAand a member of its scientific advisoryboard.1. Israelachvili, J., and Wennerstro¨m, H. (1996).Role of hydration and water structure inbiological and colloidal interactions. Nature379, 219–225.2. Kreder, M.J., Alvarenga, J., Kim, P., andAizenberg, J. (2016). Design of anti-icingsurfaces: smooth, textured or slippery? Nat.Rev. Mater. 1, 15003.3. Boinovich, L.B., Emelyanenko, A.M.,Emelyanenko, K.A., and Modin, E.B. (2019).Modus Operandi of Protective and Anti-icingMechanisms Underlying the Design ofLongstanding Outdoor Icephobic Coatings.ACS Nano 13, 4335–4346.4. Lv, J., Song, Y., Jiang, L., and Wang, J. (2014).Bio-inspired strategies for anti-icing. ACSNano 8, 3152–3169.5. Shen, Y., Wu, Y., Tao, J., Zhu, C., Chen, H.,Wu, Z., and Xie, Y. (2019). SprayingFabrication of Durable and TransparentCoatings for Anti-Icing Application: DynamicWater Repellency, Icing Delay, and IceAdhesion. ACS Appl. Mater. Interfaces 11,3590–3598.6. Schutzius, T.M., Jung, S., Maitra, T., Graeber,G., Ko¨hme, M., and Poulikakos, D. (2015).Spontaneous droplet trampolining on rigidsuperhydrophobic surfaces. Nature 527,82–85.7. Kim, P., Wong, T.-S., Alvarenga, J.,Kreder, M.J., Adorno-Martinez, W.E., andAizenberg, J. (2012). Liquid-infusednanostructured surfaces with extreme anti-ice and anti-frost performance. ACS Nano6, 6569–6577.8. Jung, S., Tiwari, M.K., Doan, N.V., andPoulikakos, D. (2012). Mechanism ofsupercooled droplet freezing on surfaces.Nat. Commun. 3, 615.9. Liu, K., Wang, C., Ma, J., Shi, G., Yao, X.,Fang, H., Song, Y., andWang, J. (2016). Januseffect of antifreeze proteins on ice nucleation.Proc. Natl. Acad. Sci. 113, 14739.10. He, Z., Wu, C., Hua, M., Wu, S., Wu, D., Zhu,X., Wang, J., and He, X. (2020). BioinspiredMultifunctional Anti-icing Hydrogel. Matter 2,723–734.

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Bio-inspired hydrogels as multi-task anti-icing hydrogel coatings

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