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Functional hydrogel coatings

Functional hydrogel coatings Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 National Science Review 8: nwaa254, 2021 REVIEW doi: 10.1093/nsr/nwaa254 Advance access publication 5 October 2020 MATERIALS SCIENCE 1,2,3 1,2 4 1,∗ Junjie Liu , Shaoxing Qu , Zhigang Suo and Wei Yang ABSTRACT Center for X-Mechanics, Key Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically, Laboratory of Soft chemically and electrically compatible with living tissues. There has been intense research and development Machines and Smart of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently, Devices of Zhejiang functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various Province and substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, Department of biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness, Engineering toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings: Mechanics, Zhejiang (i) applications and functions enabled by hydrogel coatings, (ii) methods of coating various substrates with University, Hangzhou 2 different functional hydrogels with tough adhesion, and (iii) tests to evaluate the adhesion between 310027, China; State functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review. Key Laboratory of Fluid Power and Keywords: hydrogel coatings, coating methods, coating tests, adhesion, hydrogel applications Mechatronic System, Zhejiang University, Hangzhou 310027, hydrogels include tissue engineering [7], wound INTRODUCTION China; Applied dressing (Fig. 1a) [8,9], contact lenses [10] and drug Hydrogels, a family of soft matter [ 1], are aggre- Mechanics and delivery (Fig. 1b) [11,12]. Hydrogels also play key Structure Safety Key gates of water molecules and hydrophilic polymer roles in stretchable devices and soft robotics, such Laboratory of Sichuan networks. The high water content, which can ex- as muscle-like actuators (Fig. 1c) [13–15], hydro- Province, School of ceed 95% by weight ratio, enables hydrogels to dis- gel fish (Fig. 1d) [16], soft displays [ 17], stretch- Mechanics and solve and transport ions and many small molecules. able ionotronics (Fig. 1e) [18,19], skin-like sensors Engineering, The polymer networks, which are often sparsely (Fig. 1f) [20] and axon-like interconnects [21,22]. Southwest Jiaotong crosslinked, lead to hydrogels being soft and elas- University, Chengdu One critical challenge in the application of hydro- tic. Hydrogels may have originated with the begin- 610031, China and gels is achieving strong bonding between hydrogels ning of life on earth. They are ubiquitous in nature, John A. Paulson and other materials. For instance, hydrogels have from muscle and cartilage in animal tissues to xylems School of Engineering long been developed as adhesives for wound closure, and phloems in plants [2–4]. The first report on and Applied Sciences, however for decades the adhesion was limited below synthetic hydrogels used in biomedicine was pub- Kavli Institute for the order of 10 J/m [23]. A transformative advance lished in 1960 in literature [5]. Since then, the de- Bionano Science and took place when Yuk et al. adhered hydrogels to non- Technology, Harvard velopment of hydrogels with new applications and porous surfaces [24] and elastomers [25] with ad- University, Cambridge, enhanced mechanical robustness has attracted re- hesion greater than 1000 J/m . The high adhesion MA 02138, USA searchers across chemistry, physics, engineering and was achieved through the synergy of two effects: medicine. the covalent bonds between the polymer network Corresponding In industrial applications, hydrogels are relatively in the hydrogel and the substrate, and the energy- author. E-mail: new compared with metals, ceramics and many dissipating sacrificial bonds in the hydrogel. Since yangw@zju.edu.cn other forms of polymer. The diversity of hydrogels, the publication of these studies, other strategies for natural and synthetic, with different polymer topolo- achieving tough hydrogel adhesion have been re- Received 8 March gies and chemical compositions, makes them highly ported, such as stitching wet materials (tissues and 2020; Revised 22 adaptive to a vast array of applications [6]. Func- September 2020; hydrogels) with biocompatible polymer chains [26], tional hydrogels can chemically, mechanically and Accepted 23 and in situ bonding of dissimilar polymer networks electrically mimic the functions of biological tissues September 2020 by bulk modification using silanes [ 27]. Hydrogel [4]. Established medical applications of functional The Author(s) 2020. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed un der the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 Figure 1. Typical applications of hydrogels. (a) A hydrogel seals a cut on a pig heart with strong adhesion. The figure is redrawn from ref [ 9]. Reprinted with permission from AAAS (American Association for the Advancement of Science). (b) Hydrogels in drug delivery applications. The feature size of the drug-loaded hydrogel dictates the pertinent delivery routes. The figure is redrawn with permission from ref [ 12], Springer Nature. (c) A hydrogel actuator, used as a bladder detrusor, can help to shrink an animal bladder. The figure is adapted with permission from ref [ 14], John Wiley and Sons. (d) An optically and sonically camouflaged hydraulic hydrogel fish in water. The figure is adapted with permission from ref [ 16], Springer Nature. (e) A novel transparent loudspeaker made from ionically conductive hydrogel and dielectric elastomer. The figure is adapted from ref [ 33]. Reprinted with permission from AAAS. (f) Hydrogel sensors for stretch and compression testing. The figure is adapted with permission from ref [ 20], John Wiley and Sons. hydrogel machines, which focuses on devices and adhesion is a supramolecular synergy of chemistry, robotics that incorporate hydrogels. Liu et al. re- topology and mechanics [28]. viewed recent advances in hydrogel machines [34], Functional hydrogels have been successfully describing the functions enabled by hydrogel coat- coated onto various substrates of arbitrary shapes ings and various interactions between hydrogel with strong bonds [24,25,29–32]. The hydrogel- coatings and substrates. coated substrates combine the functions of both This paper reviews the emerging topic of func- the substrates and hydrogels, enabling new func- tional hydrogel coatings. Emphasis is placed on their tions and applications, for example as biocompatible functions and applications, fabrication, and methods medical devices [31], lubricious nitinol guidewire in medicine [32] and a transparent loudspeaker [33]. to evaluate their adhesion. We list the functions and In addition, hydrogel coatings play an essential role potential applications of hydrogel coatings reported as structural components in the emerging field of in literatures. Methods that allow various hydrogels Page 2 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 Table 1. Hydrogel coatings in medical area. Function/property Hydrogel Substrate/device Bonding Drug/nanoparticle Reference Drug delivery Hyaluronan and poly-D, L-Lactide cobalt-chrome, polyethylene, PA vancomycin, amikacin, [35] titanium tobramycin, gentamicin, sodium salicylate Gelatin titanium alloy PA antimicrobial peptide, laponite [36] nanosilicate PEG-heparin polyurethane CA silver nitrate [37] PMBV/PVA titanium alloy CA paclitaxel (PTX) [38] PHEMA titanium alloy PA ciprofloxacin (CIP) [ 39] PEGPLA iridium oxide PA nerve growth factor (NGF) [40] PEGDA-co-AA titanium PA silver nanoparticle [41] PEGDA stainless steel PA copper nanoparticles [42] PHEMA silicone CA nerve growth factor (NGF) [43] Lubricity Chitosan/PVA polyurethane urethral catheter CA [44] PAAm nitinol guidewire CA [32] Chitosan polyethylene tube CA [45] Chitosan endovascular catheter CA [46] Anti-biofouling PEG polyamide composite CA [47] PEG/polycarbonate silicone rubber CA [48] Zwitterionic PCB powdered carbon PA [ 49] HEMA-co-DHPMA glucose sensor PA [50] pCBAA gold/silicon dioxide PA [51] PEGMA gold CA [52] Conductive PVA/PAA iridium oxide CA [53,54] coatings for PU iridium oxide CA [55] neural electrode Alginate/PPy gold PA [56] Alginate/PEDOT gold PA [57] PVA/PEDOT platinum PA [58] AA, acrylic acid; CA, covalent anchorage; DHPMA, 2,3-dihydroxypropyl methacrylate; PA, physical attachment; PAA, poly(acrylic acid); PAAm, polya crylamide; PCB, poly(carboxybetaine); pCBAA, poly(carboxybetaine acrylamide); PEDOT, poly(3,4-ethylenedioxythiophene); PEG, poly(ethylene glycol); PEGDA, poly(ethylene glycol diacrylate); PEGMA, poly(ethylene glycol) methacrylate; PEGPLA, poly(ethylene glycol)-poly(lactic acid); PHEMA, poly(2-hydroxyethyl methacrylate); PMBV, poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate-co-p-vinylphenylboronic acid); PPy, polypyrrole; PVA, polyvinyl alcohol; PU, polyurethane. to be coated onto different types of substrate with Drug delivery arbitrary shapes are addressed. Finally, we highlight Hydrogels are appealing candidates for use as car- the thickness-dependent adhesion of hydrogel coat- riers in drug delivery systems that transport drugs ings and the experimental methods for evaluating to target sites and release them at a controlled rate. the adhesion. Conclusions and outlook are given at This drug delivery, controlled in both space and time, the end of this review. improves the accuracy of drug allocation and re- sults in fewer side effects. The feature size of a hy- drogel dictates the possible delivery routes. For ex- FUNCTIONS AND APPLICATIONS ample, a drug-loaded transdermal hydrogel patch OF HYDROGEL COATINGS releases a drug by attaching to the human body Hydrogel coatings endow the coated bulk materi- and a drug-loaded microgel transports a drug via als with new functions, while having negligible in- oral or pulmonary delivery. Details for the design fluence on the mechanical properties of the bulk of hydrogels for controlled drug delivery can be materials. The diversity of hydrogels in chemical found in a recent review [12]. The main functions components and network topologies makes them of drugs or nanoparticles loaded into hydrogel coat- appealing candidates as coating materials. The func- ings on implanted medical devices such as stents tions and applications enabled by hydrogel coatings [59] (Fig. 2a) and neural electrodes [40,43] in- can be roughly classified as medical (Table 1) and clude anti-bacterial [36,37,41,42], anti-coagulation non-medical applications (Table 2). The details of [37], anti-inflammatory activities [ 37], nutrient de- these applications will be discussed in the sequel. livery to surrounding tissues [40,43], and prevention Page 3 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 Table 2. Hydrogel coatings in non-medical area. Sensing Hydrogel Substrate Bonding Detection Output Reference Poly(HEMA-co-AAc) silicon wafer CA volatile vapor optical signal [67] Poly(AAm-co-AAc) poly(AAm-co-AAc-co silicon wafer, glass, PET, PDMS CA copper ions, glycoprotein optical signal [68] -AAene) with imidazole ligands or PBA Gelatin long-period grating (LPG) CA humidity optical signal [69] GPBS imprinted PAH gold CA glucose SPR spectrum [70] PAAm FBG PA salt concentration transmission spectrum [71] Actuation Hydrogel Substrate Bonding Stimulation Reference PAA PDMS CA pH [32] PNIPAM, PAA, hydrogel IP pH, ionic strength [72] PEODA PVA-DEEDA-borax gel BOPP HB moisture, temperature, light [73] PNIPAM Glass CA temperature [74] PAAm PDMS CA humidity [75] PNIPAM/PVA PDMAEMA/PSS IP temperature, pH [76] Anti-marine creature fouling Hydrogel Substrate Bonding Marine creature Reference PAMPA/PAAm, PVA polyethylene FA algae, sea squirts, barnacles [77] PVA-glycerol stainless steel FA barnacle (balanus albicostatus) [78] PEG stainless steel, nylon HB diatom [79] PEG glass, silicon, PS CA barnacle, algal zoospores, diatom [80] MMA-co-AA-co- glass FA barnacle [81] TBSM PEG glass CA marine bacteria and diatom [82] Oil-water separation Hydrogel Substrate Bonding Contaminant Reference DKGM Hydrogel glass fabric PA oil, organic dyes, and heavy metals [83] PAAm magnetic nickel foam PA oil, dichloromethane [84] PAAm stainless steel mesh PA vegetable oil, gasoline, diesel, and crude oil [85] Alginate PAA-g-PVDF IB crude oil [86] Alginate filter paper PA crude oil [ 87] BOPP, polypropylene; DEEDA, N, N-diethylethane-1,2-diamine; DKGM, deacetylated konjac glucomannan; GPBS, glucose phosphate barium salt; HB, hydrogen bond; IB, ionic bond; IP, interfacial penetration; NaCl, sodium chloride; PAA, polyacrylic acid; PAAm, polyacrylamide; PAH, poly(allylamine hydrochloride); PBA, phenylboronic acid; PBG, fiber Bragg grating; PDMAEMA, poly(2-(dimethylamino)ethyl methacrylate); PDMS, poly(dimethylsiloxane); PEG, polyethylene glycol; PET, poly(ethylene terephthalate); PNIPAM, poly(N- isopropylacrylamide); PS, polystyrene; PSS, poly(sodium-p-styrenesulfonate); PVA, poly(vinyl alcohol); PVDF, poly(vinylidene fluoride); SPR , surface plasmon resonance. of implant-related infection in orthopedics [35,39]. accelerated by using a biodegradable polymer net- Examples of drug-loaded hydrogel coatings are listed work to construct the hydrogel coating. Moreover, in Table 1, including the loaded drugs or nanopar- by tailoring the molecular interactions between ticles, the constituents of the hydrogel coatings and the hydrogel polymer network and the drugs—i.e. the nature of the bonding between the hydrogel covalent linkage, electrostatic interaction and hy- coatings and substrates. In order to load drugs into drophobic interaction—the drug release rate can be hydrogel coatings, the drugs are generally dispersed efficiently tuned. in the hydrogel precursor, and the subsequent cur- ing entraps the drugs inside the hydrogel coating. Lubricity Another approach is to immerse a cured hydro- Some lubricious biological surfaces, such as the gel coating in a drug solution. The release rate of cartilage of animal joints, are essentially hydrogels the loaded drugs is determined by the diffusion of consisting of fibrous collagen and proteoglycans. the drugs into the surrounding tissues and can be Page 4 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 Figure 2. Examples of hydrogels in coating applications. (a) Drug-loaded hydrogel-coated metallic stents. The figure is modified with permission from ref [59], John Wiley and Sons. (b) Slippery hydrogel coating on biomedical tubing and a Foley catheter. The figure is modified with permission from ref [31], John Wiley and Sons. (c) Lubricious hydrogel coating on a nitinol guidewire used in surgical operation. The figure is redrawn with permission from ref [32], John Wiley and Sons. (d) Biocompatible and conductive hydrogel coating on cochlear electrode arrays made from platinum. The figure is adapted with permission from ref [58], IEEE (Institute of Electrical and Electronics Engineers). (e) Stimuli-responsive hydrogel coatings in a sensing system based on the interference of light. The figure is modified with permission from ref [ 67], John Wiley and Sons. (f) Anti-oil fouling hydrogel coating on a model boat. The figure is redrawn with permission from ref [ 32], John Wiley and Sons. duct or vessel, allowing drainage and administration Synthetic hydrogels, with non-adhesive dangling of fluids or gases and access for other surgical in- chains on their surfaces, are able to achieve an struments, such as guidewire. The typical biomate- extremely low coefficient of friction, on the or- −4 rials for the construction of catheters are polymers, der of 10 [60]. The tribological behavior of hy- such as silicone and poly(vinyl chloride) (PVC), drogels is significantly different to that of solids which are inert and unreactive to body fluids but ex- and is influenced by several factors including the perience high friction with surrounding tissues and chemical structure of the hydrogel, the surface are likely to suffer biofouling. Direct insertion of an properties of the sliding surface and the measure- uncoated catheter is likely to cause trauma to the ment [61]. One explanation attributes the low fric- surrounding tissue during the operation. Lubricious tion to the presence of a hydrated layer between biocompatible hydrogel coatings enable the inser- the hydrogel and the sliding substrate [61]. The tion of catheters into tortuous anatomical pathways lubricity of the hydrogel-coated surface is impor- with reduced tissue irritation (Fig. 2b) [31]. An- tant for some medical applications, such as con- other example is metal guidewire, which is a medi- tact lenses [10], catheters [45,46,62] and medi- cal device used to deliver implants such as stents to cal guidewire [32]. For example, a catheter with a thin tube structure serves a broad range of func- a desired site. Making guidewire surface lubricious is tions in treating diseases or performing surgical pro- instrumental to easing surgical operation and allevi- cedures. Catheters are inserted into a body cavity, ating patient discomfort (Fig. 2c) [32]. Page 5 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 A hydrogel-based ionic cable mimics the function Anti-biofouling of an axon in terms of ionic conductivity [20]. A The term biofouling describes the contamination hydrogel-based ionic conductor can transmit elec- of surfaces by the adhesion of organisms and their trical signals at high speed, 16 orders of magnitude by-products [63]. Surface biofouling of implanted higher than the diffusivity of ions in solution, en- medical devices is caused by the adhesion of mi- abling the ionic conductor to transmit electrical sig- crobial or thrombotic agents due to foreign body nals with high frequency (up to 100 MHz) over a response [64]. Biofouling limits the lifetime of distance of 10 cm. Furthermore, an ionic conduc- implanted medical devices, and can even result in tor can transmit enough power to turn on a light- their removal and replacement [50]. Biofouling emitting diode, even if the resistivity of the ionic is a complex process related to the physical and conductor is several orders of magnitude higher chemical properties of the target surface. The than that of an electronic conductor [20]. In med- anti-biofouling properties of implanted medical ical applications, hydrogel ionic conductors inherit devices can be enhanced by surface modifications, the biocompatibility of the hydrogel and are ideal including controlling surface hydrophilicity and conductive coatings for neural electrodes. Neural charge, biomolecule functionalization, and drug electrode probes, which mainly consist of rigid met- elution [64]. For hydrophilic surfaces, resistance als, such as platinum, gold and iridium for the to the adhesion of fouling agents is ascribed to electrode, and silicon, polyimide and ceramic for the hydration layer formed between the coating the construction material, are electronic medical and the surrounding environment. This hydra- devices implanted into the brain or other electri- tion layer serves as a physical barrier to resist the cally excitable tissue to record electrical signals or adhesion of fouling agents. Hydrogels, as well- stimulate neurons with electrical impulses. Hydro- known hydrophilic materials, can substantially gel coatings serve as a mechanical buffer between enhance the hydrophilicity of the coated surface. a rigid neural electrode and soft tissue, attenuat- Hydrogels commonly used as anti-biofouling ing the formation of glial scars resulting from the coatings on implanted medical devices include trauma induced by the micro-motion between the poly(vinyl alcohol) (PVA), polyethylene glycol electrode and tissue [44,55,65,66]. Glial scar en- (PEG) and natural polysaccharides, such as chitosan capsulation on the electrode surface increases the and dextran [47,48,50,52]. PEG has been credited as impedance between the electrode and tissue, re- a gold standard material [64]. The positive charges sulting in the failure or degradation of the neu- on the backbone of polysaccharide hydrogels are ral signal transmission. When coated with con- thought to be effective in disrupting the lipid mem- ductive hydrogel, the chronically implanted neural brane of microbes, resulting in antimicrobial activity. electrode maintains a low impedance over 1 bil- Another category of anti-biofouling hydrogel coat- lion stimulations (Fig. 2d) [58]. The biocompat- ings is zwitterionic hydrogels, which are neutral ibility of the hydrogel coating also effectively re- but have an equal number of positive and negative duces the loss of the neural cells around the elec- charges along the polymer network [49,51]. The trode [44]. Furthermore, the conductivity between most frequently studied zwitterionic materials the electrode and tissue can be enhanced by de- are prepared by adhering poly(sulfo-betaine) and positing a layer of conductive polymer—poly(3, 4- poly(carboxybetaine) to methacrylate or acrylamide ethylenedioxythiophene) (PEDOT)—on the metal backbones. The hydration layer formed on the sur- electrode before coating with the hydrogel [56,57]. face of the zwitterionic hydrogel coatings is strongly Nerve growth factor-loaded hydrogel coating is able bonded to the coating surface through electrostatic to enhance neuron survival and promote the dif- interactions. ferentiation of neural cells around the electrode [40,43]. Conductive coatings for neural electrode Sensing Mobile ions obtained by dissolving salt in the wa- ter component of hydrogels endow hydrogels with The diversity of hydrogel chemistry enables a wide ionic conductivity. The resistivity of salt-containing range of responses, for example deformation and hydrogel can be reduced to ∼10 m, compared change of transparency, to various stimuli such with ∼18.2 Mm for pure water [19]. Living mat- as temperature, pH, magnetic field and chemicals. ter mostly uses ions to conduct electrical signals, Stimuli-responsive hydrogels are appealing candi- while machines exclusively use electrons. An emerg- dates for soft actuator and soft sensor applications. ing research field, known as hydrogel ionotron- Stimuli-responsive hydrogel coatings incorporated ics, has been rapidly evolving in recent years [19]. with other substrates or devices generate several new Page 6 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 sensing systems. Hydrogel interferometry prepared structure is the most commonly used configuration by coating stimuli-responsive hydrogel on silicon for the construction of a stimuli-responsive hydro- wafer is sensitive to volatile vapor, humidity, copper gel coating-based actuator [73,76]. The two layers ions and glycoprotein through the change of hydro- can be prepared from different stimuli-responsive gel thickness, presenting different visible structure hydrogel coatings, achieving reversible and bidirec- colors due to the interference of the light reflected tional actuation. Using the bilayer structure as a by the hydrogel and silicon wafer surface (Fig. 2e) primary element, complex configuration transfor- [67,68]. As the detection is based on the diffusion mation from 2D to 3D can be achieved via struc- of target small molecules to the hydrogel coating ture design or patterned active hydrogel coating through the thickness direction, the thin hydrogel [72,75]. Furthermore, temperature-responsive hy- coating (on the order of 100 nm) used in this sens- drogel coatings can drive the motion of fluids in mi- ing system enables a fast response to an equilibrium crofluidic devices by cyclic swelling and deswelling state. The implementation of this sensing system is stimulated by temperature [74]. For more informa- the result of the synergy of chemistry, mechanics and tion on hydrogel actuators, please refer to previous optics. A hydrogel-coated fiber Bragg grating (FBG) reviews [88]. allows the measurement of salinity. Under different saline concentrations, the hydrogel coating swells to reach different equilibrium states, resulting in varied Anti-marine creature fouling FBG stretching and the corresponding shifts in the wavelength of the reflected ‘Bragg’ signal. In prin- A submerged surface in the marine environment ciple, this sensing system can be extended for the suffers the accumulation of marine fouling organ- detection of other chemical species through modi- isms, such as algae, diatoms and barnacles, known fication of the gel chemistry with the same sensor as marine creature fouling [89]. Marine creature construction [71]. Based on surface plasmon reso- fouling slows vessels, resulting in additional energy nance spectroscopy, glucose phosphate barium salt- consumption and vessel maintenance costs. Fur- imprinted hydrogel coating on a gold surface yields thermore, the life of the hull can be shortened owing a selective sensor for glucose. When gold nanopar- to the corrosion caused by the attached organisms. ticles are incorporated into the hydrogel coating to The fouling process is affected by several physical give a higher refractive index, the sensitivity of the and chemical properties of the surface, such as the sensor is significantly enhanced, making the sensor surface tension, wettability, modulus, roughness capable of detecting glucose at a level of μg/mL in and chemistry. Coating the target surface with deionized water [70]. anti-fouling materials is the mostly widely adopted approach to achieving anti-fouling properties for submerged substrates in the marine environment [89]. The earliest coating materials include pitch, Actuation tar, wax and heavy metals such as lead. However, The stimuli-responsive hydrogels used as active ma- these materials have a low durability and limited terials for actuation feature a relatively large actua- anti-fouling performance. In the mid-1960s, a more tion deformation triggered by external stimuli, such durable tributyltin (TBT)-containing self-polishing as pH, temperature, magnetic field and hydraulic anti-fouling coating was formulated, which shows or pneumatic pressure. The stimuli responses of hy- prolonged anti-fouling efficiency for up to five years drogels are readily implemented by using a stimuli- as a result of the continuous release of TBT from the responsive polymer network for hydrogel formation coating. However, TBT was banned in 2008 owing (e.g. poly(N-isopropylacrylamide) for temperature to its adverse effects on aquatic life [ 82]. Exploring response, and polyacrylic acid for pH response); environmentally friendly, durable anti-marine embedding active elements in the hydrogel matri- creature fouling coatings has since been urgently ces (e.g. magnetic particles for magnetic field re- pursued to protect submerged surfaces and the sponse); and designing structures with chambers marine environment. Examples of such functional or channels for actuation by hydraulic or pneu- coatings include natural anti-fouling compounds, matic pressure [34]. Active hydrogels for actua- silicone and fluorocarbon polymers with low surface tion that are in coating form are mostly based on energy, and hydrogels. The anti-fouling characteris- stimuli-responsive polymer networks. Under stim- tic of hydrogel coatings is attributed to their highly uli, the conformational transformation or change of hydrated surfaces. Several functional hydrogel crosslink density of the polymer network induces coatings, such as PVA and poly(2-acrylamide- swelling/deswelling of the hydrogel, resulting in an 2-methyl-propanesulfonic acid)/polyacrylamide expansion or contraction for actuation. A bilayer (PAMPS/PAAm) double network tough hydrogel, Page 7 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 PEG and zwitterionic hydrogels, have been shown 380 μm, with rough nanostructured hydrogel [85]. to be effective in inhibiting the attachment of This hydrogel-coated mesh exhibited selective and marine creatures in experiments [77–82]. For effective ( >99%) separation of water from various example, a polyacrylamide hydrogel-coated model oil/water mixtures including vegetable oil and even boat floating in a water-filled tank contaminated crude oil. These superhydrophilic and underwater with oil shows anti-oil fouling properties (Fig. 2f) superoleophobic hydrogel-coated filtration films for [32]. Functional hydrogel coatings therefore have oil/water mixture separation have the advantages potential for application where adhesion by marine of non-fouling and cyclic usage, which make them creatures is undesired. appealing candidates in industrial oily wastewater treatment and oil spill cleanup. Oil-water separation Oily wastewater pollution is becoming a worldwide HYDROGEL COATING METHODS threat to marine and aquatic ecosystems owing to in- An ideal hydrogel coating method should achieve creasing oily wastewater from industry and frequent two goals: strong adhesion to the substrate and con- oil spill accidents [86]. The demand for reusable formation to a substrate with an arbitrary shape. oil-water separation materials with high efficiency Strong adhesion requires a strong interaction, such and low cost is becoming increasingly urgent. Due as covalent bonding, at the interface of the hydrogel to the intrinsic immiscibility of water and oil, ma- coating and substrate, and strong cohesive strength terials with extreme affinity either to water or oil of the hydrogel coating itself. The strong adhesion are promising candidates for application in oil-water prevents the delamination or fracture of the hydro- separation [90]. ‘Oil removing’ materials are super- gel coating due to cyclic swelling or repeated sliding hydrophobic or superoleophilic materials that al- against the surroundings, while a method of coating low the infiltration of oil while leaving the water on a substrate with an arbitrary shape is important. Uni- the other side [83]. Examples of these materials in- form hydrogel coatings can easily be cast on a flat clude polyester textiles, carbon-based materials, hy- substrate with a mold, but coating on a substrate with drophobic aerogels, polystyrene and polytetrafluo- a complex shape can be challenging. In the following roethylene (PTFE) coating mesh. However, these section we only describe the coating methods that materials are prone to pore clogging and surface foul- meet the above two requirements. ing by oil due to their intrinsic oleophilic properties, resulting in the degradation of separation efficiency and a short life. The surface bridge method In contrast to ‘oil removing’ materials, ‘water removing’ materials are superhydrophilic and un- The adhesion of hydrogels to other substrates by derwater superoleophobic and do not suffer the simple attaching is intrinsically low owing to the limitations of ‘oil removing’ materials. Hydrogels presence of abundant water at the interface [17]. are well-known hydrophilic materials whose water Strong hydrogel adhesion is the result of strong in- component can exceed 95% in weight. Hydrogels teraction between the polymer network of the hy- themselves as base materials for oil-water separa- drogel and the target substrate. The surface bridge tion have the disadvantage of low water flux—below method for strong adhesion complies with the fol- 2 −1 −1 30 L/m h bar —as the mesh size is on the or- lowing principle: the two ends of a bridge molecule der of 10 nm [91]. Coating commercial filtration can form a strong interaction with the hydrogel and films with hydrogels combines the advantages of the substrate separately, establishing strong bonding at high flux of the filtration film and the hydrophilic- the substrate-coating interface. ity of the hydrogel [83,84,87]. Gao et al. coated a The commonly used bridge molecules for hy- thin alginate hydrogel film onto the surface of a poly- drogel coatings are silanes or silane-coupling agents acrylic acid-grafted -poly(vinylidene fluoride) (PAA- such as 3-(trimethoxysilyl) propyl methacrylate g-PVDF) filtration membrane via layer-by-layer self- (TMSPMA) and (3-aminopropyl) triethoxysilane 2+ assembly of sodium alginate and Cu .This hy- (APTES), as shown in Fig. 3a[24,92]. The alkoxy drogel coated PAA-g-PVDF filtration film achieved groups at one end hydrolyze into silanol groups in an 2 −1 −1 a high water flux of up to 1230 L/m h bar aqueous environment and condense with hydroxyl with high separation efficiency—above 99.8%—and groups on the target surface to form siloxane bonds, an outstanding cyclic performance [86]. Instead of enabling strong bonding between the target sub- coating hydrogel on the surface of the filtration strate and the bridge molecules. The other end of membrane, Xue et al. coated micro-scale porous the APTES has an amino group that can condense metal substrates, with mesh sizes ranging from 34 to with a carboxyl group on the polymer network of Page 8 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 Figure 3. Hydrogel coating methods. (a) The surface bridge method. The bridge molecules used are (3-aminopropyl) tri- ethoxysilane (APTES) and 3-(trimethoxysilyl) propyl methacrylate (TMSPMA). The two ends of the molecules form covalent bonds with the substrate and hydrogel coating separately, enabling strong bonding of the hydrogel coating to the substrate. The figure is redrawn with permission from ref [ 24], Springer Nature. (b) and (c) The surface initiation method. Hydrophobic benzophenone photo-initiator is adsorbed onto the surface of the target substrate by diffusion or an additional primer. The subsequent curing of a hydrogel precursor on the treated substrate enables a hydrogel coating to be strongly bonded to the substrate. (b) is adapted with permission from ref [31], John Wiley and Sons. (c) is adapted with permission from ref [30], John Wiley and Sons. (d) The hydrogel paint method. A paste-like hydrogel paint is applied to a pre-treated substrate using common painting operations such as brushing and dipping. The crosslinking of the hydrogel coating and strong bonding for- mation between the coating and substrate complete in the curing process of the hydrogel paint. The figure is adapted with permission from ref [32], John Wiley and Sons. hydrogels such as alginate and hyaluronan by EDC- 1000 J/m to be achieved [24]. In principle, as long Sulfo NHS chemistry. For TMSPMA, the vinyl as the substrate is rich in surface hydroxyl groups group at the other end can participate in the poly- the method is valid. Inorganic solids, such as glasses, merization of the hydrogel-coating precursor, en- metals and ceramics naturally have hydroxyl groups abling covalent bonding between the hydrogel coat- on their clean surface exposed to the air. While for ing and the TMSPMA. some organic solids, such as elastomers and plastics, A hydrogel coating can be firmly bonded to a tar- the hydroxyl groups can be obtained by surface treat- get substrate through bridge molecules. This method ments such as oxygen plasma and UV ozone [32]. was first used by Yuk et al. for bonding tough hydro- A hydrogel precursor is usually cast on the treated gels to non-porous surfaces, such as metal, glass, sil- substrate and then cures in a mold to hold the icon and ceramics, enabling adhesion of more than shape and to isolate oxygen, which prohibits the Page 9 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 polymerization of hydrogel precursor [24,25]. In VHB (Very High Bond) and Ecoflex with the help principle, coating hydrogel on a substrate with an of an appropriate organic solvent. The benzophe- arbitrary shape can be readily achieved by the sur- none absorbed in the elastomer serves as a grafting face bridge method as long as the hydrogel pre- agent to bond the polymer network of the hydrogel cursor can be grafted onto the substrate without to that of the elastomer and as an oxygen scavenger a mold. A printable hydrogel precursor, which be- to alleviate the oxygen inhibition effect [ 25]. When haves like a paste, meets this requirement and can exposed to ultraviolet (UV) light, a benzophenone be applied to the target substrate by brushing or dip- molecule abstracts a hydrogen atom from the poly- ping. The printable hydrogel precursor can be pre- mer network of the substrate and generates a free pared by adding a rheology modifier, such as nano- radical, mediating the grafting of the hydrogel net- clay, micro-gel or long natural or synthetic polymer work to the polymer network of the elastomer [25]. chains to the water-like hydrogel precursor [93]. To The validity of the surface initiation method using spread the hydrogel precursor on the target substrate benzophenone is limited to polymers that can swell steadily, the surface energy of the substrate should be in benzophenone organic solution and supply hy- higher than the surface tension of the hydrogel pre- drogen to benzophenone for free-radical generation. cursor. The surface energy of the target surface can Two different methods of forming a hydrogel coat- be increased using numerous techniques including ing on benzophenone-adsorbed polymers have been plasma treatment, corona treatment and acid etch- adopted. The first involves the application of a paste- ing. Conversely, additives such as surfactants can be like hydrogel precursor to the target surface and then added to the hydrogel precursor to reduce its sur- exposing it to UV light for curing, as described for the face tension. The subsequent curing of the hydrogel surface bridge method [29]. Owing to the lower sur- precursor can be carried out in an oxygen-free en- face energy of most polymers compared with that of vironment, usually in a nitrogen-filled chamber. The the hydrogel precursor, which mainly comprises wa- loss of water during curing is another consideration. ter, additional treatment of the polymer surface to Humectants such as lithium chloride can be added increase its surface energy—for example with oxy- to the hydrogel precursor or the chamber humidity gen plasma—is necessary to ensure a steady spread can be kept high to alleviate the water loss. of the hydrogel precursor on the polymer. Oth- The validity of the surface bridge method in hy- erwise, the hydrogel precursor might bead on the drogel coating formation relies on the functional hydrophobic polymer surface. groups in the bridge molecules. For example, the An alternative method is to immerse the treated widely used TMSPMA works for substrates with polymers in a water-like hydrogel precursor, which abundant hydroxyl groups on the surface and hy- is a mixture of water, hydrogel monomer and hy- drogels prepared by free-radical polymerization of drophilic initiator, as shown in Fig. 3b[31]. The vinyl monomers. In contrast, APTES is suitable hydrophilic initiator initiates the polymerization of for hydrogels that are rich in carboxyl groups in hydrogel monomers within and above the surface- the polymer network. Furthermore, it is possible bound diffusion layer of the substrate. Meanwhile, to design new bridge molecules and incorporate the hydrophobic initiator initiates the polymeriza- different functions, such as on-demand breakage. tion inside the diffusion layer of the polymer, graft- For example, Li et al. designed a bridge molecule ing the hydrogel polymer chains to the network of containing a carboxylic acid group at one end, the substrate. In addition to the hydrophobic photo- which realized strong bonding with the metal sur- initiator (benzophenone), a hydrophobic thermo- face through coordination of deprotonated carboxy- initiator (benzoyl peroxide) has also been used as late groups to metal ions, and electrostatic and hy- another example of this method. Thermal initiation drophobic interactions, and a methacrylic group at is advantageous in cases where reaching the desired the other end, which chemically bonded with the hydrogel coating site with the light required to ini- hydrogel network through copolymerization. By in- tiate polymerization is challenging [31]. As a result, troducing a breakable disulfide bond in the bridge the polymer networks of the substrate and hydro- molecule, they achieved on-demand debonding of gel interpenetrate at the interface and form covalent the hydrogel–metal interface by stimulation with re- linkages, achieving strong bonding. The weakly at- duction agents such as glutathione [94]. tached polymer chains on the surface of the elas- tomer can be rinsed away with water, leaving a strongly bonded ‘hydrogel skin’ on the target sub- The surface initiation method strate. This method enables a ‘hydrogel skin’ on poly- Benzophenone is a hydrophobic photo-initiator that mers with an arbitrary shape, a tunable thickness can diffuse into the surface of polymers such as ranging from 5 to 25 μm and resistance to prolonged polydimethylsiloxane (PDMS), polyurethane, latex, shear forces. Page 10 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 The surface initiation method was further de- (ii) free-radical polymerization usually applies to an veloped by Takahashi et al. to achieve hydrogel oxygen-free environment; (iii) a liquid-like hydrogel coating of substrates that are unable to adsorb ben- precursor requires a mold to hold its shape before zophenone directly by diffusion [ 30]. Their method solidification, which hinders the implementation of consists of two steps, as shown in Fig. 3c. The target hydrogel coatings on complex structures. substrate, regardless of whether it is plastic, rubber, Recently, a new method known as the hydro- ceramic or metal, is coated with a primer, which is gel paint method has been put forward for coat- a mixture of benzophenone and poly(vinyl acetate) ing different substrates with various hydrogels, as (PVAc). This primer layer physically bonds with the delineated in Fig. 3d[32,95]. To make a hydrogel target substrate with strong adhesion. A paste-like paintable, the key is the decoupling of the three pro- hydrogel precursor is then applied to the treated sur- cesses of hydrogel coating formation given above. face, followed by photo-initiated polymerization in As schematically illustrated in Fig. 3d, the hydro- an oxygen-free environment. High adhesion of over gel paint is a solution of copolymers of hydrogel 1000 J/m was achieved. The coatings are wear resis- monomers and coupling agents, and behaves like a tant and stable after soaking in pure water in an am- viscous liquid or a common paint. The functional bient environment for 282 days. groups on the coupling agents have tunable kinetics In principle, the surface initiation method is suit- to interact with each other and with the complemen- able for hydrogel coating formation on most sub- tary functional groups on the substrate for hydrogel strates. For polymers, the hydrophobic initiators can paint curing and strong bond formation after apply- diffuse into their surface with the help of an appro- ing the hydrogel paint to the target substrate using priate solvent (e.g. ethanol, acetone). For other sub- various painting technologies. Several embodiments strates, such as metals and ceramics, an initiator con- of hydrogel paint are achieved by copolymerizing taining primer, such as poly(vinyl acetate), can be various hydrogel monomers, such as acrylamide applied on the target surface to introduce the initia- for lubricious, acrylic acid for pH-sensitive and tors. The surfaces of most polymers have low surface N-isopropylacrylamide for temperature-responsive energy compared with that of water, which hinders applications, with silane coupling agent (TMSPMA) the wetting of the hydrogel precursor on the target through vinyl groups. The condensation rate of substrate. To achieve steady spreading of a paste-like silanol groups in hydrolyzed TMSPMA molecules hydrogel precursor on the substrate or better diffu- in the copolymers and the hydroxyl groups on sion of the hydrogel monomers into the target sur- the target substrate can be tuned using pH and face, a substrate with high surface energy is preferred curing temperature. Accordingly, the shelf life or for better wetting. To meet this requirement, surface curing time of hydrogel paint can be tuned from treatment (e.g. plasma treatment, corona treatment hours to days. Furthermore, the shelf life of the and acid etching) of the target surface can be carried hydrogel paints can be significantly extended to out for a higher surface energy, or additives (e.g. sur- months by freeze drying, storing and re-dissolving in factant) can be added to the hydrogel precursor to water. lower the surface tension. The hydrogel paint strategy enables the division of labor. The paint maker is responsible for paint formation using sophisticated chemistry, while the The hydrogel paint method paint user need not be hampered by the need for so- In general, hydrogel coating formation using free- phistication and is responsible for substrate prepa- radical polymerization involves three processes: ration, painting and curing. The toxic monomer and polymerizing hydrogel monomers to polymer initiator are mostly consumed during paint forma- chains, crosslinking the polymer chains to give a tion so that the paint user does not need to han- hydrogel network, and bonding the network to the dle the toxic compounds. The tunable viscosity of target substrate. These processes occur concur- hydrogel paint, using either water content or rhe- rently in one step. For example, a polyacrylamide ology modifiers, allows it to be applied and grafted onto substrates with complex structures. The oxygen hydrogel coating on a silica substrate is completed insensitivity of hydrogel paint curing and bonding in a one-step polymerization between the vinyl with the substrate, for example through silane con- groups of the acrylamide monomers, the crosslinker densation, ensures the hydrogel coating formation is (N, N -Methylenebisacrylamide, MBAA) and the successful in an ambient environment. All of these silane molecules (TMSPMA) anchored on the advantages make the hydrogel paint method an ap- silica substrate. The disadvantages of this traditional approach are clear: (i) the handling of the toxic hy- pealing way of hydrogel coating formation in real drogel monomers and initiators is not user-friendly; applications. Page 11 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 The functional groups or coupling agents in hy- procedure for removing these undesirable small drogel paints determine their applications and va- molecules is by incubating the cured hydrogel in ex- lidities. For example, silanol groups can condense cess deionized (DI) water for several hours to days with hydroxyl groups on target substrates for strong with regular replenishment of the DI water [96–99]. bonding, and condense with each other for solidifi- The time for the diffusion of these small molecules cation of the hydrogel paint. Silane condensation is out of the hydrogel depends on the characteristic oxygen insensitive. Thus, the bonding and curing of size of the hydrogel sample L, which can be esti- hydrogel paint can be carried out without preclud- mated as L /D, where D is the diffusion coefficient ing oxygen. The hydrogel paint method of hydro- of these small molecules in DI water (typically on the −9 2 gel coating was relatively recently introduced and is order of 10 m /s at room temperature). open for further exploration and development. ADHESION OF HYDROGEL COATINGS Purification of hydrogel coatings Adhesion between the hydrogel coating and sub- Free-radical polymerization of hydrogel precursors, strate, which is the energy needed to peel away a which mainly consist of monomers, crosslinkers 2 unit area of hydrogel coating and has units of J/m , and initiators, is a prevailing method in hydrogel quantifies the resistance to debonding of the coating. preparation. The conversion efficiency of hydrogel However, rigorous characterization of the adhesion monomers to a hydrogel network does not reach of hydrogel coatings is lacking for three main rea- 100%, which leaves unreacted monomers and ini- sons: first, strong adhesion of hydrogel coatings to tiators that are toxic and may leach out of the hy- substrates is relatively new. Second, hydrogels have drogel matrix. The removal of these residual small only recently been integrated with other materials molecules before use is important in real applica- such as dielectric elastomers in hydrogel ionotronics tions, particularly in biomedical areas. The common and metals in biomedical devices. Third, the thick- ness of the hydrogel coating can be as thin as several microns, which makes testing adhesion challenging. In this section, we discuss the origin of the thickness- dependent adhesion of hydrogel coatings and the test methods used to test the adhesion. Thickness-dependent adhesion A material-specific length, termed the fractocohesive length, was recently defined [ 100]. This length is the ratio of /W , where  is the material toughness ob- tained by stretching a sample with a crack and W is the fracture work obtained by stretching a sample without a crack. The fractocohesive length is com- parable to the flaw sensitivity length of the mate- rial. The ultimate strength of the material, such as ultimate stretch, is not affected by the existence of a crack that has a feature size smaller than the flaw sensitivity length (Fig. 4a and b) [100,101]. The frac- tocohesive length can be compared with the size of the fracture process zone, which is a highly stretched Figure 4. Fractocohesive length of a hydrogel. (a) The fractocohesive length of a hy- zone in the vicinity of the crack tip. The material drogel, estimated by the ratio of /W , determines the transition flaw length of the in the fracture process zone is highly stretched, dis- hydrogel—from flaw insensitive to flaw sensitive—with the existence of a flaw. The sipating energy through inelastic processes (such figure is adapted with permission from ref [ 101], Elsevier. (b) The rupture stretch of a as viscoelasticity, poroelasticity for hydrogels and piece of polyacrylamide hydrogel is almost a constant with pre-existing flaws or cracks chain scission) and storing elastic energy by elas- of lengths smaller than the fractocohesive length (around 1 cm in this case). Otherwise, tic deformation. A recent theory indicates that both it decreases with increasing flaw size. The figure is redrawn with permission from ref the energy dissipated during loading and the release [100], Elsevier. (c) Left: schematic diagrams of the adhesion of a hydrogel coating as a of the stored elastic energy corresponding to crack function of coating thickness in a peel test. Right: experimental results for the adhesion of polyacrylamide hydrogel coatings with various coating thicknesses in peel tests. The propagation contribute to the toughness as well as figure is adapted with permission from ref [ 102], Elsevier. the adhesion of the hydrogel coating [102,103]. Page 12 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 Peeling away a hydrogel coating involves two thickness dependent adhesion of viscoelastic adhe- length scales: the thickness of the hydrogel coating sive has been well studied both in theory and experi- and the fractocohesive length of the hydrogel. The ment [104,105]. In light of thickness-dependent ad- difference between the two length scales defines two hesion, it is important to clarify the thickness of the cases: small-scale and large-scale inelasticity peel. If hydrogel coating when specifying the adhesion. the coating thickness is smaller than the fractoco- Another commonly used length scale is called the hesive length, the fracture process zone is bounded elasto-adhesive length, defined as /E , where E is by the coating thickness. We call this case the large- the Young’s modulus [106]. This length scale defines scale inelasticity peel. In this case, the adhesion in- the size of a nonlinear elastic region at the crack front creases as the coating thickness increases. Other- where linear elastic fracture mechanics (LEFM) are wise, the fracture process zone is bounded by the no longer valid. Considering the high stretchabil- fractocohesive length, which is a constant and in- ity of soft materials, the elasto-adhesive length can dependent of the coating thickness (Fig. 4c). The be greater than the fractocohesive length by several orders of magnitude. The peel test To test the strength of the adhesion of a hydro- gel coating to a target substrate, the coating must be peeled off the substrate either through the sub- strate/hydrogel interface or through the hydrogel coating. The peel test is typically used to evaluate the adhesion of adhesive tapes [107] and has been further developed to test the adhesion of metal and ceramic films [ 108,109]. Recently, this method has been adopted to test the adhesion of hydrogel coat- ings. To peel off a hydrogel coating, a flexible and inextensible backing layer (typically a polyester film with a thickness of around 100 μm) is generally at- tached to it (Fig. 5a). A peeling result is related to many factors, including the peel speed, the backing layer used, the mode of bonding the backing to the hydrogel coating, and the angle through which the hydrogel coating is peeled off. The interpretation of the peel results is based on the work balance in steady crack propagation, in which the crack propagates at a constant speed while the peel force is invariant over peel distance. Considering the constant elastic en- ergy stored in the hydrogel coating and neglecting the elastic energy stored in the flexible and inexten- sible backing, the work imparted by the steady peel force is a direct reflection of the adhesion of the hy- drogel coating. For simplicity, the backing layer can be pulled at an angle of 90 (Fig. 5a) or 180 degrees to the substrate, giving an adhesion of F /h or 2F /h, ss ss Figure 5. Test methods for the adhesion of hydrogel coatings. (a) The 90 degree peel where F is the steady peel force and h is the width ss test. A hydrogel coating is peeled off the substrate by a backing attached at an angle of the hydrogel coating. A general expression is of 90 degrees to the substrate. (b) The simple stretch test. A stretchable substrate = F /h(1 − cos θ), where θ is the peel angle ss coated with hydrogel coating is loaded by uniaxial stretch, resulting in debonding of with respect to the substrate. It is noted that the the coating under a critical load. (c) The scratch test. A rigid stylus is drawn across steady peel force is a function of the peel angle the surface of the coating, accompanied by a synchronous movement in the thickness θ [110]. At different peel angles, the deformation direction of the coating. (d) The probe pull test. A flat tipped indenter is bonded to states at the crack front of the hydrogel are differ- a hydrogel coating and then pulled away. (e) The double cantilever beam test (left) ent, resulting in different crack modes. Although the and contoured double cantilever beam test (right). A hydrogel coating is sandwiched between two cantilever beams. The two beams are pulled apart, resulting in fracture peel test gives a global result for adhesion, namely of the hydrogel coating. the energy needed to peel off a unit area of hydrogel Page 13 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 coating, it reveals nothing about the mechanism of crack propagation. If the hydrogel coating is much debonding [106]. stiffer than the substrate (for example, one bonds an An experimental challenge of the peel test is inextensible backing on the other side of the hydro- how to firmly bond the backing layer to the hy- gel coating), the stretch of the bilayer segment is neg- drogel coating without disrupting the coating or ligible, thus λ = 1. In this case, the energy release debonding during the test. Cyanoacrylates (known rate is G =−U (λ ) + P (λ − 1). This energy re- as superglues) are commonly used to achieve strong lease rate is ascribed to the release of the elastic en- bonding. Cyanoacrylate monomers diffuse into the ergy in the substrate and the change of the potential hydrogel polymer network and form polycyanoacry- energy by the applied force. The energy release rate late chains through anionic polymerization in topo- G corresponding to the propagation of the crack logical entanglement with the hydrogel polymer under critical pull force P gives the adhesion of the network. Meanwhile, the polycyanoacrylate chains hydrogel coating. cause densely packed non-covalent interaction with the non-permeable plastic backing [111]. When the hydrogel coating is very thin, the diffusion of the The scratch test cyanoacrylate into the hydrogel leads to appreciable In this test, a rigid stylus is drawn on the surface of influence on the integrity of the hydrogel coating, the coating, accompanied by a synchronous move- compromising the peel results. The diffusion depth ment in the thickness direction of the coating in a of cyanoacrylate into hydrogel has been found to continuous or stepwise manner, as shown in Fig. 5c. reach ∼10 μm[112]. The peel test requires a new The force on the stylus is recorded during scratch- approach for achieving strong bonding between the ing and the critical force at which the failure of the coating and the backing layer. Nevertheless, if the coating occurs is determined [114]. The scratch test thickness of the hydrogel coating is much greater is regarded as semi-quantitative in determining the than the diffusion depth of the cyanoacrylates, the adhesion of the coating because the critical scratch side effects induced by the adhesive are negligible. force extracted from the test is affected by numerous factors that are not adhesion-related, such as scratch- ing speed, stylus tip radius and substrate hardness, among others. Interpreting the critical scratch force The simple stretch test in terms of the adhesion using a mechanical model is This method is effective for testing weakly bonded intractable [115]. Efforts have been made for limited hydrogel coatings on stretchable substrates, for ex- cases, such as hard coating/soft substrate systems, ample a weakly bonded hydrogel coating on an and for limited failure modes, such as coating detach- elastomer such as PDMS or VHB [113]. When ment ahead of the stylus [116,117]. Careful theoret- applying stretch to the specimen, as shown in ical considerations or simulations are required to re- Fig. 5b, the coating and substrate deform and store late the experimental results to the adhesion of the elastic energy. Assuming the specimen undergoes coating. Fortunately, the scratch test has the advan- uniaxial tension, we denote the stretch in the coat- tages of being easy to use and no special specimen ing/substrate bilayer segment λ and that in the sub- preparation being required. The critical scratch force strate single layer segment λ . The combination of obtained is effective as quantitative but relative data the elastic energy stored in the specimen and the po- to evaluate adhesion between coatings [118]. tential energy of the applied force give the potential energy of the system as U = C [U (λ ) − P λ ] + (L − C )[U (λ ) − P λ ], where U is the elastic b s The probe pull test energy stored in a unit area of the substrate, U is the elastic energy stored in a unit area of the coat- In the probe pull test, a cylindrical, flat or ing/substrate bilayer, and P is the force per unit hemispherical-ended probe is bonded to the width of the hydrogel coating. The applied energy hydrogel coating. The probe then pulls the coat- release rate is G =−∂U /∂C . Accordingly, we get ing off the substrate with a constant velocity, as G = U (λ ) − U (λ ) + P (λ − λ ). Consider- schematically shown in Fig. 5d[119]. The pull b s ing that the hydrogel coating is usually thin and force as a function of pull distance is recorded, has low modulus, the bonded hydrogel coating has which is mainly characterized by four parameters: negligible influence on the deformation of the sub- the peak stress σ , the maximum extension ε , max max strate, yielding λ = λ . The energy release rate the plateau stress σ and the work of debonding is therefore simplified to G = U (λ ) − U (λ ), W . Like the adhesion obtained in the peel test, b s deb which implies that the release of the elastic energy the four parameters obtained in the probe pull test stored in the hydrogel coating contributes to the are quantitative evaluations of different hydrogel Page 14 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 coatings under various test conditions. The probe coatings also show huge potential in non-medical pull test has the same issue with bonding the probe applications, such as environmentally friendly and the hydrogel coating together as the peel test. anti-fouling coatings for marine vessels, lubricious If the probe debonds along the interface with the coatings for soft devices and ionic conductors coating during the test, the pull result fails to reflect in stretchable ionotronics. Functional hydrogel the bonding between the hydrogel coating and the coatings are expected to play a key role in various target substrate. applications. Hydrogel coating methods have started to The double cantilever beam test achieve tough adhesion in laboratories, but some gaps remain in the translation to mass production. In this test, a coating is bonded between two can- The requirement for surface bridge and initiation tilever beams, as illustrated in Fig. 5e[120]. A load methods, such as treatment of the target substrate is then applied vertically to the end of the beam. The for strong bonding and an oxygen-free chamber load is denoted P and the deflection between the with high humidity for hydrogel coating curing, are two ends of the beams is denoted δ. The coating be- accepted in research but may not be economical for tween the beams has a pre-crack of length a. Based mass production in many applications. In addition, on a simple beam model, in which the length of the the surface initiation method leads to substantial beam equals the crack length a and the boundary wastage of the hydrogel precursor. The hydrogel of the beam at the crack tip is fixed, the energy re- 2 2 paint method uses traditional painting techniques 3BP a lease rate is given by G = , with b being the 2b for coating. The paint user is only responsible for width of the specimen and B = being a con- Ebh the painting and waiting for curing and is free of any stant of the beam (E is the Young’s modulus of the involvement with the preparation of the hydrogel beam, and h/2 is the thickness of each beam). If the paint. However, the current hydrogel paint strategy modulus of the coating is much lower than that of is only suitable for functional hydrogel coatings the two bonded beams, the soft coating will give rise with single network and is unable to achieve a to a deflection of the beams in the uncracked zone. very tough hydrogel coating. The compatibility of The simple beam theory is not sufficient for this the hydrogel paint strategy with tough hydrogel case. Instead, the energy release rate G was derived formulation approaches such as double networks, from a model of a beam on an elastic foundation and nanoparticle and fiber reinforced composites, [121–123]. The shortcoming of the traditional dou- has not been investigated so far. The development ble cantilever beam test is the requirement for ac- of tough hydrogel paint requires immediate action. curate measurement of the crack length. A modified The strong adhesion between hydrogel coatings double cantilever beam test, known as the contoured and target substrates is the main mechanical con- double cantilever beam test, was subsequently de- sideration in evaluating the quality of functional vised as shown in Fig. 5e[124,125]. If the critical hydrogel coatings. The incorporation of strong energy release rate is irrelevant to the crack length interaction at the coating-substrate interface and a, one requirement is that the value of Ba /b is a the significant dissipation of energy in the bulk of constant. Thus, the height of the cantilever beams h the coating or substrate would give an adhesion of 2 3 satisfies a /h = constant, which gives a contoured more than 1000 J/m under static loading condi- profile of the beam. The energy release rate of the tions. However, the adhesion under cyclic loading coating is obtained once the critical load for crack conditions has been reported in only a few cases propagation is measured. [126–128]. It has been shown that the toughness of a piece of hydrogel under cyclic loading can be two CONCLUSIONS AND OUTLOOK orders of magnitude lower than that under static loading conditions [129]. It is therefore question- In this paper, we review the recent advances in func- able whether the adhesion of the hydrogel coating tional hydrogel coatings, with focus on coating func- under cyclic loading is the same as that under tions and applications, coating methods, and coating static loading. This question is open to substantial tests. The majority of the established applications of further investigation. Several mechanisms have functional hydrogels are in the field of biomedicine. been put forward to improve the fatigue threshold Hydrogel coating acts as a versatile solution for im- of hydrogels [130–132], and attention should proving the biocompatibility of preformed materials be paid to preparing fatigue-resistant hydrogel or devices that will interact with living tissues, such coatings. as implants for plastic surgery, artificial menisci, implanted neural electrodes and biosensors. Fur- The long-term stable adhesion of functional hy- thermore, drug-loaded and anti-bacterial hydrogel drogel coatings in harsh environments—such as in coatings can reduce the risk of infection and in- seawater and in biological conditions including body flammation of living tissues. Functional hydrogel fluids and blood—is important for their application. Page 15 of 19 Downloaded from https://academic.oup.com/nsr/article/8/2/nwaa254/5918005 by DeepDyve user on 16 February 2021 Natl Sci Rev, 2021, Vol. 8, nwaa254 However, studies in this field are limited and scat- FUNDING tered. Both the interfacial bonding between hydro- This work was supported by the National Natural Science Foun- gel coatings and substrates and the functional hy- dation of China (11525210, 91748209) and the Fundamental drogel coating itself might degrade after extended Research Funds for the Central Universities (2020XZZX005– soaking in these environments, resulting in the fail- 02) to J.L. and S.Q. This work was also supported by the National ure of the coating by delamination or fracture. For Science Foundation (DMR-14–20570) to Z.S. example, a lightly stretched siloxane bond adopted in achieving strong adhesion of a hydrogel coating will hydrolyze into two silanol groups in the aqueous AUTHOR CONTRIBUTIONS environment [133]. The design of hydrogel coatings W.Y. conceived the idea. All authors contributed to the writing of that require long-term survival should therefore be the manuscript. carefully considered. A versatile test method for measuring the ad- Conflict of interest statement . None declared. hesion of hydrogel coatings is lacking, particularly when the coatings are extremely thin. The essential REFERENCES difficulty is how to peel the coating off in a quantita- tive manner. For example, in a peel test, a hydrogel 1. Yang W, Wang H and Li T et al. X-mechanics—an endless fron- coating is peeled away from a substrate uniformly, tier. Sci China-Phys Mech Astron 2019; 62: 14601. assisted by a backing, and the steady peel force is a 2. Mow VC, Kuei S and Lai WM et al. Biphasic creep and stress direct measurement of the adhesion. The peel test is relaxation of articular cartilage in compression: theory and ex- an ideal method for obtaining the adhesion owing to periments. 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