Application Prospect of Multifunctional Hydrogel Coating in Household Field

Abstract

Hydrogel coatings are regarded as an ideal material for enhancing the health, safety, and environmental friendliness of the home environment, owing to their outstanding antifouling, flame-retardant, anticorrosive, and antibacterial properties. To fully exploit the performance advantages of hydrogel coatings in the domestic realm, this review comprehensively examines their preparation methods, the progress of modification research, and the application status in other fields. It is revealed that hydrogel coatings can not only offer benefits by dint of their inherent flame retardancy and oleophobicity but also encapsulate chemical substances within the porous structure of certain special hydrogel coatings, thereby augmenting their anticorrosive and antibacterial capabilities. Moreover, the favorable interface adhesion between hydrogel coatings and diverse substrates, along with extensive modification research, has furnished novel concepts for applications in the domestic domain, including but not limited to the multifunctional surface modification of soft furniture, kitchen and bathroom furniture, and children’s furniture. The research findings demonstrate that hydrogel coatings hold substantial potential for enhancing the functionality and environmental sustainability of household products.

1. Introduction

Currently, a comfortable and elegant indoor home environment is what people strive for in pursuit of happiness, health, and an improved life. This is due to the fact that the indoor home environment is closely intertwined with everyone’s daily existence, as people spend a considerable amount of time indoors. Consequently, a healthy, safe, “green”, and environmentally friendly indoor home setting has become a matter of concern for governments, home and material production enterprises, and consumers globally. However, for a long time, the safety and health aspects of coatings in indoor home decoration still have much room for improvement. Toxic substances like formaldehyde present in coatings used for indoor decoration and furniture products significantly impact people’s health. Additionally, as people increasingly seek efficient and convenient living environments, the demand for multifunctional furniture surface coatings is on the rise. Therefore, the development trend of multifunctional green and environmentally friendly coatings is growing strongly.

Hydrogels typically employ water as the dispersion medium and are constituted by water-soluble polymers. Crosslinking structures exist among these polymer chains, thereby forming a relatively stable network framework. Within this network, hydrophilic groups interact with water molecules, binding water molecules within the network and rendering the hydrogel soft and moist [1]. Concurrently, hydrophobic groups may be incorporated into certain hydrogel systems, which can induce hydrophobic interactions under specific conditions, exerting a significant impact on the structure and performance of hydrogels [2]. Microscopically, hydrogels generally exhibit a porous structure. As depicted in Figure 1, the SEM image of the cryogel clearly reveals its porous morphology.

In 1960, Wichterle and Lim initially synthesized crosslinked hydrogels, which sparked great interest among many scholars. Consequently, various parties conducted research on the performance of hydrogels [4]. Hydrogel coatings hold great application prospects in fields such as environmental engineering [5] and biomedicine [6,7], giving rise to a large number of relevant studies. High-strength and tough functional hydrogel coatings have emerged as a new research trend, and their application fields have expanded accordingly. Compared to other traditional coatings, hydrogel coatings not only possess advantages such as environmental friendliness, oil repellency, and flame retardancy but also achieve multifunctional effects like antibacterial and anti-corrosion through their unique structure. In comparison with various existing green coatings, they have the advantages of low cost and easy preparation, which can maximally meet consumers’ demand for multifunctional green coatings. Multifunctional hydrogel coatings are a type of coating material with multiple characteristics and functions. They are usually composed of hydrogel and other functional components and have important applications in many fields. Simultaneously, a large amount of research on hydrogel modification can also compensate for its disadvantages in swelling, mechanical strength, and interface bonding, paving the way for its application in the household field.

2. Preparation Methods of Hydrogel Coating

The methods for preparing hydrogel coatings are primarily classified into physical crosslinking and chemical crosslinking approaches, as presented in

Table 1. Comparison of the advantages and disadvantages of different preparation methods of hydrogel coatings.

	Method	Features	Advantages and Disadvantages
Physical crosslinking method	Electrostatic interaction	Mild reaction conditions and easy implementation	No initiator, non-toxic, and green, but poor thermal stability and thermal reversibility
	Crystal crosslinking	Thermal reversibility
Chemical crosslinking method	Monomer crosslinking polymerization	Based on monomers, with high structural and performance controllability	High strength and good stability, but the initiator is toxic
	Graft copolymerization	Using natural polymers as the matrix, combining the advantages of natural polymers and synthetic monomers
	Water-soluble polymer crosslinking	Based on the original polymer structure, the reaction is mild, there are multiple technological choices, and it has many applications in the fields of biology and daily life

. Given that the preparation of physical gels does not necessitate the use of toxic crosslinking agents, in recent years, double-network (DN) or multi-network hydrogels constructed via physical crosslinking have been regarded as green strategies to enhance the comprehensive performance of biomass hydrogels [8]. Nevertheless, physical gels exhibit poor thermal stability and will transition from the gel state to a solution at a certain temperature. Additionally, their mechanical strength is relatively weak, thereby restricting their application range [9]. In contrast, hydrogels prepared by the chemical crosslinking method form a three-dimensional crosslinking network through covalent bonds, possessing high strength, excellent wear resistance, acid and alkali resistance, and good thermal stability, with a wide array of application scenarios. However, this method requires the utilization of crosslinking agents, such as glutaraldehyde, genipin, and multifunctional isocyanates. Notably, these crosslinking agents are generally toxic and not in line with environmental friendliness requirements [10].

2.1. Physical Crosslinking Method

The physical crosslinking method primarily consists of the following: (1) Electrostatic interaction. This mainly involves the formation of physical hydrogel through electrostatic interaction between polyelectrolytes and multivalent ions with opposite charges, as well as the formation of polyelectrolyte complex hydrogel by electrostatic interaction between two polyelectrolytes with opposite charges [11]. For instance, Demott et al. [12] fabricated a triple-network (TN) hydrogel. The first network was anionic PAMPS, the second network was neutral p(NIPAAm-co-AAm), and the third network was cationic poly((3-acrylamidepropyl)trimethylammonium chloride) (PAPTAC). This was mainly achieved by leveraging the electrostatic repulsive interaction within the first network, the hydrophobic interaction within the second network, and the electrostatic attraction between the first and third networks. (2) Hydrogen bonding crosslinking effect. This approach utilizes hydrogen bonding within and between molecules as physical crosslinking points, which typically occurs among functional molecules containing N-H, O-H, and F-H groups [13]. Sekizkares et al. [14] employed N,N-dimethylacrylamide (DMAA) as a hydrogen bond donor and methacrylic acid (MAAc) as a hydrogen bond acceptor. The interaction between them led to the formation of a hydrogen bond, thereby preparing a superabsorbent terpolymer hydrogel with high mechanical strength based on 2-acrylamido-2-methylpropane sulfonic acid (AMPS). (3) Crystalline crosslinking effect. By altering the temperature of the solution or the properties of the solvent, the polymers with an irregular coil distribution become intertwined and arranged to form ordered microcrystals, thus generating physical crosslinks to form a gel. Xu et al. [15] selected polyvinyl alcohol (PVA) as the model system. First, they transferred PVA from a good solvent to a relatively poor solvent to construct a uniform and loosely crosslinked PVA gel, which facilitated the expansion of the polymer conformation to homogenize the network. Subsequently, they carried out salt precipitation in a salt solution, enabling the PVA chain to form a new crystal domain. As a result, the low crosslinking density network was transformed into a high crosslinking density network, and a high-strength and high-toughness hydrogel material was synthesized. (4) Hydrophobic sssociation. Amphiphilic graft polymers can form a gel through the association of the hydrophobic parts of the polymer in water. Liang et al. [16] utilized benzyl ether methacrylate (BEM), acrylamide, and acrylic acid as monomers and sodium dodecyl sulfate (SDS) as a surfactant to prepare tough physical crosslinked hydrogels. Here, BEM is a novel amphiphilic environmental monomer with long polyoxyethylene hydrophilic blocks and long alkyl hydrophobic end groups, allowing it to be evenly dispersed in water and form effective hydrophobic association.

2.2. Chemical Crosslinking Method

The chemical crosslinking method mainly comprises the following three modalities: (1) Monomer crosslinking polymerization. This refers to the technique of fabricating polymer hydrogels through the free radical homopolymerization or copolymerization of monomers in the presence of a crosslinking agent [17]. Norioka et al. [18] synthesized a PAAm hydrogel by means of free radical polymerization, with acrylamide (AAm) as the monomer and N,N’-methylenebisacrylamide (MBAA) as the crosslinking agent. On the basis of this research, the authors put forward a universal approach in the article to optimize the balance between physical and chemical crosslinking. Specifically, it involves conducting a free radical polymerization reaction with a high monomer concentration and a low crosslinking agent content. (2) Graft copolymerization. Generally, polymer hydrogels are prepared by graft copolymerizing α-olefin monomers onto natural polymers and their derivatives. Free radical-initiated graft copolymerization is one of the principal graft copolymerization methods [19]. Mondal et al. [20] carried out graft copolymerization of acrylamide, methyl methacrylate, and chemically treated bagasse. In this process, N,N-methylenebisacrylamide served as the crosslinking agent and potassium persulfate as the initiator, resulting in the preparation of a hydrogel with super adsorption and biodegradability. (3) Crosslinking of water-soluble polymers. For example, water-soluble polymers such as PVA, polyacrylamide (PAM), polyacrylic acid (PAA), poly(N-methylpyrrolidone), and polyamine can be appropriately crosslinked to yield polymer hydrogels. This method necessitates that the raw material must be a water-soluble polymer and the crosslinking agent must be a multifunctional compound or multivalent metal ion capable of reacting with the functional groups of the polymer.

In light of the advantages and drawbacks of the chemical crosslinking method and the expansion of its application domains, numerous highly promising preparation technologies have been developed: (1) Click chemical crosslinking method. Click crosslinking is characterized by high efficiency and high selectivity. It typically employs specific chemical reactions, such as the copper-catalyzed azide-alkyne cycloaddition reaction, to rapidly form covalent bonds between polymer chains and construct gels. The reaction conditions are mild, and the reaction speed is fast [21,22]. In the biomedical field, it is prevalently utilized for drug delivery, tissue engineering, and other applications [23]. Additionally, it can be employed to enhance the surface properties of materials, including hydrophilicity, lubricity, and antifouling properties [24]. (2) Photoinduced crosslinking method. A polymer solution containing a photoinitiator and crosslinkable groups is coated onto the surface of a substrate. Subsequently, the crosslinking reaction is initiated by ultraviolet light, visible light, or other light sources, leading to covalent crosslinking between polymer chains and the formation of a hydrogel coating [25]. This method does not require heating or the addition of other chemicals, is environmentally friendly, and exhibits extremely high efficiency [26]. It is predominantly used in the fields of microelectronics and tissue engineering. (3) Natural crosslinking agent crosslinking method. Compounds derived from natural sources are employed as crosslinking agents, such as genipin [27], citric acid, tannic acid, epigallocatechin gallate, vanillin, etc. These agents react with the functional groups on the polymer chain to form covalent crosslinks, thereby preparing the hydrogel coating. These crosslinking agents generally possess good biocompatibility and biodegradability and are commonly applied in the fields of biomedicine and the food industry. (4) Double crosslinking method. Chemical crosslinking is combined with other crosslinking methods (such as physical crosslinking) to form a double-crosslinked hydrogel coating, which is mainly used in wearable devices, soft robots, and biomedical fields, among others.

3. Research Progress in Modification of Hydrogel Coatings

Owing to its unique porous structure as well as its formation and preparation mechanism, hydrogel has several disadvantages that need improvement due to the absence of the advantages of many common traditional coatings and the constraints of its own structural components. Nevertheless, the mechanical properties and biochemical properties of hydrogel coatings are adjustable [28], and their application scenarios can be enriched through modification.

3.1. Advantages Strengthening of Hydrogel Coating

(1) Antifouling properties. Hydrogels can absorb a large amount of water into their three-dimensional network structure. Water molecules within can form a highly hydrated layer through surface hydrogen bonds or ion solvation, preventing the adhesion of proteins and microorganisms and thus achieving an antifouling effect [29,30]. Scholars employing physical crosslinking, such as Ren [31] and others, designed a self-cleaning mucus-like layered fibrous bionic antifouling surface (SMCAS) through the utilization of flocking technology within electrostatic interaction. They investigated the biomass of two marine microorganisms, Nitzchia closterium f. minutissima and Marinobacter lipolyticus SM19 (T), and found that the material exhibited good antifouling properties. In contrast, certain scholars modified the crosslinking agent in chemical crosslinking to achieve the combination of chemical and physical crosslinking. They proposed a modification approach that optimized performance and was environmentally friendly by employing the double crosslinking method. Hu [32] and others prepared a low-swelling-ratio hydrogel coating by adjusting the monomer ratio of acrylic acid (AA), which has a polyelectrolyte effect, and N-(3-sulfopropyl)-N-(methacryloyloxyethyl)-N,N-dimethylbetaine ammonium (SBMA), which has an antielectrolyte effect. The zwitterionic compound SBMA imparted excellent antifouling performance to the hydrogel, and Al(OH)₃ nanoparticles were added as a physical crosslinking agent to provide enhanced mechanical properties. Through anti-protein adhesion tests, researchers determined that the adsorption capacity of large proteins could be reduced to below 20%, and all the monomer raw materials, crosslinking agents, and additives were composed of low-toxicity and environmentally friendly materials.

(2) Flame retardancy. Hydrogel inherently possesses water-rich properties and water retention capacity. It can achieve efficient fire extinguishing through heat absorption cooling, dilution, and isolation of oxygen, thereby attaining a certain flame-retardant effect [33]. Many scholars have also conducted extensive strengthening and modification research on this characteristic. In terms of physical crosslinking, Zhao et al. [34] developed a PVA/PA/MXene hydrogel fire-retardant coating by exploiting hydrogen bond crosslinking. MXene was incorporated into the gel to enhance the hydrogen bond, endowing the coating with excellent self-healing performance and high water retention. The coating was applied to wood for fire-retardant purposes via a simple one-pot heating and freeze-thaw cycle. After UL-94 testing, the ignition time at the V0 level increased from 32 s to 69 s, and the heat release rate and total heat release decreased by 41.6% and 36.14%, respectively. In the context of chemical crosslinking, Ingtipi et al. [35] utilized NaOH and borax as crosslinking agents to crosslink PVA, xanthan gum (XG), and lignin nanoparticles (LNP), thereby forming a hydrogel network. PVA, XG, and LNP are polymeric materials with a certain degree of degradability. Among them, the PX3B0.4N2L20 hydrogel coating represents the optimal combination, which can elevate the limiting oxygen index (LOI) of cotton cloth from 20% to 39%. The cone calorimeter test indicates that the minimum peak heat release rate is 90.3 ± 10 kW/m², and the minimum total heat release rate is 1.56 ± 4 MJ/m². Consequently, it is an environmentally friendly, biodegradable, and highly efficient flame-retardant hydrogel coating.

(3) Corrosion resistance. The anti-corrosion mechanism of the hydrogel flame-retardant coating can be primarily categorized into two types. Firstly, it acts as a physical barrier. The hydrogel coating is capable of forming a continuous and dense protective film on the material surface, thereby impeding the contact between oxygen, water, and other corrosive media and the protected material, and diminishing the likelihood of corrosion reactions [36]. Secondly, it involves the release of corrosion inhibitors. The three-dimensional network structure of the hydrogel can encapsulate corrosion inhibitors and, simultaneously, utilize its stimulus responsiveness to fabricate an environmentally responsive anti-corrosion coating [36]. Some scholars have conducted modification experiments employing physical crosslinking. Li et al. [37] reduced Ag⁺ in situ through dopamine-conjugated gelatin to form silver nanoparticles (AgNPs). These AgNPs can concomitantly crosslink gelatin molecules, giving rise to three-dimensional porous composite hydrogels. Immersion tests were carried out in a 3.5% NaCl solution. After 30 days of immersion, the surface of the metal samples coated with the composite hydrogel was essentially free of corrosion, whereas the uncoated samples exhibited conspicuous corrosion pits. Other scholars have also resorted to chemical crosslinking. Sun et al. [38] utilized water-soluble initiators and thermal initiators (KBSPS) to initiate ultraviolet front polymerization. The addition of nano-silica enabled the hydrogel coating to form a dense hydrogen bond and rapidly synthesize a bubble-free, self-diffusing hydrogel anti-corrosion coating in an aqueous environment. They employed SEM, AFM, FTIR, EIS, IC, XPS, and an ultra-deep field microscope to characterize the surface morphology and anti-corrosion performance of the hydrogel coating. They discovered that underwater, the hydrogel coating can physically prevent corrosive media from reaching the steel surface. After seven days under osmotic pressure, its effectiveness in blocking chloride ions was 75%. In comparison with some traditional organic solvent-based initiators or initiators containing volatile and toxic components, this study opted for water-soluble photoinitiators in the process of initiating polymerization reactions. These photoinitiators can function more effectively in aqueous environments and are less prone to generating volatile organic pollutants (VOCs). Additionally, KBSPS and nano-SiO₂ possess stable chemical properties and are non-toxic and environmentally friendly.

(4) Oil repellency. The polymer skeleton of hydrogel contains a large number of hydrophilic groups, thus possessing excellent hydrophilicity. After wetting the hydrogel with water, a hydration layer is formed on its surface, which exhibits rejection of oil and realizes oil repellency [39]. Some scholars conducted strengthening research on oil-repellent hydrogel coating using physical crosslinking. Wang et al. [40], based on hydrogen bond crosslinking, adjusted the ratio of water and ethanol in the solution system to prepare a PVA/TA-Fe³⁺ coating solution with uniform fluidity. By simple immersion and utilizing the volatility of ethanol solvent, it forms a coating on the surface of stainless steel. The contact angle between the prepared coating and oil drops underwater is >150°, with excellent underwater oil repellency. Another group of scholars conducted research through chemical crosslinking. Based on the crosslinking of water-soluble polymers, You et al. [41] fabricated a phosphorylated polyvinyl alcohol (PPVA) hydrogel by phosphorylating modified PVA. Owing to the abundant hydroxyl and phosphoric acid groups in PPVA that can be crosslinked with most metal ions, a novel type of PPVA hydrogel-coated stainless steel mesh with superhydrophilicity and underwater superoleophobicity can be prepared simply by spraying a metal ion aqueous solution. This mesh can not only separate immiscible oil-water mixtures but also separate surfactant-stabilized oil-in-water emulsions with a high separation efficiency (>99.2%). In this study, environmentally friendly PVA was utilized as the starting material, and environmentally friendly reagents were selected during the phosphating process, with no byproducts generated throughout the entire process.

(5) Antibacterial properties. The unique porous structure of hydrogel is capable of accommodating various small molecules, polymers, and particles. It can incorporate three antibacterial strategies: bacteria elimination, contact killing, and antibacterial agent release. Thus, it is a promising antibacterial material [42]. Li et al. [43] employed physical crosslinking to incorporate silver quantum dots (Ag QDs) into the PVA-chitosan (CS) dual-network hydrogel matrix. Through the synergistic interaction of hydrogen bonding, the freezing and thawing process, and chain entanglement, PVA (the first network utilized for energy dissipation) and CS (the second network employed to maintain the integrity of the hydrogel) can interpenetrate with each other. The obtained silver quantum dot-doped PVA-CS DN hydrogel-coated urinary catheter exhibits strong antibacterial and antibiofilm properties. The PCA-20 hydrogel coating, which has undergone antibacterial testing, can achieve an inhibition rate of 79% and a detectable equivalent concentration of Ag (1.19 ± 0.03 μg·L⁻¹), thus confirming the persistent antibacterial activity of the PCA-20 gel coating. In the case of chemical crosslinking, Popesqu et al. [44] utilized oxalic acid (OA) to simultaneously achieve the dissolution and ionic crosslinking of CS and, in combination with the freezing and thawing process, prepared a PVA/CS/OA nanocomposite hydrogel. Silver nanoparticles (AgNPs) were embedded therein to fabricate a PVA/CS composite hydrogel. This composite hydrogel was characterized by FTIR, TGA, SEM, XRD, etc. Testing demonstrated that the composite hydrogel exhibited antibacterial activity against Staphylococcus aureus, Klebsiella pneumoniae, and Porphyromonas gingivalis and could effectively inhibit the growth and reproduction of bacteria. The PVA and CS selected in this research are relatively green and environmentally friendly raw materials. PVA has favorable biocompatibility and degradability and is non-toxic and harmless. CS is the product of the deacetylation of the natural polysaccharide chitin and possesses excellent biodegradability and biocompatibility. OA, serving as a crosslinking agent, is a common organic acid and has environmental advantages.

3.2. Disadvantages Improvement of Hydrogel Coating

(1) Anti-swelling property. Hydrogel maintains a gel state by absorbing a large amount of water. Prolonged exposure to an underwater environment can cause the internal hydrogen bond to fail due to water molecules, leading to a reduction in cohesion and easy swelling. This results in deformation, shedding, or cracking of the coating, which limits the application of hydrogel in home scenarios [45]. Some of them were modified through physical crosslinking. Zhang et al. [46], in the absence of any chemical crosslinking agent, synthesized hydrogels via photopolymerization from hydrophilic monomers such as sulfobetaine methacrylate (SBMA) and acrylic acid (AA), as well as hydrophobic monomers like sebacate diacrylate (DA), which were dissolved in a hypoeutectic solvent (DES)/H₂O binary solvent formed by 1-butyl-3-methylimidazolium chloride (BMIMCl) and solvent ketal. The presence of hydrophobic segments effectively repelled water molecules, significantly reducing the affinity of the hydrogels for water. After 30 days of immersion testing of the PSA-DA/DES hydrogels in water, in artificial seawater, and at different pH values, they maintained a relatively stable weight, with an equilibrium swelling rate of 3% in water, thus exhibiting good anti-swelling properties. Some scholars also adopted chemical crosslinking. Bi et al. [47] prepared a CS-based ionic liquid (IL) crosslinked anti-swelling DN hydrogel. Using hydroxyethyl methacrylate-butyl acrylate-acrylic acid (P(HEMA-BA-AA)) as the first network and ILs and CS as the second network, the anti-swelling hydrogel was formed through the synergistic effect of chain entanglement, hydrophobic interaction, hydrogen bonding, and electrostatic interaction. After immersion in water for 100 days, it was observed that the weight of the hydrogel changed minimally and remained essentially unchanged. The main raw materials of the hydrogel are CS and ILs. The former is a natural polysaccharide derived from the deacetylation of chitin, which is a renewable resource with favorable biocompatibility and biodegradability. The latter has lower volatility, reducing the emission of VOCs, and, overall, does not cause any harm to the environment, fulfilling the requirements for sustainable and environmentally friendly raw materials in the concept of green environmental protection.

(2) Strong mechanical properties. Ordinary hydrogel coatings are typically fragile materials, and their application on furniture surfaces is limited mainly due to an uneven network and the lack of an energy dissipation mechanism [48,49]. Therefore, enhancing the mechanical properties of hydrogel coatings is also a hot research direction. In the case of physical crosslinking, Tang et al. [50] synthesized a fully physically crosslinked DN hydrogel by means of hydrogen bonding between the first gelatin network and the second poly(N-hydroxyethyl acrylamide) network. Tensile tests were then conducted on the resultant hydrogel. The gelatin/PHEAA hydrogel thus obtained exhibited high mechanical properties, with a tensile stress of 1.93 MPa, a tensile strain of 8.22, and a tear energy of 4584 J/m². In certain instances, Lancis et al. [51] employed citric acid to crosslink the Salecan polysaccharide, thereby forming a hydrogel with favorable performance. The hydroxyl groups in the Salecan molecule reacted with the carboxyl groups of citric acid to establish a green chemical crosslinking network. Through mechanical property tests, it was determined that the mechanical properties enhanced with the increase in citric acid concentration within the Salecan and biopolymer matrix. The addition of more Salecan and citric acid enables more chain interactions, leading to the formation of a denser crosslinked network via covalent bonds, consequently improving the mechanical properties. The crosslinking agent utilized in this chemical crosslinking is citric acid, which is a common organic acid. It is characterized by easily accessible raw materials and low cost. Moreover, it is non-toxic, possesses good biodegradability, and is readily decomposed and metabolized by microorganisms in the environment without exerting long-term negative impacts on the environment. Salecan is a biopolymer typically derived from natural biomass resources, exhibiting the properties of renewability and degradability, and thus qualifies as an environmentally friendly raw material.

4. Interface Bonding Between Hydrogel Coating and Furniture Materials

There exist two primary methods for applying the hydrogel coating: one is physical modification. This method is technically straightforward. However, the hydrogel tends to absorb water and expand, thereby generating significant internal stress, which renders the coating prone to detachment [52]. The second is chemical modification. Through covalent bonding, a strong force can be established between the hydrogel coating and the substrate, ensuring the stability of the hydrogel coating [53]. This mainly encompasses the surface initiation method [54] and the surface bridging method [55]. Wood, metal, fabric, and leather constitute the principal materials for furniture. Notably, stone generally does not require painting [56]. Consequently, if the hydrogel coating is to be applied in the household domain, it is highly essential to investigate its interface bonding with these furniture substrates.

4.1. Combination of Hydrogel Coating and Wood Furniture

Currently, there is a scarcity of research instances regarding the combination of hydrogel coatings and wood. The majority of the existing applications are concentrated in the domain of wood flame retardancy. While the integration of wood and hydrogel coatings is technically viable, it nevertheless confronts several challenges.

The feasibility is manifested in the following aspects: (1) Interaction of surface functional groups. The wood surface harbors abundant functional groups such as hydroxyl (-OH) [57]. During the preparation process, the hydrogel also contains active functional groups like hydroxyl and carboxyl (-COOH). These two can interact via hydrogen bonds, thereby endowing a certain degree of adhesion for the hydrogel coating on the wood surface. For instance, the hydroxyl group on the molecular chain of the hydrogel based on PVA is capable of forming a hydrogen bond with the hydroxyl group on the wood surface, facilitating the better adhesion of the hydrogel coating to the wood surface [58]. (2) Physical adsorption. Wood exhibits a porous microstructure, and these pores can serve as physical adsorption sites for hydrogels. In the coating process, the polymer chain of the hydrogel can infiltrate into the pores of the wood. Upon curing of the hydrogel, it becomes fixed on the wood surface in an anchor-like manner, thereby reinforcing the combination between the two. Vincent et al. [59] synthesized a novel type of borate CS hydrogel through free radical polymerization by combining N-BOC-L-histidine (BOC), Boc-β-alanine (Boc-β-Ala-OH), and Carter condensation agent (BOP), respectively. This hydrogel was then coated onto the treated wood. The wood grain pores were covered, and the gel molecules penetrated into the wood, forming a network crosslinked structure within the wood and generating a robust anti-corrosion coating on the wood surface. (3) Chemical modification. The wood surface or the hydrogel can be chemically modified to augment the combination between them. For example, by introducing certain reactive groups, such as amino groups, onto the surface of the wood and subsequently selecting a hydrogel containing functional groups that can react with carboxyl groups for coating, the amidation reaction between amino groups and carboxyl groups can be exploited to form covalent bonds, thereby significantly enhancing the bonding strength between the two.

Simultaneously, numerous difficulties remain to be resolved in the combination of wood and hydrogel coating, which also represents one of the potential research hotspots. These difficulties are as follows: (1) Uneven wood surface. The surface of wood is not entirely smooth. Its texture, knots, and other structures can lead to uneven physical and chemical properties on the surface. This renders it arduous for the hydrogel coating to attain a uniform thickness and satisfactory adhesion during the coating process. In regions such as knots, the hydrogel may accumulate or prove difficult to adhere, whereas in areas with deep texture, the hydrogel may overly permeate, thereby resulting in an uneven surface coating. (2) Water absorption and dimensional stability of wood. Wood is a hygroscopic material that absorbs or releases moisture in diverse humidity environments, leading to dimensional alterations. Such dimensional changes may induce internal stress within the attached hydrogel coating, giving rise to issues such as coating cracking and peeling. Additionally, the moisture absorbed by wood may impact the curing process of the hydrogel coating or its inherent performance, thereby diminishing the stability of the combination. (3) Compatibility of the curing process of the hydrogel coating with wood. The hydrogel coating may contract, expand, or release certain small molecules during the curing process. If these processes are incompatible with the properties of the wood, for instance, if excessive shrinkage force occurs during curing, it will cause the coating to separate from the wood surface. Alternatively, the release of small molecule substances may trigger adverse reactions with wood, which can also impinge upon the binding between the two.

4.2. Combination of Hydrogel Coating and Metal Furniture

The combination of hydrogels and metal material surfaces is predominantly applied in corrosion protection [60], sensors [61], and flexible electronics [62], among other fields. It can also confer an anti-corrosion effect on the surfaces of metal furniture. However, the direct combination of metal and hydrogel is challenging. Metal and hydrogel are two materials with disparate properties. Metal exhibits a rigid and dense crystalline structure, whereas hydrogel is a soft, water-rich material possessing a three-dimensional polymer network structure. When combined directly, a weak interaction ensues, rendering it difficult to form a robust combination. At the molecular level, there is a deficiency of effective chemical bonding or physical adsorption between the metal surface atoms and the hydrogel molecular chain, leading to inadequate adhesion. Consequently, to achieve a firm combination of hydrogel coatings and metal furniture, surface treatment is requisite. Traditional surface treatment methods encompass electrochemical methods, photochemical reactions, and layer-by-layer deposition. The interface bonding mechanisms of these methods are intricate, and the interface adhesion effect is moderate. The most promising methods at present are principally classified into two categories: one is chemical bonding and reinforcement; the second is the auxiliary combination by physical effects.

Chemical bonding and reinforcement primarily involve the introduction of surface chemical anchors between the hydrogel coating and the metal substrate to establish bridges and attain robust interface bonding [63]. For instance, Xu et al. [64] deposited a viscous polydopamine/Fe³⁺ coating-SIL on the surface of a metal medical device substrate. Subsequently, with the assistance of citric acid, Fe³⁺ ions were reduced to Fe²⁺ ions, which served as an active catalyst. Finally, surface catalytic initiation self-polymerization was carried out to enable the growth of the hydrogel coating in the monomer solution at room temperature. The interface bonding strength between the substrate and the hydrogel coating was analyzed via a 90° peel test, and a hard backing waterproof gel coating was introduced to extend along the peel direction. The test results indicated that the hydrogel coating and the metal substrate exhibited good interface bonding strength.

Physical-assisted binding primarily comprises two types: hydrogen bonding and mechanical interlocking. Hydrogen bonding entails the formation of hydrogen bonds between polar groups, such as hydroxyl and carboxyl groups, on the hydrogel molecular chain and the hydroxyl or oxide layer on the metal surface, thereby enhancing the bonding strength. For instance, during the preparation of the PVA-LM hydrogel, the multiple hydrogen bonds formed between the PVA chain and LM droplets, along with tannic acid, interact with the crystalline domain of PVA, endowing the hydrogel with favorable adhesion to the metal [65]. Mechanical interlocking involves treating the metal surface to impart roughness or a microstructure, allowing the hydrogel to infiltrate and form a mechanical interlocking structure, thereby augmenting the firmness of the bond. Generally, mechanical interlocking is combined with hydrogen bonding or chemical bonding to enhance the interface bonding between the metal and the hydrogel coating from multiple aspects.

4.3. Combination of Hydrogel Coating and Upholstered Furniture (Fabric, Leather)

When fabrics and leather are combined with the coating, numerous similarities exist. The roughness and porosity on the surfaces of fabrics and leather can furnish physical attachment sites for the hydrogel coating. Additionally, the surfaces contain certain functional groups, such as hydroxyl and carboxyl groups. These functional groups are capable of interacting with the active groups in the hydrogel, and the combination methods are essentially identical. Consequently, the subsequent discussion herein will be centered around fabrics.

The coating of fabrics with hydrogels is neither entirely straightforward nor overly difficult. From the fabric’s perspective, the surface properties of the fabric itself exert a significant influence on the bonding. Fabrics with rough surfaces and pores exhibit a greater propensity for bonding with the hydrogel coating. This is because the rough surface and pores can furnish a larger contact area and mechanical anchor points, thereby facilitating better adhesion of the hydrogel. From the hydrogel’s perspective, the binding capabilities of different types of hydrogels to fabrics vary. Consequently, in the practical integration process, both aspects need to be taken into account. Additionally, to achieve a stable combination between the fabric and the hydrogel coating, appropriate methods must be selected: (1) In situ polymerization. The monomer solution is immersed onto the fabric, following which polymerization is initiated, allowing the hydrogel to be generated in situ on the fabric surface [66,67]. Xia et al. [68] prepared a PANI composite conductive fabric via in situ polymerization, using a polyester/spandex blend fabric (comprising 92% polyester and 8% spandex) with high strength and high elastic recovery capacity as the base material. The surface of this fabric was then coated with PVA and CS to fabricate a multifunctional hydrogel (PAC hydrogel), ultimately yielding a PANI conductive fabric-based hydrogel. (2) Impregnation coating. This involves immersing the fabric in a pre-prepared hydrogel solution and subsequently causing the hydrogel to adhere to the fabric surface through drying and other treatments. This method is operationally simple; however, the bonding strength may be relatively low. At present, it is predominantly utilized as an auxiliary method. (3) Layer by layer self-assembly. By alternately depositing polyelectrolytes and hydrogels with opposite charges, multi-layer structures are constructed on the fabric surface to enhance the adhesion and coating stability [69]. This method is infrequently employed in hydrogel fabrics and is mainly applied in other fabrics or hydrogel flexible sensors. Nevertheless, it represents a promising technology for preparing highly stable hydrogel fabrics.

Currently, the prospective application fields of hydrogel fabrics are intelligent wearable textiles and composite dressings. However, these applications have not yet reached commercialization and have only been partially explored. Khan et al. [70], inspired by the supercontraction of spider silk resulting from the disordered arrangement of peptide chains under high humidity, devised the hydrogel cotton yarn intelligent yarn. Han et al. [71] synthesized an environmentally friendly nanogel with durable antibacterial and anti-adhesion properties through the copolymerization of styrene, polycaprolactone hydroxyethyl methacrylate, and polyhexamethylene guanidine hydrochloride methacrylate. This nanogel was then grafted onto the surface of cotton fiber, endowing the cotton fabric with antibacterial and antibiofilm functions, which holds promise for use as a future commercial environmentally friendly antibacterial dressing.

5. Application Prospect of Hydrogel Coating in Household Field

In summary, hydrogel coatings have the potential to serve as a novel multifunctional coating within the household domain. Figure 2 illustrates the prospective roadmap of hydrogel applications in the household field.

5.1. Surface Coating of Kitchen and Bathroom Furniture

Currently, a variety of modification methods targeting the swelling defect of hydrogel coatings upon exposure to water have emerged, significantly expanding their application fields. In the context of green homes, it is anticipated that hydrogel coatings can be utilized for the surface coating of furniture in kitchen and bathroom spaces. The advantages are as follows: (1) Improved waterproof and moisture-proof performance. The humidity levels in kitchens and bathrooms are relatively high. Hydrogel coatings, with their favorable water absorption and retention properties, can form a protective film on the furniture surface. This effectively impedes the ingress of moisture, thereby preventing furniture from deformation, mildew, and decay caused by moisture. For instance, applying hydrogel coatings to the surfaces of kitchen cabinets and bathroom vanities can substantially enhance their waterproof and moisture-proof capabilities and prolong the service life of the furniture. (2) Enhanced antibacterial and antifouling functions. Kitchen and bathroom spaces are prone to bacterial growth and staining. Hydrogel coatings can attain antibacterial and antifouling functions either by incorporating antibacterial agents or leveraging their own physical structure characteristics. On the one hand, certain hydrogel coatings can inhibit the growth and reproduction of bacteria, mitigate odor generation, and maintain a clean kitchen and bathroom environment. On the other hand, their smooth surfaces make it difficult for stains to adhere, facilitating easier cleaning even in the presence of stains and reducing the effort and frequency of furniture cleaning. (3) Temperature regulation and energy conservation. Hydrogels possess a certain degree of temperature regulation ability and can absorb or release heat in response to ambient temperature changes. In the kitchen, a significant amount of heat is generated during the cooking process, and the hydrogel coating can absorb a portion of this heat, thereby reducing the kitchen temperature. In the bathroom, during winter use, the hydrogel coating can slowly release heat, enhancing user comfort. Additionally, this temperature regulation function also contributes to reducing energy consumption and achieving a certain level of energy conservation and emission reduction.

5.2. Surface Application of Soft Furniture

Based on the preceding discussion regarding the combination of hydrogels with fabric and leather interfaces, hydrogel coatings can also offer novel and innovative concepts for the enhancement of upholstered furniture. The details are as follows: (1) Functional expansion. Soft furniture is susceptible to bacterial growth and staining. Hydrogel fabrics can attain antibacterial and antifouling functions either by incorporating antibacterial agents or capitalizing on their own physical structure characteristics. This can mitigate odor generation, maintain furniture cleanliness, and simplify stain removal, thereby diminishing the complexity and frequency of cleaning procedures. Additionally, by integrating conductive materials or sensors into hydrogel fabrics, upholstered furniture can acquire pressure-sensing and monitoring capabilities. For instance, when an individual sits on a sofa, the sofa can detect the pressure distribution of the human body and transmit the relevant data to the user or other devices. This holds substantial significance in fields such as medical rehabilitation, ergonomic research, and smart home control. (2) Enhanced durability. Hydrogel fabrics possess favorable water absorption and retention properties, enabling them to form a protective film on the furniture surface. This effectively thwarts the ingress of moisture; averts furniture dampness, deformation, mildew, and decay; and prolongs the service life of the furniture. Moreover, hydrogel fabrics exhibit a certain degree of lubricity and flexibility, which can reduce the frictional forces between objects and the furniture surface, minimize the occurrence of scratches and abrasion, and ensure the furniture surface remains smooth and flawless. Even under conditions of frequent use, it can retain a favorable appearance. (3) Development of intelligence and interactivity. With the continuous advancement of science and technology, hydrogel fabrics are anticipated to be combined with other intelligent materials or technologies, such as flexible electronic components, sensors, and communication modules, to realize the intelligent functional integration of soft furniture. For example, the integration of hydrogel fabric with temperature sensors, humidity sensors, and human body sensors empowers furniture to automatically perceive environmental changes and human body states and execute corresponding responses, such as automatically adjusting temperature and humidity and providing personalized comfort settings. The flexibility and conductivity of hydrogel fabric can also be exploited to develop soft furniture with human–computer interaction capabilities. For instance, through touch, pressure, or gestural inputs, users can interact with the furniture and manipulate various functions, such as adjusting the seat height and angle and activating or deactivating massage functions, thereby introducing greater convenience and enjoyment into people’s daily lives.

5.3. Flame Retardant Coating on the Surface of Wooden Furniture

Owing to its flame-retardant properties and strong interface bonding with wood achieved through various modification experiments, the hydrogel coating exhibits resistance to detachment and wear during daily use. Wooden furniture represents a common flammable item in domestic and office settings. In the event of a fire, it can rapidly ignite and propagate. The hydrogel flame retardant coating can form a heat-insulating and oxygen-isolating protective film upon exposure to fire. This film prevents the direct contact of flames with wooden materials, decelerates the spread of the fire, affords valuable time for personnel evacuation and firefighting rescue operations, and mitigates property losses and casualties resulting from the fire. In addition to its fire prevention capabilities, the hydrogel flame-retardant coating can also, to a certain extent, withstand the erosive effects of daily environmental factors, such as moisture, humidity, and ultraviolet light, on wooden furniture. It averts moisture-induced deformation, mildew, and decay of the wood, as well as fading and aging caused by ultraviolet radiation, thereby further prolonging the service life of the furniture. Nevertheless, the retention of water within the hydrogel coating in a home environment is an issue that demands urgent resolution. Particularly during hot summers, the moisture within the coating gradually evaporates. When confronted with a fire, the flame-retardant efficacy is diminished, and the protection afforded to wooden furniture is also weakened.

5.4. Surface Coating of Preschool Furniture

Children possess relatively weak resistance and are lively in nature, prone to bumping and getting injured, and frequently have close contact with furniture. Consequently, the surface coating of children’s furniture ought to be both green and multifunctional. Incorporating the diverse advantages of hydrogel coatings, it holds significant application prospects for the surface of children’s furniture. The details are as follows: (1) Enhanced safety/collision protection: Children are liable to collide with furniture during their activities. The hydrogel coating exhibits favorable elasticity and cushioning properties, which can effectively mitigate the damage to children’s bodies resulting from collisions and diminish the risk of accidental injuries. It is analogous to installing a soft protective pad on the furniture surface. (2) Health and environmental protection. Hydrogels generally possess good biocompatibility and degradability. Their preparation process is relatively environmentally friendly, and no harmful substances are released during usage. This meets the stringent requirements of children’s furniture for environmental protection and health and is conducive to the healthy growth of children. Additionally, certain hydrogel coatings can be supplemented with antibacterial agents, which can effectively suppress the growth of bacteria, molds, and other microorganisms. This maintains the cleanliness of the furniture surface, offers a healthier living space for children, and curtails the spread of diseases caused by contact with an unclean furniture surface. (3) Educational functionality. By adjusting the formulation and process of the hydrogel coating, distinct tactile effects can be generated, such as smoothness, roughness, and graininess. These provide children with a variety of tactile experiences and foster their tactile development and perceptual abilities.

6. Conclusions

Hydrogel coatings have exhibited a broad application prospect in the realm of modern homes due to their unique performance advantages. Considerable progress has been achieved in diversifying preparation methods, enhancing performance via modification, and exploring the combination with diverse furniture materials. They possess substantial application potential in kitchens, bathrooms, soft furnishings, wooden furniture, and children’s furniture and are anticipated to augment the functionality, durability, safety, and environmental friendliness of furniture. Nevertheless, issues such as uneven surface bonding of wood, difficulties in metal bonding, and water retention of coatings still persist. In the future, it is essential to conduct continuous in-depth research, surmount the existing problems, and optimize the comprehensive performance of hydrogel coatings. This will facilitate their extensive application in the household sector, furnish robust support for the enhancement of household life quality, and propel the development of the household industry toward a more green, intelligent, and comfortable orientation.