Stimuli-responsive Hydrogels for Textile Functionalisation : A Review

This article reviews hydrogels used for the functionalisation of textile materials. Hydrogels are reviewed according to their reason for incorporation, aspects of crosslinking, stimuli-responsive characteristics and particle size. A more in-depth focus on the eff ect of hydrogel particle size is provided, where macrogels, microgels and nanogels for textile functionalisation are considered. The advantages and disadvantages of each size group are presented. Furthermore, the correlation between synthesis conditions and the sizes of hydrogel particles is discussed, in addition to the applications of macro-, microand nanogels to textile materials and their intended uses.


Introduction
Hydrogels of stimuli-responsive polymers represent an important group of high-performance hydrated polymers that can respond to di erent stimuli from the environment.eir hydration properties enable hydrogels to absorb and retain large quantities of water or other aqueous solutions in their three-dimensional polymer networks [1][2][3][4].Namely, they can retain at least 20% water relative to their dry weight [5].Because the crosslinking of the polymer network prevents the dissolution of hydrogels in water, they swell, which causes a direct increase in their volume.
e swelling of a hydrogel is directly a ected by water-polymer interactions, which in turn are a ected by the hydrophilicity of polymers: the higher the polymer hydrophilicity, the stronger the water-polymer interactions.e water in a hydrogel can be incorporated as free or bound water.Free water is located at the outermost layer and can be easily removed via mechanical compression or centrifugation.Water attached to the polymer chain is called bound water and forms hydrogen bonds with polar groups of the polymer. is water can only be removed at very high temperatures, otherwise it remains part of the hydrogel structure.Interstitial water is physically entrapped within hydrated polymer chains.Lastly, semi-bound water possess characteristics of both bound water and free water.Swelling capacity is determined by the space within the polymer network, Izvleček V preglednem članku so predstavljeni hidrogeli, ki se uporabljajo za funkcionalizacijo tekstilnih materialov.Hidrogeli se uvrščajo glede na vzrok za aplikacijo, način zamreženja, glede na odzivne lastnosti in velikost delcev.Podrobneje se članek osredotoča na vpliv velikosti delcev hidrogela, kjer se za funkcionalizacijo tekstila uporabljajo makrogeli, mikrogeli in nanogeli.Predstavljene so prednosti in slabosti vsake velikostne skupine, prav tako pa je opisana povezava med pogoji sinteze in velikostjo delcev hidrogela, aplikacija makro-, mikro-in nanogelov na tekstilni material ter njihova predvidena uporaba.Ključne besede: pametne tekstilije, hidrogeli, odzivi na dražljaje, mikrogeli, nanogeli, metode priprave while the swelling process depends on the rate of relaxation of polymer chains and on the rate of the diffusion of water molecules [6][7].
e unique property that distinguishes stimuli-responsive hydrogels from non-responsive hydrogels is their responsiveness to minimal changes in environmental conditions (temperature, pH, ionic strength, electric and magnetic eld, light, etc.), which trigger the absorption and release of water from the polymer network. is induces a reversible volume change of the polymer network from a swollen hydrogel to a collapsed gel.
e volume phase transition of a hydrogel is attributed to a change in polymer-solvent interactions induced by external stimuli.Namely, if an external stimulus alters the polymer structure from a hydrophilic to a hydrophobic state, water will be released from the hydrogel to its surroundings, and the dehydrated hydrogel will shrink as a result [8].Since changes in a polymer structure are reversible, a hydrogel will return to its initial state when the external stimulus is absent.Furthermore, the transition from hydrogel to solution can also be triggered by external stimuli.However, these structural changes are less important for textile applications of hydrogels [5].
eir superior stimuli-responsive properties results in the classi cation of hydrogels as biomimetic "smart" polymer systems, the use of which is growing exponentially in various application elds (Figure 1).Hydrogels have already been established in biotechnology and biomedicine [9][10][11][12], where they mostly serve as sca olds for tissue engineering [13]; drug, gene and protein delivery systems [14]; superabsorbents; and biosensors and bioactuators [3].Due to their reversible swelling/ shrinking ability, stimuli-responsive hydrogels have become essential in ecology, where they are used as adsorbents in wastewater treatment for the e ective removal of dyes and heavy metals [15][16][17][18][19][20][21][22][23][24], and for the ltration of wastewater from pollution caused by oil [24][25][26].Stimuli-responsive hydrogels also present great potential and opportunities in the eld of textiles, where they are applied to di erent textile substrates to create new smart functionalities, including thermoregulation and moisture management for comfort improvement, and the controlled release of active substances for wound dressing or skin care.Few review articles have been written to date on the topic of hydrogels for use in textiles [27][28][29][30][31][32], focusing on the use of hydrogels for medical textiles and textiles for increased comfort.To provide new knowledge in the use of hydrogels for chemical modi cation of textile materials, this article focuses on the comprehensive study of the hydrogel particle size in relation to synthesis conditions and the eld of application.

Classifi cation of stimuliresponsive hydrogels
Hydrogels are usually classi ed in literature according to the nature/source of a polymer, type of crosslinking, external stimuli that trigger phase transition and the size of hydrogel particles (Figure 2).

Nature of polymers
Polymers that form stimuli-responsive hydrogels can be of natural or synthetic origins.Natural polymers, which include proteins such as gelatine [33] and polysaccharides (e.g.chitosan, alginate and κ-carrageenan [34]), are classi ed as "green smart" polymers with low toxicity and biocompatibility.Unlike natural polymers, synthetic polymers are articial and are synthesised by chemical polymerisation methods.Some of the most commonly used polymers include poly(N-alkyl substituted acrylamides), poly(N-vinylalkylamides), poly(N,N-diakylamino ethyl methacrylates) [33].

Type of crosslinking
e crosslinking of the polymer network occurs during hydrogel preparation: during the process of gelation, polymer chains begin to crosslink and form larger, branched, but still soluble, polymers.Mixtures of such poly-disperse branched polymers are called "sol".
e further entanglement of polymers leads to the formation of a so-called "gel", where the crosslinking of fully branched polymers occurs.eir solubility gradually decreases with the increasing entanglement of the polymer network. is transition is referred as the "sol-gel transition", while the critical point at which a gel rst appears is called the "gel point" [8].Hydrogels are classi ed as physically or chemically crosslinked gels with regard to the type of crosslinking.In physically crosslinked gels, polymer networks are formed via physical interactions between macromolecular chains, such as van der Waals forces, ionic interactions, hydrogen bonds or hydrophobic interactions [9,[35][36][37].Physically crosslinked hydrogels can be strongly or weakly crosslinked [38].Strongly physically crosslinked hydrogels form strong junctions between polymer chains and are analogous to chemically crosslinked hydrogels.In contrast, weakly physically crosslinked gels are linked by temporary junctions between polymer chains.ey therefore have a limited life span and are constantly changing [8].Physically crosslinked hydrogels are useful for numerous biotechnological and biomedical applications because their polymerisation process is carried out without the presence of organic crosslinking agents [38].On the other hand, chemically crosslinked hydrogels form strong covalent bonds between polymer chains [5], which make them highly stable.Covalent bonds between polymer chains can be established if the reacting polymers contain functional side groups such as OH, COOH or NH 2 in their structure [39].ey possess good mechanical properties and have a relatively long degradation time [40][41][42][43].In the preparation of chemically crosslinked hydrogels, organic crosslinking agents and initiators are usually present in the polymerisation process (Figure 3) [2].However, there are also processes in which organic crosslinkers are not used [38,[44][45][46].Hydrogels can be synthesised through radical polymerisation [38], polymerisation initiated by UV light [47], enzyme-catalysed reactions [48], and γ-ray [46] or electron beam [45] irradiation.When exposed to γ-ray or electron beam radiation, radicals form along polymer chains in an aqueous solution.e radiolysis of a water molecule results in the formation of hydroxyl groups, which can react with polymer chains to lead to the formation of microradicals. is allows the formation of covalent bonds with a crosslinked structure without the addition of an organic crosslinker [38,44].

External stimuli
Based on the origin of the stimuli to which hydrogels respond, we can distinguish between physical, chemical and biological stimuli [1,5].Physical stimuli include temperature [49], light [50], ultrasound [51], magnetic elds [52] and electrical elds [53], while chemical stimuli include solvents [54], ionic strength [55], electrochemical elds [56] and pH [54].Furthermore, biological stimuli refer to the functionality of molecules such as enzymatic reactions [57] and the identi cation of a receptor molecule [58].Hydrogels that respond to multiple stimuli can be synthesised when combining di erent responsive polymers.Table 1 summarises external stimuli and their e ect on the mechanism of swelling and shrinking of different types of hydrogels.For smart textile functionalisation, temperatureand pH-responsive hydrogels are the most studied, as these two stimuli are signi cant in physiological terms [1,5,59,[60][61].Temperature-and pH-responsive hydrogels are able to interact with the user directly, since those two stimuli can occur either through a change in temperature of the immediate surroundings of a textile material or through changes in the pH of the skin or bodily excretions such as sweat, blood and urine [62].Hydrogels based on temperature-responsive polymers possess a critical solution temperature, which can be identi ed as a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST) [63].An LCST is characterised by shrinking, which means that a polymer appears in one phase below the critical temperature and undergoes phase separation when the temperature rises above the LCST, while a UCST is characterised by swelling as the temperature rises, meaning that phase separation occurs at lower temperatures, and a change to the monophasic form occurs with rising temperature.When a temperature-responsive polymer is in a monophasic state, hydrophilic interactions are predominant, while hydrophobic interactions prevail when the conditions create a biphasic state [26,64].Polymers that respond to environmental pH are polyelectrolytes with acidic or basic pendants, which can receive or emit protons in response to stimuli from the environment.As the pendants protonate or deprotonate at a speci c pH, i.e. at pK a or pH b , the electrostatic repulsion among ionic groups produces osmotic pressure, leading to a change in the volume of a polymer.Polymers responsive to pH can be polyanions, which swell at rising pH values, or polycations, which swell at falling pH values [38,[65][66].

Hydrogel particle size
Hydrogels can also be characterised by their particle size; macro-, micro-and nanogels can be formed [68][69][70][71].Macrogel particles range in size from a micrometre or more, whereas microgel particles have a diameter of 100nm to 1µm, forming colloidally stable, water-swellable polymer networks.Nanogels occur in a size of <100nm, although the de nition in the relevant literature is o en expanded to hydrogel particles with sizes of up to 200nm [68] or even higher [69], thus overlapping with the size range of microgels.
Smaller hydrogel particles create a greater surface to volume ratio, which is re ected in shorter response times and increased surface per unit.Since the dimensions of responsive hydrogel particles and the rate of the volume phase transition are inversely proportional [66], smaller particle sizes are also reected in advantages of greater control over the swelling and entrapment and release of entrapped active substances because of the greater speci c surface area of hydrogel nanoparticles [71].
3 Stimuli-responsive hydrogels for textile applications

Working prunciple of hydrogels on textile materials
To achieve comfort improvement, the active balancing of body moisture and temperature by a textile is crucial (Figure 4).A stimuli-responsive hydrogel present on textile bres can interact with the user by detecting and responding to changes in environmental conditions.When an external stimulus dictates the Table 1: External stimuli and their e ect on the mechanism of swelling and shrinking of di erent types of hydrogels [67] External

Hydrogels with electron-acceptor groups
An electron-donor compound causes the formation of an electron donor-acceptor complex, which a ects the swelling and shrinking of a hydrogel pH pH-responsive hydrogels Change in pH causes weakly acidic or basic groups within the polymer to receive of transmit protons, which a ects the swelling and shrinking of a hydrogel

Hydrogels with immobilised enzymes
Enzymatic degradation occurs in the presence of a substrate, which creates products that a ect the swelling and shrinkage of a hydrogel swelling of a hydrogel, the porosity of the textile material decreases.Such a phenomenon causes body vapour retention and consequently heat accumulation on the skin's surface (Figure 4a).In contrast, when an external stimulus dictates the shrinking of a hydrogel structure, fabric porosity increases, thus providing breathability to the textile and greater body vapour and heat transition from the skin's surface through the textile to its surroundings (Figure 4b) [59].
Stimuli-responsive hydrogels can also be utilised for the controlled release of active substances from medical and hygienic textiles (Figure 5), which are used for accelerated wound healing or skin care.Hydrogels absorb active ingredients in the presence of environmental conditions that dictate their swelling.Active substances can be retained in a hydrogel structure (Figure 5a) until environmental conditions trigger shrinkage (Figure 5b).e reversible swelling and shrinking of a hydrogel triggered by an external stimulus provides the gradual and controlled release of active substances into the environment only under speci c conditions.
In the precipitation polymerisation of hydrogels based on poly-NiPAAm, BIS was added as the crosslinking agent.However, TEMED was used in combination with APS to initiate an anionic microgel, while a cati-onic microgel was initiated by UV irradiation [88].A nanogel based on poly-NIPAAm and poly-ALA was synthesised using the same procedure, where monomers were diluted with SDS and freeze-thawed three times.KPS was added to initiate polymerisation [80].e presence of chitosan in the synthesis of a poly-NI-PAAm-based hydrogel stabilised nanogel particles [79] and acted as a surfactant, preventing particle coagulation [59].An increase in the chitosan-to-poly-Ni-PAAm ratio decreased the size of synthesised particles of the nanogel [89].Furthermore, by increasing the temperature to 80°C, the nanoparticles of poly-NI-PAAm/chitosan hydrogels could be formed without the addition of a catalyst or surfactant.In this way, particles with a diameter of 81.2 nm were obtained [90].Other microgels used for textile modi cation have been based on carboxymethylcellulose (CMC) in combination with fumaric acid as a crosslinker [91], di-phenylalanine [83], CMC and hydroxyethyl cellulose [92] and poly (ethylene glycol) (PEG) and poly(εcaprolactone) (PCL) [61].In the case of the synthesis of nanogels, PVA- [93], β-cyclodextrin (β-CD)- [94], hydrophobised-pullulan- [95] and collagen-bearing pullulan- [96] based polymers have been used.Due to the speci c nature of each polymerisation or copolymerisation, a detailed comparison between micro-and nanogel syntheses cannot be made.
e synthesis process, the quantity of the crosslinker and initiator, temperature, time, stirring speed, and the presence of surfactants and co-monomers directly a ect the size of hydrogel particles [70].In general, the presence of a crosslinker causes the formation of smaller hydrogel particles than in the absence of a crosslinker.e synthesis temperature and size of the particles are inversely proportional, as macrogels are formed near room temperature [97][98][99][100] and temperatures for microgel synthesis increase to 50°C [78,80] on average, although they may rise as high as 70°C [82], 80°C [26] or even 85°C [61].Nanogels are usually prepared at temperatures of 70°C [80][81]101].Synthesis time is drastically shortened by reducing the hydrogel particle size from macro-to microto nanogel.Smaller hydrogel particles can be created through the addition of a surfactant such as sodium dodecyl sulphate (SDS), which stabilises polymer particles early during polymerisation reaction [80][81].e particle size decreases with an increase in the surfactant concentration [86].Hydrogel particle sizes can also be reduced through the addition of ionisable anionic co-monomers [86].

Fields of hydrogel application in textile modifi cation
Stimuli-responsive hydrogels can be applied to textile substrates in the form of a solution, microcapsules, foam or gel [105].Pad-dry-cure coating is the most common and the most accessible procedure from a technological point of view.When designing synthetic bres, a hydrogel can be incorporated into bres during the spinning process [95,[106][107].Regardless of the application procedure, a uniform distribution and the minimum thickness of hydrogel particles on a textile substrate are crucial to achieving the free swelling of hydrogel particles in their hydrophilic state [59].
e initial chemical composition of bres dictates their hydrophilicity or hydrophobicity, which greatly a ects the uptake of the functional nish.Stimuli-responsive hydrogels, however, do not form covalent bonds with a textile substrate.Because chemical and physical compatibility between a textile substrate and the applied hydrogel greatly a ect the durability of the applied hydrogel [108], di erent approaches were used in the application of stimuli-responsive hydrogels on textile materials, and are described in more detail in the following paragraphs.Not only the chemical composition of a bre, but also mechanical textile properties such as the cross sectional shape of a bre and bre diameter, weave pattern, thickness [109][110] etc. greatly a ect the moisture and water vapour transmittance of a fabric, and could a ect the responsive properties of hydrogel functional fabrics.
According to the smart textile functionality provided by a hydrogel, there are two application approaches: material technology and biotechnology [59].e material technology approach, which is crucial for achieving improved textile comfort, requires the minimal e ect of a hydrogel on the physico-mechanical properties of textile materials, as well as the durability of a hydrogel on a textile surface.Both factors are directly related to the conditions of hydrogel synthesis, the hydrogel particle size and the application technique.To increase the durability of hydrogel coatings, hydrogels are applied in combination with crosslinking agents or to previously activated bres.e latter can be achieved through a low-temperature plasma treatment that provides new functional groups on bre surfaces to serve as bonding points between a hydrogel and substrate.Furthermore, the etching e ect of plasma increases the roughness of the bre surface as well as the speci c surface area of bres, resulting in the greater uptake of a hydrogel [100].e hydrogel particle size has a signi cant e ect on the mechanical properties of a textile material.e presence of microgels on a textile substrate increases the sti ness of the fabric [74].e most recent research is therefore focused on the synthesis and application of nanogels [81, 93-94, 101, 111].Nanogels combine the characteristics of hydrogels and nanoparticles [111] and result in a minimum e ect on the mechanical properties of a textile substrate.An applied nanogel coating is a homogenous, thin gel layer or particles, and therefore has a minimum effect on the performance and haptic properties of a textile substrate [94].
e biotechnology approach is more common in the preparation of medical and hygienic textiles with an incorporated hydrogel.In such cases, a textile substrate serves as a carrying material that contributes to the improvement of the mechanical properties of a hydrogel when it is in its hydrophilic, swollen state.Accordingly, the biocompatibility of a textile substrate and the maximum responsiveness of a hydrogel are crucial, while the e ect of a hydrogel on the mechanical and physical properties of a textile substrate is less important [59].

Use of hydrogels for medical application
Overall, macrogels are not as common in the textile eld due to their e ect on the mechanical properties of textile materials, although they can be useful when sti ness does not play a signi cant role.In one instance, smart wound dressings based on poly-NiPAAm macrogels and its copolymers were bound to textile substrates by gra copolymerisation, which involves generating free radicals on a substrate and subsequently polymerising monomers directly on a textile surface.Poly-NiPAAm was copolymerised with polyurethane and gra ed on a cellulose non-woven textile [97], or with N,N-methylene bisacrylamide (BIS) and gra ed to a cellulose support [98].Photo-induced gra copolymerisation of poly-NiPAAm on previously plasma-treated textile substrates has also been carried out through copolymerisation with polypropylene (PP) [99], with the addition of chitosan [100] and with a polyethylene terephthalate (PET) lm [99].Microgels based on carboxymethyl chitosan (CMCh) and PVA [112], CMC and fumaric acid [91], CMC and hydroxyethyl cellulose derivatives [92], self-assembling di-phenylalanine [83], glycol and ε-caprolactone [62], collagen [30], and polyacrylic acid and β-cyclodextrin [107] have been used.Poly-NIPAAm-based microgels have been by far the most studied because of the LCST of the polymer, which is in the body temperature range.Microgels based on poly-NIPAAm have been applied to textile substrates alone through gra polymerisation [113], or in combination with 1,2,3,4-butanetetracarboxylic acid [78] to chemically bind a microgel with the functional groups of a textile substrate.To improve its mechanical properties and decrease its tendency to coagulate, poly-NIPAAm has been synthesised in combination with other polymers such as the copolymer 2-aminoethyl methacrylate [82].To achieve a sterile wound environment, di erent antimicrobial agents have been added to microgels such as silver in various forms [91], zinc oxide [114] and a biocidal agent based on quaternary ammonium salts [113].Nanogels have been used in the eld of medical textiles as smart coatings for wound dressings, tissue engineering and for the delivery of active substances.Smart wound dressings using nanogels have been created through the application of pullulan nanogel, carrying collagen onto a NanoClik membrane made of silicone [96].at coating promoted wound healing and protected the wound from infection.Hence, bioactive molecules or proteins could potentially be incorporated into its structure.e average particle size of a nanogel was 30nm.In a di erent study, polyvinyl alcohol (PVA) nanogels were applied to a cotton fabric into which silver nanoparticles were inserted.e hydroxyl groups of the PVA stabilised the silver nanoparticles to prevent their agglomeration and further growth.A signi cant reduction in bacteria and more rapid wound healing were thus achieved.e size of nanogel particles was 10-50nm [93].Temperature-responsive poly-NiPAAm-co-allylamine (PNIPAM-co-ALA) nanogels with incorporated silver nitrate have also been gra ed onto non-woven polypropylene fabric for the purpose of designing a smart wound dressing.Nanogel particles had a diameter of 72nm at temperatures above A Review Tekstilec, 2017, 60(2),  the LCST of the poly-NiPAAm, at which bacterial growth was prevented [80].Nanogels can be incorporated into the structure of bres via electrospinning by adding nanogels to a spinning mass.Composite poly(caprolactone) micro bres have been spun with a nanogel composed of poly(vinyl caprolactam) and 2-(methacryloyloxy) ethyl acetoacetate (PVCL/AAEM) copolymers [106].For this purpose, two di erent solvent systems were used, methanol/toluene (Me/Tol) and chloroform/ dimethylformamide (Ch/DMF), which led to the di erent morphological characteristics of spunbres.Namely, bres spun using Me/Tol had a diameter of 3 mm and the nanogel particles were located in the cores of the bres, while bres spun using CH/DMF had a diameter of 1 μm with nanogel particles on the surfaces of the bres.e size of hydrogel nanoparticles in a dry state was 100nm.In another study, polysaccharide and gelatin nano bres were produced for use in tissue engineering [95].Nanogels composed of hydrophobised-pullulan were added to a spinning mass.Fibres with a 200 to 300 nm included 60-80nm nanogel particles.Hence, hydrogel-like sub-micron bres were electrospun from poly(acrylic acid) (PAA) crosslinked with b-cyclodextrin (β-CD) and thermally treated for the purpose of stabilisation [107].Fibres ranged in size from 100 nm to several microns and were used as carriers of silver nanoparticles.A highly biocidic textile surface was achieved.

Use of hydrogels for increased comfort
Both nanogels and microgels can be used for improved comfort.To achieve dual temperature-and pH-responsiveness and thus increased comfort, poly-NIPAAm has been applied in combination with chitosan (PNCS microgel).Surfactant-free emulsion polymerisation was used to prepare a microgel with a particle size of 200 nm.Furthermore, a PNCS microgel was applied to di erent textile substrates, namely cotton, polyester and polyamide.To achieve chemical bonding of the PNCS microgel and consequently greater durability, the microgel was applied to previously chemically or physically activatedbres or in combination with crosslinking agents [59].Chemical activation was achieved through carboxymethylation and amination [72][73].While carboxymethylation included the application of monochloroacetic acid to form carboxymethyl groups, amination of the cotton fabrics was performed by dyeing cotton fabric with a reactive dye followed by reduction, thereby forming amino groups on the bre surface.It was concluded that the pH responsiveness of the previously aminated, PNCS microgel-coated fabric was superior, while the temperature responsiveness of both previously activated fabrics was comparable.Oxygen, nitrogen and argon low-temperature plasma were used for the physical activation of cotton fabrics, not only to increase the number of functional groups on thebre surface but also to increase the roughness of the bres through a plasma etching e ect, thus achieving a greater contact surface between the bres and the microgel particles, and consequently greater adhesion of the PNCS microgel [115].Application of the PNCS microgel in combination with crosslinking agents, i.e. 1,2,3,4-butanetetracarboxylic acid (BTCA) [74][75] and N,N'-methylenebisacrylamide [84][85], has also been studied.In the case of BTCA, a PNCS microgel was applied to a cotton fabric, where the acid reacted with hydroxyl groups of the cellulose and chitosan through the formation of ester bonds and with the free amino groups of chitosan through the formation of amides.When the PNCS microgel was applied in combination with the N,N'-methylenebisacrylamide crosslinker, a polyester fabric previously treated with acrylic acid was used.Crosslinking was achieved through UV irradiation in the presence of a benzophenone photo initiator.In a di erent study, the successful application of a PNCS microgel on PES fabric was achieved by using sol-gel technology, where a polysiloxane matrix was formed on the bre surface in which microgel particles were physically incorporated.e matrix was formed using a vinyltrimethoxysilane sol-gel precursor in combination with hydrophilic silica nanoparticles.Due to the elastic properties of the polysiloxane matrix, the microgel particles could swell and shrink without any restrictions, while its presence increased the washing fastness of the hydrogel coating [76][77].Nanogel coated fabrics with smart thermoregulation have been tailored using a temperature-and pH-responsive nanogel based on poly-NIPAAm and chitosan (PNCS), which was incorporated onto cotton fabric in combination with a BTCA crosslinker.e nanogel swelled at lower pH values and temperatures and shrank at higher temperatures and pH values.e addition of BTCA reduced the swelling ability slightly [79].A Review Tekstilec, 2017, 60(2), 76-96

Use of hydrogels for protective properties of textile material
Textiles with protective properties can be obtained by adding di erent active substances into nanogel structures.A non-woven textile was coated with a nanogel based on poly-NIPAAm and methacrylic acid (MAA) incorporating silver nanoparticles into its structure [81] to achieve antimicrobial properties.e nanoparticles were inserted during or a er synthesis, but prior to application to textiles; less agglomeration and smaller silver nanoparticle sizes were found when they were added during synthesis.
e nanogel particle size was between 180 and 200nm.To achieve insecticidal properties with wool and other keratin bres, a nanogel composed of highly functional β-cyclodextrin (β-CD) was loaded with an insecticide, permethrin, where the size of the nanogel particles reached 100-200 nm [94].

Use of hydrogels for fi ltration
Textile materials functionalised with stimuli-responsive hydrogels can be used as ltration systems, namely for oil and water separation.Such materials could help clean the ocean in the case of a catastrophic event.An extensive review article was written on this topic by Wang [116].To achieve water/oil ltration, a temperature-and pH-responsive PDMAEMA hydrogel was applied to a stainless steel mesh to achieve active separation of water from oil/water mixtures at controlled pH values and temperatures.When the temperature was below 55°C and pH values were less than 13, water was able to pass through the textile material, and oil was inhibited.When the temperature rose above 55°C and pH levels rose above 13, the hydrogel particles shrank, and water and oil could transit through the fabric [103].Superhydrophilic to superhydrophobic transition was achieved through the application of stimuli-responsive hydrogels based on poly-NiPAAm or PAA. e poly-NiPAAm hydrogel was coated on elastic polyurethane to achieve temperature-responsive switchable superhydrophilicity to superhydrophobicity with the LCST of poly-Ni-PAAm i.e., 32°C. is textile composite exhibited excellent water/oil separation properties, mechanical strength and elasticity.e hydrogel was prepared by dissolving poly-NiPAAm and BIS in APS, and was spun into a micro bre mat by force spinning [104].Sidorenko and his team synthesised two PAA hydrogels and a tailored hydrogel array of isolated rigid setae hybrids etched to silicon to achieve smart wetting ability.One hybrid acts superhydrophobic before exposure to water and transforms to a hydrophilic state in the presence of water, while the second surface acts in the opposite manner.e wetting behaviour is reversible upon drying [102].

Conclusion
Stimuli-responsive hydrogels are an important group of materials with potential applications in various elds.ey can be classi ed by their mode of crosslinking (chemical/physical) and their responsive characteristics, where they divide into physical (temperature, light, ultrasound, magnetic and electric eld), chemical (pH, solvent and ionic strength) or biological (functionality molecule, e.g.enzymatic reactions) stimuli responsiveness.e stimuli-responsive behaviour of a hydrogel to a speci c stimulus or a combination of di erent stimuli results in a reversible volume change of the hydrogel (i.e.swelling or shrinking) as a result of its transition from a hydrophilic to a hydrophobic state or vice versa.An important classi cation of hydrogel materials is based on their size, where macro-(>1mm), micro-(100nm-1µm) and nanogels (<100nm) can be synthesised.Due to the high degree of hydration and their three-dimensional structure, which resembles natural tissues, and their biocompatibility, hydrogels are already well-established in the elds of biotechnology, biomedicine and pharmacy, and textiles.Textile material can serve as a reinforcement material to macrogels, where the primary textile properties play a minor role.In contrast, micro-and nanogels are used when hydrogels are being used as the active nish of a textile material, to provide a minimum e ect on the original properties of the textile material.A temperature-responsive hydrogel based on poly-NiPAAm and chitosan was most commonly used for textile functionalisation.To reduce the size of hydrogel particles, shorter synthesis times and higher temperatures are needed.To achieve nano-sized hydrogel particles, appropriate synthesis is needed, depending on the monomers used.With the commonly used dispersion polymerisation technique, the key to reducing the size of hydrogel particles lies in stabilising the precursor particles early in the polymerisation reaction, either by the addition of a surfactant, catalyst or an ionisable anionic comonomer, or by increasing the temperature.

Future aspects of hydrogels applied in textiles
Although important pioneering research work has already been performed regarding the applications of hydrogels for smart textiles [67,102], there are still many topics that remain largely unexplored and therefore present challenges for researchers.ey address issues regarding technology, as well as issues concerning the safe handling of such smart textiles in terms of their potential toxicity to health and the environment.Accordingly, some of the major research problems that need to be resolved in the future are the impaired handling properties of a textile material.e sti ness of a functionalised textile substrate greatly increases after the deposition of stimuli-responsive hydrogels.e use of appropriate so eners should thus be considered.e stability of a hydrogel on textile material also needs to be further improved.Accordingly, some progress has been made with the use of crosslinking agents, and through the chemical activation of bres and the physical entrapment of hydrogel particles.However, to achieve increased washing durability with a minimum e ect on the stimuli responsiveness of a hydrogel, further focus on the optimisation of application parameters is needed.In the eld of medical textiles, more in-depth understanding of the controlled release of active substances from the structure of hydrogel particles, the e ect of released compounds on the wearer and thus the potential cytotoxicity assessment of the functionalised fabric is needed.From an economical point of view, the costs of textile functionalisation using stimuli-responsive hydrogels must also be considered.Because the price of hydrogel-based nishes varies greatly depending on the chemicals used and on the complexity of the synthesis, further production optimisation will be needed to achieve the successful transfer of such stimuli-responsive nishes from a laboratory scale to industry in order to meet the demands of cost production on the one hand and the desired level of pro t on the other.Last but not least, the e ects of hydrogel based nishes on humans and the environment are also crucial.Depending on their origin, polymers composing stimuli-responsive hydrogels could be more or less cytotoxic.Because studies of the potential risk of newly developed compounds on human health and the environment still lag behind studies regarding their functionality, further focus regarding the toxicity of functionalised smart textiles during their use or a er their disposal is needed in order to make a proper riskbene t assessment.Research in the eld of hydrogel-functionalised textiles will therefore focus on the following: Synthesis and incorporation of nano-sized hy- • drogels into di erent textile materials; Use of hydrogels as carriers for di erent active

Figure 1 :
Figure 1: Schematic illustration of application elds of stimuli-responsive hydrogels

Figure 3 :
Figure 3: Schematic illustration of the preparation of chemically crosslinked hydrogels [2] causes a change in polymerpolymer and water-polymer interactions, which a ects the swelling and shrinking of a hydrogel Ultrasound Hydrogels based on ethylene-vinyl alcohol Ultrasonic waves cause an increase in temperature, which leads to the swelling and shrining of a hydrogelElectric current Hydrogels based on polyelectrolytesElectric current charges the membrane, which leads to the swelling or shrinking of a hydrogelIonic strength Ionic hydrogelsA shi in ionic strength causes a change in the concentration of ionic groups within the hydrogel, causing the swelling and shrinking of a hydrogel Chemical species

Figure 4 :Figure 5 :
Figure 5: Schematic illustration of the controlled release of active substances from a hydrogel: (a) retention of active substance in a swollen hydrogel, (b) release of active substance from a shrunken hydrogel

Figure 6 :
Figure 6: Structures of temperature-responsive polymers used for stimuli-responsive hydrogels for textile modication of di erent stimuli-• responsive hydrogels to achieve simultaneous worn comfort along with proactive protection; Improvement of durability and washing fastness • of hydrogel coatings, in terms of maintaining stimuli-responsive characteristics; Reduction of the e ect on the mechanical pro-• perties of textile materials; Health and environmental e ects of hydrogel-ba-• sed nishes, by addressing problems of toxic side e ects, as well as the biodegradability of disposed functionalised textiles and the bioaccumulation of hydrogel compounds; and Optimisation of synthesis methods to minimise • production costs.

Table 2 :
Components, size of micro-and nanogel particles and synthesis conditions of hydrogels for use in textiles