**2. Mechanical properties**

The enhancement of mechanical properties is one of the most desirable achievements in the field of hydrogel engineering and many researchers are currently working in this complex scientific area. Most hydrogels possesses very low mechanical properties, especially in the swollen state. Thus, hydrogels can be reinforced through many established kinds of methods and techniques: block copolymers, in which hydrophobic and hydrophilic domains alternate [7], increasing crosslinking density [8], by means of binary systems composed of two or more mixed polymers as interpenetrating polymer networks [9], by plasma grafting of a hydrogel onto a hydrophobic substrate [10–12], self-reinforced composite materials composed of fibers embedded in a matrix of the same polymer [13] and with the sol–gel reaction to produce nanosilica reinforcement [14]. However, more recent studies have shown new procedures to improve the mechanical properties of hydrogels with the incorporation of nanomaterials such as graphene (2010 Nobel Prize in Physics) and their derivatives: carbon nanotubes (CNT) [15], graphene oxide (GO) [16, 17], reduced graphene oxide (rGO) [18], etc.

If a crosslinker is present in the polymeric system, fully-IPN [21] result, while in the absence of crosslinking, a network having linear polymers embedded within the first crosslinked network is formed (semi- or pseudo-IPN) [22, 23]. IPNs are prepared usually in the form of simultaneous interpenetrating polymer networks (SINs), in which the precursors of both networks are mixed and the two networks are synthesized at the same time, or in the form of sequential IPNs, by swelling of a single-polymer network into a solution containing the mixture of monomer, initiator and activator, usually with a crosslinker. Thus, poly(2-hydroxyethyl methacrylate) (PHEMA) networks were greatly reinforced by poly(ethylene glycol) in SINs and semi-SINs composed of both polymers [24]. Full-IPNs and semi-IPNs of weak gelatin hydrogels were also prepared by the sequential mode of synthesis with polyacrylic acid (PAA) to be evaluated for tissue response in rats [25]. The mechanical properties of pseudo-SIPNs and pseudo-IPNs hydrogels, where the prefix pseudo denotes connectivity of the two network, showed that non-linear tensile properties of pseudo-SIPNs are rate-dependent, but for pseudo-IPNs they are not, which is a consequence of the viscoelastic behavior of a pseudo-SIPN versus elastic performance of the pseudo-IPN [26]. In that study, the mechanical properties of triple-network (TN) hydrogels synthesized from pseudo SIPNs and pseudo-IPNs showed that the presence of a loosely crosslinked third network changes the mechanical behavior of pseudo-SIPNs and pseudo-IPNs by homogenizing the stress within the sample

**Figure 1.** Schematic representation of (a) mechanical blends, (b) graft copolymers, (c) block copolymers, (d) AB-crosslinked

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copolymer, (e) semi-IPNs and (f) full-IPNs. *Modified from* [20].

IPN hydrogels are also developed with the aim of enhancing the mechanical strength and swelling/deswelling response [27]. For example, interpenetrating polymer network (IPN) hydrogels of chitosan/poly(acrylic acid) (PAA) synthesized by the UV irradiation method showed that even in the swollen state, the present chitosan/PAA IPNs possessed good

for finite deformations.

mechanical properties [28].

#### **2.1. IPNs**

Interpenetrating polymer networks (IPN) have gained greater attention during last decades, mainly due to their biomedical applications as reinforced polymer networks. In 2003, Gong et al. [19] reported their discovery of a new hydrogel architecture that produced extraordinarily materials with enhanced mechanical properties, which they termed a double-network (DN) hydrogel. The DN hydrogel was originally believed to be an interpenetrating polymer network (IPN) of a soft neutral polymer network within a more highly crosslinked network prepared by a two-step sequential free-radical polymerization. The first step consisted of the synthesis of a highly crosslinked network, and the second step involved swelling this first network with a water soluble monomer that was then polymerized within it. The second polymerization step was conducted with or without adding a crosslinking agent. Thus, the use of an IPN, which consists of two separate but interwoven polymer networks, is a chemical procedure that is often used in polymer science to control, enhance, and/or combine functional properties. These advanced multicomponent polymeric systems are composed of crosslinked polymer networks without any covalent bonds between them, where at least one of them is synthesized and/or crosslinked within the immediate presence of the other. It is important to differentiate between the six basic multicomponent polymeric structures: mechanical blends, graft copolymers, block copolymers, AB-crosslinked copolymer, semi-IPNs and full-IPNs (see **Figure 1**) [20].

Enhancement of Hydrogels' Properties for Biomedical Applications: Latest Achievements http://dx.doi.org/10.5772/intechopen.71671 93

produced massively. However, many of their potential uses required for many biomedical applications are sometimes hindered by their low mechanical strength, biological interactions, electrical and/or thermal properties, water sorption and diffusion, antimicrobial and/ or antifouling activity, porosity etc. Thus, new advanced hydrogels have been developed and are currently under intensive research to solve all these problems by means of multicomponent polymeric systems or by combination with other materials and/or nanomaterials to form

The enhancement of mechanical properties is one of the most desirable achievements in the field of hydrogel engineering and many researchers are currently working in this complex scientific area. Most hydrogels possesses very low mechanical properties, especially in the swollen state. Thus, hydrogels can be reinforced through many established kinds of methods and techniques: block copolymers, in which hydrophobic and hydrophilic domains alternate [7], increasing crosslinking density [8], by means of binary systems composed of two or more mixed polymers as interpenetrating polymer networks [9], by plasma grafting of a hydrogel onto a hydrophobic substrate [10–12], self-reinforced composite materials composed of fibers embedded in a matrix of the same polymer [13] and with the sol–gel reaction to produce nanosilica reinforcement [14]. However, more recent studies have shown new procedures to improve the mechanical properties of hydrogels with the incorporation of nanomaterials such as graphene (2010 Nobel Prize in Physics) and their derivatives: carbon nanotubes (CNT) [15],

Interpenetrating polymer networks (IPN) have gained greater attention during last decades, mainly due to their biomedical applications as reinforced polymer networks. In 2003, Gong et al. [19] reported their discovery of a new hydrogel architecture that produced extraordinarily materials with enhanced mechanical properties, which they termed a double-network (DN) hydrogel. The DN hydrogel was originally believed to be an interpenetrating polymer network (IPN) of a soft neutral polymer network within a more highly crosslinked network prepared by a two-step sequential free-radical polymerization. The first step consisted of the synthesis of a highly crosslinked network, and the second step involved swelling this first network with a water soluble monomer that was then polymerized within it. The second polymerization step was conducted with or without adding a crosslinking agent. Thus, the use of an IPN, which consists of two separate but interwoven polymer networks, is a chemical procedure that is often used in polymer science to control, enhance, and/or combine functional properties. These advanced multicomponent polymeric systems are composed of crosslinked polymer networks without any covalent bonds between them, where at least one of them is synthesized and/or crosslinked within the immediate presence of the other. It is important to differentiate between the six basic multicomponent polymeric structures: mechanical blends, graft copolymers, block

copolymers, AB-crosslinked copolymer, semi-IPNs and full-IPNs (see **Figure 1**) [20].

composites or nanocomposites with enhanced required properties.

graphene oxide (GO) [16, 17], reduced graphene oxide (rGO) [18], etc.

**2. Mechanical properties**

**2.1. IPNs**

92 Hydrogels

**Figure 1.** Schematic representation of (a) mechanical blends, (b) graft copolymers, (c) block copolymers, (d) AB-crosslinked copolymer, (e) semi-IPNs and (f) full-IPNs. *Modified from* [20].

If a crosslinker is present in the polymeric system, fully-IPN [21] result, while in the absence of crosslinking, a network having linear polymers embedded within the first crosslinked network is formed (semi- or pseudo-IPN) [22, 23]. IPNs are prepared usually in the form of simultaneous interpenetrating polymer networks (SINs), in which the precursors of both networks are mixed and the two networks are synthesized at the same time, or in the form of sequential IPNs, by swelling of a single-polymer network into a solution containing the mixture of monomer, initiator and activator, usually with a crosslinker. Thus, poly(2-hydroxyethyl methacrylate) (PHEMA) networks were greatly reinforced by poly(ethylene glycol) in SINs and semi-SINs composed of both polymers [24]. Full-IPNs and semi-IPNs of weak gelatin hydrogels were also prepared by the sequential mode of synthesis with polyacrylic acid (PAA) to be evaluated for tissue response in rats [25]. The mechanical properties of pseudo-SIPNs and pseudo-IPNs hydrogels, where the prefix pseudo denotes connectivity of the two network, showed that non-linear tensile properties of pseudo-SIPNs are rate-dependent, but for pseudo-IPNs they are not, which is a consequence of the viscoelastic behavior of a pseudo-SIPN versus elastic performance of the pseudo-IPN [26]. In that study, the mechanical properties of triple-network (TN) hydrogels synthesized from pseudo SIPNs and pseudo-IPNs showed that the presence of a loosely crosslinked third network changes the mechanical behavior of pseudo-SIPNs and pseudo-IPNs by homogenizing the stress within the sample for finite deformations.

IPN hydrogels are also developed with the aim of enhancing the mechanical strength and swelling/deswelling response [27]. For example, interpenetrating polymer network (IPN) hydrogels of chitosan/poly(acrylic acid) (PAA) synthesized by the UV irradiation method showed that even in the swollen state, the present chitosan/PAA IPNs possessed good mechanical properties [28].

"Smart" hydrogels are able to significantly change their volume/shape in response to small alterations of certain parameters of the environment. These responsive hydrogels have numerous applications, being the most of them focused on biological and therapeutic demands [29, 30], and sensing applications [31].

bioactive properties [39]. Silica can improve the mechanical properties of hydrogels through nanosilica filling or the well-known sol–gel process, which offers a new approach to the synthesis of nanocomposite materials with domain sizes approaching the molecular level [40]. Thus, for example, a biphasic matrix of a hybrid (inorganic–organic) nanocomposite materials of poly(2-hydroxyethyl acrylate) (PHEA) with a silica network obtained by an acid catalyzed sol–gel process of tetraethoxysilane (TEOS) showed a very significant improvement of the

Enhancement of Hydrogels' Properties for Biomedical Applications: Latest Achievements

Another reinforcement strategy that can be used consists of the combination of interpenetrated polymer networks and nanosilica filling. Thus, for example, poly(acrylic acid) and alginate IPN material with the incorporation of nanosilica greatly increased the compressive

attracted increasing attention in the last decade owing to its excellent electrical and thermal conductivities [44, 45] and great mechanical strength [46]. Furthermore, graphene promotes adherence of human osteoblasts and mesenchymal stromal cells [47], which render this nanomaterial and its derivatives very promising in the biomedical research field. It has been recently reported that its oxidized form, graphene oxide (GO), can greatly enhance the compression performance of alginate hydrogels even in a minuscule concentration [48]. The improvement of mechanical strength of poly(acrylamide) (PAM), which generally exhibit pronounced weakness and brittleness, by incorporating GO to the polymer matrix has also been reported [49]. GO is also a 2D nanomaterial obtained from natural graphite that can be easily exfoliated into monolayer sheets. GO has many hydrophilic oxygenated functional groups, including hydroxyl (−OH), epoxy (−C-O-C-), carbonyl (−C=O) and carboxyl (−COOH), which groups enable its dispersion in water solution [50] and render possible its use in many synthetic procedures. The diversity of unique properties of graphene oxide, including great tensile modulus (1.0 TPa), ultimate strength (130 GPa), electrical and thermal properties [51], render GO an ideal carbon nanomaterial for variety of applications toward the development of new advanced materials. Thus, the addition of GO nanosheets increased the Young's modulus and maximum stress of poly (acrylic acid)/gelatin composite hydrogels significantly as compared with control (0.0 wt. % GO). The highest Young's modulus was observed for hydrogel with GO (0.2 wt. %)/PAA (20 wt. %), whereas the highest maximum stress was detected for GO (0.2 wt. %)/PAA (40 wt. %) specimen. These results suggests that GO nanosheets could be used to improve mechanical properties of hydrogel materials, which are very promising for tissue engineering applications in regenerative medicine [17]. In the biomedical area, a very innovative strategy for three-dimensional self-assembly of graphene oxide sheets and DNA to form multifunctional hydrogels with high mechanical strength, environmental stability, and dye-loading capacity, has also been recently reported [52]. Furthermore, the promising properties of GO with the availability of oxygen-containing functional groups has led the synthesis of 3D crosslinked GO networks able to improve the mechanical properties of


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mechanical properties of the pure hydrogel [41].

Graphene (GN) is a two-dimensional monolayer of sp<sup>2</sup>

alginate hydrogels even more than single GO nanosheets (**Figure 2**) [53].

ment of many other hydrogels' properties [55–59].

Other derivatives of graphene, such as carbon nanotubes (CNTs), discovered by Iijima [54], in the form of single wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs), as well as carbon nanofibres (CNFs) are being used for reinforcing and enhance-

strength of the pure components [42].

Encapsulation of cells in interpenetrating network (IPN) hydrogels of two biocompatible materials-agarose and poly(ethylene glycol) (PEG) diacrylate with superior mechanical integrity has been developed [32].

Although IPNs based on hydrogels have been extensively reported, the combination of liquid crystalline (LC) property based hydrogels has been rarely explored. In this case, the anisotropic and molecular order of liquid crystals can be combined with the responsive isotropic properties of hydrogels. Thus, advanced stimuli-responsive materials based on interpenetrating liquid crystal-hydrogel polymer networks have been recently fabricated consisting of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid) network that provides a humidity and pH response [33].

#### **2.2. Composite hydrogels**

Diverse types of chemical modifications of hydrogels do not have a significant change of the overall mechanical strength because of the main structural skeleton of these polymers or copolymers are still weak. However, by fiber reinforcement, the addition of fabrics imparts high strength to the polymer networks, which form the main skeleton of the composites. Thus, fiber-reinforced hydrogels usually consists of a polymer matrix imbedded with high strength fibers, such as glass, aramid and carbon [34]. In such kind of materials, the mechanical properties are presumed to be improved and the biocompatible characteristics of the polymer should remain the same. Thus, hydrogels such as PHEMA, which is one of the most popular biomaterials, has been manufactured by adding various kinds of weaved and knitted fabrics and fibers, in order to improve overall qualities in advanced wound dressing usage [35]. However, in the recent decades, natural fibers as an alternative reinforcement in polymer composites have attracted the attention of many research groups due to their advantages over conventional glass and carbon fibers [36]. These natural fibers include flax, hemp, jute, sisal, kenaf, coir, kapok, banana, henequen and many others, which offer various advantages over manmade glass and carbon fibers such as low cost, low density, comparable specific tensile properties, non-abrasive to the equipment, non-irritation to the skin, reduced energy consumption, less health risk, renewability, recyclability and bio-degradability [37]. Thus, ultra-long chitin natural fibers were incorporated into hydrophobic Poly(methyl methacrylate) (PMMA) to prepare PMMA/Chitin composite hydrogels with improved properties [38]. This achievement is a significantly environmental move toward the sustainable utilization of marine-river crab shell wastes for biomedical applications in good agreement with green chemistry principles.

#### **2.3. Nanocomposite hydrogels**

Another alternative and very promising way of reinforcing hydrogels consist of the incorporation of nanomaterials such as silica, graphene and its derivatives, nanofibres or many other nanoparticles. Silica is a biocompatible material which has been reported to possess bioactive properties [39]. Silica can improve the mechanical properties of hydrogels through nanosilica filling or the well-known sol–gel process, which offers a new approach to the synthesis of nanocomposite materials with domain sizes approaching the molecular level [40]. Thus, for example, a biphasic matrix of a hybrid (inorganic–organic) nanocomposite materials of poly(2-hydroxyethyl acrylate) (PHEA) with a silica network obtained by an acid catalyzed sol–gel process of tetraethoxysilane (TEOS) showed a very significant improvement of the mechanical properties of the pure hydrogel [41].

"Smart" hydrogels are able to significantly change their volume/shape in response to small alterations of certain parameters of the environment. These responsive hydrogels have numerous applications, being the most of them focused on biological and therapeutic demands [29, 30],

Encapsulation of cells in interpenetrating network (IPN) hydrogels of two biocompatible materials-agarose and poly(ethylene glycol) (PEG) diacrylate with superior mechanical integ-

Although IPNs based on hydrogels have been extensively reported, the combination of liquid crystalline (LC) property based hydrogels has been rarely explored. In this case, the anisotropic and molecular order of liquid crystals can be combined with the responsive isotropic properties of hydrogels. Thus, advanced stimuli-responsive materials based on interpenetrating liquid crystal-hydrogel polymer networks have been recently fabricated consisting of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid)

Diverse types of chemical modifications of hydrogels do not have a significant change of the overall mechanical strength because of the main structural skeleton of these polymers or copolymers are still weak. However, by fiber reinforcement, the addition of fabrics imparts high strength to the polymer networks, which form the main skeleton of the composites. Thus, fiber-reinforced hydrogels usually consists of a polymer matrix imbedded with high strength fibers, such as glass, aramid and carbon [34]. In such kind of materials, the mechanical properties are presumed to be improved and the biocompatible characteristics of the polymer should remain the same. Thus, hydrogels such as PHEMA, which is one of the most popular biomaterials, has been manufactured by adding various kinds of weaved and knitted fabrics and fibers, in order to improve overall qualities in advanced wound dressing usage [35]. However, in the recent decades, natural fibers as an alternative reinforcement in polymer composites have attracted the attention of many research groups due to their advantages over conventional glass and carbon fibers [36]. These natural fibers include flax, hemp, jute, sisal, kenaf, coir, kapok, banana, henequen and many others, which offer various advantages over manmade glass and carbon fibers such as low cost, low density, comparable specific tensile properties, non-abrasive to the equipment, non-irritation to the skin, reduced energy consumption, less health risk, renewability, recyclability and bio-degradability [37]. Thus, ultra-long chitin natural fibers were incorporated into hydrophobic Poly(methyl methacrylate) (PMMA) to prepare PMMA/Chitin composite hydrogels with improved properties [38]. This achievement is a significantly environmental move toward the sustainable utilization of marine-river crab shell wastes for biomedical applications in good agreement with green chemistry principles.

Another alternative and very promising way of reinforcing hydrogels consist of the incorporation of nanomaterials such as silica, graphene and its derivatives, nanofibres or many other nanoparticles. Silica is a biocompatible material which has been reported to possess

and sensing applications [31].

94 Hydrogels

rity has been developed [32].

**2.2. Composite hydrogels**

**2.3. Nanocomposite hydrogels**

network that provides a humidity and pH response [33].

Another reinforcement strategy that can be used consists of the combination of interpenetrated polymer networks and nanosilica filling. Thus, for example, poly(acrylic acid) and alginate IPN material with the incorporation of nanosilica greatly increased the compressive strength of the pure components [42].

Graphene (GN) is a two-dimensional monolayer of sp<sup>2</sup> -bonded carbon atoms [43], which has attracted increasing attention in the last decade owing to its excellent electrical and thermal conductivities [44, 45] and great mechanical strength [46]. Furthermore, graphene promotes adherence of human osteoblasts and mesenchymal stromal cells [47], which render this nanomaterial and its derivatives very promising in the biomedical research field. It has been recently reported that its oxidized form, graphene oxide (GO), can greatly enhance the compression performance of alginate hydrogels even in a minuscule concentration [48]. The improvement of mechanical strength of poly(acrylamide) (PAM), which generally exhibit pronounced weakness and brittleness, by incorporating GO to the polymer matrix has also been reported [49]. GO is also a 2D nanomaterial obtained from natural graphite that can be easily exfoliated into monolayer sheets. GO has many hydrophilic oxygenated functional groups, including hydroxyl (−OH), epoxy (−C-O-C-), carbonyl (−C=O) and carboxyl (−COOH), which groups enable its dispersion in water solution [50] and render possible its use in many synthetic procedures. The diversity of unique properties of graphene oxide, including great tensile modulus (1.0 TPa), ultimate strength (130 GPa), electrical and thermal properties [51], render GO an ideal carbon nanomaterial for variety of applications toward the development of new advanced materials. Thus, the addition of GO nanosheets increased the Young's modulus and maximum stress of poly (acrylic acid)/gelatin composite hydrogels significantly as compared with control (0.0 wt. % GO). The highest Young's modulus was observed for hydrogel with GO (0.2 wt. %)/PAA (20 wt. %), whereas the highest maximum stress was detected for GO (0.2 wt. %)/PAA (40 wt. %) specimen. These results suggests that GO nanosheets could be used to improve mechanical properties of hydrogel materials, which are very promising for tissue engineering applications in regenerative medicine [17]. In the biomedical area, a very innovative strategy for three-dimensional self-assembly of graphene oxide sheets and DNA to form multifunctional hydrogels with high mechanical strength, environmental stability, and dye-loading capacity, has also been recently reported [52]. Furthermore, the promising properties of GO with the availability of oxygen-containing functional groups has led the synthesis of 3D crosslinked GO networks able to improve the mechanical properties of alginate hydrogels even more than single GO nanosheets (**Figure 2**) [53].

Other derivatives of graphene, such as carbon nanotubes (CNTs), discovered by Iijima [54], in the form of single wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs), as well as carbon nanofibres (CNFs) are being used for reinforcing and enhancement of many other hydrogels' properties [55–59].

Other nanoparticles such as clay have been employed to reinforce hydrogels such as polyvinyl alcohol (PVA) [62]. These polymer-clay nanocomposite hydrogels, fabricated for wound healing, constitute a class of materials in which the polymer matrix is reinforced by uniformly dispersed inorganic particles (usually 10 wt.% or less) having at least one dimension in the nanometer scale and exhibiting superior mechanical and thermal properties when compared

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The electrical properties are very important in some biomedical areas because it has been demonstrated that various types of electrical stimulation can regulate cell physiological activities such as division [64], migration [65], differentiation and cell death [66]. The electrical stimulation has also been useful in promoting healing for spinal cord repair and cancer therapy due to its non-invasiveness of these polymers [67–69]. Therefore, much emphasis is being done in the development of new conductive hydrogels for biomedical applications. Graphene has been considered to be very effective electrode material due to its excellent conductivity [44], but its production is still very expensive and more new composite materials are expected to be developed with its derivative graphene oxide. However, GO has a very low conductivity due to their oxygen-containing functional groups and must be modified to reduce graphene oxide (rGO) in order to develop electrically conductive hydrogels. Thus, for example, following a single-step procedure starting from a homogeneous water dispersion of GO, it is possible to undergo reduction induced by the UV radiation during the photopolymerization of a resin [70]. Recently, transparent conductive films have been produced by grafting poly(acryl amide)/poly(acrylic acid) on the GO surface followed by a reduction to rGO nanosheets by a two-step chemical reduction with increased conductivity [71] and inorganic–organic doublenetwork (DN) conductive hydrogel of rGO and poly(acrylic acid) has been prepared by a two-step synthesis with a reduction-induced *in situ* self-assembly [72]. Even more recently, a nacre-inspired nanocomposite of rGO and PAA has been prepared *via* a vacuum-assisted filtration self-assembly process (see **Figure 3**). The abundant hydrogen bonding between GO and PAA results in both high strength and toughness of the bioinspired nanocomposites, which are higher than that of pure reduced GO. Moreover, this composite also displays high electrical conductivity, which renders it very promising material in many biomedical applica-

Carbon nanotubes (CNTs) have also been attracting intensive attention because of their excellent electrical properties with a superb conductivity, remarkable mechanical properties with many potential technological applications [74]. CNTs offer the possibility of developing ultrasensitive electrochemical biosensors due to their unique electrical properties. Thus, nanofibrous membranes filled with multiwalled carbon nanotubes (MWCNT) have been electrospun from the mixture of poly(acrylonitrile-co-acrylic acid) (PANCAA) and MWCNT to develop a glucose biosensor for diabetics [75]. Other hydrogels with very promising biomedical applications consists of dielectrophoretically aligned carbon nanotubes, which control electrical and mechanical properties of gelatin methacrylate (GelMA) hydrogels [76]. The contractile muscle

to pure polymer or conventional composites [63].

tions such as flexible electrodes and artificial muscles.

**3. Electrical properties**

**Figure 2.** Confocal microscopy of (a) GO/alginate (GO/Alg) (b & d) crosslinked GO/alginate (cGO/Alg), (c) dynamic mechanical analysis (storage modulus (E′) and loss tangent (tan δ)) (e) morphology after swelling in water for 3 hours. *Modified from Ref* [53].

Reinforcement can be also conducted with plant fiber-based nanofibres by a successful fibrillation of wood pulp fibers into nanofiber bundles, which are thin enough to work as well as bacterial cellulose in maintaining the transparency of materials [60]. Other novel nanocomposite hydrogels such as those prepared with polyacrylamide (PAM) as a matrix material reinforced with natural chitosan nanofibres via *in situ* free-radical polymerization showed that these nanofibres acted as a multifunctional cross-linker and a reinforcing agent in the hydrogel polymer system producing a compression strength and a storage modulus significantly higher than those of the pure hydrogel [61].

Other nanoparticles such as clay have been employed to reinforce hydrogels such as polyvinyl alcohol (PVA) [62]. These polymer-clay nanocomposite hydrogels, fabricated for wound healing, constitute a class of materials in which the polymer matrix is reinforced by uniformly dispersed inorganic particles (usually 10 wt.% or less) having at least one dimension in the nanometer scale and exhibiting superior mechanical and thermal properties when compared to pure polymer or conventional composites [63].
