**3. Electrical properties**

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 signifi-

**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.

cantly higher than those of the pure hydrogel [61].

*Modified from Ref* [53].

96 Hydrogels

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 applications such as flexible electrodes and artificial muscles.

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

hydrogels enabled the preparation of thermally stable, soft, magnetic field-driven actuators with muscle-like flexibility [79]. Furthermore, thermal degradation can also be improved by this filling procedure. For example, the mechanical and thermal properties of a renewable and biocompatible hydrogel of gelatin were improved through cross-linking by cellulose

Enhancement of Hydrogels' Properties for Biomedical Applications: Latest Achievements

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In the biomedical field, hydrogels are hydrophilic polymers, which are able to absorb large amounts due to contact with cells or tissue in the human body. Therefore, the thermal analysis of water and its influence on the swollen hydrogel properties becomes essen-

Water sorption and diffusion of hydrogels are also very important in biomedical applications because these properties play a very important role in cell survival, especially in tissue engineering [5]. Thus, synthetic hydrogels such as PHEMA or PHEA are very important hydrophilic materials as these polymers were able to absorb and swell retaining large amounts of water within their structure [83–86]. The excellent water sorption property has made these kind of biomaterials very promising in a wide range of biomedical applications such as controlled drug delivery, tissue engineering, wound healing, etc. [6, 87]. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from cross-links between network chains [88]. However, these single-network hydrogels have weak mechanical properties in the swollen state and slow response at swelling. Therefore, although reinforcement of hydrogels is absolutely necessary, as already mentioned, the improvement of mechanical properties can significantly affect water sorption. For example, water sorption can be dramatically reduced by the reinforcement produced by the combination of hydrophilic and hydrophobic functional groups of polymers as multicomponent polymeric sys-

Reinforcement of hydrogels by GO loading can enhance significantly water sorption and diffusion. Thus, the swelling rates of graphene oxide / poly(acrylic acid-co-acrylamide) nanocomposite hydrogels increased with increase in the GO loadings to 0.30 wt. % and then decreased with further increasing GO loadings. It is worth noting that the hydrogel with only 0.10 wt. % GO exhibited significant improvement of swelling capacity in neutral medium, and could also retain relatively higher swelling rates to a certain degree in acidic and basic solutions. Furthermore, it has been reported very recently that a very low filling of GO can produce a very significant increase of water diffusion (almost 6 times faster) in crosslinked alginate (**Figure 4**) [48]. Therefore, these GO-based super-absorbent hydrogels have very potential applications in many fields such as biomedical engineering and

The mechanism of water diffusion [89] can also be altered by the reinforcement of hydrogels through any of the methods shown in Section 2. Thus, very promising biomaterials

nanowhiskers [80].

tial [12, 81, 82].

tems (**Figure 1**).

hygienic products [50].

**5. Water sorption and diffusion**

**Figure 3.** Fabrication process of rGO–PAA composites: (a) the GO nanosheets/PAA homogeneous solution was filtered by vacuum-assisted filtration into GO–PAA composites. Then after hydroiodic acid (HI) reduction, the rGO–PAA composites were obtained. (b) a digital photograph of rGO–PAA composites (c) and (d) cross-section surface morphology with different magnifications of rGO–PAA composites. *Reprinted with permission from Ref.* [73].

cells cultured on these materials demonstrated higher maturation compared with cells cultured on pristine and randomly distributed CNTs in GelMA hydrogels.
