**6. Antimicrobial and antifouling activity**

**Figure 4.** Cryo-scanning electron micrograph of crosslinked alginate synthesized with a minuscule amount of GO and 18 wt.% of calcium chloride (with respect to the mass of sodium alginate) in the swollen state after 2 minutes of immersion in water at 24 ± 0.5°C (a). Apparent diffusion coefficients of liquid water (mean ± standard deviation) in calcium alginate hydrogels with different crosslinker contents with (black columns) and without (gray columns)

0.1 wt.% of GO (b). *Reprinted with permission from Ref* [48].

100 Hydrogels

Microbial infections can lead to implant failure, which may cause major economic losses and suffering among patients despite the use of preoperative antibiotic prophylaxis and the aseptic processing of materials. Therefore, novel antimicrobial materials are urgently in need for medical uses [96]. For that reason, much effort is being done in the development of advanced hydrogels with inherent antimicrobial properties. Thus, syringe-injectable bioadhesive hydrogels prepared from mixing polydextran aldehyde and branched polyethylenimine, able to kill both Gram-negative and Gram-positive bacteria, while sparing human erythrocytes [97] and injectable conductive self-healed hydrogels based on quaternized chitosan-g-polyaniline (QCSP) and benzaldehyde group functionalized poly(ethylene glycol)-co-poly(glycerol sebacate) (PEGS-FA) with antibacterial, anti-oxidant and electroactive dressing for cutaneous wound healing have been developed [98].

Antibacterial properties can also be imparted to a hydrogel by doping in an exogenous antibiotic for eventual release [107]. In these delivery systems, the active agent is released from the polymer matrix over time. However, the material's antibiotic activity is eventually exhausted with the remaining matrix being left inactive and the remaining vehicle may become a substrate for colonization by bacterial biofilms once the payload is depleted, which can become life threatening. For this reason, secondary surgeries are typically performed to remove these empty depots as a means of preventing this type of infection. To avoid this second surgery, a hydrogel drug delivery system in which the drug release rate of vancomycin and degradation rate of the hydrogel are linked via covalent incorporation of vancomycin in the hydrogel

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However, many hydrogels themselves do not have any antimicrobial activity and therefore some fillers, and antimicrobial agents need to be incorporated by physical blending in order to produce antimicrobial materials [109]. Thus, graphene has emerged as a novel green broad-spectrum antimicrobial nanomaterial, with little bacterial resistance and tolerable cytotoxic effect on mammalian cells. It exerts its antibacterial action via physical damages through direct contact of its sharp edges with bacterial membranes and destructive extraction of lipid molecules. The antimicrobial activity of GO against two bacterial pathogens (*Pseudomonas syringae* and *Xanthomonas campestris pv. undulosa*) and two fungal pathogens (*Fusarium graminearum* and *Fusarium oxysporum*) showed that GO had a powerful effect on the reproduction of all these four pathogens because it killed nearly 90% of the bacteria and repressed 80% macroconidia germination along with partial cell swelling and lysis at 500 μgmL−1. The graphene-based nanocomposites have a wide range of biomedical applications, such as wound dressing due to its superior antimicrobial properties and good

Another strategy to design hydrogels with desired antimicrobial performance consists of adding silver nanoparticles (Ag NPs). Hence, silver nanoparticles have emerged up with diverse medical applications ranging from silver based dressings, silver coated medicinal devices,

Infections are also frequent and highly undesired occurrences after orthopedic procedures. Thus, for example, medicated hydrogels of hyaluronic acid derivatives have been developed [112] to address this problem. However, the growing concern caused by the rise in antibiotic resistance is progressively dwindling the efficacy of such drugs and the integration of silver

The combination of both previous strategies (graphene and Ag NPs) to design antimicrobial hydrogels with good water maintaining ability is of particular significance to promote the development of wound dressing. Thus, a series of hydrogels were synthesized by crosslinking of Ag/graphene composites with acrylic acid and N,N′-methylene bisacrylamide at different mass ratios. In this study, prepared hydrogel with an optimal Ag to graphene mass ratio of 5:1 exhibited much stronger antibacterial abilities than other hydrogels and showed excellent biocompatibility, high swelling ratio, and good extensibility at the same time. Besides, *in vivo* experiments indicated that this nanocomposite hydrogel could significantly accelerate the healing rate of artificial wounds in rats, and it helped to successfully reconstruct intact and

nanoparticles in hydrogels has become a very promising alternative [113].

backbone was successfully developed [108].

biocompatibility [110].

such as nanogels, nanolotions, etc. [111].

Other hydrogels such as chitosan and its derivatives has been widely used as implant coatings for its intrinsic properties such as non-toxic, osteoconductive, pH responsive, anti-microbial, biocompatible and cell adhesive [99, 100]. Nevertheless, the development of new chitosan derivatives or composites with superior antimicrobial activity is still under research. Thus, for example, a novel hydrogel coating produced by electrophoretic co-deposition of chitosan/alkynyl chitosan showed high antibacterial effect against *Escherichia coli* and *Staphylococcus aureus* [101] by disk diffusion method [102] (see **Figure 5**). Antibacterial polymer coating adhered on the surface of medical implants and devices have attracted great interests in the last decades for its ability to reduce implantassociated infections [103, 104].

Antimicrobial hydrogels formed by crosslinking polyallylamine with aldaric acid derivatives exhibited complete kill within 4 hours against *Pseudomonas aeruginosa*, *Escherichia coli*, *Staphylococcus aureus* and *Candida albicans* suspended in culture medium [105] and a facile strategy to fabricate antibacterial ultrathin hydrogel films via a layer-by-layer (LbL) technique and "click" chemistry was reported by Wang et al. [106]. This hydrogels consisted of poly[oligo(ethylene glycol)fumarate]-co-poly[dodecyl bis(2-hydroxyethyl)methylammonium fumarate] (POEGDMAM) containing multi-enes and poly[oligo(ethylene glycol)mercaptosuccinate] (POEGMS). These ultrathin films exhibited excellent antibacterial activity against both *Staphylococcus aureus* and *Escherichia coli* due to the presence of ammonium groups with long alkyl chains in the POEGDMAM.

**Figure 5.** Antimicrobial results of chitosan and alkynyl chitosan against *E. coli* and *S. aureus* by using disk diffusion method. Images (a–e) are paper disks containing 1 wt. % chitosan, 1 wt. % alkynyl chitosan (ACS1, ACS2, ACS3 and ACS4), respectively against *E. coli*; images (f–j) correspond to paper disks containing 1 wt. % chitosan, 1 wt. % alkynyl chitosan (ACS1, ACS2, ACS3 and ACS4), respectively against *S. aureus*. Alkynyl chitosan coded as ACS1, ACS2, ACS3 and ACS4 were prepared by changing the molar ratio of chitosan 0 unit to 3-bromopropyne monomer as 1:0.5, 1:1, 1:1.5 and 1:2. *Reprinted with permission from Ref* [101].

Antibacterial properties can also be imparted to a hydrogel by doping in an exogenous antibiotic for eventual release [107]. In these delivery systems, the active agent is released from the polymer matrix over time. However, the material's antibiotic activity is eventually exhausted with the remaining matrix being left inactive and the remaining vehicle may become a substrate for colonization by bacterial biofilms once the payload is depleted, which can become life threatening. For this reason, secondary surgeries are typically performed to remove these empty depots as a means of preventing this type of infection. To avoid this second surgery, a hydrogel drug delivery system in which the drug release rate of vancomycin and degradation rate of the hydrogel are linked via covalent incorporation of vancomycin in the hydrogel backbone was successfully developed [108].

to kill both Gram-negative and Gram-positive bacteria, while sparing human erythrocytes [97] and injectable conductive self-healed hydrogels based on quaternized chitosan-g-polyaniline (QCSP) and benzaldehyde group functionalized poly(ethylene glycol)-co-poly(glycerol sebacate) (PEGS-FA) with antibacterial, anti-oxidant and electroactive dressing for cutaneous

Other hydrogels such as chitosan and its derivatives has been widely used as implant coatings for its intrinsic properties such as non-toxic, osteoconductive, pH responsive, anti-microbial, biocompatible and cell adhesive [99, 100]. Nevertheless, the development of new chitosan derivatives or composites with superior antimicrobial activity is still under research. Thus, for example, a novel hydrogel coating produced by electrophoretic co-deposition of chitosan/alkynyl chitosan showed high antibacterial effect against *Escherichia coli* and *Staphylococcus aureus* [101] by disk diffusion method [102] (see **Figure 5**). Antibacterial polymer coating adhered on the surface of medical implants and devices have attracted great interests in the last decades for its ability to reduce implant-

Antimicrobial hydrogels formed by crosslinking polyallylamine with aldaric acid derivatives exhibited complete kill within 4 hours against *Pseudomonas aeruginosa*, *Escherichia coli*, *Staphylococcus aureus* and *Candida albicans* suspended in culture medium [105] and a facile strategy to fabricate antibacterial ultrathin hydrogel films via a layer-by-layer (LbL) technique and "click" chemistry was reported by Wang et al. [106]. This hydrogels consisted of poly[oligo(ethylene glycol)fumarate]-co-poly[dodecyl bis(2-hydroxyethyl)methylammonium fumarate] (POEGDMAM) containing multi-enes and poly[oligo(ethylene glycol)mercaptosuccinate] (POEGMS). These ultrathin films exhibited excellent antibacterial activity against both *Staphylococcus aureus* and *Escherichia coli* due to the presence of ammonium groups with

**Figure 5.** Antimicrobial results of chitosan and alkynyl chitosan against *E. coli* and *S. aureus* by using disk diffusion method. Images (a–e) are paper disks containing 1 wt. % chitosan, 1 wt. % alkynyl chitosan (ACS1, ACS2, ACS3 and ACS4), respectively against *E. coli*; images (f–j) correspond to paper disks containing 1 wt. % chitosan, 1 wt. % alkynyl chitosan (ACS1, ACS2, ACS3 and ACS4), respectively against *S. aureus*. Alkynyl chitosan coded as ACS1, ACS2, ACS3 and ACS4 were prepared by changing the molar ratio of chitosan 0 unit to 3-bromopropyne monomer as 1:0.5, 1:1, 1:1.5

wound healing have been developed [98].

102 Hydrogels

associated infections [103, 104].

long alkyl chains in the POEGDMAM.

and 1:2. *Reprinted with permission from Ref* [101].

However, many hydrogels themselves do not have any antimicrobial activity and therefore some fillers, and antimicrobial agents need to be incorporated by physical blending in order to produce antimicrobial materials [109]. Thus, graphene has emerged as a novel green broad-spectrum antimicrobial nanomaterial, with little bacterial resistance and tolerable cytotoxic effect on mammalian cells. It exerts its antibacterial action via physical damages through direct contact of its sharp edges with bacterial membranes and destructive extraction of lipid molecules. The antimicrobial activity of GO against two bacterial pathogens (*Pseudomonas syringae* and *Xanthomonas campestris pv. undulosa*) and two fungal pathogens (*Fusarium graminearum* and *Fusarium oxysporum*) showed that GO had a powerful effect on the reproduction of all these four pathogens because it killed nearly 90% of the bacteria and repressed 80% macroconidia germination along with partial cell swelling and lysis at 500 μgmL−1. The graphene-based nanocomposites have a wide range of biomedical applications, such as wound dressing due to its superior antimicrobial properties and good biocompatibility [110].

Another strategy to design hydrogels with desired antimicrobial performance consists of adding silver nanoparticles (Ag NPs). Hence, silver nanoparticles have emerged up with diverse medical applications ranging from silver based dressings, silver coated medicinal devices, such as nanogels, nanolotions, etc. [111].

Infections are also frequent and highly undesired occurrences after orthopedic procedures. Thus, for example, medicated hydrogels of hyaluronic acid derivatives have been developed [112] to address this problem. However, the growing concern caused by the rise in antibiotic resistance is progressively dwindling the efficacy of such drugs and the integration of silver nanoparticles in hydrogels has become a very promising alternative [113].

The combination of both previous strategies (graphene and Ag NPs) to design antimicrobial hydrogels with good water maintaining ability is of particular significance to promote the development of wound dressing. Thus, a series of hydrogels were synthesized by crosslinking of Ag/graphene composites with acrylic acid and N,N′-methylene bisacrylamide at different mass ratios. In this study, prepared hydrogel with an optimal Ag to graphene mass ratio of 5:1 exhibited much stronger antibacterial abilities than other hydrogels and showed excellent biocompatibility, high swelling ratio, and good extensibility at the same time. Besides, *in vivo* experiments indicated that this nanocomposite hydrogel could significantly accelerate the healing rate of artificial wounds in rats, and it helped to successfully reconstruct intact and thickened epidermis during 15 day of healing of impaired wounds [114]. In the same way, acrylic acid (AA) grafted onto poly(ethylene terephthalate) (PET) film through gamma-ray induced graft copolymerization with silver nanoparticles on the surface showed strong and stable antibacterial activity [115].

**7. Porosity**

last decades (see **Figure 6**).

Porous polymers have received an increased level of research interest because of their potential to merge the properties of both porous materials and polymers [119]. Porous polymers have potential applications in many fields such as gas storage and separation materials [120, 121], drug delivery [122], catalysts [123], supports for electrochemical sensing [124], low-dielectric constant materials [125], packing materials in chromatography [126], scaffolds or three-dimensional porous matrices for tissue engineering in regenerative medicine [5, 41, 127, 128] and many others. These high value applications have driven much emphasis on development of reliable methods for preparation of porous polymers with designed pore architectures in the

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Tissue engineering holds great promise for regeneration and repair of diseased tissues, making the development of new porous supports as scaffolds for tissue regeneration a topic of great interest in biomedical research. Hydrogels have emerged as leading candidates for engineered tissue scaffolds due to their good biocompatibility and similarities to native extracellular

**Figure 6.** Different porous polymers prepared by different preparation methods to obtain diverse pore architectures.

Reprinted with permission from Ref [12, 41, 48, 128–130].

It is highly desired to have a hydrogel material bearing the excellent antifouling property/biocompataiblity to prolong the lifetime of implanted materials, switchable antimicrobial property to eliminate infection and inflammation, and good mechanical properties to avoid the failure of the implanted material. We hypothesize derivatives of zwitterionic carboxybetaine with hydroxyl group(s) can switch between the lactone form (anti-microbial) and the zwitterionic form (anti-fouling) and the intramolecular hydrogen bonds will enhance the mechanical property of the zwitterionic hydrogel. It is highly desired to have a hydrogel material bearing the excellent antifouling property/biocompataiblity to prolong the lifetime of implanted materials, switchable antimicrobial property to eliminate infection and inflammation, and good mechanical properties to avoid the failure of the implanted material. We hypothesize derivatives of zwitterionic carboxybetaine with hydroxyl group(s) can switch between the lactone form (antimicrobial) and the zwitterionic form (anti-fouling) and the intramolecular hydrogen bonds will enhance the mechanical property of the zwitterionic hydrogel.

On the other hand, the surface of hydrogels must be modified to make it resistant to protein adsorption and cell adhesion to avoid fouling. Thus, there is a need for coatings with antifouling properties that are able to improve the performances of implanted biomedical devices. Thus, the antifouling properties of poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) hydrogels were improved by the surface grafting of a brush of poly(oligoethylene glycol methyl ether acrylate) [poly(OEGA)] [116]. Novel antifouling highly wettable hydrogels with superior mechanical and self-healing properties have also been developed by UV-initiated copolymerization of non-fouling zwitterionic carboxybetaine methacrylamide (CBMAA-3) and 2-hydroxyethyl methacrylate (HEMA) in the presence of uniformly dispersed clay nanoparticles (Laponite XLG) in water [117].

Therefore, it would be highly desired to have a hydrogel material bearing the excellent antifouling property/biocompataiblity to prolong the lifetime of implanted materials, switchable antimicrobial property to eliminate infection and inflammation, and good mechanical properties to avoid the failure of the implanted material. Thus, derivatives of zwitterionic carboxybetaine have been developed with hydroxyl group(s), which can switch between the lactone form (antimicrobial) and the zwitterionic form (anti-fouling) [118]. Besides, the intramolecular hydrogen bonds enhance the mechanical property of the zwitterionic hydrogel.

Nevertheless, the rapid emergence of antibiotic resistance in pathogenic microbes is becoming an imminent global public health problem because they are highly prone to develop resistance through mutation and the treatment with conventional antibiotics often leads to resistance development leaving the bacterial morphology intact. Therefore, much research is currently being done in the development of new antimicrobial hydrogels because they have been demonstrated to be very effective in preventing and treating multidrug-resistant infections.
