Antimicrobial hydrogels: promising materials for medical application

Introduction

Nowadays, with the rapid development of biomaterials and medical devices, health care-associated infections (HAIs) have posed severe problems on clinicians. For example, in the US, the annual costs associated with HAIs are estimated to be up to $33 billion.¹ The rapid emergence of antibiotic resistance in pathogenic microbes is becoming an imminent global public health problem.² According to a report in Lancet, most acute sequelae and global mortality were caused predominantly by infectious diseases.³ Medical devices may bring HAIs to patients in hospital. These biomaterials and medical devices including joint implants, wound dressings, catheters, cardiac pacemakers and contact lenses bring implant-associated infection, calling for an urgent need of inherent antimicrobial biomaterials and medical devices. Among all antimicrobial materials, heavy metals and natural extracts have been used for a long time since first discovered. However, these materials still have inherent disadvantages that restrict their application and efficacy. They fight against microbes as well as normal cells which cause damage to normal organs and tissues of patients.⁴ Antibiotics emerged in antimicrobial history 80 years ago when penicillin was discovered by Sir Alexander Fleming.² For all these decades, antibiotics have brought us consolation until the existence of drug-resistant bacterium was discovered. At the beginning of antibiotic resistance development, conventional antibiotics such as penicillin and methicillin were noneffective to resistant strains. Now, vancomycin-resistant and linezolid-resistant strains have emerged. This has led to ceaseless demands for novel antibiotics, putting clinicians in a dilemma whether to test a novel multi-resistant strain with another antibiotic.⁵ Synthetic antimicrobial agents such as salicylate, chlorhexidine, isothiazolinones, thiosemicarbazones, octenidine and even quaternary ammonium compounds also faced progressive threats with the development of drug resistance.⁶ According to the Darwinian view of the role of antibiotics, it is widely accepted that antibiotics and antibiotic-resistant genes act as weapons and shields in shaping the structures of microbial communities.⁷ Nowadays, antibiotic resistance is considered as bacteria’s specific response to an injury caused by antibiotics, which means it cannot be totally avoided even if we create a new antibiotic agent.⁸ Increasing rates of antibiotic resistance, drug allergies and antibiotic shortages further complicate the choice of antibacterial agents.⁹ Problems that the traditional antimicrobial agents faced include drug resistance, overdose and cytotoxicity. These problems urgently call for an efficient and safe delivery system of drug release, which can delay the release of toxic antimicrobial agents and reduce the risk of bacterial drug resistance. Apart from antibiotics, other antimicrobial materials also have their own problems in clinical application. In recent years, antimicrobial peptides (AMPs) have been reported to have antimicrobial properties (especially short sequences) because of their ionic structure; so, it is difficult to induce resistance of bacterium or formation of biofilm.^10–13 However, AMPs are also hemolytic, toxic and easy to lose efficacy and hence, AMPs need an effective drug delivery system to avoid these side effects.^12,14 Besides, antimicrobial amylolytic polymers, antimicrobial polysaccharides and other antimicrobial components have also been reported, which can be frameworks of biomedical polymers.^15,16 Yet, how to make these biomaterials play the greatest role in fighting against HAIs remains a problem.

In these cases, a novel drug delivery system with absorbability and delayed release performance is needed. The nanocarrier system or nano-drug delivery systems (DDS) can carry the antibiotic as well as protect it. Nanomaterials with inherent antimicrobial activity or nanomaterials that can improve the efficacy and safety of antimicrobial drugs are called nanoantimicrobials (NAMs). They could be an effective alternative to conventional antibiotics by the provision of improved bioavailability, protection, mucoadhesion, absorption, controlled release and target delivery for the encapsulated or surface-adsorbed drugs.¹⁷ A set of organic, inorganic and hybrid materials can be identified in the NAM family.¹⁸ Among all the NAMs, hydrogel is a three-dimensional cross-linked polymeric network that can swell dramatically in an aqueous medium such as body fluids, while maintaining its structure and controlling drug release.^19,20 Hydrogels can also be triggered by stimulations such as changes in pH, temperature, enzyme catalysis, ultraviolet gamma irradiation and even inflammation.²¹ Hydrogel can be coated on urinary catheters, central venous catheters,²² contact lenses, joint and dental implants^23,24 and local injection for drug release and wound healing.²⁵ Moreover, some types of hydrogels also have inherent antimicrobial properties.^26,27 Combined with nanomaterials such as hydrogel, the antibacterial agent may be used at a lower dose than when administered systemically, thus overcoming the problem of resistance and diminishing other undesirable side effects to some extent.²⁸ These characteristics have drawn remarkable attention in the pharmaceutical and medical fields especially for antimicrobial application (Figure 1). According to the development of antimicrobial agents, the progresses of antimicrobial hydrogels in recent years are shown in the following section.

Figure 1 The different applications of hydrogels.

Hydrogel loaded with metal nanoparticles

Heavy metals have been used to fight against microbes for a long time. Silver, gold, copper and zinc were all reported to be used in this area. Among these metals, silver is most widely used due to its good antibacterial property and relatively low toxicity. However, other metals, such as gold, copper and zinc, have their own advantages and antibacterial spectrums.

Sliver nanoparticles (Ag NPs)

Silver have been regarded as an antimicrobial agent for thousands of years, before people knew about the word “microorganisms”. Silver bowls, water vessels, spoons and other containers were used to preserve water, food and wine in their condition.^4,29 Silver powder was applied in wound healing and treatment of ulcers, which was first documented in medical history by Hippocrates.⁴ Silver still plays an important role in biomedical areas such as wound dressings, textiles, bone implants etc.³⁰ Thanks to the development of nanoscience and technology, nowadays silver is mainly applied in the form of nanoparticles.^31,32 Ag NPs have antimicrobial activity against a wide spectrum of microbes (probably due to their multiple mechanisms of antimicrobial action), including activity against drug-resistant bacteria, fungi (such as Candida albicans) and viruses.^33–36 Ag NPs are emerging as efficient antimicrobial agents because of their different mechanisms of sterilization,^32,37,38 although no final conclusion about mechanisms has been made. Recent studies suggest that the primary mechanism of the antibacterial action of Ag NPs is to release silver ion (Ag⁺). Particle-specific activity of Ag NPs cannot be ignored, which indicates that the mechanism of antibacterial action differs between Ag⁺ and AgNPs.³⁹ The most universally accepted hypothesis is that the Ag⁺ released from Ag NPs interact with cysteine in certain regions of proteins on bacterial membranes, causing K⁺ loss from inside and the disruption of cellular transport systems, which finally leads to bacterial cell death (Figure 2).^40,41 Other studies show that Ag⁺ interact with proteins of the cell wall and plasma membrane of bacteria.³¹ Combination of Ag⁺ with negatively charged membrane perforates the membrane, thus allowing cytoplasmic contents to flow out of the cell, dissipating the H⁺ gradient across the membrane and sometimes causing cell death.⁴² If the bacteria have not been killed yet, these contacts allow Ag⁺ to move through the cell wall and the plasma membrane. Finally, Ag⁺ functions as an extra antimicrobial agent in the cytoplasm of the bacterial cell.³⁴ Despite widespread use of Ag⁺, bacterial resistance to Ag⁺ has been found rare and developed slowly, especially compared to resistance to antibiotics, which makes it a potential antimicrobial agent to solve the problem of antibiotic resistance. Again, this is presumably due to the multiple mechanisms of antimicrobial action of Ag described earlier, whereas antibiotics usually have only one mechanism of action.^34,36,42 As is known to all, Ag NP-based hydrogels have so many merits that they performed better on Gram-negative bacteria than Gram-positive bacteria because Gram-negative bacteria have low resistance of the cellular membranes compared with the peptidoglycan cellular walls of Gram-positive bacteria.⁴⁰ But, it has also been argued that Gram-negative bacteria are less sensitive than Gram-positive bacteria to Ag⁺, because Ag⁺ binds to the negatively charged lipopolysaccharide (LPS) of the outer membrane of Gram-negative bacteria more strongly than to the peptidoglycan layer of Gram-positive bacteria. By this argument, Ag⁺ is trapped in the LPS and is less likely to enter a Gram-negative cell than a Gram-positive cell.^31,34

Figure 2 Transmission electron microscope image of Escherichia coli cells treated with silver nanoparticles in liquid Luria-Bertani medium: (A) membrane of E. coli; (B) nanoparticles accumulated in the membrane and penetrated the cell (arrows). Note: Reprinted from Adv Drug Deliv Rev. 65(13–14). Pelgrift RY, Friedman AJ, Nanotechnology as a therapeutic tool to combat microbial resistance1803–1815, Copyright (2013), with permission from Elsevier.31

In this review, we concentrate on the hydrogels that are loaded with Ag NPs. There are mainly two types of hydrogel matrices: one is the natural polymer (including modified natural polymer) and the other is the synthetic polymer. The most common natural polymers are polysaccharides. Polysaccharides mainly include alginate, chitin, chitosan (CS) and carboxymethyl cellulose (CMC). Alginate is a natural derivative linear copolymer that can form hydrogel via methods such as Ca²⁺ ionic interaction. Ag NPs were incorporated into alginate microbeads through electrochemical synthesis by Stojkovska et al,^43,44 which showed antibacterial activity against Staphylococcus aureus and Escherichia coli. Although alginate has been already commercially in use for wound dressings, Ag NPs on alginate have high tendency to aggregate. Obradovic et al⁴⁵ optimized the technique for the production of Ag/alginate microbeads by freezing–thawing based on alginate, poly(vinyl alcohol) (PVA) and poly(N-vinylpyrrolidone) (PVP) to reduce the aggregation. Ghasemzadeh et al⁴⁶ also attempted to use alginate/PVA as a hydrogel matrix with sodium borohydride as a reducing agent. Madhusudana Rao et al⁴⁷ went one step further by fabricating sodium alginate-based semi-interpenetrating polymer network (IPN) hydrogels for delivery of Ag NPs, and the hydrogel exhibited good antibacterial activity. The degree of cross-linking and nature of semi-IPN polymer chains are key factors in regulating the size, shape and release of nanoparticles.⁴⁸ Neibert et al⁴⁹ described a method to enhance mechanical strength of alginate hydrogel loaded with Ag NPs by chemical cross-linking, which is more favorable for epidermal regeneration while maintaining antibacterial properties.⁵⁰ Many animal experiments on alginate hydrogel loaded with Ag NPs have been conducted, which means this kind of antimicrobial hydrogel has been studied thoroughly.

Other important polysaccharides used as antimicrobial hydrogels are chitin and CS. It is notable that both chitin and CS have antimicrobial and metal-binding properties. Chitin- or CS-based hydrogels such as CS/2-glycerophosphate/nanosilver hydrogel and silver molybdate nanoparticle/chitin matrix (Ag₂Mo₂O₇/chitin) hydrogel also provide green synthetic process and excellent antibacterial performance against E. coli.^51,52 The other polysaccharide hydrogels include iota-carrageenan-based Ag NP hydrogel and Ag NP-loaded PVA/gum acacia (GA) hydrogel,^53,54 both iota-carrageenan and GA are well-known polysaccharides with rich production in nature. Both the hydrogels showed good antibacterial activity against Gram-negative bacterium E. coli. Sodium CMC is another kind of biocompatible and biodegradable polysaccharide polymer which can effectively work as both reducing and stabilizing agents. It has been reported that CMC can be cross-linked by epichlorohydrin as an antimicrobial hydrogel matrix, and it can also be added into CMC and starch-composed hydrogel network as a component,^55,56 both systems work well as antimicrobial hydrogels. Ranga Reddy et al⁵⁷ demonstrated that the natural polysaccharide gelatin has contributed an excellent property for anchoring and stabilizing the Ag NPs and formulating poly (gelatin–acrylamide) silver nanocomposite hydrogels for inactivation of bacteria. The natural hydrogels have weak antimicrobial properties, but they can be good carriers for Ag NPs, and other antibiotic agents. Moreover, they can be extracted from natural materials easily.

As for a synthetic matrix for Ag NP hydrogels, there is a large diversity, but most of them are poly(acrylamide) (PAM), acrylic acid, poly(ethylene glycol) (PEG), PVA, pyrrolidone and their derivatives. The main advantage of using this template is that the morphology and size of the nanoparticles can be easily controlled by changing the amount of cross-linker and monomer of the hydrogel network.^48,58,59 For example, PAM/PVA hydrogel–Ag NPs fabricated by Varaprasad et al⁶⁰ can obtain Ag NPs of 2–3 nm size in gel networks, which exhibit higher antibacterial activity on E. coli compared with Ag NPs alone and Ag⁺-bonded hydrogels. Styrene sulfonic acid sodium salt was incorporated into hydrogels to form poly(acrylamide-styrene sulfonic acid sodium salt) Ag NP hydrogel, and the most sensitive strain it can deal with was Bacillus subtilis.⁵⁸ PAM is also used to form semi-interpenetrating network hydrogels composed of pluronic and PAM by simultaneous free-radical cross-linking polymerization and served as nanoreactors for the synthesis of Ag NPs.⁵⁹ PAM mixed with itaconic acid (IA) or starch to form Ag NP-loaded hydrogels was also reported to have good antibacterial properties while providing a green process of synthesis.^61,62 Poly(N-isopropylacrylamide) (PNIPAM) is the second commonly used matrix in Ag NP hydrogels. James et al,⁶³ Manjula et al⁶⁴ and Zafar et al⁶⁵ used PNIPAM as a main component to synthesize Ag NP hydrogels. James et al⁶³ synthesized PNIPAM-co-allylamine nanogels and grafted them onto non-woven polypropylene. Hydrogels made by Manjula et al⁶⁴ were reduced with neem leaf (Azadirachta indica) extracts, providing another green process. During the fabrication, emphasis was placed on green techniques, in order to make it environmentally friendly. Zafar et al⁶⁵ mixed Ag NPs with N-isopropylacrylamide-based nanogels which had a peak of lower critical solution temperature (LCST) that is close to the human body temperature. This increases the possibility in practical medical application. All these three hydrogels demonstrated conspicuous antibacterial properties. Hydrogels of 2-acrylamido-2-methylpropane sulfonic acid sodium salt containing Ag NPs have been proved to have no cytotoxicity while exhibiting better antimicrobial ability than commercial Acticoat™ (Smith & Nephew, London, UK) and PolyMem Silver^® (Ferris Mfg. Corp., Fort Worth, TX, US),^66,67 which can give us more confidence in exploitation of Ag NP hydrogels. However, some researchers would like to try some new ways, such as cross-linking fumaric acid (FA) and CMC. These hydrogel-based silver nanocomposites were coated on cotton fabric for antibacterial property, and the result was promising.⁶⁸ Paladini et al⁶⁹ used in situ photochemical reaction to coat Ag NPs on the fibers of hydrogel and demonstrated their antibacterial capabilities by any hydrogel blend on E. coli and S. aureus.

As for other Ag NP hydrogels, different matrices bring different characteristics and different processes of synthesis, all these creative points offered us a unique view on the way to more advanced antimicrobial biomaterials. Poly(acrylic acid co-poly(ethylene glycol)methyl ether acrylate)/Ag NP composite hydrogels were developed by Lee et al,⁷⁰ offering a novel promising bioadhesive patch or wound dressing materials with their inherent good electrical conductivity. Thermoplastic PEG-polyhedral oligosilsesquioxane (POSS) hydrogels were synthesized from multiblock PEG-POSS polyurethanes by Wu et al,⁷¹ and their antimicrobial property lasts over 10 days. PVA/PVP-based hydrogels containing Ag NPs fabricated by Eid et al⁷² were reported to be high, uniformly distributed, and stable. Poly(methacrylic acid) (PMAA) hydrogel reduced with borohydride by Bajpai et al⁷³ and poly(2-hydroxyethyl methacrylate/IA)/Ag NP hydrogels synthesized with gamma irradiation by Micic et al⁷⁴ showed antimicrobial activity against E. coli. The pH-sensitive poly(methyl methacrylate-methacrylic acid)/Ag NP hydrogels synthesized with free radical cross-linking by Wei et al⁷⁵ can be potentially smart antimicrobial biomaterials. All the abovementioned hydrogels displayed enhanced antimicrobial ability against E. coli, S. aureus, Pseudomonas aeruginosa and even B. subtilis. Some of them even acquired longer antimicrobial duration than antibiotics.⁷⁶ The antimicrobial ability and cytotoxicity can be regulated by diverting the amount of components, which may turn out to be potentially smart antimicrobial biomaterials.

A novel antibacterial coating made of poly(L-lysine)/hyaluronic acid multilayer films and liposomes loaded with Ag⁺ was designed in 2008.⁷⁷ The strong antibacterial effect was attributed to the diffusion of silver ions from the AgNO₃ coating, which resulted in a bactericidal concentration of silver ions aggregated around the membrane of the bacteria. Similarly, other small antimicrobial chemicals such as antibiotics can be loaded in liposomes in hydrogels to reach the aim of delayed drug delivery. Malcher et al opened a new route to modify surfaces with small chemicals which cannot permeate phospholipid membranes.⁷⁷

The most interesting Ag NP hydrogels are hydrogels synthesized with water-soluble PEG polymers, which contain reactive catechol moieties. Synthesis of this hydrogel was inspired by mussel adhesive proteins. This biomimetic material has a strong potential for antibacterial tissue adhesives and biomaterial coatings because of the material-independent adhesive properties of catechols.⁷⁸ Another new hydrogel with Ag NPs was called reduced graphene oxide (GO)-based Ag NP-containing hydrogel. This composite was fabricated in situ through the simultaneous reduction of GO and noble metal precursors within the GO gel matrix.⁷⁹ This new kind of hydrogel has already been used in waste water cleansing due to its antimicrobial and antifouling properties inspiring the idea of clinical application. For example, this hydrogel can be used to deal with a polluted wound as a wound dressing.

However, serum albumin also reduces the antibacterial effects of Ag NP-embedded hydrogels.⁸⁰ The gene toxicity of Ag NPs has also been reported, and balances between anti-reactive oxygen species (ROS) response and DNA damage; and mitosis inhibition and chromosome instability, might play significant roles in silver-induced toxicity.⁸¹ Therefore, the vital issues are: improvement of the antimicrobial ability against Gram-positive bacteria, minimization of gene toxicity, and reduction of serum albumin when designing Ag NP-based hydrogels. More non-toxic and environmentally friendly synthetic processes such as the idea of size-controllable synthesis of Ag NPs with tobacco mosaic virus (TMV) as a biomediator without external reducing agents⁸² should be developed. In recent studies, more hydrogels loaded with Ag NPs have been discovered. Researchers have improved their properties, such as strong antimicrobial properties and prolonged release. All these developments and improvements ensure the clinical potential of the hydrogels. To provide clarity, all the hydrogels with Ag NPs are recorded in Table 1.

Table 1 Information of hydrogels with Ag NPs Abbreviations: Ag+, silver ions; Ag NPs, silver nanoparticles; CMC, carboxymethyl cellulose; CS, chitosan; FA, fumaric acid; GA, gum acacia; HEMA, 2-hydroxyethyl methacrylate; IA, itaconic acid; IPN, interpenetrating polymer network; LCST, lower critical solution temperature; MIC, minimal inhibition concentration; MRSA, methicillin-resistant S. aureus; MW, molecular weight; NaCMC, CMC sodium salt; NIPAM, N-isopropylacrylamide; PAM, poly(acrylamide); PBS, phosphate buffered saline; PEG, poly(ethylene glycol); PMAA, Poly(methacrylic acid); PNIPAM, poly(N-isopropylacrylamide); POSS, polyhedral oligosilsesquioxane; PVA, poly(vinyl alcohol); PVP, poly(N-vinylpyrrolidone); QPVA, quaternized PVA; RGO, reduced graphene oxide; TEM, transmission electron microscope; TMV, tobacco mosaic virus; UV, ultraviolet; VRE, vancomycin-resistant Enterococcus.

Gold nanoparticles (Au NPs)

Gold is universally considered as biologically inert but Au NPs have a diversity of biological functions.⁸³ Au NPs can be designed into different sizes and be functionalized with desired polymers, thus they are realized as biocompatible materials.⁸⁴ Au NPs can be attached to the bacterial membrane, which leads to the leakage of bacterial contents or penetration of the outer membrane and the peptidoglycan layer, resulting in bacterial death.⁸⁵ However, compared with Ag NPs, studies on antimicrobial Au NP hydrogels alone are rare. In a recent study by Brown et al,⁸⁶ Au NPs lack antibacterial activity alone. However, Au NP with ampicillin bound to the surface (Au NP-AMP) killed multiple drug-resistant bacteria, including methicillin-resistant S. aureus (MRSA), P. aeruginosa, Enterobacter aerogenes and E. coli K-12 substrain DH5-alpha (pPCR-Script AMP SK⁺).⁸⁶ Though N-isopropylacrylamide-based hydrogels containing Au NPs and pH-responsive PMAA hydrogel microcapsules carrying Au NPs had already been reported,^87,88 their antimicrobial property had not been studied until Gao et al⁸⁹ demonstrated that hydrogel containing Au NP-stabilized liposomes for antimicrobial application displayed excellent antibacterial properties on S. aureus without skin toxicity in a mouse model. In the research of Ribeiro et al,⁹⁰ silk fibroin/nanohydroxyapatite hydrogel modified with in situ-synthesized Au NPs showed antimicrobial activity. Compared with Ag NPs, no toxicity against osteoblastic cells was found, which means Au NPs could be used for bone regeneration.⁹⁰ Moreover, Jayaramudu et al⁹¹ used acrylamide (AM) and wheat protein isolate (WPI) to develop biodegradable gold nanocomposite hydrogels. The results indicated that these biodegradable gold nanocomposite hydrogels can be used as potential candidates for antibacterial applications.⁹¹ Through combination of bimetallic (Ag, Au) hydrogel nanocomposites, Ranga Reddy et al⁹² took it one step further to enhance their antimicrobial activity. Varaprasad et al⁹³ even prepared dual-metallic (Ag⁰–Au⁰) nanoparticles via a green process with mint leaf extract, which exhibited significant antibacterial activity against Bacillus and E. coli (Figure 3). Although the antimicrobial ability of Au NPs is weaker than that of Ag NPs, the Au NPs have their own advantages. The antibacterial spectrum of Ag NPs is broad, including MRSA. Moreover, the hydrogels with Au NPs showed negligible interference to bone regeneration. These properties of hydrogels with Au NPs make them promising materials in clinical orthopedic surgery.

Figure 3 Gao et al synthesized hydrogel containing Au NP-stabilized liposomes for antimicrobial application (A) illustrations of hydrogel containing nanoparticle-stabilized liposomes for topical antimicrobial delivery; (B) bacteria incubated with AuC–liposome hydrogel (PEGDMA 0.8 vol%) at pH = 4.5; (C) a zoomed-in image of (B). Note: The scale bars in (B and C) represent 1 μm. Reproduced from Gao W, Vecchio D, Li J, et al. Hydrogel containing nanoparticle-stabilized liposomes for topical antimicrobial delivery. ACS Nano. 2014;8(3):2900–2907.89

Zinc oxide nanoparticles (ZnO NPs)

There are also many other metal nanoparticles with antimicrobial activities besides silver and gold,^94,95 but only a few are embedded into hydrogels. Among these, zinc is the most popular antimicrobial agent.^96,97 ZnO NPs are used in many cosmetic materials because of their well-known antibacterial activity and non-cytotoxicity at an appropriate concentration.⁹⁸ ZnO NPs combat microbes through multiple mechanisms. Resistance to ZnO NPs is rarely reported.³¹ Some of the mechanisms are as follows: 1) ZnO NPs bind to bacterial cell membranes tightly and destroy both the lipids and proteins of the membrane causing increased membrane permeability and cell lysis; and^31,33,95 2) ZnO NPs also cause the formation of Zn²⁺ ions and ROS, including hydrogen peroxide (H₂O₂), which damage the bacterial cell.^33,35

ZnO NPs are effective against both Gram-positive and Gram-negative bacteria because of their antibacterial activity against high temperature-resistant and high pressure-resistant bacterial spores.⁹⁹ Similar to Ag NPs, ZnO NPs were incorporated into PNIPAM as antimicrobial hydrogel coatings, which was demonstrated to be a promising candidate for novel biomedical device coatings.^100,101 Yadollahi et al¹⁰² synthesized CMC/ZnO nanocomposite hydrogel through the in situ formation of ZnO NPs within swollen CMC hydrogels which demonstrated their antibacterial effects against E. coli and S. aureus bacteria. Nanocomposite hydrogels with IPN structure based on PEG methyl ether methacrylate-modified ZnO (ZnO-PEGMA) and 4-azidobenzoic agarose (AG-N3) exhibited an increasing anti-adhesive property and bactericidal activity toward Gram-negative E. coli and Gram-positive S. aureus.¹⁰³ Moreover, the ZnO hydrogels showed great potential in drug carrying and wound healing in some recent studies.^103–105 CMC and CS hydrogels were also reported to be used as a hydrogel matrix for ZnO NPs.^98,99,106 CMC hydrogels exhibited antibacterial activity against both Gram-positive and Gram-negative bacteria, and CS hydrogels were confirmed eligible wound dressing materials.^103,107,108 Although the antibacterial ability of ZnO NPs is relatively weak, the low cytotoxicity still indicated that ZnO NPs have potential in clinical use. Moreover, ZnO NPs have a positive effect on bone regeneration,¹⁰⁹ which means ZnO NPs are promising materials in orthopedic surgery.

Other metal nanoparticle-based antimicrobial hydrogels

There are many other metal nanoparticles combined with hydrogels, which have been studied in recent years. Their antibacterial mechanisms are shown in Figure 4.³¹ Apart from these commonly used metal nanoparticles, cytocompatible nickel nanoparticle-loaded chitin hydrogels were developed against S. aureus,¹¹⁰ and antibacterial cobalt-exchanged natural zeolite/PVA hydrogel was proved to have antibacterial activity against E. coli.¹¹¹ Although copper-containing NPs (Cu NPs/CuO NPs) have weaker antibacterial effects than Ag NPs, they have a greater range of microbicidal activities against both fungi, especially Saccharomyces cerevisiae, and bacteria, including E. coli, S. aureus and Listeria monocytogenes.^112–114 CMC/CuO nanocomposite hydrogels and CS hydrogel loaded with copper particles demonstrated excellent antibacterial effects against E. coli and S. aureus without causing any toxicity in recent studies.^115,116 It was reported recently that magnesium-containing nanoparticles, including magnesium halogen-containing nanoparticles (MgX₂ NPs) and magnesium oxide-containing nanoparticles (MgO NPs), also combat microbes through multiple mechanisms.^117–119 Hezaveh and Muhamad¹²⁰ loaded MgO NPs to hydrogels prepared from hydroxyalkyl κ-carrageenan derivatives, thus controlling the drug delivery in gastrointestinal tract studies. This may enlighten us with the idea that we can load hydrogels with metal nanoparticles or other ingredients to adjust the release of other drugs in the same system. Different from Ag NPs and Au NPs, other metal nanoparticles might need further exploitation as many of these kinds of metals or their alloys appear more in designing and fabricating modern medical biomaterials. The hydrogels with other metal nanoparticles are recorded in Table 2.

Figure 4 Multiple mechanisms of antimicrobial action of Ag NPs, ZnO NPs, copper-containing nanoparticles and Mg NPs are separately exhibited. Note: Reprinted from Adv Colloid Interface Sci. 166(1–2). Dallas P, Sharma VK, Zboril R, Silver polymeric nanocomposites as advanced antimicrobial agents: classification, synthetic paths, applications, and perspectives, 119–135, Copyright 2011, with permission from Elsevier.37 Abbreviations: Ag NPs, silver nanoparticles; Mg NPs, magnesium-containing nanoparticles; NP, nanoparticle; ROS, reactive oxygen species; UV, ultraviolet; ZnO NPs, zinc oxide nanoparticles.

Table 2 Information of hydrogels with other metal nanoparticles Abbreviations: AG, agarose; Ag NPs, silver nanoparticles; AG-N3, 4-azidobenzoic agarose; AG-N3, 4-azidobenzoic agarose; AM, acrylamide; Au NPs, gold nanoparticles; CMC, carboxymethyl cellulose; CMCh, carboxymethyl chitosan; CS, chitosan; CuO/Cu NPs, copper-containing NPs; EDA, ethylenediamine; HA, hydroxyapatite; HT, hydrothermally treated; IPN, interpenetrating polymer network; κC, κ-carrageenan; MgO NPs, magnesium oxide-containing nanoparticles; MRSA, methicillin-resistant S. aureus; NaCMC, CMC sodium salt; PMAA, poly(methacrylic acid); SF, silk fibroin; UV, ultraviolet; ZnO NPs, zinc oxide nanoparticles; ZnO-PEGMA, poly(ethylene glycol) methyl ether methacrylate-modified ZnO.

Hydrogel with metal nanoparticles might be a way to solve antibiotic resistance. There are several advantages of these antimicrobial materials. First, metal nanoparticles could be good substitutes for antibiotics. Despite widespread use of metal nanoparticles, bacterial resistance has been rarely reported. This is presumably due to the multiple mechanisms of antimicrobial action (Table 3), while antibiotics usually have only one mechanism of action. Second, the small size of particles allows them to pass through peptidoglycan cell walls and cell membranes, getting into the cytoplasm of bacterial cells easily. Third, metal nanoparticles are stable in quality, which means they could go on to kill other microbial cells after being released from dead cells. Metal nanoparticles could bring sustainable antimicrobial effect in this way. Finally, hydrogels can offer delivery system for local application. Antibacterial property improves with increasing concentration of nanoparticles. The concentration of metal nanoparticles could be high at the infection zone. All the abovementioned advantages indicate that hydrogels with nanoparticles can help to solve the present-day challenges of antimicrobial medicine.

Table 3 Antimicrobial mechanism of nanoparticles Abbreviations: Ag NPs, silver nanoparticles; Amp, ampicillin; Au NPs, gold nanoparticles; CuO/Cu NPs, copper-containing NPs; MgO NPs, magnesium oxide-containing nanoparticles; MgX2 NPs, magnesium halogen-containing nanoparticles; PVA, polyvinyl alcohol; ROS, reactive oxygen species; ZnO NPs, zinc oxide nanoparticles.

Hydrogel loaded with micromolecular drugs

Micromolecular drugs include various antibiotic agents, such as antibiotics, biological extracts and synthetic antimicrobial drugs. All these drugs have been used for their great antimicrobial properties clinically. Usually, they are systematically used in the hospital. Once carried by hydrogels, they can be used locally around the focus, and are a good way to reduce the dosage and the appearance of resistance.

Antibiotics

Though antibiotics were discovered later than metal antimicrobial agents in human history, they are undoubtedly the most commonly used, and the most effective antimicrobial agents until now.¹²¹ The drug-resistant effect of antibiotics becomes the biggest obstacle on the development and application of antibiotics. In recent years, there have been several new antibiotic approvals as well as renewed interest in second- and third-line antibiotics because of the concerns mentioned earlier.⁹ Almost all recent antibiotic resistance appeared in the year when the resistant bacterium were discovered (Figure 5). In recent studies, only one antibiotic, teixobactin, has no resistant bacteria strains.¹²² It is very effective to Gram-positive bacteria. However, the antibacterial spectrum of teixobactin does not include Gram-negative bacteria.¹²³ Moreover, the lack of selection of resistance to teixobactin in vitro should be viewed with great caution before large scale of clinical use.¹²⁴ Although the Governments of US and European Union tried to make it attractive, most pharmaceutical companies have stopped, or severely limited, investments to discover or develop new antibiotics to treat the increasing prevalence of infections caused by multidrug-resistant bacteria.¹²⁵ The reason is that return on investment has been mostly negligible for antibiotics with US Food and Drug Administration (FDA) certification in the last few decades.¹²⁶ There are two main ways to overcome antibiotic resistance, one is manufacture of novel antibiotics, and the other is minimizing the dosage to decrease antibiotic resistance.¹²⁷ To achieve the second aim, local antibiotic administration has drawn increasing attention in recent years to improve the treatment effects. Antibiotic-loaded systems can deliver an adequate local bactericidal dose directly to the infected site, without significantly overtaking the systemic toxicity level.¹²⁸ Hydrogels, as a kind of local administration matrix, offer high surface area to volume ratio and the capacity to design their physical properties such as porosity to match natural tissue. Recent studies have shown that a combination of synthetic antimicrobial polymers and antibiotics could potentially evade problems of drug resistance by taking advantages of the polymer’s membrane-lytic mechanism. Meanwhile, polymer toxicity is mitigated as the co-usage of antibiotics allows for a smaller amount of polymer in use.² So, it is easy for hydrogels to selectively load drug molecules with controlled release at the desirable site and to offer accurate prolonged release.^31,129–131 The antibiotics in common use for antimicrobial hydrogels are as follows.

Figure 5 Development of antibiotics and appearance of drug resistance are summarized chronologically referring to Huh and Kwon,35 Andersson and Hughes,132 Rodriguez-Rojas et al,133 van Hoek et al,134 Molton et al.135 Abbreviations: E. coli, Escherichia coli; K. pneumoniae, Klebsiella pneumoniae; MRSA, methicillin resistant S. aureus; S. aureus, Staphylococcus aureus; VISA, vancomycin intermedicate resistant S. auereus; VRE, vancomycin-resistant Enterococcus; VRSA, vancomycin-resistant S. aureus.

Ciprofloxacin

Ciprofloxacin is a fluoroquinolone antibacterial agent, which is active against a broad spectrum of Gram-positive and Gram-negative bacteria.¹³⁶ It is the gold standard for various topical applications, such as eye and skin infections.¹³⁷ Ciprofloxacin is also a recommended treatment for Shigella infections. However, ciprofloxacin-resistant Shigella sonnei are being increasingly isolated in Asia and sporadically reported on other continents.^138,139 The mechanism of ciprofloxacin depends upon blockage of bacterial DNA duplication by binding itself to DNA gyrase, thereby causing double-stranded ruptures in the bacterial chromosomes, so resistance to this drug develops slowly.¹⁴⁰ Minimal toxicity of ciprofloxacin is related to dosage, and excessive doses can cause damages to tissues, whereas hydrogels can solve this problem as a local delivery system.

Ciprofloxacin can be self-assembled with a tripeptide into an antimicrobial nanostructured hydrogel to enable abundant drug to be carried along with prolonged release.^129,137 Modified hydrogel coatings were reported to prevent titanium implant-associated infections.¹²⁸ A hydrogel generated by polymerizing aminophenyl boronic acid in PVA with ciprofloxacin was reported to treat wound healing in diabetes patients.¹⁴¹ It has been reported that diseases associated with the colon such as constipation may be treated with hydrogels containing laxative psyllium and ciprofloxacin.¹⁴² Hosny¹³⁶ demonstrated that a liposomal hydrogel containing ciprofloxacin improved maximum ocular availability through albino rabbit cornea. In the research of Zhou et al,¹⁴³ porous scaffolds of PVA were prepared by quenching in liquid nitrogen and the freeze drying method, from different concentration aqueous solutions loaded with ciprofloxacin were fabricated. Complete inhibition of microorganism growth revealed the sustaining release of ciprofloxacin. Other researchers used dextrin and poly-based hydrogel as a carrier for ciprofloxacin, and the results suggest that hydrogel was a promising candidate for controlled release of ciprofloxacin.^144,145 These studies indicate that the hydrogels loaded with ciprofloxacin have great potential of clinical administration, especially for infectious diseases due to their excellent antimicrobial properties and prolonged effect.

Gentamicin

Gentamicin is a traditional broad spectrum antibiotic used for the treatment of skin and soft tissue infections. However, systemic toxicity, especially for kidney, and low plasma concentration hinder its application.¹⁴⁶ To avoid the side effects, gentamicin is often used locally nowadays.^147,148 Local administration of functional gentamicin hydrogels offers an efficient solution. Gentamicin-loaded PVA and PVA-AAm hydrogels cross-linked by sterculia have many biomedical properties such as blood compatibility, tensile strength, burst strength, water vapor permeability and oxygen diffusion. It can be a kind of potent antimicrobial wound dressings.^149,150 Superabsorbent polysaccharide gentamicin hydrogels based on pullulan derivatives also brought a broadened view about antimicrobial hydrogels. It may become one of the important applications in the future with the ability to expand to 4,000% of its volume.¹⁵¹ Phospholipid-modified solid lipid microparticles encapsulating gentamicin were loaded into three polymeric hydrogels. Poloxamer 407 microgels were proved to have the most desirable properties in terms of fast antibacterial activity, in vitro diffusion-dependent permeation, spread ability, pH and viscosity.¹⁵² This implied that the same drug can reach different diffusion speeds on hydrogels of different matrices. Hydrogel based on the copolymer poly(N-isopropylacrylamide-co-dimethyl-γ-butyrolactoneacrylate-co-Jeffamine^® M-1000 acryl amide) (PNDJ) with delivery in >6 weeks was loaded with gentamicin. This hydrogel might decrease treatment failure for orthopedic infection.¹⁵³ Inspiringly, Wu et al¹⁵⁴ found that gentamicin sulfate (GS)-loaded carboxymethyl-chitosan (CMCh) hydrogel cross-linked by genipin was an effective and simple approach to achieve combined antibacterial efficacy and excellent osteoblastic cell responses, which has great potential in orthopedic applications. In open orthopedic surgeries, gentamicin-loaded thermosetting composite hydrogels, which were prepared combining CS with bovine bone substitutes (Orthoss^® granules, Orange, CA, USA), beta-glycerophosphate as a cross-linker and lyophilized to obtain moldable composite scaffolds (moldable composite scaffold loaded with gentamicin [mCSG]), were considered to reduce the infection risks.¹⁵⁵ The hydrogels have broken the limit of gentamicin application since the effective dosage can be decreased. Other antibiotics with serious side effects can also be used with hydrogels.

Vancomycin

Vancomycin, a macromolecular glycopeptide antibiotic, is considered as the last defense of infection clinically, especially for methicillin-resistant Staphylococcus.^156,157 But now even vancomycin-resistant Enterococcus (VRE) has been found in different regions.^158–160 As mentioned earlier, hydrogels as a delivery system are able to protect and enhance the validity of vancomycin. Syringeable pluronic-α-cyclodextrin (CD) supramolecular gels,¹⁶¹ hydrogel of thiolated CS cross-linked with maleic acid-grafted dextrin,¹⁶² thermosensitive hydrogel of CS/gelatin/β-glycerol phosphate,¹⁶³ hydrogel of oligo(PEG fumarate)/sodium methacrylate (OPF/SMA) charged copolymers as biocompatible matrices,¹⁶⁴ poly(β-amino ester) (PBAE) hydrogels mixed with PEG (MW = 400) diacrylate (PEGDA) and diethylene glycol diacrylate (DEGDA)¹⁶⁵ and hydrogels achieved by photo cross-linking of methacrylated dextran and poly(L-glutamic acid)-g-hydroxyethyl methacrylate are all studied, and they exhibited excellent antimicrobial properties and desirable release capacity.¹⁶⁶ The most common pathogen of osteomyelitis is S. aureus, especially MRSA. Vancomycin is always used in the treatment of osteomyelitis because it is the most effective antibiotic against MRSA. The combination of hydrogels and vancomycin is a good material, which can prevent osteomyelitis clinically.

Synthetic antimicrobial drugs

Here, synthetic antimicrobial drugs refer to the nitroimidazoles, sulfanilamide groups and other frequently used antibiotics through de novo synthesis, not including semi-synthetic antibiotics or biological extract. Synthetic drugs have many advantages because of their special chemical structures, but they bring risks and damage to normal tissue for the same reason too. So, stable and safe delivery systems become necessary.¹⁶⁷ Nitroimidazoles can have an effect on anaerobic bacteria and amoeba, so they are often used for the digestive system.¹⁶⁸ Ornidazole has been loaded on hydrogels composed of CMCh for colon-targeted delivery, and its release can be controlled by a change in pH.^163,169 Das et al^130,144,170 used dextrin and poly-based hydrogel as a carrier for ornidazole, and the result suggests that the hydrogel was a promising vehicle. Hydrogels based on dextrin grafted with poly(2-hydroxyethyl methacrylate) by embedding N,N-methylene bisacrylamide as a cross-linker can also be a good candidate for an orally administered drug delivery system for the colon region.¹³⁰ Metronidazole (MTZ) containing PMAA nanogel as an oral dosage form for gastrointestinal infection¹⁷¹ and tinidazole containing hydrogels based on CS have also been studied.¹⁷² Moreover, floating pH-sensitive CS hydrogels containing MTZ were more effective against Helicobacter pylori than the commercially available oral MTZ tablets.¹⁷³ CS/gelatin/β-glycerol phosphate hydrogels could maintain sustained release of MTZ in concentrations that are effective for eliminating pathogenic bacteria over time.¹⁶³ Chlorhexidine is considered a promising antimicrobial agent and possesses a broad spectrum of activity against bacteria.¹⁷⁴ Chlorhexidine thermosensitive hydrogel¹⁷⁵ and chlorhexidine diacetate containing thermoresponsive hydrogel copolymers exhibit novel application of this traditional sterilization agent.¹⁷⁶ In recent research studies, the micrometer-sized β-CD-based hydrogel (bCD-Jef-MPs) system also achieved sustained release of chlorhexidine digluconate, thus treating periodontitis lesions became effective.¹⁷⁷ The prolonged release has made it possible to decrease its dosage. Therefore, its side effects were reduced. Octenidine, as an external application, has become active wound dressings with minimized side effects after being loaded on bacterial nanocellulose.¹⁷⁸ Thiosemicarbazone, an antimicrobial drug used in ophthalmic diseases, was loaded on poly(2-hydroxyethyl methacrylate)-conjugated beta-CD or directly cross-linked hydroxypropyl-beta-CD to explore novel materials for fabrication of soft contact lenses.¹⁷⁹ In the study by Sittiwong et al,¹⁸⁰ the drug release rate of sulfanilamide-loaded PVA hydrogels could be controlled through the drug size, matrix pore size, electrode polarity and applied electric field. As for wound therapy, immobilization of cetylpyridinium chloride to PVA hydrogels offers suppressed release;¹⁸¹ chloramine-T and sulfadiazine sodium salt-loaded hydrogels composed of PVA, PVP and glycerin showed an excellent swelling capacity;¹⁸² a novel polyvinyl–pyrrolidone–iodine hydrogel in wound therapy was found to be able to enhance epithelialization and reduce loss of skin grafts;¹⁸³ poly(N-hydroxyethyl acrylamide)/salicylate hydrogels provide both antimicrobial and antifouling functions;¹⁸⁴ and isothiazolinones delivered in alginate hydrogel sphere achieved long-term antibacterial activity by improvement of the alkali and heat resistances.¹⁸⁵ All these evidence showed that novel applications of synthetic drugs and hydrogels can avoid risks and side effects. The combination of synthetic drugs and hydrogels offers us a an effective clinical antimicrobial method.

Other antibiotics

Besides the aforementioned most commonly used antibiotics, there are many other antibiotics loaded in hydrogels, such as amoxicillin, ampicillin, cephalosporin etc. Each of them has its own antibacterial spectrum and advantages. Amoxicillin trihydrate, a common treatment for peptic or gastric ulcers caused by H. pylori infection,¹⁸⁶ loaded in κ-carrageenan hydrogels containing CaCO₃ and NaHCO₃ or CS/poly-gamma-glutamic acid nanoparticle pH-sensitive hydrogels was well protected from the gastric juice, thus facilitating drug effects specifically at the site of infection.^187,188 The similar results of in vivo studies by Moogooee et al¹⁸⁹ showed that the amoxicillin-loaded hydrogels enhance drug concentration at the topical site than powder amoxicillin, meaning that therapeutic concentration can be achieved at a much lower dose which may reduce the adverse effects of amoxicillin in high doses. Ampicillin sodium-loaded PVA–alginate physically cross-linked hydrogel exhibited both Gram-positive and Gram-negative antimicrobial properties and improved hemolysis.¹⁹⁰ Cephalosporin belongs to beta-lactamase, and it is a widely used β-lactamase-resistant and broad spectrum antibiotic.¹⁹¹ Cefixime (CFX)-loaded CS/PEG hydrogel exhibited controlled release of drug and antibacterial activity against Gram-negative bacteria (E. coli) and Gram-positive bacteria (S. aureus).¹⁹² Cefditoren pivoxil hydrogels with gastroretentive effect were achieved,¹⁹³ and cefazolin containing methoxy PEG-co-poly(lactic acid-co-aromatic anhydride) hydrogels offered a stable release without initial burst.¹⁹⁴ Levofloxacin-loaded hyaluronic acid hydrogels were reported to be able to chase bacteria within the cells for both S. aureus and P. aeruginosa strains.¹²⁷ In order to eradicate bacterial biofilm and avoid possible intestinal obstructions, Islan et al¹⁹⁵ reported a smart autodegradable hydrogel containing alginate lyase (AL) and levofloxacin, which induced the reduction of drug toxicity and enhancement of drug bioavailability. A hydrogel based on (-)-menthol, which is a traditional cooling compound tailed by an amino acid derivate through an alkyl chain, can provide innoxious environment to living cells and deliver lincomycin to the local infection site.¹⁹⁶ O-Carboxymethyl CS (O-CMCS) hydrogels synthesized from CS and monochloroacetic acid were reported as a promising carrier for antibiotics, which showed significant antibacterial activities against E. coli and S. aureus while loaded with lincomycin.¹⁹⁷ Doxycycline was also loaded on an in situ thermally sensitive hydroxypropyl-β-CD hydrogel for ophthalmic delivery.¹⁹⁸ Controlled release of doxycycline from CS-gelatin hydrogels cross-linked with transglutaminase was observed in other research, indicating that it is a potential carrier for cell delivery.¹⁹⁹ Mupirocin appears to be one of the promising antimicrobials, as it is well tolerated in topical administration with very few side effects. Liposomes-in-hydrogel delivery system for mupirocin solved the problem of controlled and prolonged release of mupirocin, which offered an improved burn therapy, and substantial efforts have been devoted in the literature to prove its antibiofilm activity against S. aureus biofilms and non-toxicity against keratinocytes.^200,201 The methoxypoly(ethylene glycol)-co-poly(lactic-co-glycolic acid) (mPEG-PLGA) hydrogel containing teicoplanin was reported effective for treating osteomyelitis in rabbits.²⁰² There are a plenty of reports about different antibiotics loaded in hydrogels, but the three mentioned earlier are the mostly used ones. However, other antibiotics and hydrogel offered us with more choices when facing different bacterial infections. Meanwhile, it decreases the risk of antibiotic resistance. The controlled release of antibiotics is another advantage of hydrogels. The stable and continuous release without initial burst would ensure prolonged antimicrobial effect which can satisfy clinical demand. To offer an easier query, most of the hydrogels with antibiotics are recorded in this review (Table 4).

Table 4 Information of hydrogels with antibiotic agents Abbreviations: c-Dxt/PAA, dextrin and poly(acrylic acid); CFX, cefixime; CG, chitosan–gelatin; CHX-dg, chlorhexidine digluconate; CS, chitosan; CS/G/β-Gp, chitosan/gelatin/β-glycerol phosphate; DOX, doxycycline; Dxt, dextrin; EDC, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride; KPS, potassium persulfate; mPEG, methoxy poly(ethylene glycol); mPEG-PLGA, methoxypoly(ethylene glycol)-co-poly(lactic-co-glycolic acid); mPEG-PLCPHA, methoxy polyethylene glycol-co-poly(lactic acid-co-aromatic anhydride); MIC, minimal inhibition concentration; MRSA, methicillin-resistant S. aureus; MTZ, metronidazole; NHS, N-hydroxysuccinimide; O-CMCS, O-carboxymethyl CS; OPF/SMA, oligo(poly(ethylene glycol)fumarate)/sodium methacrylate; PBAE, poly(β-amino ester); PNDJ, poly(N-isopropylacrylamide-co-dimethyl-c-butyrolactone acrylate-co-Jeffamine® M-1000 acrylamide); PEG, poly(ethylene glycol); P (NIPAM-AA-HEM), N-Isopropylacrylamide-acrylic acid-hydroxyethyl methacrylate; PMAA, poly(methacrylic acid); PVA, poly(vinyl alcohol); SA, salicylate; TCP, tricalcium phosphate; UV, ultraviolet; GP, glycerophosphate; IZD, inhibition zone diameter.

Biological extracts

Biological extracts include extracts from vegetations and animals, some of these extracts have a long history of application, and others were discovered in recent years.²⁰³ For example, the therapeutic efficiency of herbal extracts and ingredients has been limited by various factors, including the lack of targeting capacity and poor bioavailability. Hydrogel is a promising carrier for the extracts of herbal medicine in recent studies.²⁰⁴ Following are reports of hydrogels loaded with various natural extracts. Seaweed extract-based hydrogel was reported as a novel antimicrobial wound dressing, and no seaweed-derived antimicrobials have been used in wound dressings ever before.²⁰⁵ Combinations of agar and carrageenan–PVA hydrogel wound dressing have been proved to be useful in treating burns, other external wounds and non-healing ulcers of diabetes.²⁰⁶ Hydrogels extracted and assembled from dermis samples containing basement membrane proteins vital to skin regeneration, including laminin β3, collagen IV and collagen VII, were applied as a barrier against bacteria in wound healing.²⁰⁷ Though according to some research studies alginate does not display antimicrobial properties, it can be an ideal wound dressing due to its morphology, fiber size, porosity, degradation and swelling ratio.^205–208 Allicin–CS complexes were proved to have antibacterial activity against spoilage bacteria, and they may be used as an antimicrobial agent in foods.²⁰⁹ CS-based hydrogel film loaded with ethyl acetate Salix alba leaves extract showed no cytotoxicity and excellent antibacterial ability against Salmonella typhi and Candida guilliermondii.²¹⁰ Achyrocline satureioides is a medicinal plant widely used in South America, which exhibits a well-documented antioxidant activity against Gram-positive and Gram-negative bacteria, as well as a set of yeast molds.²¹¹ Curcumin is non-toxic and bioactive agent with multifunction; it is found in turmeric and has been applied for centuries as a remedy to various ailments.²¹² However, low aqueous solubility and poor bioavailability limit the application of curcumin, and thus curcumin nanoparticles and hydrogels were developed. Ag NPs–curcumin hydrogels for wound dressing were also reported, exhibiting good antibacterial properties and sustained release, which indicate enormous prolonged therapeutic value.^213,214 A polysaccharide extracted from Aloe vera, Acemannan, has various medical properties, such as antibacterial property, and it can accelerate healing of lesions.²¹⁵ Some studies demonstrated its antibacterial activity against both susceptible and resistant H. pylori strains.²¹⁶ Alginate hydrogels containing Aloe vera were applied in clinical wound care treatment due to their antimicrobial and anti-inflammation capacity.²¹⁷ Essential oils, such as lavender, thyme oil, peppermint, tea tree, rosemary, cinnamon eucalyptus, lemongrass and others, have been found to possess particular antimicrobial properties, mainly in response to the overwhelming concern of consumers over the safety of synthetic food additives.^218,219 Essential oils encapsulated in sodium alginate were reported to be qualified as disposable wound dressings.²²⁰ For those extracts from animals, honey was the most easily acquired; a Malaysian honey, Gelam honey, was incorporated into a hydrogel system to produce a functional wound dressing.²²¹ Besides honey, bee propolis loaded into hydrogels has good antibacterial ability, making it a good wound dressing for skin wound healing.²¹⁷ Another bee derivative is bee venom peptide, namely melittin, and its copolymer interactions on thermosensitive PLGA-PEG-PLGA hydrogel can be used as delivery systems for peptide drugs.²²² Lysozymes, derived from normal tears with their inherent antibacterial properties, were deposited on hydrogel contact lenses that exhibit marked activity.²²³ Vitamin E is also an important antioxidant, biodegradable hydrogel from vitamin E-functionalized polycarbonates for antimicrobial applications; it displayed excellent compatibility with human dermal fibroblast loaded with cationic polymers and/or fluconazole at minimum biocidal concentrations.²²⁴ Lignins and lignin-derived compound model polymer, dehydrogenate polymer (DHP) in alginate hydrogel, have shown strong antimicrobial and wound healing activity.²²⁵ These biological extracts are easier to get and more readily accepted. Excellent biocompatibility and good antibacterial properties also make them promising antimicrobial biomaterials in the future. However, with the studies ongoing, we still need to test its properties once we find a new natural extract. We cannot evaluate it’s antimicrobial properties and it’s safety until we conduct some experiments about it as it is newly found.

Hydrogels with inherent antibacterial activity

Here, hydrogels with inherent antibacterial activity refer to polymers of these hydrogels that exhibit antimicrobial activity by themselves or those whose biocidal activity is conferred through their chemical modification, not including hydrogels that incorporate antimicrobial organic compounds or active inorganic systems.^6,15,226 These hydrogels developed in recent years can be regarded as novel antimicrobial agents without traditional defects.⁷ The main types of these hydrogels are as follows.

Antimicrobial polymers

Antimicrobial polymers are non-stimulated or potential antimicrobials. Some of the antimicrobial polymers can form hydrogels. For non-stimulated polymers, most commonly there are certain components in the chemical structures which can play a role in antimicrobial activity. These polymers could be prepared by several routes such as in situ synthesis within a hydrogel to obtain antimicrobial activities.²²⁷ Novel hydrogels composed of thermoresponsive PNIPAM and redox-responsive poly(ferrocenylsilane) (PFS) macromolecules exhibited strong antimicrobial activity while maintaining a high biocompatibility with cells.²²⁸ Jiang et al²²⁹ synthesized the quaternary ammonium salt of gelatin using 2,3-epoxypropyl trimethylammonium chloride (EPTAC) as the antimicrobial polymer ingredient of the hydrogel. pH-sensitive and temperature-sensitive hydrogels based on 2-hydroxyethyl methacrylate (HEMA) and IA copolymers were proved to have great potential for biomedical applications, especially for skin treatment and wound dressings with excellent results of microbe penetration test.²³⁰ Antimicrobial property of an antifouling hydrogel prepared by the photopolymerization of PEGDA and a monomer containing ammonium salt (RNH3Cl) in the presence of a photoinitiator was also demonstrated by a study employing E. coli.²³¹ As for potential antimicrobial polymers, light is one of the most important factors. Photodynamic porphyrin anionic hydrogel copolymers were reported and showed great promise to the prevention of intraocular lens-associated infectious endophthalmitis.²³² Another photodynamic pHEMA-based hydrogels exhibit light-induced bactericidal effect via release of NO.²³³ All mechanisms of these antimicrobial polymers provide not only novel antimicrobial materials but also novel delivery and release methods, which can be a turn on–off switch. Although not all the ingredients can be used as antibacterial agents, they provide us with the reference. In further research studies, we may design the antimicrobial hydrogels with some functional structures or ingredients that are able to function with bacteria. As for the other parts, we shall keep them for some other properties, such as anti-inflammation or antifouling.

Antimicrobial polypeptides

AMPs are an abundant and diverse group of molecules produced by multicellular organisms as a defense mechanism against competing pathogenic microbes.²³⁴ They are recognized as a possible source of panacea for the treatment of antibiotic-resistant bacterial infections,^13,235 because AMPs have strong antimicrobial activity against a very broad spectrum of microorganisms, including Gram-positive and Gram-negative bacteria, fungi and virus.^5,236 Although agreement about the specific mechanism of AMPs has not been reached until now, it is known that AMPs work with membranes and finally lead to bacteria killing (Figure 6).²³⁵ However, AMPs have their own disadvantages. They are not stable and easy to degrade. Moreover, antimicrobial properties of natural AMPs are not as good as antibiotics. To overcome all these disadvantages, researchers have designed some recombinant AMPs with short chains, which have improved antibacterial property. The hydrogels can also be good media for AMPs to prevent self-degradation.

Figure 6 Mode of action for intracellular antimicrobial peptide activity. In this figure Escherichia coli was shown as the target microorganism from Brogden. Note: Reprinted by permission from Springer Nature, Nat Rev Microbiol, Brogden KA, Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria? 2005;3(3):238–250, Copyright 2005.235

At first, relatively simple AMPs were loaded on hydrogels, and then AMPs with certain structures or even self-assembled AMPs were developed. Mitra et al developed dipeptide-based amphiphile hydrogel with good antibacterial activities and greater cell specificities.¹⁴ Peptide-based hairpin hydrogels were reported, respectively, by Salick et al with MAX1 peptides and Veiga et al with arginine-rich peptides; both of them are self-assembly peptides exhibiting potent antibacterial activity.^237,238 A Gram-positive antibacterial activity possessing peptide (KIGAKI)3-NH2 with hairpin and self-assembly structure was incorporated with hydrogels by Liu et al.²³⁹ Highly active AMP CKRWWKWIRW-NH2 was immobilized to the surface of poly(ethylene terephthalate) hydrogel, thus establishing bactericidal activity against S. aureus and S. epidermidis.²⁴⁰ Poly-lysine, a popular AMP that has been reported by Zhou et al, was applied in photopolymerized antimicrobial hydrogels, which can be promising coatings for medical devices and implants (Figure 7).^241,242 In the research studies conducted by Jiang et al,²⁴³ cationic multidomain peptides (MDPs) demonstrated a better antimicrobial activity in hydrogels than in solution. AMP maximin-4-loaded poly(2-hydroxyethyl methacrylate) hydrogels,²⁴⁴ L-cysteine- and silver nitrate-loaded hydrogels were proved to have qualified antibacterial activity.²⁴⁵

Figure 7 Morphological observation of various microorganisms seeded on hydrogels by scanning electron microscope. Left columns (control), right columns (antimicrobial hydrogels). Note: Reprinted from Biomaterials. 32(11). Zhou C, Li P, Qi X, et al, A photopolymerized antimicrobial hydrogel coating derived from epsilon-poly-l-lysine, 2704–2712, Copyright 2011, with permission from Elsevier.242 Abbreviations: C. albicans, Candida albicans; E. coli, Escherichia coli; F. solani, Fusarium solani; P. aeruginosa, Pseudomonas aeruginosa; S.aureus, Staphylococcus aureus; S. marcescens, Serratia marcescens.

Although AMPs still have disadvantages, such as tissue toxicity and hemolysis,^246,247 they also exhibited a higher antimicrobial biocompatibility index value compared with synthetic drugs with similar structures,^248,249 and a lot of studies have attempted to improve the biocompatibility.^234–236 A cell adhesive polypeptide and PEG hydrogel with inherent antibacterial activity was developed by Song et al as a potential scaffold for cutaneous wound healing.²⁵⁰ Moreover, a protein anchor developed to immobilize functional protein to PEGDA microspheres by Buhrman et al demonstrated a novel method to maintain therapeutic efficacy without toxicity.²⁵¹ In the study of Xie et al,²⁵² in situ forming biodegradable hydrogel (iFBH) system conjugated and functionalized with AMPs offered excellent bacteria inhibition and promoted wound healing without cytotoxicity. Interestingly, nanostructured hydrogels with D-amino acids for peptide self-assembling demonstrated better antimicrobial activity without cytotoxicity.²⁵³ These studies have brought us the possibility of applying the AMPs as antibiotic agents in the hospital. However, there is still a long way to go due to the fact that AMPs are not stable and they degrade easily. Whether AMPs can be kept in the hydrogels for a long time still needs further studies.

Amphoteric ion hydrogels

Amphoteric ion hydrogels work in the similar way to AMPs. They are synthetic mimics (polymers) of AMPs; the feature of the mechanisms includes electrostatic interactions that facilitate binding of polymers with anionic bacterial membrane. The resulting amphiphilic interactions physically destroy the membrane structure, leading to cell death.²⁵⁴ This is also the mechanism of some types of drugs. However, we concentrate on novel amphoteric hydrogels functioning in the same way. A plethora of antimicrobial synthetic cationic polymers have been reported, including poly(acrylate) and poly(norbornene) systems, poly(arylamide)s poly-β-lactams and polycarbonates.^255–262 Jiang and Cao^263,264 are the frontrunners in this area and have published several works and reviews on zwitterionic polymers such as poly(carboxybetaine) (pCB) and poly(sulfobetaine) (pSB) in the construction of antimicrobial hydrogels. Mi and Jiang²⁶⁵ reported a new antimicrobial and non-fouling zwitterionic hydrogel through using the antibacterial salicylate anion with the negative charge to initialize its zwitterionic state. Quaternary ammonium group was one of the most famous antimicrobial materials; an in situ antimicrobial and antifouling hydrogel was fabricated from polycarbonate and PEG through Michael addition by Liu et al.²⁶⁶ When combined with hydrogels, amphiphiles work as effectively as AMPs. Polyampholytic hydrogels with high antibacterial activity exhibited water absorbency, making them a good carrier for water-soluble agents.²⁶⁷ Potent antimicrobial hydrogels were formed with anti-inflammatory N-fluorenyl-9-methoxycarbonyl (Fmoc) amino acid/peptide-functionalized cationic amphiphiles and exhibited efficient antibacterial activity against both Gram-positive and Gram-negative bacteria.²⁶⁸ To achieve the bifunctional aim of antibacteria and antifouling, a zwitterionic hydrogel is conjugated with an antimicrobial agent salicylate. This hydrogel can reach one-salicylate-per-monomer drug delivery while still maintaining non-fouling property at protein and bacteria levels.²⁶⁵ For amphiphiles, biocompatibilities may be an obstacle to overcome. Dutta et al²⁶⁹ developed cholesterol-based amino acid containing hydrogels with the aim to improve the biocompatibility of these amphiphilic molecules. In their studies, Ag NPs were synthesized in situ. The amphiphile-Ag NP soft nanocomposite exhibited notable antimicrobial property. Apart from disinfection of normal Gram-positive and Gram-negative bacteria, an antimycobacterial supramolecular hydrogel based on amphiphiles was developed by Bernet et al,²⁷⁰ which retains specific, chain length-dependent antimicrobial and antimycobacterial activity, while showing practically negligible antiproliferative cytotoxic effects. With good antibacterial properties and negligible cytotoxicity, the clinical application of amphoteric ion hydrogel still needs to be developed. These hydrogels may be a promising material to solve the problem of antibiotic resistance.

Antimicrobial polysaccharides

Antimicrobial polysaccharides are usually natural polymer or its derivatives such as starch and CS, which are being recently used for the preparation of hydrogels because of their non-toxicity, biodegradability, biocompatibility and abundance in nature.^271,272 Some of these polysaccharides have inherent antimicrobial activity, the most popular one is CS. CS has wide antibacterial spectrum of activity and high killing rate against Gram-positive and Gram-negative bacteria and low toxicity toward mammalian cells.¹⁶ As for bacteria, polysaccharide capsule plays a key role in dampening the effects of environment on bacteria. In particular, the capsule protects bacteria from osmotic stress, ensuring the cells maintain viable cytoplasmic turgor.²⁷³ CS can be dissolved in weakly acidic solution and release NH₂⁺, which could bind with negative charge to achieve bacteria stasis.²⁷⁴ As for the polymers composed mainly of CS, semi-interpenetrating CMCh/poly(acrylonitrile) hydrogels were reported to have clearly better antibacterial activity with more CMCh, and hydrogel coating by electrophoretic co-deposition of CS/alkynyl CS exhibited better antibacterial activities than pure CS hydrogel.^19,275 In the study by Straccia et al,²⁷⁶ alginate hydrogels coated with CS hydrochloride showed intrinsic antimicrobial activity against E. coli. Quaternary ammonium CS/PVA/polyethylene oxide (PEO) hydrogels were reported to exhibit a pronounced inhibitory effect against S. aureus and E. coli.²⁷⁷ As for the polymers containing CS which is just an antibacterial modification, PNIPAM/polyurethane copolymer hydrogel after CS modification exhibited good antibacterial activity.²⁷⁸ CS-grafted hydrogels containing mica nanocomposite produced a rougher surface while maintaining antibacterial activity.²⁷⁹ The CS hydrogels have already been used clinically as wound dressings due to their good antihemorrhagic properties. The antibacterial ability suggests that the clinical usage of CS hydrogels can be further developed in the future.

Peptide-based hydrogels

Several notable peptide-based antimicrobial hydrogels have also been reported in recent years. Different from hydrogels loaded with AMPs, peptide-based hydrogels refer to those hydrogels that were synthesized with amino acid or peptides as ingredients in their structure. For example, Salick et al²³⁷ designed a β-hairpin hydrogel scaffold based on the self-assembling 20-residue peptide for tissue regeneration purposes, whereby the hydrogel itself possessed intrinsic broad-spectrum antibacterial activity. Two years after the development of the β-hairpin hydrogel, the same group reported another injectable β-hairpin hydrogel based on a different 20-residue peptide, which is capable of killing MRSA on contact.²⁸⁰ In the work of Schneider et al,²³⁸ the role of arginine in the structure of antibacterial peptide was highlighted which worked as instructions for the following research studies. Moreover, recently, Liu et al²³⁹ also designed a Gram-positive antibacterial peptide-containing hydrogel material which can self-assemble in response to external stimuli such as pH, ionic strength and heat. Debnath et al²⁶⁸ reported a class of Fmoc-protected peptide hydrogelators that contained terminal pyridinium moieties, known as possessing antibacterial properties due to their propensity for penetrating cell membranes. All of the peptide hydrogels tested were effective at killing both Gram-positive and Gram-negative bacteria.²⁶⁸ A related AMP hydrogel was designed by Hughes et al.²⁸¹ They exploited enzymatic hydrolysis mechanisms inside E. coli cells to trigger an intracellular molecular self-assembly of amphiphilic peptide hydrogelators.²⁸¹ Song et al²⁵⁰ developed all-synthetic polypeptide hydrogels with antibacterial activity by cross-linking poly(Lys)x(Ala)y copolymers with six-armed N-hydroxysuccinimide (NHS)-terminated PEG. Zhou et al²⁴² modified epsilon-poly-L-lysine (EPL), an AMP produced by Streptomyces albulus, with methacrylamide moieties, and it was then cross-linked with PEGDA to form antibacterial hydrogels. Besides their antibacterial applications, these peptide-based hydrogels have offered inspiration of hydrogel design for us in the future. We can design antimicrobial hydrogels according to the different active structures of antimicrobial drugs. Therefore, the hydrogels would have excellent antimicrobial capacity. The hydrogels with inherent antibacterial activity are in Table 5.

Table 5 Information of hydrogels with inherent antibacterial activity Abbreviations: Ag NPs, silver nanoparticles; Fmoc, N-fluorenyl-9-methoxycarbonyl; GST, glutathione S-transferase; PEG, poly(ethylene glycol); PEGDA, polyethylene glycol diacrylate; MIC, minimal inhibition concentration.

Hydrogels with synergistic effect

Hydrogels with synergistic effect refer to hydrogels containing two or more antimicrobial agents combined to reach more powerful antimicrobial effect. There are two main types of antimicrobial biomaterials that are commonly reported to be incorporated into hydrogels with synergistic effect: metal nanoparticles group and antibiotics group. Those containing both metal nanoparticles and antibiotics are assigned to the antibiotics group because antibiotics feature prominently in clinical practice.

Synergistic effective hydrogels containing metal nanoparticles

Metal nanoparticles in synergistic effective hydrogels were mainly Ag NPs. Ag NPs can be loaded on synthetic amphiphilic or amino acid-based hydrogels, and they can also be loaded with biological extracts.²⁸² Reithofer et al²⁸³ synthesized size-controlled, stable Ag NPs within ultrashort peptide hydrogels with great potential for applications in wound healing due to their low silver content, sustained Ag NP release and biocompatibility. Novel Ag NP composite systems are more suitable for biomedical applications because of their good biocompatibility with biological molecules, cells, tissues and so on.²⁸⁴ In situ-synthesized Ag NPs on amphiphilic hydrogels by Dutta et al²⁸⁵ exhibited improved biocompatibility and antimicrobial efficacy, which has promising applications in biomedicine and tissue engineering. The same laboratory also reported in situ-synthesized Ag NP in self-assemblies of amino acid-based amphiphilic hydrogel in the same year, exhibiting normal growth of mammalian cells on its surface while being lethal toward both Gram-positive and Gram-negative bacteria.²⁸⁶ Some researchers synthesized antimicrobial Ag NPs and impregnated them into antifouling zwitterionic hydrogels, thus getting mussel-inspired, antifouling, antibacterial hydrogels with great potential in wound healing applications (Figure 8).²⁸⁷ Both bactericidal hydrogels based on L-cysteine and silver nitrate and Ag(I)–glutathione hydrogel which exhibited improved cytocompatibility were reported in 2011,^245,288 offering more possibilities on potential application in biomedical field such as burn wound dressings. For other combinations, Ag NP–curcumin composite hydrogels demonstrated that incorporation of curcumin into these hydrogel nanocomposites would further enhance their antibacterial efficacy. The entrapped Ag NPs and curcumin molecules proved sustained release, which could be exerted in enormous prolonged therapeutic values.²¹³ Anjum et al²⁸⁹ reported a composite hydrogel for wound dressing containing nanosilver along with aloe vera and curcumin. It showed better antimicrobial nature, wound healing and infection control compared with the control group.²⁸⁹ Synergistic effective hydrogels containing metal nanoparticles show great antibacterial ability and large antibacterial spectrum. According to distinct antimicrobial pathways, it is impossible to develop antimicrobial resistance. These materials are promising for hospital application in the future.

Figure 8 A new strategy that uses catecholic chemistry to synthesize antimicrobial silver nanoparticles impregnated into antifouling zwitterionic hydrogels. Notes: On the top is the schematic illustration of the combination of AgNPs and antifouling hydrogel. In the middle, Photographs show the changes in color of hydrogels by changing the pH because of reaction that converts the Ag+ into solid AgNPs. The bottom section shows the surface structure and the morphology of hydrogel via scanning electron microscopy. Reprinted with permission from GhavamiNejad A, Park CH, Kim CS. In situ synthesis of antimicrobial silver nanoparticles within antifouling zwitterionic hydrogels by catecholic redox chemistry for wound healing application. Biomacromolecules. 2016;17(3):1213–1223. Copyright (2016), American Chemical Society.287

Synergetic effective hydrogels containing antibiotics

Hydrogels containing antibiotics exhibit more potent antimicrobial properties and biocompatibility when combined with other antimicrobial materials. As for traditional gentamicin, a novel controlled release zinc oxide/gentamicin–CS composite gel with potential application in wounds care was reported. ZnO, gentamicin and CS are all antimicrobial agents, but the composite gel can significantly improve minimal inhibition concentrations (MICs) of Gram-positive and Gram-negative bacteria compared with only gentamicin (Figure 9).²⁹⁰ Bacterial cellulose polymers functionalized by RGDC (R: arginine; G: glycine; D: aspartic acid; C: cysteine)-grafting groups and gentamicin offer a creative method for novel antimicrobial composite though it was not hydrogel.²⁹¹ To cure keratitis, Paradiso et al²⁹² added levofloxacin and chlorhexidine to vitamin E-loaded silicone hydrogel contact lenses and found that drug loaded in the lenses can be controlled to achieve a daily release in vivo. Ciprofloxacin is one of the most effective antibiotics used clinically, and it has become the gold standard for various topical applications such as skin and eye infections. It was reported to be able to be combined with different materials from metal nanoparticles to amphiphiles.¹³⁷ Ciprofloxacin loaded into an antimicrobial nanostructured self-assembly tripeptide hydrogel was reported by Marchesan et al,¹²⁹ which is meaningful to the design of cost-effective nanomaterials. In their design, drug incorporation in the delivery could lead to prolonged release and novel antimicrobial formulations.¹²⁹ Release of ciprofloxacin loaded on PVA-based super paramagnetic nanocomposites can be magnetically mediated, which provides novel approach of release though no hydrogel formation was studied in this article.¹⁴⁰ In in vivo studies, dextrin polymer hydrogels impregnated with amikacin and clindamycin were applied in dogs whose tibial plateau leveling osteotomy implants were removed due to suspected surgical site infection, and no signs of inflammation or infection in any dog were found at the 12th week.²⁹³ Quaternized gellan gum-based particles for controlled release of ciprofloxacin demonstrated another potential dermal application.²⁹⁴ Besides, tetracycline hydrochloride Ag NP composite hydrogels were developed to inhibit bacteria in simulated colon environment.²⁹⁵ All these synergetic effective composite hydrogels offer possible approaches for minimum of antibiotics dosage. Combination with other antibacterial ingredients can be a good way to solve the antibiotic resistance and side effects. Meanwhile, the antibacterial spectrum is enlarged, indicating that synergetic effective composite hydrogels have great potential clinically. However, synergistic effects occur when two or more drugs work together to form a stronger response than individually, known as 1+1>2 effect. In most of the abovementioned studies, researchers were more likely to describe additive effect. When the different antimicrobial ingredients were put together in hydrogel, the antibacterial spectrum was boarder and the antimicrobial effect became better compared with hydrogels loaded with one agent separately, whereas we could not tell if the overall effect was synergistic. We would like to see whether the two antimicrobial ingredients would exhibit synergistic effect or only additive effect in further study.

Figure 9 Graphical representation of MICs obtained after growing S. aureus and P. aeruginosa in the presence of different concentrations of gentamicin and ZnO/gentamicin–chitosan. Note: Reprinted from Int J Pharm. 463(2). Vasile BS, Oprea O, Voicu G, et al, Synthesis and characterization of a novel controlled release zinc oxide/gentamicin-chitosan composite with potential applications in wounds care, 161–169, Copyright 2014, with permission from Elsevier.290 Abbreviations: MICs, minimal inhibition concentrations; S. aureus, Staphylococcus aureus; P. aeruginosa, Pseudomonas aeruginosa.

Summary and prospect

Recent advances in natural and synthetic hydrogels have either intrinsic antimicrobial properties or act as carriers for antibiotics. Hydrogels as antimicrobial biomaterials can be an alternative and amendable solution other than the traditional antibiotic treatment since too many drug-resistant bacteria were developed due to misuse of antibiotics and other antimicrobial drugs. Controlled and prolonged release, local administration, stimulated switch on–off release, enhanced mechanical strength and improved biocompatibility are important advantages which a broad diversity of hydrogels can bring. Antimicrobial hydrogels can be applied to a broad spectrum such as wound dressings, urinary tract coatings, contact lens, treatment of osteomyelitis, catheter-associated infections, gastrointestinal infections and so on, finally conquer formidable problems in traditional therapy. Novel antimicrobial biomaterials, novel combination of these materials and novel approaches will bring us brand new prospects and promising further in anti-infection treatment.

For treating microbial infections, it is crucial that antimicrobial components can be released from gels to enter immune cells and kill the pathogenic microbes from inside. Hydrogels loaded with antibiotics, metal nanoparticles, antimicrobial polymers and peptides can release the antimicrobial agents in a sustained manner, which is important to treat infections effectively and prevent biofilm formation. Biodegradable antimicrobial polymer-loaded or peptide-loaded gels are more attractive than gels encapsulated with antibiotics or metal nanoparticles because antibiotics easily develop drug resistance, and it is relatively more difficult to mitigate toxicity of metal nanoparticles due to their non-degradability.

Antimicrobial hydrogels could help to solve the present-day challenges of antimicrobial medicine, including antibiotic resistance. The mechanisms are as follows: 1) the antimicrobial hydrogels could be used locally, which would avoid the side effect of systemic application; 2) the hydrogels, as a novel drug delivery system providing sustainable release of antimicrobial drugs, could offer prolonged antimicrobial effect and avoid screening of resistant bacteria; 3) according to the multiple mechanisms of nanoparticles and other antibacterial ingredients, it is difficult for bacteria to develop resistance aiming at only one target; and 4) different ingredients might exhibit synergistic effect. This would bring broader antibacterial spectrum and better antimicrobial effect.

Hydrogels have offered us a new way to fight against antibiotic resistance in clinical application. However, the controlled release of drugs cannot be accurate in the existing hydrogels. Some of the hydrogels degrade too fast to prolong the effect. Moreover, the antibacterial property of hydrogels is usually weak. Most of them cannot be used as antimicrobial materials alone. Some hydrogels would react with drugs they load, thus limiting their practical application. In the future, these problems still call for more research studies to be solved.

As for the antimicrobial spectrum of antimicrobial hydrogels, lots of them were determined by the antimicrobial ingredients they carried. Some of the materials were only tested with specific bacteria. Some of the hydrogels were examined with both Gram-positive (usually S. aureus) and Gram-negative bacteria (E. coli). The result indicated that the antimicrobial properties of the materials was different against various bacteria. Rarely, researchers have reported the entire antimicrobial spectrum of antibacterial hydrogels in their articles. We hope that researchers could carry out more studies about the antibacterial properties of materials against different bacteria. This will help us to find out if activity against one particular bacterium is limited in scope or that nanomaterial might have broader utility.

For future clinical applications, it is critical to test antimicrobial hydrogels against clinically isolated microbes, especially multidrug-resistant strains and evaluate the in vitro and in vivo biocompatibility of hydrogels and encapsulated cargo. With rational design, synthetic polymer chemistry and comprehensive in vitro and in vivo evaluation, hydrogel systems with broad-spectrum antimicrobial activity against multidrug-resistant microbes, high selectivity and negligible toxicity would find great potential in the prevention and treatment of infections.

Disclosure

The authors report no conflicts of interest in this work.

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