**6. Antibacterial activity**

In biomedicine, bacterial 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 antibacterial materials are urgently needed for medical uses [90]. However, acrylics itself do not have antibacterial activity intrinsically, and therefore some fillers and antibacterial agents need to be incorporated by physical blending in order to produce an acrylic-based antibacterial material [91]. Thus, graphene has emerged as a novel green broad-spectrum antibacterial material, 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 graphene-based nanocomposites have a wide range of biomedical applications such as wound dressing due to its superior antibacterial properties and good biocompatibility [92].

In the field of dental materials, since methyl methacrylate was firstly used in tooth restoration in 1937, methacrylate monomers having good biocompatibility and adhesive property have been extensively used as dental materials [91]. The most commonly used methacrylate monomers in commercial dental resin-based materials are methyl methacrylate (MMA), 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)-phenyl]propane (Bis-GMA), 1,6-bis-[2-methacryloyloxyethoxycarbo nylamino]-2,4,4-trimethylhexane (UDMA) and tri-ethylene glycol dimethacrylate (TEGDMA) [93]. However, the dental materials produced with these monomers are not of antibacterial nature, which is very important in this biomedical field. Thus, another strategy to design acrylic hydrogels with desired antibacterial performance consists of adding silver nanoparticles (Ag NPs). This modification produced a strong antibacterial activity against *Escherichia coli* and also improved the mechanical properties of acrylic resins for dental applications. Such antibacterial effects were mainly attributed to the release of silver ions upon immersion of the dental composite in water, which appeared to be fairly nontoxic to humans [94]. Poly(methyl methacrylate) (PMMA) nanofibers containing silver nanoparticles were synthesized by radical-mediated dispersion polymerization and also showed enhanced antimicrobial efficacy compared to that of silver sulfadiazine and silver nitrate at the same silver concentration [95].

Infections are also frequent and highly undesired occurrences after orthopedic procedures. Besides, the growing concern caused by the rise in antibiotic resistance progressively decreased the efficacy of such drugs. Thus, in this area, the integration of silver nanoparticles in the polymeric mineralized acrylic-based nanocomposites also provides antibacterial activity against bacteria [96].

The combination of both previous strategies (graphene and Ag NPs) to design antibacterial 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 days of healing of impaired wounds [97]. 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 [98].
