In dentistry, liposomes have been used topically to control the oral biofilm (preventing caries and gingivitis), to treat oral lesions and periodontitis, and in photodynamic therapy. Liposomes are synthetic nano-sized vesicles consisting of one or more phospholipid bilayers, able to accommodate hydrophilic and lipophilic molecules. Liposomes may be formulated with a range of characteristics including different size, charge and drug retention, which can be customized for a given drug and target site [53, 54]. In early 80 s, Mezei and Gulasekharam [55, 56] have shown the applicability of liposomes as drug carriers for the topical administration using triamcinolone as a model drug. Later, the potential of liposomes as drug carriers to the ulcerated oral mucosa was investigated in vivo in hamsters using radioactive triamcinolone acetonide palmitate . Liposomes were shown to increase local and decrease systemic drug concentration. In addition, the authors suggested that liposomes decrease drug diffusion into neighboring tissues and localize the drug in the area of inflammation. Proteoliposomes with surface-bound succinylated concanavalin A were prepared to deliver triclosan for elimination of Streptococcus sanguis biofilms . It was shown that triclosan delivered in liposome was a more effective growth inhibitor than free triclosan. Further, reactive liposomes were prepared encapsulating the enzymes, glucose oxidase (GO) and GO in combination with horse radish peroxidase (HRP) to eliminate the biofilms of the oral bacterium Streptococcus gordonii . Increased bacterial inhibition was observed with the reactive liposomes. Antibacterial activity in the presence of saliva was also observed with the reactive liposomes.
Immunoliposomes were developed to increase the specificity and affinity of bactericide delivery to a specific model bacterium . Antibacterial immunoliposomes were prepared using covalently bound antibody, extended to the cell surface of the bacterium Streptococcus oralis and chlorhexidine and triclosan were incorporated as the bactericides. For short exposure times to the biofilms, several times enhanced growth inhibition of S. oralis was obtained with immunoliposomes when compared to free bactericides.
Variable results have been reported in regard to relation between the surface charge of the liposomes and their effect on biofilms, most likely due to the differences between test methods used in the studies. Nguyen et al.  have reported that negatively charged liposomes, specifically targeting for the teeth, appeared to be the most suitable for use in the oral cavity because these liposomes were found to be the least reactive with the components of parotid saliva. On the other side, Sugano et al  have investigated the behavior of cationic liposomes on S. mutans in planktonic cells and biofilms and they reported that cationic liposomes have higher affinity not only to oral bacterial cells, but also biofilms than conventional liposomes. It was demonstrated microscopically that cationic liposomes interacted with the negative charge on the bacterial surface and penetrated the deep layers of biofilms.
Lectin-conjugated liposomes were prepared using wheat germ agglutinin (WGA) to serve as bioadhesive drug carrier that can rapidly bind to oral epithelial cells within minutes, and stay on the cells to provide sustained, localized drug release for the management of oral ulcerative lesions and other related complications . A significant reduction in oral cell damage was obtained when the bacterially infected cells were treated with amoxicillin-loaded WGA liposomes compared to the untreated controls.
Erjavec et al  have investigated liposome formulations of varying composition and size to identify a suitable carrier for drug delivery to oral mucosal lesions by assessing the effects of a hyperaemic drug on the oral mucosa using in vivo EPR oximetry. They have reported that multi-lamellar liposomes made from hydrogenated soy lecithin appeared to be the most appropriate for local drug delivery to oral mucosa.
Liposome formulations have been widely investigated for treatment of periodontitis. It was shown in vivo that local delivery of liposome-encapsulated superoxide dismutase and catalase suppressed periodontal inflammation in experimentally induced periodontitis beagle dogs . As an adjunctive treatment for chronic periodontitis, liposome formulation for an antimicrobial drug, minocycline was developed and investigated in vitro on murine macrophages (ANA-1) . Liposomes were shown to have stronger and longer inhibition effect on LPS-stimulated TNF-α secretion of macrophages cell when compared to that of solution of the drug.
pH-responsive quaternary ammonium chitosan (TMC) - liposome formulations loaded with doxycycline were developed for periodontal treatment . The periodontitis healing capacity of the developed formulations was evaluated in rats. The formulations showed antimicrobial activity against P. gingivalis and Prevotella intermedia, strong inhibition on biofilm formation and prevented alveolar bone absorption in vivo.
Periodontal therapy usually requires also local anesthesia. A liposomal lidocaine/prilocaine, thermosetting anesthetic gel formulation delivered into periodontal pocket was investigated for pain control during scaling and root planing (anti-infective periodontal therapy) in 40 volunteers with moderate to severe chronic periodontitis . It was reported that the intra-pocket anesthetic gel would be a good option for anxious patients, or those who have a fear of needles.
Micelles are self-assembling colloidal systems obtained by the aggregation block or graft amphiphilic copolymers . Micelles have found applications in dentistry for a targeted - delivery of antimicrobials to the tooth surface against biofilm formation. Chen et al , have used alendronate terminated Pluronic copolymers to prepare triclosan-loaded tooth-binding micelles and demonstrated that micelles were able to inhibit initial biofilm growth of S. mutans. The use of alendronate as a binding moiety, however, has raised concerns on the safety of these tooth-binding micelles therefore, the same group has replaced alendronate with diphosphoserine and conjugated it to the chain termini of Pluronic P123 and combined it with another biodegradable tooth-binding moiety, pyrophosphate (PPi) . Tooth-binding potential and binding stability as well as anti-biofilm activity against S. mutans of the developed micelles were found to be significant. Recently a multifunctional matrix for the treatment of periodontitis and enhancement of regeneration of the periodontal tissue was prepared from vitamin E containing hydrogel made of alginate and gelatin, and doxycycline HCl containing methoxy poly(ethylene glycol)-block-polycaprolactone micelles . A sustained drug release and enhanced antimicrobial activity were observed against E. coli and S. aureus.
Hydrogels have a three-dimensional porous and interconnected structures composed of hydrophilic, cross-linked macromolecules that absorb water, aqueous solutions, or physiological fluids, but remain insoluble due to their network structure [73, 74]. They provide a biocompatible microenvironment for cell attachment and proliferation, and possess many unique advantages on the targeted delivery systems for hydrophilic and hydrophobic agents and other biomolecules. Localized application is possible with hydrogels and they can be tailored to release the drug for a long time by controlling the hydrogel architectures, network pores, and gelation mechanisms (physical and chemical gelation). Synthetic (poly(hydroxyethyl methacrylate) (polyHEMA, PHEMA); polyethylene glycol and derivatives, poly(vinyl alcohol), polyvinylpyrrolidone, polyimide, polyacrylate, polyurethane)  and natural (chitosan, alginate, collagen, gelatin etc.)  polymers have been used for preparation of hydrogels. Most of these polymers exert also adhesive properties which enables a longer retention of the system on application site. Hydrogels have found applications in dentistry for regenerative therapies to provide recovery of the function of tissues lost due to oral and dental pathologies of infection as well as traumatic and neoplastic origin [77–79]. Furthermore, various hydrogel formulations have been used for treatment of oral lesions and also for delivery of antimicrobials, anaesthetics, antiinflammatory drugs [80–86]. Our group has investigated gel formulations based on chitosan, which is a material widely investigated in dental field both for its bioactive properties such as wound healing, tissue regeneration, antimicrobial and as a biocompatible, bioadhesive biopolymer for delivery of drugs, especially the anti-inflammatory and antimicrobial molecules [79, 87]. Chitosan gel itself has been shown to exhibit antimicrobial activity against various dental patogens . Antimicrobial activity was found to depend on the properties of the chitosan used (source -animal or non-animal, molecular weight, solubility, degree of deacetylation etc.) as well as the type of the strains tested. Furthermore, when incorporated with various antimicrobial drugs such as chlorhexidine [82, 89], nystatin , moxifloxacin , metronidazole  and anti-inflammatory drug such as atorvastatin [80, 92], the effect of the drug was found to be enhanced in presence of chitosan, besides the improved retention time and prolonged drug release. Chitosan gel itself has also been shown in vivo in human to be promising for periodontal tissue regeneration .
Hydrogels for the anesthetic drug lidocaine hydrochloride were prepared for buccal application using chitosan glutamate, or its binary mixture with glycerin. The anesthetic activity of mucoadhesive hydrogels was assessed in healthy volunteers in comparison to commercial semisolid formulations. Prolonged release of drug, which resulted in local anesthetic activity lasting for 20 to 30 min upon application was obtained. The developed hydrogels were suggested as potential delivery system reducing the pain symptoms that characterize aphthosis and other mouth diseases .
Hydrogels exert appropriate syringeability properties which makes it suitable for administration into periodontal pocket . The injectable thermosensitive hydrogels have gained more attention, especially for unapproachable periodontal pockets. An injectable thermogel system for the treatment of oral mucosa-related ulcers was developed by Luo et al . These thermogels were formed from a series of chitosan-based conjugates, composed of a chitosan backbone and synthetic side chains of thermosensitive poly(N-isopropylacrylamide) (PNIPAAM). Ulcer healing was investigated in vivo in rats and the antibacterial activity against Staphylococcus aureus as well as proliferation promotion, hemostasis effect of the developed formulation was demonstrated. Ji et al  have developed a thermosensitive hydrogel based on chitosan, quaternized chitosan and β-glycerophosphate loaded with 0.1% w/w chlorhexidine. Higher antimicrobial activity against P. gingivalis and Prevotella intermedia was obtained with gels prepared using quaternized chitosan when compared to that with chitosan.
A thermo-reversible poly-isocyanopeptide (PIC), which is a water soluble polymer forming a gel at very low polymer concentrations with good injectability properties, and has a sol-gel transition temperature of 15–18 °C , was investigated as a hydrogel for delivery of doxycycline and/or lipoxin A 4 for antimicrobial and anti-inflammatory treatment . The PIC hydrogel facilitated the drug for around 4 days in vitro. When applied in dogs, local or systemic adverse effects were observed. The subgingival bacterial load and pro-inflammatory interleukin-8 level were shown to reduce with the hydrogel formulations. Gingival clinical attachment was improved when compared to mechanical debridement.
Dong et al  have incorporated metronidazole loaded microcapsules into a poly(vinyl alcohol) injectable hydrogel by dynamic covalent bonding and ionic interaction through a 4-carboxyphenylboronic acid bridge. The developed formulation exhibited desirable antibacterial activity against P. gingivalis and Fusobacterium nucleatum for 1 week period on the rats.
Hydrogels have been used also to deliver antimicrobial peptides (AMPs), which are one of the most well-studied classes of biofilm eradication agents [101, 102]. AMPs are a diverse group of host-defense molecules that include defensins, cathelicidins, histatins, neuropeptides, peptide hormones, and many other proven and putative peptides. In the oral cavity, the AMPs are produced by the salivary glands and the oral epithelium . AMPs are effective defensive weapons and have been shown to modify cellular functions such as chemotaxis, apoptosis, gene transcription and cytokine production. Further, they play role in stimulation of wound healing and angiogenesis. Due to their antibacterial, anti-inflammatory and/or immune modulatory actions, they are used to control oral infections [104–110]. Sani et al  have developed a hydrogel based on a visible-light-activated naturally derived polymer (gelatin) and an antimicrobial peptide (AMP) for treatment of peri-implant diseases. An enhanced antimicrobial activity against P. gingivalis was obtained with the gels.
In oral mucosal conditions related to immunological pathogenesis, clinical studies have shown that topical immunomodulators such as cyclosporine, tacrolimus and pimecrolimus are also effective when compared to the steroids which are the conventionally used drugs [111–116]. In order to enhance their activities, these immunomodulators were incorporated into bioadhesive gels. For the treatment of oral lichen planus, clobetasol and cyclosporin adhesive gels based hydroxyethyl cellulose were applied twice a day on dried lesions for two months and significant healing was observed with the gels .
Currently, there are commercially available products based on hydrogels. A two syringe mixing system (Atridox) is a subgingival controlled-release product composed of the syringe A: Atrigel® Delivery System, which is a bioabsorbable, flowable polymeric formulation composed of 36.7% poly(DLlactide) (PLA) dissolved in 63.3% N-methyl-2-pyrrolidone (NMP) and syringe B containing doxycycline hyclate . Upon contact with the crevicular fluid, the liquid product solidifies and then allows for controlled release of drug for a period of 7 days. In addition, numerous gel formulations of metronidazole are also available on the market.
Nanomaterials and polymeric nanoparticles
Materials in nano size and drug-incorporated nanoparticles as well as their combination have found wide applications in dentistry for prevention, diagnosis, therapeutic, restoration and tissue regeneration purposes [52, 119–121].
Metallic nanoparticles such as silver, gold and zinc oxide due to their broad-spectrum antibacterial activity have been used to eliminate the biofilms in the oral cavity [122–127]. The large surface area and high charge density of these nanoparticles enable them to interact with the negatively-charged surface of bacterial cells to a greater extent resulting in enhanced antimicrobial activity. In order to enhance the antimicrobial activity, these metals have been combined with other antimicrobial agents such as chlorhexidine . Recently, the antimicrobial efficacy of silver and gold nanoparticles with diode laser was investigated against S. mutans in teeth sample, and the greatest reduction in colony-forming units (CFU) was observed with the combination of silver nanoparticles with diode laser group .
Metallic nanoparticles combined with polymers or coated onto biomaterial surfaces have been shown to exhibit superior antimicrobial properties in the oral cavity [128, 130, 131]. Besides silver, gold and zinc oxide, bismuth subsalicylate nanoparticles have also been shown to inhibit the growth of several periodontal pathogens including A. actinomycetemcomitans, C. gingivalis, and P. gingivalis .
Further, mesoporous silica nanoparticles, which have a porous structure with large surface area, have been investigated as anti-biofilm agents [133, 134]. When combined with another antimicrobial such as chlorhexidine, antibacterial activity against S. mutans, F. nucleatum, A. actinomycetemcomitans and P. gingivalis was shown to be enhanced .
Recently, graphene family nanomaterials, due to their superior mechanical, chemical, and biological properties, have gained great attention in dentistry. Graphene oxide (GO), as the derivative of graphene, was investigated for its antimicrobial property against various dental pathogens including S. mutans, Fusobacterium nucleatum, P. gingivalis, and GO nanosheets were reported to be highly effective in inhibiting the growth of dental pathogens . It was also shown by transmission electron microscopy that the cell wall and membrane of bacteria lost their integrity and the intracellular contents leaked out after they were treated by GO. Furthermore, graphene oxide (GO) has been widely investigated as a nanodelivery system for variety of drugs [137, 138], which makes it a promising material for treatment of infections in the oral cavity.
In the past decade, the application of antimicrobial photodynamic therapy (aPDT) on oral infectious diseases has attracted great interest. The bacteria can be killed when induced with light in presence of a sensitizing agent, by means of generation of cytotoxic, reactive oxygen species (ROS) . There are a number of sensitizers that interact with bacterial cell and generate ROS such as methylene blue, erythrosine, indocyanine green, eosin-Y, psoralen, toluidine blue ortho . Erythrosine has been applied with white light (500–650 nm) which successfully killed S. mutans and inhibited biofilm formation . A dental light with haematoporphyrin sensitizer was investigated against S. mutans, A. actinomycetemcomitans and E. faecalis and it was found that the sensitizer can penetrate gram-positive bacteria cell, whilst by A. actinomycetemcomitans, the sensitizer is taken up in presence of 10% EDTA . Furthermore, dental LEDs with blue light absorbing photosensitizer were demonstrated to disrupt E. faecalis biofilm depending on the concentration of sensitizer . However, due to the hydrophobic characteristics of the photosensitizers, aPDT was not very effective on the viability of biofilms, hence, nanomaterials (metal and metal oxide nanoparticles) or polymeric nanoparticles have been used in order to enhance the antimicrobial performance of aPDT [143, 144].
The photosensitizer indocyanine green (ICG) was incorporated into chitosan nanoparticles and
A. actinomycetemcomitans ATCC 33384 strain was treated with these nanoparticles, which was excited with a diode laser . The expression of rcpA gene which is involved in biofilm formation of A. actinomycetemcomitans was found to be significantly downregulated upon using nanoparticles for aPDT, indicating a promising approach for control of periodontal pathogens. Similarly, indocyanine green was incorporated into PLGA nanoparticles coated with chitosan for aPDT . A significantly higher antibacterial activity against P. gingivalis was observed. De Freitas et al  have investigated the effect of aPDT on human dental plaque bacteria using methylene blue (MB)-loaded poly(lactic-co-glycolic) nanoparticles in a clinical pilot study with 10 adult human subjects with chronic periodontitis. Patients were treated either with ultrasonic scaling and scaling and root planing (US + SRP) or ultrasonic scaling + SRP + aPDT with MB-nanoparticles. The clinical study demonstrated the safety of aPDT. At month three, more profound effect (28.82%) on gingival bleeding index was observed in ultrasonic SRP + aPDT group when compared to ultrasonic SRP.
In literature there are numerous studies on polymeric nanoparticles used to deliver drugs into oral cavity for treatment of oral infections [148–152]. Due to their versatile characteristics such as surface charge, dimension and hydrophobicity, it has been possible to prepare tailor-made polymeric nanoparticles for an enhanced local treatment of oral infections (see Tables 1–3)