**4. Biological properties of PMMA**

#### **4.1. Antimicrobial activity**

As mentioned above, the current techniques for processing base denture produce porosities, which allow bacterial colonization [30, 31]. One way to approach this issue is the covering up of the PMMA surface. Since the introduction of nanoparticle-based antimicrobial agents, these have generated really huge interest. Diverse mechanisms for explaining the activity of antimicrobial agents have been discussed, such as the release of ions from the nanoparticle surface, the internalization through cell wall, the production of reactive oxygen species [57], and the destruction of cell wall by the nanometric pillars on the surfaces, among others [58]. For instance, the wing surfaces of insects such as dragonflies and cicadas exhibit a texture that is formed by nanopillars, which are very effective against certain type of pathogenic microorganisms [58, 59]. The possibility of developing surfaces that have antibacterial effects quickly became the subject of study [60].

The characteristics of the surfaces of certain objects make them excellent places for proliferation of pathogenic microorganisms and thereby prevent the bacterial adhesion. The main characteristics of polymer surfaces related to microbial adhesion are chemical composition and topography [61].

Different surface modifications have been suggested to reduce the adhesion of pathogenic microorganisms. At present, one of the most effective methods is the surface modification with metallic antibacterial agents such as silver, copper, and zinc oxide at nanometric scale [62, 63]. It has been demonstrated that the oxidized state on surfaces (through electrochemical anodization) shows a significant decrease of some bacterial strains present in the oral cavity and bacteria involved in the process of biofilm formation [64].

Besides, polymeric glycol-based coatings have been proposed in order to immobilize the molecules on the surface of the substrate. Thus, this prevents bacterial adhesion. The modification of the surface topography generates an unfavorable surface chemistry for the adhesion of certain microorganisms and therefore the colonization of surfaces [65, 66].

The arrangement of polymeric coatings with antibacterial agents such as nanoparticles has been studied. The best alternative is that the nanoparticles have to be contained in the polymer matrix, so that their release acts at the level of biofilm formation [67]. According to the type of antimicrobial agent and disposition on the surface, it may offer more than one function eradicating an acute infection and even providing extended periods of suppression of bacterial proliferation [68, 69]. The action can be differentiated depending upon its mechanism in passive coatings (coatings that prevent bacterial adhesion), contact-killing coatings, and active coatings with the ability to release the antibacterial agent incorporated [70]. A coating includes different antimicrobial agents, such as moieties, nanoparticles, and antibiotics for specific pathogens [66, 71]. Silver nanoparticles as a cover on PMMA decrease the roughness from 566.7 nm (without nanoparticles) to 104.08 nm (with nanoparticles) (**Figure 4**). On the other hand, Ziad *et al*. found that Nystatin modifies the roughness of PMMA so that this could influence the antifungal agents on the PMMA surface [66].

In summary, the best conditions for higher values of both elastic modulus and flexural strength correspond to the specimens processed at 80 kHz and 100% of power (group 4). The processing of PMMA with water bath or microwave processing generated similar values for elastic modulus and flexural strength. Ultrasound can be used to process the acrylic resin (Opticryl®) as an alternative technique for PMMA processing with similar results to those obtained using water bath or microwave processing (control groups). The correlation coefficient between the elastic modulus and the flexural strength indicates a weak correlation but statistically significant association between these two variables. The sign of the coefficient is positive, this means that as the values of the elastic modulus increase, those of the flexural

As mentioned above, the current techniques for processing base denture produce porosities, which allow bacterial colonization [30, 31]. One way to approach this issue is the covering up of the PMMA surface. Since the introduction of nanoparticle-based antimicrobial agents, these have generated really huge interest. Diverse mechanisms for explaining the activity of antimicrobial agents have been discussed, such as the release of ions from the nanoparticle surface, the internalization through cell wall, the production of reactive oxygen species [57], and the destruction of cell wall by the nanometric pillars on the surfaces, among others [58]. For instance, the wing surfaces of insects such as dragonflies and cicadas exhibit a texture that is formed by nanopillars, which are very effective against certain type of pathogenic microorganisms [58, 59]. The possibility of developing surfaces that have

The characteristics of the surfaces of certain objects make them excellent places for proliferation of pathogenic microorganisms and thereby prevent the bacterial adhesion. The main characteristics of polymer surfaces related to microbial adhesion are chemical composition

Different surface modifications have been suggested to reduce the adhesion of pathogenic microorganisms. At present, one of the most effective methods is the surface modification with metallic antibacterial agents such as silver, copper, and zinc oxide at nanometric scale [62, 63]. It has been demonstrated that the oxidized state on surfaces (through electrochemical anodization) shows a significant decrease of some bacterial strains present in the oral cavity

Besides, polymeric glycol-based coatings have been proposed in order to immobilize the molecules on the surface of the substrate. Thus, this prevents bacterial adhesion. The modification of the surface topography generates an unfavorable surface chemistry for the adhesion of certain

strength also increase.

52 Acrylic Polymers in Healthcare

**4.1. Antimicrobial activity**

and topography [61].

**4. Biological properties of PMMA**

antibacterial effects quickly became the subject of study [60].

and bacteria involved in the process of biofilm formation [64].

microorganisms and therefore the colonization of surfaces [65, 66].

These results show that PMMA with antimicrobial agent are potentially useful for their application in dentures for the future. Not many studies have been carried out and there is still scope for further study in this area.

In addition to the antibacterial effect, PMMA-metal oxide nanoparticles have been synthesized with the purpose of improving PMMA's flexural strength as well [63]. With this aim, several works have been carried out by incorporating TiO2 nanoparticles and assessing the dependence of the flexural strength on the TiO<sup>2</sup> nanoparticle concentration. It was observed that by increasing the concentration of nanoparticles, the flexural strength of PMMA value increases. In some cases, better flexural strength value was found in comparison with PMMA alone [72, 73]. Studies on the improvement of tensile strength concluded that increasing the TiO2 nanoparticle concentration provided better tensile strength up to some concentration and then the strength decreases [74]. Recently, Totu *et al*. developed a PMMA-TiO2 material with improved antibacterial activity, for manufacturing 3D-printed dental prosthesis [75].

Other metal oxide nanoparticles that have also been used for their integration to PMMA are the iron dioxide nanoparticles [25]. These nanoparticles improved the antimicrobial and mechanical properties of the acrylic resins. Nanopigmented particles incorporated into PMMA also have been shown to be non-cytotoxic (against fibroblast *in vitro*) and to exhibit good physical

**Figure 4.** Surface roughness of (a) PMMA uncoated and (b) coated with silver nanoparticles by spin coating.

and mechanical properties as well [24]. In both the cases, specimens exhibited good mechanical and physical properties and were not non-cytotoxic showing similar appearance to commercial acrylic resins.
