**Strategies to Reduce Biofilm Formation in PEEK Materials Applied to Implant Dentistry—A Comprehensive Review**


Received: 1 July 2020; Accepted: 15 September 2020; Published: 16 September 2020

**Abstract:** Polyether-ether-ketone (PEEK) has emerged in Implant Dentistry with a series of short-time applications and as a promising material to substitute definitive dental implants. Several strategies have been investigated to diminish biofilm formation on the PEEK surface aiming to decrease the possibility of related infections. Therefore, a comprehensive review was carried out in order to compare PEEK with materials widely used nowadays in Implant Dentistry, such as titanium and zirconia, placing emphasis on studies investigating its ability to grant or prevent biofilm formation. Most studies failed to reveal significant antimicrobial activity in pure PEEK, while several studies described new strategies to reduce biofilm formation and bacterial colonization on this material. Those include the PEEK sulfonation process, incorporation of therapeutic and bioactive agents in PEEK matrix or on PEEK surface, PEEK coatings and incorporation of reinforcement agents, in order to produce nanocomposites or blends. The two most analyzed surface properties were contact angle and roughness, while the most studied bacteria were *Escherichia coli* and *Staphylococcus aureus*. Despite PEEK's susceptibility to biofilm formation, a great number of strategies discussed in this study were able to improve its antibiofilm and antimicrobial properties.

**Keywords:** biofilms; biofilm inhibition; dental implants; bacteria; peri-implantitis; polyether-ether-ketone

### **1. Introduction**

The diverse microbiome that harbors in the oral cavity plays an important role in health maintenance through the development of the immune response and inhibition of the pathogen colonization [1]. However, under certain circumstances, normal microbiota may be responsible for many oral diseases [2,3]. Oral dysbiosis triggers important changes, reducing the number of beneficial bacteria and favoring the growth of potential pathogens [4]. This is particularly worrying in susceptible individuals affected by periodontitis, a biofilm related disease characterized by alveolar bone resorption, which may lead to tooth mobility and tooth loss [5,6]. In fact, periodontal patients who were rehabilitated with dental implants are more predisposed to develop peri-implant diseases, for which poor plaque control also acts as a primary etiologic factor [7].

In a systematic review carried out in 2017, [8] patient-level data and implant-level data indicated that peri-implantitis was present in 9.25% and 19.83% of analyzed cases, respectively, while mucositis affected 29.48% of patients and 46.83% of implants analyzed [8]. Since there was no consensus on the best treatment protocol [9], biofilm prevention becomes not only desirable but necessary [10]. This can be achieved at the clinical level through favorable implant position and adequate prosthetic design, accompanied by oral hygiene education and regular appointments [10]. Still, at the research level, there is an incessant demand for investigations to develop materials with either antibiofilm or antimicrobial surfaces, or both, through manipulation of surface topographical properties (i.e., contact angle and roughness), or by the incorporation of antibiofilm agents, which can be evaluated through physicochemical analysis [11–14].

Since the demonstration of titanium osseointegration by Branemark et al. (1981) [15], this material has been widely used in Implant Dentistry, revolutionizing oral rehabilitation modalities [16]. However, under certain circumstances, such as therapeutic treatment of peri-implantitis [17] or wear-corrosion, metallic debris is released resulting in prejudicial effects to peri-implant tissues. It had been proved that those metal particles stimulate molecular mechanisms such as enhancement of proinflammatory cytokines and osteoclasts activity, as well as infiltration of inflammatory cells with cytotoxic and genotoxic effects [18]. Hence, there is a growing interest in the development of an alternative material that can be used in dental implants and as implant abutments [19–21].

Within this context, the thermoplastic biocompatible polymer polyether-ether-ketone (PEEK) stands out, with several desired properties to Implant Dentistry improvements, such as mechanical and chemical resistance, stability at high temperatures (enabling sterilization) and natural white pigmentation (favorable for esthetics) [22,23]. Several methods have been studied to establish an effective adhesion of PEEK to resin-matrix composite in restorative dentistry, which is useful to the esthetic of provisional restorations [24]. Moreover, its production process is very versatile, as PEEK is compatible with many reinforcement agents and surface coatings, which can be used to improve its mechanical and biological properties [25–27]. Currently, PEEK is safely used in Implant Dentistry as provisional abutments, healing screws, prosthetic transfers and frameworks [20,23,28]. Nevertheless, as reported by Khonsari et al. [29], there are cases in which PEEK dental implants had been employed in patients and poor osseointegration led to severe infectious complications and subsequent implant loss.

Figure 1 ilustrates the propositions exposed above. In order to develop a PEEK-based dental implant or even to convert the available applications from provisional to definitive (i.e., PEEK-based prosthetic components), additional research is necessary. Therefore, a comprehensive literature review was carried out aiming to investigate available strategies to reduce biofilm formation on PEEK materials for Implant Dentistry applications.

**Figure 1.** (**A**) Biofilm formation on titanium implants, underneath an implant-supported total prosthesis; (**B**) bone defects around dental implants at posterior lower jaw, a sequel of peri-implantitis; (**C**) metallic debris being released to peri-implant tissues during peri-implantitis treatment (implantoplasty); (**D**) PEEK healing screw (FGM, Brazil); (**E**) PEEK temporary abutments (Straumann, Switzerland) that support esthetic restorations; (**F**) PEEK prosthetic transfers (FGM, Brazil); (**G**) PEEK abutment cap (Straumann, Switzerland).

### **2. Strategies to Reduce Biofilm Formation in PEEK Materials Applied to Implant Dentistry**

A full strategy with inclusion and exclusion criteria, as well as the flow chart of selected studies, are available as Supplementary Data. From a total of 376 studies initially found during the literature search, 33 were chosen for full text reading based on titles. Thereafter, 31 studies fulfilled the inclusion criteria of this review. Tables 1 and 2 reveal comprehensive information on pure and modified PEEK, respectively.

### *2.1. Study Characteristics*

Amongst the included studies, 5 involved in vitro associated to in vivo (animal) investigations, while 26 were restricted to in vitro studies. In vivo (human) studies did not fulfill inclusion criteria of this review. Regarding PEEK modification strategies, 6 studies analyzed pure PEEK compared to other materials [30–35] (e.g., titanium, silicon, gold, silver, zinc oxide, zirconia, silicon nitride) and none of them revealed special antibiofilm or antimicrobial properties of PEEK material. A total of 25 studies used strategies to reduce biofilm and bacterial colonization on PEEK, which were able to successfully confer either antibiofilm or antimicrobial properties, or both, to the material. Regarding applications aimed at the investigated materials, orthopedic, dental and the treatment of bone defects were the most commonly mentioned, followed by the development of biomaterials in general.



**1.**ofunmodifiedPEEK




a A decrease in free energy favors stability. b Contact angle≥90◦ means that the material is hydrophobic, and<90◦means that it is hydrophilic.


**Table 2.**Descriptive analysis of modified PEEK materials.



**Table 2.** *Cont.*





**Table 2.***Cont.*


**Table 2.** *Cont.*



**Table2.***Cont.*



**Table 2.** *Cont.*



**Table 2.** *Cont.*

### *2.2. Available Strategies to Reduce Biofilm Formation on PEEK Materials*

Strategies are summarized and illustrated at Figure 2 and are listed as follows:


### *2.3. Microbiological Analysis*

The most commonly investigated bacteria were *Escherichia coli* and *Staphylococcus aureus*, but other microorganisms such as *Streptococcus sanguinis*, *Streptococcus oralis*, *Streptococcus faecalis*, *Streptococcus gordonni*, *Streptococcus epidermidis*, *Pseudomonas aeruginosa*, *Aggregatibacter actinomycetemcomitans*, *Porphyromonas gingivalis*, *Fusobacterium nucleatum*, *Enterococcus faecalis*, *Candida albicans*, *Actinomyces naeslundii*, *Streptococcus mutans* and *Staphylococcus epidermidis* were also studied. Microbiological analysis was very heterogenic, and several methods were used, which are summarized in Tables 1 and 2. Among the included methods, it should be highlighted that the most recurrent ones were plate-counting, for the determination of average colony forming units (CFU/mm2); Real-Time Polymerase Chain Reaction (RT-PCR) and Live/Dead cells analysis, followed by FE-SEM and confocal laser scanning microscopy; bacterial growth inhibition zone tests; crystal violet assays; longevity and stability of antibacterial activity and agar diffusion assay.

### *2.4. Physicochemical and Topographical Characterization*

With the exception of 7 papers [32,37,40,41,45,56,60], all the other studies analyzed surface topographical aspects, such as either or both contact angle and surface roughness. The physicochemical and additional characterization of included papers was achieved by energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), porosity evaluation through drainage method, dynamic differential scanning calorimetry (DSC), X-ray diffractograms (XRD), Hydrogen nuclear magnetic resonance (1H-NMR), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR) and UV spectrophotometer.

**Figure 2.** Summary of some available strategies to improve PEEK biological properties. (**A**) SEM image of a sulfonated PEEK membrane; (**B**) SEM image of bioglass particles; (**C**) photography of PEEK coated with adhesive film; (**D**) SEM image of natural amorphous silica fibers; (**E**) photography of PEEK powder and PEEK cylinders manufactured through compression molding; (**F**,**G**) SEM images of MC3T3 osteoblasts on zirconia surface; L929 fibroblasts on PEEK surface; (**H**) undesired biofilm formation on material surface; (**I**) PEEK provisional abutment (Straumann, Switzerland).

### **3. Discussion**

Investigations have demonstrated that peri-implantitis is a heterogeneous infection, in which periodontopathogens and opportunistic microorganisms act simultaneously [62–64]. Moreover, the disease has been associated to specific immunological alterations on peri-implant crevicular fluid levels of proinflammatory, anti-inflammatory and osteoclastogenesis-related chemokines [65]. Several studies analyzed in this review [30–35] investigated biofilm and antimicrobial properties of pure PEEK, demonstrating that the polymer is susceptible to biofilm colonization. Within a context in which PEEK clinical applications in Implant Dentistry are increasing [28], strategies to modify its surface to enhance its antimicrobial/antibiofilm properties are crucial.

It becomes even more important to improve PEEK materials with the above-mentioned properties when considering that biofilms are organized polymicrobial communities that offer bacteria protection against environmental factors and antibiotic treatments [66,67]. In vitro analysis of submucosal biofilm samples of 120 peri-implantitis sites revealed that 71.7% exhibited bacterial pathogens resistance to one or more of tested antibiotics (clindamycin, amoxicillin, doxycycline or metronidazole) [68]. Therefore, the identification of compounds capable of inhibiting biofilm formation or disrupt biofilm organization emerges as an attractive alternative to avoid peri-implant related infections [69,70]. It is important to notice that this approach is not expected to completely eliminate biofilm formation, but it is a very effective way of modifying oral ecology instead, reducing the number of pathogenic bacteria and

favoring the growth of mutualistic species. By doing so, the host organism is provided with just the necessary advantage to defeat the pathogens using its own resources.

Additionally, it is important to analyze PEEK surface properties and its influence on biologic systems. For example, the PEEK hydrophobic surface associated to its bio inertness is a major concern when prospecting for the expansion of its application in Implant Dentistry [28,71], as this type of surface typically reduces cellular adhesion and does not promote osseointegration [22]. Numerous modifications have been proposed to overcome those limitations, such as blending with bioactive particles such as titanium dioxide, hydroxyapatite and fluorapatite [72–74]. Interestingly, the present review exposed that some of those strategies showed the favorable additional effect of reducing biofilm formation [41,44,54]. For example, a very promising candidate to replace metallic implants is carbon fiber reinforced PEEK (CFRPEEK) [75], which has similar elastic modulus to the human cortical bone [22]. One of the studies included in this review [44] proposed a dual zinc and oxygen plasma immersion ion implantation to modify CFRPEEK. Despite the fact that this strategy made the surface far more hydrophobic (contact angle shifted from 66.6◦ to 144.1◦ after surface modification), it also improved both osteogenic and antibacterial activities, as evaluated through MC3T3-E1 and rat bone mesenchymal stem cell development and through *Staphylococcus aureus*, *MRSA* and *Staphylococcus epidermidis* inhibition [44]. Those findings provide positive perspectives of the development of PEEK surfaces enhanced with bioactive and antibiofilm properties, which is favorable for PEEK-based dental implant development.

Bone cell activity on the PEEK surface is very important to achieve proper osseointegration on dental implants, but considering that an imperative application for PEEK in Implant Dentistry is as implant abutments [76], the gingival sealing must be analyzed as well, since it provides protection to implants against infections by potential pathogens [10]. Among the studies included in this review describing strategies for PEEK modification through the incorporation of antibiofilm agents, the embedding of lactams through the PEEK sulfonation process is worth mentioning [45]. Lactams are compounds analogous to furanones, which were initially isolated from the algae *Delisea pulchra*, and had been proved to be effective against *Streptococus mutans* biofilms [77]. An in vitro study [26] demonstrated that PEEK sulfonation positively interferes with the ability of fibroblasts L929 to spread over the surface of the material [26]. This corroborates previous indications that PEEK sulfonation is a suitable process for the development of modified PEEK abutments with embedded antibiofilm compounds.

In addition to the mentioned in vitro studies indicating these strategies as promising approaches to develop clinical materials biofilm resistant, an in vivo (human) investigation also revealed that PEEK healing abutments did not affect important parameters of peri-implant health, such as marginal bone loss and soft tissue recession, during a three-month evaluation period [78]. Therefore, it seems plausible to associate PEEK inherent favorable properties with adequate strategies to maximize its biological properties and consequently achieve even better clinical outcomes in the near future.

### **4. Conclusions**

Within the scope of the present review, it may be concluded that pure PEEK is susceptible to biofilm formation and that several strategies presented here are able to significantly improve its antibiofilm and antimicrobial properties. Those strategies include the PEEK sulfonation process, incorporation of therapeutic and/or bioactive agents in the PEEK matrix or on the PEEK surface, PEEK coatings and incorporation of reinforcement agents to produce nanocomposites and/or blends. Since the use of PEEK in Implant Dentistry is increasing, those modifications are necessary in order to enable patients to benefit from these new materials which present great potential to prevent infections. Therefore, it is expected that further in vivo studies, both in animals and humans, will make available PEEK-based dental implants and improved implant abutments for clinical practice applications.

**Author Contributions:** R.S.B.—Conceptualization, formal analysis, investigation, methodology, writing—original draft preparation; L.G.L.—validation, formal analysis, investigation; C.Â.M.V.—conceptualization, data curation, visualization, supervision; C.A.M.B.—conceptualization, resources, visualization, project administration; A.d.L.P.—conceptualization, writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was supported by a grant from the ITI Foundation, Switzerland.

**Acknowledgments:** Authors express their gratitude to CAPES and FAPEU in Brazil. Additionally, authors are grateful to Felipe Ouriques, Bruna Barbosa Côrrea, Mario Eduardo Escobar-Ramos, Maria Elisa Galarraga-Vinueza, Mariane Beatriz Sordi and Patrícia Rabelo Monich due to their collaboration on images that enabled the creation of the present figures. Authors are also thankful to Camila Rodrigues de Souza due to her efforts in the language review.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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*Review*
