Template assisted surface microstructuring of flowable dental composites and its effect on microbial adhesion properties
Introduction
Since the introduction of resin-based dental materials around the middle of the last century, composite restorations in dentistry became indispensable because of the patients’ esthetic demands and ease of composite processing [1]. Composites are a mixture of organic and/or inorganic fillers surrounded by a monomer matrix which can be set-on-command by photopolymerization e.g. with blue LED lamps [2]. Depending on their filler particles sizes composites can for example be categorized into four different groups: macro-, micro-, hybrid- and nanofiller composites. In addition, they can be categorized according to their rheological properties into flowable and non-flowable composites.
Despite their advantages, such as good esthetic properties, absence of mercury and adhesive bonding to teeth, dental composites still have some drawbacks, such as for example polymerization shrinkage, their tendency to absorb water [3] and the onset of secondary caries caused by microbes on teeth filled with composite materials [4].
A challenge arises through the surface treatment of composites. Inappropriate finishing procedures may result in increased surface roughness [5], [6], [7]. An important property in relation to the structural surfaces roughness of dental composites is the adhesion of oral microbes to the composites. Surface roughness influences bacterial colonization [8], particularly on composite materials [5]. Smooth surfaces are preferred clinically, because of their relatively low bacteria adherence [9]. Carlén et al. reported, however, a polished hybrid composite to accumulate more bacteria than the unpolished one [5].
Some investigations showed that microbes adhere stronger on composites surfaces than to the natural tooth covered by a pellicle [10] or in comparison to other dental materials. A threshold level of composite surface roughness of Ra = 0.2 μm has been discussed, below which no further reduction in microbial accumulation could be expected [11], however, no convincing explanation for this has been given. Although surface roughness seems to be an important factor for microbial accumulation on dental composites, materials properties, such as filler-size [11] shape and content [12], composite surface tension [10], chemical surface composition [13], protein adsorption [5] and others seem to be important factors as well.
It has been predicted that future commercial dental composites will possess antimicrobial properties [14] and the quantity of scientific literature addressing this subject has grown strongly over the last years [15]. Approaches to equip resin based dental materials with antimicrobial properties include silver [16] or zinc oxide nano particles [17], silver-supported antibacterial materials [18], zinc oxide eugenol [19], quaternary ammonium functionalities [20], [21], alkylated ammonium chloride derivatives [22], chlorhexidine diacetate (CHXA) [23], carolacton [24] and others. The addition of antimicrobial agents to composites, however, may lead to reduced mechanical properties of the composites and in many cases the antimicrobial effect of the composites is not sustainable [14].
A new and promising approach to reduce the microbial adhesion to different biomaterials surfaces uses specific micro or nano surface topographies or patterns [25], [26], [27], [28], [29]. The microbial adhesion reduction mechanisms of these materials surfaces are still an enigma, but some authors assume that an unfavorable physical interaction between microbes and the materials surface is responsible for their antimicrobial effect [28], [29].
Reducing microbial adhesion to materials with this approach has a number of advantages since it uses neither antibiotics nor other chemical antimicrobial agents or compounds. Hence these materials cannot lead to antibiotic resistance of microbes or negative side effects of drug release, such as cytotoxicity to body cells.
Based on these findings, the question arises if a surface structuring approach is also feasible for reducing microbial adhesion to dental materials, such as dental composites. Little is known about the interaction of microbes and flowable composites. Due to their rheological properties flowables seem to be, however, the ideal materials for surface patterning.
It was, therefore, the aim of this current study to test the hypotheses that (i) different surface microstructures can be created on composites by a novel straightforward approach potentially suitable for clinical application and (ii) that these surface structures have a statistically significant effect on microbial adhesion properties when compared with flat control samples of the same composite. To the best of our knowledge, neither has been attempted previously and may, if successful, lay the foundations for a new way of functional surface structuring of dental composites.
Section snippets
Dental composites
Six different composites were first tested for their structurability by polydimethylsiloxane (PDMS) templates: the nanohybride composites CLEARFIL MAJESTY Posterior (CMP; Kuraray Europe GmbH, Frankfurt, Germany), Grandio Flow (GF; Voco GmbH, Cuxhaven, Germany), Premise (P; Kerr Corporation, Orange, USA), Tetric EvoFlow (TEF; Ivoclar Vivadent AG, Schaan, Liechtenstein), Venus Diamond Flow (VDF; Heraeus Kulzer GmbH, Hanau, Germany) and the microhybrid composite XFlow (XF; Dentsply International,
Results
The SEM micrographs (Fig. 2a–f) of the initial template assisted surface microstructuring experiment of the composites presented in Table 1 reveal the successful pattern creation for GF (Fig. 2b) with a good surface pattern definition of the cubes. Although a cube surface structure was also partially accomplished with the CMP composite (Fig. 2a), the surface pattern definition is clearly poorer compared to the GF surfaces. In some surface areas of the CMP samples the structures (cubes) are
Discussion
Structure and properties of dental composites surfaces depend on their composition and the finishing procedures (i.e. polishing) applied to the composite, which in turn may affect microbial adhesion properties. In the current study, we developed a novel approach to create different composite surface structures and affected through this the microbial adhesion properties of the composites.
More microbes adhere and accumulate to dental composite than to other restorative dental materials in vitro
Conclusion
We introduced a straightforward and innovative approach to create different microstructures on dental composite surfaces. The surface structured composites did differ in their microbial adhesion properties from flat control surfaces, an important factor in this being the geometry of the patterns. With this we opened a new route of composite surface structuring that may lead to a new range of properties of dental composite surfaces. Factors limiting the surface structurability of dental
Acknowledgements
KDJ gratefully acknowledges the partial financial support of the Deutsche Forschungsgemeinschaft (DFG), grant reference INST 275/241-1 FUGG, and the TMBWK, grant reference 62-4264 925/1/10/1/01. We are grateful to Dr. Markus Beyer and Max Hennig for support in some of the experiments.
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These authors contributed equally.
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Present address: ICFO – The Institute of Photonic Sciences, Mediterranean Technology Park, Av. Carl Friedrich Gauss 3, 08860 Castelldefels (Barcelona), Spain.