IMPROVING POLYMETHYL METHACRYLATE RESIN USING A NOVEL NANO-CERAMIC COATING
Su Huang,1 Arghya Bishal,2 Randold Binns,3 Ghaith Dawish,3 Vanlentim Barao,4 Cortino Sukotjo,3
Christine D. Wu,5 Christos G. Takoudis,1,2 Bin Yang,3 Stephen Campbell3
1 Department of Chemical Engineering, College of Engineering, UIC, Chicago, IL, USA
2 Department of BioEngineering, College of Engineering UIC, Chicago, IL, USA
3 Department of Restorative Dentistry, College of Dentistry, UIC, Chicago, IL, USA
4 University of Campinas (UNICAMP) - Piracicaba Dental School, Brazil
5 Department of Pediatric Dentistry, College of Dentistry, UIC, Chicago, IL, USA
Polymethyl-Methacrylate (PMMA) resin has been extensively used as a denture base material due to its aesthetics, processability, and reparability. However, it exhibits poor wear resistance, high water absorption and promotes biofilm adhesion. TiO2 films (30 nm thickness) were successfully deposited by ALD method on PMMA. After coating, the surface wettability decreases to less than 5°, leading to a super-hydrophilic surface. Anti-bacterial results showed that TiO2-coated PMMA surfaces reduced the amount of adherent Candida by 63-56%. Brushing test results indicated the coating layer is still stable on the PMMA substrate even after 6000 brushing strokes (equivalent to 5 years) with 50g brushing force. According to XPS results, coated samples still contained the same amount of TiO2 even after aging test. Bending strength between coated and non-coated samples showed no statistically significant difference.
PMMA resin deteriorates over time after exposure to water, foods and oral bacteria, resulting in the formation of denture biofilm that has been associated with denture stomatitis, and systemic co-morbidities. Atomic Layer deposition (ALD) is a nano-thin film deposition technique based on the reaction of gas phase reactants. With these sequential, self-limiting surface reactions, ALD can provide precise film thickness control, exceptional smooth and conformal deposition surfaces, as well as tunable and accurate film composition. In this study, TiO2 film was deposited on PMMA surface by ALD technique to add functionality to acrylic resin surface. The coating may increase surface wear resistance, surface hydrophilicity, self-cleaning ability and extend the longevity of acrylic resin. In addition, it may help to reduce diffusion of pathogens into the acrylic resin, adherence of biofilm and colonization of microorganisms on denture surfaces. These functionalities provide the potential for daily disinfection of acrylic prostheses thereby reducing the impact of oral pathogenic organisms on oral and systemic health.
PMMA samples (Lucitone 199, Dentsply International, York, PA) were fabricated in a denture flask according to manufacture's recommendation with dimension 2 cm x 2 cm x 1 mm. These substrates were then pre-cleaned with 5% sodium hydroxide (NaOH) solution and de-ionized water in an ultrasonic bath at room temperature. Samples were then dried with N2 gas to remove the excess water. Titanium dioxide films were grown from tetrakis(dimethylamido)titanium (TDMAT) and ozone by the ALD method at low temperatures between 60 and 65℃. Argon was used as the purging gas in between successive precursor pulses. One deposition cycle consisted of a 0.5 s TDMAT pulse, 10 s purge, 1 s ozone pulse and 10 s purge (Fig. 1). Three hundred deposition cycles lead to a thickness of 30 nm TiO2 film.
A sessile drop method was used to perform water contact angle measurements in order to monitor the surface wettability. The anti-bacterial property of ceramic-PMMA material was evaluated using plate-counting.
An Instron Model 1125 test frame with MTS ReNew© Upgrade Package and a crosshead speed of 1mm/s was used to determined the bending strength of coated and non-coated PMMA.
Oral-B Sensi-Soft Manual Toothbrushes were used at an applied load per brush head of 50 g. The specimens were subjected to 3000 strokes with linear brushing movements followed by 5 month water storage and another 3000 strokes of brushing cycles.
For the aging test, coated samples were treated with Polident solution and/or sonication for four hours to reveal the physical and chemical effect on the nano-thin TiO2 films.
WCA measurements were conducted using a Rame’-Hart NRL CA Goniometer (M#100-0, S#2067). A micro-syringe (Hamilton, 802RN) was used to place a 5 μL DI water droplet onto PMMA and TiO2-PMMA surfaces. The results showed that before deposition, the water contact angle (WCA) was 67°(Fig.2a); however, after coating with TiO2 nano-thin film, the WCA value of surface changed to less than 5°(Fig.2b), leading to a super-hydrophilic surface.
Surface roughness (Ra) of the sample surface was tested with an Optical Profilometer (Bruker-Nano Contour GT-K). The surface of PMMA samples were polished with silicon carbide grinding paper with grit P800 to P5000 before deposition. According to the literature, the standard surface roughness of PMMA denture base material should be no more than 0.2 µm. In our study, it was observed that before and after deposition, the Ra value of samples surfaces were around 0.1 µm, which meet the standards requirement. However, after the brushing test for PMMA samples, the surface roughness increased dramatically to 3 μm, while samples coated with TiO2 were still able to meet the standard value (Fig. 3).
The flexural strength of PMMA substrate is 139 ±11MPa (Mean±SD), which is consistent with the literature values. The flexural strength of TiO2-PMMA is 130 ±37MPa, which proves that the nano-thin layer coating will not change the mechanical strength of PMMA.
The effect of surface modification on attachment of C. albicans was examined with the adhesion test after 6 hours, and biofilm formation after 12 hours with a prolonged incubation at 37°C. The number of eluted C. albicans was then determined by colony forming units (CFU). It was found that TiO2-coated PMMA surfaces reduced the amount of adherent Candida by 63% (Fig. 4a). In addition, the reduction of biofilm formation is 56% after the deposition of TiO2 nano-thin fillm (Fig. 4b).
As shown in Fig. 5, TiO2-coated PMMA were divided by 4 groups: (1)TiO2-PMMA, (2)TiO2-PMMA soaked in DI water for 1 hour, (3)TiO2-PMMA Soake in Polident solution for 4 hours, (4) TiO2-PMMA sonicated in Polident solution for 1 hour. X-ray photoelectron spectroscopy (XPS) result reveals that even after hours of physical and chemical aging, the ALD coated nano-thin TiO2 film is still adherent to the PMMA.
By depositing TiO2 ceramic film on PMMA surface, many of the drawbacks of PMMA can be eliminated while retaining the mechanical properties of PMMA. In addition, coating provides self-cleaning, non-stick properties and disinfectant capabilities. The increased smoothness and hydro-philic surface provided by the ceramic coating can lead to a significant reduction in microorganism adhesion and diffusion of pathogens into the acrylic resin base. This facilitates the removal of pathogenic factors such as biofilm/plaque from acrylic prostheses, thereby reducing oral pathogenic organisms and their impact on oral and systemic health. The potential impact on the large growing patient population is significant.
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This study is supported by the ACPEF 2014 GSK Innovator Award; Wach fund 2015, College of Dentistry, University of Illinois at Chicago; and the NSF CBET 1067424 and EEC 1062943. The authors thank Dr. W Li for his assistance in the microbiology assays.