Titanium dioxide nanoparticles (TiO2NP) have gained interest in the dental field because of their multiple uses in addition to their antimicrobial effect. One of the applications in dentistry involves the incorporation into poly methyl methacrylate (PMMA) resin. However, there is a lack of evidence on their effects on the behavior of the resulting nanocomposite. Therefore, the present review aims to screen literatures for data related to PMMA/TiO2 nanocomposite to figure out the properties of TiO2 nanoparticles, methods of addition, interaction with PMMA resin matrix, and finally the addition effects on the properties of introduced nanocomposite and evidence on its clinical performance. Regardless of the latest research progress of PMMA/TiO2 nanocomposite, the questionable properties of final nanocomposite and the lack of long-term clinical evidence addressing their performance restrict their wide clinical use. A conclusive connection between nanoparticle size or addition method and nanocomposite properties could not be established.
The numerous advantages of poly methyl methacrylate (PMMA) make it the most dominant polymer used as denture base material. The ease of processing, low cost, light weight, stability in the oral cavity, and aesthetic properties are of these advantages [
The science and applications of nanotechnology are constantly evolving as we witness new products being introduced into the market. This comes with great responsibility to insure the safety, efficiency, and applicability of such new technologies. Although nanomaterials generally offer superior properties, their mechanical properties fall short in comparison to pure materials. Lately, worldwide research showed several advancements in the nanocomposite field after the extensive research on mechanical and physical properties of these nanocomposites [
Up to November of 2018, database search on Google scholar, PubMed, and Scopus was conducted using the key words “Denture base; Polymethylmethacrylate (PMMA); TiO2 nanoparticles; Physical properties”. Forty-eight articles were found (Figure
Study design.
TiO2NP has proved to have antimicrobial properties. Moreover, it is a cheap biocompatible material, chemically stable, free of toxicity, resistant to corrosion with high strength, and high refractive index [
Its photocatalytic ability promoted it to be known as an antimicrobial agent encouraging its addition to biomaterials [
Resulting properties of the hybrid material (PMMA/TiO2NP) will depend on the dispersion of the nanoparticles within the matrix, which is directly related to the added amount [
Addition of TiO2NP to liquid monomer is another method of filler incorporation within acrylic resin [
An additional method called twin-screw extraction process was developed to disperse the particles into the PMMA [
Hence, the properties of nanocomposites depend on the interactions between the polymer matrix and the filler, suggesting the importance of functionalized TiO2NP [
In addition to NP size and shape, its interaction with PMMA matrix is considered a main factor of reinforcement effectiveness, which depends on the surface characterization of the NP. Chatterjee (2010) studied the interaction between PMMA and TiO2NP and found that they interact chemically and physically [
The dispersion of the TiO2NP within the matrix hinders polymer chain movements due to the strong adhesion between the TiO2NP and PMMA. As a result, better modulus is seen with TiO2NP-PMMA composite materials [
As mentioned earlier, PMMA is the most predominant polymer-based material for dentures. However, its mechanical and surface properties are low [
TiO2NP were found to have an intrinsic antimicrobial property due the production of cytotoxic oxygen radicles [
Anehosur et al. (2012) found that the addition of 3 wt% TiO2NP to PMMA produced a positive antimicrobial effect. It had the ability to reduce microbial number, which prevents quorum sensing thereby halts plaque formation on PMMA/TiO2 nanocomposite surface [
Few studies reported that the addition of as much as 5wt% TiO2NP to the PMMA is needed to achieve the antimicrobial effect [
Surface microtopography of the denture is a very important feature in microbial adhesion and plaque formation and subsequently denture stomatitis.
To establish a relation between contact angle (wetting) and the amount of TiO2NP filler in the PMMA resin, Hashem et al. (2017) found that it is dependent on filler amount. The addition of 1wt% TiO2NP led to a reduction in surface wetting while the addition of more filler improved wetting. This finding confirms the effect of fillers on the surface of composite material compared to the pure counterpart [
While few investigations were made on surface roughness of PMMA/TiO2 nanocomposite, a significant increase was reported compared to pure PMMA [
Alwan and Alameer (2015) concluded that 3% addition of TiO2 caused a significant increase in surface hardness compared to pure PMMA [
In contrast to previous studies, superior flexural properties were reported with different concentrations of TiO2NP added to PMMA than those of normal PMMA [
In a study by Mosalman et al. (2017), various percentages of TiO2NP (0.5, 1, and 2wt %) were added to pure PMMA and found that the flexural strength of all groups stayed unchanged. Samples with 0.5wt% TiO2NP showed only 3.75% improvement in flexural strength compared to pure PMMA. Young’s modulus for all groups was improved with the highest value seen in samples containing 2wt% TiO2NP [
Contrary to previous studies, Chatterjee in 2010 found that nanocomposite with filler content as high as 5wt% and 15wt% had a 59% and 95% increase in tensile modulus, respectively, compared to that of pure PMMA [
It is noteworthy that the reinforcing filler material should ideally improve the mechanical properties without causing an adverse reaction to the aesthetics [
Conventional acrylic resins have voids and porosities that allow water molecules exchange which could be the leading cause for water sorption and solubility [
Therefore, the addition of TiO2NP fills the microvoids and polymer interstitial spaces decreasing the ability of composite material to absorb water. In addition to that TiO2NP are insoluble in water and these particles partly replace the hydrophilic matrix, which decreases water uptake. The use of silane-coupling agent in silanization process of TiO2NP could lead to a reduction in the amount of water that reaches the inner layers of polymer matrix [
The addition of 3% TiO2NP to PMMA had no effect on thermal conductivity [
Chatterjee (2010) in another study reported that the addition of 5wt% TiO2NP caused a 23% increase in
The thermal stability of TiO2NP added to PMMA inhibits the degradation of the resin and improves the thermal stability of the composite material [
It is knows that PMMA shrinks upon polymerization causing dimensional changes in the final product. Dimensional stability is also affected by coefficient of thermal expansion. The result of a study by Hashem et al. (2017)[
Little work has been done on the effects of TiO2NP on the viscoelastic behavior (creep-recovery and relaxation) behavior of PMMA matrix. Recently, in a study by Alrahlah done to investigate viscoelastic properties of TiO2NP-modified PMMA denture base composite, it was found that the creep-recovery and relaxation behaviors of PMMA were significantly improved due to the addition of TiO2NP. Also, the improvement further increased as the concentration of the nanofiller changed from 1% to 3%. This improvement in the behavior indicates the role of the nanoparticles in increasing the stiffness of the PMMA matrix owing to the reduction in its molecular mobility and free volume [
It is imperative to avoid prolonged contact between oral mucosa and materials with high electrical conductivity. Metallic particles present in restorative materials my produce a galvanic effect in the highly conductive oral environment causing oral discomfort, changes in cell proliferation, and immune markers [
It is well understood that advancements in biomaterial science affect the progression of technologies in any field including dental prostheses. The introduction of nanomaterials had significantly changed the clinical and technological aspects of dentistry. In this paper, the latest research progress on the applications of TiO2NP in prosthodontics was reviewed. It clearly shows varying responses of physical and mechanical properties of the modified materials where a number of properties improved, others deteriorated, and few did not change. Their level of effectiveness as shown in the literature is diverse, being more or less effective than pure materials. Therefore, to attain removable prostheses with improved properties and acceptable clinical performance, the material of manufacture (acrylic resin) can be enhanced by adding proper percentage of nanofiller, initial surface treatment of the nanoparticles, and appropriate selection of addition method. Authors hope that this review article would provide some valuable elicitation for future scientific and technological innovations in the related field.
Based on this review, TiO2NP were found to be enhancers in some aspects, modifiers in some, and insignificant in others. The effect depends mainly on NP size, addition method, surface treatment, and loading percentage. Although the size of NP ranged between 5 nm and 350 nm, the results of the studies were not justified based on the nanofiller size, and a clear link between size and effect was not established. For that, further investigations to relate the resulting properties to nanoparticle size are required.
Variations in the results were mainly related to filler mode of addition. The addition of nanoparticles to acrylic monomer was considered more effective owing to better dispersion of NP within the monomer. In this method, the dominant improvement was noticed in mechanical properties while physical properties were slightly affected. It is worth noting that with this technique the polymer: monomer ratio may be affected. Therefore, mixing the nanofiller with acrylic powder has been suggested and studied. Till now, no study can be found that compares between the effects of different modes of addition (nanofillers addition to powder or monomer). Based on this review, further investigations of the above-mentioned point are necessary as well as the proper way to establish the proportion of polymer/monomer ratio for each method; hence nanofiller addition interferes with the manufacture’s recommended ratios.
The percentage of addition also plays a role in resulting properties. While the range of addition was very broad (0.5–30wt%), low percentages resulted in improved properties compared to higher percentages. Simple addition of 1–2wt% ratios exhibited improved properties, while increasing the filler content more than 5wt% significantly weakened the final nanocomposites. In fact, the bonding between TiO2NP and resin matrix is a critical factor to achieve the desired properties of nanocomposite. As showed in Table
TiO2NP applications in denture base and its effect on the tested properties.
Authors /year | Particle size | Addition percentage | Type of Acrylic | Nanocomposite preparation | Properties tested | Specimen size | Effects (Increase/Decrease/Unchanged) |
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Chatterjee, 2010 [ | 5 nm | 0%-15wt% | PMMA from Scientific Polymer Products (Ontario, NY) | (i) Measured TiO2NP mixed with PMMA for 5-10 min | (i) Tensile modulus | 10 x 6 x 0.3 mm | (i) Improvement in tensile modulus. |
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Chatterjee, 2010 [ | 5 nm | 0% | PMMA from Scientific Polymer Products (Ontario, NY) | (i) Measured TiO2NP mixed with PMMA for 5-10 min | (i) Glass transition temperature ( | 10 x 6 x 0.3 mm | (i) |
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Anehosur et al., 2012 [ | 31 nm | 3.0% | DPI heat cure acrylic resin, (India) | (i) Visible light activated TiO2NP were mixed with methyl methacrylate monomer. | (i) Microbial inhibitory effect against S. Aureus | 5 x 5 x 2 mm | (i) 3w% of TiO2 shows antimicrobial activity against S. Aureus. |
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Sodagar et al., 2013 [ | 21 nm | 0% | Selecta Plus (self-cure acrylic resin) | (i) TiO2NP were added to acrylic monomer. | (i) Flexural strength | 50 x 10 x 3.3 mm | (i) Flexural strength decreased as the filler content increased. |
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| (i) Flexural modulus | (i) No change in flexural modulus. | |||||
Hamouda and Beyari, 2014 [ | 21 nm | 5.0% | Conventional heat cure acrylic resin (Acroston, WHN, England) and high impact (Metrocryl Hi, Metrodent, LTD, England) | (i) TiO2NP were mixed thoroughly with acrylic powder by hand. | (ii) Flexural strength | 65 x 10 x 2.5 mm | (ii) Flexural strength and toughness decreased. |
(iv) Monomer release | (iii) No difference between control and TiO2 reinforced regarding monomer release. | ||||||
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Nazirkar et al., 2014 [ | 7 nm | 0% | DPI heat cure acrylic resin | (i) TiO2NP added to acrylic monomer. | (i) Flexural strength | 65 x 10 x 3.3 mm | (i) Flexural strength decreased as the TiO2 amount increased. |
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Shirkavand and Moslehifard, 2014 [ | <25 nm | 0% | Heat cure acrylic resin from Ivoclar Vivadent | (i) TiO2NP were mixed with the acrylic resin polymer in an amalgamator for 20 min. | (i) Tensile strength | 60 x 12 x 4 mm | (i) Tensile strength and elastic modulus improved with 1% TiO2NP. |
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Harini et al., 2014 [ | | 0% | Clear heat cure acrylic resin | (i) Nanoparticles were incorporated into monomer by ultrasonic dispersion. | (i) Flexural strength | 65 x 10 x 3 mm | (i) Flexural strength improved with TiO2 addition, significant difference noticed with 5%. |
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Safi, 2014 [ | 5.0% | Heat cure denture base acrylic (Superacryl plus, Czechoslovakia) | (i) Nanoparticles added to monomer and sonically dispersed. | (i) Coefficient of thermal expansion | 15 x 6 mm | (i) Decrease in coefficient of thermal expansion. | |
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Alwan, and Alameer, 2015 [ | <50 nm size | 0% | Heat cure acrylic resin | (i) Silanized TiO2NP were added to monomer and sonicated. | (i) Impact strength | 80 x 10 x 4 mm | (i) Increased |
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Ahmed et al., 2016 [ | 46 nm | 0% | Conventional heat cure acrylic resin (Implacryl, Vertex) and high impact heat cure acrylic resin (Vertex-Dental, Netherlands) | (i) TiO2NP were added into acrylic resin. | (i) Flexural strength | 50 x 10 x 3 mm | (i) Decreased with TiO2 addition. |
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Sodagar et al., 2016 [ | 21 nm | 0.5% | Selecta Plus (self-cure acrylic resin) | (i) Nanoparticles were added to acrylic monomer and stirred | (i) Antimicrobial properties | 20 x 20 x 1 mm | (i) TiO2 reduced microbial growth at both concentrations at 90 min under UVA exposure |
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Ahmed et al., 2017 [ | <25 nm | 0% | Heat cure acrylic resin from Dentsply International Inc., (Chicago, IL, USA) | (i) TiO2NP were added to acrylic polymer and mixed using amalgam capsule. | (i) Flexural strength | 65 x 10 x 2.5 mm | (i) Increased with both filler percentages. |
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Hashem et al., 2017 [ | 90 nm | 0% | Self-cure acrylic resin from Eco-crylcold, | (i) TiO2NP were mixed with the monomer. | (i) Flexural modulus and flexural strength | 30 x 8 x 1 mm | (i) Increased linearly |
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Ghahremani et al., 2017 [ | 20-30 nm | 0% | SR Triplex Hot, heat cure acrylic resin (Ivoclar Vivadent Inc. Schaan, Liechtenstein) | (i) TiO2NP were mixed with acrylic resin powder in an ultrasonic mixer. | (i) Tensile strength | 60 x 12 x 3.9 mm | (i) Increased |
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Totu et al., 2017 [ | 65-170 nm | 0% | PMMA+PEMA for 3D printing (eDent 100, EnvisionTec GmbH Gladbeck, Germany) | (i) Nanoparticles were added into PMMA solution with continuous stirring and ultrasonic mixing for 1 hour. | (i) Antimicrobial effect ( | (i) 0.4, 1% and 2.5% inhibited candida growth | |
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Aziz, 2018 [ | 30 nm | 0% | High impact heat cure acrylic resin (Vertex-Dental, Netherlands)- | (i) TiO2NP were dispersed in monomer and sonicated at 120W and 60 KHz for 3 minutes. | (i) Impact strength | 80 x 10 x 4 mm | (i) Increased |
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Alrahlah et al., 2018 [ | 80-100 nm | 0% | Heat cure acrylic resin (Lucitone 550, Dentsply Int. Inc. Pa, USA) | | 50 x 10 mm discs cut in different sizes for different tests | | |
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Karci et al., 2018 [ | 13 nm | 0% | (i) Auto-polyerized (Heraeus Kulzer, Newbury | (i) TiO2NP were mixed with acrylic resin powder using ball milling at 400 rpm for 2 hours | (i) Flexural strength | 65 x 10 x 3 mm | (i) Increased for heat- and auto-polymerized acrylic at 1% |
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Totu et al., Totu et al., 2017 [ | “Anatase phase” | 0% | (i) PMMA-MA | (i) TiO2 modified by methacrylic acid then manually mixed with PMMA mixture | (i) Thermal stability | Stereolithographic dentures | (i) Increased (improved) |
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Totu et al., 2018 [ | “Anatase phase” | 0% | PMMA for 3D printing (eDent 100, EnvisionTec GmbH Gladbeck, Germany) | (i) Resistance | (i) Decreased with 1.0%, 2.0%, 2.5% and 5% |
According to Table
Titanium nanoparticles remain under focus because of the antimicrobial efficacy against
Even with this number of studies on recently introduced TiO2NP into PMMA, no clear evidence on the clinical applicability of this nanocomposite has been demonstrated. Further investigations are required to interpret and confirm the chemical structure changes in PMMA/TiO2 nanocomposite and also to determine the need from otherwise for surface treatment as well as the proper percentage of addition that will not affect the final product adversely.
Research to improve upon existing nanomaterials is still ongoing with emphasis on efficiency. Although the science behind nanotechnology is intriguing, the lack of long-term evidence addressing their clinical performance restricts their wide clinical use. Overall, there is an essential requirement to investigate the durability of these PMMA/TiO2 composites in different environmental conditions to extend the applicability of these hybrid materials.
Based on the current review, we could conclude that the addition of TiO2NP to PMMA denture bases is still questionable for working as a reinforcing material and requires further investigations following the ADA specifications. These investigations must explore the chemical and structural changes happening in the nanocomposite after TiO2 addition. The improvements in properties mentioned above were dominantly seen with lower concentrations of TiO2 and the increase in the amount of added nanoparticles caused adverse effects on resulting PMMA/TiO2 composite.
In conclusion, there is an essential need to investigate the clinical performance and durability of these nanocomposites in different conditions simulating the oral environment to verify their applicability and provide an insight of possible future researches in this field.
The authors declare that they have no conflicts of interest.