1. Introduction

As dental resins enter into the new era of development, the choice of suitable materials becomes more diverse as they have broadened the range of dental products in all areas of dentistry. Prosthetic dentistry has also broadened its range of many promising materials for denture fabrication. Many different types of materials were used for fabrication of denture base. Prior to 1940, vulcanite was the most widely used denture base polymer. This is a highly cross-linked nature rubber, which is difficult to pigment and tends to become unhygienic due to uptake of saliva. Various other materials have been used in denture construction, including cellulose products, phenol-formaldehyde, vinyl resin, polyamine polymers. However, they have suffered from a variety of problems [1, 2].

In the 1930s, Walter Wright and the Vernon brothers at the Rohm and Haas Company in Philadelphia developed poly (methyl methacrylate) (PMMA), a hard plastic. Although many other materials were utilized for dental prosthetics none could come close to that of PMMA, and nowadays more than 90% of dentures are fabricated from this acrylic polymer [3]. The popularity of PMMA is associated with its favorable working characteristics, processing ease, accurate fit, stability in oral environment, superior esthetics, and use with minimum and inexpensive equipment [4].

This chapter presents a comprehensive review of various fibers and fillers utilized in PMMA denture base resin and their influence on the physical and mechanical properties. The review involves relevant data and sources from scientific papers, reviews, and abstracts published in dental literature. The search for published materials was conducted using both general and specialist databases, such as Google Scholar, Research Gate, and PubMed, with the aid of specific keywords including denture base, PMMA, reinforcement, nanoparticles, fibers, and fillers.

2. The criteria for an ideal denture base material

The requirements of a denture base material can be conveniently categorized into physical, chemical, mechanical, biological, and miscellaneous properties.

2.1 Physical properties

Ideal denture base material should be capable of matching the appearance of the natural oral soft tissue. The importance of this requirement varies considerably, depending on whether the base will be visible when the patient opens his mouth. A polymer, which is used to construct a denture base, ought to have a value of glass transition temperature (Tg), which is high enough to prevent softening and distortion during service. Although the normal temperature in the mouth varies from 32 to 37°C, account must be taken of the fact that patients sometimes take hot drinks higher than this temperature, and also clean their dentures in very hot water despite being advised not to do so.

The base is supposed to have good dimensional stability in order for the shapes of the denture not to change over a period of time. In addition to distortion, which may occur due to thermal softening, other mechanisms such as relief of internal stresses, and water absorption may contribute to dimensional instability. The material should ideally have a low value of specific gravity and dentures should be as light as possible.

The denture base should have radiopaque characteristics leading to its event of detection using normal diagnostic radiographic techniques. Early radiological detection of the denture or fragment of denture is immense help in deciding the best course of treatment.

2.2 Mechanical properties

The denture base should be rigid and has high modulus of elasticity. A high value of elastic limit is required to ensure that stresses encountered during bite and mastication do not cause permanent deformation. It ought to have sufficient flexural strength to resist bending and fracture. The base material should have an adequate fatigue life and high impact strength. Denture base materials need to possess sufficient abrasion and indentation resistance to prevent excessive wear of material by abrasive denture cleaners or foodstuffs.

2.3 Chemical properties

A denture base material should be chemically inert. It should be naturally insoluble in oral fluids and should not absorb water or saliva since this may alter the mechanical properties of the material and cause unhygienic denture.

2.4 Biological properties

In the unmixed or uncured states, the denture base material should not be harmful to the technician involved in its handling. Furthermore, it has to be non-toxic and non-irritant to the patient. The base should neither promote nor sustain the growth of bacteria and fungus.

2.5 Miscellaneous properties

An ideal denture base material ought to be relatively inexpensive and has a long shelf life so that the material can be purchased in bulk and stored without deteriorating. The material should be easy to manipulate and fabricate without having to resort to expensive processing equipment. It should be easy to repair on the occasion of fractures.

2.5.1 Radiopacity of denture base material

Many attempts were tried to achieve the radiopacity into PMMA as denture base material. These efforts include the addition of finely divided metal such as powdered dental amalgam or gold powder, simple halogen-containing molecules such as tetrabromoethane or organo metallic such as triphenly bismuth. Additionally, the incorporation of insoluble inorganic heavy metal salts such as BaSO₄, BaF₂, BiCl₃, BiBr3, or finely divided glasses containing barium or bismuth [5].

All these techniques exhibit specific disadvantages, which had precluded their use. Out of all the heavy metal inorganic salts and glasses that have been investigated, only BaSO₄ has achieved conventional exploitation as an x-ray opacifying agent for both denture base and also as the opacifying medium in methacrylate.

As far as denture base is concerned the resin produced is opaque white and even when tinted, the translucent of nature gum tissue cannot be mimicked so it was never poplar [6]. Moreover, the x-ray opacity was poor and not only simply a question of adding more BaSO₄ as not only did the esthetic qualities suffer further but the mechanical properties began to deteriorate as well [5, 6]. There is another radiopacifier described based on iodine-methacrylate, where the contrast agent was introduced via the liquid component and via iodine-containing copolymer [7]. Since the conversion of monomers during curing is incomplete, free monomer will always be present and as a result, there is always a risk for in situ release of iodine-containing methacrylates. Because nothing is known about the toxic effects of such a monomer, this poses an unknown and presumably unacceptable risk for the patient. Therefore, a covalently bound x-ray opacifier would eliminate many of the inherent difficulties that others have found toward the successful development of an x-ray opaque PMMA denture base material.

Lewis et al. [8] studied radiopacifying particle reinforced PMMA. Their approach for enhancing the properties of the polymer and reducing the production of wear particles and debris was directed toward the mechanical reinforcement through the radiopacifier particles. They investigated the improvement of the interface adhesion by establishing covalent chemical bonding between the inorganic fillers (oxide particles) and the PMMA matrix. This was achieved through the preliminary treatment of the fillers surface with a silane bonding agent as 3-(trimethoxysilyl) propyl methacrylate (γ-MPS), capable of later copolymerizing with the (co) monomers.

Abboud et al. [9] studied the mechanical characterization of acrylic resin prepared from γ-MPS treated alumina particles, which could act simultaneously as radiopacifying and reinforcing agents. They found that for some formulations, the compressive strength and modulus reached 150 and 3400 MPa respectively. Those formulations require high concentration of silanated alumina particles (over 35%wt) and such composites are unprocessable due to the lack of liquid monomer for simultaneously wetting the filler surface and dissolving the PMMA beads.

Mohamed [10] studied the ability of HA and alumina in acting as both radiopacifying and reinforcement agents in PMMA denture base material. The effects of the particles microstructures, surface treatment of fillers with γ- MPS were also evaluated. The author concluded that the incorporation of γ- MPS treated ceramic fillers into PMMA matrix in general, has increased the flexural modulus, tensile strength, tensile modulus, and fracture toughness of PMMA denture base material.

2.5.2 Fracture toughness

Fracture toughness is an intrinsic characteristic of a material concerning resistance to crack propagation. It is a measure of the energy required to initiate and propagate a crack in a material, which may lead to catastrophic failure. Fractures are usually classified as brittle fracture and ductile fracture. In brittle fracture, the materials behave elastically up to the point of failure. There is hardly observable deformation of the materials prior or during breakage [11].

The fracture surfaces are relatively smooth and largely perpendicular to the direction of the applied stress. The two surfaces can be fit together quite accurately. Ductile fracture implied that large permanent deformation has occurred before failure, requiring a significant greater amount of energy absorption by the part before failure. Since the result of permanent deformation, the fracture surfaces do not match, and the cross-sectional area at the location of the fracture is reduced from the original value.

The common measurement of fracture toughness is called the critical stress intensity factor (KIC). The KIC is the critical value of material that fracture occurs when an applied stress intensity factor on the material is greater than the critical value. The stress intensity factor K, is a measure of applied stress associated with crack size. When the KIC value is exceeded at the crack tip, the material will then fracture spontaneously leading to complete failure. Below the KIC value the crack can still grow slowly. Because the KIC is measured as stress intensity and not just as a stress, its units are MN/m1.5 [12].

The fracture toughness test is very efficient and other parameters can also be derived from it including modulus of elasticity and the plasticity of the material. The fracture toughness is closely related to fatigue strength [12].

Factors that contribute to stress concentration enable the initiation and propagation of cracks, thereby influencing the rate of failure. As described by Yee [13] the observed energy loss during the impact test depends on four factors; (a) the energy to bend the specimen up to the point of crack initiation, (b) the energy to propagate the crack through the specimen, (c) the kinetic energy of the fractured specimen, and (d) the vibrational or otherwise dissipated energy. As previously reported in the literature, the fracture toughness test appears to be more reliable and advantageous than impact testing when determining the influence of variation in the material composition [14].

Zappini et al. [15] determined the fracture toughness of denture base resins and compared the results with impact strength measurements. Seven heat-polymerized denture base resins were chosen for the study. They concluded that the specimen geometry and testing configuration influenced the impact strength measurements and the fracture toughness method seemed to be more suitable than impact strength measurements to demonstrate the effects of resin modifications. Moreover, the differences between conventional and so-called “high impact” denture base resins were more clearly demonstrated with fracture toughness measurements.

3. Filler reinforced denture base

Reinforcement in particulate denture base may comprise of either flexible particulate rubber, such as in high impact polystyrene (HIPS), or rigid mineral fillers, including hydroxyapatite, alumina calcium carbonate, barium titanate, and others. Numerous factors contribute to the properties of particulate-filled denture base, including the type of filler, type of matrix, filler-matrix interaction, filler volume fraction, filler-filler interaction, voids, and defects. It has been observed by Harper et al. that smaller particle size yields better properties enhancement than larger particle size. Geometrically balanced particles, such as spherical glass beads, produce isotropic composites, while platelet and angular-shaped particles not only introduce anisotropy to the mechanical properties but also increase the viscosity of the melt [16].

The composite function employs a continuous phase matrix that serves the purpose of retaining the particulate filler while also providing it with a protective enclosure against both mechanical and chemical damage. Additionally, the matrix acts as a medium for overall stress distribution, allowing for applied loads to be passed on to the filler. It is crucial that the matrix possess the ability to distribute the particulate filler evenly in order to prevent agglomeration.

The mechanical and chemical properties of the composites, as well as their suitability for high temperature environments, are largely determined by the type of matrix utilized. The interaction between the filler and matrix occurs at the interface, which is a small region located between the contact surfaces of the two materials. This interaction plays a critical role in determining the extent to which the load is shared between the matrix and the filler. An effective interface is essential for the transfer of stress from the matrix to the filler, which possesses superior rigidity and strength. This mechanism allows for the reinforcement of the overall composites and thus enables them to endure higher levels of applied stress, resulting in improved properties. The interaction between filler and matrix, particularly in the case of mineral fillers such as hydroxyapatite and alumina with non-polar organic matrices, can be further improved with the use of coupling agents, typically silane coupling agents [17].

The silane coupling agent is comprised of two distinct components, each of which exhibits a strong affinity toward either the filler or the matrix. As a result, it functions as a bridge of sorts that enhances the interaction between the filler and matrix through the formation of a chemical bond between the two phases, as opposed to relying solely on a mechanical interlock [18].

Debnath et al. [19] investigated methacrylic resin-based dental composites treated with silane coupling agent to provide the interfacial phase that holds together the organic polymer matrix with the reinforcing inorganic phase. In this study, fiber pullout tests were used to measure the interfacial bond strength at the fiber-matrix interface. Glass fibers (approximately 30 μm diameter, 8 cm length, MoSci) were silanated using various concentrations (1, 5, and 10%) of either 3-methacryloxypropyl-trimethoxysilane (MPS) or glycidoxypropyl trimethoxy silane (GPS) in acetone (99.8%). Rubber (poly (butadiene/acrylonitrile), amine terminated, (Mw 5500) molecules were also attached to the fiber surface via GPS molecules. A positive correlation was found between the amount of silane on the filler surface and the property loss after soaking. Rubber treatment provided improvement in interfacial strength. 5% MPS samples had the highest strength both in soaked as well as unsoaked samples.

The filler volume fraction, also known as the filler content, plays a crucial role in determining the properties of a composite material. Typically, as the amount of filler increases, the modulus also increases, but this is accompanied by a greater brittleness in the material. The reason for this is that as more filler is added, the polymer content decreases [20]. In order to achieve the maximum loading of filler while maintaining the desired tensile strength and toughness of the polymer, it is essential that the filler is evenly distributed and that the interface between the filler and matrix is of high quality. Furthermore, as the amount of filler increases, it becomes increasingly important to consider the interactions between the individual filler particles [21].

The filler-filler interaction deals with the affinity of fillers toward each other and toward composite matrix. When the filler-filler interaction is stronger than the filler-matrix interaction, it leads to the clustering of fillers in a particular region. This, in turn, acts as a site of stress concentration which can result in premature failure or cracking. To mitigate this issue, the use of dispersing agents and effective mechanical mixing can be employed to decrease the filler-filler interaction. Additionally, voids and defects caused by the presence of moisture-containing fillers or during processing also have a significant impact on the properties of composite materials. In fact, they exhibit an effect that is equivalent to filler agglomeration, serving as a source of stress concentration and a site for crack initiation, ultimately leading to premature failure, as noted by Atkins and Mai [22].

3.1 Fibers

The filled of acrylic resin with fibers has been documented as a means of enhancing the flexural and impact strength, also to the fatigue resistance of the resin [23]. Numerous investigations have been carried out utilizing various kinds of fibers, including but not limited to nylon, polyethylene, polyamide fiber, and glass fiber, which are widely used owing to its biocompatibility and exceptional esthetic and mechanical characters [24].

3.1.1 Glass fiber

The utilization of glass fiber as a reinforcing agent has been discovered to produce a significant enhancement in the flexural strength, impact strength, fracture toughness, and Vickers hardness of acrylic resin, as evidenced by previous studies [25]. Moreover, it has been observed that deformation of the denture base is reduced to less than 1%. Researchers have found that the manner in which the glass fiber interacts with the denture base affects its properties. In particular, it has been reported that positioning the glass fiber in close proximity to the surface of the denture base leads to an improvement in flexural strength, fracture toughness, and flexural modulus. Conversely, placement of the glass fiber in neutral stress areas only enhances flexural toughness, while placement in the compressive side results in an increase in surface flexural modulus [26].

On the other hand, the findings of a study indicate that the impregnation of glass fiber into acrylic resin does not have an impact on its linear dimensional stability [27]. Additionally, the use of preimpregnated and silane-treated glass fiber, specifically with 3-(Trimethoxysilyl) propyl methacrylate (TMSPM), has been shown to increase both flexural and impact strength of the acrylic resin [28]. Furthermore, the introduction of silanized glass fiber to heat-cured and light-cured resins has been deemed biocompatible. Notably, the utilization of fiber-reinforced nanopigmented PMMA has been found to reduce porosity and Candida albicans adherence [29].

3.1.2 Polyamide fiber

Polyamide fibers, including both Nylon and Aramid fibers, were used to reinforce denture base. Research has shown that Aramid fiber reinforcement enhances the biocompatibility of the resin, while simultaneously increasing its flexural strength and modulus [30]. Nevertheless, it is worth noting that an increase in fiber concentration can lead to a decrease in the resin’s hardness, which is considered a disadvantage. Additionally, the yellow color of the Aramid fiber is also regarded as a drawback [31]. On the other hand, Nylon has been found to improve the fracture resistance of PMMA due to its high resistance to continual stress. As a result, incorporating Nylon fiber into PMMA has been shown to increase its structural elasticity [32].

3.1.3 Polyethylene and polypropylene fibers

The incorporation of polyethylene fiber into PMMA resulted in a significant increase in impact strength, with even greater improvements observed through the application of fiber surface treatment, as previously noted in studies [33]. In contrast, while woven polyethylene fiber reinforcement has been shown to enhance the elastic modulus and toughness of PMMA, the practicality of fiber etching, preparation, and positioning has been reported to be difficult [34]. The use of polypropylene fiber, with surface treatment leading to further enhancements in impact strength [35]. In fact, plasma-treated polypropylene fibers have been found to provide the highest impact strength, offering a viable option for strengthening acrylic resin and reducing the likelihood of fracturing. Incorporating silanized polypropylene fiber into heat-cured PMMA resin has also been shown to significantly improve transverse, tensile, and impact strengths, although it should be noted that wear resistance may be negatively impacted [36].

3.1.4 Natural fibers

Two natural fibers, namely oil palm empty fruit bunch (OPEFB) and ramie fiber, have been utilized to reinforce denture base resins. The incorporation of OPEFB has led to a significant increase in the flexural strength and flexural modulus of acrylic resin, as evidenced by previous research [37]. Short ramie fiber, on the other hand, has been observed to increase the flexural modulus of acrylic resin relative to conventional PMMA. However, the flexural strength has decreased due to weak interfacial bonding between the filler and matrix. It should be noted that the long form of ramie fiber poses a disadvantage, as it necessitates additional work, such as cutting and preparation [38].

3.1.5 Hybrid reinforcement

The idea to reinforce PMMA with a variety of fibers was initially introduced by Vallittu [39]. This combination could include varied types of fibers, metal oxides, ceramics, and fibers with metal oxides or ceramic materials. The integration of hybrid fiber reinforcement has effectively increased the flexural strength and toughness of the reinforced acrylic resin. Similar results were noted through the inclusion of metal oxides and ceramics, specifically NPs, in PMMA. Moreover, this method provides improved surface roughness, tensile strength, flexural modulus, hardness, thermal conductivity, radiopacity, and reduced shrinkage. Additionally, it exhibits antibacterial properties without any cytotoxicity. A combination of fibers and other fillers has also resulted in an elevation of impact strength, hardness, surface roughness, and thermal conductivity, as well as compressive and fatigue strengths [40, 41, 42, 43].

3.2 Fillers

Numerous investigations have demonstrated that the utilization of fillers amplifies the potency of denture base resin and markedly ameliorates its characteristics. The addition of nanofillers has been recommended as a means to augment the properties of PMMA. Nanofillers, owing to their elevated surface area, small dimensions, and uniform distribution, have been shown to enhance the thermal properties of PMMA and heighten its thermal stability in comparison to pure PMMA. The characteristics of the resin, which is reinforced by nanofillers, are contingent upon the magnitude, morphology, category, and concentration of the supplementary particles [44, 45, 46, 47].

3.2.1 Metal oxides

Several investigations have indicated that the augmentation of PMMA with metal oxides has led to enhancements in both the physical and mechanical properties of the material, as well as the tactile sensations experienced by patients in response to hot and cold stimuli [48]. Consequently, the addition of metal fillers to denture base resin was expected to result in improved food sensation and healthier oral mucosa. Nevertheless, certain researchers have noted that the incorporation of metal oxides into acrylic resin has been observed to have a deleterious impact on the strength of the denture base, owing to stress concentration in the vicinity of the embedded metal particles and their weak adhesion to the polymer. Several techniques, including sandblasting, silanization, and metal adhesive resins, have been recommended to enhance the bond between the acrylic resin and metal surface [49, 50].

3.2.2 Alumina (Al₂O₃)

The incorporation of alumina particles into acrylic resin has been found to have a positive impact on the properties of denture base, as reported previously [51]. Furthermore, the addition of alumina powder to acrylic resin has been shown to enhance its thermal conductivity and mechanical properties, according to previous research [52]. Additionally, the reinforcement of PMMA with Al₂O₃ has been found to increase the flexural strength, impact strength, tensile strength, compressive strength, and surface hardness of the resin. Moreover, the inclusion of aluminum in PMMA has been found to significantly reduce warpage. However, some studies have reported that the addition of aluminum decreases both the impact and tensile strength of PMMA. It has also been observed that the flexural properties of acrylic resin can be improved by treating Al₂O₃ particles with a coupling agent. Similarly, the treatment of aluminum particles with silane has been found to significantly increase the compressive, tensile, and flexural strength as well as the wear resistance of reinforced denture base resin, as reported in several studies [53, 54, 55].

The impact of alumina reinforced denture resin on surface roughness and water sorption has been investigated. One study observed no significant changes in these properties. However, another study found that the addition of Al₂O₃ resulted in a decrease in water sorption and solubility, while yet another study reported an increase in water sorption [56]. Additionally, the thermal stability of PMMA was found to increase upon the addition of Al₂O₃ nanoparticles. The thermal properties and flexural strength of acrylic resin were also shown to improve with the addition of silanized Al₂O₃ NPs, while water sorption and solubility decreased. Finally, biocompatibility was demonstrated when alumina NPs were added to both microwave-treated and untreated PMMA powder in a different study [57, 58].

The utilization of alumina-reinforced PMMA is hindered by the occurrence of resin discoloration, thereby restricting its application to non-visible areas. While the introduction of Al₂O₃ into PMMA resulted in a considerable enhancement of thermal conductivity, the flexural strength values of PMMA remained unaltered to a significant extent [59, 60].

3.2.3 Zirconia (ZrO₂)

The inclusion of zirconia (ZrO₂) fillers into PMMA resulted in a notable increase in its flexural strength, as reported by various studies [61, 62]. However, it was also observed that there was a slight reduction in the flexural strength, which could be attributed to the clustering of particles within the resin, leading to material weakness. Furthermore, the incorporation of ZrO₂ into PMMA led to a significant improvement in the impact strength, fracture toughness, and hardness, as documented in one study. Nonetheless, one study found that the impact strength and surface hardness of zirconia-reinforced resin did not increase significantly compared to unreinforced PMMA. In fact, a decrease in both impact strength and surface hardness was reported in some cases. It is important to note that the addition of ZrO₂ had a significant impact on the thermal conductivity of PMMA, which increased considerably. However, studies have yielded varying results with respect to the effect of ZrO₂ on the water sorption and solubility of PMMA. While some studies suggest that adding ZrO₂ led to a significant decrease in the water sorption and solubility of PMMA, other studies found an insignificant difference in water solubility and an increase in water sorption within the limit of ADA specifications [63, 64].

The inclusion of zirconia nanoparticles has been proposed as a means to enhance the mechanical properties of PMMA. The addition of zirconia nanoparticles to PMMA has been shown to increase its impact strength, flexural strength, compressive strength, fatigue strength, fracture toughness, and hardness. Furthermore, it has been suggested that zirconia nanoparticles may have antifungal properties and could potentially serve as a preventive measure for patients who are susceptible to fungal infections. However, one study has reported a negligible increase in the hardness of nano-ZrO₂/PMMA, and no significant alteration in its surface roughness [65, 66, 67].

The impact of the inclusion of ZrO₂ nanoparticles (NPs) on the color properties of PMMA was not found to possess any significant color alterations. To enhance the bond strength between ZrO₂ NPs and PMMA, a silane coupling agent was employed, resulting in an increase in the acrylic resin’s flexural strength and impact strength; however, its tensile strength was not enhanced. Nonetheless, a study discovered that the incorporation of silanized ZrO₂NPs improved the tensile strength and fatigue strength of PMMA. Furthermore, the addition of silanized ZrO₂ NPs to acrylic resin resulted in a significant increase in hardness and a slight increase in surface roughness, while apparent porosity, water sorption, and solubility decreased. Additionally, zirconia nanotubes were found to exhibit a superior reinforcing effect compared to zirconia NPs. However, surface treatment would decrease the reinforcing effect of ZrO₂ nanotubes compared to ZrO₂ NPs. Specifically, flexural strength was optimized when 2 wt% untreated ZrO₂ nanotubes were incorporated into PMMA [68, 69, 70].

3.2.4 Hydroxyapatite (HA)

The utilization of bio-ceramic systems based in HA has proven to be a significant category of bioactive materials that can effectively promote bone regeneration and consequently facilitate a robust interface fixation between host tissues and medical or dental devices. The incorporation of HA can substantially improve the properties of denture base materials, particularly in terms of their radiopaque nature. Research has demonstrated that the addition of 5 and 10% HA that has been treated with γ-MPS significantly enhances the flexural, flexural toughness, tensile strength, and hardness of PMMA denture base resin. Furthermore, it has been observed that the radiopacity of denture base material is also elevated [9, 10, 16, 17].

The introduction of HA fillers to PMMA results in superior mechanical properties, including an increase in flexural strength and flexural modulus of PMMA. This is primarily due to the enhanced interfacial interaction between the HA filler and the PMMA matrix that is brought about by the treatment with γ-MPS. However, the immersion of PMMA in water has been found to cause a reduction in the flexural properties due to water’s plasticizing effect, which weakens the bonding between the HA filler and the PMMA matrix. In contrast, the addition of HA NPs has been shown to increase both the fatigue and compression strength of PMMA resin in comparison with pure PMMA, in addition to inducing a significant increase in thermal conductivity [11, 12].

3.2.5 Titanium (TiO₂)

Numerous investigations have been conducted to examine the impact of the inclusion of TiO₂ on the attributes of PMMA. It has been determined that the introduction of TiO₂ particles may enhance the flexural strength, fracture toughness, and hardness of PMMA as well as its thermal conductivity. Furthermore, the incorporation of TiO₂ into PMMA has been shown to cause a noteworthy increase in impact strength, while simultaneously leading to a significant reduction in water sorption and solubility [71].

In contrast, certain investigations have indicated that the incorporation of TiO₂ into PMMA does not enhance its flexure strength due to the clustering of particles within the resin, thereby compromising its structural integrity. However, research has demonstrated that the introduction of TiO₂ nanoparticles into PMMA can influence its thermal properties (such as a reduction in thermal expansion coefficient and contraction) and mechanical stability (with a decrease in E-modulus), although it may result in a reduction in flexural strength and toughness.

To optimize the properties of PMMA composite, it is crucial to ensure strong adhesion between the resin matrix and filler particles, which can be achieved by employing a titanium coupling agent to reinforce titanium-reinforced PMMA. Furthermore, the addition of silanized TiO₂ nanoparticles to PMMA has been observed to improve its impact strength, transverse strength, and surface hardness, while reducing water sorption and solubility. However, it also increased surface roughness after adding 3 wt% of silanized TiO₂ nanoparticles to acrylic resin [11, 72]. Additionally, the incorporation of apatite-coated titanium dioxide and fluoridated apatite-coated titanium dioxide into PMMA, followed by ultraviolet irradiation, has been found to effectively prevent candida adhesion due to their antifungal properties, thereby promoting appropriate denture hygiene. However, the addition of BaTiO₃ as a radiopacifier to PMMA resulted in a slight decline in fracture toughness properties. Although PMMA/BaTiO₃ composite material has demonstrated thermal stability, its increased density could compromise denture retention [11].

3.2.6 Silicon dioxide (SiO₂)

Numerous investigations have been carried out to examine the impact of the addition of SiO₂ on the properties of PMMA. It has been determined that the mechanical and thermal properties of PMMA can be improved by incorporating SiO₂ nanoparticles (NPs). The introduction of SiO₂ NPs has led to an enhancement in both the transverse and impact strength of PMMA. Additionally, surface hardness has been observed to increase with a higher concentration of SiO₂ NPs. However, it has been discovered that a low concentration of SiO₂ NPs can lead to an improvement in both hardness and fracture toughness. Conversely, an increase in SiO₂ NPs content results in agglomeration and crack propagation, which can reduce both hardness and fracture toughness [73, 74].

The inclusion of surface-treated SiO₂ has been found to enhance the flexural strength of PMMA, albeit it does not affect the hardness. In contrast, a recent study has revealed that silica NPs have an adverse effect on the flexural strength of PMMA. The reinforcement of acrylic resin with glass flakes, which are silica-based fillers, has been found to improve its fracture toughness. Furthermore, the use of silane coupling has resulted in further enhancement of the resin’s properties. Micas, a group of lamellar silicate minerals, have also been proposed as a means of improving the properties of resin. These minerals are characterized by their high aspect ratio and have been observed to enhance the mechanical, thermal, and dimensional properties of PMMA. The incorporation of mica has been found to increase the hardness of acrylic resin, while its flexural strength has been reduced due to the weak bond between mica and the acrylic resin [75, 76, 77].

The incorporation of fluoride glass fillers into PMMA has been observed to reduce microbial adhesion, albeit with a slight increase in the surface roughness of the denture base resin. This was documented in Refs [78]. Additionally, the utilization of nanoclay as an additive in composite and acrylic polymers has been noted to enhance their properties. It was found that the introduction of nanoclay particles into PMMA led to an improvement in its thermal conductivity, albeit at the expense of its flexural strength, as reported in Ref. Furthermore, the placement of silicon carbide filler powders in the palatal region of dentures has been shown to enhance the thermal conductivity of PMMA without compromising its strength or increasing its weight. This was detailed in Ref. [79].

3.2.7 Polyetheretherketone (PEEK)

Polyetheretherketone (PEEK) is a semi-crystalline engineering plastic that exhibits remarkable mechanical and thermal properties. With its impressive advantages, including its lightweight nature, non-toxicity, corrosion resistance, and low modulus closely resembling that of natural bone, PEEK has emerged as a highly promising clinical implant for orthopedic applications [80, 81].

Recently, PEEK has been introduced to enhance the general mechanical performance of the PMMA resin base. The PMMA resin has been combined with TiO₂ and PEEK, leading to a significant improvement in both the average bending strength and the flexural modulus [82]. In the realm of dental implant applications, Chen et al. [83] have found PEEK compounds to be promising restorative materials. Moreover, a recent study Schwitalla et al. [84] has demonstrated that the mechanical properties of PEEK were unaffected by artificial saliva solution with different pH values over a period of 30 days at 25°C. There are some studies that suggest PEEK as a potential restorative material in the oral cavity [85, 86].

Chen et al. [83] developed a practical and convenient protocol for light-curing resin utilized in the 3D industry. This protocol enhances the antibacterial and mechanical properties of PMMA resin through the combination of nanofillers of surface-modified TiO₂ and micro-fillers of PEEK. The study’s findings demonstrated that PMMA composite resins reinforced with TiO₂–1%-PEEK-1% exhibited the most optimized properties. The researchers concluded that the incorporation of 1% of TiO₂ would be an effective amount to enhance both the mechanical and antibacterial properties of PMMA composite resin. In addition, the PMMA (TiO₂–1%-PEEK-1%) composite resin’s printed model presented a smooth surface and precise resolution. This indicates that the functional dental restoration material would be a suitable light-curing resin in the 3D industry [83].

3.2.8 Silver (Ag)

Several studies have reported that the inclusion of silver nanoparticles (AgNPs) in denture base acrylic resin exhibits antifungal properties. This effect is particularly noticeable at high concentrations and has been shown to act as a latent antifungal material, with low-releasing Ag+ [84, 85, 86, 87]. Conversely, Wady et al. [88] found that the incorporation of silver NPs in PMMA did not impact the adhesion of C. albicans and biofilm accumulation. The addition of silver to PMMA is known to possess antimicrobial properties, which can reduce microbial adhesion and colonization. As such, it may be beneficial for immune-compromised and geriatric patients. Furthermore, PMMA reinforced with silver has been shown to increase flexural and fatigue strength, as well as improve thermal conductivity. Another study revealed that the incorporation of 0.5% antimicrobial silver-zinc zeolite in heat-cured acrylic resin did not affect its impact and transverse strength, surface hardness, or surface roughness. However, it did result in a significant decrease in water sorption and an increase in water solubility. It has been suggested that the mechanical properties of denture base resin may be negatively affected by the addition of silver, depending on its percentage [88, 89].

Incorporating silane-treated Ag particles significantly increased the compressive strength of PMMA. Also, addition of 10 and 20 wt% silane-treated silver fillers enhanced the tensile and flexural strength of PMMA. The inclusion of silane-treated silver particles demonstrated a noteworthy increase in the compressive strength of PMMA. Furthermore, when 10 and 20 wt% silane-treated silver fillers were added, both the tensile and flexural strength of PMMA were enhanced. The introduction of silver powder to PMMA also led to a significant increase in thermal conductivity, while the flexural strength values of PMMA remained unaltered. The physical and mechanical properties of PMMA were improved by the incorporation of silver nanoparticles (NPs), including increased thermal conductivity and compressive strength [90].

Hence, the application of Ag NPs is recommended in the palatal region of maxillary acrylic resin dentures. Additionally, it has been determined that PMMA-silver NPs do not exhibit cytotoxicity. However, the tensile strength of PMMA did not undergo significant changes after the inclusion of 0.2% Ag NPs in comparison with unmodified PMMA. Nevertheless, a significant decrease in tensile strength was observed after the incorporation of 2% Ag NPs. Moreover, PMMA-Ag NPs have been reported to exhibit poor color stability. The addition of AgNPs to acrylic denture base material can enhance its viscoelastic properties [91].

The potential benefits of incorporating nano-gold (Au), platinum (Pt), and palladium (Pd) into PMMA denture base have been suggested in recent studies. However, the available literature on the effects of adding nano-gold to PMMA remains limited. Encouragingly, it has been observed that the inclusion of Au NPs can significantly enhance the flexural strength and thermal conductivity of PMMA, resulting in nearly double the value of pure PMMA and potentially increasing patient satisfaction. Additionally, the addition of Pt NPs may improve the mechanical properties of PMMA and provide an antimicrobial effect [92].

The results of the investigation revealed that the utilization of Pt led to a notable increase in the bending deflection of PMMA. Furthermore, it was observed that palladium had a positive impact on the bending strength of the material, in contrast to silver and gold, which both exhibited the lowest level of bending strength. Additionally, the incorporation of gold and palladium proved to enhance the Vickers hardness of PMMA, whereas the introduction of platinum was shown to have a decreasing effect on this property [93].

The halloysite nanotube, a naturally occurring silica-based mineral, was initially introduced by Abdallah [94] in 2016 as a means to enhance the properties of PMMA. It was observed that the addition of halloysite nanotube in small proportions led to an increase in the hardness of PMMA, however, there was no significant improvement in the flexural strength and Young’s modulus. The use of carbon family fillers, specifically carbon fillers, to reinforce PMMA, is not a common practice due to issues such as biological complications, suboptimal esthetics, and challenges in handling and polishing. However, nano-carbon has emerged as a prominent sector of nanotechnology in recent times [92].

3.2.9 Nano-carbon

The introduction of carbon nanotubes at a rate of 1% into PMMA led to a significant increase in both the impact strength and flexural strength of the resin, albeit at the cost of decreased hardness, according to one study. Another study found that the addition of 1.5% single-walled carbon nanotubes had a significant effect on the impact and transverse strength of PMMA, yet led to a noticeable reduction in surface hardness. In contrast, a separate study revealed that the addition of single-walled carbon nanotubes had an insignificant effect on the flexural strength of PMMA. Furthermore, the incorporation of 0.5 and 1% multiple-wall carbon nanotubes (MWCNTs) into PMMA resulted in an improvement in both the flexural strength and resilience of the resin, but with higher concentrations of MWCNTs, the fatigue resistance was seen to decrease [93].

3.2.10 Nano-diamonds

The exceptional characteristics of nano-diamonds, specifically their high level of hardness and thermal conductivity, have led researchers to explore their potential for enhancing the mechanical properties of PMMA. The incorporation of NDs into PMMA resulted in a significant increase in impact strength, as well as an improvement in fracture toughness, albeit only at the lowest concentration of NDs. Additionally, the scratch resistance of PMMA was shown to be enhanced through the use of heat-treated NDs. However, the agglomeration of the nanoparticles was identified as a primary drawback, as it could potentially serve as a site for stress concentration [95].

4. Advanced perspective dental prosthesis

The employment of digital dentistry technology has emerged as the prevailing trend in dental prosthetics across the globe in contemporary times. The advent of digital three-dimensional (3D) printing technology has significantly enhanced the diagnostic rate and demonstrated remarkable potential in the realm of personalized medicine. In comparison to the conventional manufacturing process, 3D-printed prostheses boast shorter production cycles and higher precision, thereby optimizing the comfort of denture patients. Presently, approximately 75% of 3D printing dental applications utilize light-curing technology, wherein light-curing resins are extensively used as fillers and restorative materials in stomatology. PMMA is a widely used commercial light-curing resin in the 3D printing industry owing to its low odor, low irritancy, good flexibility, and cost-effectiveness. However, the inherent limitations of PMMA, including substantial shrinkage rate during light-curing, brittleness, poor mechanical properties, and low antibacterial activity, among others, have restricted its widespread clinical application. Hence, further research and development are imperative to enhance the existing properties of 3D printing denture base materials [96, 97].

5. Conclusion

Although PMMA has been utilized for constructing denture bases and has become the preferred material for such fabrication, it exhibits low fracture resistance, specifically under fatigue failure within the oral cavity and impact failure outside of it. Consequently, various reinforcement agents, such as fibers or filler particles in micro and nano scales, have been investigated to enhance the properties of denture base materials. This has resulted in the expanded use of dental composites in numerous applications, which in turn has spurred the demand for continued research and improvement to enhance their properties and performance. One promising approach involves employing a nanofiller modified with a silane coupling agent to improve the dispersibility of fillers in a polymer, thereby enhancing the mechanical properties of the composite resin. The future of manufacturing applications in three-dimensional (3D) printing and CAD/CAM technology of denture base resins with improved properties will necessitate the development of materials suitable for 3D printing technology and denture base applications.

Conflict of interest

The author declares no conflict of interest.

Hayati Filik

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