Abstract
Objectives
To present an overview of fiber-reinforced composites (FRCs) that are a group of non-metallic dental biomaterials used in several fields of dentistry.
Methods
A range of relevant publications from the past half century are surveyed, with emphasis upon recent publications.
Results
FRCs vary according to the type of fiber fillers and orientation of fibers, the latter being responsible for several properties which can vary from isotropic to anisotropic. The length of the fibers, i.e. the aspect ratio of the filler, is another factor or variable that contributes to the properties and the development of new types of composite resins for restorative and prosthetic dentistry, as well as to reconstructive medicine.
Significance
Understanding the anisotropic nature of FRCs from the perspective of dental applications has increased in recent years. This review describes some fiber orientation related anisotropic properties of FRCs which contribute to the increased use of FRCs in clinical dentistry.
1
Introduction
Using high aspect ratio fillers in composite resins can significantly change the resin’s physical properties in comparison to using particulate fillers which provide isotropic properties for the material. High aspect ratio fillers have a high ratio of the length of the filler to its cross-sectional diameter. In composites, high aspect ratio fillers are fibers but shorter filament like fillers of whiskers can be also considered high aspect ratio fillers . Different types of fibers with various orientations and lengths have been utilized for decades in engineering applications to construct devices with high strength and fracture toughness. The first glass fiber-reinforced boat was produced in 1937 in Russia and since then the anisotropicity of fiber-reinforced materials has been utilized in everyday life and recently in dentistry and medicine.
In natural constructs, reinforcing fibers of cellulose can be found, e.g. in wood, where the length of the oriented polysaccharide based cellulose fibers and shorter branched hemicellulose fibers is between 0.8 and 2.3 mm . Chiral and crystalline cellulose fibers in wood are embedded in the lignin matrix, which is a natural biopolymer of aromatic alcohols. Other examples of natural systems that contain high aspect ratio components include bone and dentin . Osseous tissue of bone and dentin is composed of a hard, lightweight composite, where mineral calcium phosphate in the chemical arrangement of hydroxyapatite forms the inorganic matrix for the protein origin collagen fibers, which are formed from thinner type I fibrillar collagen. The collagen fibers with a diameter of 20–400 nm consist of collagen molecules, which are stabilized by four cross-linking covalent bonds per molecule . The length of collagen fiber is around 23 μm . High aspect ratio fibers in collagen provide high tensile strength and fracture toughness for bone and dentin, whereas hydroxyapatite is responsible for the compressive properties. Both collagen fibers and crystals of hydroxyapatite are oriented best to withstand physiological loading conditions. Mineralized collagen fibers provide toughness through crack-tip shielding through osteons, especially with lower strain rates .
Man-made high aspect ratio fillers have been used since ancient times to reinforce bricks and buildings . Modern fiber-reinforced composites (FRCs) are used in applications where high static and dynamic strength and fracture toughness, especially in relation to weight, are desired properties. For example, dental and medical devices are typically subjected to hundreds of thousands of loading cycles by the masticatory system or the weight of the body during physical exercise . FRCs are typically designed to have the highest possible reinforcing efficiency against the direction of stress, and thus, they often represent anisotropic material in terms of mechanical properties. However, some other properties, such as optical properties, surface physical properties, thermal properties and polymerization contraction properties are related to the orientation of the fibers in the FRC. From the point of view of high fatigue resistance and toughness, FRC is part of a group of choice materials for dental and medical needs. FRCs in dentistry were first developed in the early 1960s but an increase in the number of published scientific papers occurred in the early 1990s . Currently, FRCs are used in fixed prosthodontics, restorative dentistry, periodontology, orthodontics and in repairs of prosthetic devices . There are also cranial implants made of glass FRCs and attempts to develop oral and orthopedic implants are ongoing . This is a review of the current status of knowledge of some anisotropic properties of FRCs used in dentistry.
2
Reinforcing efficiency of fibers
The majority of dental FRCs are presently produced from glass fibers due to their surface chemistry, which allows for their adhesion to the resin matrix via silane coupling agents, and to the transparency of their fibers . Glass fibers do not cause severe problems related to the appearance of the restoration. Out of the several different types of fibers (carbon/graphite, aramid, polyethylene), glass fibers have been adopted for use in dentistry and medicine. The most common glasses, namely E-glass and S-glass, and their behavior as components of dental FRCs were recently reviewed in more detail in another publication .
FRC is a material which contains at least two phases, one of which is characterized by its high aspect ratio, i.e. ratio between the length and diameter ( l / d ). Among many parameters (interfacial adhesion, elongation of fibers, fiber volume fraction) which contribute to the reinforcing efficiency of fibers, length and orientation of fibers are important in terms of the isotropicity–anisotropicity of the material . FRCs are classified as short discontinuous and long continuous FRCs; which have different mechanical properties although the fiber volume fraction could remain the same . By changing continuous unidirectional fibers to longitudinally oriented discontinuous short fibers of lower aspect ratio, ultimate tensile strength of the composite is reduced ( Fig. 1 ). In this case both continuous and discontinuous FRCs are anisotropic. Changing the orientation of short fibers so they lay randomly causes the tensile strength to reduce even more, and the FRC material becomes isotropic. Consequently, strength, unlike stiffness of continuous FRCs, cannot be attained by discontinuous short fiber systems even with high aspect ratios . Failure types of discontinuous short FRCs including cracking of the polymer matrix, debonding of the fiber and fracture of the fiber ( Fig. 2 ). Depending on the length of the fiber, aspect ratio of the fiber, interfacial fracture energy (adhesion of fibers to the matrix) and fiber volume fraction, some of the failure types are more common. Dependence between the orientation and length of the fibers is also described by Krenchel’s factor, in which the reinforcing efficiency factor for fiber reinforcement goes against the known direction of stress . Reinforcing fibers in the direction of the stress provides the highest reinforcing efficiency and finally axial failure of the fibers and polymer matrix will occur ( Fig. 3 ). If stress is applied perpendicular to the fibers, the fibers do not reinforce the FRC, and transverse failure of the polymer matrix occurs.
The effect of the aspect ratio of fibers is also related to “critical fiber length”, which can be defined as the minimum length of the fiber. It allows for the development of sufficient stress to cause the fiber to fail at its midpoint. Interfacial fracture energy of the adhesive interface between the fibers and polymer matrix versus the tensile strength of the fiber has an impact on the critical fiber length. It has been concluded that for advanced FRCs, the critical fiber length could be as much as 50 times the diameter of the fiber. To eliminate fiber debonding under tensile stress, FRCs with longer fibers could exhibit strength similar to the corresponding continuous FRC . The diameter of glass fibers presently used in dental FRCs is 15–18 μm and the critical fiber length should be, therefore, between 750 and 900 μm . A recently introduced short fiber-reinforced composite for filling applications has fiber lengths within this range .
2
Reinforcing efficiency of fibers
The majority of dental FRCs are presently produced from glass fibers due to their surface chemistry, which allows for their adhesion to the resin matrix via silane coupling agents, and to the transparency of their fibers . Glass fibers do not cause severe problems related to the appearance of the restoration. Out of the several different types of fibers (carbon/graphite, aramid, polyethylene), glass fibers have been adopted for use in dentistry and medicine. The most common glasses, namely E-glass and S-glass, and their behavior as components of dental FRCs were recently reviewed in more detail in another publication .
FRC is a material which contains at least two phases, one of which is characterized by its high aspect ratio, i.e. ratio between the length and diameter ( l / d ). Among many parameters (interfacial adhesion, elongation of fibers, fiber volume fraction) which contribute to the reinforcing efficiency of fibers, length and orientation of fibers are important in terms of the isotropicity–anisotropicity of the material . FRCs are classified as short discontinuous and long continuous FRCs; which have different mechanical properties although the fiber volume fraction could remain the same . By changing continuous unidirectional fibers to longitudinally oriented discontinuous short fibers of lower aspect ratio, ultimate tensile strength of the composite is reduced ( Fig. 1 ). In this case both continuous and discontinuous FRCs are anisotropic. Changing the orientation of short fibers so they lay randomly causes the tensile strength to reduce even more, and the FRC material becomes isotropic. Consequently, strength, unlike stiffness of continuous FRCs, cannot be attained by discontinuous short fiber systems even with high aspect ratios . Failure types of discontinuous short FRCs including cracking of the polymer matrix, debonding of the fiber and fracture of the fiber ( Fig. 2 ). Depending on the length of the fiber, aspect ratio of the fiber, interfacial fracture energy (adhesion of fibers to the matrix) and fiber volume fraction, some of the failure types are more common. Dependence between the orientation and length of the fibers is also described by Krenchel’s factor, in which the reinforcing efficiency factor for fiber reinforcement goes against the known direction of stress . Reinforcing fibers in the direction of the stress provides the highest reinforcing efficiency and finally axial failure of the fibers and polymer matrix will occur ( Fig. 3 ). If stress is applied perpendicular to the fibers, the fibers do not reinforce the FRC, and transverse failure of the polymer matrix occurs.
The effect of the aspect ratio of fibers is also related to “critical fiber length”, which can be defined as the minimum length of the fiber. It allows for the development of sufficient stress to cause the fiber to fail at its midpoint. Interfacial fracture energy of the adhesive interface between the fibers and polymer matrix versus the tensile strength of the fiber has an impact on the critical fiber length. It has been concluded that for advanced FRCs, the critical fiber length could be as much as 50 times the diameter of the fiber. To eliminate fiber debonding under tensile stress, FRCs with longer fibers could exhibit strength similar to the corresponding continuous FRC . The diameter of glass fibers presently used in dental FRCs is 15–18 μm and the critical fiber length should be, therefore, between 750 and 900 μm . A recently introduced short fiber-reinforced composite for filling applications has fiber lengths within this range .
3
Fiber-reinforced composite as an adhesive substrate
Use of FRCs in dental applications requires covering the fiber-rich part of the construction by some other resin system in order to adhere (adhesives and luting cements) the fiber-rich FRC to the dental tissues. This also protects the fibers and provides a polishable surface for the FRC device (veneering composites, filling composites, removable dentures) . In these applications, the orientation of fibers in the load-bearing part of the construction is selected according to the loading conditions of an FRC system, which can best resist the mechanical stresses of the masticatory system. The surface of the substrate onto the adhesive resin, luting cement or veneering composite resin is adhered and can have different orientations of fibers as shown in Fig. 4 .
In the case that the surface of the substrate is ground, and the reinforcing glass fibers are exposed, an average of 50% of the surface area of the substrate is composed of the polymer matrix and 50% are glass fibers. The exposed glass fibers are those which have lost the surface silanation due to the grinding. This can occur with prefabricated FRC root canal posts and frameworks in fixed dental prostheses, which are polymerized and mechanically shaped before veneering. Repairs to FRC constructs also expose the fibers to the surface. Bonding mechanisms and bonding efficiency to the cross-linked polymer matrix and interpenetrating polymer matrix have been reported . When the orientation of exposed fibers is considered, the highest bonding strength to the FRC surface is obtained with perpendicularly exposed fibers ( Fig. 5 ). Fibers in-a-plane, which were along the direction of debonding stress, reached almost the same value of bonding strength as perpendicularly oriented fibers, whereas the in-a-plane fibers, which were opposite to the debonding stress, gave the lowest bonding strength ( Figs. 4 and 5 ). Discontinuous short fibers can be used to reinforce dental fillings . A layer of discontinuous short FRC, which replaces the missing part of dentin, is covered with a layer of particulate filler composite resin to achieve a wear resistant and polishable surface. FRC layer substructures and the particulate filler composite resins form a bilayered composite structure in which randomly oriented and slightly protruded discontinuous fibers reinforce the interface of the composite layers.
