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High-aspect ratio fillers: Fiber-reinforced composites and their anisotropic properties Pekka K. Vallittu ∗ Department of Biomaterials Science and Turku Clinical Biomaterials Centre – TCBC, Institute of Dentistry, University of Turku, Turku, Finland

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To present an overview of fiber-reinforced composites (FRCs) that are a group of

Received 21 June 2014

non-metallic dental biomaterials used in several fields of dentistry.

Received in revised form

Methods. A range of relevant publications from the past half century are surveyed, with

11 July 2014

emphasis upon recent publications.

Accepted 14 July 2014

Results. FRCs vary according to the type of fiber fillers and orientation of fibers, the latter being

Available online xxx

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

Keywords:

to the properties and the development of new types of composite resins for restorative and

Fiber

prosthetic dentistry, as well as to reconstructive medicine.

Composites

Significance. Understanding the anisotropic nature of FRCs from the perspective of dental

Fiber-reinforced composite

applications has increased in recent years. This review describes some fiber orientation

Polymer

related anisotropic properties of FRCs which contribute to the increased use of FRCs in

Anisotropicity

clinical dentistry. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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 [1]. 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 [2]. 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 [3]. 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



Correspondence to: Institute of Dentistry, University of Turku, Lemminkäisenkatu 2, FI-20520 Turku, Finland. Tel.: +358 2 333 8332. E-mail address: [email protected]fi http://dx.doi.org/10.1016/j.dental.2014.07.009 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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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 [4]. The length of collagen fiber is around 23 ␮m [5]. 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 [6]. Man-made high aspect ratio fillers have been used since ancient times to reinforce bricks and buildings [1]. 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 [7]. 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 [8–15]. Currently, FRCs are used in fixed prosthodontics, restorative dentistry, periodontology, orthodontics and in repairs of prosthetic devices [16–26]. There are also cranial implants made of glass FRCs and attempts to develop oral and orthopedic implants are ongoing [27,26,28,29]. 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 [30–32]. 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 [33]. 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 [1]. FRCs are

Fig. 1 – Influence of the aspect ratio (l/d) of fibers and their orientation to the tensile stress ( t ) and modulus of elasticity (E) with the same volume fraction of fibers (Vf ) [34].

classified as short discontinuous and long continuous FRCs; which have different mechanical properties although the fiber volume fraction could remain the same [34]. 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 [34]. 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 [35]. Reinforcing fibers in the direction of the stress provides the highest reinforcing efficiency and finally axial failure of the fibers and polymer

Fig. 2 – Schematic presentation of the types of failures (crack of the matrix, debonding of the fiber, fracture of the fiber) of discontinuous short fiber-reinforced composite (Xo = embedded fiber length which as debonded, d = diameter of fiber, L on fiber length, f = volume fraction of fibers). Arrows show the direction of load.

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Fig. 4 – Schematic presentation of alternatives to bond particulate filler composite resin (PFC) to the surface of fiber-reinforced composite (FRC) (upper row) and bonding fiber-reinforced composite (FRC(bidir)) = bidirectional fibers, FRC (rand) = randomly oriented fibers, FRC(vert) = vertical unidirectional fibers) to the surface of enamel (E) and dentin (D). Bonding values are shown in Fig. 5.

Fig. 3 – Schematic presentation of types of failures (from top to bottom: axial tensile failure, transverse tensile failure, shear failure) of a continuous unidirectional fiber-reinforced composite. Arrows show the direction of force.

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 [36]. The diameter of glass fibers presently used in dental FRCs is 15–18 ␮m and the critical fiber length should be, therefore, between 750 and 900 ␮m [37]. A recently introduced short fiberreinforced composite for filling applications has fiber lengths within this range [38,39].

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) [17,19,40,41]. 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 [42]. 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 ina-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 [43] (Figs. 4 and 5). Discontinuous short fibers can be used to reinforce dental fillings [39]. 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.

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Fig. 5 – Shear bond strength (MPa) of material combinations described in Fig. 4 [43–45].

In direct applications of FRCs, e.g. during fabrication of direct fixed dental prostheses or periodontal splints, the fibers are adhered directly to the surface of the dentin or enamel. In direct applications, the orientation of fibers can vary from bidirectional weaves to veils of randomly oriented fibers to continuous longitudinal fibers (Fig. 5). Bonding efficiency of FRCs with different orientations of fibers varies only slightly from each other (Fig. 5) [44,45].

4.

Fig. 6 – Anisotropic linear polymerization contraction strain (microstrain) of fiber-reinforced composites (FRC) with various orientations of fibers and the direction of measuring the strain in relation to the long axis of the fiber. Control material is isotropic particulate filler composite resin [53].

Polymerization contraction

Polymerization contraction is one of the shortcomings related to dental composite resins, which contributes to the longevity of restorations [46–48,50,51]. Reinforcing fibers not only improve mechanical properties of the composites but can also control the polymerization contraction. Polymerization contraction strain of FRCs has been measured by conventional strain gauges and optical Bragg grating sensors [49,52]. Both strain gauges and Bragg sensors have shown the dependence of the polymerization contraction on the direction of the reinforcing fibers. Fig. 6 summarizes isotropic polymerization contraction strains of particulate filler composite resin and FRCs with varied orientations of fibers. Hygroscopic expansion of resin composites is also dependent to the fiber orientation (Fig. 7) [49,53]. FRCs with longitudinal fiber orientation demonstrate no contraction in the direction of fibers; minor thermal expansion can be found, which is caused by an increase in temperature due to exothermic polymerization reactions. Anisotropicity in polymerization contractions can be beneficial in tailoring the properties of filling composite resins in order to minimize gaps between fillings and tooth substances. An observation was published recently on the use of individually formed FRC root canal posts, i.e. FRC material, which is directly polymerized in a root canal with a high configuration factor (c-factor = 953) [54]. It was expected that the higher polymerization contraction in the transverse direction to the fibers of the FRC could have caused debonding and gap formation between the FRC material and root canal dentin [55]. However, the results showed

Fig. 7 – Anisotropic hygroscopic expansion of the fiber-reinforced composite (FRC) compared to isotropic expansion of particulate filler composite resin and unfilled resin [49].

the opposite: direct polymerization of the FRC in the root canal with the composite resin luting cement demonstrated better adaptation and sealing between the post and root canal compared to the posts, which were polymerized chairside before cementation. This was explained by the elastic properties of the FRC post interpenetrating polymer network (IPN) polymer matrix. The lower modulus of elasticity of the IPN polymer matrix was assumed to be able to absorb polymerization contraction stress during polymerization and cause less stress to the cement–tooth interface [42,56].

5. Effect of fiber orientation on optical properties Optical properties of FRCs are also fiber direction related; they are utilized in fiber optics of communication systems and in transmission of light in illumination applications. Fiber

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optics can also be used in sensor technology, for instance in Braggs grating sensors to measure dimensional changes with high precision [57]. Optical fibers have a transparent core, which is covered with a transparent cladding material with a lower index of refraction in order to keep the light in the core by total internal reflection. Applications in dentistry, which are utilizing optical properties of fibers can be found in restorations of root canal treated teeth [58–61]. There are some prefabricated FRC root canal posts available which transmit the polymerization light through the post and enables lightinitiated polymerization to occur in the composite resin luting cement [59,60]. In the case of individually formed FRC root canal posts, the resin matrix of the post needs to be polymerized as well as the resin based luting cement. Transmission of light to the resin system surrounding the fibers and to the fiber-rich area in general is based on reflectance and the scattering of the light. Scattering of light includes the deflection of a ray of the light from a straight path. The deflection of light can be caused by irregularities in the propagation medium, at the interface between two media, or from differences in the refraction indices of light transmitting fibers in the surrounding resin system. Scattering of this type is referred to as diffuse reflection. There are studies related to the optical properties of dimethacrylate resin systems in the context of lightinitiated polymerization of resin systems. It has been shown that polymerization reactions of bisphenol-A-dimethacrylate (bisGMA)–triethylene glycol dimethacrylate (TEGDMA) resin systems increase the refraction index from 1.515 to 1.550 with a bisGMA–TEGDMA ratio of 50:50, and to 1.580 with bisGMA–TEGDMA ratio of 99:1 (Fig. 7) [62]. This is due to the reorganization of the molecular structure during polymerization. At the same time, the extinction coefficient at the 470 nm wavelength of the resin system decreases due to reducing the amount of initiator camphorquinone until the extinction coefficient is dominated by the copolymer only. Simultaneous changes in the refraction index and extinction coefficient of the resin system, and the refractive index of E-glass fiber (1.556) and boron free E-glass fiber (1.547) [63], enhance light to scatter out from the glass fibers once the polymerization of the resin system has started and proceeded to a particular stage (Fig. 7). This is according to the Snell’s law of relationship between the angles of incidence and refraction of light passing through a boundary between two isotropic media; Eglass and resin system of bisGMA–TEGDMA. In practice, this phenomenon is utilized in the polymerization of continuous unidirectional E-glass fiber composite prepreg with an optimized bisGMA–TEGDMA ratio inside the root canal when an individually formed root canal post is prepared. It is known that surface irregularities of the fibers enhance scattering. It has been demonstrated in vitro that the depth of lightinitiated polymerization of the bisGMA–TEGDMA resin matrix of FRC root canal post is 18 mm for the degree of monomer conversion of 55%. This exceeds the length of the suggested individual FRC post (ca. 5 mm), which has shown to provide the highest load-bearing capacity for a root-post-crown system [64]. Optical properties of glass FRCs also play a role in the appearance of FRC-reinforced restorations. A recent study showed that continuous unidirectional and bidirectional Eglass fibers should be veneered with a layer of particulate

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Fig. 8 – Index of refraction and extinction coefficient of a monomer system of bisGMA–TEGDMA in volume ratios of 50:50 and 99:1 plotted against the light curing time. The gray area demonstrates the refraction index of E-glass fibers with different compositions [62,63].

filler composite resin with a thickness of more than 2 mm in order to eliminate the risk of color shade mismatch [65] (Fig. 8).

6. Other anisotropic properties and conclusions Research on man-made FRCs demonstrates that they mimic structurally constructs of biological systems. Natural anisotropic systems are not only anisotropic in their final constructs. Natural systems function anisotropically during the creation of tissues and structures from cellular activity. Anisotropic substrates for cells can direct cell morphology and growth. Studies related to continuous unidirectional Eglass FRC in implant applications have shown that surface wettability, surface energy and cellular responses are also dependent on the direction of the exposed glass fibers [66,67]. Microscopic investigation has shown fibroblast cells to orient themselves parallel to the direction of E-glass fibers. This was likely due to the hydrophilic surface of the glass fiber when compared to the polymer matrix. Approaches of tailoring fiber-containing scaffolds for tissue engineering applications are ongoing. One promising alternative could be to add fibrous scaffold to the surface of the implant construction in order to enable anisotropic tissue growth to the implant. It can be concluded that FRCs with high-aspectratio fillers have several properties which relate to the direction of the fibers. In the case of unidirectional fibers, both continuous and discontinuous fibers provide anisotropic properties for the composite. Dependent on the application, anisotropicity should be carefully considered when designing FRC devices. More research is needed for detailed information on anisotropicity and its influence on the function of FRC devices in clinical applications.

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Acknowledgements Author expresses gratitude to the researcher team of the BioCity Turku Biomaterials Research Program (www.biomaterials.utu.fi) and principal financing partners of the FRC research: Finnish Agency for Technology and Innovation (TEKES), Academy of Finland and European Commission (Grant: NEWBONE NMP3-CT-006-026279-2).

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Please cite this article in press as: Vallittu PK. High-aspect ratio fillers: Fiber-reinforced composites and their anisotropic properties. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.07.009

High-aspect ratio fillers: fiber-reinforced composites and their anisotropic properties.

To present an overview of fiber-reinforced composites (FRCs) that are a group of non-metallic dental biomaterials used in several fields of dentistry...
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