Reinforcement of Dental Composites by Electrospun Nanofibers - Research Paper Example

 Reinforcement of Dental Composites by Electrospun Nanofibers   In Partial Fulfillment of the Requirements   INTRODUCTION: Our investigation will attempt to address the question concerning the effectiveness of electrospun nanofibers to bind with a commercially used polyesteramide (Hybrane, 0.3% w/w DSM), a hyperbranched dendritic polymer, embedded within the recent most, efficient, light cured, polyacrylate resin based matrix used in dental formulations. It will also throw light on the extent of enhancement in the mechanical strength of the dental composites due to the electrospun nanofibers. Quite a lot of work has been performed on the characterization of different types of dental composites composed of polyacrylate resin matrix equipped with various forms of hybrid fillers in order to understand their physical and chemical properties along with their mechanical compatibility to suit adverse oral environment. Our work draws support from the background researches concerned with the application of nanoscaled reinforcing materials and organic-inorganic hybrid nanocomposites synthesized by conventional soft chemistry procedures on various innovative industrial products. The principal look out of the following investigation is also based on the previous studies conducted to understand the intricate mechanisms behind the phenomenon of dissipation of energy that is hypothesized to be one of the definitive factors for hiking the index of fracture toughness in solid composites. (Composites Science and Technology, 2008) Past researches show that this energy dissipating phenomenon implicating increase in the fracture toughness is far more definitive and effect enticing for nanocomposites compared to ordinary composites having micron scaled fibers. In the following investigation we attempt to understand and interpret the reasons associated with mimicking such kind of mechanical strength and structured toughness after reinforcing the resinous polyacrylate matrices of the dental composites with hyperbranched polymeric nanofibers. Our research also draws its motivation from the investigations attempting to analyze and understand the reasons for the rupture of artificial acrylic dentures. (Saudi Dental Journal, 2006) Though such research is always critical and is undoubtedly clouded by several other unaccountable factors, like, the inherent clinical design of the prosthesis, deformations of the dental base, climatic wear and tear of the metal supports, conditions leading to accidental damages along with the gender, age and total span of usage of the wearer; the plausible causes arising from the discrepancies in the mechanical index of the denture material itself, was the only object of our interest for pursuing further investigations along relevant lines. Therefore, it is undoubtedly clear that the primary objective of this research is to record and understand the effects of the covalently bonded nanoscaled organic reinforcements on the performance and durability index of the recently used dental composites. By designing composites of polyacrylate resin matrices modified with commercial dendritic polymers, like polyesteramide and polyester, we need to create a variation from the previous investigations by incorporating electrospun polyamide nanofibers into the system and test their effect on the overall mechanical strength of these composites. EXECUTIVE SUMMARY: The nanofibers will be synthesized by electrospinning three different polymers that are Poly Vinyl Alcohol (PVOH), Poly-L-Lactide Acid (PLLA) and Nylon 6 (Polycaprolactam or Polyamide or PA6) and they are to be used with the Hybrane modified acrylate resin matrix of the composites at different weight percents and particle diameters. The compressive strength, diametric tensile strength, linear shrinkage index and flexural strength will be measured at different concentrations and diameters. The morphology of the nanofibers that are expected to reinforce the dental composites will be ascertained with Atomic Force Microscopy and Scanning Electron Microscopy. The overall assay will comprise of several steps including the fabrication of the composites, the procurement of the materials used for making the dental formulations, the electrospinning and the testing procedure for recording the mechanical strength of the nanocomposites. The investigation will involve the use of Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) for studying the three dimensional topology and the internal topography of the composites. The data obtained for both the mechanical reinforcement along with the surface and internal topography will be subjected to ANOVA or Analysis of Variance for taking into account their statistical differences and nature of correlation. BACKGROUND RESEARCH & LITERATURE REVIEW: Dental amalgams were originally used as restoratives and dental prosthetics for its high mechanical strength, durability, surface hardness, low thermal expansion coefficient and also for the inherent property of liquid mercury to solidify at a considerably faster rate leading to the formation of a number of multi-phase alloys, appreciably stable at room temperature. (SCENIHR, 2007) There had been a number of investigations to find out better replacements of conventional dental-amalgams, composed of a critical mixture of metals, for tooth fillings and other forms of dental restoratives. Past studies have proved the pressure withstanding ability and abrasion resistance of this restoring material under varying extents of packing pressure simulating the natural mastication process. (Journal of Dental Research, 1977) However, the dental amalgams, though sufficiently strong and durable with considerable mechanical and tensile strength in order to withstand the high masticatory pressure cannot mimic the appearance and texture of dental tissue. Moreover these dental amalgams having 50% Mercury, 35% Silver and almost 15% Tin, are suspected to have serious health implications due to Mercury’s potential to incite conditions of cytotoxicity and neurotoxicity. In fact the Mercury used in the amalgam is not held securely by the metal bonds in the teeth and have a tendency to leak out in copious amounts affecting the body leading to imbalances in neurotransmission, untimely immune reactions, perturbations in cellular enzymatic processes by binding with the cysteine side chains in the amino acids and initiation of allergic response. Recent researches carried out to investigate on new avenues for improvements on the bio-compatibility and durability of dental restoratives and prosthetics indicate that composites with acrylate resin matrices are effective to be used in dental formulations. The need for further enhancement in mechanical properties, biological compatibility and power to withstand the adversities of oral environment led researchers to investigate along different lines in order to come up with the best possible formulation. Silica filled light cured acrylate resins are the main constituents of the polymeric matrices that are recently used for devising long lasting, bio-chemically effective dental composites. These dental composites are the most suitable alternatives for the dental amalgams, having serious health implications. These composites are composed of a number of different chemical constituents formulated together at various concentrations. Ideally, the composites are composed of an organic matrix material, which polymerizes from a wide range of diacrylate monomers and oligomers, like Bisphenol-A-glycidylmethacrylate (Bis-GMA), Triethylenglycoldimethacrylate (TEGDMA), Urethane dimethacrylate (UDMA) and are sufficiently stable at room temperature. (Jamison, P. 121) Various inorganic filler materials, such as silica (SiO2), alumina (Al2O3) along with mixture of glass and sodium fluoride (NaF) are incorporated into these matrices along with a coating of trialkoxysilane that acts as anchor, due to its stable carbon-carbon linkages, between the monomeric matrices and the fillers, prior to the polymerization process. The nanoscaled inorganic fillers with their particle sizes small enough to be lesser than the wavelength of visible light are much better substitutes of the earlier used 5 to 10 μm macro sized filler particles, mainly comprising of colloidal silica being difficult to polish and had low resistance to wear and tear. Other relevant researches on nanocomposites devised from both nano-clusters and nanofillers exhibited fracture resisting power presumably due to both higher diametric and compressive strengths than commercialized conventional hybrid fillers. (Journal of American Dental Association, 2003) Essentially, the nanofillers having much smaller particle size neither absorb nor scatter the visible light and exhibit high radio-opacity ideally mimicking dental appearance and texture complying with the oral esthetics. Smaller size also permits them to be loaded into the acrylate matrices at greater much quantities leading to lower shrinkage during polymerization of the oligomeric and monomeric matrix particles. (Walmsley, P. 68) Various additional components are added to the composites already composed of the polymerized acrylate matrix, filler particles and suitable coupling agents; these include certain organic compounds and inorganic oxides that function as pigments to generate a wide range of shades for these dental restoratives. Besides etchants, resins and certain primers are used for adhesion of these fabricated composites to the dental tissues during the prosthesis and restoration procedures. In spite of the fact that polyacrylate resin made prosthetics effectively mimic the esthetics of the oral environment; certain evidences based on in-vitro analysis indicate, these restorative materials along with their hybrid fillers do also have cytotoxic implications to both pulp and gingival cells. The organic matrix of most dental composites can release a host of different organic and inorganic materials including fillers like silica and alumina, silanized fragments and acrylate monomers on wear and damage in order to actuate certain cytotoxic and allergic responses. (Benson, P. 63) A certain type of glass ionomer cement after being freshly mixed displayed cytotoxic character when analyzed by determining the counts of fibroblasts and macrophages. Its toxic nature was also manifested by assaying the changes in the staining kinetic and levels of enzyme. (Cell Biochemistry & Function, 1983) Due to the suspected toxicity of almost all kinds of dental restoratives like, resinous composites, glass ionomers, casting alloys and ceramics, bio-compatibility is considered to be a prime most attribute of effective dental formulations. Low polymerization shrinkage ensuring greater adhesion of the formulation to the restored cavity is one of the necessary attributes of a good prosthesis. The nanofiber reinforced Hybrane modified acrylate resin matrix of the composite has a greater compatibility in restoration, thereby exhibiting lower polymerization shrinkage. This attribute along with other notable characteristics like, abrasion resistance, surface hardness, low moisture absorbing ability, moderately low coefficient of thermal expansion and lower viscosity before application, definitely ensures much lesser chance of stress induced damage for composites, where the conventional Bisphenol-A-glycidylmethacrylate (Bis-GMA) monomers originally used for the matrix are replaced by nanoscaled silesquioxane methacrylate monomers (POSS-MA) that are polyhedral and oligomeric by nature. (Dental Materials, 2005) However in this investigation the dental composites that are to be used for nanofiber reinforcement are composed of conventional formulations using Bis-GMA and Triethylenglycoldimethacrylate (TEGDMA) acrylate resins for matrices. With regard to the literature from background research, this will specifically be done to carefully study the reinforcing effects of electrospun nanofibers on the formulations without the interrupting influence of the POSS-MA matrix for improving the mechanical strength of the composites. Such a conventional acrylate resin matrix will provide us with a unique opportunity to record the extent of performance enhancement, if any, solely due to the nanofiber reinforcements. We also plan to incorporate hyperbranched polyesteramide moiety into the Bis-GMA and TEGDMA matrices when fabricating the dental composites as per the past researches on high mechanical withstanding power and performance index of reinforced acrylate based composites and dental adhesives, formulated with such commercialized hyperbranched polymers. (Journal of Adhesion Science and Technology, 2004) The hyperbranched polymers when added to the Bis-GMA, HEMA and TEGDMA matrices filled with silica based inorganic fillers, not only enhanced the compressive strength and lowered the linear shrinkage of the composites, but also significantly improved the adhesion of the restorative to the dental surfaces. Similar researches were carried out with carbon nanofibers for bone prosthetic implants, where its adhesive nature was found to be pronounced with osteoblasts or the bone forming cells, compared to the fibroblasts, chondrocytes and smooth muscle cells. (Materials Research Society, 2003) Further evidences indicated that the high degree of surface roughness of the nanophase carbon fibers is responsible for such an increased tendency for adhesion to the osteoblasts and similar cell surfaces. Finally in the conclusion of this review, let us defend our reason to use the technology of electrospinning to synthesize the reinforcing nanofibers mainly from Poly Vinyl Alcohol (PVOH) and Poly-L-Lactide Acid (PLLA). Researches indicate that the morphology and the characteristics of Nylon 6 nanofibers processed by electrospinning technology as analyzed by Scanning Electron Microscopy revealed a positive correlation between the density of entangled chains and its molecular mass. This definitely signifies that substantial quantity of nanofibers can be processed from lower concentrations of starting polymers by electrospinning procedure. (Journal of Applied Polymeric Science, 1959) There have been recent developments in the novel electrospinning technology that is helping researchers to come up with ultra fine and continuous nanofibers with a wide range of diameters and large surface area to facilitate adhesion to the cell surfaces and act as scaffolding systems in biomedical engineering. (Applied Biochemistry and Biotechnology, 2005) We will likewise attempt to use electrospun nanofibers from PVOH and PLLA solutions with standardized fiber diameters required to fabricate the dental composite. ADDRESSING THE RESEARCH QUESTIONS: The key question that actuates our investigation formulated itself from a resource full of unaccounted evidences and hypothesized conclusions drawn from past researches on the morphology, attributes and mechanism of polyacrylate resin based dental composites, spanning over a decade. Throughout our whole course of study we will attempt to address the three central facts that are the essential triggers of our research. The first question is whether these novel electrospun PVOH and PLLA nanofibers from organic polymers can at all significantly reinforce the polyesteramide modified Bis-GMA and TEGDMA dental composites. This is in fact an analytic rather than a descriptive study as our attempts will be to design experiments on measuring the extent of reinforcement of the composites by nanofiber incorporation and test the standard hypothesis dealing with the subsequent performance enhancement of those dental nanocomposites. Our next obvious concern will be to record the exact concentration and diameter of the nanoparticles at which, they really influence the mechanical strength and performance of these dental composites. We intend to provide a solution to this question by following the lines of previous researches attempted to determine the optimal mass fractions at which the electrospun nanofiber impregnations proved promising for reinforcing the composites. (Polymer, 2007) This is to be done by applying various concentration ranges (by weight percents) of polymeric PVOH and PLLA solutions for electrospinning the desired nanofibers in order to fabricate the polyacrylate composites. The final challenge of the investigation is to gather conclusive evidences to support the hypothesis regarding the exact nature and mechanism of adhesion of the reinforcing fibers to the matrix of the composite. It has also been hypothesized that the hyperbranched polyesteramide moieties modify the acrylate resin of the matrix by entering into a kind of nanophase and restrict the motion of the acrylate framework, thereby ensuring greater mechanical strength and lower polymerization shrinkage. We intend to study the extent of mechanical enhancement of the composites modified by these hyperbranched polymers in order to provide some evidential support to the hypothesis. RAISING THE ANALYTIC HYPOTHESES: There is a central idea that is governing not only our own investigation but also a number of parallel endeavors, is supported by a platform of thought, reinforced by past studies with dental formulations. The polyacrylate frameworks modified with various commercialized forms of hyperbranched organic polymers or nanofillers or even organic-inorganic hybrid moieties together constitute ideal dental composites for detailed analyses. Investigations have been carried out in two distinct fields one of which is dependable on the other. One school concentrated on the idea of reinforcing the polyacrylate resinous matrices with nanofillers and observed the effect of the impregnation on the overall compressive strength of the composites. The other focused on the use of hyperbranched polymers to restrict the motion of the acrylate framework and thereby increase the mechanical strength and lower the polymerization shrinkage of the composites. Our research tries to unite the propositions developed by the two schools and finally establish a conclusive evidential point, in other words, conceptualize or if not, at least hypothesize that electrospun nanofibers from organic polymers do have the potency to increase the overall mechanical strength of polyacrylate dental composites fabricated with hyperbranched supporting moieties. We plan to fulfill our objective by devising experiments that will separately and collectively signify the importance of hyperbranched polymeric modifications and electrospun nanofiber impregnations in reinforcing the polyacrylate composites. MATERIALS AND METHODS: Let us enumerate the materials needed in our investigation along with the methodologies that are to be applied at the various levels of the work in order to conduct the required analysis for understanding the effect of electrospun nanofibers of fabricated composites. The dental formulations are to be done using the standard Bis-GMA and TEGDMA matrices modified with Hybrane, a hyperbranched polyesteramide suspension by DSM adjusted at 0.3 % concentration by weight. The nanofibers will be electrospun from three different of organic polymers: 1) Poly Vinyl Hydroxide (PVOH), 2) Poly-L-Lactide Acid (PLLA) and 3) Polycaprolactam or Polyamide or Nylon 6 (PA6). The polymeric compounds for generating the ultra fine nanofibers by the electrospinning procedure will be chosen based on two different parameters, namely, their biochemical compatibility and the presence suitable functional groups to react with the Hydroxyl groups and other basic side chains of the acrylate resin in order to effectively anchor with the matrix for effectively reinforcing the composites. The electrospinning procedure will be organized in a Biomedical Device Laboratory using the usual closed chamber fitted with a vacuum pump for adjusting the changes in the atmospheric pressure during the controlled spinning procedure. A Harvard Infusion Pump will be fitted with a syringed device for controlling the solution flow into the chamber. The fluid will be charged by using a Gamma source model functioning at a high voltage. De-ionized water and methyl chloride are to be used as solvents for the spinning of PVOH and PLLA. The spinning process is to be repeated a multiple number of times in order to produce appreciable quantity of ultra fine nanofibers for the composite fabrication and the subsequent testing sessions. Intermittently High Resolution Scanning Electron Microscopy (HRSEM) can be performed to assess the topography and the detailed morphology of the nanofibers for reinforcing the composites. The nanofibers used for matrix impregnation must undergo surface treatment with γ-Methacryloxypropyl-tri-methoxy-silane maintained in 4:1 ethanol-water solution (by weight) and subsequent drying at 1200C for 30 minutes. This procedure will enhance their adhesion to the matrix for fabricating the composites. The components that are to be used for fabricating the polyacrylate resin matrix are tabulated below: Serial Number Component Used Given Chemical Composition 1 Organic matrix Monomeric or Oligomeric Acrylate Derivatives 2 Photosensitizers used for polymerizing acrylate resin Camphorquinone (CQ), Ethyl-4-dimethylaminobenzoate (EDB), etc 3 Polymerization Initiators Tertiary amine, Benzoyl peroxide, etc 4 Dendritic modifiers Hybrane, DSM or Polyesteramide After properly fabricating and treating the dental composites by standard procedures the subsequent tests will be performed for assessing their compressive strength (CS), viscosity prior to the curing process, linear shrinkage (LS) during polymerization, diametric tensile strength (DTS), flexural strength (FS) and overall biochemical compatibility of the Hybrane, DSM modified polyacrylate resin matrix with varying concentrations of nanofibers electrospun from PVOH, PLLA and PA6 polymers. Three different groups will be organized based on the respective impregnations of PVOH, PLLA and PA6 nanofibers into the polyacrylate resin matrix forming three types of reinforced dental composites. A corresponding control group will be maintained for each of the categories of the composites for comparing the results obtained from the standardized tests performed to assess the overall mechanical strength and the performance index of the dental composites. The following table indicates the details of the methods that will be employed for assaying the performance index of the nanocomposites: Serial Number Property Assayed Equipment Used 1 Compressive Strength (CS) Lloyd Testing Machine 2 Flexural Strength (FS) Lloyd Testing Machine 3 Linear Shrinkage (LS) Glass Capillary Tube & Optical Microscope 4 3D- Topography of Dental Composites Atomic Force Microscopy (AFM) 5 Internal Structure of Dental Composites High Resolution Scanning Electron Microscope (HRSEM) The compressive strength (CS) is to be measured using a Lloyd Testing Machine with a 1.0 mm/min crosshead speed. Before subjected to the testing procedure the reinforced polyacrylate resin matrix and the cross-linking agent are to be stored separately at a temperature of 40C. After sufficient storage the two components are to be mixed for polymerization in cylindrical Teflon made mold having 8.01 nm length and 4.0 0.1 nm diameter. After being subjected to polymerization the components will be maintained at room temperature for an hour and then will be kept under water at approximately 370C for a day. After the total procedure they are to be measured accordingly for the required data. The assay for flexural strength (FS) will be carried out using rectangular shaped Teflon molds having 25 x 2 x 2 mm dimension. The components will be first stored at ambient temperature for an hour and then at 370C for a day before being subjected to the testing procedure. A glass capillary tube of 4.15 mm diameter and an optical microscope of x10 magnification capacity are needed for measuring the linear shrinkage (LS) during the polymerization and then the components will be immersed in water at a temperature around 370C for a day. After each and every testing procedure Atomic Force Microscopy and High Resolution Scanning Electron Microscopy will be carried out to assess the respective surface and internal topography of the reinforced composites. PROBABLE LIMITATIONS: In spite of the organized lay out of the series of assays to be performed there are certain possible limitations in the methodology of the overall investigation procedure. The first and foremost problem that we expect to face in our work is with the standardization of all the devices that are to be used in the assay. There can even be problems in fabricating and maintaining the reinforced composites in a proper way by preventing unwanted damages, etches and discrepancies in the formulation when preparing them for the subsequent testing procedure. The next momentous problem can arise during correlating the readings obtained for a series of different assays performed on the various categories of both the modified and controlled composites. The most obvious uncertainty that is bound to be associated with the investigation is regarding the investigation of the nature and extent of interaction of the electrospun nanofibers with the resin matrix in the composites for theorizing the possible mechanistic reasons behind the reinforcement and scaffold. References: 1. Williams, David. and Jong, Wim. De. (2007). The safety of dental amalgam and alternative dental restoration materials for patients and users. Scientific Committee on Emerging and Newly Identified Health Risks, available at: http://ec.europa.eu/health/ph_risk/committees/04_scenihr/docs/scenihr_o_011.pdf (accessed on November 24, 2008) 2. Harrison, A. (1977). Effect of packing pressure on the abrasion resistance of dental amalgams. Journal of Dental Research, 56(6), 613 - 5. 3. Meryon, S.D. and Browne, R.M. (1983). In vitro cytotoxicity of a glass ionomer cement of a new generation. Cell Biochemistry and Function, 2, 43 - 48. 4. James S. 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