Abstract
Objective
Conventional dental composites with randomly dispersed inorganic particles within a polymer matrix fail to recapitulate the aligned and anisotropic structure of the dentin and enamel. The aim of the study was to produce a biomimetic composite consisting of a ceramic preform with graded and continuously aligned open pores, infiltrated with epoxy resin.
Methods
The freeze casting technique was used to obtain the hierarchically structured architecture of the ceramic preforms. Optical and scanning electron microscopy (SEM) and differential thermal analysis and thermogravimetry (TG–DTA) were used to characterize the samples. Three point bending test and compression test were also performed.
Results
All analysis confirmed that the biomimetic composite was characterized by a multi-level hierarchical structure along the freezing direction. In the bottom layers close to the cooling plate (up to 2 mm thick), a randomly packed ceramic with closed pores were formed, which resulted in incomplete infiltration with resin and resultant poor mechanical proprieties of the composite. Above 2 mm, all ceramic samples showed an aligned structure with an increasing lamellae spacing (wavelength) and a decreasing wall thickness. Mechanical tests showed that the properties of the composites made from ceramic preforms above 2 mm from cooling plate are similar to those of the dentin.
Significance
The fabrication processing reported in this work offers a viable route for the fabrication of biomimetic composites, which could be potentially used in a range of dental restorations to compete with the current dental composites and ceramics.
1
Introduction
Enamel is the highest mineralized tissue of the body and is composed in 96% of hydroxyapatite crystallites, surrounded by an organic matrix, mainly tyrosine-rich amelogenin protein (trap). Its basic structure, the enamel rod, is characterized by a main body of about 5 μm but, its diameter increases toward the enamel surface. The dentin, on the contrary, is composed of 70% hydroxyapatite (by weight), 20% of organic material, and 10% of water. It is characterized by closely packed tubules that run across its entire thickness, each containing a cytoplasmatic extension of odontoblasts. They follow a S-shaped course and are directed occlusally . The diameter of the tubules decreases from the pulpal end (>2.0 μm) at the enamel–dentin junction (<0.5 μm) . The density of the tubules changes with the age (it is higher in younger subjects) and along the thickness of the dentin: at the pulpal surface it is about 50,000 per mm 2 but at the dentin-enamel junction it decreases to 20,000 per mm 2 .
The microstructure of both dentin and enamel is characteristic for its anisotropy due to its aligned architecture . Anisotropy is an important feature not only for aesthetic and light propagation, but also for mechanical properties, like elastic modulus ( E ), shear modulus ( G ) and Poisson’s ratio .
The ultimate goal of biomimetic research in restorative dentistry is to produce synthetic materials that mimic both microstructure and mechanical performance of the dentinal tissues .
For more than 100 years, dental ceramics have been used for indirect restorations such as crown and bridges . Although a natural tooth appearance, they possess much higher elastic modulus ( E ) than that of enamel (e.g. zirconia and alumina, E : 200–380 GPa; enamel, E : 20–84 GPa). In addition, they are abrasive to the opposing dentition due to their high hardness, producing sensitivity and occlusal imbalance . Furthermore, chipping is a major problem with ceramic restorations owing to their brittleness, and repairs are usually carried out with resin composite materials .
Composite materials are used in more than 95% of all anterior tooth direct restorations and in about 50% of all posterior tooth direct restorations . They are characterized by mechanical properties similar to the dentin . However, despite extensive efforts, current dental composites and ceramics tend to suffer from common limitations: these materials are both isotropic and unable to mimic the aligned tissue structure of the dentine and enamel ( Fig. 1 (A)) . Their inorganic filler particles are characterized by a random arrangement in the organic polymer matrix of composites.
The aim of this study was to produce a biomimetic composite for possible indirect dental restorations, which are characterized by a multi-level hierarchical structure of its inorganic component, infiltrated with organic resin ( Fig. 1 (B)). The advantages of this new material could be similar to those of resin composites, i.e., a less invasive tooth preparation, without high level of abrasion on antagonizing teeth, the possibility to repair alteration with the same resin from which it is made and the chemically compatibility with adhesive resin cements . Furthermore the new biomimetic composite will be characterized by the same anisotropy of the natural dentin.
The porous ceramic preforms with graded lamellar structures were fabricated using a double cooling freeze casting technique, which is a versatile processing technique that can be used to fabricate porous lamellar ceramics with complex hierarchical structures. This complex process involves a large number of processing parameters which provide unique means to manipulate the architecture of freeze-cast materials at multiple length scales from nanometres to millimetres in a single processing step .
2
Materials and methods
2.1
Slurry preparation
Alumina powder used in this work was composed of 20% of Ronaflair White Sapphire (Merck Performance Materials, USA) and 80% alumina (Almatis AC, Inc. USA). RonaFlair ® white sapphire aluminium oxide powder characterized by a unique platelet-like morphology and particle size of <16 μm has been used to induce possible epitaxial grain growth during ceramic sintering. Alumina powder (10 vol%) was mixed with 0.60 wt% of dispersant (Dolapix CE 64, Zschimmer & Schwarz GmbH, Germany) in distilled water. 2 wt% polyvinyl alcohol (PVA, Sigma–Aldrich) was used as a binder. The slurry was ball-milled in a zirconia media for about 48 h and then balls were removed.
2.2
Freeze casting
The process for freeze casting was carried out as previously described . Briefly, freezing of the slurry was accomplished by pouring it into a teflon mold (diameter: 60 mm, height: 65 mm) and cooled using a custom built freezing setup. The mold was placed between two copper rods that were cooled by liquid nitrogen and cold finger, respectively. Band heaters were attached to the copper rods in order to control the cooling rate and temperature gradient. The cooling rate was set to 10 °C/min from 24 °C to 0 °C and 2 °C/min from 0 °C to −10 °C. Then the samples were freeze-dried in a Modulyo Freeze Dryer (Edwards, UK) for 24 h at −55 °C and 0.1 mbar. Thereafter the green bodies were carefully removed from molds and placed in a laboratory chamber furnace (Model BRF17/4M, Elite Thermal Systems Ltd., UK). The sintering rate of temperature was 2 °C/min from 25 °C to 600 °C and 10 °C/min from 600 °C to 1600 °C for 2 h. In order to allow the passive cooling, the samples were removed from the furnace after 24 h.
2.3
Infiltration of epoxy resin
A self-contained vacuum impregnation system (Cast N’Vac Castable Vacuum System, BUEHLER, USA) was used to backfill samples with an epoxy resin (Specfix, Struers, UK) containing 0.5% of methylene blue dye for better imaging contrast. The trapped air was first removed from the freeze cast ceramic preform, the epoxy resin was then infiltrated into the pores of ceramic. Curing was carried out at 40 °C for 24 h.
2.4
Cutting of samples
An Accutom-50 (Struers, UK) precision cutting machine was used to prepare the samples. For optical and scanning electron microscope (SEM) observations and TG–DTA analysis, samples were cut at every 1 mm in seven different positions along the freezing direction.
2.5
Polishing of samples
A grinding machine (Tegra Pol 15, Struers, UK) and three different types of SiC papers (p80, p1200 and p2400, Struers) were used under water cooling at a speed of 600 rpm for 30 s each with a pressure of 10 N. The ceramic samples were then sonicated in a U300 ultrasonic water bath (Ultrawave Ltd., UK) in distilled water and air-dried.
2.6
Samples characterizations
For qualitative optical and SEM evaluation, representative specimens of 1 mm × 1 mm × 1 mm were observed. Optical microscope quantification of the cross sections was performed at 4× magnification objective lens with a Nikon Eclipse E600 microscope. The images captured with a digital camera coupled with the microscope were analyzed using an ImageJ 1.34s image-analysis software (National Institute of Health, USA). The software was used to measure and calculate mean values of lamellae spacing (wavelength), wall thickness and the resin/ceramic ratio of the samples.
The SEM observation was performed with a Phenom Desktop scanning electron microscope (FEI) at 5 kV (Phenom-World BV, The Netherlands).
2.7
Differential thermal analysis and thermogravimetry (TG–DTA)
TG/DTA analysis was performed on 5 different samples of 1 mm spacing in freezing direction to measure the volume fraction of resin in each layer. The samples size was 1 mm × 1 mm × 1 mm.
Prior to TG/DTA analysis, a double weighing with an electronic balance (mod. E42, Gibertini srl Milano, Italy) and a TG/DTA scale was made, then all samples underwent TG–DTA cycles.
Simultaneous thermal analyzer was used to measure the mass change and heat effects (TG–DTA) of composites using TG/DTA 6300 (Model TG/DTA 6300, Seiko Instruments Inc. Torrance, CA, USA). As previously described for dental composites , samples were heated at a constant rate of 10 °C/min, from 25 to 450 °C under nitrogen atmosphere (flow rate: 100 mL/min). At the end of the thermal cycle the residual weight of the samples were recorded and analysed.
2.8
Three point bending test
The preparation of the three point bending test specimens was carried out according to EN ISO 4049 . The size of specimen was (25 ± 2) mm × (2 ± 0.1) mm × (2 ± 0.1) mm.
Two groups of samples were obtained from different positions of the freeze-cast ceramic/polymer composites:
- –
Specimens obtained from 0 to 2 mm above the cooling plate.
- –
Specimens obtained from 3 to 5 mm above the cooling plate.
The three point bending test was performed at room temperature with a universal testing machine (Lloyd LRX, UK) at a crosshead speed of 1 mm/min and a distance between supports of 20 mm. The modulus of elasticity and the flexural strength were calculated as previously described .
2.9
Compression test
Five bar specimens from the bottom and five from the top of the sample (1.75 mm × 1.75 mm cross sectional area × 3.50 mm height) were prepared by cutting the ceramic/polymer composites using an Accutom-50 and then polished. A testing machine (Lloyd LRX, UK) was used at a crosshead speed of 1 mm/min. Compressive strength was calculated by dividing the failure load ( F m ) by the cross-sectional area. The mechanical properties were determined according to the EN ISO 604:2003 standard .
2
Materials and methods
2.1
Slurry preparation
Alumina powder used in this work was composed of 20% of Ronaflair White Sapphire (Merck Performance Materials, USA) and 80% alumina (Almatis AC, Inc. USA). RonaFlair ® white sapphire aluminium oxide powder characterized by a unique platelet-like morphology and particle size of <16 μm has been used to induce possible epitaxial grain growth during ceramic sintering. Alumina powder (10 vol%) was mixed with 0.60 wt% of dispersant (Dolapix CE 64, Zschimmer & Schwarz GmbH, Germany) in distilled water. 2 wt% polyvinyl alcohol (PVA, Sigma–Aldrich) was used as a binder. The slurry was ball-milled in a zirconia media for about 48 h and then balls were removed.
2.2
Freeze casting
The process for freeze casting was carried out as previously described . Briefly, freezing of the slurry was accomplished by pouring it into a teflon mold (diameter: 60 mm, height: 65 mm) and cooled using a custom built freezing setup. The mold was placed between two copper rods that were cooled by liquid nitrogen and cold finger, respectively. Band heaters were attached to the copper rods in order to control the cooling rate and temperature gradient. The cooling rate was set to 10 °C/min from 24 °C to 0 °C and 2 °C/min from 0 °C to −10 °C. Then the samples were freeze-dried in a Modulyo Freeze Dryer (Edwards, UK) for 24 h at −55 °C and 0.1 mbar. Thereafter the green bodies were carefully removed from molds and placed in a laboratory chamber furnace (Model BRF17/4M, Elite Thermal Systems Ltd., UK). The sintering rate of temperature was 2 °C/min from 25 °C to 600 °C and 10 °C/min from 600 °C to 1600 °C for 2 h. In order to allow the passive cooling, the samples were removed from the furnace after 24 h.
2.3
Infiltration of epoxy resin
A self-contained vacuum impregnation system (Cast N’Vac Castable Vacuum System, BUEHLER, USA) was used to backfill samples with an epoxy resin (Specfix, Struers, UK) containing 0.5% of methylene blue dye for better imaging contrast. The trapped air was first removed from the freeze cast ceramic preform, the epoxy resin was then infiltrated into the pores of ceramic. Curing was carried out at 40 °C for 24 h.
2.4
Cutting of samples
An Accutom-50 (Struers, UK) precision cutting machine was used to prepare the samples. For optical and scanning electron microscope (SEM) observations and TG–DTA analysis, samples were cut at every 1 mm in seven different positions along the freezing direction.
2.5
Polishing of samples
A grinding machine (Tegra Pol 15, Struers, UK) and three different types of SiC papers (p80, p1200 and p2400, Struers) were used under water cooling at a speed of 600 rpm for 30 s each with a pressure of 10 N. The ceramic samples were then sonicated in a U300 ultrasonic water bath (Ultrawave Ltd., UK) in distilled water and air-dried.
2.6
Samples characterizations
For qualitative optical and SEM evaluation, representative specimens of 1 mm × 1 mm × 1 mm were observed. Optical microscope quantification of the cross sections was performed at 4× magnification objective lens with a Nikon Eclipse E600 microscope. The images captured with a digital camera coupled with the microscope were analyzed using an ImageJ 1.34s image-analysis software (National Institute of Health, USA). The software was used to measure and calculate mean values of lamellae spacing (wavelength), wall thickness and the resin/ceramic ratio of the samples.
The SEM observation was performed with a Phenom Desktop scanning electron microscope (FEI) at 5 kV (Phenom-World BV, The Netherlands).
2.7
Differential thermal analysis and thermogravimetry (TG–DTA)
TG/DTA analysis was performed on 5 different samples of 1 mm spacing in freezing direction to measure the volume fraction of resin in each layer. The samples size was 1 mm × 1 mm × 1 mm.
Prior to TG/DTA analysis, a double weighing with an electronic balance (mod. E42, Gibertini srl Milano, Italy) and a TG/DTA scale was made, then all samples underwent TG–DTA cycles.
Simultaneous thermal analyzer was used to measure the mass change and heat effects (TG–DTA) of composites using TG/DTA 6300 (Model TG/DTA 6300, Seiko Instruments Inc. Torrance, CA, USA). As previously described for dental composites , samples were heated at a constant rate of 10 °C/min, from 25 to 450 °C under nitrogen atmosphere (flow rate: 100 mL/min). At the end of the thermal cycle the residual weight of the samples were recorded and analysed.
2.8
Three point bending test
The preparation of the three point bending test specimens was carried out according to EN ISO 4049 . The size of specimen was (25 ± 2) mm × (2 ± 0.1) mm × (2 ± 0.1) mm.
Two groups of samples were obtained from different positions of the freeze-cast ceramic/polymer composites:
- –
Specimens obtained from 0 to 2 mm above the cooling plate.
- –
Specimens obtained from 3 to 5 mm above the cooling plate.
The three point bending test was performed at room temperature with a universal testing machine (Lloyd LRX, UK) at a crosshead speed of 1 mm/min and a distance between supports of 20 mm. The modulus of elasticity and the flexural strength were calculated as previously described .
2.9
Compression test
Five bar specimens from the bottom and five from the top of the sample (1.75 mm × 1.75 mm cross sectional area × 3.50 mm height) were prepared by cutting the ceramic/polymer composites using an Accutom-50 and then polished. A testing machine (Lloyd LRX, UK) was used at a crosshead speed of 1 mm/min. Compressive strength was calculated by dividing the failure load ( F m ) by the cross-sectional area. The mechanical properties were determined according to the EN ISO 604:2003 standard .
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