Highlights
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Laser-sintering of thin glass-ceramic films on enamel was successfully achieved.
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Bonding between enamel and the glass-ceramic coating was proved.
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Method holds great potential to protect susceptible teeth against caries and erosion.
Abstract
Objectives
The established method of fissure-sealing using polymeric coating materials exhibits limitations on the long-term. Here, we present a novel technique with the potential to protect susceptible teeth against caries and erosion. We hypothesized that a tailored glass-ceramic material could be sprayed onto enamel-like substrates to create superior adhesion properties after sintering by a CO 2 laser beam.
Methods
A powdered dental glass-ceramic material from the system SiO 2 -Na 2 O-K 2 O-CaO-Al 2 O 3 -MgO was adjusted with individual properties suitable for a spray coating process. The material was characterized using X-ray fluorescence analysis (XRF), heating microscopy, dilatometry, scanning electron microscopy (SEM), grain size analysis, biaxial flexural strength measurements, fourier transform infrared spectroscopy (FTIR), and gas pycnometry. Three different groups of samples (each n = 10) where prepared: Group A, powder pressed glass-ceramic coating material; Group B, sintered hydroxyapatite specimens; and Group C, enamel specimens (prepared from bovine teeth). Group B and C where spray coated with glass-ceramic powder. All specimens were heat treated using a CO 2 laser beam process. Cross-sections of the laser-sintered specimens were analyzed using laser scanning microscopy (LSM), energy dispersive X-ray analysis (EDX), and SEM.
Results
The developed glass-ceramic material (grain size d50 = 13.1 mm, coefficient of thermal expansion (CTE) = 13.3 10 −6 /K) could be spray coated on all tested substrates (mean thickness = 160 μm). FTIR analysis confirmed an absorption of the laser energy up to 95%. The powdered glass-ceramic material was successfully densely sintered in all sample groups. The coating interface investigation by SEM and EDX proved atomic diffusion and adhesion of the glass-ceramic material to hydroxyapatite and to dental enamel.
Significance
A glass-ceramic material with suitable absorption properties was successfully sprayed and laser-sintered in thin films on hydroxyapatite as well as on bovine enamel. The presented novel technique of tooth coating with a dental glass-ceramic using a CO 2 -laser holds a great potential as a possible method to protect susceptible teeth against caries and erosion.
1
Introduction
Tooth-preserving preventative measures such as composite-based fissure sealing materials are moderately effective. They have been shown to offer effective protection against caries in the first 48 months, however beyond that period it is also clear that this protection is no longer effective . In fact, the failure rate is 30–55% after 5 years . A reason for the failure rate is an insufficient adhesive bonding between the filling material and the tooth surface. Therefore tooth decay can often form below the sealing material itself. In addition to tooth decay, an increase of another type of enamel loss is observed. Erosion, an attribute of acid-based attacks, can cause pronounced destruction of tooth enamel (demineralization), which is exacerbated by mechanical influences such as tooth brushing . In addition to composite-based fissure sealing materials there are also fluoride-based varnishes which help prevent diseases of tooth enamel. This method was developed in the 1960s and has been in application as a successful prevention measure ever since . Recent publications indicate that bioglasses also show great potential in effective protection against caries . Through ion release, the potential of glasses as fillers in tooth-coating materials may also contribute to the prevention of tooth enamel demineralization . Even if it is known that dental glass-ceramics can show corrosion and surface degradation in saliva, glasses are able to withstand low and high pH buffer solutions fulfill the required ISO standards 6872 for dental ceramics, and moreover show lower substance loss compared to human tooth enamel .
For any intended use, a glass-ceramic powder needs to be geometrically formed before it is further compacted by a sintering process. Typical ceramic handling processes are pressing, molding, and casting . Spray coating is also a promising option, where glass-ceramic slurries can be finely sprayed through a nozzle and used to produce thin coating layers. For example, it has been reported that via spray coating defined glass coating green layers for dental implants were produced in order to achieve sintered thin films which were adequately connected in the interface .
In clinical applications as well as in dental patient care the implementation of laser technology is developing rapidly. However, while specific laser applications are efficient and produce successful outcomes, the adjustable laser parameters need to be set for each use in other application fields. In dental patient care it has been shown that with an appropriate set of parameters it is possible with a CO 2 laser beam treatment can lead to decrease enamel caries progression by 81% without causing surface and subsurface damage . To achieve the desired physiochemical and photo-optical reactions it is important to set all laser parameters according to the expected material behavior. For example, in order to receive a densely sintered glass-ceramic material made of a powdered glass, a variation of specifically adjusted laser parameters must be set. It was shown that it is in principle possible to densify a dental porcelain powder by using a laser beam, however phase transformation in densified glass-ceramic bodies can cause undesirable side effects .
An even more challenging goal is to densify a dental porcelain powder using the same technique in order to produce a protective film. In the case of material setting prior to a spray- and laser-coating process, some important variables such as the particle size distribution, the material- and wavelength-dependent light absorption, the sintering temperature, as well as the material-dependent thermal expansion of the layer have to be considered. Furthermore, laser-dependent process variables such as energy input or the chosen scanning strategy should be considered in particular before laser processing, in order to yield a controlled and optimized sintering application. Investigations have shown that pore formation is strongly dependent on the chosen laser beam input as well as pore size distribution prior to laser treatment . With combined laser sources some manufacturing challenges such as solidification cracking due to large temperature gradients can be avoided . But it should be clear that any heat treatment or prior material setting, such as the phase setting of fine-grained and dispersed leucite crystals can transform or change into a leucite-free zone in the glass matrix . Aspects of material processing and phase transformations could result in changes in material properties after sintering. The temperature reached strongly depends on the energy of the laser beam and the material absorption. To reach proper temperatures for an optimized sintering process both variables need to be precisely chosen and iteratively adapted.
The aim of this study was to adjust a powdered glass-ceramic material suitable for a spray coating and a subsequent laser-sintering process in order to achieve thin and densified protective layers on hydroxyapatite and enamel substrates. Therefore, a customized glass frit was synthesized and characterized with regard to particle size distribution, material and wavelength-dependent light absorption, and material-dependent thermal expansion. To investigate boundary conditions, the laser beam inputs were calculated, the temperatures reached during laser sintering were measured, and imaging methods were examined and compared. We hypothesized that we could produce densified thin and adherent films on enamel-like substrates by laser-sintering a powdered glass-ceramic material as a novel preventive technique for treating teeth susceptible to caries and erosion.
2
Materials and methods
2.1
Preparation of the glass-ceramic powder
The stoichiometrically weighed and mixed glass batch composition was (w/w%): 54.04% SiO 2 , 3.74%, Al 2 O 3 , 7.01% K 2 CO 3 , 7.42% CaO, 27.37% Na 2 CO 3 , and 0.42% MgO. The glass batch was heated in a platinum crucible in an electric furnace (Therm-AIX, Typ 312476, Aachen, Germany) at 10 °C/min to 1500 °C (4 h hold) and quenched in 10 l of distilled water. The frit was dried in a compartment dryer (VWR, DL 53 DRY-Line, Langenfeld, Germany) at 110 °C for 12 h and ground with a planet mill (Pulverisette6, FRITSCH, Idar-Oberstein, Germany) for 90 min at a rotation speed of 300 rpm. ZrO 2 balls with diameters of 20 and 10 mm were used for the pulverization process and the glass was subsequently sieved with a mash size of 125 μm. The glass was heat-treated again in the furnace from 23 °C at a rate of 10 °C/min to 650 °C (1 h hold), then ramped to 1120 °C (1 h hold) and air-quenched. After the frit was ground and sieved again under the same conditions it was ready to use.
2.2
Characterization of the glass-ceramic powder
To control the stoichiometrically calculated composition before and after the heat treatments, X-ray fluorescence analysis (PW2405, Philips, Almelo, NL) was used. In order to determine its fusibility and characteristic temperatures in accordance to international standards (DIN 51730:2007, ISO 540), the frit was investigated with a heating microscope (Hesse Instruments, Osterode am Harz, Germany). To characterize a further material-specific property, the coefficient of thermal expansion (CTE) was measured with a dilatometric device (DIL 402 E, NETZSCH-Gerätebau, Selb, Germany) in the range of 50–500 °C within a sapphire calibration (10 K/min). This value is given by the ratio of the change in length as a function of the temperature and is important in coating behaviors. Further investigations were made by SEM (FEI ESEM XL30 FEG, Philips, Eindhoven, NL), grain size analysis (Mastersizer 2000, Malvern, Worcestershire, UK) and gas pycnometry (Upyc 1200e, QUANTACHROME, Odelzhausen, DE) to characterize figurative and quantitative properties of the powdered material as well as the bulk density. A FTIR device (FTIR- Spektrometer, PerkinElmer LAS, Rodgau Jügesheim, Germany) was used to investigate optical properties in order to calculate the percental absorption of the laser beam.
2.3
Specimen fabrication and preparation
Group A specimens were powder pressed tablets compressed by a hydraulic pump (MP 150, Maassen, Reutlingen, Germany) at 3 tons (approx. 2.5 kg/cm 2 ). The powder of each sample was first wet with one droplet (approx. 50 μl) of deionized water. A stainless steel cylindrical mold (d = 12 mm) was filled with 0.3 g glass ceramic powder and uniaxially compacted for one minute in order to reach constant pressure. After compressing the specimens, the tablets remained for at least 24 h before sintering. Group B specimens were hydroxyapatite tablets with the same weight of 0.3 g which were compressed with the same hydraulic and the same steel cylindrical mold for the same time. As a binder a droplet (approx. 50 μl) of 50% H 2 O, 35% PAF35Optapix and 15% DolapixCE64 was used. The compressed specimens were heated by an electric furnace (Therm-AIX, Typ 312476, Aachen, Germany) at 10 °C/min to 150 °C (30 min hold), then ramped to 1150 °C (4 h hold), then ramped to 300 °C (30 min hold) and cooled down to 23 °C. Afterwards the samples were dry-handled on 4000 grade (grit) paper. Group C specimens were tooth samples (extracted bovine incisors) prepared as previously reported . Shortly after extraction and root removal, the samples were cut out of the vestibular anterior tooth, prepared in standardized samples of 4 × 5 × 3 mm and stored in 0.1% thymol solution (pH 7, at 4 °C). The enamel surfaces were prepared stepwise on 800, 2500, and 4000 grade abrasive papers. Groups B and C were additionally spray coated with the glass-ceramic powder prepared earlier. Therefore, 70 g of powdered glass-ceramic were mixed with 120 ml distilled water and 0.5 g binder (Kukident Super-Haftpulver, Reckitt Benckiser, Heidelberg, Germany) to receive a slurry for the spray coating process. The glass ceramic slurry was continuously mixed during an atomized and controlled spray coating process with a 0.8 mm nozzle (Mod.970/5, Schlick, Untersiemau, Germany). The nozzle was operated with a continuous pressure of 1 bar and a table track spray distance of 6.5 cm which ensured a homogeneous spray mist over the samples.
2.4
Laser based sintering
All samples were irradiated using a CO 2 laser (Firestar V30, SYNRAD, Mukilteo, USA) (wavelength λ = 10.6 μm) operating in pulse mode with a maximum output power of 30 W. The laser beam was focused onto the surface (spot size of 0.63 mm in diameter). The sintering process was executed by multiple line scans and in all samples the last 2 mm were not irradiated in order to serve as a reference area. All groups were irradiated with the same set of parameters including a scan speed of 126 mm/s, a frequency of 2 kHz, and a pulse distance of d x = d y = 0.063 mm. During the laser sintering process the surface temperature was measured with a thermal imaging camera (Variotherm, Jenoptik, Jena, Germany).
2
Materials and methods
2.1
Preparation of the glass-ceramic powder
The stoichiometrically weighed and mixed glass batch composition was (w/w%): 54.04% SiO 2 , 3.74%, Al 2 O 3 , 7.01% K 2 CO 3 , 7.42% CaO, 27.37% Na 2 CO 3 , and 0.42% MgO. The glass batch was heated in a platinum crucible in an electric furnace (Therm-AIX, Typ 312476, Aachen, Germany) at 10 °C/min to 1500 °C (4 h hold) and quenched in 10 l of distilled water. The frit was dried in a compartment dryer (VWR, DL 53 DRY-Line, Langenfeld, Germany) at 110 °C for 12 h and ground with a planet mill (Pulverisette6, FRITSCH, Idar-Oberstein, Germany) for 90 min at a rotation speed of 300 rpm. ZrO 2 balls with diameters of 20 and 10 mm were used for the pulverization process and the glass was subsequently sieved with a mash size of 125 μm. The glass was heat-treated again in the furnace from 23 °C at a rate of 10 °C/min to 650 °C (1 h hold), then ramped to 1120 °C (1 h hold) and air-quenched. After the frit was ground and sieved again under the same conditions it was ready to use.
2.2
Characterization of the glass-ceramic powder
To control the stoichiometrically calculated composition before and after the heat treatments, X-ray fluorescence analysis (PW2405, Philips, Almelo, NL) was used. In order to determine its fusibility and characteristic temperatures in accordance to international standards (DIN 51730:2007, ISO 540), the frit was investigated with a heating microscope (Hesse Instruments, Osterode am Harz, Germany). To characterize a further material-specific property, the coefficient of thermal expansion (CTE) was measured with a dilatometric device (DIL 402 E, NETZSCH-Gerätebau, Selb, Germany) in the range of 50–500 °C within a sapphire calibration (10 K/min). This value is given by the ratio of the change in length as a function of the temperature and is important in coating behaviors. Further investigations were made by SEM (FEI ESEM XL30 FEG, Philips, Eindhoven, NL), grain size analysis (Mastersizer 2000, Malvern, Worcestershire, UK) and gas pycnometry (Upyc 1200e, QUANTACHROME, Odelzhausen, DE) to characterize figurative and quantitative properties of the powdered material as well as the bulk density. A FTIR device (FTIR- Spektrometer, PerkinElmer LAS, Rodgau Jügesheim, Germany) was used to investigate optical properties in order to calculate the percental absorption of the laser beam.
2.3
Specimen fabrication and preparation
Group A specimens were powder pressed tablets compressed by a hydraulic pump (MP 150, Maassen, Reutlingen, Germany) at 3 tons (approx. 2.5 kg/cm 2 ). The powder of each sample was first wet with one droplet (approx. 50 μl) of deionized water. A stainless steel cylindrical mold (d = 12 mm) was filled with 0.3 g glass ceramic powder and uniaxially compacted for one minute in order to reach constant pressure. After compressing the specimens, the tablets remained for at least 24 h before sintering. Group B specimens were hydroxyapatite tablets with the same weight of 0.3 g which were compressed with the same hydraulic and the same steel cylindrical mold for the same time. As a binder a droplet (approx. 50 μl) of 50% H 2 O, 35% PAF35Optapix and 15% DolapixCE64 was used. The compressed specimens were heated by an electric furnace (Therm-AIX, Typ 312476, Aachen, Germany) at 10 °C/min to 150 °C (30 min hold), then ramped to 1150 °C (4 h hold), then ramped to 300 °C (30 min hold) and cooled down to 23 °C. Afterwards the samples were dry-handled on 4000 grade (grit) paper. Group C specimens were tooth samples (extracted bovine incisors) prepared as previously reported . Shortly after extraction and root removal, the samples were cut out of the vestibular anterior tooth, prepared in standardized samples of 4 × 5 × 3 mm and stored in 0.1% thymol solution (pH 7, at 4 °C). The enamel surfaces were prepared stepwise on 800, 2500, and 4000 grade abrasive papers. Groups B and C were additionally spray coated with the glass-ceramic powder prepared earlier. Therefore, 70 g of powdered glass-ceramic were mixed with 120 ml distilled water and 0.5 g binder (Kukident Super-Haftpulver, Reckitt Benckiser, Heidelberg, Germany) to receive a slurry for the spray coating process. The glass ceramic slurry was continuously mixed during an atomized and controlled spray coating process with a 0.8 mm nozzle (Mod.970/5, Schlick, Untersiemau, Germany). The nozzle was operated with a continuous pressure of 1 bar and a table track spray distance of 6.5 cm which ensured a homogeneous spray mist over the samples.
2.4
Laser based sintering
All samples were irradiated using a CO 2 laser (Firestar V30, SYNRAD, Mukilteo, USA) (wavelength λ = 10.6 μm) operating in pulse mode with a maximum output power of 30 W. The laser beam was focused onto the surface (spot size of 0.63 mm in diameter). The sintering process was executed by multiple line scans and in all samples the last 2 mm were not irradiated in order to serve as a reference area. All groups were irradiated with the same set of parameters including a scan speed of 126 mm/s, a frequency of 2 kHz, and a pulse distance of d x = d y = 0.063 mm. During the laser sintering process the surface temperature was measured with a thermal imaging camera (Variotherm, Jenoptik, Jena, Germany).
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