Polymerization kinetics of experimental bioactive composites containing bioactive glass

Comparing ISO 4049 and Fourier-transform infrared spectroscopy for assessing ambient light sensitivity in dental resin composites

This study evaluated the sensitivity of dental resin composites to ambient light using the ISO 4049 standard and Fourier-transform infrared (FTIR) spectroscopy. It aimed to compare the working times measured by these two methods to assess FTIR's potential as an alternative to ISO 4049 for determining premature polymerization. Eight dental composites (three bulk-fill and five conventional) were exposed to simulated ambient light. Working time was assessed using the method described in ISO 4049 standard, which identifies premature polymerization via visually identifiable macroscopic changes. FTIR spectroscopy enabled an analysis of the progress of polymerization by measuring the degree of conversion of methacrylate C=C bonds. Working times were compared across composites and between methods, with statistical analyses performed using ANOVA and Pearson correlation. All composites met the minimum ISO 4049 working time of 60 s. However, significant differences in working times were noted between ISO 4049 and FTIR, with FTIR generally overestimating the working time. Pearson correlation indicated moderate alignment between the methods, particularly at higher working times, though FTIR was less sensitive to early polymerization changes compared to ISO 4049. In conclusion, FTIR spectroscopy shows potential for assessing ambient light sensitivity in dental resin composites, yet it lacks the sensitivity of ISO 4049 in detecting early-stage polymerization. This study underscores the need for standardized, sensitive methods in assessing resin composites' working times, especially as material compositions evolve in modern dentistry.

Emerging technologies for the evaluation of spatio-temporal polymerisation changes in flowable vs. sculptable dental resin-based composites

BACKGROUND

This study presents a novel multi-technique approach that integrates micro-CT and optical photothermal infrared spectroscopy (O-PTIR) to evaluate polymerisation differences, so-called spatio-temporal polymerisation properties, between flowable and sculptable dental resin-based composites.

METHODS

Ten commercially available dental composites were investigated, including flowable and sculptable counterparts from the same manufacturer. Eight parameters were evaluated: short-term polymerisation characteristics (degree of conversion after 5 min, maximum polymerisation rate, time to reach maximum polymerisation rate) was measured using ATR-FTIR with real-time monitoring; changes in the degree of conversion with depth were evaluated with O-PTIR, 3D visualisation of shrinkage patterns, overall volumetric shrinkage, depth-specific shrinkage, and porosity were measured using micro-CT; surface morphology with detailed measurements of elemental composition was characterised using SEM/EDX; light transmittance was analysed with a NIST-referenced spectrometer.

RESULTS

The study found that the increase in filler weight and volume ratio reduced the degree of conversion and polymerisation shrinkage, while moderately influencing the maximum polymerisation rates. The time to reach maximum polymerisation rates and light transmittance were not dependent on the filler amount. O-PTIR assessed a depth-dependent decrease in the degree of conversion for both composite types, with flowable composites generally showing a greater decrease in the degree of conversion than sculptable composites, except for bulk-fill composites. Micro-CT scans showed significantly higher flowable shrinkage values than their sculptable counterparts, highlighting the performance differences between the two types of composites.

CONCLUSIONS

The findings of this study have practical implications for the selection and use of dental composites. Flowable composites, despite their higher degrees of conversion and polymerisation rates, also exhibit higher volumetric shrinkage, which can be detrimental for clinical applications. The new measurement methods used in this study provide a comprehensive overview of the polymerisation behaviour of commercially available dental composites, offering valuable insights for material optimisation.

3D micro-CT and O-PTIR spectroscopy bring new understanding of the influence of filler content in dental resin composites

BACKGROUND

Dental resin composites' performance is intricately linked to their polymerisation shrinkage characteristics. This study compares polymerisation shrinkage using advanced 3D micro-computed tomography (micro-CT) and traditional 2D linear assessments. It delves into the crucial role of filler content on shrinkage and the degree of conversion in dental resin composites, providing valuable insights for the field.

METHODS

Five experimental dental composite materials were prepared with increasing filler contents (55-75 wt%) and analysed using either 3D micro-CT for volumetric shrinkage or a custom-designed linometer for 2D linear shrinkage. The degree of conversion was assessed using Optical Photothermal Infrared (O-PTIR) and Fourier-Transform Infrared (FTIR) spectroscopy. Light transmittance through a 2-mm layer was evaluated using a NIST-calibrated spectrometer. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) examined surface morphology and elemental distribution. Correlation between the investigated parameters was determined using Spearman correlation analyses.

RESULTS

The study found significant differences in polymerisation-related properties among different filler content categories, with volumetric shrinkage consistently demonstrating higher mean values than linear shrinkage across most groups. Volumetric shrinkage decreased with increasing curing depth, showing no direct correlation between filler content and shrinkage levels at different curing depths. The results highlighted a strong negative correlation between filler content and degree of conversion, volumetric and linear shrinkage, as well as maximum shrinkage rate. Light transmittance showed a moderate correlation with the filler content and a weak correlation with other tested parameters.

CONCLUSIONS

This study underscores the importance of considering both volumetric and linear shrinkage in the design and analysis of dental composite materials. The findings advocate optimising filler content to minimise shrinkage and enhance material performance. Integrating micro-CT and O-PTIR techniques offers novel insights into dental composites' polymerisation behaviour, providing a foundation for future research to develop materials with improved clinical outcomes.

The influence of copper-doped mesoporous bioactive nanospheres on the temperature rise during polymerization, polymer cross-linking density, monomer release and embryotoxicity of dental composites

OBJECTIVES

Composites with copper-doped mesoporous bioactive nanospheres (Cu-MBGN) were developed to prevent secondary caries by imparting antimicrobial and ion-releasing/remineralizing properties.

METHODS

Seven experimental composites containing 1, 5 or 10 wt% Cu-MBGN, the corresponding inert controls (silica) and bioactive controls (bioactive glass 45S5) were prepared. The temperature rise during light curing, cross-linking density by ethanol softening test, monomer elution and their potential adverse effects on the early development of zebrafish Danio rerio was investigated.

RESULTS

Materials combining Cu-MBGN and silica showed the highest resistance to ethanol softening, as did the bioactive controls. Cu-MBGN composites showed significant temperature rise and reached maximum temperature in the shortest time. Bisphenol A was not detected, while bis-GMA was found only in the control materials and TEGDMA in the eluates of all materials. There was no increase in zebrafish mortality and abnormality rates during exposure to the eluates of any of the materials.

CONCLUSIONS

The composite with 5 wt% Cu-MBGN combined with nanosilica fillers showed the lowest ethanol softening, indicating the polymer's highest durability and cross-linking density. Despite the TEGDMA released from all tested materials, no embryotoxic effect was observed.

Influence of the Bracket Material on the Post-Cure Degree of Conversion of Resin-Based Orthodontic Adhesive Systems

The aim of this study was to examine the influence of the orthodontic bracket material on the short-term and long-term post-cure development of the degree of conversion (DC) of resin-based orthodontic adhesive systems. Five commercially available materials characterized by different compositions and curing modes (light-curable or dual-curable) were tested under three different light curing conditions: without brackets (control group, CO), and in the presence of metal brackets (MB group) or ceramic brackets (CB group). Fourier-transform infrared spectroscopy was used to determine the post-cure DC development, both after "short-term" periods (2, 6, and 10 min) and "long-term" periods (1, 7, and 28 days). The short-term DC values ranged from 43.9% to 76.1%, and the long-term DC values were higher and ranged from 54.3% to 85.3%. The MB group demonstrated significantly lower short-term DC values compared to the CO and the CB groups, while the CB group had statistically similar or slightly lower DC values compared to the CO group. Long-term DC values in the MB and the CB groups were statistically lower or similar compared to the CO group, which depended on the post-cure time. The results indicated that the post-cure DC development was highly material-dependent and affected by the presence of different types of bracket material.

Synergistic effect of ion-releasing fillers on the remineralization and mechanical properties of resin-dentin bonding interfaces

In modern restorative dentistry, adhesive resin materials are vital for achieving minimally invasive, esthetic, and tooth-preserving restorations. However, exposed collagen fibers are found in the hybrid layer of the resin-dentin bonding interface due to incomplete resin penetration. As a result, the hybrid layer is susceptible to attack by internal and external factors such as hydrolysis and enzymatic degradation, and the durability of dentin bonding remains limited. Therefore, efforts have been made to improve the stability of the resin-dentin interface and achieve long-term clinical success. New ion-releasing adhesive resin materials are synthesized by introducing remineralizing ions such as calcium and phosphorus, which continuously release mineral ions into the bonding interface in resin-bonded restorations to achieve dentin biomimetic remineralization and improve bond durability. As an adhesive resin material capable of biomimetic mineralization, maintaining excellent bond strength and restoring the mechanical properties of demineralized dentin is the key to its function. This paper reviews whether ion-releasing dental adhesive materials can maintain the mechanical properties of the resin-dentin bonding interface by supplementing the various active ingredients required for dentin remineralization from three aspects: phosphate, silicate, and bioactive glass.

Influence of bioactive particles and onium salt on the physicochemical properties of experimental infiltrants

This study evaluated physicochemical properties of experimental infiltrants after addition of hydroxyapatite nanoparticles (HAp) or 58S bioactive glass (BAG) and diphenyliodonium hexafluorophosphate (DPI). The resin matrix was composed of TEGDMA/Bis-EMA (3:1), 0.5 mol% CQ, and 1 mol% EDAB. The blends received or not 0.5 mol% DPI and 10% wt BAG or HAp. Icon was used as commercial control. The groups were characterized by XRD, FT-IR spectrometry, and SEM before and after simulated body fluid (SBF) immersion for up to 7 days. Polymerization kinetics (n =3 ), water sorption and solubility (n=10), and viscosity (n = 3) were surveyed. For polymerization kinetics, the samples were polymerized for 5 min and the data were obtained from 40 s and 5 min. Statistical analysis was made using ANOVA and Tukey's test (a = 0.05). After 7 days of SBF immersion, XRD and FT-IR showed that the HAp crystalline phase was present only in the HAp groups. A lower degree of conversion (DC) and polymerization rate were observed for the Icon and BAG groups, whereas HAp showed higher values. For the BAG group, DPI increased polymerization rate and DC in 40 s. After 5 min, all groups presented DC above 80%. In groups with particles, the HAp groups exhibited higher viscosity, whereas DPI groups showed a decrease in viscosity. Icon had the highest water sorption. To conclude, BAG neither improved the physicochemical properties studied, nor did it show bioactive properties. The addition of DPI reduced viscosity caused by particle addition and also attenuated the DC decrease caused by BAG addition. The addition of bioactive particles to infiltrants should be seen with caution because they increase viscosity and may not bring major clinical improvements that justify their use. DPI might be indicated only if any component is added to the infiltrant to act as a compensation mechanism.

Effect of inorganic fillers on the light transmission through traditional or flowable resin-matrix composites for restorative dentistry

OBJECTIVES

The aim of this in vitro study was to evaluate the light transmission through five different resin-matrix composites regarding the inorganic filler content.

METHODS

Resin-matrix composite disc-shaped specimens were prepared on glass molds. Three traditional resin-matrix composites contained inorganic fillers at 74, 80, and 89 wt. % while two flowable composites revealed 60 and 62.5 wt. % inorganic fillers. Light transmission through the resin-matrix composites was assessed using a spectrophotometer with an integrated monochromator before and after light curing for 10, 20, or 40s. Elastic modulus and nanohardness were evaluated through nanoindentation's tests, while Vicker's hardness was measured by micro-hardness assessment. Chemical analyses were performed by FTIR and EDS, while microstructural analysis was conducted by optical microscopy and scanning electron microscopy. Data were evaluated using two-way ANOVA and Tukey's test (p < 0.05).

RESULTS

After polymerization, optical transmittance increased for all specimens above 650-nm wavelength irradiation since higher light exposure time leads to increased light transmittance. At 20- or 40-s irradiation, similar light transmittance was recorded for resin composites with 60, 62, 74, or 78-80 wt. % inorganic fillers. The lowest light transmittance was recorded for a resin-matrix composite reinforced with 89 wt. % inorganic fillers. Thus, the size of inorganic fillers ranged from nano- up to micro-scale dimensions and the high content of micro-scale inorganic particles can change the light pathway and decrease the light transmittance through the materials. At 850-nm wavelength, the average ratio between polymerized and non-polymerized specimens increased by 1.6 times for the resin composite with 89 wt. % fillers, while the composites with 60 wt. % fillers revealed an increased ratio by 3.5 times higher than that recorded at 600-nm wavelength. High mean values of elastic modulus, nano-hardness, and micro-hardness were recorded for the resin-matrix composites with the highest inorganic content.

CONCLUSIONS

A high content of inorganic fillers at 89 wt.% decreased the light transmission through resin-matrix composites. However, certain types of fillers do not interfere on the light transmission, maintaining an optimal polymerization and the physical properties of the resin-matrix composites.

CLINICAL SIGNIFICANCE

The type and content of inorganic fillers in the chemical composition of resin-matrix composites do affect their polymerization mode. As a consequence, the clinical performance of resin-matrix composites can be compromised, leading to variable physical properties and degradation.

Additive Manufacturing of Bioactive Glass and Its Polymer Composites as Bone Tissue Engineering Scaffolds: A Review

Bioactive glass (BG) and its polymer composites have demonstrated great potential as scaffolds for bone defect healing. Nonetheless, processing these materials into complex geometry to achieve either anatomy-fitting designs or the desired degradation behavior remains challenging. Additive manufacturing (AM) enables the fabrication of BG and BG/polymer objects with well-defined shapes and intricate porous structures. This work reviewed the recent advancements made in the AM of BG and BG/polymer composite scaffolds intended for bone tissue engineering. A literature search was performed using the Scopus database to include publications relevant to this topic. The properties of BG based on different inorganic glass formers, as well as BG/polymer composites, are first introduced. Melt extrusion, direct ink writing, powder bed fusion, and vat photopolymerization are AM technologies that are compatible with BG or BG/polymer processing and were reviewed in terms of their recent advances. The value of AM in the fabrication of BG or BG/polymer composites lies in its ability to produce scaffolds with patient-specific designs and the on-demand spatial distribution of biomaterials, both contributing to effective bone defect healing, as demonstrated by in vivo studies. Based on the relationships among structure, physiochemical properties, and biological function, AM-fabricated BG or BG/polymer composite scaffolds are valuable for achieving safer and more efficient bone defect healing in the future.

Effect of polymerization mode on shrinkage kinetics and degree of conversion of dual-curing bulk-fill resin composites

OBJECTIVES

To assess the behavior of dual-cure and conventional bulk-fill composite materials on real-time linear shrinkage, shrinkage stress, and degree of conversion.

MATERIALS AND METHODS

Two dual-cure bulk-fill materials (Cention, Ivoclar Vivadent (with ion-releasing properties) and Fill-Up!, Coltene) and two conventional bulk-fill composites (Tetric PowerFill, Ivoclar Vivadent; SDR flow + , Dentsply Sirona) were compared to conventional reference materials (Ceram.x Spectra ST (HV), Dentsply Sirona; X-flow; Dentsply Sirona). Light curing was performed for 20 s, or specimens were left to self-cure only. Linear shrinkage, shrinkage stress, and degree of conversion were measured in real time for 4 h (n = 8 per group), and kinetic parameters were determined for shrinkage stress and degree of conversion. Data were statistically analyzed by ANOVA followed by post hoc tests (α = 0.05). Pearson's analysis was used for correlating linear shrinkage and shrinkage force.

RESULTS

Significantly higher linear shrinkage and shrinkage stress were found for the low-viscosity materials compared to the high-viscosity materials. No significant difference in degree of conversion was revealed between the polymerization modes of the dual-cure bulk-fill composite Fill-Up!, but the time to achieve maximum polymerization rate was significantly longer for the self-cure mode. Significant differences in degree of conversion were however found between the polymerization modes of the ion-releasing bulk-fill material Cention, which also exhibited the significantly slowest polymerization rate of all materials when chemically cured.

CONCLUSIONS

While some of the parameters tested were found to be consistent across all materials studied, heterogeneity increased for others.

CLINICAL RELEVANCE

With the introduction of new classes of composite materials, predicting the effects of individual parameters on final clinically relevant properties becomes more difficult.

Biomimicry and 3D-Printing of Mussel Adhesive Proteins for Regeneration of the Periodontium-A Review

Innovation in the healthcare profession to solve complex human problems has always been emulated and based on solutions proven by nature. The conception of different biomimetic materials has allowed for extensive research that spans several fields, including biomechanics, material sciences, and microbiology. Due to the atypical characteristics of these biomaterials, dentistry can benefit from these applications in tissue engineering, regeneration, and replacement. This review highlights an overview of the application of different biomimetic biomaterials in dentistry and discusses the key biomaterials (hydroxyapatite, collagen, polymers) and biomimetic approaches (3D scaffolds, guided bone and tissue regeneration, bioadhesive gels) that have been researched to treat periodontal and peri-implant diseases in both natural dentition and dental implants. Following this, we focus on the recent novel application of mussel adhesive proteins (MAPs) and their appealing adhesive properties, in addition to their key chemical and structural properties that relate to the engineering, regeneration, and replacement of important anatomical structures in the periodontium, such as the periodontal ligament (PDL). We also outline the potential challenges in employing MAPs as a biomimetic biomaterial in dentistry based on the current evidence in the literature. This provides insight into the possible increased functional longevity of natural dentition that can be translated to implant dentistry in the near future. These strategies, paired with 3D printing and its clinical application in natural dentition and implant dentistry, develop the potential of a biomimetic approach to overcoming clinical problems in dentistry.

Impact of silanization of different bioactive glasses in simplified adhesives on degree of conversion, dentin bonding and collagen remineralization

OBJECTIVE

To analyze simplified adhesive containing pure or silanized bioglass 45S5 (with calcium) or Sr-45S5 (strontium-substituted) fillers applied on dentin and to evaluate the microtensile bond strength (µTBS), interface nanoleakage, degree of conversion of adhesive, collagen degradation and remineralization.

METHODS

Ambar Universal adhesive (FGM) was doped with 10 wt% bioactive glasses to form following groups: Control (no bioglass), 45S5 (conventional bioglass 45S5), Sr-45S5 (Sr-substituted bioglass 45S5), Sil-45S5 (silanized bioglass 45S5) and Sil-Sr-45S5 (silanized bioglass Sr-45S5). Adhesives were applied after dentin acid-etching using phosphoric acid at extracted human molars. Resin-dentin sticks were obtained and tested for µTBS, nanoleakage at 24 h or 6 months. Degree of conversion was measured using micro-Raman spectroscopy. Dentin remineralization was assessed by FTIR after 6-month storage in PBS. Hydroxyproline (HYP) release was surveyed by UV-Vis spectroscopy. Statistical analysis was performed using ANOVA and Tukey's test (p < 0.05).

RESULTS

Regarding µTBS, Sr-45S5 and 45S5 presented higher and stable results (p > 0.05). Control (p = 0.018) and Sil-Sr-45S5 (p < 0.001) showed µTBS reduction after 6-month aging. Sil-Sr-45S5 showed higher HYP release than that obtained in the 45S5 group. Sil-45S5 showed mineral deposition and increase in µTBS (p = 0.028) after 6-months. All experimental adhesives exhibited higher degree of conversion compared to Control group, except for 45S5. All adhesives created gap-free interfaces, with very low silver impregnation, except for Sil-Sr-45S5.

SIGNIFICANCE

The incorporation of silanized 45S5 bioglass into the universal adhesive was advantageous in terms of dentin remineralization, bonding performance and adhesive polymerization. Conversely, Sil-Sr-45S5 compromised the µTBS, interface nanoleakage and had a negative impact on HYP outcomes.

A Narrative Review of Bioactive Glass-Loaded Dental Resin Composites

This review aims to provide a comprehensive analysis of the characterizations of bioactive glass (BAG)-loaded dental resin-based composite materials. Online databases (Web of Science, PubMed, and Science Direct) were used to collect data published from January 2011 to January 2022. Only BAG-containing resin adhesive and resin restorative composites are discussed in this narrative review. BAG-loaded resin composites exhibit excellent mineralization ability reflecting enhanced ion release, pH elevation, and apatite formation, especially regarding high BAG loading. This aids the anti-demineralization and remineralization of teeth. Furthermore, BAG-loaded resin composites demonstrated in vitro biocompatibility and antibacterial performance. It has been suggested that BAG fillers with small particle sizes and no more than 20 wt% in terms of loading amount should be used to guarantee the appropriate mechanical properties of resin composites. However, most of these studies focused on one or some aspects using different resin systems, BAG types, and BAG amounts. As such, this makes the comparison difficult, and it is essential to find an optimal balance between different properties. BAG-loaded resin composites can be regarded as bioactive materials, which present major benefits in dentistry, especially their capability in the bacterial inhibition, cell biocompatibility, anti-demineralization, and remineralization of teeth.

Using Copper-Doped Mesoporous Bioactive Glass Nanospheres to Impart Anti-Bacterial Properties to Dental Composites

Experimental dental resin composites containing copper-doped mesoporous bioactive glass nanospheres (Cu-MBGN) were developed to impart anti-bacterial properties. Increasing amounts of Cu-MBGN (0, 1, 5 and 10 wt%) were added to the BisGMA/TEGDMA resin matrix containing micro- and nano-fillers of inert glass, keeping the resin/filler ratio constant. Surface micromorphology and elemental analysis were performed to evaluate the homogeneous distribution of filler particles. The study investigated the effects of Cu-MBGN on the degree of conversion, polymerization shrinkage, porosity, ion release and anti-bacterial activity on S. mutans and A. naeslundii. Experimental materials containing Cu-MBGN showed a dose-dependent Cu release with an initial burst and a further increase after 28 days. The composite containing 10% Cu-MBGN had the best anti-bacterial effect on S. mutans, as evidenced by the lowest adherence of free-floating bacteria and biofilm formation. In contrast, the 45S5-containing materials had the highest S. mutans adherence. Ca release was highest in the bioactive control containing 15% 45S5, which correlated with the highest number of open porosities on the surface. Polymerization shrinkage was similar for all tested materials, ranging from 3.8 to 4.2%, while the degree of conversion was lower for Cu-MBGN materials. Cu-MBGN composites showed better anti-bacterial properties than composites with 45S5 BG.

Effect of citric acid erosion on enamel and dentin and possible protection by a novel bioactive borate adhesive system

OBJECTIVES

This study examined the ability of a borate adhesive to protect enamel/dentin surfaces from acidic erosion and its effect on the shear bond strength (SBS) of enamel/dentin to resin composite.

MATERIALS AND METHODS

180 human enamel/dentin specimens were utilized. Enamel buccal surfaces were etched with phosphoric-acid then divided into: (EBG) borate glass adhesive group; (ERS) resin-adhesive system group; (EF) fluoride gel 1.23% group, and enamel control (EC) group; followed by bonding to orthodontic-buttons. The dentin specimens were conditioned by EDTA (Ethylene-diamine-tetra-acetic acid) and divided into: (DBG) borate glass resin, (DRS) resin adhesive; (DDA) group had a dentin-desensitizing agent VivaSens (VivaDent, Liechtenstein) and (DC) control group. The treated enamel/dentin specimens had their SBS to composite. The enamel/dentin specimens were exposed to 1% citric acid (18 min). Enamel/dentin specimens were examined by (SEM/EDS) scanning-electron-microscope equipped with electron-dispersive-spectroscopy and (FTIR/ATR). Analysis-of-Variance (ANOVA) was used to compare the SBS and Wilcoxon-signed-rank test was used to compare the enamel/dentin areas protected by the applied agents before/after erosion (p = 0.05).

RESULTS

There was no significance difference in SBS among all groups except for (DDA) group that showed significant decrease p < 0.05. (EBG) and (DBG) groups were the only groups significantly protected enamel and dentin from erosion p < 0.05. FTIR/ATR showed that erosion altered the chemical structure of (DRS), (DDA), and (DC) groups but did not affect the other enamel/dentin groups. Degree of conversion of the borate-adhesive system was acceptable.

CONCLUSION

The Borate adhesive system released calcium and phosphate compounds that decreased the erosive activity of the citric acid resulting in protecting simulated dentin-hypersensitive areas and enamel from erosion without affecting the SBS to resin-composite.

CLINICAL SIGNIFICANCE

A Borate adhesive system can be adopted as a therapeutic agent in a fully integrated program for protecting dentin-hypersensitive areas and in enamel next to orthodontic fixed appliances.

Polymerization Kinetics and Development of Polymerization Shrinkage Stress in Rapid High-Intensity Light-Curing

This study investigated polymerization kinetics, linear shrinkage, and shrinkage stress development for six contemporary composite materials of different viscosities cured using radiant exitances of 1100–2850 mW/cm². Real-time measurements of degree of conversion, linear shrinkage, and shrinkage stress were performed over 5 min using Fourier-transform infrared spectrometry, a custom-made linometer, and a custom-made stress analyzer, respectively. For most tested variables, the factor “material” had a higher effect size than the factor “curing protocol”. Maximum polymerization rate and maximum shrinkage stress rate were the most affected by changes in curing conditions. In contrast, no significant effects of curing conditions were identified within each material for shrinkage stress values measured at the end of the 5 min observation period. Linear shrinkage and shrinkage stress values measured after 5 min were closely correlated (R = 0.905–0.982). The analysis of polymerization kinetics suggested that the two composites specifically designed for rapid light-curing responded to higher radiant exitances differently than other composites. Polymerization kinetics and shrinkage stress behavior of contemporary restorative composite materials of different viscosities were overall more affected by material type than differences in curing conditions. Subtle differences in polymerization kinetics behavior shown by the two composites specifically designed for rapid high-intensity light-curing did not translate into significant differences in the development of polymerization shrinkage stress.

Impact of Copper-Doped Mesoporous Bioactive Glass Nanospheres on the Polymerisation Kinetics and Shrinkage Stress of Dental Resin Composites

We embedded copper-doped mesoporous bioactive glass nanospheres (Cu-MBGN) with antibacterial and ion-releasing properties into experimental dental composites and investigated the effect of Cu-MBGN on the polymerisation properties. We prepared seven composites with a BisGMA/TEGDMA (60/40) matrix and 65 wt.% total filler content, added Cu-MBGN or a combination of Cu-MBGN and silanised silica to the silanised barium glass base, and examined nine parameters: light transmittance, degree of conversion (DC), maximum polymerisation rate (R_max), time to reach R_max, linear shrinkage, shrinkage stress (PSS), maximum PSS rate, time to reach maximum PSS rate, and depth of cure. Cu-MBGN without silica accelerated polymerisation, reduced light transmission, and had the highest DC (58.8 ± 0.9%) and R_max (9.8 ± 0.2%/s), but lower shrinkage (3 ± 0.05%) and similar PSS (0.89 ± 0.07 MPa) versus the inert reference (0.83 ± 0.13 MPa). Combined Cu-MBGN and silica slowed the R_max and achieved a similar DC but resulted in higher shrinkage. However, using a combined 5 wt.% Cu-MBGN and silica, the PSS resembled that of the inert reference. The synergistic action of 5 wt.% Cu-MBGN and silanised silica in combination with silanised barium glass resulted in a material with the highest likelihood for dental applications in future.

The influence of inorganic fillers on the light transmission through resin-matrix composites during the light-curing procedure: an integrative review

PURPOSE

The objective of this study was to perform an integrative review on the effect the inorganic fillers on the light transmission through the resin-matrix composites during the light-curing procedure.

METHOD

A bibliographic review was performed on PubMed using the following search terms: "fillers" OR "particle" AND "light curing" OR "polymerization" AND "light transmission" OR "light absorption" OR "light irradiance" OR "light attenuation" OR "light diffusion" AND "resin composite." The search involved articles published in English language in the last 10 years.

RESULTS

Selected studies reported a decrease in biaxial strength and hardness in traditional resin-matrix composites in function of the depth of polymerization. However, there were no significant differences in biaxial strength and hardness recorded along the polymerization depth of Bulk-Fill™ composites. Strength and hardness were enhanced by increasing the size and content of inorganic fillers although some studies revealed a progressive decrease in the degree of conversion on increasing silica particle size. The translucency of glass-ceramic spherical fillers promoted light diffusion mainly in critical situations such as in the case of deep proximal regions of resin-matrix composites.

CONCLUSIONS

The amount of light transmitted through the resin-matrix composites is influenced by the size, content, microstructure, and shape of the inorganic filler particles. The decrease of the degree of conversion affects negatively the physical and mechanical properties of the resin-matrix composites.

CLINICAL RELEVANCE

The type and content of inorganic fillers in the chemical composition of resin-matrix composites do affect their polymerization. As a consequence, the clinical performance of resin-matrix composites can be compromised leading to variable physical properties and degradation. The polymerization mode of resin-matrix composites can be improved according to the type of inorganic fillers in their chemical composition.

Preparation of Ca doped wrinkled porous silica (Ca-WPS) for the improvement of apatite formation and mechanical properties of dental resins

The purpose of this work was to fabricate and characterize Ca doped wrinkled porous silica (Ca-WPS), and evaluate their effect on the mineralization and mechanical properties of resin composites as functional fillers. Ca-WPS were prepared by sol-gel method and characterized by scanning electron microscopy, transmission electron microscopy and N₂ adsorption-desorption measurements. The mineralization properties of the prepared Ca-WPS particles and the resin composites with different amount of Ca-WPS were evaluated by simulated body fluid (SBF) immersion method. The mechanical properties (flexural strength, flexural modulus, compressive strength and microhardness) of the dental resins containing unimodal Ca-WPS fillers and bimodal Ca-WPS fillers with nonporous silica were evaluated by a universal testing machine. Results showed that after immersing in SBF for 5 d, apatite formed on the surface of Ca-WPS and composites containing Ca-WPS fillers, indicating the excellent mineralization property of the prepared Ca-WPS. The mechanical properties of the dental resins increase with the increase of the proportion of unimodal Ca-WPS fillers. The dental resins with bimodal Ca-WPS fillers showed better mechanical properties than the group with only nonporous fillers at the same filler loading (60 wt%). Among all the samples, the dental composites filled with bimodal fillers (mass ratio of Ca-WPS: nonporous silica = 10:50, total filler loading 60 wt%) exhibited the best mechanical performance. The flexural strength, flexural modulus, compressive strength and microhardness of these samples were 26.96%, 42.75%, 16.04% and 54.1% higher than the composites with solid silica particles alone, respectively. Thus, the prepared Ca-WPS could effectively improve the apatite formation and mechanical properties of resin composites.

Polymerization kinetics of experimental resin composites functionalized with conventional (45S5) and a customized low-sodium fluoride-containing bioactive glass

This study aimed to investigate polymerization kinetics and curing light transmittance of two series of experimental dental resin composites filled with 0–40 wt% of either 45S5 bioactive glass (BG) or a customized low-Na F-containing BG. Polymerization kinetics in 0.1-mm and 2-mm thick layers were investigated through real-time degree of conversion measurements using a Fourier transform infrared (FTIR) spectrometer. FTIR spectra were continuously collected at a rate of 2 s⁻¹ during light-curing (1340 mW/cm²). Light transmittance through 2-mm thick composite specimens was measured using a UV–Vis spectrometer at a rate of 20 s⁻¹. Unlike BG 45S5, which led to a dose-dependent reduction in the rate and extent of polymerization, the customized low-Na F-containing BG showed a negligible influence on polymerization. The reduction in light transmittance of experimental composites due to the addition of the low-Na F-containing BG did not translate into impaired polymerization kinetics. Additionally, the comparison of polymerization kinetics between 0.1-mm and 2-mm thick layers revealed that polymerization inhibition identified for BG 45S5 was not mediated by an impaired light transmittance, indicating a direct effect of BG 45S5 on polymerization reaction. A customized low-Na F-containing BG showed favourable behaviour for being used as a functional filler in light-curing dental resin composites.

Short- and Long-Term Dentin Bond Strength of Bioactive Glass-Modified Dental Adhesives

This study investigated the short- and long-term effects of dental adhesives doped with nano-sized bioactive glass 45S5 (BAG) on the resin-dentin interfacial bond strength. Two etch-and-rinse adhesives (Adper Scotchbond Multi-Purpose (ASB) and Solobond Plus (SB)) and one self-etch adhesive (Clearfil SE Bond (CF)) were doped with different concentrations of BAG (5, 10, and 20 wt%). The unmodified (0 wt% BAG) commercial adhesives served as control groups. Dentin of 120 molars (n = 10 per group) was treated with the different adhesives, followed by buildups with a conventional composite restorative material. From each tooth, 14 sticks were prepared for micro-tensile bond strength (µTBS) testing. The sticks were stored in simulated body fluid at 37 °C and tested after 24 h or six months for µTBS and failure mode. Data were analyzed using Kruskal-Wallis tests in combination with post-hoc Conover-tests and Wilcoxon signed-rank tests at a level of significance of α = 0.05. After 24 h and six months, both etch-and-rinse adhesives with a low BAG content (up to 10 wt% for ASB and 5 wt% for SB) showed similar µTBSs as their respective control groups (0 wt% BAG). CF showed a significant decrease in µTBS even after addition of 5 wt% BAG. At a high concentration of added BAG (20 wt%), all three adhesives showed a significant decrease in µTBS compared to the unmodified controls. The CF control group showed significantly lower µTBS after 6 months of storage than after 24 h. In contrast, the µTBS of all CF groups modified with BAG was unaffected by aging. In conclusion, the tested etch-and-rinse adhesives can be modified with up to 5 wt% (SB), or 10 wt% (ASB) of BAG without reducing their short- and long-term dentin bond strength. Moreover, the addition of nano-sized BAG may prevent long-term bond strength deterioration of a self-etch adhesive.

Incorporation of Copper-Doped Mesoporous Bioactive Glass Nanospheres in Experimental Dental Composites: Chemical and Mechanical Characterization

Experimental dental resin composites incorporating copper-doped mesoporous bioactive glass nanospheres (Cu-MBGN) were designed to impart antibacterial and remineralizing properties. The study evaluated the influence of Cu-MBGN on the mechanical properties and photopolymerization of resin composites. Cu-MBGN were synthesized using a microemulsion-assisted sol-gel method. Increasing amounts of Cu-MBGN (0, 1, 5, and 10 wt %) were added to the organic polymer matrix with inert glass micro- and nanofillers while maintaining a constant resin/filler ratio. Six tests were performed: X-ray diffraction, scanning electron microscopy, flexural strength (FS), flexural modulus (FM), Vickers microhardness (MH), and degree of conversion (DC). FS and MH of Cu-MBGN composites with silica fillers showed no deterioration with aging, with statistically similar results at 1 and 28 days. FM was not influenced by the addition of Cu-MBGN but was reduced for all tested materials after 28 days. The specimens with 1 and 5% Cu-MBGN had the highest FS, FM, MH, and DC values at 28 days, while controls with 45S5 bioactive glass had the lowest FM, FS, and MH. DC was high for all materials (83.7-93.0%). Cu-MBGN composites with silica have a potential for clinical implementation due to high DC and good mechanical properties with adequate resistance to aging.

Radical photoinitiation with LEDs and applications in the 3D printing of composites

Radical initiation upon LED light irradiation is discussed herein as well as its application in additive manufacturing. The ability of manufacturing complex structures, freedom of design, low energy consumption, fast prototyping, and excellent spatial resolution are the main benefits of the 3D printing technology by photopolymerization. Therefore, the 3D printing of composites through photopolymerization processes is developing rapidly in the academia and industry, and has been a turning point of additive manufacturing (AM). In the present review, an overview of radical initiation with LEDs (i.e., the photopolymerization LED technology, the photoinitiating systems, and the polymerizable media) and of the main 3D printing methods by photopolymerization, materials, and their applications in different fields has been carried out. As a challenging topic, the issue of light penetration in a filled matrix for the access to composites is discussed, including the light transmittance of the composite, the mismatch of the refractive index between the filler and the monomer, the factors of the filler, and the adverse influence of low light penetration on the 3D printing process. In particular, the popular applications of 3D printing by photopolymerization in biomedical science, electronic industry, materials for adsorption, and 4D printing are discussed. Overall, this review gives an overview of the 3D printing of polymer matrix composites through photopolymerization processes as a benchmark for future research and development.

The use of bioactive glass (BAG) in dental composites: A critical review

OBJECTIVE

In recent years, numerous studies have analyzed the role of bioactive glass (BAG) as remineralizing additives in dental restorative composites. This current review provides a critical analysis of the existing literature, particularly focusing on BAGs prepared via the melt-quench route that form an "apatite-like" phase when immersed in physiological-like solutions.

METHODS

Online databases (Science Direct, PubMed and Google Scholar) were used to collect data published from 1962 to 2020. The research papers were analyzed and the relevant papers were selected for this review. Sol-gel BAGs were not included in this review since it is not a cost-effective manufacturing technique that can be upscaled and is difficult to incorporate fluoride.

RESULTS

BAGs release Ca²⁺, PO₄^3- and F^- ions, raise the pH and form apatite. There are numerous published papers on the bioactivity of BAGs, but the different glass compositions, volume fractions, particle sizes, immersion media, time points, and the characterization techniques used, make comparison difficult. Several papers only use certain characterization techniques that do not provide a full picture of the behavior of the glass. It was noted that in most studies, mechanical properties were measured on dry samples, which does not replicate the conditions in the oral environment. Therefore, it is recommended that samples should be immersed for longer time periods in physiological solutions to mimic clinical environments.

SIGNIFICANCE

BAGs present major benefits in dentistry, especially their capacity to form apatite, which could potentially fill any marginal gaps produced due to polymerization shrinkage.

Polymerization shrinkage behaviour of resin composites functionalized with unsilanized bioactive glass fillers

Previous work has shown that partial replacement of reinforcing fillers with unsilanized silica particles can diminish polymerization shrinkage stress of dental resin composites. The aim of the present study was to investigate whether such an effect can be attained by using unsilanized bioactive glass (BG). Incorporating BG fillers into resin composites is interesting due to their potential for exerting caries-preventive effects. Experimental light-curable composites with a total filler load of 77 wt% were prepared. Reinforcing fillers were partially replaced with 0-60 wt% of BG 45S5 and an experimental low-sodium fluoride-containing BG. The following properties were investigated: linear shrinkage, degree of conversion, shrinkage stress, maximum shrinkage stress rate, and time to achieve maximum shrinkage stress rate. The diminishing effect of BG 45S5 on shrinkage stress was mediated by a decrease in degree of conversion caused by this BG type. In contrast, as the degree of conversion remained unaffected by the experimental BG, the resulting shrinkage behaviour was governed by the effect of varying amounts of silanized and unsilanized fillers on material's viscoelastic properties. The replacement of silanized reinforcing fillers with unsilanized BG did not reduce polymerization shrinkage stress unless the reduction was attained indirectly through a diminished degree of conversion.

Polymerization and shrinkage stress formation of experimental resin composites doped with nano- vs. micron-sized bioactive glasses

This study investigated the effect of adding bioactive glass 45S5 (BG) of different particle sizes to dental composite on resin polymerization and shrinkage stress formation. Commercial flowable composite was mixed with either 15 wt% BG fillers (nanometric, micrometric, or hybrid BG) or inert barium glass. Real-time linear polymerization shrinkage and shrinkage stress were recorded, and the degree of conversion was measured using FTIR spectroscopy. The commercial (unmodified) composite developed significantly higher linear shrinkage and shrinkage stress than the groups with 15 wt% added inert or BG fillers. After adding inert barium glass, the composite showed significantly higher linear shrinkage than when micrometric BG was added. The addition of bioactive or inert glass fillers did not affect the degree of conversion. Shrinkage stress can be reduced by adding inert or bioactive fillers (nano- and/or microparticulate BG) without affecting monomer conversion.

A New Customized Bioactive Glass Filler to Functionalize Resin Composites: Acid-Neutralizing Capability, Degree of Conversion, and Apatite Precipitation

This study introduced an experimental bioactive glass (BG) with a lower Na₂O content than conventional BG 45S5 (10.5 wt% vs. 24.5 wt%), additionally containing CaF₂ (12 wt%) and a network connectivity similar to that of BG 45S5. A series of experimental composites functionalized with 5-40 wt% of the novel BG was prepared and compared to a corresponding series of experimental composites functionalized with 5-40 wt% of BG 45S5. Commercial acidneutralizing materials (alkasite, giomer, and glass ionomer) were used as references. The capabilities of the materials to neutralize hydrochloric acid (pH = 2.6) and lactic acid (pH = 4.5) were evaluated by real-time pH measurements over 1 h. The degree of conversion and precipitation of calcium phosphate were also investigated. Data were analyzed using one-way and Welch ANOVA at an overall level of significance of 0.05. The acid-neutralizing potential of the experimental BG incorporated into resin composites was generally comparable to that of BG 45S5, and better than that of a giomer and glass ionomer. Fluorine was identified in the precipitate that developed on the composites functionalized with the experimental BG, suggesting a capability of forming fluorapatite. Unlike the 45S5 composition, the experimental BG did not impair the degree of conversion of resin composites. The novel BG filler is therefore an interesting candidate for future investigations of caries-preventive resin composites, and their potential clinical applicability for restorative, preventive, and orthodontic purposes.

Degree of conversion of experimental resin composites containing bioactive glass 45S5: the effect of post-cure heating

Resin composites containing reinforcing inert glass fillers combined with bioactive glass (BG) can aid in the prevention of secondary caries, which is a major cause of failure of contemporary composite restorations. A series of previous studies on experimental resin composites filled with BG 45S5 has demonstrated that methacrylate resin polymerization can be impaired by the addition of unsilanized BG, leading to lower degrees of conversion (DC). In order to distinguish whether the polymerization inhibition is caused by a direct (temperature-independent) effect of BG or an indirect (temperature-dependent) effect of restricted mobility of reactive species, this study used Raman spectroscopy to evaluate the DC values of experimental composites post-cured at 37 °C and 150 °C. The potential of BG to adversely affect DC was highly dependent on the resin system. The highest DC reduction was observed in the resin system based on ethoxylated bisphenol A dimethacrylate (Bis-EMA), followed by bisphenol A glycidyl methacrylate (Bis-GMA). In contrast, the DC for urethane dimethacrylate (UDMA) was not compromised by BG. Increasing the mobility of reactive species by heating at 150 °C showed limited potential for increasing the DC in the Bis-EMA and Bis-GMA resin systems, indicating a direct inhibitory effect of BG on polymerization.

The effect of excitation laser power in Raman spectroscopic measurements of the degree of conversion of resin composites

OBJECTIVES

To evaluate the effect of excitation laser power in Raman spectrometry by comparing the spectra and the degree of conversion (DC) values obtained using excitation powers between 300 and 1000mW.

METHODS

Five commercial and three experimental resin composites were light cured at 1200mW/cm² for 10-20s from a commercial blue-violet LED dental curing unit. Raman spectra were collected from composite specimens within 9min after light-curing. The excitation laser (1064nm) was focused on the spot of 0.4mm in diameter. The following powers were used for specimen excitation (mW): 300, 400, 600, 800, and 1000. From Raman spectra, the DC values were calculated and compared among different laser powers. Also, vector-normalized Raman spectra collected using the lowest excitation power (300mW) were compared to those collected using the maximum excitation power (1000mW).

RESULTS

Varying the excitation laser power between 300 and 1000mW resulted in statistically significant differences in both the DC values and the intensity of particular spectral features. The effect of varying laser power on Raman spectra and obtained DC values was material-dependent. The DC values measured within an individual material using different laser powers varied between 3.2 and 7.2% (absolute DC difference). The spectral bands affected by variations in laser power were assigned to symmetric and asymmetric stretching of -CH₂ (2900-3100cm^-1), symmetric stretching of aliphatic CC (1640cm^-1) and scissoring of C-H (1458cm^-1).

SIGNIFICANCE

The DC can be artificially elevated through increasing excitation laser power. This effect should be considered in Raman spectroscopic evaluations of DC in specimens during ongoing post-cure polymerization.

A Chemical Approach to Optimizing Bioactive Glass Dental Composites

The chemical microenvironment surrounding dental composites plays a crucial role in controlling the bacteria grown on these specialized surfaces. In this study, we report a scanning electrochemical microscopy (SECM)-based analytic technique to design and optimize metal ion-releasing bioactive glass (BAG) composites, which showed a significant reduction in biofilm growth. SECM allows positioning of the probe without touching the substrate while mapping the chemical parameters in 3-dimensional space above the substrate. Using SECM and a solid-state H⁺ and Ca²⁺ ion-selective microprobe, we determined that the local Ca²⁺ concentration released by different composites was 10 to 224 µM for a BAG particle size of <5 to 150 µm in the presence of artificial saliva at pH 4.5. The local pH was constant above the composites in the same saliva solution. The released amount of Ca²⁺ was determined to be maximal for particles <38 µm and a BAG volume fraction of 0.32. This optimized BAG-resin composite also showed significant inhibition of biofilm growth (24 ± 5 µm) in comparison with resin-only composites (53 ± 6 µm) after Streptococcus mutans bacteria were grown for 3 d in a basal medium mucin solution. Biofilm morphology and its subsequent volume, as determined by the SECM imaging technique, was (0.59 ± 0.38) × 10⁷ µm³ for BAG-resin composites and (1.29 ± 0.53) × 10⁷ µm³ for resin-only composites. This study thus lays the foundation for a new analytic technique for designing dental composites that are based on the chemical microenvironment created by biomaterials to which bacteria have been exposed.