Biodegradable polymeric ion–releasing fillers

Abstract

Background

Dentists spend considerable time treating issues stemming from caries, especially recurrent caries. European Union regulations require microplastics to be biodegradable. The goal of this research is to develop biodegradable, ion-releasing fillers that could be incorporated into dental materials that aid in remineralization of teeth.

Methods

Polymeric ion–releasing fillers (PIRFs) that released calcium from a dental composite were prepared. These fillers were synthesized from a biodegradable polymer obtained from a renewable resource (alginate) and are capable of releasing calcium ions. Synthetic variables included the concentration of reactants, emulsifying agents, and applied shear rate. PIRFs were incorporated into dental formulations and categorized by scanning electron microscopy, ion release, degree of conversion, depth of cure, and viscosity measurements.

Results

Scanning electron microscopic images confirmed the successful isolation of the PIRFs from synthesis and subsequent mixing into a bisphenol A glycidyl methacrylate and triethylene glycol dimethacrylate continuous phase that was converted into a dental composite. The degree of conversion and depth of cure measurements were consistent with those of a typical dental material, showing the viability of adding PIRFs to dental composites. The effect of mixing the PIRFs on the viscosity of the continuous phase was measured. The addition of up to 5 wt/wt% of these fillers did not significantly increase viscosity, allowing for inclusion into a wide array of dental materials. Incorporation led to pseudoplastic behavior, in which the viscosity decreased with increased shear rate. The incorporation of PIRFs leads to rheopectic behavior at low shear rates and thixotropic behavior at higher rates.

Conclusions

Biodegradable PIRFs are a promising approach to help prevent caries.

Graphical abstract

Why Is This Important?

The prevention of caries and recurrent caries remains an important goal in dentistry. The use of fluoride has been shown to reduce the occurrence of caries effectively, yet the challenge remains. It has been shown that the presence of calcium ions can enhance the effectiveness of fluoride treatments. One way to introduce calcium ions to promote remineralization is to include polymeric fillers in dental materials that release the ion. However, the inclusion of certain microplastic fillers into composites creates stress on the environment. In this study, a new biodegradable filler that is prepared from a renewable resource is reported. Alginate sourced from seaweed is readily biodegradable and used in the food industry as a thickening agent, stabilizer, and emulsifier. In this research, micrometer-sized polymeric fillers containing high concentrations of calcium ions were prepared. The conditions for preparation are described. In addition, the impact of these new fillers is characterized in terms of the impact it has on inclusion in a dental material’s handling characteristics and polymerization properties. These new fillers can be incorporated into a wide range of dental materials to help prevent caries.

Introduction

Secondary caries is one of the most prevalent issues in dentistry. Resin-based dental materials prone to polymerization shrinkage can result in margins between the tooth and the material, resulting in sites susceptible to recurrent caries on which it is almost impossible for the average person to perform proper oral hygiene.^1,2 This creates the possibility for new caries lesions that can become a detriment to patient health. The European Union (EU) has developed new laws and regulations concerning the efficacy of the biodegradability of materials and microplastics.³ These stricter regulations translate to the rise of biodegradable materials originating from renewable resources, such as sodium alginate.4, 5, 6 Classically used as a thickening agent in the food industry, sodium alginate is a water-soluble linear polysaccharide that originates from plants such as brown algae and kelp.7, 8, 9 This natural polymer has already been widely investigated for use in various medical applications such as drug formulation and delivery, showing how it can be used throughout the body.¹⁰ The use of sodium alginate and its combination with several natural and synthetic polymers for use in drug delivery systems has been reviewed.¹¹ By combining sodium alginate with natural, synthetic, or petrochemical-based polymers, researchers were able to enhance mechanical properties.12, 13, 14 Medicinal applications of alginate, especially for means of drug delivery, have been reviewed. In dentistry, sodium alginate has been used as an impression material, since it is relatively inexpensive, malleable, and easy to use.^15,16

Other examples of biodegradable materials with medical applications have been reported. Bovine serum albumin has been encapsulated into poly-(d,l-lactide-co-glycolide) microspheres using an oil-to-oil, nonaqueous method.¹⁷ The protein could be encapsulated in poly-(d,l-lactide-co-glycolide) under various synthetic conditions. In addition, biodegradable microspheres that have the potential to release drugs and proteins into the body have been reported.¹⁴ Microspheres using chitin and poly(lactic-co-glycolic acid) and chitin and poly(lactic acid) blends for the protein delivery system were described. Cellulose-based materials have also been examined for similar purposes as cellulose is both versatile and abundant. Cellulose-based materials have shown potential for both controlled drug delivery and antibacterial activity.^18,19

Hydroxyapatite makes up most of the mineral components of bone and teeth.²⁰ When this structure dissociates into individual calcium, phosphate, and hydroxyl ions in the oral environment, it is known as demineralization. The dissociation process primarily occurs when the pH of the oral cavity falls below critical pH.²¹ This can happen under acid attack, which can stem from both consumed acid (ie, through food and drink) and acid produced by bacteria present in the mouth.²² The process of restoring these mineral components to the oral environment is known as remineralization. This cycle perpetuates naturally in the oral environment but can be influenced by external factors, such as diet and dental hygiene.

The addition of calcium and phosphate ions into the oral cavity has been known to aid in remineralization of tooth structure and slow the demineralization process.^23,24 Research has shifted from reactive to preventive measures against this disease. Multiple ion-releasing glasses and fillers have been thoroughly studied. Casein phosphopeptide-amorphous calcium phosphate (ACP) and tricalcium phosphate are 2 types of calcium phosphates that have been investigated.²⁵ Similar to other calcium phosphates, casein phosphopeptide-ACP and other derivatives of ACP are reported to dissociate and release calcium and phosphate ions when the pH of the oral cavity drops below critical pH.26, 27, 28 Ideally, these dissociated ions would precipitate on the surface of tooth enamel, aiding in remineralization. A review of calcium phosphate nanocoatings considered a wide range of similar materials, such as bioglass, carbon nanotubes, silica, and ceramic oxide, all of which are being used in the dental field.²⁹ Ion-releasing glasses behave similarly to ACP in an acidic environment, in which the filler can release calcium or phosphate ions into the oral cavity.30, 31, 32 Another widely studied remineralization material concerns nano-HAP.^33,34 HAP particles are molecularly similar to naturally occurring apatite found in the tooth. These materials are synthesized with the intention that, when added to the oral environment, nano-HAP will be able to chemically bond to the tooth to replace degrading natural apatite and aid in remineralization. Several researchers have considered these materials for remineralization.35, 36, 37, 38 These treatments are regarded as fluoride-free methods of remineralization. Despite these numerous efforts, secondary caries remains one of the leading problems within dentistry.

In previous studies, polyurethane-based microcapsules were synthesized to be filled with aqueous, bioactive calcium, phosphate, and fluoride ions.^39,40 It was found that these microcapsules could release ions through the process of simple diffusion through the polyurethane membrane.^39,40 These reports confirmed that the microcapsules slowly release remineralizing ions over an extended period.^39,40 The presence of these aqueous ions is critical for remineralization as the ions are already dissolved and thus in a bioavailable state.

A common approach to introducing more highly functioning materials involves the inclusion of fillers. With consideration of the EU’s new environmental regulations prohibiting the inclusion of nonbiodegradable microplastics, we found that ion-releasing fillers can be prepared with sodium alginate. This allows the plastic component of these fillers to not only be biodegradable, but also sourced from an easily renewable resource. Many types of dental materials promote remineralization. A filler that releases calcium ions can be included in a wide range of materials, including over-the-counter dentifrices, to enhance fluoride uptake or promote remineralization in a fluoride-free formulation. In terms of materials used in dental offices, calcium-releasing fillers could be added to the acrylic continuous phase of materials such as sealants, baseliners, restorative materials, or cement to promote remineralization at interfaces most susceptible to caries. The approach in our study results in a filler that releases calcium by diffusion, that does not require an acid to etch or dissolve filler that can weaken physical properties.

Our study aimed to synthesize and characterize these polymeric ion–releasing fillers (PIRFs) that are biodegradable and sourced from a renewable resource. PIRFs were incorporated into colloids and characterized by the degree of conversion, viscosity, and depth of cure measurements. The purpose of the characterization was to see how the dental materials would be affected by the addition of PIRFs and to determine the potential for use in dental materials.

Methods

PIRF synthesis

A 5 M calcium chloride (CaCl₂) solution was mixed with gum arabic emulsifying agent in an IKA LR1000 reactor. Sodium alginate solution (1 g/100 mL water) was added dropwise using a syringe pump to ensure alginate was introduced 1 drop at a time, preventing streams of alginate. The initial temperature of the synthesis was 25 °C. Because of heat introduced from mixing, the final temperature of the synthesis was 50 ± 5 °C. A ThermoScientific ST16 centrifuge was used to isolate synthesized PIRFs. Synthetic variables reported in the Table included the shear rate of the reactor mixer, the concentration of the emulsifying agent, and the concentration of alginate. The effect on PIRF size and yield was determined. The size of PIRFs was calculated by taking the average size of PIRFs from 10 scanning electron microscopic (SEM) images that included at least 10 PIRFs per image. The cross-sectional diameter of each PIRF was measured using software included with the SEM. The yield reported refers to the mass of PIRFs that were isolated after the synthesis.

Table.

Initial synthetic conditions, including reactant solution volumes, emulsifying agent, and shear rate for the production of polymeric ion–releasing fillers.^∗

Volume of Calcium Chloride	Volume of Sodium Alginate	Emulsifying Agent	Shear Rate†	Stable Polymeric Ion–Releasing Fillers Yield	Mean (SD) Size
375 mL	125 mL	6.25 g gum arabic	1,884	34.7 g	2.40 (2.02) μm
375 mL	240 mL	6.25 g gum arabic	1,884	92.7 g	3.21 (3.65) μm
375 mL	240 mL	3.12 g gum arabic	1,884	95.5 g	3.09 (3.81) μm
375 mL	240 mL	12.5 g gum arabic	1,884	58.7 g	2.50 (2.86) μm
375 mL	240 mL	6.25 g gum arabic	471	12.0 g (micro) 68.0 g (macro)	3.05 (3.77) μm 3 mm
375 mL	240 mL	6.25 g gum arabic	942	16.5 g (micro) 71.2 g (macro)	4.62 (4.64) μm 3 mm
375 mL	240 mL	6.25 g gum arabic	1,413	25.3 g (micro) 60.0 g (macro)	4.30 (2.87) μm 3 mm
375 mL	240 mL	6.25 g locust bean gum	1,884	60.1 g	2.06 (0.03) μm
375 mL	125 mL	0.78 g xanthan gum	1,884	52.2 g	1.61 (0.36) μm
375 mL	125 mL	0.39 g guar gum	1,884	42.0 g	1.56 (0.03) μm

^∗The synthesis is characterized by yield and particle size.

^†Shear rate for a rotor diameter of 0.025 m (units: seconds⁻¹).

Dental material formulations

Depending on the formulation, a different quantity of PIRFs and glass fillers were added to a 65% bisphenol A glycidyl methacrylate (BisGMA) and 35% triethylene glycol dimethacrylate (TEGMA) continuous phase. The PIRFs (0, 2.5, 5, or 10 wt/wt%) and Schott UltraFine (Schott) 1.5 μm silanated (1.5% silanated) (0, 20, or 50 wt/wt%) were added to the formulations. Aerosil OX50 (Evonik Industries AG) fumed silica (unsilanated) was mixed under vacuum. Camphorquinone (0.5 wt/wt%) and ethyl-4-dimethylaminobenzoate (0.5 wt/wt%) photoinitiators were mixed into the formulations.

SEM

Dental composite formulations prepared from acrylic resin and PIRFs (with and without glass filler) were prepared on a Cressington-108 Sputter Coater (Cressington Scientific Instruments Ltd). Cross-section images of composites were taken on a Hitachi TM3000 tabletop SEM (Hitachi High-Tech America, Inc) to image PIRFs to determine the morphologic shape and mean size of the polymeric fillers after synthesis and isolation by centrifugation. SEM images were used to determine how PIRFs were dispersed in the continuous phase in the presence of glass fillers during the formulation processes.

Ion release

Calcium ions being released from a dental composite prepared with PIRFs into ultrapure water were measured. The release rate of calcium ions is highly sensitive to the surface area to volume ratio of the material exposed to water. Composites formulated with 5 wt/wt% PIRFs with no glass fillers in a 65% BisGMA to 35% TEGMA continuous phase were loaded into washers that were affixed to glass microscope slides (Fisher Finest premium microscope slides [Thermo Fisher Scientific Inc], 3 in × 1 in × 1 mm). Each experiment consisted of 20 slides with 3 washers each (nylon standard flat washers [Washers USA]; outer dimension: 15.875 mm; inner dimension: 9.525 mm; thickness: 0.81 mm; surface area: 71.3 mm²; total volume: 57.9 mm³), for a total of 60 washers. These slides were placed in a slide holder and submerged in 200 mL of ultrapure water. One milliliter aliquots were obtained immediately on the first day at time zero and subsequently on days 1, 4, 7, 14, 30, and 90. The volume taken from the slide dish during each aliquot was refreshed with the same volume of ultrapure water. An Orion Calcium Electrode (Thermo Fisher Scientific Inc) was used to determine the concentration of calcium ions released from the composite as a function of time. The concentration of calcium was determined by comparing calcium measured in aliquots relative to CaCl₂ solution standards. The release profile was measured in triplicate.

Degree of conversion

The degree of conversion of formulations was analyzed on a Nicolet iS5 ATR-FTIR (Thermo Fisher Scientific Inc) crystal as a function of time. The dental material sample was cured with a VALO Grand curing light (Ultradent Products Inc) at a power of 1,000 mW/cm² in 5-second intervals. Individual spectra were obtained every 5 seconds for 60 seconds. Absorbance values (abs) at wavelengths 1,640 cm⁻¹ and 1,720 cm⁻¹ were obtained, and the relative difference was used to find the degree of conversion (DC) at 5-second intervals (t), illustrated by the equation below:

$$\mathrm{D}\mathrm{C}=1-\left(\frac{\left(\raisebox{1ex}{$\mathrm{abs}1640$}\!\left/ \!\raisebox{-1ex}{$\mathrm{abs}1720$}\right.\right)t\sec }{\left(\raisebox{1ex}{$\mathrm{abs}1640$}\!\left/ \!\raisebox{-1ex}{$\mathrm{abs}1720$}\right.\right)0\sec }\right)$$ (1)

Measurements were performed in triplicate. Plotted values represent the mean value. The SD was within 5% of the mean. Error bars were omitted on the plot for clarity. As just the trend of PIRF inclusion on the degree of conversion was being determined, a deeper statistical analysis was not performed on the data.

Depth of cure

The depth of cure measurements (n = 5) of composites followed ISO4049 and was determined by filling a 12-mm cylindrical mold with formulation and curing 1 side for 20 seconds.⁴¹ A VALO Grand curing light at a power of 1,000 mW/cm² was used to cure the sample. The uncured part of the sample was scraped off the bottom. A micrometer was used to determine the remaining length, and this value was halved to obtain depth of cure measurements. As just the trend of PIRF inclusion on the depth of cure was being determined, a deeper statistical analysis was not performed on the data.

Viscosity

The viscosity of uncured material containing fillers was measured using a Brookfield DVT3 rheometer (AMETEK Inc) at room temperature using a small sample chamber and SC4-18 spindle. The formulations were sheared at 3 speeds: 1, 10, and 25 rpm for 90 seconds each. Viscosity measurements were performed in duplicate. Nearly identical data was obtained for each formulation measurement performed in duplicate.

Results

PIRF synthesis

PIRFs were synthesized under a variety of conditions that varied in volumes of CaCl₂ and sodium alginate, emulsifying agent concentrations, and shear rate of the reactor rotor. The PIRF yield and average size were measured and reported in the Table. When the shear rate of the rotor in the reactor was reduced below 1,884 seconds⁻¹, a bimodal size distribution was produced. A fraction of macrosized PIRFs were synthesized under those specific conditions. When the quantity of sodium alginate was doubled, the stable PIRF yield increased. This is to be expected as the 2 variables are positively correlated with each other. The concentration of emulsifying agent had a substantial effect on the yield of the PIRFs, in which there appears to be an optimal concentration based on the other variables. Other emulsifying agents with a polysaccharide backbone were tested at albeit lower concentrations (because of the viscosity thickening nature) than gum arabic, which also resulted in the synthesis of stable PIRFs.

SEM

PIRFs were isolated by centrifugation. This resulted in PIRFs separating from the supernatant and forming a pellet that was able to be isolated and incorporated into various formulations. Because of the g-forces exerted on the PIRFs during the centrifugation isolation process and subsequent formulation mixing, SEM was used to show that PIRFs can withstand these forces. Figure 1A depicts a representative cross-sectional SEM image of material formulated with 20 wt/wt% PIRFs (collected from the second synthesis listed in the Table) in a BisGMA and TEGMA continuous phase that shows PIRFs do not agglomerate when mixing and subsequent polymerization of colloid into composite. Figure 1B shows a cross-sectional SEM of a dental material consisting of BisGMA and TEGMA continuous phase mixed with 20 wt/wt% glass filler and 2.5 wt/wt% PIRFs (also from the second synthesis listed in the Table).

Figure 1.

A. Scanning electron microscopic image of the cross-section of a dental material containing 20 wt/wt% polymeric ion–releasing fillers in a bisphenol A glycidyl methacrylate and triethylene glycol dimethacrylate continuous phase. B. Scanning electron microscopic image of a cross-section of a dental material containing 2 wt/wt% polymeric ion–releasing fillers and 20 wt/wt% glass filler in a bisphenol A glycidyl methacrylate and triethylene glycol dimethacrylate continuous phase.

Calcium ion release

Composites loaded with 5 wt/wt% calcium-containing PIRFs were soaked in ultrapure water to determine if calcium was released from the composite. Release of calcium ion per gram of formulation as a function of time from composite is reported in Figure 2. There was an initial burst release of calcium over the first 4 days. Afterward, a more steady-state release of calcium ions was observed from days 4 through 90.

Figure 2.

Normalized concentration of calcium ions released from a composite loaded with 5 wt/wt% calcium ion-releasing fillers. The plot is the concentration of the calcium ions in ppm per gram of composite as a function of time in days. The error bars represent the SD.

Degree of conversion

FTIR-ATR was used to determine polymerization kinetics for each dental material loaded with PIRFs (0-10 wt/wt%). Figure 3 shows how the addition of PIRFs affects the degree of conversion for every set of samples. The degree of conversion typically decreased as the amount of PIRFs increased, regardless of glass loading. As seen in Figure 3, all formulations showed the highest increase in rate within the first 5 seconds of curing. After that, the percentage conversion increased more gradually from 20 through 60 seconds.

Figure 3.

The degree of conversion as a function of time for dental formulations with PIRF loadings from 0 through 10 wt/wt% with 0 wt/wt% glass (A), 20 wt/wt% glass (B), and 50 wt/wt% glass (C). PIRF: polymeric ion–releasing fillers.

Depth of cure

Depth of cure is an important test for dental materials as dental materials may be up to 4 mm thick during photocuring. For the varying glass loading samples displayed in Figure 4, as PIRF loading increased, the average depth of cure of material decreased, regardless of glass loading. All composites had a depth of cure 4 mm or greater.

Figure 4.

The depth of cure after 20 seconds as a function of PIRF loading for dental formulations with 0 wt/wt% glass (A), 20 wt/wt% glass (B), and 50 wt/wt% glass (C). The error bars represent the SD. PIRF: polymeric ion–releasing fillers.

Viscosity

Handling characteristics of dental materials is a critical property. Figure 5 depicts that the viscosity of the colloid increased as PIRF loading increased. When comparing Figures 5A and 5B, as glass was added, the viscosity increased. Formulations appear to be non-Newtonian as a function of time, with 10 wt/wt% PIRF-loaded samples exhibiting much stronger pseudoplastic behavior as a function of time.

Figure 5.

The viscosity as a function of time of a dental formulation with 0 wt/wt% glass filler (A) and 20 wt/wt% glass filler (B) with PIRF loading from 0 through 10 wt/wt%. Colloids were sheared at a rate of 1 rpm (1.32 seconds⁻¹) for 90 seconds, then 10 rpm (13.2 seconds⁻¹) for 90 seconds, and then 25 rpm (33 seconds⁻¹). PIRF: polymeric ion–releasing fillers.

Discussion

Our study aimed to synthesize and characterize PIRFs for the purpose of aiding in remineralization. In consideration of the synthesis, when added into a CaCl₂ solution, the monovalent sodium ion in the sodium alginate is replaced with the divalent calcium ion, allowing the alginate to crosslink and form spherical fillers insoluble in the aqueous phase.^42,43 Referencing the Table, as 3-mm–sized drops of sodium alginate hit the surface of the CaCl₂ solution, rotor speed must deliver enough kinetic energy to break droplets into micrometer-sized drops at a rate faster than calcium cross-linking the alginate PIRFs. When insufficient energy was present, a bimodal size distribution was produced that resulted in microsized and macrosized PIRFs. One reason this might have occurred is insufficient energy being delivered to the solution during the synthesis. When the sodium alginate solution was added to the reactor, not all droplets broke into micrometer-sized droplets faster than the crosslinking reaction between calcium ions and alginate. This could have led to the synthesis of macrosized PIRFs. There was no statistically significant difference between the mean sizes of microsized PIRFs in any of the syntheses.

For PIRFs to promote remineralization in the oral environment, calcium ions need to be released from the composite. Calcium ion release was measured from a composite with 5 wt/wt% PIRFs in a BisGMA and TEGMA blend. In Figure 2, there was evidence of a significant release of the calcium ions as a function of time. There was an initial burst release of ions from the material over the first 4 days relative to the last 86 days of the experiment. The greater rate of release at the beginning could be due to a greater concentration gradient between calcium concentration contained in the composite and the water bath it soaked in at the start of the experiment compared with the concentration gradient that existed after soaking for 4 days. There would have been greater osmotic potential at the start of the experiment because of initial ultrapure water having no calcium ions present at time zero compared with the soaking solution having significant calcium after 4 days. Considering that the solubility constant product of enamel is typically on the order of 10⁻⁵⁵, low levels of calcium being released can be significant in terms of keeping a margin saturated with calcium to prevent secondary caries from occurring in the oral environment.

The rate of polymerization is a key performance variable for dental materials. Typically, dental composites must cure within 20 through 60 seconds in the oral environment. The degree of conversion of BisGMA and TEGMA continuous phases was measured with different loadings of glass and PIRFs. Figure 3 shows degree of conversion of the polymer decreases as PIRF loading increases for all glass loadings. Conversion of each glass variation decreases as glass loading increases. Higher quantities of filler (whether glass or PIRF) resulted in a lower overall degree of conversion. The addition of PIRFs with no glass filler did not significantly affect the polymerization rate with 2.5 wt/wt% loading. In addition, 5 through 10 wt/wt% PIRFS in the absence of glass resulted in a decreased rate. In the presence of glass fillers (20-50 wt/wt%), the addition of PIRFs decreased the degree of conversion. These results suggest that the use of these PIRFs would require more attention to be paid to the refractive index of both the glass filler and continuous phase used in the dental composite, along with the choice and concentration of initiator and coinitiator.

The depth of cure of dental composites after exposure to light during photocuring is an important feature of all light-cured dental materials. Most dental composites are not cured more than 4 mm in thickness at a time; many are 2 mm or less. The depth of cure was measured for dental colloids that contained both glass and PIRFs. Figure 4 shows depth of cure of composites formulated with PIRFs with different loadings of glass. As seen in Figure 4, PIRF samples with a higher weight of concentration had a lower average depth of cure. Similar to the degree of conversion measurements, if the refractive index of the PIRFs is different from the continuous phase and glass fillers used, this would likely be expected. However, in all cases, the depth of cure still exceeded 4 mm in depth in 20 seconds. This suggests that PIRFs are promising fillers for use in dental composites.

Viscosity of dental materials is an important handling characteristic for dentists to consider. As seen in Figure 5, unfilled material exhibited Newtonian behavior as viscosity did not change as a function of shear rate or time. With 0 wt/wt% glass filler, Figure 4A shows at a constant mixing speed of 1 rpm, material with 0 through 10 wt/wt% PIRFs displays rheopectic properties as viscosity increases as a function of time. At higher speeds of 10 and 25 rpm, the 0 through 10 wt/wt% PIRF-loaded materials displayed thixotropic properties as viscosity decreased as a function of time. This pattern persisted with glass-filled samples, as seen in Figure 5B. The biggest difference between different wt/wt% PIRF samples is the magnitude of the behavior. Both 5 and 10 wt/wt% PIRF samples displayed non-Newtonian behavior, but the viscosity of the 10 wt/wt% PIRF sample was relatively higher overall. This could be due to intermolecular interactions between hydroxyl groups on sodium alginate of the PIRFs. Hydroxyl groups can hydrogen bond decreasing the movement between PIRFs. As higher quantities of PIRFs are added, those intermolecular interactions also increase in quantity.^44,45

All filled samples exhibited pseudoplastic or shear-thinning properties as a function of shear rate. The addition of glass particles into formulations resulted in non-Newtonian properties of colloids. The 0 through 5 wt/wt% PIRF with 0 wt/wt% glass samples exhibited Newtonian properties, whereas 0 through 5 wt/wt% PIRF with 20 wt/wt% glass samples almost exclusively exhibited non-Newtonian properties, possibly because of physical interactions between particles within the colloid. Generally, as particles are introduced into colloidal material, viscosity will increase. This is because in a highly filled system there is constant friction and movement between particles.⁴⁶ These physical barriers to movement increase the viscosity of the dental material, as seen in Figure 5.

Conclusions

The development of a biodegradable filler capable of slow-releasing bioavailable ions is a novel approach to promoting remineralization. These PIRFs can withstand the shear force of formulation and centrifugation during the synthesis process. By using materials that are both biodegradable and sourced from renewable resources, the PIRFs follow the environmental regulations set by the EU and present an environmentally friendly opportunity for composites with the potential to remineralize hard tissue in the oral environment.

Disclosure

None of the authors reported any disclosures.