Development of a multifunctional adhesive system for prevention of root caries and secondary caries

Highlights

  • A novel adhesive for prevention of tooth root caries and secondary caries was developed containing a protein-repellent agent, an antibacterial monomer, and remineralization nanoparticles for the first time.

  • A single coat of this adhesive on tooth root dentin produced a coating thickness of approximately 70 μm and completely sealed all dentinal tubules to provide a barrier against biofilm acids.

  • In addition, this adhesive resin had a 95% reduction in protein adsorption, was strongly antibacterial, and reduced biofilm colony-forming units by four orders of magnitude.

  • Therefore, this novel multifunctional adhesive with strong protein-repellent, antibacterial and remineralization properties is promising to coat tooth roots to prevent root caries, as well as to bond restorations in tooth cavities to inhibit secondary caries.

  • The combined use of triple agents (protein-repellent agent, antibacterial agent, and remineralization agent) may have wide applicability in bonding agents, cements, sealants and composites to inhibit caries.

Abstract

Objectives

The objectives of this study were to: (1) develop a novel adhesive for prevention of tooth root caries and secondary caries by possessing a combination of protein-repellent, antibacterial, and remineralization capabilities for the first time; and (2) investigate the effects of 2-methacryloyloxyethyl phosphorylcholine (MPC), dimethylaminohexadecyl methacrylate (DMAHDM), and nanoparticles of amorphous calcium phosphate (NACP) on dentin bond strength, protein-repellent properties, and dental plaque microcosm biofilm response.

Methods

MPC, DMAHDM and NACP were added into Scotchbond Multi-Purpose primer and adhesive. Dentin shear bond strengths were measured. Adhesive coating thickness, surface texture and dentin–adhesive interfacial structure were examined. Protein adsorption onto adhesive resin surface was determined by the micro bicinchoninic acid method. A human saliva microcosm biofilm model was used to investigate biofilm metabolic activity, colony-forming unit (CFU) counts, and lactic acid production.

Results

The resin with 7.5% MPC + 5% DMAHDM + 30% NACP did not adversely affect dentin shear bond strength ( p > 0.1). The resin with 7.5% MPC + 5% DMAHDM + 30% NACP produced a coating on root dentin with a thickness of approximately 70 μm and completely sealed all the dentinal tubules. The resin with 7.5% MPC + 5% DMAHDM + 30% NACP had 95% reduction in protein adsorption, compared to SBMP control ( p < 0.05). The resin with 7.5% MPC + 5% DMAHDM + 30% NACP was strongly antibacterial, with biofilm CFU being four orders of magnitude lower than that of SBMP control.

Significance The novel multifunctional adhesive with strong protein-repellent, antibacterial and remineralization properties is promising to coat tooth roots to prevent root caries and secondary caries. The combined use of MPC, DMAHDM and NACP may have wide applicability to bonding agents, cements, sealants and composites to inhibit caries.

Introduction

The prevalence and severity of tooth root caries increases with aging, and this is a growing public health issue due to the rapid increase in the elderly population coupled with substantial increases in tooth retention in seniors . Gingival recession due to aging, periodontal disease or traumatic tooth-brushing habits can increase the susceptibility to root caries . In addition, low salivary flow in seniors and patients with dry mouths further contribute to biofilm and plaque buildup and the occurrence of root caries . Indeed, root caries occurrence in the United States was reported to increase rapidly with aging, from 7% among young people, to 56% in seniors (≥75 years of age) . In addition, secondary caries has been suggested in previous studies as a primary reason for dental restoration failures . The replacement of the failed restorations accounts for 50–70% of all restorations performed . The annual cost for tooth cavity restorations was approximately $46 billion in 2005 in the United States . Hence, dental caries is a significant burden, and it is highly desirable to develop effective methods to prevent root caries and secondary caries.

Dental plaque formation is a prerequisite for the occurrence of root caries and secondary caries . Dental plaque are aggregates of microorganisms, which are formed due to the attachment of bacteria to tooth surface and to each other in the oral environment . Once the tooth root surface or resin restoration surface are exposed in the oral cavity, they are coated with a salivary pellicle that comprises a layer of selectively adsorbed salivary proteins that mediate the binding of microorganisms . The adherence of bacteria to the salivary pellicle is an initial step in biofilm formation . Therefore, inhibiting protein adsorption and bacterial adherence is a promising approach for suppressing plaque formation and preventing root caries and secondary caries.

The prevention of root caries have been attempted by the daily usage of fluoride solutions or toothpastes , professional application of fluoride gel/varnishes , or the use of chlorhexidine solutions/varnishes . However, these treatments are temporary and success depends on the compliance of patients . Efficient and simple single-visit methods to prevent root caries are currently not available . Recently, coating of tooth root surface with adhesives was investigated as a preventive treatment against root caries, as it provides a strong physical barrier with the formation of a hybridized layer . Besides coating tooth roots, adhesives are also used to bond composite in tooth cavities. However, while composites are the principal material for cavity restorations due to their excellent aesthetics and direct-filling capability , resins in vivo tend to accumulate more plaque than other restorative materials . Furthermore, microgaps can be observed at the tooth-restoration interfaces . Microleakage can occur and biofilms at the restoration margins can produce acids and cause secondary caries . While adhesive compositions and bonding procedures have been improved , further improvements could be achieved by developing a protein-repellent and antibacterial bonding agent to combat biofilms at the tooth-restoration margins. To date, a dentin adhesive that possesses a combination of protein-repellent and antibacterial capabilities has not been reported.

Quaternary ammonium methacrylates (QAMs) were incorporated into adhesives . Antibacterial adhesives reduced biofilm viability and acid production. The antibacterial activity increased when the alkyl chain length (CL) was increased from 5 to 16 . Dentin adhesive containing a new monomer dimethylaminohexadecyl methacrylate (DMAHDM) with CL of 16 had the strongest antibacterial activity .

2-Methacryloyloxyethyl phosphorylcholine (MPC), a methacrylate with a phospholipid polar group in the side chain, is one of the most common biocompatible and hydrophilic biomedical polymers . It has been shown that hydrophilic material surfaces are more resistant to protein adsorption than hydrophobic surfaces . The MPC polymer coating rendered the surfaces extremely hydrophilic and prevented the adsorption of proteins . Recently, MPC was incorporated into dentin bonding agents and composites, achieving a strong protein-repellent capability . It would be highly desirable to combine MPC with DMAHDM to develop a novel dental adhesive with a combination of protein-repellent and antibacterial capabilities, in order to repel protein and inhibit bacteria attachment, thereby preventing root caries and secondary caries.

Exposure of root dentin after gingival recession is common among seniors because the thin cementum can be lost due to tooth-brushing or biofilm acids . Dentin differs from enamel due to the smaller mineral crystallites with a higher carbonate content . Moreover, dentin mineral is more soluble than enamel mineral . Demineralization on tooth root is approximately twice as rapid as that of enamel . Therefore, it is beneficial for the adhesive that is used to coat tooth roots to also possess remineralization capability. Calcium phosphate (CaP) composites released supersaturating levels of calcium (Ca) and phosphate (P) ions and remineralized tooth lesions in vitro . Recently, nanoparticles of amorphous calcium phosphate (NACP) with a size of 116 nm were synthesized via a spray-drying technique and filled into composites and adhesives . These nanocomposites achieved Ca and P ion release similar to those of traditional CaP composites, while possessing much better mechanical properties .

Previous studies focused on the development of adhesive systems possessing antibacterial and/or remineralization properties . To date, there has been no report on the development of a dental adhesive possessing protein-repellent, antibacterial, and Ca and P ion release capabilities. The objective of this study was to develop an adhesive for prevention of root caries and secondary caries by combining MPC with DMAHDM and NACP in the adhesive. Such an adhesive is promising for use in coating tooth roots as well as bonding restorations in tooth cavities. Recent studies showed that dental resins containing NACP remineralized enamel lesions in vitro , and inhibited secondary caries in a human in situ study . In the present study, the protein-repellent and antibacterial effects of this adhesive system were evaluated, and its dentin coating and bonding capability was investigated. It was hypothesized that: (1) incorporating MPC, DMAHDM and NACP into the adhesive would not compromise the dentin bond strength; (2) adhesive with MPC-DMAHDM-NACP would produce hermetic sealing of root dentin with a relatively thick resinous layer as a barrier against biofilm attack; (3) the MPC-DMAHDM-NACP containing adhesive would have much less protein adsorption than commercial control; and (4) the MPC-DMAHDM-NACP containing adhesive would greatly reduce biofilm viability, acid production and colony-forming unit (CFU) counts, compared to commercial control.

Materials and methods

MPC incorporation into bonding agent

Scotchbond multi-purpose (3M, St. Paul, MN), referred to as “SBMP”, was used as the parent bonding system. According to the manufacturer, SBMP etchant contained 35% phosphoric acid. SBMP primer single bottle contained 35–45% 2-hydroxyethylmethacrylate (HEMA), 10–20% copolymer of acrylic and itaconic acids, and 40–50% water. SBMP adhesive contained 60–70% BisGMA and 30–40% HEMA.

MPC (Sigma-Aldrich, St. Louis, MO) was synthesized via a method reported by Ishihara et al. . The MPC powder was mixed with SBMP primer at MPC/(SBMP primer + MPC) mass fraction of 7.5%. This mass fraction was selected from previous study showing that 7.5% MPC yielded the strongest protein-repellent property without compromising the dentin bond strength . MPC was added into primer and magnetically stirred with a bar at a speed of 150 rpm (Bellco Glass, Vineland, NJ) for 24 h to completely dissolve MPC in primer . Similarly, 7.5% of MPC was also mixed into the SBMP adhesive.

DMAHDM incorporation into bonding agent

DMAHDM was synthesized using a modified Menschutkin reaction where a tertiary amine was reacted with an organo-halide . A benefit of this reaction is that the reaction products are generated at virtually quantitative amounts and require minimal purification. Briefly, 10 mmol of 2-(dimethylamino) ethyl methacrylate (DMAEMA, Sigma-Aldrich) and 10 mmol of 1-bromohexadecane (BHD, TCI America, Portland, OR) were combined with 3 g of ethanol in a 20 mL scintillation vial. The vial was stirred at 70 °C for 24 h. The solvent was then removed via evaporation, yielding DMAHDM as a clear, colorless, and viscous liquid . The SBMP primer was first mixed with MPC as described above. Then, DMAHDM was mixed into the SBMP-MPC primer, at DMAHDM/(SBMP primer + DMAHDM) mass fraction of 5%. DMAHDM mass fractions of 7.5% or higher were not used due to a decrease in dentin bond strength when combined with MPC in preliminary study. Similarly, 5% DMAHDM was incorporated into the SBMP-MPC adhesive.

NACP incorporation into bonding agent

A spray-drying technique was used to synthesize NACP (Ca 3 [PO 4 ] 2 ) as described previously . The NACP had a mean particle size of 116 nm . NACP were incorporated into the SBMP adhesive, but not into the primer, because preliminary study showed that adding NACP into primer decreased the dentin bond strength . NACP was mixed into the adhesive at NACP/(SBMP adhesive + NACP) = 0, 20, 30, and 40% by mass. NACP mass fraction of less than 20% was not used due to the need for sufficient ion release. NACP of more than 40% was not used due to a decrease in dentin bond strength .

Dentin shear bond strength testing

As listed in Table 1 , five groups were used for dentin shear bond strength testing. The results showed that group 5 had significantly lower dentin bond strength, while all other groups had dentin bond strengths similar to SBMP control. Therefore, group 5 was not included in subsequent experiments. Groups 1–4 were used in SEM examination, and protein adsorption and biofilm assays.

Table 1
Compositions of primer and adhesive. a
Group Primer Adhesive Group name
1 SBMP primer SBMP adhesive SBMP control
2 SBMP primer + 7.5% MPC + 5% DMAHDM SBMP adhesive + 7.5% MPC + 5% DMAHDM 7.5MPC + 5DMAHDM
3 SBMP primer + 7.5% MPC + 5% DMAHDM SBMP adhesive + 7.5% MPC + 5% DMAHDM + 20% NACP 7.5MPC + 5DMAHDM + 20NACP
4 SBMP primer + 7.5% MPC + 5% DMAHDM SBMP adhesive + 7.5% MPC + 5% DMAHDM + 30% NACP 7.5MPC + 5DMAHDM + 30NACP
5 SBMP primer + 7.5% MPC + 5% DMAHDM SBMP adhesive + 7.5% MPC + 5% DMAHDM + 40% NACP 7.5MPC + 5DMAHDM + 40NACP

a MPC = 2-methacryloyloxyethyl phosphorylcholine; DMAHDM = dimethylaminohexadecyl methacrylate; NACP = nanoparticles of amorphous calcium phosphate.

Extracted caries-free human molars were used. The tips of the crowns were cut off using a diamond saw (Isomet, Buehler, Lake Bluff, IL). The cut surface of the tooth was ground using 320 grit SiC paper until occlusal enamel was completely removed . After etching for 15 s and rinsing with water , a primer was applied, and the solvent was removed with air. An adhesive was applied and air-blown for 5 s to produce a thin adhesive layer , and then light-cured for 10 s (Optilux-VCL401, Demetron, Danbury, CT). A stainless-steel iris with a central opening (diameter = 4 mm, thickness = 1.5 mm) was held against the adhesive-treated dentin surface. The opening was filled with a composite (TPH, Caulk/Dentsply, Milford, DE) and light-cured for 60 s . The bonded specimens were stored in distilled water at 37 °C for 24 h . The dentin shear bond strength, S D , was measured as previously described . A chisel was held parallel to the composite–dentin interface and loaded via a Universal Testing Machine (MTS, Eden Prairie, MN) at 0.5 mm/min until the composite–dentin bond failed. S D = 4 P /( πd 2 ), where P is the load at failure and d is the diameter of the composite . Ten teeth were tested for each group.

Fabrication of root dentin slabs

Root dentin slabs were fabricated using bovine instead of human root dentin, since the development and inhibition rates of caries are similar in both tissues , and bovine teeth are easier to obtain. The specimen preparation procedure followed previous studies . Extracted bovine incisors with intact roots were used. The root was separated from the crown at 1–2 mm below the cemento-enamel junction using a water-cooled diamond saw (Isomet) . Then, root dentin slabs were prepared from buccal and lingual root surfaces with a water-cooled diamond saw . The slabs were mounted with sticky wax on plexiglas blocks to facilitate handling, and were serially polished to remove the cementum and flatten the surfaces . The polishing procedure was performed with wet SiC papers up to 4000 grit. The polished slabs were examined microscopically (20×), to ensure that a smooth dentin surface was obtained . The root slabs were cleaned by sonication for 10 min in distilled water . The final dimensions of the root dentin slabs were approximately 5 mm in length, 3 mm in width, and 2 mm in thickness . One dentin surface of 5 mm × 3 mm faced the cementum, while the other 5 mm × 3 mm surface faced the pulp. Primer and adhesive were applied to the dentin surface that faced the cementum with open dentinal tubules, in order to simulate the clinical application of using an adhesive to cover roots with missing cementum and open tubules .

Scanning electron microscopic (SEM) examinations

Because adhesives can provide a barrier to protect the exposed roots from acid attacks , and a thicker coating is expected to provide a stronger barrier, the coating thickness was measured. Root dentin surface (with cementum removed) was covered with a single coat of an adhesive via the micro-brush supplied in the package, following the manufacturer’s instructions. First, the root dentin slab surface was etched with etchant for 15 s and rinsed with water, and dried with air . A primer was applied to the dentin surface and left for 20 s, and air-blown for 5 s . Then, 20 μL of adhesive was applied using pipette and rubbed in for 10 s with the micro-brush . The adhesive was light-cured for 10 s (Optilux). The different viscosity of the four adhesive systems produced different coating thicknesses.

Cross-sectional cutting would yield information on coating thickness and the resin–dentin interface; however, the adhesive coating on dentin could chip and dislodge in the cutting process. To avoid adhesive coating fracture and to enable the measurement of coating thickness, a composite (TPH) was placed on the top of the adhesive coating and light-cured for 60 s, to facilitate the subsequent cutting and the measurement of the adhesive coating thickness . The samples were cut through the center perpendicularly to the coated surface via a diamond saw (Isomet) with copious water. The cross-section was polished with increasingly finer SiC papers up to 4000 grit . The specimens were air dried and sputter-coated with gold, and examined in SEM (Quanta 200, FEI, Hillsboro, OR). The thickness of adhesive coating on root dentin was measured for six specimens. Four readings were taken at random locations on each specimen, yielding a total of 24 measurements per group.

To examine the interfaces, additional specimens were treated with 50% phosphoric acid for 30 s and 5% NaOCl for 10 min before being prepared for SEM observation . To observe the adhesive surface texture, root dentin surfaces were coated with one of the adhesives in the same manner as described above (without TPH and without cutting). The surfaces were then air-dried and sputter-coated with gold for SEM observations .

Measurement of protein adsorption onto resin surface

To test the protein-repellent properties of groups 1–4, resin disks were fabricated. The cover of a sterile 96-well plate (Costar, Corning Inc., Corning, NY) was used as molds following a previous study . Briefly, 10 μL of a primer was placed in the bottom of each dent of the 96-well plate. After drying with a stream of air, 20 μL of adhesive was applied to the dent and photo-polymerized for 30 s (Optilux), using a Mylar strip covering to obtain a disk of 8 mm in diameter and 0.5 mm in thickness. The cured resin disks were immersed in 200 mL of distilled water and magnetically stirred with a bar at a speed of 100 rpm for 1 h to remove any uncured monomers, following a previous study . The disks were then sterilized with ethylene oxide (Anprolene AN 74i, Andersen, Haw River, NC) and de-gassed for 3 days .

The amount of protein adsorbed on resin disks was determined by the micro bicinchoninic acid (BCA) method . First, the disks were immersed in phosphate buffered saline (PBS) for 2 h. Then they were immersing in 4.5 g/L bovine serum albumin (BSA) (Sigma-Aldrich) solution at 37 °C for 2 h. This solution had a concentration of 4.5 g of BSA per 1 L of PBS, following previous studies . The disks then were rinsed with fresh PBS by stirring at a speed of 300 rpm for 5 min (Bellco Glass, Vineland, NJ), immersed in sodium dodecyl sulfate (SDS) 1 wt% in PBS, and sonicated at room temperature for 20 min to completely detach the BSA adsorbed onto the disk surface. A protein analysis kit (micro BCA protein assay kit, Fisher Scientific, Pittsburgh, PA) was used to determine the BSA concentration in the SDS solution. Briefly, 25 μL of the SDS solution was mixed with 200 μL of the BCA working reagent in a 96-well plate, which was incubated at 60 °C for 30 min . Then the 96-well plate was cooled down to room temperature and the absorbance at 562 nm was measured via a microplate reader (SpectraMax M5, Molecular Devices, Sunnyvale, CA). Standard curves were prepared using the BSA standard. From the concentration of protein, the amount of protein adsorbed on the disk surface was calculated . Six disks were evaluated for each group.

Saliva collection for dental plaque microcosm biofilm model

The dental plaque microcosm model using human saliva as inoculum has been approved by the University of Maryland Baltimore Institutional Review Board (# HP-00050407). Saliva is ideal for growing dental plaque microcosm biofilms in vitro , with the advantage of maintaining much of the complexity and heterogeneity of the dental plaque in vivo . Saliva was collected from 10 healthy adult donors having natural dentition without active caries or periopathology, and without the use of antibiotics within the last 3 months . The donors did not brush teeth for 24 h and abstained from food and drink intake for 2 h prior to donating saliva. Stimulated saliva was collected during paraffin chewing and was kept on ice. An equal volume of saliva from each of the 10 donors was combined. The saliva was diluted in sterile glycerol to a concentration of 70%, and stored at −80 °C .

Dental plaque microcosm biofilm formation and live/dead assay

The saliva–glycerol stock was added, with 1:50 final dilution, to a growth medium as inoculum . The growth medium contained mucin (type II, porcine, gastric) at a concentration of 2.5 g/L; bacteriological peptone, 2.0 g/L; tryptone, 2.0 g/L; yeast extract, 1.0 g/L; NaCl, 0.35 g/L, KCl, 0.2 g/L; CaCl 2 , 0.2 g/L; cysteine hydrochloride, 0.1 g/L; hemin, 0.001 g/L; vitamin K1, 0.0002 g/L, at pH 7 . Each resin disk was placed into a 24-well plates, 1.5 mL of inoculum was added to each well, and incubated at 37 °C in 5% CO 2 for 8 h. Then, the disks were transferred to new 24-well plates with fresh medium. After 16 h, the disks were transferred to new 24-well plates with fresh medium and incubated for 24 h. This totaled 2 days of incubation, which was shown to form mature biofilms on resin .

Disks with biofilms grown for 2 days were rinsed with phosphate buffered saline (PBS) and live/dead stained using the BacLight live/dead bacterial viability kit (Molecular Probes, Eugene, OR) . Live bacteria were stained with Syto 9 to produce a green fluorescence. Dead bacteria were stained with propidium iodide to produce a red fluorescence. Disks were examined using an inverted epifluorescence microscope (Eclipse TE2000-S, Nikon, Melville, NY). Six specimens were evaluated for each group. Three randomly chosen fields of view were photographed from each disk, yielding a total of 18 images for each group.

MTT assay of metabolic activity

Resin disks with 2-day biofilms were transferred to a new 24-well plate for the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay . MTT is a colorimetric assay that measures the enzymatic reduction of MTT, a yellow tetrazole, to formazan. One milliliter of MTT dye (0.5 mg/mL MTT in PBS) was added to each well and incubated at 37 °C in 5% CO 2 for 1 h. Metabolically active bacteria reduced the MTT to purple formazan. Disks were transferred to a new 24-well plate, and 1 mL of dimethyl sulfoxide (DMSO) was added to solubilize the formazan crystals. The plate was incubated for 20 min with gentle mixing at room temperature in the dark. Then, 200 μL of the DMSO solution from each well was collected, and its absorbance at 540 nm was measured via a microplate reader (SpectraMax M5). A higher absorbance is related to a higher formazan concentration, which indicates a higher metabolic activity in the biofilm .

Lactic acid production and colony-forming unit (CFU) counts of biofilms

Disks with 2-day biofilms were rinsed with cysteine peptone water (CPW) to remove loose bacteria . Disks were transferred to 24-well plates containing buffered peptone water (BPW) plus 0.2% sucrose. The disks were incubated in 5% CO 2 at 37 °C for 3 h to allow biofilms to produce acid . The BPW solutions were collected for lactate analysis using an enzymatic method. The 340-nm absorbance was measured with the microplate reader. Standard curves were prepared using a standard lactic acid (Supelco, Bellefonte, PA).

Disks with biofilms were transferred into tubes with 2 mL CPW, and biofilms were harvested by sonication and vortexing (Fisher, Pittsburgh, PA) . Three types of agar plates were prepared. First, tryptic soy blood agar culture plates were used to determine total microorganisms . Second, mitis salivarius agar (MSA) plates with 15% sucrose were used to determine total streptococci . This is because MSA contains selective agents including crystal violet, potassium tellurite and trypan blue, which inhibit most Gram-negative bacilli and most Gram-positive bacteria except streptococci, thus enabling the streptococci to grow . Third, cariogenic mutans streptococci are known to be resistant to bacitracin, and this property is used to isolate mutans streptococci from the highly heterogeneous oral microflora . Therefore, MSA agar culture plates plus 0.2 units of bacitracin per milliliter was used to determine mutans streptococci . The bacterial suspensions were serially diluted and spread onto agar plates for CFU analysis.

Statistical analysis

All data were first checked for normal distribution with the Kolmogorov–Smirnov test. One-way and two-way analyses of variance (ANOVA) were performed to detect the significant effects of the variables. Tukey’s multiple comparison test was used to compare the data at a p -value of 0.05.

Materials and methods

MPC incorporation into bonding agent

Scotchbond multi-purpose (3M, St. Paul, MN), referred to as “SBMP”, was used as the parent bonding system. According to the manufacturer, SBMP etchant contained 35% phosphoric acid. SBMP primer single bottle contained 35–45% 2-hydroxyethylmethacrylate (HEMA), 10–20% copolymer of acrylic and itaconic acids, and 40–50% water. SBMP adhesive contained 60–70% BisGMA and 30–40% HEMA.

MPC (Sigma-Aldrich, St. Louis, MO) was synthesized via a method reported by Ishihara et al. . The MPC powder was mixed with SBMP primer at MPC/(SBMP primer + MPC) mass fraction of 7.5%. This mass fraction was selected from previous study showing that 7.5% MPC yielded the strongest protein-repellent property without compromising the dentin bond strength . MPC was added into primer and magnetically stirred with a bar at a speed of 150 rpm (Bellco Glass, Vineland, NJ) for 24 h to completely dissolve MPC in primer . Similarly, 7.5% of MPC was also mixed into the SBMP adhesive.

DMAHDM incorporation into bonding agent

DMAHDM was synthesized using a modified Menschutkin reaction where a tertiary amine was reacted with an organo-halide . A benefit of this reaction is that the reaction products are generated at virtually quantitative amounts and require minimal purification. Briefly, 10 mmol of 2-(dimethylamino) ethyl methacrylate (DMAEMA, Sigma-Aldrich) and 10 mmol of 1-bromohexadecane (BHD, TCI America, Portland, OR) were combined with 3 g of ethanol in a 20 mL scintillation vial. The vial was stirred at 70 °C for 24 h. The solvent was then removed via evaporation, yielding DMAHDM as a clear, colorless, and viscous liquid . The SBMP primer was first mixed with MPC as described above. Then, DMAHDM was mixed into the SBMP-MPC primer, at DMAHDM/(SBMP primer + DMAHDM) mass fraction of 5%. DMAHDM mass fractions of 7.5% or higher were not used due to a decrease in dentin bond strength when combined with MPC in preliminary study. Similarly, 5% DMAHDM was incorporated into the SBMP-MPC adhesive.

NACP incorporation into bonding agent

A spray-drying technique was used to synthesize NACP (Ca 3 [PO 4 ] 2 ) as described previously . The NACP had a mean particle size of 116 nm . NACP were incorporated into the SBMP adhesive, but not into the primer, because preliminary study showed that adding NACP into primer decreased the dentin bond strength . NACP was mixed into the adhesive at NACP/(SBMP adhesive + NACP) = 0, 20, 30, and 40% by mass. NACP mass fraction of less than 20% was not used due to the need for sufficient ion release. NACP of more than 40% was not used due to a decrease in dentin bond strength .

Dentin shear bond strength testing

As listed in Table 1 , five groups were used for dentin shear bond strength testing. The results showed that group 5 had significantly lower dentin bond strength, while all other groups had dentin bond strengths similar to SBMP control. Therefore, group 5 was not included in subsequent experiments. Groups 1–4 were used in SEM examination, and protein adsorption and biofilm assays.

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Development of a multifunctional adhesive system for prevention of root caries and secondary caries
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