NIH Public Access Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2014 March 26.
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Published in final edited form as: Polymer Prepr. 2010 January 1; 51(1): 173–174.
Physicochemical Evaluation of an Experimental Endodontic Sealer J. M. Antonucci1 and D. Skrtic2 1 Polymers Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD 2
Paffenbarger Research Center (PRC), American Dental Association Foundation (ADAF), Gaithersburg, MD
Introduction
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Amorphous calcium phosphate (ACP)-based dental materials have tremendous appeal due to: (a) their potential to remineralize defective tooth structures and (b) their high biocompatibility. For the last decade we have systematically investigated structurecomposition-property relationships of ACP fillers with a variety of methacrylic resins in order to develop strategies that better control ACP’s dispersion in the polymer matrix and aid in our understanding of the complex interactions at the inorganic filler/organic matrix interface that are critical to the physicochemical performance of these composites. As a result of this research, prototypes of ACP-based composites intended for dental use as pit and fissure sealants and as orthodontic adhesives were formulated [1, 2]. In both cases, aqueous solution solubility of ACP enabled the release of supersaturating levels of calcium and phosphate ions intra-lesionally and shifted the solution thermodynamic driving forces toward the formation of apatite [3, 4]. Our in vitro experiments indicated that ACP can sustain these supersaturation conditions over extended periods of time [5,6].
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In this study, we evaluate an experimental ACP composite formulated for endodontic application to serve as a biocompatible, easy-to-manipulate root canal sealer. The hypothesis was that urethane dimethacrylate (UDMA) resin system (designated UPHM) consisting of UDMA, a poly(ethyleneglycol)-extended UDMA (PEG-U), 2-hydroxyethyl methacrylate (HEMA), and methacryloyloxyethyl phthalate (MEP) as well as ACP/UPHM composites would yield copolymers with a high degree of vinyl conversion (DVC) without the excessive polymerization shrinkage (PS) and/or stress development (PSS), while maintaining desirable ion release profiles, i.e., the remineralizing potential of the composite. It is further hypothesized that ACP/UPHM composite will exhibit relatively high water sorption (WS) accompanied with significant hygroscopic expansion (HE) of composite specimens. The latter may offset the high PS developed in these composites. It is expected that the relatively high WS will not diminish composites’ mechanical strength more than it is customarily seen in other types of ACP methacrylate composites [7, 8]. To test the above hypotheses, we prepared dual-cure (DC; light + chemical cure) UPHM resins (due to the nature of the intended application as endodontic sealer, light cure alone may be clinically inadequate), fabricated ACP/UPHM composites, measured DVC attained in copolymers and composites, determined PS and PSS, WS, HE, mechanical strength and ion release from composites upon mid-term exposure to aqueous milieu. The results of the physicochemical
Disclaimer. Certain commercial materials and equipment are identified in this article to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by NIST or ADAF or that the material or equipment identified is necessarily the best available for the purpose. “Official contribution of NIST; not subject to copyrights in USA”.
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evaluation will determine whether the experimental composite is a suitable candidate for quantitative assessment of leachables and cellular responses before being recommended for clinical testing.
Experimental Formulation of the resins The experimental resin was formulated from the commercial UDMA, HEMA and MEP monomers and an oligomeric urethane dimethacrylate co-monomer, PEG-U (Table 1). They were hand-blended into UDMA:PEG-U:HEMA:MEP resin. After combining all the monomers, their mixture was magnetically stirred (38 rad/s; in the absence of blue light. Homogenized monomer mixture was divided into two equal parts by mass which were activated for DC by adding the required amounts of 1850 IRGACURE and BPO to one part and DHEPT to the other part. ACP synthesis and characterization
NIH-PA Author Manuscript
As-made zirconia-hybridized ACP (designated am-ACP) was synthesized as described in detail in [1, 2]. Grinding of am-ACP was performed according to the procedure detailed in [9]. Both am- and ground (g-ACP) filler were characterized by X-ray diffraction (XRD), Fourier-transform Infrared (FTIR) spectroscopy, particle size distribution (PSD) analysis and scanning electron microscopy (SEM) as detailed in [1–4]. Preparation of copolymer and composite specimens Copolymer (unfilled resin) and composite specimens alike were placed into teflon molds, each opening was covered with a Mylar film and glass slide, and the entire assembly was clamped in place by spring clips. DC formulations were prepared by mixing equal amounts of (1850IRGACURE+BPO)- and DHEPT-containing UPHM resin and then irradiating the mixture for 30 s at an intensity of 450 mW/cm2 with a dental curing unit (Dentsply Spectrum 800, Dentsply Caulk, Milford, DE, USA). Composites were prepared by handmixing (1850IRGACURE+BPO)- and DHEPT-containing UPHM resin (mass fraction 60 %) and either am- or g-ACP (mass fraction 40 %). The paste was mixed until a uniform consistency was achieved, with no remaining visible particulates. Two components were than combined in 1:1 mass ratio and light cured in identical manner as copolymers. Control composite specimens were prepared with the unsilanized Sr-glass (Denstply Caulk, Milford, DE, USA). To achieve handling properties comparable to ACP-composites, 70 mass % glass needed to be blended with the resin.
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Degree of vinyl conversion (DVC) To determine the degree of vinyl conversion (DVC) attained after polymerization of the copolymers and composites, a near infrared (NIR) spectroscopic technique was employed [13]. The change in the =CH2 absorption band at 6165 cm−1 in the overtone region was used to assess the DVC in paired unpolymerized and polymer samples. NIR spectra of the specimens were acquired before cure, and 5 h, 24 h and 5 d post-cure. The decrease in integrated peak area following polymerization was used to calculate DVC. Polymerization shrinkage (PS) The PS of composite resin samples was measured by a computer-controlled mercury dilatometer (PRC-ADAF, Gaithersburg, MD, USA). Composite pastes were cured using a 60 s/30 s exposure and data acquisition of (60 min+30 min). PS of a specimen corrected for temperature fluctuations during the measurement was plotted as a function of time. The
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overall shrinkage (volume fraction, %) was calculated based on the known mass of the sample [(50 to 100) mg] and its density. The latter was determined by means of the Archimedean displacement principle using an attachment to a microbalance (YDK01 Density Determination Kit; Sartorius AG, Goettingen, Germany). Polymerization stress (PSS) A cantilever beam tensometer (PRC-ADAF, Gaithersburg, MD, USA; [10]) was used to assess the PSS of the composites. The corresponding software program was also developed at the PRC-ADAF. The deflection of the cantilever beam was measured with a linear variable differential transformer. The force was calculated from a beam length (12.5 cm) and a calibration constant (3.9 N/μm). PSS was obtained by dividing the measured force by the cross sectional area of the sample (diameter = 6 mm). A cavity design factor (C-factor), defined as the ratio of bonded composite area (the silanated ends of the silica rods) to the unbonded area (the compliant plastic enclosure), of 1.33 (h = 2.25 mm) was maintained in all experiments. Water sorption (WS) and hygroscopic expansion (HE)
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A minimum of five disk specimens/group (cylindrical disk geometry) were initially dried over CaSO4 until a constant mass was achieved (± 0.1 mg) and then exposed either to an air atmosphere of 75 % relative humidity (RH) at 23 °C by keeping them suspended over a saturated aqueous NaCl slurry in closed systems or immersed in 25 mL of 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-buffered, pH=7.4, 0.13 mol/L NaCl solution at 23 °C. Their mass (determined gravimetrically) was recorded at different times of RH exposure and immersion. In addition, diameter and thickness (measured by micrometer) were determined for the immersed specimens. Mass and dimensional data were used to calculate relative mass (Δ mass, %) and volume (Δ vol (%) = hygroscopic expansion, HE) changes of the samples at time t vs. the initial mass and volume, respectively. Biaxial flexure strength (BFS) To compare the mechanical strength of dry (kept in the air 24 h at 23 °C) and wet (up to 3 months immersion in NaCl solution at 23 °C) copolymer and composite specimens, their biaxial flexure strength (BFS) was determined using a computer-controlled Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA) operated by Instron Merlin Software series 9. BFS of disk specimens (n=5/group) supported along a lower support circle was determined under a static load utilizing a piston-on-three-ball loading arrangement with a cross head speed of 0.5 mm/min. The failure stress was calculated according to [11].
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Mineral ion release Kinetics of the mineral ion release from ACP composite specimens was assessed at 23 °C, in magnetically stirred, HEPES-buffered (pH=7.40) NaCl solutions (25 mL solution/ specimen). The immersing solution was replaced at predetermined time intervals (up to 3 months). Ca and PO4 solution levels were determined by atomic emission spectroscopy (Prodigy High Dispersion inductively coupled plasma optical emission spectrometer; Teledyne Leeman Labs, Hudson, NH, USA). The ion activity product (IAP) and the saturation ratio (SR) with respect to enamel attained in the immersing solutions as a result of Ca and PO4 ion release from composites was calculated according to the following expression [1, 2]: SRenamel) =(IAP/Ksp)1/n, where Ksp is the corresponding thermodynamic solubility product (pKSP = −log(KSP) that equals 108.6 for enamel and n is the number of ions in the IAP (n=18). SR values > 1.00 indicate solution supersaturated with respect to enamel.
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Statistical data analysis
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Experimental data were analyzed by ANOVA (α = 0.05). Significant differences between the groups were determined by all pairwise multiple comparisons (Tukey-test). One standard deviation (SD) is identified in this paper for comparative purposes as the estimated uncertainty of the measurements.
Results and Discussion The results of the physicochemical evaluation of DC UPHM copolymers and their am- and g-ACP composites are summarized in Table 2.
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Am-ACP composites yielded the DVC values comparable to those attained in corresponding copolymers. DVC values achieved with g-ACP composites were higher than or equal to DVC of DC copolymers. The PS could only be measured with the light-cure formulation. In dual-cure systems, a paste hardened within 10 min of mixing the chemically activated components. Some degree of contraction therefore occured before the sample was even placed in dilatometer, making the material unsuitable for measurement by this method. Relatively high PS values measured in light-cured am- and g-ACP composites ((7.1 ± 0.3) % and (6.9 ± 0.1) %, respectively) significantly exceeded the PS values of glass-control composite [(4.4 ± 0.1) %]. However, normalizing PS values of the glass control to the resin/ filler ratios of ACP composites brought them in line, with an average of 7.7 %. The PSS that developed in am- and g-ACP composites was significantly higher than the PSS in control Sr-glass composites. Plateau, WSmax values, reached within the initial 336 h of exposure to 75 % RH decreased in the following order: am-ACP composite > (g-ACP composite, copolymer) > Sr-glass composite. Specimen mass increases upon immersion in saline solutions were roughly (2.5 to 3.0) times higher compared to the corresponding specimens exposed to RH. The WSmax values of the immersed specimens and the accompanying HE values decreased in the following order: (am- and g-ACP composite > copolymer) > Srglass composite. In both dry state and after aqueous immersion, copolymers exhibited significantly higher BFS than ACP- or glass composites. Significantly, differences between the BFS of dry and wet DC am- and g-ACP composites were not statistically significant (therefore, only wet BFS values are shown in Table 2). Kinetics profiles of Ca and PO4 release ACP UPHM composites revealed that within 3 mo of aqueous immersion, no plateau (maximum) concentration of either ion had yet been reached. The supersaturation conditions attained with both am-ACP as well as g-ACP composite specimens were significantly above the minimum necessary for the potential mineral re-deposition.
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A high DVC seen in UPHM formulations expectedly resulted in elevated PS values, since the PS is directly related to DVC [12]. While significantly higher than the PS values reported for the commercial restoratives [(1.9 to 4.1) %] and flowable composites [(3.6 to 6.0) %], the PS values of ACP/UPHM composites only slightly exceeded the lower end values seen in adhesive resins [(6.7 to 13.5) % [1, 2, 8]]. This relatively high PS may be attributed to the intensified hydrogen bonding in UPHM resins with relatively high amount of HEMA (17 mass %). The measured PSS values compare well with the PSS that developed in am-ACP composites based on binary UDMA/HEMA composites [9]. This finding would suggest that the actual stress developed as a consequence of polymerization shrinkage in UPHM matrices was not elevated by the simultaneous inclusion of PEG-U and HEMA in UDMA-based resin matrix. The shrinkage of ACP UPHM composite specimens should be more than compensated for by the HE (up to 14 vol %) they undergo during aqueous immersion. The potential beneficial effects of HE have been reported by other researchers [13, 14].
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Conclusions NIH-PA Author Manuscript
DC UPHM and its ACP composites formulated for endodontic uses attained high DVC accompanied by relatively high PS and moderate PSS. The high DVC attained in these composites suggests that a minimal possibility for leachability of un-reacted monomeric species and, consequently, minimal adverse cellular response to these composites. These experimental materials also exhibited relatively high levels of WS, accompanied by a significant HE. The latter may be useful in combating stresses that develop in these materials. Ion release profiles of the experimental materials confirmed their potential for regeneration of mineral-deficient tooth structures. Their moderate to low BFS, similar to that of ACP orthodontic adhesive, did not diminish the potential for the proposed application as an endodontic sealer. Micro-leakage and quantitative leachability studies should be performed to further establish the viability of these experimental materials.
Acknowledgments Support from the NIDCR (grant DE 13169; NIDCR/NIST Interagency Agreement YI-DE-7005-01) and contribution of monomers from Esstech, Essington, PA, USA are gratefully acknowledged.
References NIH-PA Author Manuscript
1. Skrtic D, et al. J Res Natl Inst Stands Technol. 2003; 108(3):167. 2. Antonucci, JM.; Skrtic, D. Polymers for Dental and Orthopedic Applications. Shalaby, W.; Salz, U., editors. CRC Press; Boca Raton, FL: 2007. p. 217-242. 3. Skrtic D, et al. Acta Biomaterialia. 2006; 2:85. [PubMed: 16701862] 4. O’Donnell JNR, et al. J Bioact Comp Polym. 2008; 23:207. 5. Skrtic D, et al. J Dent Res. 1996; 75(9):1679. [PubMed: 8952621] 6. Langhorst SE, et al. Dent Mater. 2009; 25:884. [PubMed: 19215975] 7. Skrtic D, et al. Biomaterials. 2004; 25:1141–1150. [PubMed: 14643587] 8. Skrtic D, Antonucci JM. J Biomat Appl. 2007; 21:375–393. 9. O’Donnell JNR, Skrtic D. J Biomim Biomater Tissue Eng. 2009 in press. 10. Lu H, et al. J Mater Sci: Mater Med. 2004; 15:1097. [PubMed: 15516870] 11. ASTM F394–78 (re-approved 1991). 12. Silikas N, et al. Dent Mater. 2000; 16(4):292. [PubMed: 10831785] 13. Huang C, et al. J Dent. 2002; 30:11. [PubMed: 11741730] 14. Momoi Y, McCabe JF. Brit Dent J. 1994; 176:91. [PubMed: 7599006]
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Table 1
The composition of dual cure (DC) UPHM resin used in the study.
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Monomers/components of the polymerization initiator system
Acronym
Mass %
urethane dimethacrylate
UDMA
47.2
poly(ethyleneglycol)-extended UDMA
PEG-U
29.1
2-hydroxyethyl methacrylate
HEMA
16.8
methacryloyloxyethyl phthalate
MEP
2.9
bis(2,6-dimethoxynezoyl)-2,4,4- triethylpentyl phosphine oxide & 1-hydroxycyclohexyl phenyl ketone
1850IRGACURE
1.0
benzoyl peroxide
BPO
2.0
2,2′-dihydroxyethyl-p-toluidine
DHEPT
1.0
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Table 2
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Physicochemical characteristics of the experimental copolymers and composites (mean value±standard deviation (SD)). Property
Copolymer
am-ACP composite
g-ACP composite
Sr-glass composite
79.3±3.4 81.9±2.2
76.0±3.4 81.2±2.6
85.4±3.2 81.9±2.2
85.8±3.0 86.5±1.0
PS* (vol %)
Nd
7.1±0.3
6.9±0.1
4.4±0.1
PSS (MPa)
Nd
3.7±0.3
3.6±0.2
2.2±0.1
WSmax (%) RH immersion
2.5±0.5 6.6±0.5
2.9±0.1 8.2±0.5
2.6±0.2 9.3±0.4
2.2±0.1 5.2±0.2
HE (vol %)
8.2±1.2
13.7±1.2
14.4±1.1
4.3±0.4
BFS (MPa) wet
124±41
42±4
47±9
69±6
n/a
4.6±0.2
3.7±0.1
n/a
DVC (%) 24 h 5d
SRenamel
nd=not determined; n/a not applicable. *
Indicated PS values were measured with the light-cure UPHM formulations.
NIH-PA Author Manuscript NIH-PA Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2014 March 26.
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Published in final edited form as: Polymer Prepr. 2010 January 1; 51(1): 173–174.
Physicochemical Evaluation of an Experimental Endodontic Sealer J. M. Antonucci1 and D. Skrtic2 1 Polymers Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD 2
Paffenbarger Research Center (PRC), American Dental Association Foundation (ADAF), Gaithersburg, MD
Introduction
NIH-PA Author Manuscript
Amorphous calcium phosphate (ACP)-based dental materials have tremendous appeal due to: (a) their potential to remineralize defective tooth structures and (b) their high biocompatibility. For the last decade we have systematically investigated structurecomposition-property relationships of ACP fillers with a variety of methacrylic resins in order to develop strategies that better control ACP’s dispersion in the polymer matrix and aid in our understanding of the complex interactions at the inorganic filler/organic matrix interface that are critical to the physicochemical performance of these composites. As a result of this research, prototypes of ACP-based composites intended for dental use as pit and fissure sealants and as orthodontic adhesives were formulated [1, 2]. In both cases, aqueous solution solubility of ACP enabled the release of supersaturating levels of calcium and phosphate ions intra-lesionally and shifted the solution thermodynamic driving forces toward the formation of apatite [3, 4]. Our in vitro experiments indicated that ACP can sustain these supersaturation conditions over extended periods of time [5,6].
NIH-PA Author Manuscript
In this study, we evaluate an experimental ACP composite formulated for endodontic application to serve as a biocompatible, easy-to-manipulate root canal sealer. The hypothesis was that urethane dimethacrylate (UDMA) resin system (designated UPHM) consisting of UDMA, a poly(ethyleneglycol)-extended UDMA (PEG-U), 2-hydroxyethyl methacrylate (HEMA), and methacryloyloxyethyl phthalate (MEP) as well as ACP/UPHM composites would yield copolymers with a high degree of vinyl conversion (DVC) without the excessive polymerization shrinkage (PS) and/or stress development (PSS), while maintaining desirable ion release profiles, i.e., the remineralizing potential of the composite. It is further hypothesized that ACP/UPHM composite will exhibit relatively high water sorption (WS) accompanied with significant hygroscopic expansion (HE) of composite specimens. The latter may offset the high PS developed in these composites. It is expected that the relatively high WS will not diminish composites’ mechanical strength more than it is customarily seen in other types of ACP methacrylate composites [7, 8]. To test the above hypotheses, we prepared dual-cure (DC; light + chemical cure) UPHM resins (due to the nature of the intended application as endodontic sealer, light cure alone may be clinically inadequate), fabricated ACP/UPHM composites, measured DVC attained in copolymers and composites, determined PS and PSS, WS, HE, mechanical strength and ion release from composites upon mid-term exposure to aqueous milieu. The results of the physicochemical
Disclaimer. Certain commercial materials and equipment are identified in this article to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by NIST or ADAF or that the material or equipment identified is necessarily the best available for the purpose. “Official contribution of NIST; not subject to copyrights in USA”.
Antonucci and Skrtic
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evaluation will determine whether the experimental composite is a suitable candidate for quantitative assessment of leachables and cellular responses before being recommended for clinical testing.
Experimental Formulation of the resins The experimental resin was formulated from the commercial UDMA, HEMA and MEP monomers and an oligomeric urethane dimethacrylate co-monomer, PEG-U (Table 1). They were hand-blended into UDMA:PEG-U:HEMA:MEP resin. After combining all the monomers, their mixture was magnetically stirred (38 rad/s; in the absence of blue light. Homogenized monomer mixture was divided into two equal parts by mass which were activated for DC by adding the required amounts of 1850 IRGACURE and BPO to one part and DHEPT to the other part. ACP synthesis and characterization
NIH-PA Author Manuscript
As-made zirconia-hybridized ACP (designated am-ACP) was synthesized as described in detail in [1, 2]. Grinding of am-ACP was performed according to the procedure detailed in [9]. Both am- and ground (g-ACP) filler were characterized by X-ray diffraction (XRD), Fourier-transform Infrared (FTIR) spectroscopy, particle size distribution (PSD) analysis and scanning electron microscopy (SEM) as detailed in [1–4]. Preparation of copolymer and composite specimens Copolymer (unfilled resin) and composite specimens alike were placed into teflon molds, each opening was covered with a Mylar film and glass slide, and the entire assembly was clamped in place by spring clips. DC formulations were prepared by mixing equal amounts of (1850IRGACURE+BPO)- and DHEPT-containing UPHM resin and then irradiating the mixture for 30 s at an intensity of 450 mW/cm2 with a dental curing unit (Dentsply Spectrum 800, Dentsply Caulk, Milford, DE, USA). Composites were prepared by handmixing (1850IRGACURE+BPO)- and DHEPT-containing UPHM resin (mass fraction 60 %) and either am- or g-ACP (mass fraction 40 %). The paste was mixed until a uniform consistency was achieved, with no remaining visible particulates. Two components were than combined in 1:1 mass ratio and light cured in identical manner as copolymers. Control composite specimens were prepared with the unsilanized Sr-glass (Denstply Caulk, Milford, DE, USA). To achieve handling properties comparable to ACP-composites, 70 mass % glass needed to be blended with the resin.
NIH-PA Author Manuscript
Degree of vinyl conversion (DVC) To determine the degree of vinyl conversion (DVC) attained after polymerization of the copolymers and composites, a near infrared (NIR) spectroscopic technique was employed [13]. The change in the =CH2 absorption band at 6165 cm−1 in the overtone region was used to assess the DVC in paired unpolymerized and polymer samples. NIR spectra of the specimens were acquired before cure, and 5 h, 24 h and 5 d post-cure. The decrease in integrated peak area following polymerization was used to calculate DVC. Polymerization shrinkage (PS) The PS of composite resin samples was measured by a computer-controlled mercury dilatometer (PRC-ADAF, Gaithersburg, MD, USA). Composite pastes were cured using a 60 s/30 s exposure and data acquisition of (60 min+30 min). PS of a specimen corrected for temperature fluctuations during the measurement was plotted as a function of time. The
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overall shrinkage (volume fraction, %) was calculated based on the known mass of the sample [(50 to 100) mg] and its density. The latter was determined by means of the Archimedean displacement principle using an attachment to a microbalance (YDK01 Density Determination Kit; Sartorius AG, Goettingen, Germany). Polymerization stress (PSS) A cantilever beam tensometer (PRC-ADAF, Gaithersburg, MD, USA; [10]) was used to assess the PSS of the composites. The corresponding software program was also developed at the PRC-ADAF. The deflection of the cantilever beam was measured with a linear variable differential transformer. The force was calculated from a beam length (12.5 cm) and a calibration constant (3.9 N/μm). PSS was obtained by dividing the measured force by the cross sectional area of the sample (diameter = 6 mm). A cavity design factor (C-factor), defined as the ratio of bonded composite area (the silanated ends of the silica rods) to the unbonded area (the compliant plastic enclosure), of 1.33 (h = 2.25 mm) was maintained in all experiments. Water sorption (WS) and hygroscopic expansion (HE)
NIH-PA Author Manuscript
A minimum of five disk specimens/group (cylindrical disk geometry) were initially dried over CaSO4 until a constant mass was achieved (± 0.1 mg) and then exposed either to an air atmosphere of 75 % relative humidity (RH) at 23 °C by keeping them suspended over a saturated aqueous NaCl slurry in closed systems or immersed in 25 mL of 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-buffered, pH=7.4, 0.13 mol/L NaCl solution at 23 °C. Their mass (determined gravimetrically) was recorded at different times of RH exposure and immersion. In addition, diameter and thickness (measured by micrometer) were determined for the immersed specimens. Mass and dimensional data were used to calculate relative mass (Δ mass, %) and volume (Δ vol (%) = hygroscopic expansion, HE) changes of the samples at time t vs. the initial mass and volume, respectively. Biaxial flexure strength (BFS) To compare the mechanical strength of dry (kept in the air 24 h at 23 °C) and wet (up to 3 months immersion in NaCl solution at 23 °C) copolymer and composite specimens, their biaxial flexure strength (BFS) was determined using a computer-controlled Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA) operated by Instron Merlin Software series 9. BFS of disk specimens (n=5/group) supported along a lower support circle was determined under a static load utilizing a piston-on-three-ball loading arrangement with a cross head speed of 0.5 mm/min. The failure stress was calculated according to [11].
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Mineral ion release Kinetics of the mineral ion release from ACP composite specimens was assessed at 23 °C, in magnetically stirred, HEPES-buffered (pH=7.40) NaCl solutions (25 mL solution/ specimen). The immersing solution was replaced at predetermined time intervals (up to 3 months). Ca and PO4 solution levels were determined by atomic emission spectroscopy (Prodigy High Dispersion inductively coupled plasma optical emission spectrometer; Teledyne Leeman Labs, Hudson, NH, USA). The ion activity product (IAP) and the saturation ratio (SR) with respect to enamel attained in the immersing solutions as a result of Ca and PO4 ion release from composites was calculated according to the following expression [1, 2]: SRenamel) =(IAP/Ksp)1/n, where Ksp is the corresponding thermodynamic solubility product (pKSP = −log(KSP) that equals 108.6 for enamel and n is the number of ions in the IAP (n=18). SR values > 1.00 indicate solution supersaturated with respect to enamel.
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Statistical data analysis
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Experimental data were analyzed by ANOVA (α = 0.05). Significant differences between the groups were determined by all pairwise multiple comparisons (Tukey-test). One standard deviation (SD) is identified in this paper for comparative purposes as the estimated uncertainty of the measurements.
Results and Discussion The results of the physicochemical evaluation of DC UPHM copolymers and their am- and g-ACP composites are summarized in Table 2.
NIH-PA Author Manuscript
Am-ACP composites yielded the DVC values comparable to those attained in corresponding copolymers. DVC values achieved with g-ACP composites were higher than or equal to DVC of DC copolymers. The PS could only be measured with the light-cure formulation. In dual-cure systems, a paste hardened within 10 min of mixing the chemically activated components. Some degree of contraction therefore occured before the sample was even placed in dilatometer, making the material unsuitable for measurement by this method. Relatively high PS values measured in light-cured am- and g-ACP composites ((7.1 ± 0.3) % and (6.9 ± 0.1) %, respectively) significantly exceeded the PS values of glass-control composite [(4.4 ± 0.1) %]. However, normalizing PS values of the glass control to the resin/ filler ratios of ACP composites brought them in line, with an average of 7.7 %. The PSS that developed in am- and g-ACP composites was significantly higher than the PSS in control Sr-glass composites. Plateau, WSmax values, reached within the initial 336 h of exposure to 75 % RH decreased in the following order: am-ACP composite > (g-ACP composite, copolymer) > Sr-glass composite. Specimen mass increases upon immersion in saline solutions were roughly (2.5 to 3.0) times higher compared to the corresponding specimens exposed to RH. The WSmax values of the immersed specimens and the accompanying HE values decreased in the following order: (am- and g-ACP composite > copolymer) > Srglass composite. In both dry state and after aqueous immersion, copolymers exhibited significantly higher BFS than ACP- or glass composites. Significantly, differences between the BFS of dry and wet DC am- and g-ACP composites were not statistically significant (therefore, only wet BFS values are shown in Table 2). Kinetics profiles of Ca and PO4 release ACP UPHM composites revealed that within 3 mo of aqueous immersion, no plateau (maximum) concentration of either ion had yet been reached. The supersaturation conditions attained with both am-ACP as well as g-ACP composite specimens were significantly above the minimum necessary for the potential mineral re-deposition.
NIH-PA Author Manuscript
A high DVC seen in UPHM formulations expectedly resulted in elevated PS values, since the PS is directly related to DVC [12]. While significantly higher than the PS values reported for the commercial restoratives [(1.9 to 4.1) %] and flowable composites [(3.6 to 6.0) %], the PS values of ACP/UPHM composites only slightly exceeded the lower end values seen in adhesive resins [(6.7 to 13.5) % [1, 2, 8]]. This relatively high PS may be attributed to the intensified hydrogen bonding in UPHM resins with relatively high amount of HEMA (17 mass %). The measured PSS values compare well with the PSS that developed in am-ACP composites based on binary UDMA/HEMA composites [9]. This finding would suggest that the actual stress developed as a consequence of polymerization shrinkage in UPHM matrices was not elevated by the simultaneous inclusion of PEG-U and HEMA in UDMA-based resin matrix. The shrinkage of ACP UPHM composite specimens should be more than compensated for by the HE (up to 14 vol %) they undergo during aqueous immersion. The potential beneficial effects of HE have been reported by other researchers [13, 14].
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Conclusions NIH-PA Author Manuscript
DC UPHM and its ACP composites formulated for endodontic uses attained high DVC accompanied by relatively high PS and moderate PSS. The high DVC attained in these composites suggests that a minimal possibility for leachability of un-reacted monomeric species and, consequently, minimal adverse cellular response to these composites. These experimental materials also exhibited relatively high levels of WS, accompanied by a significant HE. The latter may be useful in combating stresses that develop in these materials. Ion release profiles of the experimental materials confirmed their potential for regeneration of mineral-deficient tooth structures. Their moderate to low BFS, similar to that of ACP orthodontic adhesive, did not diminish the potential for the proposed application as an endodontic sealer. Micro-leakage and quantitative leachability studies should be performed to further establish the viability of these experimental materials.
Acknowledgments Support from the NIDCR (grant DE 13169; NIDCR/NIST Interagency Agreement YI-DE-7005-01) and contribution of monomers from Esstech, Essington, PA, USA are gratefully acknowledged.
References NIH-PA Author Manuscript
1. Skrtic D, et al. J Res Natl Inst Stands Technol. 2003; 108(3):167. 2. Antonucci, JM.; Skrtic, D. Polymers for Dental and Orthopedic Applications. Shalaby, W.; Salz, U., editors. CRC Press; Boca Raton, FL: 2007. p. 217-242. 3. Skrtic D, et al. Acta Biomaterialia. 2006; 2:85. [PubMed: 16701862] 4. O’Donnell JNR, et al. J Bioact Comp Polym. 2008; 23:207. 5. Skrtic D, et al. J Dent Res. 1996; 75(9):1679. [PubMed: 8952621] 6. Langhorst SE, et al. Dent Mater. 2009; 25:884. [PubMed: 19215975] 7. Skrtic D, et al. Biomaterials. 2004; 25:1141–1150. [PubMed: 14643587] 8. Skrtic D, Antonucci JM. J Biomat Appl. 2007; 21:375–393. 9. O’Donnell JNR, Skrtic D. J Biomim Biomater Tissue Eng. 2009 in press. 10. Lu H, et al. J Mater Sci: Mater Med. 2004; 15:1097. [PubMed: 15516870] 11. ASTM F394–78 (re-approved 1991). 12. Silikas N, et al. Dent Mater. 2000; 16(4):292. [PubMed: 10831785] 13. Huang C, et al. J Dent. 2002; 30:11. [PubMed: 11741730] 14. Momoi Y, McCabe JF. Brit Dent J. 1994; 176:91. [PubMed: 7599006]
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Antonucci and Skrtic
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Table 1
The composition of dual cure (DC) UPHM resin used in the study.
NIH-PA Author Manuscript
Monomers/components of the polymerization initiator system
Acronym
Mass %
urethane dimethacrylate
UDMA
47.2
poly(ethyleneglycol)-extended UDMA
PEG-U
29.1
2-hydroxyethyl methacrylate
HEMA
16.8
methacryloyloxyethyl phthalate
MEP
2.9
bis(2,6-dimethoxynezoyl)-2,4,4- triethylpentyl phosphine oxide & 1-hydroxycyclohexyl phenyl ketone
1850IRGACURE
1.0
benzoyl peroxide
BPO
2.0
2,2′-dihydroxyethyl-p-toluidine
DHEPT
1.0
NIH-PA Author Manuscript NIH-PA Author Manuscript Polymer Prepr. Author manuscript; available in PMC 2014 March 26.
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Table 2
NIH-PA Author Manuscript
Physicochemical characteristics of the experimental copolymers and composites (mean value±standard deviation (SD)). Property
Copolymer
am-ACP composite
g-ACP composite
Sr-glass composite
79.3±3.4 81.9±2.2
76.0±3.4 81.2±2.6
85.4±3.2 81.9±2.2
85.8±3.0 86.5±1.0
PS* (vol %)
Nd
7.1±0.3
6.9±0.1
4.4±0.1
PSS (MPa)
Nd
3.7±0.3
3.6±0.2
2.2±0.1
WSmax (%) RH immersion
2.5±0.5 6.6±0.5
2.9±0.1 8.2±0.5
2.6±0.2 9.3±0.4
2.2±0.1 5.2±0.2
HE (vol %)
8.2±1.2
13.7±1.2
14.4±1.1
4.3±0.4
BFS (MPa) wet
124±41
42±4
47±9
69±6
n/a
4.6±0.2
3.7±0.1
n/a
DVC (%) 24 h 5d
SRenamel
nd=not determined; n/a not applicable. *
Indicated PS values were measured with the light-cure UPHM formulations.
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