Article
Intraoral Temperature Triggered Shape-Memory Effect and Sealing Capability of A Transpolyisoprene-Based Polymer Gakuji Tsukada 1, *, Ryuzo Kato 2, : , Masayuki Tokuda 1, : and Yoshihiro Nishitani 1, : Received: 20 September 2015 ; Accepted: 29 October 2015 ; Published: 9 November 2015 Academic Editor: Wei Min Huang 1
2
* :
Department of Restorative Dentistry and Endodontology, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima 890-8544, Japan;
[email protected] (M.T.);
[email protected] (Y.N.) Department of Information Science and Biomedical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40, Korimoto, Kagoshima 890-0065, Japan;
[email protected] Correspondence:
[email protected]; Tel./Fax: +81-99-275-6192 These authors contributed equally to this work.
Abstract: In dentistry, pure gutta-percha (trans-1,4-polyisoprene (TPI)) is widely used as a main component of root canal filling materials. TPI has an interesting shape memory formed through cross-linking, and this characteristic is expected to be very effective for development of novel dental treatments; in particular, modification of the shape recovery temperature to the intraoral temperature (37 ˝ C) will enhance the applicability of the shape-memory effect of TPI in root canal filling. In this study, trial test specimens consisting of varying proportions of TPI, cis-polyisoprene, zinc oxide, stearic acid, sulfur and dicumyl peroxide were prepared and the temperature dependence of their shape recovery, recovery stress and relaxation modulus were measured. Additionally, their sealing abilities were tested using glass tubing and a bovine incisor. As the ratio of cross-linking agent in the specimens increased, a decrease in recovery temperature and an increase in recovery stress and recovery speed were observed. In addition, the test specimen containing the highest concentration of cross-linking agent showed superior sealing ability under a thermal stimulus of 37 ˝ C in both sealing ability tests. Keywords: shape-memory polymer; gutta-percha; trans-1,4-polyisoprene; root canal filling material; thermal property; crystallinity; sealing ability; cis-polyisoprene
1. Introduction In dentistry, gutta-percha is one of the most popular materials for obturating the root canal space. This root canal filling treatment is one of the most important procedures in endodontic treatment, because microleakage of the root canal resulting from an inadequate seal can permit the passage of bacteria, fluids, and chemical substances [1,2]. Lateral condensation and the warmed gutta-percha technique are the main root canal filling methods performed clinically using gutta-percha [3–5]. The lateral condensation technique, which fills the root canal space using one master gutta-percha point and a number of accessory gutta-percha points with side-to-side filling using a dental spreader, has been widely used clinically for many years. The size of the master point is usually identical to that of the K-file used for final enlargement of the root canal, and the apical seat is then prepared slightly above the apical foramen to prevent extrusion of the master point from the apical foramen. Resistance (tug-back) upon withdrawing the master point inserted up to the apical seat is
Polymers 2015, 7, 2259–2275; doi:10.3390/polym7111512
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Polymers 2015, 7, 2259–2275
used to confirm whether the tip of the master point is suitable for the shape of the enlarged root canal. Additionally, this technique is always used in combination with a root canal sealer. The advantage of the technique is easy control of the filling operation, and hence the method is still widely taught in dental schools [6]. However, it is difficult to fit the master points to the irregular shape of the root canal using only tug-back resistance, and poor sealing has been reported [7]. The warmed gutta-percha technique, in which the thermoplasticity of the material is used to obturate the root canal space, has been developed to overcome the disadvantages of the lateral condensation technique [8,9]. Since the proposal of vertical condensation of warmed gutta-percha [10], injectable thermoplasticized gutta-percha has been introduced [11] and a variety of techniques using warmed gutta-percha have been developed. Several studies have compared the sealing ability of the warmed gutta-percha and lateral condensation techniques [4,12,13]. However, it has been found that warmed gutta-percha techniques are not necessarily superior to the lateral condensation procedure [14,15], and the use of root canal sealer is frequently required in warmed gutta-percha methods [16,17]. In addition, problems such as shrinkage during setting [18–20] and overextension of the gutta-percha and the root canal sealer from the apical foramen [21,22] have been reported. As a consequence, warmed gutta-percha techniques have not yet replaced the lateral condensation method. Gutta-percha is a composite material [23–25], and pure gutta-percha (trans-1,4-polyisoprene (TPI)) has been used as the main component of dental gutta-percha for a number of years because of its very low toxicity [26,27] and ease of handling [6]. TPI is incorporated into gutta-percha in a non-cross-linked state. TPI is a crystalline polymer [28–32] that is easily cross-linked using sulfur or peroxide, with mechanical properties dependent on its crystallinity [20,23]. These properties lead to TPI demonstrating interesting shape-memory properties [31–37], and this shape-memory polymer has been commercialized under the product name SMP-2 (Kuraray Corporation, Kashima, Japan). In our previous study [32], we confirmed the shape-memory capacity of SMP-2, and this function was expected to be effective for close filling of the root canal. However, the shape recovery temperature of SMP-2 is somewhat (about 10–15 ˝ C) higher than the intraoral temperature (37 ˝ C) [32] and application of SMP-2 as a root canal filling material has been found to be difficult. Adjustment of the shape recovery temperature to the intraoral temperature (37 ˝ C) would greatly improve the operability of SMP-2 as a root canal filling material. Hence, confirmation of the possibility of adjustment of the shape recovery temperature to 37 ˝ C for various TPI composites, and examination of the sealing ability using the shape-memory effect of TPI-based polymers is necessary. The purpose of this study was to prepare trial samples consisting of a TPI-based shape-memory polymer and examine methods for adjusting the thermal properties of this polymer to identify a formulation that yielded shape recovery at 37 ˝ C. The sealing ability of the shape-memory polymers was also measured using two methods: glass tubing and a bovine incisor. The bovine incisor was used to test both the lateral condensation method and the warmed gutta-percha for a comparison of sealing ability. 2. Experimental Section 2.1. Shape-Memory Polymer Preparation Trial materials for the study were prepared, and the component ratios of the ingredients are shown in Table 1. TPI (TP-301, Kuraray Corporation, Kyoto, Japan) was used as the main base polymer, and cis-1,4-polyisoprene (Sigma-Aldrich Corporation, St. Louis, MO, USA [CPI]) was used as the co-cross-linking polymer. Zinc oxide and stearic acid (both from Wako Pure Chemical Industry, Ltd., Osaka, Japan) were used as accelerator activators, and sulfur and dicumyl peroxide (both from Nacalai Tesque Inc., Kyoto, Japan) were used as cross-linking agents. TPI, zinc oxide, stearic acid is used as a component of gutta-percha. The amounts of zinc oxide and stearic acid were identical for all specimens (Table 1), whereas the amounts of TPI, CPI, sulfur and dicumyl peroxide were changed for each specimen. Specimen TPI:100-S:0.5 material was prepared as a standard material,
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Stearic acid Sulfur Dicumyl peroxide
1 0.5 3
1 0.5 3
1 0.5 3
1 0.5 3
1 0.75 4.5
1 1.0 6.0
1 1.25 7.5
Test specimens were prepared in almost accordance with a previously reported method [32]. Polymers 2015, 7, 2259–2275 First, non-cross-linked compounds were prepared by kneading the raw ingredients using a rubber
roller at 75 ± 5 °C for 11 min. Split metal molds were prepared to form cylindrical test specimens and trial points (Figure S1A,B in Supplementary Materials). The non-cross-linked compound was heated and at test specimens were prepared by stepwise variation of the proportions of TPI and CPI to yield 90 °C for 10 min to soften the material, placed in the metal mold, and pressed at 196 N. The mold specimens TPI:85-S:0.5, TPI:70-S:0.5 and TPI:50-S:0.5. A further set °C of for specimens prepared containing the non-cross-linked compound was heated from 24 to 150 15 min in was an oven and by stepwise variation quantity of cross-linking added (sulfur and peroxide) to then held at 150 of °C the for 30 min under a nitrogen gas agents atmosphere to cross-link the dicumyl TPI and CPI. The yieldmold specimens TPI:100-S:0.75, TPI:100-S:1.0 TPI:100-S:1.25. Specimen compositions are shown was then cooled to room temperature and the sample was removed from the mold. Hence, cylindrical test specimens (6.0 mm diameter, 9.5 mm length) (Figure S2A, Supplementary Materials) in Table 1. and trial points having a 0.087 taper ratio (Figure 1A) were prepared. The tip of the trial point was Table 1. Test materials anddiameter component of their ingredients. compound ingredients are spherically formed and its wasratios 2.8 mm. Thermal propertyThe measurements and a sealing given as parts perglass hundred bywere weight of the rubber polymer. ability test using tubing performed on each cross-linked cylindrical test specimen, and a sealing ability test using a bovine incisor was performed on the trial point. For measurement of the Code TPI:70-S:0.5 TPI:50-S:0.5 Materials) TPI:100-S:0.75 TPI:100-S:1.0 TPI:100-S:1.25 thermal properties, TPI:100-S:0.5 the original TPI:85-S:0.5 shape (Figure S2A, Supplementary was compressed to be Trans-1,4-polyisoprene 100 85 70 50 100 100 100 as deformed as shown in Figure S2B, Supplementary Materials and subsequently allowed to recover, Cis-1,4-polyisoprene 0 15 30 50 0 0 0 shown Figure S2D, Supplementary for the sealing Zincin oxide 30 30 Materials.30In contrast,30 30 ability test, 30 the deformed 30 Stearic acid and shape 1 recovery 1patterns shown 1 1 S2A,C,D1(Supplementary 1 1 shape fixation in Figure Materials) Sulfur 0.5 0.5 0.5 0.5 0.75 1.0 1.25 were used. Similar results were obtained from3 five replicate determinations and6.0median values Dicumyl peroxide 3 3 3 4.5 7.5 are reported.
Figure of the original (B) deformed and (C) recovery Figure 1.1. Photograph Photograph of trial the point: trial (A) point: (A)shape; original shape; shape; (B) deformed shape;shape. and (C) recovery shape. 3
Test specimens were prepared in almost accordance with a previously reported method [32]. First, non-cross-linked compounds were prepared by kneading the raw ingredients using a rubber roller at 75 ˘ 5 ˝ C for 11 min. Split metal molds were prepared to form cylindrical test specimens and trial points (Figure S1A,B in Supplementary Materials). The non-cross-linked compound was heated at 90 ˝ C for 10 min to soften the material, placed in the metal mold, and pressed at 196 N. The mold containing the non-cross-linked compound was heated from 24 to 150 ˝ C for 15 min in an oven and then held at 150 ˝ C for 30 min under a nitrogen gas atmosphere to cross-link the TPI and CPI. The mold was then cooled to room temperature and the sample was removed from the mold. Hence, cylindrical test specimens (6.0 mm diameter, 9.5 mm length) (Figure S2A, Supplementary Materials) and trial points having a 0.087 taper ratio (Figure 1A) were prepared. The tip of the trial point was spherically formed and its diameter was 2.8 mm. Thermal property measurements and a sealing ability test using glass tubing were performed on each cross-linked cylindrical test specimen, and a sealing ability test using a bovine incisor was performed on the trial point. For measurement of the thermal properties, the original shape (Figure S2A, Supplementary Materials) was compressed to be deformed as shown in Figure S2B, Supplementary Materials and subsequently allowed to 2261
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recover, as shown in Figure S2D, Supplementary Materials. In contrast, for the sealing ability test, the deformed shape fixation and shape recovery patterns shown in Figure S2A,C,D (Supplementary Materials) were used. Similar results were obtained from five replicate determinations and median values are reported. 2.2. Measurement of Shape Recovery Ratio at a Fixed Constant Temperature for the Standard Material The shape recovery of the TPI:100-S:0.5 material prepared as the standard was investigated under a range of temperature conditions. Shape recovery ratios were measured from 37–55 ˝ C and the test procedure was as follows: the axial length of the test specimen was measured at 24 ˝ C (original length, Figure S2A, Supplementary Materials). The specimen was then softened by heating in a water bath at 90 ˝ C for 3 min, then axially compressed by 30%. Cooling the specimen in a water bath at 0 ˝ C fixed the compressed dimensions. The specimen was then taken out of the water and allowed to equilibrate to 24 ˝ C to stabilize the dimensions. The length of the deformed specimen was then measured (deformed length, Figure S2B, Supplementary Materials). The specimen was heated in a thermostat-controlled box maintained at a constant temperature. After heating for 10 min at each temperature, the test specimen was removed from the box and immediately immersed in a water bath at 0 ˝ C for 10 min. The specimen was then placed at room temperature at 24 ˝ C to equilibrate. The length of the test specimen was remeasured (recovery length, Figure S2D, Supplementary Materials) and the shape recovery ratio (SRR) was calculated as follows: SRR p%q “
Lr ´ Ld ˆ 100 L0 ´ Ld
(1)
where Lr = recovery length, Ld = deformation length and L0 = original length. 2.3. Measurement of Shape Recovery under A Constant Rate Rising Temperature All cylindrical test specimens were softened by heating in a water bath at 90 ˝ C for 3 min and then axially compressed by 30% in length. Following this compressive deformation, the dimensions were fixed by cooling in a water bath at 0 ˝ C. The test specimen was removed from the water bath and allowed to warm to 10 ˝ C (Figure S2B, Supplementary Materials), then heated from 10–80 ˝ C at 1.0 ˝ C/min. Changes in length were measured using a laser displacement meter (LC-2100; Keyence Corporation, Osaka, Japan) across a range of temperatures to investigate the shape recovery from the deformed shape. 2.4. Measurement of Recovery Stress under a Constant Rate Rising Temperature The stress generated by shape recovery from the deformed shape was measured using a compression testing machine (AG-100: Shimadzu Corporation, Kyoto, Japan). The test specimens were softened by heating in a water bath at 90 ˝ C for 3 min, then axially compressed by 30% in length. The compressive deformation was fixed by cooling in a water bath at 0 ˝ C for 10 min and the test specimen was removed from the water bath (Figure S2B, Supplementary Materials). The test specimen was then placed on a table installed in the testing machine, surrounded by a temperature-controlled box, and allowed to warm to 10 ˝ C. An indenter, which was connected to the load cell, was placed in contact with the top of the deformed test specimen. The position of the indenter was fixed while the test specimen was heated at a rate of 1.0 ˝ C/min from 10–80 ˝ C. The stresses generated by heating were measured. 2.5. Measurement of Relaxation Modulus at a Fixed Constant Temperature The relaxation modulus was measured using a compression testing machine (TG-50KN: Minebea Corporation Ltd., Nagano, Japan). All specimens were placed on a platen enclosed in a thermostat-controlled box; the temperature of the specimen was changed by changing the
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temperature of the box. Measurements were made at constant temperatures in the range 4–80 ˝ C. The compressive strain (γ) was 0.05, the measurement time was 10 s, and the relaxation modulus (Er (t)) was calculated as follows: f ptq Er ptq “ (2) γ where f(t) is the compressive stress after t seconds from the start of the measurement. In this study, f(5) was used. 2.6. Measurement of7,the Shape Recovery Ratio at a Fixed Constant Temperature of 37 ˝ C Polymers 2015, page–page To examine degree of shape fromfrom thethe deformed at the intraoral temperature where f(t)the is the compressive stressrecovery after t seconds start of theshape measurement. In this study, (37 ˝ C), the shape recovery ratio of all specimens was measured at a constant temperature of 37 ˝ C. f(5) was used. The test procedure was as follows: the axial length of the specimen was deformed by 30% and fixed 2.6. Measurement of the Shape Recovery Ratio at a Fixed Constant Temperature of 37 °C in the same way as described in Section 2.2. The axial length of the specimen was measured at 24 ˝ C To examine the degree length, of shape recovery the deformed shape at the intraoral temperature (original length and deformed Figure from S1A,B, Supplementary Materials), and no dimension (37 °C), the shape recovery ratio of all specimens was measured at a constant temperature of 37 °C. ˝ changes were recorded. The specimen was heated in a thermostat-controlled box at 37 C. After The test procedure was as follows: the axial length of the specimen was deformed by 30% and fixed heating for each time period (1–10 min), the specimen was removed from the box and immediately in the same way as described in Section 2.2. The axial length of the specimen was measured at 24 °C ˝ (original length and deformed length, Figure S1A,B, Supplementary Materials), and no dimension immersed in a water bath at 0 C for 10 min. The specimen was then removed from the water bath recorded. The specimen The was heated 37 °C. After (recovery ˝ C to equilibrate. and againchanges placedwere at 24 lengthinofa thermostat-controlled the test specimen box wasatremeasured heating for each time period (1–10 min), the specimen was removed from the box and immediately length), and the shape recovery rate (SRR) at each time was calculated as given by Equation (1). immersed in a water bath at 0 °C for 10 min. The specimen was then removed from the water bath and again placed at 24 °C to equilibrate. The length of the test specimen was remeasured (recovery
2.7. Sealing Ability Usingrecovery Glass Tubing length), andTest the shape rate (SRR) at each time was calculated as given by Equation (1).
Cross-linked cylindrical specimens (Figure S2A, Supplementary Materials) were softened by 2.7. Sealing Ability Test Using Glass Tubing heating in a water bath at 90 ˝ C for 3 min, then inserted into a glass tube (inner diameter 4.6 mm) Cross-linked cylindrical specimens (Figure S2A, Supplementary Materials) were softened by ˝ C for 10 min to fix the deformation. The specimen in the glass and cooled in ainwater heating a waterbath bath atat900°C for 3 min, then inserted into a glass tube (inner diameter 4.6 mm) and tube wascooled removed from the thedeformation. deformedThe testspecimen specimen in a water bath at 0water °C for bath, 10 min and to fix the in thewas glassremoved tube was from the ˝ removed from the water bath, and the deformed test specimen was removed from the glass tube. The allowed to glass tube. The deformed specimen was then exposed at room temperature (24 C) and deformed specimen was then exposed at room temperature (24 °C) and allowed to equilibrate to equilibrate to stabilize the dimensions, and the fixation of the deformed shape was confirmed (Figure stabilize the dimensions, and the fixation of the deformed shape was confirmed (Figure S2C, S2C, Supplementary SupplementaryMaterials). Materials).
2. Photograph the glass tube forthe the sealing sealing test: (A)(A) glassglass tube; tube; (B) glass withtube deformed Figure 2. Figure Photograph of theofglass tube for test: (B)tube glass with deformed test specimen inserted; (C) flattened deformed glass tube; and (D) flattened deformed glass tube with test specimen inserted; (C) flattened deformed glass tube; and (D) flattened deformed glass tube with deformed test specimen inserted. deformed test specimen inserted.
Another glass tube (inner diameter 5.5 mm in the upper section and 4.3 mm in the lower) was prepared for this test (Figure 2A) and stored in an incubator at 37 °C. A test specimen (diameter 4.6 mm) Another glass tube (inner diameter 5.5 mm in the upper section and 4.3 mm in the lower) was was inserted into the larger section of the tube and pushed up to the transition zone to the smaller ˝ C. A test specimen (diameter prepared inner for this test (Figure 2A) and in antoincubator 37 seat diameter. This transition zone stored was intended model the at apical and insertion of the deformed test specimen stopped at thisof point 2B). The specimen tube waszone to the 4.6 mm) was inserted into thewas larger section the(Figure tube and pushed up in to the theglass transition then compressed axially at 0.981 N for 5 min using an indenter, in an incubator at 37 °C (Figure 3A,B). If the shape of the test specimen was recovered by the thermal stimulus at 37 °C, the test specimen 5 2263
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smaller inner diameter. This transition zone was intended to model the apical seat and insertion of the deformed test specimen was stopped at this point (Figure 2B). The specimen in the glass tube was then compressed at 0.981 N for 5 min using an indenter, in an incubator at 37 ˝ C (Figure 3A,B). Polymers 2015, 7, axially page–page Polymers 2015, 7, page–page If the shape of the test specimen was recovered by the thermal stimulus at 37 ˝ C, the test specimen thus sealed the glass tube (Figure 3C,D). In addition,the the sealing ability test was performed using thus thus sealed the glass tube (Figure 3C,D). abilitytest testwas wasperformed performed using a sealed the glass tube (Figure 3C,D).InInaddition, addition, the sealing sealing ability using a deformed flattened glass tube (Figure 2C). This glass tube was prepared by pressing the original deformed flattened glass glass tube tube (Figure 2C).2C). ThisThis glass tube waswas prepared byby pressing a deformed flattened (Figure glass tube prepared pressingthe theoriginal original glass glass tube (Figure 2A) under heat until just before the deformed specimen could no longer be inserted tube glass (Figure under until before the deformed specimen could beinserted inserted up tube2A) (Figure 2A)heat under heatjust until just before the deformed specimen couldno no longer longer be up to the transition zone (Figure 2D). up to the transition zone (Figure 2D). to the transition zone (Figure 2D).
Figure 3. (A) Schematic representation of the filling method for the glass tube; (B) photograph of the Figure 3. (A) Schematic representation of the filling method for the glass tube; (B) photograph of the 3. (A) representation of the filling method for after the glass tube; (B) photograph of the fillingFigure method forSchematic the glass tube; (C)(C) schematic filling glass tube the test filling method for the glass tube; schematicrepresentation representation after filling thethe glass tube withwith the test filling and method for the glass tube; (C) schematic representation after filling the glass tube with the test specimen; (D) photograph after filling the glass tube with the test specimen. specimen; and (D) photograph after filling the glass tube with the test specimen. specimen; and (D) photograph after filling the glass tube with the test specimen.
As shown in Figure 4A,B, the sealingability ability of of the specimen was tested by by immersion of the As shown in Figure 4A,B, the thespecimen specimenwas was tested immersion As shown in Figure 4A,B, thesealing sealing ability of the tested by immersion of theof the lower, wider end of the tube containing the specimen in water at 37 °C, while the upper end was ˝ lower, wider endend of the tube containing in water wateratat3737 while upper end was lower, wider of the tube containingthe the specimen specimen in °C, C, while the the upper end was filled with 1% fuchsin basic solution (Nacalai Tesque Inc., Kyoto, Japan). Electrical resistance was 1% fuchsin basic solution(Nacalai (Nacalai Tesque Tesque Inc., Electrical resistance was was filledfilled withwith 1% fuchsin basic solution Inc., Kyoto, Kyoto,Japan). Japan). Electrical resistance measured using a digital multimeter to evaluate water leakage. measured using a digital multimetertotoevaluate evaluate water water leakage. measured using a digital multimeter leakage.
Figure 4. Sealing ability test: (A) schematic representation and (B) photograph.
Figure 4. Sealing abilitytest: test:(A) (A) schematic schematic representation and (B)(B) photograph. Figure 4. Sealing ability representation and photograph.
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2.8. Sealing Ability Test Using Bovine Incisor 2.8. Sealing Ability Test Using Bovine Incisor To examine the sealing ability in an environment similar to the human root canal, the sealing Totest examine the sealing ability an canal environment similar to the human canal, sealing ability was performed using thein root of a bovine incisor, which has aroot shape andthe properties ability test was performed using the root canal of a bovine incisor, which has a shape and properties resembling that of the root canal of the human tooth. The size of the trial point was referenced to that resembling that of canal of the and human The size of thematerial trial point was referenced to that of the root canal ofthe theroot bovine incisor, onlytooth. the TPI:100-S:1.25 sample was prepared for of the root canal of the bovine incisor, and only the TPI:100-S:1.25 material sample was prepared for this experiment. The trial point (Figure 1A) was softened by heating in a water bath at 90 °C for ˝ C for 3 min, this experiment. The trial point (Figure 1A) was softened by heating in a water bath at 90 3 min, and then inserted into the metal mold (Figure S1C, Supplementary Materials) and compressed and then inserted into the metal mold (Figure S1C, Supplementary Materials) andfixed compressed using using an indenter for deformation. Following deformation, the dimensions were by cooling in an indenter for deformation. Following deformation, the dimensions were fixed by cooling in a water a water bath at 0 °C for 3 min. The mold was removed from the water bath and the trial point was bath at 0 ˝from C forthe 3 min. The mold was at removed from the water bath trial point was removed removed mold and placed room temperature at 24 °C.and No the change in dimensions was ˝ from the mold and at room temperature 24 taper C. Noratio change was spherical recorded recorded (Figure 1B).placed The deformed points had aat 0.087 and in itsdimensions tip was formed (Figure The deformed a 0.087were taper obtained. ratio and its was formed (1.8 mm (1.8 mm1B). diameter). Fresh points bovinehad incisors Thetipcrowns, pulp spherical and periodontal diameter). Fresh bovine incisors were The crowns, and periodontal membrane were membrane were removed and the rootobtained. canals enlarged usingpulp a steel dental bur until the deformed removed and the root canals enlarged using a steel dental bur until the deformed point (Figure 1B) point (Figure 1B) could inserted without resistance into the root canal and stopped at the apical seat. could inserted without resistance into the root canal and stopped at the apical seat. Nail varnish was Nail varnish was then applied to the entire root surface excluding the apical foramen (Figure 5). then applied to the entire root surface excluding the apical foramen (Figure 5).
Figure 5. Photograph of the root of the bovine incisor prepared for the sealing-ability test with the trial point: (A) lateral side; (B)root apical side; and (C) cervical side. for the sealing-ability test with the Figure 5. Photograph of the of the bovine incisor prepared trial point: (A) lateral side; (B) apical side; and (C) cervical side.
The deformed trial point (Figure 1B) was inserted into the root canal (Figure 6A), which was The trialatpoint (Figure 1B) was inserted into the rootusing canalan (Figure 6A),for which was stored in deformed an incubator 37 ˝ C, and compression was maintained indenter 5 min at stored an incubator 37 °C, compression was of maintained using an indenter forroot 5 min at 0.981 N.inThe root canal at space wasand filled after recovery the trial point (Figure 6B), and canal 0.981 The root canal space was filled after recovery the trial point (Figure 6B), and root canal sealerN. was not used together. For comparison with theofcurrent preferred root canal filling method sealer was not used together. Forability comparison with current preferred root canal filling method performed clinically, the sealing test was alsothe performed on bovine root canals filled using performed clinically, the sealing ability test was also performed on bovine root canals filled using the the lateral condensation technique and the warmed gutta-percha technique. To compare the sealing lateral condensation technique and the warmed gutta-percha technique. To compare the sealing ability of the gutta-percha, root canal sealer was not used. For the lateral condensation technique, ability ofcanal the gutta-percha, root canalwas sealer was notto used. condensation technique, the the root of the bovine incisor enlarged the For sizethe of lateral the #140 K-file and the apical seat root canal of the bovine incisor was enlarged to the size of the #140 K-file and the apical seat added, added, then the root canal was irrigated with 3%EDTA solution and 3% sodium hypochlorite solution. then themaster root canal with 3%EDTA solution sodiuminto hypochlorite solution. A #140 pointwas (GCirrigated Corporation, Tokyo, Japan) wasand then3% inserted the root canal and A #140 master point (GC Corporation, Japan) was then inserted the apical root canal and tug-back tug-back confirmed (Figure 6C). In Tokyo, this study, the sealing ability into to the foramen by the confirmed (Figure In this study, the sealing to theFor apical the master point only, master point only, 6C). confirmed by tug-back, was ability examined. theforamen warmedby gutta-percha technique, confirmed by tug-back, was examined. For the root was the root canal was enlarged and irrigated in warmed a similargutta-percha manner, andtechnique, the root the canal of canal a bovine enlarged and irrigated in a similar manner, and the root canal of a bovine incisor was filled with three 2265 7
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kinds gutta-percha: II (Obtura Corporation,Obtura Fenton,II MO, USA), ObturationFenton, Gutta Hard incisorofwas filled withObtura three kinds of gutta-percha: (Obtura Corporation, MO, kinds gutta-percha: Obtura II(Toyo (Obtura Corporation, Fenton, MO, USA), Obturation Gutta Hard (Toyo Chemical Corporation, Tokyo, Japan) and Ultrafil Regular Set (Hygenic Corporation, Akron, USA), of Obturation Gutta Hard Chemical Corporation, Tokyo, Japan) and Ultrafil Regular (Toyo Chemical Corporation, Japan) and Ultrafil Regular Set these (Hygenic Corporation, Akron, OH, USA). The root canalAkron, ofTokyo, the OH, bovine incisor was filled with gutta-perchas using the Set (Hygenic Corporation, USA). The root canal of the bovine incisor was filled with OH, USA). The root canal of the bovine incisor was filled with these gutta-perchas using the respective dedicated using devices inrespective accordancededicated with the manufacturers' instructions. filled root was these gutta-perchas the devices in accordance with The the manufacturers' respective dedicated devices in accordance with thethe manufacturers' instructions. The filled immersed in aThe 1% filled fuchsin solution without soaking cut surface (Figure 7). The immersed was instructions. root was immersed in a 1% fuchsin solution without soaking the cutroot surface immersed in a 1% fuchsin solution without soaking the cut surface (Figure 7). The immersed root was taken out (Figure 7B) after 24 h for the lateral condensation method, at 4 days for the warmed gutta(Figure 7). The immersed root was taken out (Figure 7B) after 24 h for the lateral condensation taken out (Figure 7B) after 24 h for the lateral condensation method, at 4 days for the warmed guttapercha method, and 10 warmed days trial point. After that, root was using a method, at 4 days foratthe gutta-percha method, and the at 10 days forcut thelongitudinally trial point. After that, percha method, and at 10 days for the trial point. After that, the root was cut longitudinally using a low-speed diamond saw (Isomet, Buehler Co., Lake Bluff, IL, USA) and the sealing ability was the root was cut longitudinally using a low-speed diamond saw (Isomet, Buehler Co., Lake Bluff, IL, low-speed diamond saw (Isomet, Buehler Co., Lake Bluff, IL, USA) and the sealing ability was confirmed using a dye penetration. USA) and the sealing ability was confirmed using a dye penetration. confirmed using a dye penetration.
Figure Figure 6. 6. (A) (A)Schematic Schematicrepresentation representation of of the the filling filling method method for for the the root root canal canal of of the the bovine bovine incisor; incisor; Figure 6. (A) Schematic representation of the filling method for the root canal of the bovine incisor; (B) schematic representation after filling the root canal of the bovine incisor; and (C) confirmation of (B) schematic representation after filling the root canal of the bovine incisor; and (C) confirmation (B) schematic representation after filling the root canal of the bovine incisor; and (C) confirmation of tug-back. of tug-back. tug-back.
Figure 7. (A) Schematic representation of filled root of bovine incisor immersed in 1% fuchsin basic Figure 7. 7.solution; (A)Schematic Schematic representation of the filled root of bovine incisorafter immersed in from 1% fuchsin fuchsin basic Figure (A) representation filled root bovine incisor immersed in 1% basic staining and (B) photograph of root ofof bovine incisor removal 1% fuchsin staining solution; and (B) photograph of the root of bovine incisor after removal from 1% fuchsin basic staining and (B) photograph of the root of bovine incisor after removal from 1% fuchsin basic staining solution. staining solution. basic staining solution.
3. Results 3. 3. Results Results 3.1. Shape Recovery Ratio under A Fixed Constant Temperature for the Standard Material 3.1. Shape Shape Recovery Recovery Ratio Ratio under under A A Fixed Fixed Constant Constant Temperature Temperature for 3.1. for the the Standard Standard Material Material The shape recovery ratio of the TPI:100-S:0.5 standard material at each fixed constant The shape shape recovery ratio 8. ofShape the TPI:100-S:0.5 TPI:100-S:0.5 standard material at each each fixed constant The recovery ratio of the material at fixed constant temperature is shown in Figure recovery wasstandard not observed between 37 and 43 °C. From ˝ C. temperature is shown in Figure 8. Shape recovery was not observed between 37 and temperature is shown in Figure 8. Shape with recovery was not and observed between andat43over °C.43 From 44 °C, the shape recovery ratio increased temperature reached almost37 100% 53 °C 44 °C, the (Figure 8). shape recovery ratio increased with temperature and reached almost 100% at over 53 °C (Figure 8). 2266 8 8
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From 44 ˝ C, the shape recovery ratio increased with temperature and reached almost 100% at over ˝ C (Figure Polymers 2015, 7, page–page 53 8).
Figure 8. Shape recovery ratio under a fixed constant temperature for the standard material. Figure 8. Shape recovery under a fixed constant temperature for standard the standard material. Figure 8. Shape recovery ratioratio under a fixed constant temperature for the material.
3.2. Shape Recovery under a Constant Rate Rising Temperature
3.2. Shape Recovery under a Constant 3.2. Recovery a Constant RateRate Rising Temperature TheShape changes inunder shape recovery forRising the Temperature different proportions of TPI to CPI are shown in
Figure 9A and for different proportions cross-linking agent 9B. In all The changes ininshape recovery for the proportions of in TPIFigure to CPI inspecimens, Figure changes shape recovery forofdifferent the different proportions of TPI are to shown CPI are shown 9A in the length of each specimen increased slightly at lower temperatures (Figure 9A,B). In Figure 9A, marked and for different proportions of cross-linking agent in Figure 9B. In all specimens, the length of each Figure 9A and for different proportions of cross-linking agent in Figure 9B. In all specimens, the specimen increased at lower temperatures (Figure 9A,B).(Figure In 30 Figure 9A, increases in32 to lengthinoflength each specimen increased slightly lower temperatures 9A,B). Inmarked Figure 9A, marked increases are slightly observed from 24 toat34 °C for TPI:50-S:0.5, to 38 °C for TPI:70-S:0.5, ˝ C for TPI:50-S:0.5, 30 to 38 ˝ C for TPI:70-S:0.5, 32 to 40 ˝ C for length are observed from 24 to 34 increases in length are observed from 24 to 34 °C for TPI:50-S:0.5, 30 to 38 °C for TPI:70-S:0.5, 32 40 °C for TPI:85-S:0.5 and 33 to 42 °C for TPI:100-S:0.5. As the proportion of CPI increased, theto shape ˝ C for TPI:100-S:0.5. As the proportion of CPI increased, the shape recovery TPI:85-S:0.5 and 33 to 42 40 °C temperature for TPI:85-S:0.5 33 toa42 °C for TPI:100-S:0.5. As the proportion of CPIdecreased. increased, the shape 9B, recovery atand which marked increase in length was observed In Figure temperature at which a marked in length was observed decreased. In Figure24 recovery temperature at which aincrease marked increase length wasfor observed decreased. In9B, Figure marked increases in length were observed at 22in to 29 °C TPI:100-S:1.25, tomarked 31 9B, °C for increases in length were observed 22 to 29 ˝ at C for to 31 ˝ C for TPI:100-S:1.0 marked increases in length wereatobserved 22 TPI:100-S:1.25, to 29 °C for 24 TPI:100-S:1.25, 24 to 31 °C and for TPI:100-S:1.0˝ and 27 to 34 °C for TPI:100-S:0.75. As the proportion of cross-linking agent increased, 27 to 34 C forand TPI:100-S:0.75. As TPI:100-S:0.75. the proportion of agent increased, theagent shapeincreased, recovery TPI:100-S:1.0 27 to 34 °C for Ascross-linking the proportion of cross-linking the shape recovery temperature at which a marked increase in length was observed decreased. temperature at whichtemperature a marked increase in length was increase observedin decreased. At higher temperatures, the shape recovery at which a marked length was observed decreased. At higher temperatures, specimen behaved similarly. Because asignificant significant were not each specimen behavedeach similarly. Because a significant changes wereanot observedchanges inchanges the measured At higher temperatures, each specimen behaved similarly. Because were not ˝ C, theyvalues observed inabove the measured above 60 °C, they werenot notshown showninin Figure values 60 measured were not shown Figure observed in the values above 60in °C, they 9. were Figure 9. 9.
Figure 9. Changes in shape recovery from deformation as a function of temperature for (A) TPI:50-S:0.5, TPI:70-S:0.5, TPI:85-S:0.5 and TPI:100-S:0.5 (various to CPI); and for Figure 9. Changes in shape recovery from deformation functionofofTPI of temperature temperature Figure 9. Changes in shape recovery from deformation as asproportions a afunction (B) TPI:100-S:1.25, of cross-linking for (A) TPI:50-S:0.5,TPI:100-S:1.0, TPI:70-S:0.5,TPI:100-S:0.75 TPI:85-S:0.5 and and TPI:100-S:0.5 TPI:100-S:0.5(various (variousamounts proportions to (A) TPI:50-S:0.5, TPI:70-S:0.5, TPI:85-S:0.5 and TPI:100-S:0.5 (various proportions of TPIoftoTPI CPI); and agents). CPI); and (B) TPI:100-S:1.25, TPI:100-S:1.0, TPI:100-S:0.75 and TPI:100-S:0.5 (various amounts of (B) TPI:100-S:1.25, TPI:100-S:1.0, TPI:100-S:0.75 and TPI:100-S:0.5 (various amounts of cross-linking cross-linking agents).
agents). 3.3. Recovery Stress under a Constant Rate Rising Temperature
Changes recovery stress generated through recovery from the deformed shape are shown in 3.3. Recovery Stressinunder a Constant Rate Rising Temperature 2267
Figure 10A,B. Figure10A shows the effect of changing the proportion of TPI to CPI, and Figure 10B Changes recovery stress generated recovery fromInthe are stress shown in shows the in effect of changing the amountthrough of cross-linking agent. alldeformed specimens,shape recovery
Figure 10A,B. Figure10A shows the effect of changing the proportion of TPI to CPI, and Figure 10B
Polymers 2015, 7, 2259–2275
3.3. Recovery Stress under a Constant Rate Rising Temperature Changes in recovery stress generated through recovery from the deformed shape are shown in shows the effect of changing the proportion of TPI to CPI, and Figure 10B shows the effect of changing the amount of cross-linking agent. In all specimens, recovery stress increased increased slightly slightly at at lower lower temperatures. temperatures. In In specimens specimens of of varying varying polymer polymer composition, composition, marked marked increases in recovery stress were observed in the following temperature ranges: ˝ C for increases in recovery stress were observed in the following temperature ranges: 26 26 to to 33 33 °C for TPI:50-S:0.5, 30 to 37 °C for TPI:70-S:0.5, 32 to 38 °C for TPI:85-S:0.5 and 33 to 40 °C for TPI:100-S:0.5 ˝ ˝ ˝ TPI:50-S:0.5, 30 to 37 C for TPI:70-S:0.5, 32 to 38 C for TPI:85-S:0.5 and 33 to 40 C for TPI:100-S:0.5 (Figure (Figure 10A). 10A). At At higher higher temperatures, temperatures, the the highest highest recovery recovery stress stress value value was was seen seen in in test test material material TPI:100-S:0.5, followed by TPI:85-S:0.5, TPI:70-S:0.5, and TPI:50-S:0.5 (Figure 10A). In specimens TPI:100-S:0.5, followed by TPI:85-S:0.5, TPI:70-S:0.5, and TPI:50-S:0.5 (Figure 10A). In specimens with with varying amounts of cross-linking agent, marked increases in recovery stress were observed in the varying amounts of cross-linking agent, marked increases in recovery stress were observed in the following temperature ranges: 22 to 30 °C for TPI:100-S:1.25, 25 to 33 °C for TPI:100-S:1.0, and 29 to ˝ ˝ following temperature ranges: 22 to 30 C for TPI:100-S:1.25, 25 to 33 C for TPI:100-S:1.0, and 29 36 °C for TPI:100-S:0.75 (Figure 10B). At higher temperatures, the highest recovery stress value was to 36 ˝ C for TPI:100-S:0.75 (Figure 10B). At higher temperatures, the highest recovery stress value seen in test material TPI:100-S:1.25, followed by TPI:100-S:1.0, TPI:100-S:0.75, and TPI:100-S:0.5. was seen in test material TPI:100-S:1.25, followed by TPI:100-S:1.0, TPI:100-S:0.75, and TPI:100-S:0.5. As the proportion of CPI increased, recovery stress decreased at high temperatures (Figure 10A). As the proportion of CPI increased, recovery stress decreased at high temperatures (Figure 10A). In contrast, as the proportion of cross-linking agent increased, recovery stress increased at high In contrast, as the proportion of cross-linking agent increased, recovery stress increased at high temperatures (Figure 10B). Because a significant changes were not observed in the measured values temperatures (Figure 10B). Because a significant changes were not observed in the measured values above 60 °C, they were not shown in Figure 10. above 60 ˝ C, they were not shown in Figure 10. Polymers 7, page–page Figure 2015, 10A,B. Figure 10A
Figure 10. 10. Changes Changesininshape shaperecovery recoverystress stress asas aa function function of of temperature temperature for for (A) (A) TPI:50-S:0.5, TPI:50-S:0.5, Figure TPI:70-S:0.5, TPI:85-S:0.5 TPI:85-S:0.5 and and TPI:100-S:0.5 TPI:100-S:0.5 (varying (varying proportion proportion of of TPI TPI to to CPI); CPI); and and (B) (B) TPI:100-S:1.25, TPI:100-S:1.25, TPI:70-S:0.5, TPI:100-S:1.0, TPI:100-S:0.75 TPI:100-S:0.75 and and TPI:100-S:0.5 TPI:100-S:0.5 (various (various amounts of cross-linking agents). TPI:100-S:1.0,
3.4. 3.4. Relaxation Relaxation Modulus Modulus under under aa Fixed Fixed Constant Constant Temperature Temperature Changes modulus after 5 s 5(Esr(5)) in Figure 11A,B.11A,B. Figure Figure 11A shows Changes in inthe therelaxation relaxation modulus after (Erare (5))shown are shown in Figure 11A the effect changing the proportion of TPI to CPI, andtoFigure of changing shows theof effect of changing the proportion of TPI CPI, 11B and shows Figurethe 11Beffect shows the effectthe of amount of cross-linking agent. In all specimens, a higher E r (5) value was observed at lower changing the amount of cross-linking agent. In all specimens, a higher Er (5) value was observed temperatures. In specimens of varying polymerpolymer composition, markedmarked decreases in Er(5)in were at lower temperatures. In specimens of varying composition, decreases Er (5) ˝ C for observed in theinfollowing temperature ranges: 22 to °C ˝for TPI:50-S:0.5, were observed the following temperature ranges: 22 40 to 40 C for TPI:50-S:0.5,2626toto42 42 °C for ˝ ˝ TPI:70-S:0.5, 28 to 42 °C for TPI:85-S:0.5 and 32 to 44 °C for TPI:100-S:0.5 (Figure 11A). At higher TPI:70-S:0.5, 28 to 42 C for TPI:85-S:0.5 and 32 to 44 C for TPI:100-S:0.5 (Figure 11A). At higher temperatures, temperatures, the the highest highest EErr(5) (5)value valuewas wasseen seenin intest testmaterial materialTPI:100-S:0.5, TPI:100-S:0.5,followed followedby byTPI:85-S:0.5, TPI:85-S:0.5, TPI:70-S:0.5, TPI:70-S:0.5, and and TPI:50-S:0.5 TPI:50-S:0.5 (Figure (Figure 11A). 11A). In In specimens specimens with with varying varying proportions proportions of of cross-linking cross-linking ˝C agent, r(5) thethe following temperature ranges: 22 to agent, marked markeddecreases decreasesininEE wereobserved observedinin following temperature ranges: 22 30 to °C 30 for r (5)were ˝ ˝ TPI:100-S:1.25, 22 to22 32to°C32forCTPI:100-S:1.0, and 24and to 44 (Figure(Figure 11B). At11B). higher for TPI:100-S:1.25, for TPI:100-S:1.0, 24 °C to for 44 TPI:100-S:0.75 C for TPI:100-S:0.75 At temperatures, the highest E r (5) value was seen in test material TPI:100-S:1.25, followed by higher temperatures, the highest Er (5) value was seen in test material TPI:100-S:1.25, followed by TPI:100-S:1.0, ofof CPI increased, Er(5) decreased at TPI:100-S:1.0, TPI:100-S:0.75, TPI:100-S:0.75,and andTPI:100-S:0.5. TPI:100-S:0.5.As Asthe theproportion proportion CPI increased, Er (5) decreased high temperatures (Figure 11A). In contrast, as the of cross-linking agentagent increased, Er(5) at high temperatures (Figure 11A). In contrast, asproportion the proportion of cross-linking increased, increased at high temperatures (Figure 11B). Because a significant changes were not observed in the measured values above 60 °C, they were not shown in Figure 11. 2268
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2015, 7, page–page EPolymers at high temperatures (Figure 11B). Because a significant changes were not observed in r (5) increased Polymers 2015, 7, page–page ˝
the measured values above 60 C, they were not shown in Figure 11.
Figure 11. Changes in Er(5) as a function of temperature for (A) TPI:50-S:0.5, TPI:70-S:0.5, TPI:85-S:0.5 Figure 11.Changes Changes as a function temperature for TPI:50-S:0.5, TPI:70-S:0.5, Figure 11. in Erin (5) E asr (5) aproportion function of of temperature for (A)and TPI:50-S:0.5, TPI:70-S:0.5,TPI:100-S:1.0, TPI:85-S:0.5 and TPI:100-S:0.5 (varying TPIof to CPI); (B)(A) TPI:100-S:1.25, TPI:85-S:0.5 and TPI:100-S:0.5 (varying proportion of TPI to CPI); and (B) TPI:100-S:1.25, TPI:100-S:1.0, and TPI:100-S:0.5 (varying proportion of TPI to CPI); and (B) TPI:100-S:1.25, TPI:100-S:1.0, TPI:100-S:0.75 and TPI:100-S:0.5 (various amounts of cross-linking agents). TPI:100-S:0.75 and and TPI:100-S:0.5 TPI:100-S:0.5 (various (various amounts amounts of of cross-linking TPI:100-S:0.75 cross-linking agents). agents).
3.5. Measurement of the Shape Recovery Ratio at 37 °C ˝C 3.5. Measurement Measurement of the Shape Recovery Ratio at 37 °C The shape recovery ratio of all specimens at 37 °C is shown in Figure 12. The shape recovery ˝ C is shown in Figure 12. The shape recovery ratio recovery ratio at at 37 recovery ratioofofallallspecimens specimens 37 °C is shown in Figure 12. The shapeThe recovery ratioThe of shape specimen TPI:100-S:1.25 was highest and reached almost 100% after 4 min. shape of specimen TPI:100-S:1.25 was highest reached after 4100% min. after The shape recovery ratio ratio of specimen TPI:100-S:1.25 was and highest andalmost reached almost 4 min. The shape recovery ratio of TPI:100-S:1.0 reached almost 100% after 100% 7 min. In contrast, specimens TPI:50-S:0.5 of TPI:100-S:1.0 reached almost 100% after 7 min. In contrast, specimens TPI:50-S:0.5 and TPI:70-S:0.5 recovery ratio of TPI:100-S:1.0 reached almost 100% after 7 min. In contrast, specimens TPI:50-S:0.5 and TPI:70-S:0.5 showed a slow shape recovery speed, and a 100% recovery ratio was not reached showed slowRatios shape recovery speed, and a 100% recovery ratio was recovery not reached after 10not min. Ratios and showed a slow shape recovery speed, aTPI:70-S:0.5 100% was reached afterTPI:70-S:0.5 10 amin. of 67.7% for TPI:50-S:0.5 and 60.0%and for wereratio reached after 10 min of 67.7% for TPI:50-S:0.5 and 60.0% for TPI:70-S:0.5 after min (Figure TPI:85-S:0.5, after 10 min. Ratios of 67.7% for TPI:50-S:0.5 and were 60.0%reached for TPI:70-S:0.5 were reached 10 min (Figure 12). TPI:85-S:0.5, TPI:100-S:0.75 and TPI:100-S:0.5 showed no 10 recovery at 37 12). °C. after ˝ C. TPI:100-S:0.75 and TPI:100-S:0.5 showedand no TPI:100-S:0.5 recovery at 37showed (Figure 12). TPI:85-S:0.5, TPI:100-S:0.75 no recovery at 37 °C.
˝ C. Figure Figure12. 12. Shape Shape recovery recoveryratio ratioof ofall allmaterials materialsunder underaafixed fixedconstant constanttemperature temperatureof of37 37 °C. Figure 12. Shape recovery ratio of all materials under a fixed constant temperature of 37 °C.
3.6. 3.6. Sealing Sealing Ability Ability using using Glass Glass Tubing Tubing 3.6. Sealing Ability using Glass Tubing A A photograph photograph of of the the sealing sealing ability ability test test using using the the glass glass tube tube without without deformation deformation isis shown shown in in Figure 13A. For test material TPI:100-S:1.25, no dye penetration was observed after three months A photograph of the sealing ability test using the glass tube without deformation is shown in Figure 13A. For test material TPI:100-S:1.25, no dye penetration was observed after three months and and year (Figure 13A,B). A photograph the sealing abilitytest testusing usingthe theafter deformed glass tube isis Figure 13A. For test material TPI:100-S:1.25, no dye penetration was observed three months and one one year (Figure 13A,B). A photograph of of the sealing ability deformed glass tube shown in Figure No penetration was after month, no one year 13A,B). A photograph theobserved sealing ability test usingindicating the deformed glassleakage. tube is shown in(Figure Figure13C. 13C. Nodye dye penetrationof was observed afterone one month, indicating nowater water leakage. In addition, electrical insulation was confirmed after three months, indicating no water shown in Figure 13C. No dye penetration was observed after one month, indicating no water leakage. In addition, electrical insulation was confirmed after three months, indicating no leakage. In insulation was confirmed three months, indicating no water Foraddition, other testelectrical materials, dye penetration and waterafter leakage were observed immediately afterleakage. the start For other test materials, dye penetration and water leakage were observed immediately after the start of the test. of the test. 2269
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For other test materials, dye penetration and water leakage were observed immediately after the Polymers 2015, 7, page–page start of the test. Polymers 2015, 7, page–page
Figure Photographofofglass glasstube tubesealed sealed polymer shape recovery: months; Figure 13. 13. Photograph byby polymer shape recovery: (A) (A) afterafter threethree months; and and (B) after one year; (C) photograph of the flattened deformed glass tube sealed by polymer shape Figure 13.year; Photograph of glass tubeofsealed by polymer shape recovery: (A) after three andshape (B) after one (C) photograph the flattened deformed glass tube sealed bymonths; polymer recovery afterone one month. (B) after (C) photograph of the flattened deformed glass tube sealed by polymer shape recovery after oneyear; month. recovery after one month.
3.7. 3.7. Sealing Sealing Ability Ability using using aa Bovine Bovine Incisor Incisor
3.7. Sealing Ability using a Bovine Incisor
Photographs the sealing sealingability abilitytest testusing usinga abovine bovine incisor shown in Figures 14 and 15. Photographs of of the incisor areare shown in Figures 14 and 15. For Photographs of the sealing ability test using a bovine incisor are shown in Figures 14 and 15. For For lateral condensation method, penetration was observedininthe theentire entireroot rootcanal canal after after 24 24 hh the the lateral condensation method, dyedye penetration was observed the lateral condensation method, dye penetration was observed in the entire root canal after 24 h (Figure 14A). For the warmed gutta-percha method, dye penetration length from apical foramen of (Figure 14A). ForFor thethe warmed dyepenetration penetration length from apical foramen of (Figure 14A). warmedgutta-percha gutta-percha method, method, dye length from apical foramen of 6.52 0.12 h for Obtura II, 2.41 ˘ 0.66 h for Obturation Gutta Hard and 2.64 ˘ 0.37 h for Ultrafil 6.52 ±˘ 0.12 h for Obtura II, 2.41 ± 0.66 h for Obturation Gutta Hard and 2.64 ± 0.37 h for Ultrafil Regular 6.52 ± 0.12 h for Obtura II, 2.41 ± 0.66 h for Obturation Gutta Hard and 2.64 ± 0.37 h for Ultrafil Regular Regular Set14B–D) (Figure 14B–D) were after observed after four days.for In contrast, for the trial TPI:100-S:1.25 Set (Figure were observed Incontrast, contrast, for the trial TPI:100-S:1.25 point, almost Set (Figure 14B–D) were observed afterfour four days. days. In the trial TPI:100-S:1.25 point, almost point, almost no dye penetration was observed after 10 days (Figure 15A–C). The trial can no penetration dye penetration was observedafter after10 10days days (Figure The trial point can can adapt to a variety no dye was observed (Figure15A–C). 15A–C). The trial point adapt topoint a variety adapt a variety of such root canal such as atubular relatively regular tubular root canal (Figure 15A), a ofto root canal shapes such ashapes relatively regular root canal (Figure 15A), a gradually tapered of root canal shapes asas a relatively regular tubular root canal (Figure 15A), a gradually tapered root canal (Figure 15B) and a steeply tapered root canal (Figure 15C). In addition, root fracture by gradually tapered root canal (Figure 15B) and a steeply tapered root canal (Figure 15C). In addition, root canal (Figure 15B) and a steeply tapered root canal (Figure 15C). In addition, root fracture by recovery by stress was notstress observed any of the roots. root fracture recovery was observed in any of the roots. recovery stress was not observed ininnot any of the roots.
Figure 14. Photograph of the results of the dye penetration test after four days: (A) lateral Figure 14. Photograph of the results of the dye penetration test after four days: (A) lateral condensation method, and warmed gutta-percha method (B) for Obtura II; (C) for Obturation Gutta condensation method, and warmed gutta-percha method (B) for Obtura II; (C) for Obturation Gutta Hard;Hard; and and (D) (D) for Ultrafil Regular Set. for Ultrafil Regular Set. Figure 14. Photograph of the results of the dye penetration test after four days: (A) lateral condensation method, and warmed gutta-percha method (B) for Obtura II; (C) for Obturation Gutta Hard; and (D) for Ultrafil Regular Set. 2270 12
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Figure 15. Photograph of the results of the dye-penetration test for the trial point at 10 days: Figure 15. Photograph of the results of the dye-penetration test for the trial point at 10 days: (A) Relatively regular tubular root canal; (B) gradually tapered root canal; and (C) steeply tapered (A) Relatively regular tubular root canal; (B) gradually tapered root canal; and (C) steeply tapered root canal. root canal.
4. Discussion Discussion 4. Most endodontic endodontic failures failures can can be be attributed attributed to to poorly poorly obturated obturated root root canals canals [38,39]. [38,39]. Therefore, Therefore, an an Most important property of a root canal filling material is the ability to seal the internal space of the root important property of a root canal filling material is the ability to seal the internal space of the root canal tightly tightly over over aa long long period. period. In In addition, addition, any any filling filling material material should should have have excellent excellent operability. operability. canal The internal shape of the root canal cannot be observed directly; thus the root canal filling procedure The internal shape of the root canal cannot be observed directly; thus the root canal filling procedure relies strongly strongly on on the the intuition intuition of of the the dentist. dentist. Therefore, Therefore, the the development development of of aa new new root root canal canal filling filling relies method that does not depend on the dentist’s technique is needed. Additionally, dentists desire method that does not depend on the dentist’s technique is needed. Additionally, dentists desire a a new, easy method that reliably canal. As such, we have sought newcanal root new, easy method that willwill reliably sealseal the the rootroot canal. As such, we have sought a newaroot canal filling technique gutta-percha to replace existing methodsusing usinglateral lateralcondensation condensation or or filling technique usingusing gutta-percha to replace existing methods warmed gutta-percha. warmed gutta-percha. In our ourprevious previousstudy, study, confirmed the shape-memory capacity of SMP-2, a TPI-based In wewe confirmed the shape-memory capacity of SMP-2, a TPI-based polymer polymer [32]. For ideal clinical application of shape-memory polymer as a root canal filling material, [32]. For ideal clinical application of shape-memory polymer as a root canal filling material, the shape the shape deformation of theshould polymer be fixed at long-term at refrigeration temperature, the deformation of the polymer be should fixed long-term refrigeration temperature, the deformed deformed shape be maintained duringtreatment dental treatment room temperature, and recovery shape should beshould maintained during dental at room attemperature, and recovery to the ˝ C). to the original memorized shape should occur upon exposure to the intraoral temperature (37 original memorized shape should occur upon exposure to the intraoral temperature (37 °C). Furthermore, shape shaperecovery recoveryshould shouldbeberelatively relatively rapid and material should against the Furthermore, rapid and thethe material should pushpush against the root root canal gently to seal it permanently without rootfracture. fracture.Therefore, Therefore,we wesought sought to to identify identify canal wall wall gently to seal it permanently without root formulation parameters to alter the thermal properties of this polymer. In this study, TPI:100-S:0.5 formulation parameters to alter the thermal properties of this polymer. In this study, TPI:100-S:0.5 was prepared prepared as as aa standard standard material, material, with with components components and and content content ratios ratios determined determined with with reference reference was to Ishii’s Ishii’s report report [31], [31], to to aa study study of of SMP-2 SMP-2 [32] [32] and and to to the the components components of of commercial commercial gutta-percha gutta-percha to products for root canal filling material [23–25]. For TPI:100-S:0.5, the temperature at which the shape shape products for root canal filling material [23–25]. For TPI:100-S:0.5, the temperature at which the ˝ recovery ratio ratio began began to to increase increase was C, and and 100% 100% shape shape recovery recovery within within 10 10 min min was was observed observed at at recovery was 44 44 °C, ˝ C (Figure 8). Based on these results, since it was not possible to induce shape recovery at the over 52 over 52 °C (Figure 8). Based on these results, since it was not possible to induce shape recovery at the intraoral temperature temperature using using this this composition, composition, the the composition composition was was changed changed to to attempt attempt to to decrease decrease intraoral the shape shape recovery recovery temperature. the temperature. A previous study showed showed that that co-cross-linking co-cross-linking with with CPI CPI decreases decreases the the shape shape recovery recovery A previous study temperature of TPI and and CPI CPI are are cis-trans cis-trans isomers isomers of of temperature of cross-linked cross-linked TPI-based TPI-based polymers polymers [31]. [31]. TPI polyisoprene, which contains double bonds; the two polymers can therefore be co-cross-linked. polyisoprene, which contains double bonds; the two polymers can therefore be co-cross-linked. Although the the chemical chemical formula formula of of these these polymers polymers is is identical, identical, their their properties properties are are significantly significantly Although different. The Thepolymer polymerform formofof TPI is relatively elongated, so polymer strands are close together different. TPI is relatively elongated, so polymer strands are close together and and easily cohere to form crystalline structures [28–32]. This crystallization of TPI can resist easily cohere to form crystalline structures [28–32]. This crystallization of TPI can resist forces that forces that return the to shape, its original shape, memorized in the cross-linking reaction [31,32]. return the material to material its original memorized in the cross-linking reaction [31,32]. In contrast, In contrast, CPI, rubber, or natural rubber, is flexible extremely at the double bond,the making the structure polymer CPI, or natural is extremely at flexible the double bond, making polymer structure irregular (non-crystalline), with many gaps between polymer strands; intermolecular forces irregular (non-crystalline), with many gaps between polymer strands; intermolecular forces within
CPI are weak. Also, TPI has a high elasticity at 37 °C through crystallization, while CPI appears more 2271 13
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within CPI are weak. Also, TPI has a high elasticity at 37 ˝ C through crystallization, while CPI appears more syrup-like at this temperature. The decrease in shape recovery temperature with increasing proportions of CPI (Figure 9A) is likely due to the weakened crystallization of TPI when co-cross-linked with CPI. However, compounding with CPI likely decreases the recovery stress (Figure 10A). In addition, the shape recovery speed was slow (Figure 12). The sealing ability test using glass tubing showed a poor seal, likely caused by these thermal properties. Thus, co-cross-linking TPI with CPI does not appear to be suitable method for creating a root canal filling material. The other approach to altering the shape recovery temperature of the cross-linked TPI-based polymers was to vary the cross-linking density of TPI. Shape recovery of TPI-based polymers occurs by melting of the crystalline region, which fixed the deformation [32]. In Figure 8, the shape recovery temperature of the standard specimen was lower than that of the transition temperature (melting point: 67 ˝ C) of TPI [30]. Based on these results, increasing the degree of cross-linking of TPI was expected to be effective in decreasing the transition temperature. As shown in Figure 9B, as the proportion of cross-linking agent increased, the shape recovery temperature decreased, and the recovery stress increased (Figure 10B). An increase in cross-linking points and tightening of the mesh structure inhibits the movement of TPI polymer strands and decreases the molecules that contribute to crystallization. Moreover, the forces that act to recover the original shape memorized by cross-linking become stronger. In addition, recovery stress increases (Figure 10B) and the shape recovery speed increases (Figure 12). Based on these results, this modification seemed promising for application as a root canal filling material. The results shown in Figures 9 and 10 were measured under a rising temperature at a constant rate of 1 ˝ C/min. For TPI:100-S:0.5, the temperature range of marked increases in length (33 to 42 ˝ C ) and in recovery stress (33 to 40 ˝ C) was lower than the constant temperature used for measurement of the shape recovery ratio (Figure 8). Under a rising temperature conditions (1 ˝ C/min), it was presumed that the transition temperature was shifted to the low temperature side, because recrystallization of the specimen after the melting of the crystalline region was suppressed by more heat was supplied in order to raise the specimen temperature constantly (1 ˝ C/min) against the supercooling phenomenon by the endothermic reaction with crystalline melting. The results shown in Figures 8, 11 and 12 were measured under fixed temperature conditions. Specimen TPI:100-S:0.5 showed an increased shape recovery ratio at 44 ˝ C (Figure 8), this temperature (44 ˝ C) is the point at which the elastic modulus changed from a marked decrease to a slight decrease (Figure 11A,B). The crystallinity of TPI was considered to be almost melted at 44 ˝ C, the TPI:100-S:0.5 material was presumed to exhibit rubber elasticity, shape recovery was spontaneously induced, and its recovery ratio increased with increasing temperature and rubber elasticity (Figure 8). One of the most important themes of this study was to identify a formulation of TPI polymer that yields shape recovery at 37 ˝ C; the results shown in Figure 12 suggest these characteristics for test specimens TPI:50-S:0.5, TPI:70-S:0.5, TPI:100-S:1.25 and TPI:100-S:1.0. For TPI:100-S:1.25 and TPI:100-S:1.0, the elastic modulus at 37 ˝ C had stabilized and the specimens had transformed into a rubber state (Figure 11B) and were presumed to be able to recover their shape spontaneously. In contrast, specimens TPI:50-S:0.5 and TPI:70-S:0.5 had an elastic modulus that was close to the point of stabilization, and shape recovery could be induced, however the rubber elasticity modulus was quite low and the shape recovery speed was slow (Figure 12). For TPI:85-S:0.5, TPI:100-S:0.5 and TPI:100-S10.75, since the elastic modulus at 37 ˝ C was markedly decreasing and repeated crystalline melting and recrystallization was occurring (Figure 11AB), shape recovery was presumed not to be induced. Another important theme in this study was to investigate the sealing capability of the trial material. In this study, two sealing ability tests were performed. The first used glass tubing instead of a natural root canal to allow direct observation of the behavior of the test specimen and changes in dye penetration. The internal diameter of the larger section of glass tubing was 5.5 mm, the diameter of the original test specimen was 6.0 mm (0.5 mm larger than the tube), and it was deformed to
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4.6 mm (0.9 mm smaller, allowing easy insertion into the glass tube) (Figure 3A,B). Additionally, the internal diameter of the smaller section of the glass tubing was 4.3 mm, and the transition zone between these internal diameters stopped further penetration of the deformed test specimen, even if the specimen was compressed using an indenter (Figure 3A,B). In addition, natural root canals often do not have a regular tubular shape, and irregular shapes such as flattened root canals are frequently observed. Therefore, the sealing ability test was also performed using a flattened glass tube. Specimen TPI:100-S:1.25 containing the highest concentration of cross-linking agent showed the highest recovery stress (Figure 10B) and highest recovery speed (Figure 12) among all specimens showing close sealing. In addition, this test specimen could fit both the non-deformed glass tube and the flattened deformed glass tube and sealed the inner space tightly. Thus, TPI with a high cross-linking density may be suitable as a root canal filling material. Another sealing ability test using a bovine incisor was also performed, because the shape and surface properties of the bovine root canal are similar to those found in a human incisor, and bovine incisors are easy to obtain. The results of the sealing ability test using the bovine incisor indicated that, for the lateral condensation method, the apical foramen could not be sealed tightly using tug-back confirmation. For the warmed gutta-percha method, the gutta-percha fitted the root canal better than in the lateral condensation method, but leakage of dye was observed as a result of shrinkage of melted gutta-percha and extrusion of gutta-percha. The difference in dye penetration length was presumed to be mainly due to differences in the crystallization temperature and crystallization rate of the TPI [20]. For the trial point, the shape-memory effect successfully sealed the prepared root canal of the bovine incisor without use of a sealer. Additionally, as shown in Figure 15, this trial point adapted to various tapered root canal shapes without extrusion. These results suggest the applicability of the TPI based shape-memory polymer as a root canal filling material. In future studies, a trial point that can be applied to the root canal of the human tooth will be developed, it is also necessary to investigate the root canal sealer suitable for this trial point, and in addition, analysis of the stress on the tooth and the periodontal tissue by the shape-memory is also necessary for the prevention of the root fracture. 5. Conclusions With the TPI-based shape-memory polymer, it was possible to recover the deformed shape under the thermal stimulus of 37 ˝ C by the approach of increasing the proportion of CPI or cross-linking agent. However, as the proportion of CPI increased, recovery stress decreased. In contrast, as the proportion of cross-linking agent increased, recovery stress increased. The material containing the highest concentration of cross-linking agent showed superior sealing ability under a thermal stimulus of 37 ˝ C in sealing ability tests, and it appears to be promising for application as a root canal filling material with excellent operability and sealing ability. Supplementary Materials: 2073-4360/7/11/1512/s1.
The
Supplementary
Materials
can
be
found
at
www.mdpi.com/
Acknowledgments: The authors thank Kuraray Corp. for providing TP-301. This research was supported by Grants-in-Aid for Scientific Research #14571818. Author Contributions: Gakuji Tsukada and Ryuzo Kato conceived and designed the experiments; Gakuji Tsukada performed the experiments; Ryuzo Kato analyzed the data; Masayuki Tokuda and Yoshihiro Nishitani contributed reagents/materials/analysis tools; Gakuji Tsukada wrote the manuscript. All authors revised the manuscript and approved the final version. Conflicts of Interest: The authors declare no conflict of interest.
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