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Fluoride release and microhardness of one-step adhesives in
two immersion solutions
Somphone Phanthavong, DDS, PhD,a Naoko Harada, DDS, PhD,a Hidekazu Sonoda,
DDS, PhD,a Toru Nikaido, DDS, PhD,a Patricia N. R. Pereira, DDS, PhD,b and Junji
Tagami, DDS, PhDa,caCariology and Operative Dentistry, Department of Restorative Sciences, Graduate School, Tokyob
Medical and Dental University, Department of Operative Dentistry, School of Dentistry, University
cof North Carolina at Chapel Hill, Chapel Hill, NC, USA, and Center of Excellence Program for
Frontier Research on Molecular Destruction and Reconstruction of Tooth and Bone, Tokyo
Medical and Dental University, Tokyo, Japan
Purpose: To measure the amount of fluoride-release and microhardness of fluoride-releasing one-stepadhesives; Xeno CF Bond, Reactmer Bond, and One-Up Bond F in two different solutions.
Materials and Methods: For fluoride-release measurement, five samples for each group were stored indeionized water or demineralizing solution for one day. A resin-modified glass ionomer cement, Fuji II LC(Improved), was used as a control. Fluoride ion release was analyzed with a fluoride ion-specific electrode.
For microhardness measurement, the specimens prepared in the same manner as described above werestored in two different solutions or dry condition for one day. After dividing into halves, the microhardnessof the specimen was measured using a nanoindentation tester.
Results: Amount of fluoride-release and microhardness of the adhesives were significantly different
according to the immersion solutions and the materials. The adhesives demonstrated significantly lessamounts of fluoride-release than that of Fuji II LC (Improved). Microhardness of the specimens immersedin solutions were significantly higher than those stored in dry condition for all the adhesives.
Conclusion: The amount of fluoride-release was significantly less than that of the resin-modified glassionomer cement. Microhardness of the adhesives increased after 1-day storage in the media probably due
to subsequent acid-base reaction and cross-linking of polymers. (Int Chin J Dent 2004; 4: 1-7.)
Clinical Significance: Amount of fluoride-release and microhardness of the fluoride-containing adhesives
were material and immersion solution dependent. The adhesives demonstrated the capacity for
fluoride-release, although the amount of fluoride-release was significantly less than that of theresin-modified glass ionomer cement.
Key Words: fluoride-release, microhardness, one-step adhesives.

Introduction
Recurrent or secondary caries has been one of the main reasons for the failure and replacement of
restorations. As an attempt to prevent recurrent caries, fluoride (F)-containing restorative materials havebeen developed.1 Examples of these materials are amalgam,2 glass ionomer cements,3 resin-modified glassionomer cements,4,5 compomers,6,7 and resin composites.8 These materials release fluoride in different
amounts,9 and the cariostatic effect of fluoride on enamel and root dentin caries has been demonstrated inseveral studies in vitro10,11 and in vivo.12
Dentin adhesives for direct composite restorations have been widely accepted. Recently, fluoride has
been incorporated to dentin bonding systems. Consequently, F-containing dentin adhesives would bondeffectively to tooth structure, contribute to reduction of microleakage,13 and possibly prevent recurrent
caries due to F-release.14 Although there are several reports on measurement of F-release from F-releasing
restorative materials,6,9 studies evaluating F-release and a change of microhardness from adhesive systems

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are lacking. The purpose of this study was to measure the amount of F-release and microhardness of three
F-releasing one-step adhesives in two different solutions.

Materials and Methods
Specimen Preparation
The materials, manufacturers and batch numbers used in this study are listed in Table 1. Three adhesives,
Xeno CF Bond (XB, Dentsply-Sankin, Tokyo, Japan), Reactmer Bond (RB, Shofu, Kyoto, Japan), One-Up
Bond F (OB, Tokuyama Dental, Tokyo, Japan), were evaluated. A resin-modified glass ionomer cement,
Fuji II LC (Improved) (F II LC, GC, Tokyo, Japan) was used as a control in measurement offluoride-release.
Table 1. Materials used in this study.
Material (Code)

Batch

Xeno CF Bond
(XB)

060911060911

Reactmer Bond(RB)
One-Up Bond F(OB)

Composition

Manufacturer

Universal
Catalyst

HEMA, Ethanol, Water
Pyrophosphate ester, UDMA, PEM-F,
Microfiller, Catalyst

Dentsply-Sankin
Tokyo, Japan

099900

Bonding A

Shofu Inc.
Kyoto, Japan

099900

Bonding B

Aluminum fluorosilicate glass,
F-PRG filler, Water, Initiator
4-AET, 4-AETA, UDMA, Initiator, etc

001

Bonding A

Tokuyama Dental
Tokyo, Japan

501

Bonding B

Phosphoric monomer, MAC-10,
Catalyst, Monomer
HEMA, Water, Aluminum fluorosilicate
glass, Catalyst

Fuji II LC (Improved) 050491(F IILC)
310391

PowderLiquidFluoro-aluminosilicate glass, Shade pigments GC Corp.
Acrylic-maleic acid copolymer, HEMA,Tokyo, Japan
Water, etc

HEMA, 2-hydroxyethyl methacrylate; UDMA, urethane dimethacrylate; PEM-F, Penta cyclophosphazene fluoride; F-PRG
filler, Full reaction type of pre-reacted-glass ionomer filler; 4-AET, 4-acryloxyethyltrimellitate, 4-AETA, 4-acryloxyethyl
trimellitate anhydride; MAC-10, 11-methacryloxy-1,1-undecane dicarboxylic acid.

The materials were placed in a vinyl mold, through which a 3 mm x 0.36 mm hole (diameter x height)
was made. The materials were mixed in accordance with the manufacturer’s instruction prior to application.
Following this, the mold was covered with a plastic matrix strip, pressed flat with a glass slide andlight-cured for 20 s per site on the top and the bottom sides. Each specimen was placed in a bottle
containing 8 mL of either deionized water (DW) or demineralizing solution (DS; pH 4.5) at 37˚C.
The demineralizing solution was formulated according to van Dijk et al.15 and ten Cate et al.16
composition: 2.2 mmol/L CaCl2, 2.2 mmol/L NaH2PO4, and 50 mmol/L acetic acid adjusted to pH value of
4.5 with NaOH.
Measurement of Fluoride Release
After storage in each solution, specimens were rinsed with 1 mL of DW or DS, which was added to 8 mLof storage solution. The specimens were then placed in a new solution of either DW or DS. One milliliter2

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Int Chin J Dent 2004; 4: 1-7.

of total ions strengths adjustment buffer (TISAB) was added to the 8 mL of old storage solution and 1 mLof rinse solution, making a total volume of 10 mL, which was measured for concentration of fluoride ions.
The F-ion release was measured with an ion-specific electrode (combination electrode fluoride 960900;
Orion Research, Boston, MA, USA) and calculated in parts per million (ppm). Theoretically, the limit of
F-ion measurement is 0.02 ppm with this ion-specific electrode.
Ten disks were prepared for each material and divided into two groups. Means and standard deviations
for five samples of each group were calculated. Statistical analysis of the amount of fluoride release fromeach material was done using one- and two-way analysis of variance (ANOVA) and Fisher’s PLSD test at
the 95% level of confidence.
Microhardness Measurement
Sample preparation for a nanoindentation testing is similar to that for the F-release measurement. The
specimens were stored in either DW or DS for one day as described above. Also, the specimens were kept
dry at room temperature for one day, which were used as a dry control. Following this, the disc wascarefully cut into two halves at the center of the disc using a safety razor. The specimens were thenembedded in epoxy resin (EPON 815, Nisshin EM, Tokyo, Japan). After 24 hours of polymerization of theresin, the surfaces were polished with waterproof silicon carbide papers (grit #600, 800, and 1,000) anddiamond pastes (particle size 6, 3, 1, and 0.25 µm). Specimens were set on the stage of the nanoindentation
tester (ENT-1100, Elionix, Tokyo, Japan).
The instrument used for this experiment was a depth sensing computer controlled instrument with athree-sided pyramidal diamond probe as previously reported.17 After setting a sample on the stage of thetesting machine, the instrument was programmed to create the position of indentations. Five indentations
were performed around the center of the polished surface for each specimen. The load on the indenter was
0.049 N (5 gf).

After the performing the indentation, nanohardness were calculated by an attachedcomputer. Indentation tests were carried out on ten specimens for each group. After the tests, the geometryof the indentation marks was confirmed on the attached display.
Fifteen disks were prepared for each material and divided into three groups. Means and standard
deviations for five samples of each group were calculated. Statistical analysis of microhardness of one-stepbonding agent was done using one- and two-way ANOVAs and Fisher’s PLSD test at the 95% level ofconfidence.

Results
Table 2 shows the amount of fluoride released from FII LC and F-releasing dentin adhesive materials in
DW and DS after one day, respectively.
Analysis with two-way ANOVA revealed that the amount of F-release of FII LC and the other
F-releasing adhesive materials tested was significantly different in according to the immersion solution(F=170.091, p<0.0001) and the materials (F=38.582, p<0.0001).
As expected, FII LC had the highest amounts of F-release in both DW and DS solutions, and a3

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significant differences were found when compared to those obtained with dentin adhesive materials(p<0.05).
The tendency of the amount of F-release varied for each adhesive material. For XB, the amount of
F-release in DW (0.06 ppm) was higher than that in DS (0.03 ppm) (p<0.05). For RB and OB, there wereno significant differences in the amounts of F-release between in DW and in DS (p>0.05).
Table 2. Fluoride release from each material after one day (ppm).
Material

Deionized water

Demineralizing solution

F II LC
XB
RB
OB

0.13 (0.019)
0.06 (0.009) A
0.02 (0.003) A, C
0.03 (0.005) A, D

0.34 (0.064)
0.03 (0.005) B
0.03 (0.003) B, C
0.03 (0.006) B, D

Mean (SD), n=10. No statistically significant differences within columns are shown by superscript letters (p>0.05).

Table 3. Microhardness of one-step bonding agent after one day (MPa).
Material

Dry

Deionized water

Demineralizing solution

RB
OB
XB

292.76 (25.71)
223.78 ( 3.47)A
207.55 ( 2.04)A

309.80 (14.80)D
243.06 ( 4.39)B, E
231.22 ( 1.33)B

312.55 (18.06)D
245.92 ( 6.22)C, E
250.71 ( 1.73) C

Mean (SD), n=10. No statistically significant differences within columns are shown by superscript letters (p>0.05).

Table 3 indicates the microhardness of the one-step adhesives after 1-day storage in the dry, water andacidic conditions, respectively.

Analysis with two-way ANOVA revealed that the microhardness of

F-releasing adhesive materials tested was significantly different in according to the storage condition(F=44.561, p<0.0001) and the materials (F=359.094, p<0.0001).

The microhardness of Dry groups

indicated significantly lower than those of DW and DS in all the adhesives (p<0.05). For RB and OB, there
were no significant differences in the microhardness between DW and DS (p>0.05), while themicrohardness of DS was significantly higher than that of DW for XB group (p<0.05). The microhardnessof RB indicated the highest in the three adhesives for each condition.

Discussion
Recently, the materials for one-step restorative adhesives with F-releasing potential have beendeveloped.18

These systems are simple to handle, bond to dentin and show sustained F-release.19,20

Bonding agents are being used as bases in restorative dentistry because of their ability to provide efficient
cavity sealing to prevent microleakage.21
The source of fluoride from F II LC was from the gel layer of fluoro-aluminosilicate glass. However, thesources of fluoride from the F-release dentin adhesive materials were different for each material. For XB,
the source of F-release was from F-release methacrylate, PEM-F. For RB, the sources of fluoride are from4

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Int Chin J Dent 2004; 4: 1-7.

both pre-reacted glass ionomer (PRG) filler and aluminum fluorosilicate glass. The PRG filler is speciallysynthesized by the acid-base reaction of glass-ionomer cement. The acidic monomer of 4-AET in theadhesive is believed to promote release of fluoride from the PRG filler and aluminum fluorosilicate glass.
For OB, the source of F-release was from the aluminum fluorosilicate glass.
In order to simulate the clinical conditions, the specimen size of adhesive materials for F-releasing
measurement in this study was much smaller and thinner than the previous one for conventional glassionomer cement, compomer or F-releasing resin composite. In clinical situation, the thickness of theadhesives might be much less than the size used in this study. It was reported that the maximum thicknessof the bonding agent used in the resin coating of composite inlays at the cavity walls were between 63 µm
and 145 µm.22 However, the sample size influenced the amount of F-release from that, which was close to
the limit of the F-measuring electrode (0.02 ppm). And also, it is very difficult and technique sensitive to
make such a thin specimen. Therefore, the sample size was decided as thinner as possible in considerationof both the limitation of the F-measuring devices and the handling difficulties to make the specimens.
Generally, greatest amounts of F-release from F-releasing restorative materials occur in the first 24 hours,
especially the first four hours.9 The amount of fluoride loss may be associated with the setting andmaturation reactions of the materials.3,23 The amount of F-release from the resin-modified glass ionomer
cement was significantly greater than those from dentin adhesives.
A greater amount of F-release from FII LC was obtained in DS compared with DW, which was similar tothe previous studies by Forss et al.24 and Forsten,4 documenting higher amounts of F-release in acidic
solutions.

It was believed that this greater release of fluoride was caused by the dissolution of the

GIC-matrix in the demineralizing solution. Karantakis et al.9 reported that the drop of pH by use of lactic
acid solution also affected the F-release to such a degree for resin-modified glass ionomer cements,
compomers, and resin composites.
However, there were no significant differences of released fluoride between DW and DS for RB and OB.
Surprisingly, the amount of F-release in DW was significantly higher than that in DS for XB. The sourceof fluoride from XB is the F-releasing monomer, PEM-F. The release of fluoride may be more stable in anacidic condition than in a basic condition because pKa of PEM-F is around 2.0 (unpublished data).
Therefore, XB may release more fluoride in DW than in DS.
Most of the materials used in this study contain a hydrophilic resin monomers like HEMA, which forms
poly-HEMA and has strong affinity to water with its hydrophilic hydroxyl group causing F-release to beable to take place.25 Several investigators have attributed this to the composition and content of the fluoride
source and hydrophilic monomers.26 For dentin adhesive materials, the F-source surrounded by a resin
matrix may have difficulty being in contact with deionized water or demineralizing solution.27 The freemovement of deionized water or the solution is probably limited in the cured resin matrix.28 Differences
may also be explained by the mechanism of F-release, as the release partly occurs by diffusion throughpores and crack. Therefore, it could be expected that F-release will differ among different materials, as wasfound in this study. The polymerized resin matrix may be a barrier to release of fluoride ions, but further

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Int Chin J Dent 2004; 4: 1-7.

study is necessary to understand the mechanism of the F-release from dentin adhesive materials.
Munack et al.29 reported that surface microhardness of compomers dropped significantly due to wet
storage, and the type of the storage media did not influence microhardness significantly. Therefore, it was
expected that microhardness of the one-step adhesives would decrease in the media.

However,microhardness of the one-step adhesive systems in DS or DW was significantly higher than that of Drygroup.

Though the reason for the higher values is unclear, the subsequent acid-base reaction andcross-linking of polymers might continue after initial polymerization under water, leading increased
microhardness.
Okada et al.30 reported microhardness of the conventional glass ionomer cement and the compomer
increased 40 days storage in distilled water, while that of the resin composite decreased. Urabe et al.17
reported that microhardness of F-releasing bonding resin and Protect Liner F decreased after 6 months
storage in DW and DS. Because of the F-release from the adhesives for a long time, microhardness would
decrease. It seems that further long-term study should be done to evaluate the stability of the F-releasing
adhesive materials.

Conclusion
The fluoride-releasing dentin adhesive materials used in this study demonstrated the capacity for
F-release in both solutions, although the amount of F-release from the materials was significantly less thanthat of the resin-modified glass ionomer cement. Microhardness of the adhesives increased after 1-day
storage in the media probably due to subsequent acid-base reaction and cross-linking of polymers.
Acknowledgements
The authors express their gratitude to Dr. Timothy F. Watson, Guy’s, King’s & St. Thomas’ Dental Institute, KCL,
Guy’s Hospital, for reading the manuscript and his critical comments. The authors also wish to acknowledge thecompanies, Dentsply-Sankin, GC, Shofu, and Tokuyama Dental, for providing materials used in this study.
This research was supported by the grant for Center of Excellence Program for Frontier Research on Molecular
Destruction and Reconstruction of Tooth and Bone in Tokyo Medical and Dental University.

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Reprint request to:
Dr. Toru Nikaido
Cariology and Operative Dentistry, Department of Restorative Sciences
Graduate School, Tokyo Medical and Dental University
1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-0195 E-mail: nikaido.ope@tmd.ac.jp
Received October 15, 2003. Revised December 8, 2003. Accepted December 30, 2003.
Copyright ©2004 by the Editorial Council of the International Chinese Journal of Dentistry.

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