Dental Materials (2005) 21, 895–910
www.intl.elsevierhealth.com/journals/dema
Chemical aspects of self-etching enamel–dentin
adhesives: A systematic review
Norbert Moszner, Ulrich Salz*, Jörg Zimmermann
Research and Development, Ivoclar Vivadent AG, FL-9494 Schaan, Liechtenstein
Received 29 March 2005; accepted 10 May 2005
KEYWORDS
Dental adhesives;
Hydrolytic stability;
Adhesive monomers;
Phosphonic acids;
Polymerizable
phosphates;
Cross-linkers;
Methacrylates, bis(acrylamide)s
Summary Objectives: The paper gives an overview on the components and the
polymer chemical aspects of currently used self-etching enamel–dentin primers/adhesives. In addition, the contribution of new adhesives monomers and cross-linkers
exhibiting enhanced hydrolytic stability than methacrylates to improve the
performance of single-bottle adhesives is discussed.
Sources: Information from original scientific papers or reviews about enamel–dentin
adhesives, the patent literature concerning dental adhesives and manufacturer
information of commercial self-etching adhesives were included in this review.
Data: The most efficient self-etching enamel–dentin adhesives are based on strongly
acidic adhesive monomers, containing dihydrogenphosphate, phosphonic acids or
carboxylic acid groups. Serious problems of single-bottle water-based, strongly
acidic self-etching enamel–dentin adhesives arise both from the hydrolytic instability
of the methacrylate monomers used and the side reaction of the applied initiator
components.
Conclusions: The stability of the self-etching enamel–dentin adhesives can be
improved by using new acrylic ether phosphonic acids or mono- or difunctional
acrylamides, while more stable and compatible components have to be developed in
the future.
Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction
The concept of self-etching adhesives is based on
the use of polymerizable acidic monomers that
simultaneously condition and prime dentin and
enamel. Therefore, the self-etching primers
* Corresponding author. Tel.: C41 423 2353421; fax: C41 423
2331279.
E-mail address: [email protected] (U. Salz).
eliminate the technique-sensitive rinsing step to
remove the phosphoric acid from enamel and
dentin. The clinical requirements for self-etching
enamel–dentin adhesives are the same as for
adhesives used in combination with the acid etch
technique. Removal of the weak smear layer on top
of the dentin and creation of an adequate etch
pattern on the enamel in a clinically relevant period
of time (i.e. 15–30 s). Inward diffusion of comonomers into etched enamel and dentin by resin
tag formation in the etch pattern and dentinal
0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dental.2005.05.001
896
N. Moszner et al.
layer [12], or in the words of Nakabayashi, nonhybridized, decalcified dentin does not exist.
Due to the changes in chemistry, bonding
mechanism, number of components, application
technique, and clinical effectiveness in the
development of dental adhesive systems, generations of adhesives have been redefined [13]
(Fig. 2). According to this classification, selfetching dentin–enamel adhesives have been classified as the seventh generation [14].
This paper is intended to outline the components
and the polymer chemical aspects of self-etching
enamel–dentin primers/adhesives currently used. In
addition, the contribution of new adhesive monomers and cross-linkers exhibiting better hydrolytic
stability than methacrylates to improve the performance of single-bottle adhesives is discussed.
tubules and intertubular dentin penetration by
formation of the so called hybrid layer [1]. This
concept of monomer diffusion into water-filled
spaces between denuded collagen fibrils was first
reported by Nakabayashi et al. [1], and later
confirmed by Inokoshi et al. [2] and his collaborators [3,4].
The most widely used self-etching adhesive
systems involve two application steps: the conditioning of dentin and enamel with a self-etching
primer, followed by the application of an adhesive
resin. Nowadays, one-component self-etching
adhesives are increasingly introduced to the
market. However, most of the currently available
self-etching primers/adhesives are methacrylatebased (Fig. 1) with a pH-value in the 1.5–2.5 range
[5]. Under these strong acidic conditions, esters
such as 2-hydroxyethyl methacrylate (HEMA),
triethyleneglycol dimethacrylate (TEGDMA),
methacryloyloxydecyl dihydrogen phosphate
(MDP) or HEMA-phosphate, are hydrolytically
degraded [6,7]. Another disadvantage of onecomponent self-etching adhesives is seen in their
relatively high water uptake, resulting in the
formation of water trees at the interface [8,9].
One of the first acidic monomers that was
developed is 4-methacryloyloxyethyl trimellitate
anhydride (4-META) [10,11], which is still very
often used in self-etching primer systems. Clinically,
it has been shown that self-etching systems exhibit,
a low technique sensitivity with regard to the
conditions of the dentin surface (dry, wet, moist
etc.), resulting in a very low level of postoperative
sensitivity. It is obvious that the low risk of postoperative sensitivity has to do with the fact that
dentin decalcification and the penetration of dentin
by the acidic and co-monomers occur up to the same
depth. No nanoleakage is experienced in the hybrid
Monomers in self-etching enamel–dentin
adhesives
General aspects
The monomers contained in commercial selfetching adhesives can be divided into three main
groups according to their function: (a) self-etching
adhesive monomers; (b) cross-linking monomers
and (c) additional monofunctional co-monomers
(Fig. 1).
All these monomers have to meet the following
general requirements:
/ High rate of free-radical homopolymerization or copolymerization with the other
monomers in the adhesive.
/ Optimal solubility in the adhesive composition, i.e. the monomers should be conveniently miscible with aqueous solutions
Self-etching adhesive
monomers e.g. HEMAphosphate
O
Monofunctional co-monomers e.g. HEMA
O
O
O
O P OH
OH
Self-etching
enamel-dentin
adhesives
Cross-linking monomers e.g. TEGDMA
O
OH
Additives: photoinitiator, solvents, stabilizers, filler etc.
O
O
O
O
O
O
Figure 1
Components of currently available self-etching enamel–dentin primers/adhesives.
Chemical of self-etching adhesives
1955
1st Generation
2nd Generation
3rd Generation
4th Generation
5th Generation
2001
Figure 2
/
/
/
/
6th Generation
897
Cavity primers with low bond strength
Dentin-enamel bonding agents with improved bond strength to etched enamel
Etching of dentin + partially removal and
modification of smear layer
Total-etch technique and formation of
the hybrid layer and resin tags
Simplification of clinical procedure: onebottle systems and self-etching primers
One-step bonding systems with proper
bond to enamel and dentin
Evolution of bonding systems from the first generation to current bonding materials of the sixth generation.
of acetone and ethanol, which are mainly
used as solvents in commercial self-etching
adhesives, and the other monomers and
additives.
Sufficient stability of both the monomer and
the formed polymer. Stability of the monomer does not only comprise the stability
against premature polymerization, but also
the stability against degradation by oxygen,
heat, light and, of course, water during
storage.
Minimal water uptake and low swelling
degree of the formed polymer and the
monomers should not impair the sufficient
mechanical strength of the adhesive layer.
Low polymerization shrinkage or at least
contribution of the monomer to reduce
shrinkage or thermal stress in the adhesive
layer.
Low oral toxicity and cytotoxicity of the
monomers and the prerequisite that they
may not show any mutagenic or carcinogenic effect with and without metabolic
activation.
These general requirements can be fulfilled with
structure-designed monomers containing one or
more polymerizable groups and additional
functional groups, which are combined by a
tailor-made spacer group. Among the various
free-radically polymerizable groups (Fig. 3),
methacrylate functions show sufficient reactivity.
However, acrylates are more reactive and they may
increase the toxicological risk of the monomers.
Additionally, acrylates tend to produce side
effects, e.g. the Michael addition of nucleophilic
compounds, which leads to a loss of their polymerizability. Vinyl or styryl monomers are less
reactive in the free-radical photopolymerization
and often it is more difficult to synthesize
additional functionalized derivatives of these
monomers. In this context, functionalized (meth)
acrylamides provide easier access to synthetics and
qualify for the synthesis of monomers with
enhanced hydrolytic stability under acidic conditions. Allyl monomers exhibit a low tendency
towards homopolymerization and, if mixed with
other monomers, show a degradative chain transfer
reaction, which may impair the polymerization
behaviour of the whole adhesive mixture. Moreover, 1,2-disubstituted ethylene derivatives, such
as maleic acid, do not undergo homopolymerization
and, therefore, can only be used with suitable
copolymerizable monomers.
The structure of the spacer groups is important
since this may combine two or three polymerizable
groups in the case of cross-linkers or a polymerizable group with an adhesive group or functional
group in the case of self-etching adhesive monomers
or monofunctional co-monomers. The design of the
spacer group enables the exertion of a specific
influence on monomer properties, such as the
behaviours regarding volatility, solubility, viscosity,
wetting and penetration. Moreover, the structure of
the spacer groups also influences the properties of
the resulting polymers, such as hydrophilicity,
swelling properties, flexibility or stiffness. Thus,
the substitution of e.g. two carbon atoms by oxygen
atoms in an octamethylene group improves the
water solubility and the wetting behaviour of the
corresponding monomer, whereas a xylylidene
spacer containing eighth carbon atoms results in a
more rigid and hydrophobic monomer (Fig. 4).
Self-etching adhesive monomers are monomer
components in a self-etching enamel–dentin
(Meth)acrylate
(Meth)acrylamide
CH 3 (H)
CH2
C
CH 3 (H)
CH2
CO O
R: Alkyl, Aryl
C
CO N
R
Vinyl
CH2
CH
Figure 3
adhesives.
Styryl
CH2
CH
Allyl
CH2
CH CH2
Polymerizable groups in monomers for
898
N. Moszner et al.
Acid groups:
O
R
CH2
COOH
O
O
O
P OH
OH
CH2
Figure 4 Examples of spacer groups R in adhesive
monomers.
/ Capability of self-etching the enamel surface in a relatively short time while forming
a surface with increased roughness that
enables micromechanical bonding of the
adhesive on enamel.
/ Optimal wetting and film-forming behaviour on the tooth surface and the capability of penetrating, for example, into the
dentinal tubules.
/ Fast ionic or covalent interaction with
components of the dental hard tissue,
e.g., the formation of low soluble calcium
salts or formation of covalent bonds with
collagen.
To achieve these specific requirements, a great
number of acidic monomers with the potential for
generating a self-etching enamel–dentin adhesive
have been described in the literature. In general,
these self-etching adhesive monomers are bifunctional molecules containing at least the
following components: first, a polymerizable
group P, which can react both with the other
monomers of the adhesive and the restorative
material by copolymerization, second, an acid
adhesive group AD capable of both etching the
dental hard tissues and interacting with the tooth
substance, and, finally, a spacer group R designed
to influence, e.g. the solubility, flexibility and
wetting properties of the adhesive monomer
(Fig. 5). Suitable adhesive groups are acidic groups
(Fig. 6), in particular, phosphonic acid and mono-or
Spacer group influences flexibility,
solubility, wetting behavior etc.
Adhesive group to
create the bond to
P
R
AD
dentin and enamel
Self-etching adhesive monomer
Figure 5 General structure of a self-etching adhesive
monomer.
S OH
OH
COOH
COOH
Polymerizable group
O P OH
OH
Chelating groups:
adhesive, which are responsible for the specific
interaction of the adhesive with the dental hard
tissue. Therefore, they should meet, beside the
above mentioned general requirements, the
following additional demands:
O
N
COOH
OH
Covalent coupling groups:
O
C
OH
H
SH
O
O
O
Figure 6 Adhesive groups AD enable chemical adhesion
to enamel or dentin.
dihydrogenphosphate groups, which form stronger
acids than the corresponding carboxylic acids. The
general potential of the acidic monomers to etch
e.g., enamel, largely depends on the acidity of the
monomers that increases in the following order:
carboxylic acids!phosphonic acids!acid phosphates!sulfonic acid. However, the application
of polymerizable sulfonic acids in present selfetching adhesives only concerns 2-acrylamido2-methylpropane sulfonic acid. The reason for this
is that suitable self-etching adhesive monomers
should not only exhibit appropriate etching properties, but also meet the full set of requirements.
The chemical adhesion of monomers on the hard
tissue of a natural tooth can be realized by the
formation of primary chemical bonds, such as
covalent or ionic bonds, generated by the reaction
of suitable adhesive groups with components of the
dental hard tissue. As a consequence, ionic bonds
are formed by the acidic groups reacting with the
main inorganic component of the dental hard
tissue, which is hydroxyapatite. In addition, chelating groups, which we can find, for example, in
salicylic acid or aminodiacetic acids (Fig. 6), enable
the formation of coordinative linkages to the
calcium ions of enamel or dentin. Furthermore,
additional reactive groups in the monomeric acids
may result in establishing covalent bonds between
the collagen fibers in dentin and the self-etching
adhesive monomer. As dentinal collagen contains
reactive groups, which are amino or hydroxyl
groups in particular, the reaction of dentin with
e.g. aldehyde or anhydride groups may establish
covalent bonds with the collagen fibers if conditions
are mild. Moreover, secondary valence forces, such
as attraction forces of molecular (Van der Waals
forces) and induced dipoles (London dispersion
forces), or attraction forces caused by the formation of hydrogen bridges or charge-transfer
Chemical of self-etching adhesives
O
899
O
GDMP
O
O
O
O
O
O P OH
O
O
OH
O
O
O
O
(CH2)10
O P OH
MDP
OH
O
O
O
O
O
MEP
OH
PENTA-P
O
O
O P OH
O
O P OH
OH
O
Figure 7
O
O
O P O
O
OH
MEP-P
Examples of polymerizable acidic phosphates used in dentin adhesives.
interactions may additionally contribute to the
adhesion (physical adhesion).
Adhesive monomers in commercial
self-etching enamel–dentin adhesives
Phosphorus-containing monomers
In general, phosphorus-containing monomers, e.g.
phosphonic acids or acidic phosphates, are capable
of etching enamel and dentin. In addition, these
monomers promote monomer diffusion into the
acid-conditioned and underlying intact dentin.
Among the phosphorus-containing monomers,
mainly polymerizable acidic phosphates are used.
One of the first chemical compounds proposed to
improve bonding with human dentin [15,16] was the
glycerol dimethacrylate ester of phosphoric acid
(GDMP) (Fig. 7). Further examples of acidic methacrylate phosphates conveniently applied to improve
bonding on dentin are the reaction products of
phosphorus oxychloride with bis-GMA [17], methacryloyloxyethyl phenyl hydrogen phosphate
(MEP-P), MDP, methacryloyloxypropyl dihydrogen
phosphate (MPP), methacryloyloxyethyl dihydrogen
phosphate (MEP, HEMA-phosphate), and dipentaerythrolpentaacryloyl dihydrogen phosphate (PENTAP), which are summarized in Fig. 7. The synthesis
and dental adhesive application of MEP-P and its
p-methoxy derivative was first described by Nakabayashi et al. [18,19]. The polymerizable acidic
phosphates shown in Fig. 7 were first used in dental
adhesives of the second generation. These commercially available compounds are mostly mixtures of
different phosphoric acid derivatives. Thus, Reinhart et al. [20], showed, that, e.g. commercially
available MEP additionally contained both di(DMEP) and tri-HEMA ester (TMEP) of phosphoric
acid, as well as pyrophosphates (PMEP) and even
free phosphoric acid (Fig. 8). Obviously, the easily
separable triester TMEP is a non-acidic monomer,
and therefore, not able to etch enamel. Different
acidic esters can easily be distinguished using
31
phosphorous nuclear magnetic resonance
(31P NMR) spectroscopy. Chemical shifts of the
phosphorus atoms of the different phosphoric
acids range between 10 and 20 ppm and depend on
the pH-value [21]. Therefore, the number of signals
in the 31 P NMR spectrum of phosphoric acid
monomers mostly corresponds to the number of
present chemical species. For example, the 31P NMR
spectrum of crude product of the MEP shows three
signals at 10.2, 9.0 and 3.1 ppm, which can be
assigned to the phosphorus atoms of the three acidic
methacrylates DMEP, MEP and PMEP. The polymerizable acidic phosphates are generated by the
reaction of phosphorus oxychloride (POCl3) with
the corresponding OH-group containing methacrylate. For example, the reaction of POCl3 with
10-hydroxydecyl methacrylate in the presence of
triethyl amine (TEA) at K30 to K40 8C resulted in
MDP with a purity of 92–95% [22] (Fig. 9). The
different solubility of barium or calcium salts of free
phosphoric acid and acidic phosphates enables the
separation of free phosphoric acid from polymerizable acidic phosphates.
O
O
R O P O R
R O P O R
O
R O P OH
OH
MEP
O
OH
R TMEP
O
O
R O P O P O R
OH
O
OH
DMEP
R
O
PMEP
Figure 8
Various esters of HEMA and phosphoric acid.
900
N. Moszner et al.
O
O
POCl3 +
O
OH
O
TEA
O
9
O P OH
9
OH
MDP
Figure 9
Phosphorylation of 10-hydroxydecyl methacrylate with POCl3.
From the literature [23], it is known that monoand dialkylesters are actually stronger acids than
phosphoric acid itself and that dialkyl hydrogen
phosphates are stronger acids than the corresponding monoalkyl dihydrogen phosphates. However,
the phosphoric acid ester bonds in the diesters are
under acidic conditions, less hydrolytically stable
than in the monoesters, which is the case e.g. when
the pH-value reaches 1–2. In the case of acidic
methacrylate phosphates, an additional hydrolytic
instability results from the hydrolysis of the
methacrylate ester bond taking place in the
presence of water, which is frequently used as cosolvent in self-etching enamel–dentin adhesives and
is catalyzed by hydrogen ions of the phosphoric acid
group. For MEP, we found [24,25] that the
hydrolysis of both the methacrylate and phosphate
ester bonds resulted in the formation of
methacrylic acid (MAA) (A) and 2-hydroxyethyl
methacrylate (HEMA) (B), as is shown in Fig. 10. In
general, the hydrolytic stability of the phosphoric
acid esters increases in the following order: dialkyl
hydrogen phosphate!trialkyl phosphate!monoalkyl dihydrogen phosphate. Therefore, hydrolysis
of the phosphate ester bond in the monoester MEP
was not expected. Furthermore, we found that in
the case of longer alkylene spacers, for example, in
the case of MDP, the phosphate ester bond was
significantly more stable. Nevertheless, aqueous
solutions of MDP are not hydrolytically stable, if
stored at room temperature for weeks [7], and
therefore, they have to be stored in a refrigerator.
In summary, MDP, MEP, DMEP and GDMP are used as
monomers in the current self-etching enamel–
dentin adhesives.
Polymerizable carboxylic acids
While a number of free-radically polymerizable
unsaturated carboxylic acids form the main
adhesive monomers in the dentin adhesives up to
the 4th generation, their use in self-etching primers
is restricted to only a few compounds, above all,
4-methacryloyloxyethyl trimellitic acid (4-MET),
4-META, and 10-methacryloyloxydecyl malonic
acid (MAC-10) (Fig. 11). Aqueous solutions of
these polymerizable unsaturated carboxylic acids
show a pH-value below 2.0 as well and as a
consequence enable the surface of enamel and
dentin to be etched.
Monofunctional and cross-linking monomers
in commercial adhesives
Besides the truly adhesive acidic monomers, the
currently used self-etching enamel–dentin
adhesives contain further mono - and difunctional
non-acidic methacrylates, which are instrumental
in influencing the properties of the liquid adhesive,
such as miscibility, viscosity, wetting and filmforming behaviour, monomer penetration and
polymerization reactivity, as well as the properties
of the polymerized solid adhesive layer, such as
mechanical strength, water uptake or stability
against hydrolytic or enzymatic degradation.
Among the monomethacrylates, HEMA is the most
frequently applied monomer, while similar monomers such as 2-hydroxypropyl methacrylate (HPMA)
or 2-hydroxy-3-phenoxypropyl methacrylate
(HPPMA) are hardly used (Fig. 12). HEMA is a
water soluble, low viscous monomer that improves
the miscibility and solubility of the polar and nonpolar adhesive components and the wetting behaviour of the liquid adhesive on the dental hard
tissue. In addition, it is well known [26] that HEMA
may stabilize the collagen fibril network and
improves the dentinal permeability and monomer
diffusion. However, HEMA is not hydrolytically
stable and forms associates in concentrated solutions, which promote its hydrolysis [27]. Furthermore, it has to be considered that a high content of
monofunctional monomers in the adhesive impair
its polymerization rate. This may lead to a
significant lower density of cross-linkage in the
polymerized adhesive layer, which may result in
OH
+
P
O OH
O
HO
H2O
OH MAA
O
A
O
O
O
O
HO
P
OH
+
OH
O
B
HO
P
OH
MEP
OH
H2O
O
HEMA
O
Figure 10
Hydrolysis of MEP in the presence of water.
Chemical of self-etching adhesives
901
O
O
O
COOH
O
O
O
O
O
4-MET
O
O
4-META
COOH
O
COOH
O
CH
MAC-10
O
Figure 11
COOH
Structure of self-etching polymerizable carboxylic acids.
O
O
O
HEMA
Figure 12
OH
O
OH
O
HPMA
O
O
HPPMA OH
OH-group containing methacrylates used in dentin adhesives.
weakened adhesive bonds, in particular, after
storage and swelling in water.
Cross-linking dimethacrylates are used in
enamel–dentin adhesives to generate the formation
of the polymer network, which leads to a number of
favorable effects. First, the polymerization rate
strongly increases because of the gel-effect.
Second, the mechanical properties of a polymer
network are improved in comparison to linear
polymers. Finally, the formed cross-linked layer is
not water soluble and the degree of swelling
decreases with increasing polymer network density.
The most popular cross-linking dimethacrylates
used in enamel–dentin adhesives are 2,2-bis[4(2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (Bis-GMA), 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (UDMA),
glycerol dimethacrylate (GDMA), and TEGDMA
(Fig. 13). The dimethacrylates demonstrate different properties such as viscosity, polarity and water
solubility, polymerization shrinkage, film formation
behaviour and reactivity. Thus, Bis-GMA shows high
reactivity, however, it exhibits also high viscosity
and low water solubility. In contrast, TEGDMA and
GDMA are dimethacrylates featuring low viscosity
and improved solubility in water. Unfortunately,
all these dimethacrylates are not hydrolytically
stable in aqueous acidic solutions and degrade
under formation of corresponding diols and
methacrylic acid.
New phosphorus containing monomers
with improved hydrolytic stability
The problem of the hydrolytic instability of
methacrylate phosphates can be solved by applying
monomers that contain more hydrolytically stable
bonds between the polymerizable group and the
strongly acidic phosphorus group. Phosphonates
may be used for this purpose. In dentistry, organic
phosphonates are well known and used in
OH
OH
O
O
O
O
Bis-GMA
O
O
O
O
O
O
HN
NH
UDMA
O
O
O
O
O
O
O
O
O
O
TEGDMA
O
O
O
O
GDMA
OH
Figure 13
Cross-linking dimethacrylates used in dentin adhesives.
902
N. Moszner et al.
O
O
CH2
CH P OH
VPA
CH2
CH
OH
CH2
P OH
OH
VBPA
Figure 14 Structure of the monomeric phosphonic
acids VPA and VBPA.
dentifrices to reduce the formation of supragingival
dental calculus because they act as calcium
sequestrants or inhibit the crystal growth of
calcium phosphates [28]. A first evaluation of
polymerizable phosphonates for dental adhesives
was carried out by Anbar et al. [29–31]. They proved
that
vinylphosphonic
acid
(VPA)
and
4-vinylbenzylphosphonic acid (VBPA) (Fig. 14) or
corresponding copolymers can improve the
adhesion of filling composites on etched enamel,
and decrease the adsorption of proteins on enamel.
Unfortunately, VPA and VBPA are less reactive than
methacrylates in radical polymerization. In this
context, we were able to synthesize a number of
new acrylic ether phosphonic acids (AEPA, Fig. 15)
with improved hydrolytic stability and reactivity in
the free-radical polymerization, in which the
polymerizable methacrylate group and the strong
acidic phosphorus group are connected through
a hydrolytically stable ether bond (Fig. 15) [7,24,
32–34]. The synthesized phosphonic acids dissolve
well in water, acetone or ethanol. A 20 wt% solution
of the phosphonic acids shows a pH-value that is
lower than 2.0, e.g. 1.12 in the case of EAEPA (ethyl
2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate).
Due to their acidity, the monomers are able to etch
O
enamel or dentin. The etch pattern produced with
the polymerizable phosphonic acids is similar to
that generated by commercial etching gels based on
35 wt% phosphoric acid. Among the synthesized
monomers, EAEPA and MAEPA (2,4,6-trimethylphenyl 2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate) featured the best dentin adhesive
properties, whereas the corresponding carboxylic
acid group containing phosphonic acid CAEPA (2-[4(dihydroxyphosphoryl)-2-oxabutyl]acrylic acid) or
the nitrile derivative NAEPA (2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylonitrile) exhibited significantly less adhesive action. This could be explained
on the basis of e.g., the lower solubility (CAEPA) or
radical polymerizability (NAEPA) of the monomers.
CAEPA and NAEPA are hydrolytically stable in
aqueous solutions at room temperature. In contrast
to these monomers, the carboxylic ester bond in
EAEPA tends to hydrolyze with formation of ethanol
and the phosphonic acid CAEPA. Therefore, we
synthesized the phosphonic acid monomer MAEPA,
which was completely stable under aqueous
conditions at 42 8C during 4 months of investigation
[34–36]. The improved hydrolytic stability of MAEPA
in comparison to EAEPA is caused by the steric
hindrance of hydrolysis regarding the methyl groups
in the 2- and 6-position of the phenyl group.
Recently, a number of methacrylates of hydroxyalkylphosphonates have been proposed, as e.g.,
MAPA-1 [37] or the difunctional monomer MAPA-2
(Fig. 16) [38]. However, these monomers are not
hydrolytically stable, because they undergo
hydrolysis if water is present, and form methacrylic
O
O
O
OH
P
O
O
HO
EAEPA
CAEPA
P
O
OH
O
O
OH
OH
OH
MAEPA
Structure of various acrylic ether phosphonic acids (AEPA).
O
O
O
O
O
P
OH MAPA-1
OH
O
O
O
OH
O
OH
HO P O
OH
Figure 16
P
O
NAEPA
Figure 15
OH
OH
O
O
NC
P
O
OH
O P OH
MAPA-2
OH
Structure of the methacryloyloxyalkyl phoshonic acids MAPA-1 and MAPA-2.
Chemical of self-etching adhesives
O
O
DMHD
Figure 17
MAMPA.
O
O
P OH
OH
903
N
P OH
NH
MAMPA
OH
N
DMBAAH
O
N
Structure of the phosphonic acids DMHD and
acid. In this context, the phosphonic acids DMHD
and MAMPA (Fig. 17) described in a recent patent
[39] should be hydrolytically stable. Nevertheless,
DMHD is not an appropriate monomer for radical
polymerization. Unfortunately, results of adhesion
measurements were not presented in the patent.
Monofunctional and cross-linking monomers with improved hydrolytic stability
Stability studies showed [7] that conventional
dimethacrylates such as TEGDMA or GDMA undergo
rapid hydrolysis under acidic aqueous conditions.
Therefore, they cannot be used for the preparation
of water based single-bottle adhesives that remain
stable if stored at room temperature. From the
literature it is well known [40] that carbamides are
more hydrolytically stable than esters because their
carbonyl group exhibits a lower reactivity. Thus,
vigorous conditions and strong catalysts such as
concentrated sulfuric or phosphoric acids are often
required to bring hydrolysis on. Thus, it can be
expected that difunctional acryl- or methacrylamides exhibit enhanced hydrolytic stability
compared to dimethacrylates. A number of bis(acrylamide)s, e.g. N,N 0 -ethylenebisacrylamide or
N,N 0 -methylenebisacrylamide, are commercially
available and used as cross-linking agents for the
preparation of poly(acrylamide) gels. However,
these are solids and they feature a water solubility
of less than 5%. Moreover, they are practically
insoluble in ethanol or acetone. We were able to
synthesize a number of new bis(acrylamide)s
(Fig. 18), which are liquids and completely soluble
in water and ethanol and show an improved
hydrolytic stability [41]. Examples of suitable
bis(acrylamide)s are N,N 0 -dimethyl-1,3-bis(acrylamido)-hexane (DMBAAH), N,N 0 -dimethyl-1,3-bis
(acrylamido)-propane (DMBAAP), and N,N 0 -diethyl1,3-bis(acrylamido)-propane (DEBAAP). All monomers in particular, which contained N,N 0 -disubstituted carbamide groups, demonstrated excellent
solubility compared to bis(acylamide)s with N,N 0 monosubstituted carbamide groups. This means
that the hydrogen bonding in these monomers
between the CO–NH groups impair their solubility
in organic solvents. In order to investigate the
N
DMBAAP
O
N
O
O
N
DEBAAP
O
Figure 18 Structure of bis(acrylamide)s with improved
hydrolytic stability.
reactivity of the synthesized monomers, gelation
experiments were carried out. It could be seen that
the water soluble bis(acrylamide)s featured a
similar reactivity than the cross-linking dimethacrylates as e.g. GDMA. Bis(methacrylamide)s are
less reactive than the corresponding bis(acrylamide)s. In contrast to GDMA, the water soluble
bis(acrylamide)s as e.g. DEBAAP feature a significantly higher hydrolytic stability. The hydrolytic
stability of the monomers was examined by means
of 1 H NMR spectroscopic measurements. As
expected, the monomer DEBAAP remained stable
in aqueous ethanol in the presence of 20 wt% of
phosphoric acid after it had been stored at 37 8C for
a test period of 4 months. GDMA was not stable and
methacrylic acid started to build up after 1 day
under comparable conditions. In this context, it has
to be mentioned that many of the synthesized
water soluble bis(acrylamide)s exhibit lower cytotoxicity than the corresponding hydrophilic
dimethacrylates. Cytotoxicity was determined
with in vitro single cell gel electrophoresis (Comet
Assay [42]). So, the XTT50-value for the DEBAAP
monomer was 880 mg/ml while it reached 25 mg/ml
for TEGDMA. The obtained results confirmed [7]
that the water soluble bis(acrylamide)s as e.g.
DEBAAP can be used to substitute TEGDMA or GDMA
in water based single-bottle self-etching enamel–
dentin adhesives containing new, strongly acidic
acrylic phosphonic acids, which results in adhesives
with a storage stability at room temperature more
than 2 years.
Analogous to the dimethacrylates, the monofunctional monomers and HEMA in particular also
have to be substituted in water based single-bottle
self-etching enamel–dentin adhesives with appropriate monomers that exhibit enhanced hydrolytic
stability, as is the case e.g. with (meth)acrylamides.
HEMA substitutes are, for example, N-methylolacrylamide (HMAM) or N-methylolmethacrylamide
(HMMAM) (Fig. 19), which are more effective in
904
N. Moszner et al.
[47,48], and N-methacryloyl glycine (NMGLY) [6].
The investigation of NMGLY showed that its
hydrolytic stability was higher than that of HEMA.
O
O
NH
NH
OH
HMMAM
OH
HMAM
O
O
NH
OH
OH
N
Initiator systems and solvents for
self-etching adhesives
MHEAM
HEMAM
Figure 19 Structure of HEMA-substitutes
improved hydrolytic stability.
with
Initiators
As in the case of composite filling or luting
materials, adhesives can be photopolymerized, or
the polymerization can be initiated by a redox
initiator system. An intrinsic problem of selfetching enamel–dentin adhesives is the acid–base
reaction of the acidic monomers with amines often
used in the initiator systems [49–52], such as the
camphorquinone/amine system in visible (VIS) light
curing adhesives (Fig. 21) or the amine/peroxide
system in chemically curing adhesives (Fig. 22). In
both cases, the concentration of the amine, and
therefore, also of the formed amine radical, which
is responsible for the initiation of polymerization,
decreases [53,54]. Retarded polymerization not
only occurs in the adhesive layer, but also in the
oxygen-inhibited surface zone that bonds the
composite to the adhesive [55–57]. The acid–base
reaction results in an equilibrium between the
protonized and unprotonized form of the amine and
the acid. Therefore, the amine concentration needs
to be exactly adjusted to the concentration of acid
in self-etching systems. Chemically cured adhesives
conjunction with suitable dentin primers than HEMA
[43]. It is, however, well known [44] that N-methylol
compounds are degradable under formation of
formaldehyde which is a toxicological risk. Therefore, monomers with a longer spacer between the
OH group and the polymerizable (meth)acrylamide
group would be advantageous. In fact, we found out
[45] that e.g. both N-(2-hydroxyethyl)-methacrylamide (HEMAM) and N-methyl-N-(2-hydroxyethyl)acrylamide (MHEAM) feature improved hydrolytic
stability and very low cytotoxicity. They are,
therefore, suitable to substitute HEMA in water
based single-bottle self-etching enamel–dentin
adhesives containing new, strongly acidic adhesive
monomers.
Finally, it should be mentioned that a number
of COOH-group-containing (meth)acrylamides
(Fig. 20) were investigated to be used as monomers
for enamel and dentin adhesives. Examples for
these are N-methacryloyl-1-aminosalicylic acid
(MASA) [46], N-acryloyl aspartic acid (NAASP)
COOH
O
NH
O
NH
NH
OH
COOH
O
NAASP
NMGLY
MASA
Figure 20
CH3
N
CO OC2H5
CH3
O
Accelerator
O
R`
N CH2
hν
C O
R
nπ∗
R
C O
1* ISC
R
[Photoinitiator]: 0.1-0.2 wt.-%;
Figure 21
COOH
Structure of COOH-containing (meth)acrylamides.
Camphorquinone
R
COOH
R
R
R
3 * R`
C O
R
(max): 468 nm
Electron and
proton transfer
C O H
R
R`
+
N CH
R
R`
Amine radical
Initiator system for the photopolymerization of VIS-light curing adhesives.
Chemical of self-etching adhesives
905
O
O
O
O
C O
C O O C
O C
+
+
R
N
CH2 R`
R
N
CH2 R`
CH2 R`
CH2 R`
-H
-H
+H
R
N
H
Figure 22
R
CH2 R`
CH2 R`
N
CH R`
CH2 R`
Amine radical
Acid–base reaction of an amine/peroxide redox initiator system for radical polymerization.
are even more sensitive due to thicker oxygen
inhibition layers [57,58]. This has resulted in
alternatively using less sensitive redox initiatiors
containing reductants such as barbituric [59],
ascorbic [60], sulfinic acids [61], or combinations
of them with amine/peroxide systems [52,62]. The
initiator components are often contained in a thin
coat on purpose-designed applicators to facilitate
the application and prolong the storage stability of
the material [63,64].
In the case of photoinitiators, Norrish type I
systems, which do not require amine accelerators
and absorb light in the wavelength range of
400–500 nm, are needed. Acylphosphine oxides
were introduced on the market some years ago as
a new class of a-cleavage photoinitiators [65]. The
absorption characteristics of acylphosphine oxides
differ from most other photoinitiators of the
a-cleavage type in that they feature enhanced
absorption in the near UV/VIS range. This is very
important for their use in dental materials, e.g.
dental adhesives. The chemical structures of some
acylphosphine oxides are shown in Fig. 23. A
disadvantage of acylphosphine oxides is that they
are not compatible with most of the new dental
LED polymerization light units and that acylphosphine oxides are prone to undergoing solvolytic
cleavage of the carbon–phosphorus bond in the
presence of nucleophilic compounds such as water
or alcohols [65].
Solvents
Dentin consists of approx. 70% hydroxyapatite, 20%
organic material and 10% water [66]. To achieve a
thorough wetting of this moist substrate, the use of
hydrophilic adhesive components will be necessary.
Decalcification of enamel and dentin, and removal
of the smear layer, which is created during cavity
preparation on top of the dentin, are ionic
processes. Calcium ions are chelated by acidic
monomers, and parts of collagen fibres are solubilized or hybridized [67]. For these ionic processes
water is required, and therefore, self-etching
adhesives or primers are generally water based. In
contrast to the acid etch technique [68–72], selfetching adhesives/primers do not exhibit an equal
sensitivity to moisture on dentin surfaces due to
their water base [73]. However, excess water could
decrease the content of the adhesives’ monomeric
components within the collagen network and
probably interfere with their polymerization,
which would then result in a lower cross-linking
density of the formed hybrid layer [74]. Co-solvents
like ethanol are added to self-etching adhesives,
which form an azeotropic mixture with water and
thus accelerate the surface dehydration by means
of air syringe drying. Excessive dehydration of
dentin, however, negatively affects the mechanical
substrate properties [75]. If acetone is used as a cosolvent, phase separation and precipitation of
O O
O O
O O
C P O
C P
C P
O
Acylphosphonat
Figure 23
O
Acylphosphine oxide
Acyldibenzoxaphosphorine oxide
Examples of acylphosphine oxide a-cleavage photoinitiators.
906
adhesive components can occur. During solvent
evaporation the ratio water–acetone is changing.
Further additives for dentin–enamel
adhesives
Fillers
Various dental adhesive systems with a significant
filler amount are available on the market.
Compared to composite filling materials, the filler
content and composition of dental adhesives play a
minor role. Nevertheless, the whole range of
different fillers from spherical glass fillers, silicates
to nanometer-sized pyrogenic silica as well as ion
leachable glass fillers are used for adhesive
formulations [76–78]. Adhesive systems are influenced by inorganic fillers in several ways. Especially
for total etch adhesives, the enhanced physical
properties of the bonding agent by filler particles
are discussed, on the one hand, as a possibility for
improving the bond strength of an adhesive. On the
other hand also the negative effects of fillers, e.g.
on the penetration behaviour of adhesives, are
discussed in the literature [79]. Based on these
controversial results various investigations, where
different total etch adhesive systems were tested,
proved that maximal bond strength can be found
with a filler content of between 10 and 40 wt% [80].
However, the influence of fillers in adhesive
formulations as stress buffers remains unpredictable and has not been confirmed in clinical trials
[81,82].
In contrast to this, the newest self-etching
bonding systems, where adequate wetting of the
surface and penetration of the acidic monomers is
essential for reliable adhesion, contain only small
amounts of fillers. Especially in single-step systems,
silica fillers are often used as thickener to increase
the viscosity; thus, a film is reliably formed, which
results in an adequate thickness of the film. Effects
like over-thinning and incomplete polymerization
due to oxygen inhibition can be prevented [83–85].
In addition to conventional fillers, some adhesive
systems contain filler particles featuring specific
properties like the capability to release fluoride
ions or to provide radiopacity. For the controlled
release of fluoride ions fillers like desiccated
siliceous hydrogel fillers, derived from fully reacted
glass ionomer reactions and fluoroaluminosilicate
glass, were used [86]. Fluorides releasing selfetching adhesive systems are also available on the
market. Surface modified, i.e. polysiloxane-encapsulated sodium fluoride particles, are used as
N. Moszner et al.
a source for fluoride ions in e.g. Clearfil Protect
Bond (Kuraray Medical Inc., Japan) [87]. There are
different reasons for the development of fluoridereleasing adhesives. Fluoride is applied because of
its anticariogenic activity resulting in an increased
resistance of enamel and dentin to acid attacks
[88], and the inhibition effect it exhibits to the
carbohydrate metabolism in dental plaque [89]. In
addition to this, it has been shown that the
incorporation of fluoride to the adhesive resins
increases the durability of the dentin bond. Several
studies proved that the bond strength of systems
containing fluoride did not decrease after longterm water storage, whereas the bond strength of
comparable resins without fluoride decreased
during the same time of immersion [90].
Additives which facilitate the application
of adhesives and antimicrobial agents
Some manufacturers add different dyes to their
adhesive formulation. In two-component systems,
dyes are used with the intention to clearly indicate
to the clinician that if both components have been
properly mixed by featuring a colour change. Two
systems are available on the market (One-Up Bond
F, Tokuyama, Japan, and Tyrian SPE; Bisco, USA),
where the colour change occurs through an acid
indicator dye. In One-Up Bond F system the colour
turns from yellow to pink when the neutral
component containing the dye (yellow) is mixed
with the acidic component. The colour is also useful
for facilitating the control of homogeneous tooth
coverage and the prevention of pooling. When the
adhesive layer is light-cured, the colour of both
systems fades.
The reasons for adding antimicrobial agents to
dental adhesive formulations are manifold. On the
one hand, identification and complete removal of
carious dentin in clinical trials is often difficult to
assess, as no convenient and objective method for
caries detection is at hand [91,92]. It is, thus,
intricate to be able to control the absence of
bacteria, and adding antibacterial components
seemed to be a promising way. On the other hand,
dental plaque adheres to the surface of the tooth
and restorative materials. Oral bacteria penetrating through micro gaps in between restoration and
tooth may generate secondary caries and pulp
damage. Therefore, antibacterial components are
added to adhesive systems in several ways to ensure
the biological sealing of the restoration. Nowadays,
total etch adhesive systems are well established.
These systems use phosphoric acid to condition the
prepared tooth surface by removing the smear layer
Chemical of self-etching adhesives
907
and decalcify the tooth surface. The infiltration of
adhesive resins into the decalcified tooth structure
results in a hybrid zone, which provides hermetic
sealing of the surface [93]. In addition, several
studies have shown that phosphoric acid exhibits a
short-term antimicrobial activity during the acid
etch procedure, and also total etch dentin bonding
components as long as they are uncured [94]. New
self-etching adhesive systems incorporate the
smear layer, and microbial contaminations from
the smear layer are thus also incorporated. To
integrate antimicrobial effects into self-etching
primers appears to be attractive for this reason.
During the last years, evidence that several selfetching adhesives possess inherent antibacterial
activities, comparable to the previous total etch
systems, was supplied [95]. Different studies have
generally assigned this effect to the low pH-value of
self-etching primers [96,97] which can be compared
to the antibacterial effect of phosphoric acid used
in total etch systems [98]. However, bonding resins
from two-step self-etching adhesive systems also
featured some antibacterial effects. The reason for
this is difficult to evaluate due to overlapping
factors, such as the antibacterial activity which
some fluoride salts or monomers and initiator
derivatives exhibit [99]. In addition to these
inherent antibacterial properties of adhesives,
manufacturers add various antibacterial components to adhesive formulations. For many years,
glutaraldehyde has been a well-known component
in adhesive systems used as a disinfectant, which
features antibacterial properties. It is applied in
total etch multi-bottle systems like Syntac (Ivoclar
Vivadent, Liechtenstein) or Gluma Bond (Heraeus
Kulzer, Germany) as well as in i-Bond, a onecomponent self-etching adhesive system of the
latest generation (Heraeus-Kulzer, Germany)
[100,101]. Due to the fact that glutaraldehyde
cannot be polymerized into the adhesive matrix,
the antibacterial effect persists after polymerization because the molecules are leaching. In
contrast, a polymerizable antibacterial monomer
MDPB (12-methacryloyloxydodecylpyridiunium bromide) was introduced by Kuraray Company
(Fig. 24). If this monomer is used, the adhesive
system will provide antibacterial effects before
polymerization, whereas the polymerized adhesive
CH3
CH2
C
CO O
(CH2)12
N
Br
MDPB
Figure 24 Structure of MDPB, a polymerizable antimicrobial agent.
will feature only bacteriostatic activity as a contact
antimicrobial [102].
Conclusion
The commercial self-etching enamel–dentin
adhesives consist of a mixture of self-etching
adhesive monomers, cross-linking monomers and
additional monofunctional co-monomers. Thus, the
most efficient adhesives available at the moment
are based on strongly acidic adhesive monomers,
containing dihydrogenphosphate, phosphonic acid
or carboxylic acid groups. These monomers enable
effective etching of the dental hard tissues,
promote wetting and monomer penetration into
the rough enamel and porous dentin surface
structure, and form a strong and durable adhesive
layer by free-radical copolymerization with the
other monomeric components of the adhesive. At
the moment, serious problems of water-based,
highly acidic self-etching enamel–dentin adhesives,
in particular single-bottle adhesives arise from the
hydrolytic instability of the methacrylate monomers used, and the side effects of the applied
initiator components. The application of new
components with improved hydrolytic stability,
which is the case with e.g. acrylic ether phosphonic
acids or mono- or difunctional acrylamides, may
help to solve the problem. Moreover, more stable
and compatible components have to be developed
in the future with regard to initiators. Finally, welldesigned additives, such as nanofillers or antibacterial monomers, may also contribute to enhance
the performance of self-etching enamel–dentin
adhesives.
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