J. Biomedical Science and Engineering, 2011, 4, 147-157 JBiSE
doi:10.4236/jbise.2011.43021 Published Online March 2011 (http://www.SciRP.org/journal/jbise/).
Published Online March 2011 in SciRes. http://www.scirp.org/jour nal/JBiSE
Synthesis and evaluation of a novel antibacterial dental resin
composite with quaternary ammonium salts
Yiming Weng1, Xia Guo1, Voon Joe Chong1, Leah Howard1, Richard L. Gregory2, Dong Xie1
1Department of Biomedical Engineering, Purdue School of Engineering and Technology Indiana University-Purdue University at
Indianapolis, Indianapolis, USA;
2Department of Oral Biology, School of Dentistry, Indiana University Indianapolis, Indianapolis, USA.
Received 16 February 2011; revised 26 February 2011; accepted 1 March 2011.
ABSTRACT
The novel quaternary ammonium bromide (QAB)-
containing oligomers were synthesized and applied
for d e ve loping an antibacterial resin composite. Com-
pressive strength (CS) and S. mutans (an oral bacte-
ria strain) viability were used to evaluate the me-
chanical strength and antibacterial activity of the
formed composites. All the QAB-modified resin com-
posites showed significant antibacterial activity and
mechanical strength reduction. Increasing chain length
and loading significantly enhanced the antibacterial
activity but dramatically reduced the CS as well. The
30-day aging study showed that the incorporation of
the QAB accelerated the degradation of the compos-
ite, suggesting that the QAB may not be well suitable
for development of antibacterial dental resin com-
posites or at least the QAB loading should be well
controlled, unlike its use in dental glass-ionomer ce-
ments. The work in this study is beneficial and valu-
able to those who are interested in studying antibac-
terial dental resin composites.
Keywords: QAB; Substitute Chain Length; Antibacterial;
S. Mutans Viability; CS; Dental Resin Composites
1. INTRODUCTION
Long-lasting restoratives and restoration are clinically
attractive because they can reduce patents’ pain and ex-
pense as well as the number of their visits to dental of-
fices [1-4]. In dentistry, both restorative materials and
oral bacteria are believed to be responsible for the resto-
ration failure [2]. Secondary caries is found to be the
main reason to the restoration failure of dental restora-
tives including resin composites and glass-ionomer ce-
ments [1-4]. Secondary caries that often occurs at the
interface between the restoration and the cavity prepara-
tion is primarily caused by demineralization of tooth
structure due to invasion of plaque bacteria (acid-pro-
ducing bacteria) such as Streptococcus mutans (S. mu-
tans) in the presence of fermentable carbohydrates [4].
To make long-lasting restorations, the materials should
be made antibacterial. Although numerous efforts have
been made on improving antibacterial activities of dental
restoratives, most of them have been focused on release
or slow-release of various incorporated low molecular
weight (MW) antibacterial agents such as antibiotics,
zinc ions, silver ions, iodine and chlorhexidine (CHX)
[5-9]. Yet release or slow-release can lead or has led to a
reduction of mechanical properties of the restoratives
over time, short-term effectiveness, and possible toxicity
to surrounding tissues if the dose or release is not properly
controlled [5-9].
Materials containing quaternary ammonium salt (QAS)
or phosphonium salt (QPS) groups have been studied
extensively as an important antimicrobial material and
used for a variety of applications due to their potent an-
timicrobial activities [10-14]. These materials are found
to be capable of killing bacteria that are resistant to other
types of cationic antibacterials [15]. The examples of
QAS-containing materials as antibacterials for dental
restoratives include incorporation of a methacryloy-
loxydodecyl pyridinium bromide (MDPB) as an anti-
bacterial monomer into resin composites [12], use of
methacryloxylethyl cetyl ammonium chloride (DMAE-
CB) as a component for antibacterial bonding agents
[16,17], and incorporation of quaternary ammonium poly-
ethyleneimine (PEI) nanoparticles into composite resins
[18,19]. All these studies found that QAS-containing ma-
terials did exhibit significant antibacterial activities. In
this study, we proposed to synthesize the novel QAS-
containing oligomers for developing antibacterial dental
resin composites.
The objective of this study was to synthesize new
quaternary ammonium salt (QAS)-containing oligomers,
incorporate them to dental resin composites, and evalu-

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 148
ate the effects of these new oligomers on the mechanical
strength and antibacterial activity of the formed compos-
ites.
2. MATERIALS AND METHODS
2.1. Materials
Bromoethane, bromohexane, bromododecane, bromo-
hexadecane, 2-dimethylaminoethanol (DMAE), 2-hdro-
xyethyl methacrylate (HEMA), 1,2,4,5-benzenetetra-
carboxylic dianhydride (BTCDA), 3,3’,4,4’-benzophe-
nonetetracarboxylic dianhydride (BPTCDA), 4,4’-(4,4’-
isopropylidenediphenoxy)-bis(phthalic anhydride) (IPD-
PBisPA), triethylene glycol dimethacrylate (TEGDMA),
bisphenol A glycerolate dimethacrylate (BisGMA), dl-
camphoroquinone (CQ), 2-(dimethylamino)ethyl metha-
crylate (DMAEMA), pyridine, N,N'-dicyclohexylcar-
bodiimide (DCC), N-methylpyrrolidone (NMP) and hex-
ane were used as received from VWR International Inc
(Bristol, CT) without further purifications. The untreated
glass fillers for Herculite XRV (0.7 microns) were used
as received from Sybron Dental Specialties (Newport
Beach, CA).
2.2. Synthesis and Characterization
2.2.1. Synthesis of the Polymerizable Oligomers
Bearing Quaternary Ammonium Bromide
(QAB)
The polymerizable oligomer bearing quaternary ammo-
nium bromide (QAB) was synthesized via three steps:
synthesis of the hydroxyl group-containing QAB, cou-
pling the QAB onto the oligomer, and introduction of the
methacrylate groups onto the oligomer. Briefly, 1) to a
flask containing DMAE (0.01 mol) in methanol, bro-
mododecane (0.013 mol) was added. The reaction was
run at room temperature overnight. After most of metha-
nol was removed, the mixture was washed with hexane 3
times. The formed 2-dimethyl-2-dodecyl-1-hydroxyethy-
lammonium bromide (or namely B12) was purified by
dissolving in methanol and washing with hexane several
times before drying in a vacuum oven; 2) to a flask con-
taining BPTCDA (0.01 mol) in NMP, B12 (0.013 mol)
was added in the presence of pyridine. After the reaction
was run at 60˚C for 4 h, the mixture was precipitated
from hexane, followed by washing with hexane 3 times;
3) the purified product BPDQAB (0.01 mol, an adduct
of BPTCDA and B12) in NMP was used to react with
HEMA (0.013 mol) in the presence of DCC (0.013 mol)
and pyridine (1.5% by weight of HEMA). After the reac-
tion was run at room temperature overnight, the mixture
was precipitated with hexane, followed by washing with
hexane several times. The purified oligomer BPDQAB-
DMA (an adduct of BPDQAB and HEMA) was then
dried in a vacuum oven at room temperature prior to use.
The other two oligomers, BDQABDMA (an adduct of
BDQAB and HEMA) and IPDPDQABDMA (an adduct
of IPDPDQAB and HEMA), were synthesized the same
as shown above. The structures of three starting dianhy-
drides, TEGDMA and BisGMA as well as the synthesis
scheme are shown in Figure 1.
2.2.2. Character i zation
The chemical structures of the synthesized oligomers
were characterized by Fourier transform-infrared (FT-IR)
spectroscopy and nuclear magnetic resonance (NMR)
spectroscopy. The proton NMR (1HNMR) spectra were
obtained on a 500 MHz Bruker NMR spectrometer
(Bruker Avance II, Bruker BioSpin Corporation, Biller-
ica, MA) using deuterated dimethyl sulfoxide and chlo-
roform as solvents and FT-IR spectra were obtained on a
FT-IR spectrometer (Mattson Research Series FT/IR 1000,
Madison, WI).
O
O
O
O
O
O
C
CH3
CH3
OO
O
O
O
O
O
O
C
OO
O
O
O
O
O
C
CH3
CH3
OOCH
2CHCH2OOCC
CH3
CH2 CCOOCH2CHCH2
CH3
CH2
OHOH
CCOOCH2CH2OCH2CH2OCH2CH2OOCC CH2 CH2
CH3 CH3
BTCDA BPTCDA
IPDP BisPA
TEGDMA
BisGMA
(a)
C
OO
O
O
O
O
O
Br
C
O
COOCH2CH2
COOH HOOC
CH2CH2OOC N
CH3
CH3
R
N
CH3
CH3
R
Br
Br
C
O
COOCH2CH2
COO OOC
CH2CH2OOC N
CH3
CH3
R
N
CH3
CH3
R
CH2CH2OOCC CH2
CH3
CCOOCH2CH2 CH2
CH3
Br
HO CH2CH2N(CH3)2
HOC H2CH2N(CH3)2R
Br
+
+
+
RBr
HEMA
where R = (CH2)nCH3 and n = 1, 5, 9 and 13
BPTCDA
BPDQAB
BPDQABDMA
(b)
Figure 1. Structures and synthesis scheme: (a) Structures of
BTCDA, BPTCDA, IPDPBisPA, TEGDMA and BisGMA; (b)
Synthesis scheme for preparation of the polymerizable quater-
nized oligomer BPDQABDMA.

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 149
2.3. Evaluation
2.3.1. Sample Preparation f or Mec hanical Stre n gth
Tests
The experimental resin composites were formulated with
a two-component system (liquid and powder) [20]. The
liquid was formulated with the newly synthesized oli-
gomer, BisGMA, TEGDMA, CQ and DMAEMA. The
synthesized oligomer, BisGMA and TEGDMA were
mixed in a ratio of oligomer/BisGMA/TEGDMA =
30/35/35 (oligomer = 30%) unless specified. CQ (1.0%
by weight) and DMAEMA (2.0%) were added for photo-
initiation. The untreated glass Herculite XRV (0.7 mi-
crons) powders were used as fillers and treated with γ-
(trimethoxysilyl)propyl methacrylate, following the pub-
lished protocol [21]. A filler level at 75% (by weight)
was used throughout the study.
Specimens were fabricated by thoroughly mixing the
liquid with the treated fillers at room temperature ac-
cording to the published protocol [20,21]. Briefly, the
cylindrical specimens were prepared in glass tubing with
dimensions of 4 mm in diameter by 8 mm in length for
compressive strength (CS), 4 mm in diameter by 2 mm
in length for diametral tensile strength (DTS), and 4 mm
in diameter by 2 mm in depth for antibacterial tests. The
rectangular specimens were prepared in a split Teflon
mold with dimensions of 3 mm in width by 3 mm in
thickness by 25 mm in length for flexural strength (FS)
test. All the specimens were exposed to blue light
(EXAKT 520 Blue Light Polymerization Unit, EXAKT
Technologies, Inc., Oklahoma City, OK) for 2 min, fol-
lowed by removing from the mold or conditioned in dis-
tilled water at 37˚C for 24 h prior to testing, unless
specified.
2.3.2. Strength Me a sur e m ents
CS, DTS and FS tests were performed on a screw-driven
mechanical tester (QTest QT/10, MTS Systems Corp.,
Eden Prairie, MN), with a crosshead speed of 1 mm/min.
The FS test was performed in three-point bending with a
span of 20 mm between supports. Six to eight specimens
were tested to obtain a mean value for each material or
formulation in each test. CS was calculated using an
equation of CS = P/r2, where P = the load at fracture
and r = the radius of the cylinder. DTS was determined
from the relationship DTS = 2P/dt, where P = the load
at fracture, d = the diameter of the cylinder, and t = the
thickness of the cylinder. FS was obtained using the ex-
pression FS = 3 Pl/2bd2, where P = the load at fracture, l
= the distance between the two supports, b = the breadth
of the specimen, and d = the depth of the specimen.
2.3.3. MIC Test for the Synthesized QAB
The minimal inhibitory concentration or MIC of the
synthesized QAB was determined following the pub-
lished protocol with a slight modification [22]. Briefly,
colonies of S. mutans (UA159) were suspended in 5 ml
of Tryptic soy Broth (TSB) prior to MIC testing. Two-
fold serial dilutions of the synthesized QAB were pre-
pared in TSB, followed by placing in 96-well flat-bottom
microtiter plates with a volume of 250 μl per well. The
final concentration of the QAB ranged from 1.563 to 2 ×
104 µg/ml. The microtiter plate was then inoculated with
S. mutans suspension (cell concentration = 5 × 105
CFU/ml) and incubated at 37˚C for 48 h prior to MIC
testing. The absorbance was measured at 595 nm via a
microplate reader (SpectraMax 190, Molecular Devices,
CA) to assess the cell growth. Chloehexidine (CHX) and
dimethylsulfoxide were used as positive and negative
controls, respectively. Triple replica was used to obtain a
mean value for each QAB.
2.3.4. Antibacterial Test for the Formed Cements
The antibacterial test was conducted following the pub-
lished procedures [23]. S. mutans was used for evalua-
tion of antibacterial activity of the studied cements.
Briefly, colonies of S. mutans (UA159) were suspended
in 5 ml of Tryptic soy Broth (TSB), supplemented with
1% sucrose, to make a suspension with 108 CFU/ml of S.
mutans, after 24 h incubation. Specimens pretreated with
ethanol (10 sec) were incubated with S. mutans in TSB
at 37˚C for 48 h under anaerobic condition with 5% CO2.
After equal volumes of the red and the green dyes
(LIVE/DEAD BacLight bacterial viability kit L7007,
Molecular Probes, Inc., Eugene, OR, USA) were com-
bined in a microfuge tube and mixed thoroughly for 1
min, 3 μl of the dye mixture was added to 1 ml of the
bacteria suspension, mixed by vortexing for 10 sec,
sonicating for 10 sec as well as vortexing for another 10
sec, and kept in dark for about 15 min, prior to analysis.
Then 20 μl of the stained bacterial suspension was ana-
lyzed using a fluorescent microscope (Nikon Microphot-
FXA, Melville, NY, USA). Triple replica was used to
obtain a mean value for each material.
2.3.5. Statistical Analysis
One-way analysis of variance (ANOVA) with the post
hoc Tukey-Kramer multiple-range test was used to de-
termine significant differences of mechanical strength
and antibacterial tests among the materials in each group.
A level of α = 0.05 was used for statistical significance.
3. RESULTS
3.1. Characterization
The characteristic peaks (cm-1) from the FT-IR spectra
shown in Figure 2 for DMAE, bromododecane, B12,
BPTCDA and BPDQAB (an adduct of BPTCDA and
B12) are listed in Table 1. The appearance of both peaks
at 3600-3200 and 1632 for = N+ = groups

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 150
-N(CH
3
)
2
-CH
3
-OH
-CH
2
-&-CH
3
-CH
2
-&-CH
3
-CH
2
-&-CH
3
-CO
3
C- -C=O
=N
+
=
=N
+
=
=N
+
=
-COOH -
C
=
O
a
b
c
d
e
4000 3500 3000 25002000 1500 1000 500
Wavenumber (c
m
-1
)
Figure 2. FT-IR spectra for DMAE, bromododecane, B12, BPTCDA and BPDQAB (an adduct of BPTCDA and
B12): (a) DMAE; (b) bromododecane; (c) B12; (d) BPTCDA and (e) BPDQAB.
Table 1. The characteristic peaks from the FT-IR spectra shown in Figure 2.
Material The characteristic
p
eaks
(
c
m
–1
)
DMAE
3399 (O-H stretching), 2944 (C-H stretching on -CH2-), 2861 (C-H stretching on -CH3), 2820 and 2779 (C-H
stretching on –N(CH3)2), 1459, 1364 and 1268 (C-H deformation on -CH2-), 1090 (O-H deformation), and 1040
as well as 776 (C-N deformation)
Bromo-
dodecane
2924 (C-H stretching on -CH2-), 2854 (C-H stretching on -CH3), 1465, 1377 and 1255 (C-H deformation on
-CH2-), and 722 as well as 647 (C-Br deformation)
B12 3600-3200 (= N+ = stretching), 2917 (C-H stretching on -CH2-), 2850 (C-H stretching on -CH3), 1632 (= N+ =
deformation), 1470 (C-H deformation on -CH2-), and 1090 as well as 730 (O-H deformation)
BPTCDA
2910 (=C-H stretching on phenyl groups), 1855 and 1782 (two -C=O stretching on five-membered ring acid
anhydride), 1717 (-C=O stretching on ketone), 1610 and 1570 (-C=C- stretching on phenyl groups), 1495 (-C=O
deformation vibration), 1231 (other vibrations on five-membered ring acid anhydrides), and 1388, 1135, 1069,
917, 715 and 625 (-C=C-and -=C-H stretching, out-of-plane and other vibrations on phenyl groups)
BPDQA
B
3320 (=N+= stretching), 3600-2600 (O-H stretching on -COOH), 2924 (C-H stretching on -CH2-), 2854 (C-H
stretching on -CH3), 1726 and 1669 (-C=O stretching on esters), 1587 (-C=O stretching on ketone), 1467 (-C=O
deformation vibration), and 1388, 1135, 1069, 917, 715 and 625 (-C=C-and -=C-H stretching, out-of-plane and
other vibrations on phenyl groups)
and disappearance of the peaks at 1040 and 776 for C-N
groups confirmed the formation of B12. The disappear-
ance of the peaks at 1855 and 1782 for anhydrides as
well as appearance of a broad peak at 3600 - 2600 for
-COOH, a relatively sharp peak at 3320 for = N+ = and
strong peaks at 2924 and 2854 for -CH3 and -CH2 groups
confirmed the formation of BPDQAB.
The characteristic peaks (cm-1) from the FT-IR spectra
shown in Figure 3 for HEMA, BPDQAB, BPDQAB-
DMA, BDQABDMA and IPDPDQABDMA are listed in
Table 2. The disappearance of the peak at 3428 (-OH
from HEMA) as well as appearance of the peaks at

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 151
-OH
-CH
2
-&-CH
3
-CH
2
-&-CH
3
-C=C-
=N
+
=
-COOH
-C=O
-CH
2
-&-CH
3
-C=C-
=N
+
= -C=O
a
b
c
d
e
4000 3500 3000 2500 20001500 1000 500
Wavenumber (cm
-1
)
Figure 3. FT-IR spectra for HEMA, BPDQAB, BPDQABDMA (an adduct of BPDQAB and HEMA), BDQAB-
DMA (an adduct of BDQAB and HEMA) and IPDPDQABDMA (an adduct of IPDPDQAB and HEMA): (a)
HEMA; (b) BPDQAB; (c) BPDQABDMA; (d) BDQABDMA and (e) IPDPDQABDMA.
Table 2. The characteristic peaks from the FT-IR spectra shown in Figure 3.
Material The characteristic peaks (cm-1)
HEMA 3428 (O-H stretching), 2957 (C-H stretching on -CH2-), 2889 (C-H stretching on -CH3), 1719 (-C=O
stretching on ester), and 1637 (-C=C stretching)
BPDQAB
3320 (=N+= stretching), 3600-2600 (O-H stretching on –COOH), 2924 (C-H stretching on -CH2-),
2854 (C-H stretching on -CH3), 1726 and 1669 (-C=O stretching on esters), 1587 (-C=O stretching on
ketone), 1467 (-C=O deformation vibration), and 1388, 1135, 1069, 917, 715 and 625 (-C=C-and
-=C-H stretching, out-of-plane and other vibrations on phenyl groups)
BPDQABDMA
3328 (=N+= stretching), 2924 (C-H stretching on -CH2-), 2854 (C-H stretching on -CH3), 1726 (-C=O
stretching on esters), 1647 (C=C stretching on methacrylates), 1587 (-C=O stretching on ketone), 1467
(-C=O deformation vibration), and 1388, 1135, 1069, 917, 715 and 625 (-C=C-and -=C-H stretching,
out-of-plane and other vibrations on phenyl groups)
BDQABDMA Similar to BPDQABDMA
IPDPDQABDMA Similar to both BPDQABDMA and BDQABDMA.
3600-3200 for = N+ =, 2923 and 2853 for –CH2- and
-CH3 and 1647 for C = C groups confirmed the formation
of three polymerizable quaternized oligomers.
The characteristic chemical shifts (ppm) from the
1HNMR spectra shown in Figure 4 for DMEA, bromo-
dodecane, B12, BPTCDA and BPDQABDMA are listed
in Ta b le 3. The appearance of all the new peaks in the
spectrum, especially at 5.82 and 6.25 for carbon-carbon
double bond and 7.82-8.40 for phenyl groups confirmed
the successful attachment of HEMA and B12 onto the
BPTCDA.
3.2. Evaluation
Table 4 shows the code, description and MIC of the

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 152
Figure 4. 1HNMR spectra for DMAE, bromododecane, B12, BPTCDA and BPDQABDMA: (a) DMAE;
(b) bromododecane; (c) B12; (d) BPTCDA and (e) BPDQABDMA.

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 153
Table 3. The characteristic chemical shifts from the 1HNMR spectra shown in Figure 4.
Material The characteristic chemical shifts (ppm)
DMEA 4.40 (-OH), 3.42 (-CH2OH), 2.30 (-CH2N-) and 2.10 (H3CN-)
Bromododecane 3.51 (-CH2Br), 1.80 (-CH2CH2Br), 1.38 (-CH2-, all) and 0.89 (-CH3)
B12 5.25 (-OH), 3.82 (-CH2OH), 3.40 (-CH2N(CH3)2), 3.08 (H3CN-), 1.55
(-CH2CH2N(CH3)2), 1.25 (-CH2- all) and 0.89 (-CH3)
BPTCDA 7.85-8.40 (-H, all from the phenyl groups) and 2.50 (TMS)
BPDQABDMA 7.85-8.40 (-H, all from the phenyl groups), 5.82 and 6.25 (=C-, from methacrylates) and all the other chemical shifts
similar to those shown on B12
Table 4. Codes, description, MIC values of the synthesized QAB.
Code
Q
AB1Chain len
g
th MIC
(μg
/ml
)
2
B2 2-Dimethyl-2-ethyl-1-hydroxyethylammonium bromide 2 20,000
B6 2-Dimethyl-2-hexyl-1-hydroxyethylammonium bromide 6 1,000
B12 2-Dimethyl-2-dodecyl-1-hydroxyethylammonium bromide 12 25
B16 2-Dimethyl-2-hexadecyl-1-hydroxyethylammonium bromide 16 1.563
synthesized QAB. The MIC values ranged from 1.563 to
2 × 104 µg/ml for B16 to B2.
Figure 5 shows the effect of the substitute chain
length on the synthesized oligomers on CS and S. mu-
tans viability of the experimental resin composite. The
mean CS value (MPa) was in the decreasing order of B2
> B6 > B12 > B16, where there were no statistically
significant differences between B2 and B6, between B6
and B12, and between B12 and B16 (p > 0.05). Increas-
ing the substitute chain length on the oligomer decreased
the CS values of the resin composite. The mean S. mu-
tans viability was in the decreasing order of B2 > B6 >
B12 > B16, where all the resin composites were signify-
cantly different from each other (p < 0.05).
Figure 6 shows the effect of different oligomers on
CS and S. mutans viability of the resin composite. The
mean CS value (MPa) of the dry resin composite was in
the decreasing order of A > B > C > D, where there were
no statistically significant differences among B, C and D
(p > 0.05). The mean CS value (MPa) of the wet resin
composite (the composite after conditioning in distilled
water for 24 h) was in the decreasing order of A > D > C
> B, where there were no statistically significant differ-
ences among B, C and D (p > 0.05). The mean S. mutans
viability was in the decreasing order of A > D > C > B,
where there were no statistically significant differences
between B and C and between C and D (p > 0.05).
Figure 7 shows the effect of the oligomer loading on
CS and S. mutans viability. Both mean CS value (MPa)
and S. mutans viability were in the decreasing order
of10% > 20% > 30% > 50% > 70%, where all the resin
composites were significantly different from each other
in either category (p < 0.05).
Figure 8 shows the effect of aging of both unmodified
and QAB-modified resin composites on CS and S. mu-
tans viability. The mean CS value (MPa) was in the de-
creasing order: (A) Unmodified composite: 1 d > 7 d >
30 d, where there were no statistically significant differ-
ences between 1 d and 7 d (p > 0.05); (B) QAB-modified
composite: 1 d > 7 d > 30 d, where all were significantly
different from each other (p < 0.05). The mean S. mutans
viability values were statistically the same within 30
days for either unmodified or QAB-modified composite
(p < 0.05).
Table 5 shows the property comparison of the un-
modified and modified resin composites. These proper-
ties include yield strength (YS), compressive modulus
(M), CS, diametral tensile strength (DTS), flexural
strength (FS) and antibacterial activity.
4. DISCUSSION
Currently there is a growing interest in preventing or
reducing biofilm formation in many biomedical areas. In
preventive restorative dentistry, secondary caries is a
critical issue and prevention of secondary caries plays a
key role in long-lasting restorations [1-4]. Secondary
caries is found to be the main reason to the restoration

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 154
0
50
100
150
200
250
300
350
B2B6B12 B16
CS (MPa)
0
10
20
30
40
50
60
70
80
90
100
S. mutans viability (%)
Viability CS
Figure 5. Effect of the substitute chain length on the synthe-
sized QAB on CS and S. mutans viability of the resin com-
piosite: B2, B6, B12 and B16 represent the substitute chain
length on the synthesized QAB (see codes and description in
Table 1). The composite was composed of BPDQAB-
DMA/BisGMA/TEGDMA at a ratio of 20:40:40 (by weight or
BPDQABDMA = 20%). Specimens were tested directly for CS
and incubated with S. mutans for 48 h for antibacterial testing.
0
50
100
150
200
250
300
350
400
450
ABCD
CS (MPa)
0
10
20
30
40
50
60
70
80
90
100
S. mutans viability (%)
CS1 CS2 Viability
Figure 6. Comparison among the resin composites having
different QAB-containing oligomers via CS and S. mutans
viability testing: A, B, C and D stand for the resin composites
composed of BisGMA/TEGDMA = 50/50 (by weight), BDQA-
BDMA/BisGMA/TEGDMA = 30/35/35, BPDQABDMA/Bis-
GMA/TEGDMA = 30/35/35 and IPDPDQABDMA/BisGMA/
TEGDMA = 30/35/35, respectively. CS1 and CS2 represent the
CS for day and wet resin composites. QAB = B12. Specimens
were tested directly for CS and incubated with S. mutans for
48 h for antibacterial testing.
0
50
100
150
200
250
300
350
10% 20% 30%50% 70%
CS (MPa)
0
10
20
30
40
50
60
70
80
90
100
S. mutans viabiliy (%)
CS Viability
Figure 7. Effect of the QAB loading on CS and S. mutans
viability: BPDQABDMA = 10, 20, 30, 50 and 70%, where
BisGMA/TEGDMA = 50/50. QAB = B12. Specimens were
tested directly for CS and incubated with S. mutans for 48 h for
antibacterial testing.
0
50
100
150
200
250
300
350
400
1 d7 d30 d
CS (MPa)
0
10
20
30
40
50
60
70
80
90
100
S. mutans viability
CS (RC)CS (QAS-RC)
Viability (RC)Viability (QAS-RC)
Figure 8. Effect of aging on CS and S. mutans viability:
BPDQABDMA = 30%; BisGMA/TEGDMA = 50/50; QAB =
B12. The specimens were conditioned in distilled water for 1
day, 7 days and 30 days, followed by direct testing for CS and
incubating with S. mutans for 48 h for antibacterial testing.
failure of dental restoratives [1-4]. Secondary caries that
often occurs at the interface between the restoration and
the cavity preparation is mainly caused by demineraliza-
tion of tooth structure due to invasion of plaque bacteria
(acid-producing bacteria) such as S. mutans in the pres-
ence of fermentable carbohydrates [4]. Therefore, pre-
venting these bacteria from invasion to natural tooth is

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 155
Table 5. Comparison of properties of the unmodified and modified resin composites.
Material1 YS2 [MPa] M3 [GPa] UCS4 [MPa] DTS5 [MPa] FS Viability (%)
RC 155.8 (11)a, 6 7.16 (0.33)b 365.5 (15)c 63.7 (1.6)d 114.6 (8.7)
RC (24h) 153.7 (6.2)a 7.09 (0.15)b 359.5 (12)c 64.7 (2.7)d 112.8 (10)
86.8 (2.2)
QAB-RC 125.1 (5.2) 6.19 (0.06) 240.9 (9.2) 45.5 (2.8) 83.5 (5.6)
QAB-RC (24h) 69.1 (3.6) 3.82 (0.07) 204.4 (14) 34.4 (4.7) 70.6 (8.5)c 54.3 (1.5)
1RC and QAB-RC stand for the day specimens of unmodified and QAB-modified resin composites, whereas RC (24h) and QAB-RC (24h) represent the wet
specimens after conditioning in distilled water at 37 oC for 24 h; 2YS = CS at yield; 3M = compressive modulus; 4UCS = ultimate CS; 5DTS = diametral tensile
strength; 6Entries are mean values with standard deviations in parentheses and the mean values with the same superscript letter were not significantly different
(p > 0.05). Specimens for bacterial viability test were directly tested after incubating with S. mutans for 48 h.
the key to long-lasting dental restorations when the mi-
croleakage or materials failure occurs at the interface.
Quaternary ammonium salts (QAS) and their constructed
materials represent a new trend of antimicrobial agents
in biomedical applications [10,13]. QAS can be incur-
porated in many ways, including mixing with fillers,
copolymerizing with other monomers and grafting onto
the polymer skeletons [10-14]. The advantage of using
QAS is that they can kill the microorganisms by simple
contact. The mechanism of QAS to kill bacteria is be-
lieved to disrupt the surface membrane of bacteria by
changing membrane permeability or surface electrostatic
balance [11,18]. In this regard, we purposely synthesized
the new QAB-containing oligomers, incorporated them
into the resin composite and evaluated the CS and anti-
bacterial activity of the formed composite.
It has been noticed that chain length on QAS has a
significant effect on its antibacterial activity [11,14].
Generally speaking, there are four main processes for
QAS to kill bacteria and they are 1) adsorption onto the
negatively charged bacterial cell surface; 2) penetrating
through the cell wall; 3) binding to the cytoplasmic
membrane; and 4) disrupting the cytoplasmic membrane
[14]. It has also been found that both positive charge
density and substitute chain length are the key to the
biocidal ability, because the high positive charge density
may enhance the driving force and the long substitute
chain may strongly interact with the cytoplasmic mem-
branes [14]. From Tab le 1, it is apparent that increasing
the substitute chain length significantly increased the
biocidal activity of the synthesized QAB. The QAB with
16-carbon substitute chain (B16) was the highest in MIC
whereas the one with 2-carbon chain (B2) was the low-
est. In fact, the trend for the biocidal activity of the QAB
in this study was similar to those described elsewhere
[11,14], i.e., the longer the substitute chain, the higher
the biocidal activity. The same trend was also observed
for the resin composites having the QAB-containing
oligomers with different chain length. As shown in Fig-
ure 5, increasing the substitute chain length significantly
decreased the S. mutans viability. However, the CS value
was also decreased. The decrease in CS can be attributed
to the fact that simply introducing the hydrocarbon CH2
unit that does not contain any strong primary bonds such
as C = C bond or secondary bonds such as dipole-dipole
or hydrogen bond could reduce mechanical strengths
[24].
From the results in Figure 6, it is evident that intro-
duction of the QAB significantly increased the antibac-
terial activity (or decreased the S. mutans viability) of
the resin composite. As compared to the unmodified
composite A, the QAB-modified B, C and D signify-
cantly killed the S. mutans from 31 to 42%. Meanwhile,
their CS values were also significantly decreased with a
reduction of 34-36% for dry composites and 35-47% for
wet composites. The significant decrease in strength for
the dry composite can be attributed to the introduction of
the QAB. The QAB synthesized in this study is nothing
but a quaternized salt with a long-chain hydrocarbon
attached, which does not contribute any strength en-
hancement but rather reduces the amount of C = C in-
stead [24]. That is why a significant decrease in CS has
been observed. Regarding the dry and wet composites,
the unmodified composite A behaved very differently
from the QAB-modified B, C and D. No change in CS
was found for the composite A after 24 h in water. On
the other hand, statistically significant differences were
found between the dry and wet composites for either B,
or C or D. This significant decrease in CS can be attrib-
uted to the hydrophilic nature of the QAB-modified
composite. The QAB by nature is a quaternary ammo-
nium salt (QAS) bearing both positive and negative
charges, which absorb water [25]. Since water serves as
a plasticizer in the material [26], the QAS-containing
material behaves like a hygrogel more or less [27]. No
wonder the QAB-modified composites in this study
showed decreased CS values after conditioning in water.
Furthermore, the wet composite B seems to show more
decrease in CS than either wet C or wet D, which may
be attributed to the fact that B contains more QAB in

Y. M. Weng et al. / J. Biomedical Science and Engineering 4 (2011) 147-157 156
one mole due to its lower molecular weight.
The effect of the oligomer loading on CS and antibac-
terial activity is shown in Figure 7. Apparently, the more
the QAB-containing oligomer added the lower the CS
value and the higher the antibacterial activity. With the
oligomer increasing from 10 to 70%, the CS value and S.
mutans viability were decreased from 17 to 78% and 16
to 78%, respectively. To keep the CS value close to 250
MPa and S. mutans viability close 50%, we chose the
formulation with 30% of the QAB-containing oligomer
to study the aging of the modified composite. We tested
the CS and S. mutans viability of both unmodified and
modified composites after conditioning in distilled water
for 1 day, 7 days and 30 days. As shown in Figure 8,
there was nearly no change in S. mutans viability for
either unmodified or modified composites, suggesting
that there might be no leachable from the modified
composite. On the other hand, however, a dramatic de-
crease in CS (MPa) was observed for the modified com-
posite with the results of 241 for 1 day, 183 for 7 days
and 155 for 30 days. In contrast, statistically significant
difference was found only between 1 day (335 MPa) and
30 days (302 MPa) for the unmodified composite. It is
known that dental resin composites show a certain degree
of degradation due to water sorption caused by two hy-
droxyl groups pendent on BisGMA and three -CH2CH2O-
units on TEGDMA (see structures in Figure 1(a) [28].
The absorbed water can hydrolyze the silane bond that is
used to couple resin with fillers, de-bond the resin-filler
interface and thus reduce the mechanical strengths with
time [28]. That may be why the unmodified composite
showed a decrease in CS after conditioning in water for
30 days. Regarding the QAB- modified composite, the
significant decrease in CS should be attributed to the
hydrophilic nature of the QAB incorporated. As compared
to two hydroxyl groups on BisGMA, two QAB groups
attached to the newly synthesized oligomer would ab-
sorb water even more aggressively because of the ionic
charges they carry [27,28]. These ionic charges can ac-
celerate the interfacial de-bonding. That may be why a
dramatic reduction in CS was observed. Unlike those
QAS-modified dental glass-ionomer cements [29], the
above negative effect to dental resin composites should
be cautiously weighed while the positive effect of QAS
is beneficial in reducing bacteria. In our previous work
related to glass-ionomer cements, we found that QAS
did not degrade the cement during the 30-day aging al-
though it reduced the initial strength as well [29].
Finally we compared YS, M, CS, DTS, FS and anti-
bacterial activity between unmodified and modified
composites. The QAB-modified composite was 20 and
55% in YS, 14 and 46% in modulus, 34 and 43% in CS,
29 and 46% in DTS and 27 and 37% in FS lower than
the unmodified composite, respectively, in dry and wet
states. On the other hand, however, the QAB-modified
composite was much higher (37% higher) in antibacte-
rial activity than the unmodified composite.
5. CONCLUSIONS
We have synthesized several novel QAB-containing oli-
gomers and used them for formulation of antibacterial
resin composites. All the QAB-modified composites
showed significant antibacterial activity and mechanical
strength reduction. It was found that increasing chain
length and loading significantly enhanced the antibacte-
rial activity but also dramatically reduced the CS. The
30-day aging study showed that the incorporation of the
QAB accelerated the degradation of the composite, sug-
gesting that the QAB may not be well suitable for de-
velopment of antibacterial dental resin composites or at
least the QAB loading should be well controlled, unlike
its use in dental glass-ionomer cements. The authors
believe that the work in this study is beneficial and
valuable to those who are interested in studying antibac-
terial dental resin composites.
6. ACKNOWLEDGEMENTS
This work was sponsored by NIH challenge grant (RC1) DE020614.
Ms. Cunge Zheng was acknowledged for her assistance in MIC testing.
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