Materials Sciences and Applicatio n, 2011, 2, 1070-1075
doi:10.4236/msa.2011.28144 Published Online August 2011 (http://www.SciRP.org/journal/msa)
Effects of Methacrylic Acid on
Physical/Mechanical Properties and
Biocompatibility of Urethane-Based Denture
Biomaterials
Zhengbing Cao1, Xinbo Sun1, Chih-Ko Yeh2, Yuyu Sun1*
1Biomedical Engineering Program, University of South Dakota, Sioux Falls, USA; 2 Department of Comprehensive Dentistry, Uni-
versity of Texas Health Science Center at San Antonio and Geriatric Research, Education and Clinical Center, Audie L. Murphy
Division, South Texas Veterans Health Care System, San Antonio, USA.
Received May 3rd, 2011; revised June 8th, 2011; accepted June 15th, 2011.
ABSTRACT
Candida-associated denture stomatitis (CADS) is a significant clinical concern. We have demonstrated that ure-
thane-based denture biomaterials with 10% methacrylic acid (MAA) could bind and then slowly relea se antifu nga l drug
for months. Drugs on the resins could be repeatedly quenched/recharged, and in subsequent recharging, they could be
changed/switched to more potent/effective ones. However, the physical/mechanical properties and biocompatibility of
the new MAA-based resins are currently unknown. The objective of the current study is to evaluate the effects of co-
polymerization with MAA on physical/mech anical properties and biocompatibility o f urethane-based dentu re resin ma-
terials. MAA and diurethane dimethacrylate (UDMA) were copolymerized using initiator a zobisisobu tyro nitrile (AIBN).
Water sorption and solubility were assessed with the specifications of ISO (International Standards Organization) test
method 1567, flexural strength and modulus were measured according to ASTM D-790, and biocompatibility was pre-
liminarily evaluated in cytotoxicity assay using mouse 3T3 fibroblast cells with the trypan blue method. The results
demonstrated that copolymerization of UDMA with up to 10% MAA did not negatively affect water sorption/solubility,
flexural strength/modulus, and biocompatibility. With 20% MAA, however, the mechanical properties of the resulting
resins were significantly decreased. To sum up, UDMA-MAA copolymers with up to 10% MAA had adequate physi-
cal/mechanical properties for dentu re materials with no side effects on cell viability. The UDMA-MAA denture bioma-
terials have a good potential to be used clinically for managing CADS and other related infectious cond itions.
Keywords: Antifungal, Denture Biomaterials, Physical Properties, Biocompatibility, Cytotoxicity
1. Introduction
Dentures are invaluable to the nutritional intake, speech,
appearance, and quality of life of partial or full edentu-
lous patients [1,2]. Unfortunately, because of the coloni-
zation and biofilm formation of Candida species on den-
ture surfaces [3-5], the use of these prostheses often leads
to Candida-associated denture stomatitis (CADS), a
non-specific inflammatory reaction to microbial antigens,
toxins and enzymes produced by the colonizing micro-
organisms. CADS is a common, recurring disease that
affects up to 67% of denture wearers [4-7], and can lead
to other oral health problems such as caries and perio-
dontal diseases, gastrointestinal and pleuropulmonary
infections, compromised quality of life, and even death
[8-10]. Management of CADS includes denture clean-
ing/disinfection, appropriate denture wearing hab-
its/hygiene, use of tissue conditioners/liners, and topi-
cal/systemic antifungal therapy [11,12]. However, none
of these can completely prevent or eliminate Candida
colonization and biofilm formation, and the reinfection
rate is high, particularly in the elderly and those who
are immunocompromised or medically compromised
[13-15].
An alternative approach is to impregnate denture bio-
materials with antifungal drugs that elute from the device
and impair microbial growth [11-13,16,17]. A high anti-
fungal concentration can be achieved (at least initially) in

Effects of Methacrylic Acid on Physical/Mechanical Properties and Biocompatibility of Urethane-Based 1071
Denture Biomaterials
the near vicinity of the denture surface, generally ex-
ceeding the minimum inhibition concentration (MIC) and
minimum fungicidal concentration (MFC) required for
susceptible species. However, there are still no antifungal
denture materials that are effective for long-term (months
to years) use. A primary reason is that the impregnating
approaches cannot incorporate enough antifungal agents
into dentures to maintain the MIC/MFC near denture
surfaces for extended use. Further, the releasing patterns
of the impregnated antifungal agents are not optimized:
regardless of whether active infection is present, the
dentures have a high antifungal release initially, followed
by an exponential decrease in the antifungal agents re-
leased. After a short period of time (days to weeks), the
antifungal agents released do not reach the critical con-
centrations, and inhibitory effects are lost.
We have developed a rechargeable, “click-on/click
-off” technology to extend antifungal duration and con-
trol drug release behaviors by copolymerizing methacry-
lic acid (MAA) with denture resin monomer diurethane
dimethacrylate (UDMA). The anionic MAA moieties in
the UDMA denture materials acted as a “rechargeable
battery” to bind and then slowly release cationic antifun-
gal drugs such as miconazole and chlorhexidine diglu-
conate for weeks to months. The drug-containing denture
materials could be “quenched” by treating them with
EDTA and recharged with the same or other more po-
tent/effective antifungal drugs to enhance antifungal po-
tency and/or minimize the risk of microbial resistance.
MAA-based acrylic denture resins have been investi-
gated to potentially reduce Candida adhesion [16], but no
information is available about MAA-containing ure-
thane-based denture biomaterials. We are interested in
this system because UDMA-based urethane resins are
rapidly gaining popularity as denture base biomaterials.
Moreover, UDMA contains two acrylate double bonds,
which are expected to allow a high-level of crosslinking
of UDMA itself with the acrylate structure of MAA into
the denture resins so as to “neutralize” the potential nega-
tive effects of MAA on physical/mechanical properties
(see Figure 1). The detailed drug binding/drug releasing
kinetic studies and antifungal activities of the experi-
mental resins have been reported elsewhere [18,19]. The
objective of the current study is to evaluate the effects of
copolymerization of UDMA with MAA on physi-
cal/mechanical properties and biocompatibility (cytotox-
icity) of the urethane-based denture biomaterials.
2. Experimental
All the chemicals were purchased from Sigma-Aldrich.
Azobisisobutyronitrile (AIBN) was purified by recrystal-
lization from methanol. Bald/c mouse 3T3 fibroblast
cells were obtained from American Type Culture Collec-
tion (ATCC).
Fabrication of UDMA and UDMA-MAA denture
biomaterials: Disc-shaped (13 mm in diameter and 1
mm in thickness) and bar-shaped (65 × 10 × 4 mm)
UDMA-MAA denture resins were prepared by free radi-
cal copolymerization of MAA with UDMA in aluminum
molds. MAA weight percentage in the monomer mixture
varied at 0% (100% UDMA controls), 5%, 10%, and
20%, and the weight percentage of AIBN was kept at 1%
of the monomer mixture (MAA plus UDMA). Polymeri-
zation was carried out in a laboratory heat-curing unit at
70˚C for 3 h under N2 gas protection. The specimens
were ejected from the mold and visually examined to
ensure that they were free of voids.
Water sorption and solubility: The effects of co-
polymerization of UDMA with MAA on water sorption
and solubility of the resulting resins were tested follow-
ing the specifications of ISO (International Standards
Organization) test method 1567. For each MAA content
(0%, 5%, 10%, and 20%), disc specimens were stored in
a desiccator at 37˚C ± 1˚C for 23 h. The specimens were
then transferred to a second desiccator at 23˚C ± 2˚C for
1 h and weighed [20]. The cycles of desiccation (i.e.,
37˚C ± 1˚C, 23 h and 23˚C ± 2˚C, 1 h) were repeated
until the weight reached a constant mass (m1).The weight
(m1) of the dried specimen was determined using an
electronic scale. Thickness and diameter of the speci-
mens were measured using a digital caliper, rounded to
the nearest 0.01 mm, and these measurements were used
to calculate the volume (mm3) of each specimen. The
dried specimens were immersed in distilled water at 37˚C
± 1˚C for 7 days, and then dried with a clean towel and
weighed again (m2). The difference in weights m2 and m1
divided by the volume of the specimen was defined as
the amount of water absorbed (µg/mm3) [20,21]. Fol-
lowing the weighing for sorption, the specimens were
dried to a constant mass (m3) using the protocol previ-
ously described for m1 determination. The value m3 was
subtracted from m1, and divided by the volume of the
specimen. The value obtained represented the solubility
of the specimens (µg/mm3). Data was analyzed with
1-way ANOVA followed by post-hoc Bonferroni tests.
Mechanical properties of the denture biomaterials:
Bar-shaped denture biomaterials were immersed in dis-
tilled water at 37˚C for 60 days before evaluation for
mechanical properties. The flexural strength and modulus
of the specimens were measured according to ASTM
D-790 with a mechanical testing system (MTS 370, MTS
Systems Corp., MN, USA). The distance between the
specimen supports was set at 50 mm. The loading force
was applied to the specimen at a cross-head speed of 5

Effects of Methacrylic Acid on Physical/Mechanical Properties and Biocompatibility of Urethane-Based
Denture Biomaterials
1072
mm/min until the specimen fractured [22-24]. Data was
analyzed with 1-way ANOVA followed by post-hoc
Bonferroni tests.
Effects of the denture biomaterials on mammal cell
viability: Biocompatibility of the new denture biomate-
rials was preliminarily evaluated with the cytotoxicity
assay using the trypan blue dye exclusion method [25].
The disc samples were sterilized with UV and placed into
a 96-well plate. The bald/c mouse 3T3 fibroblast cells
were cultured in DMEM/high glucose medium supple-
mented with 10% fetal bovine serum at 37˚C in a hu-
midified air atmosphere with 5% CO2. Cells (1.25 × 104)
were seeded in each well containing a disc of the denture
materials in 200 µL of the medium. Disc-free wells were
also seeded with the cells to serve as controls. After cul-
turing for one and three days, the cells on the plate and
denture materials were trypsinized and exposed for 5 min
to 0.2% trypan blue solution (diluted from 0.4% solution;
Sigma-Aldrich). The numbers of stain-positive (dead and
dying cells) and stain-negative cells in each culture were
counted in a hemocytometer chamber. The data were
analyzed with Student’s t-test for statistical significance
[25].
3. Results and Discussion
Antifungal denture biomaterials have been reported in a
number of studies [11-13,16,17]. However, the antifun-
gal action of those dentures is short-lived (days to weeks),
and there are still no antifungal denture materials that can
provide long-term (months to years) infection-responsive
protection against CADS. We have developed a re-
chargeable, “click-on/click-off” technology to extend
antifungal duration and control drug release behaviors by
copolymerizing MAA with UDMA [18,19]. In this study,
we investigated the effects of copolymerization with
MAA on water sorption/solubility, mechanical properties
and biocompatibility (cytotoxicity) of the UDMA-base
new denture biomaterials.
3.1. Water Sorption and Solubility
UDMA resins are hydrophobic polymers with low water
sorption and solubility, which are desirable physical
properties for denture biomaterials. On the other hand,
MAA is a hydrophilic monomer with a high polarity and
high tendency of water sorption. However, in our study,
copolymerizing UDMA with up to 20% MAA did not
have significant effects on water sorption and solubility
(Table 1). Without MAA, the pure UDMA resin control
showed a water sorption value of 28.95 ± 3.25 µg/mm3,
and a water solubility value of 22.83 ± 2.81 µg/mm3 (n =
5). These physical parameters were not significantly af-
fected by copolymerization with 5% - 20% of MAA into
UDMA. These results could be attributable to the high
crosslinking capability of UDMA that contains two
acrylate groups. The small amount of MAA in the cured
UMDA has no profound interruption of the highly
crosslinked network (Figure 1). This crosslinked net-
work could restrict the access of water molecules and
reduce the extent of swelling so as to “neutralize” the
potential increase effect in water sorption/solubility
caused by MAA. According to the ISO test method 1567
(Dentistry-Denture base polymers), water sorption of the
denture base materials should not exceed 32 mg/mm3,
and the soluble substances eluted during storage in water
should not exceed 1.6 mg/mm3. The water absorption
and solubility values of the control denture biomaterials
(100% of UDMA) and the experimental denture bioma-
terials (UDMA-MAA copolymers; MAA content: 5% -
20%) were much lower than these upper limits (Table 1).
3.2. Mechanical Properties
Flexural strength and modulus are among the most im-
portant mechanical properties of denture biomaterials.
The effects of copolymerization with MAA on flexural
strength and modulus of the new denture materials are
shown in Table 2. Under our experimental conditions,
the UDMA resin control had a flexural strength of 112.4
± 2.67 Mpa, and a flexural modulus of 2.43 ± 0.16 Gpa.
No significant effects on flexural strength and modulus
were observed with up to 10% MAA in the new denture
biomaterials. However, the copolymer of 20% MAA
with UDMA was too brittle/weak for practical use and
was not tested. The American Dental Association speci-
fication No. 12 sets the minimum values of flexural
strength and flexural modulus for denture base materials
at 65 MPa and 2 GPa, respectively. Both the control
biomaterial (100% of UDMA) and the experimental
biomaterials with up to 10% MAA meet these require-
ments.
3.3. Cell Viability
Biocompatibility of the new denture biomaterials was
preliminarily assessed with cytotoxicity assay (Table 3).
U-U-U-U-U
U-U-U-U-U
U-U-U-U-U
UU
UU
With only UDMA (U )
U-A-U-U-U
U-U-A-U-U
A-U-U-U-U
UU
UU
With UDMA (U ) and
a small amout of MAA (A)
Figure 1. Crosslinked networks in the denture biomaterials

Effects of Methacrylic Acid on Physical/Mechanical Properties and Biocompatibility of Urethane-Based 1073
Denture Biomaterials
Table 1. Water sorption (Wsp) and water solubility (Wsl) of the UDMA-MAA copolymers (n = 5).
MAA content in the copolymer Wsp ± SD (μg/mm3) Wsl ± SD (μg/mm3)
0% 28.95 ± 3.25 22.83 ± 2.81
5% 29.15 ± 2.52 23.03 ± 2.42
10% 29.24 ± 4.65 23.81 ± 2.31
20% 30.43 ± 2.27 24.58 ± 3.35
Table 2. Flexural strength and modulus of the UDMA-MAA copolymers (n=5)*.
MAA content in the copolymers Flexural Strength (Mpa) (Means ± SD) Flexural Modulus (Gpa)
(Means ± SD)
0 % 112.4 ± 2.67 2.43 ± 0.16
5% 113.1± 3.23 2.41± 0.19
10 % 109.1 ± 3.07 2.39 ± 0.21
*: Copolymers with 20% MAA were too weak that could be easily broken by hands, which were removed from mechanical testing.
Table 3. Cell viability with bald/c mouse 3T3 fibroblast cells in the trypan blue assay.
Samples % of undamaged cells after 1 Day
(Means ± SD)
% of undamaged cells after 3 Days
(Means ± SD)
Cell-only control 98.2 ± 7.7 94.8 ± 6.4
Pure UDMA 90.1 ± 5.8 92.1 ± 5.3
UDMA with 5% MAA 87.3 ± 8.7 89.4 ± 5.9
UDMA with 10% MAA 89.2 ± 6.2 90.9 ± 2.0
UDMA with 20% MAA 86.7 ± 8.2 82.4 ± 8.4
As demonstrated by the trypan blue assay, the viability of
bald/c mouse 3T3 fibroblast cells was not significantly
affected by the presence of 5% or 10% MAA even after 3
days of continuous contact. Of all the cells exposed to the
MAA-based resins, only a few had trypan blue-stained
nuclei (indicating cell death), and when viewed by
phase-contrast microscopy, the stained cells were of the
same size and shape as the unstained cells (images not
shown). Cultures exposed to UDMA discs with 20%
MAA had slightly higher percentage of damaged cells,
but the differences were not statistically significant.
These findings are not surprising because the UDMA-based
biomaterials are currently used as denture base materials.
The MAA-based polymers are the major components of
dental glass-ionomer cements [26-29] and have been
successfully used as biocompatible and bioadhesive car-
riers for controlled release of drugs, peptides, and pro-
teins [30-32]. Furthermore, MAA is covalently bound
onto the denture biomaterials so that it does not diffuse
away from the denture materials. All these factors may
contribute to the very low cytotoxic effect of the newly
formulated denture biomaterials.
4. Conclusions
In conclusion, this study showed that copolymerization
with up to 10% MAA has no detrimental effect on the
physical/mechanical properties and biocompatibility
(cytotoxicity) of the UDMA-based new denture biomate-
rials. Our previous studies have demonstrated that
UDMA biomaterials with 10% MAA could provide sus-
tained antifungal drug release for a long period of time
(weeks to months), drugs in the biomaterials could be
repeatedly quenched/recharged, and in subsequent re-
charging, drugs could be changed/switched to more po-
tent/effective ones [18,19]. Thus, the antifungal medica-
tion on the UDMA-MAA denture materials could be
added or removed (“click-on/click-off”) based on the
presence or absence of Candida infection. The results
from the current study provided additional information
about the physical/mechanical properties and biocom-
patibility (cytotoxicity), further suggesting the clinical
potential of the UDMA-MAA experimental denture
biomaterials for clinical applications. Our future studies
will move to animal model tests and clinical trials to
further evaluate the safety, effectiveness, and cost-
effective- ness of the new rechargeable, infection-
responsive antifungal denture materials for managing
CADS, a significant clinical concern, particularly for the
elderly and the immunocompromised/medically com-
promised patients.

Effects of Methacrylic Acid on Physical/Mechanical Properties and Biocompatibility of Urethane-Based 1074
Denture Biomaterials
5. Acknowledgements
This study was sponsored by NIH, NIDCR (Grant num-
ber R03 DE018735).
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