# Performance Enhancement of Dental Composites Using Electrospun Nanofibers

Authors: H. Dodiuk-Kenig; K. Lizenboim; S. Roth; B. Zalsman; W. A. McHale; M. Jaffe; K. Griswold
Journal: Journal of Nanomaterials
 (2008)

---

## Abstract

The objective of the present study is to investigate the effect of electrospun nanofiber reinforcement on the properties of commercially available, hyperbranched polymer modified (Hybrane, 0.3 wt.% DSM) dental formulations. The emergence of functionalized nanoscale reinforcements having large surface area (hundreds of square meters/gram) has enabled the design of novel nanocomposites with new and complex structures leading to enhanced mechanical and physical properties. Electrospun nanofibers from a range of polymer chemistries (PVOH, PLLA, Nylon 6) have been investigated as a reinforcing phase at levels between 0.01 and 0.3 wt.%, with and without a silane coupling agent surface treatment.
The experimental results indicate that 0.05 wt.% reinforcement with 250 nm diameter PVOH nanofibers leads to a 30% improvement in compressive strength, coupled with a shrinkage reduction of about 50%. Electrospun fiber reinforcement by other chemistries or at other diameters showed either no property improvement or led to property loss.


---

## Body

## 1. Introduction

A main challenge for centuries has been the development and selection of
biocompatible, long-lasting, direct-filling tooth restoratives and prosthetic
materials that can withstand the adverse conditions of the oral environment. Polymer matrix composites, comprised of silica-filled UV curable acrylate
resins, have emerged as desirable materials for these applications.Key properties of dental composites include
low viscosity prior to cure, biocompatibility, low polymerization shrinkage
(both to insure good prosthesis adhesion and to eliminate unfilled space for
infection or other contamination), high mechanical properties (especially
fracture toughness, compressive strength, and fatigue), surface hardness,
abrasion resistance, low moisture uptake, low coefficient of thermal expansion
(in the range of body temperature), ease of handling in the oral environment,
and the ability to match the esthetics of the patient’s teeth [1–4].The current state-of-the-art dental
composites contain modified acrylate resin matrices filled with micro-/nanoscaled ceramic particles. It has been shown
that the performance of dental composites can be improved through the use of
nanotechnology [5–10], including the use of covalently anchored nanoscaled
organic moieties to an inorganic network [6], and the incorporation of nanoscaled
monomethacrylate functionalized polyhedral oligomeric silsesquioxanes (POSSs)
into an acrylate resin system [11].
Novel polymeric dental restorative composites have been explored, in
which polyhedral oligomeric silsesquioxane methacrylate (POSS-MA) monomers
were used to partially (or completely) replace the commonly used base monomer,
Bis-GMA [10]. Fong explored the use of polyamide nanofibers as a toughening
agent in dental acrylate
resins. The best performance reported to date for acrylate-based dental
composites was achieved by the Dodiuk-Kenig group through the modification of acrylate resin
by the incorporation of hyperbranched
moieties into the matrix resin [9]. It has been hypothesized that these
hyperbranched moieties assemble to form nanophases that retard motions in the
acrylate backbone, leading to improved mechanical performance and reduced
polymerization shrinkage [9]. This improved resin system has been commercialized
by BJM Ltd. and distributed worldwide by the Premier Dental Company.Many groups have worked extensively
in the area of electrospinning [12–15], focusing on both process understanding
and nanofiber applications.The objective of this study is to investigate the impact of
nanofiber reinforcement on the
performance of state-of-the-art dental composites.


## 2. Materials and Methods

### 2.1. Composite Fabrication

Nanofiber-reinforced hyperbranch modified acrylate resin
composites were prepared in the Israeli laboratories of BJM Ltd. This proprietary
process involves the processing of nanofiber mats to produce individual
nanofibers (aspect ratio of about 1000), which can then be conveniently
dispersed in the acrylate matrix resin at the chosen concentration.


### 2.2. Materials

The dental formulations utilized in this study were based on
the standard used bisphenyl
glycidylmethacrylate (Bis-GMA) and triethylenglycoldimethacrylate (TEGDMA)
acrylate resin system. In all cases, the resin composition was modified by the
addition of 0.3 wt.% of hyperbranch polyesteramide into the acrylate matrix
polymer. Electrospun nanofibers of a variety of chemistries (PVOH, PLLA,
Nylon6) where chosen as a reinforcing phase for the base resin. Criteria for
choice were as follows:(i)
biocompatibility,(ii)
functional groups available for reaction with the base resin (e.g., OH groups). In each case, the
same hyperbranched polymer-modified acrylate matrix resin was reinforced with
electrospun nanofibers of a given chemistry and given concentration. All
samples were analysed for composite compressive strength (ISO 9917), flexural
strength (ISO 4049), diametral tensile strength, and linear shrinkage.
Nanofibers were incorporated into the resin over a broad weight fraction range
(0.05–1% wt.) to establish the relationship between amount of reinforcement and
composite performance.The materials used in this study are shown in Tables1(a)
and 1(b).Table 1(a)
Description of the matrix resin
ComponentChemical compositionOrganic resin matrixAcrylic mono- or multifunctional monomers and olygomersPhotosensitizers for the light curing processCamphorquinone (CQ) and Ethyl-4-dimethylaminobenzoate (EDB)Initiators for polymerizationTertiary amine and Benzoyl peroxideDendritic modifier componentHyperbranched polyesteramide(b)
Description of electrospun nanofibers
Nanocomponent TypeNanocomponentBrand name, manufacturerDescriptionElectrospun NanofibersPVOHMedical Device Concept Laboratory, Department of Engineering, NJIT, USAElectrospun Polyvinyl Alchohol NanofibersPLLAMedical Device Concept Laboratory, Department of Engineering, NJIT, USAElectrospun Poly-L-Lactic Acid NanofibersPA6Elmarco s.r.o., Czech RepublicNylon 6 (Polyamide 6) Nanofibers


### 2.3. Electrospinning

Electrospun
nanofibers were produced in the Medical Device
Concept Laboratory (Department of Biomedical Engineering, New Jersey Institute
of Technology, USA).
Electrospinning was performed within a closed chamber, fitted to a mild vacuum
to minimize atmospheric changes during spinning. Solution was supplied to a
standard syringe with a Harvard infusion pump and charging of the fluid was
effected through use of a Gamma high-voltage source model RR60-0.3R. The
solvent for PVOH spinning was deionized water; PLLA was spun from methylene
chloride. Takeup was accomplished on a flat aluminum plate. More than 50
repeats of each spinning condition were performed to generate sufficient fiber
for composite fabrication and testing. Fiber diameters were determined by averaging
of multiple SEM micorographs from multiple runs. Typical SEM micrographs are
shown in Figures1(a) and 1(b).(a) SEM image of PVOH nanofibers (250 nm diameter), 200 nm magnification. (b) SEM image of PLLA nanofibers (250 nm diameter), 300 nm magnification.
(a)(b)The nanofibers were either used as
produced or were silane surface treated with a 1 wt.%γ-Methacryloxypropyl-tri-methoxy-silane in 80 : 20 wt by wt ethanol: water solution and dried at 120°C for 30 minutes
prior to their incorporation into the resin matrix.


### 2.4. Testing Methods

The compressive strength was determined
using a Lloyd Testing Machine (Model LR 10K, Lloyd Instruments, serial no.
9211) in accordance with ISO 9917. The crosshead speed was 1.0 mm/min and ten
specimens were prepared for each composite formulation. The two resin parts
(nanofiber-reinforced matrix and crosslinking agent) were stored separately at
4°C prior to preparation. The specimens were prepared by polymerization in a
cylindrical Teflon mold of 4.0±0.1 mm diameter and 8.0±0.1 mm length. After
polymerization, the specimens were taken out of the mold and held at room
temperature for 1 hour and then immersed in water at (37° ± 1)°C for 24 hours
prior to measurement.The flexural strength was measured using
rectangle specimens25×2×2 mm produced in split Teflon molds in accordance with
ISO 4049, 9917. The specimens were stored for an hour at ambient temperature
and then stored in distilled water at 37°C for 24 hours. Testing
was performed using a Lloyd Testing Machine (Model LR 10K, Lloyd Instruments, serial
no. 9211) equipped with a load cell of 10 N and Bencor multi-T testing device
as a flexural test apparatus (described in ISO/TS 11405:2003).
Each specimen was placed under continuous loading at 1.0 mm per minute.Linear shrinkage measurements were performed
using glass capillary tubes (4.15 mm diameter) which were filled with composite
material and self polymerized by a free-radical mechanism. The length (height
of the specimens) was measured by using an optical microscope “WILD Herrbrugg,”
magnification×10, with the intrinsic scale accurate to 0.1 mm. The
measurements were carried out during the polymerization process as a function
of time (5 minutes, 30 minutes, 60 minutes) then immersed in water at (37° ± 1)°C
for 24 hours. The value of linear shrinkage (%) was calculated (l−l0/l0).Atomic force microscope (AFM) was used to
characterize the three-dimensional topography of the various dental composite
systems evaluated.A high-resolution scanning electron
microscope (HRSEM) was employed to characterize the internal structure and
topography of the various dental composite systems (JEOL Ltd., 2004, 2005).All data was statistically analyzed by the analysis
of variance (ANOVA) method to ensure the
statistical significance of
differences noted experimentally.


## 2.1. Composite Fabrication

Nanofiber-reinforced hyperbranch modified acrylate resin
composites were prepared in the Israeli laboratories of BJM Ltd. This proprietary
process involves the processing of nanofiber mats to produce individual
nanofibers (aspect ratio of about 1000), which can then be conveniently
dispersed in the acrylate matrix resin at the chosen concentration.


## 2.2. Materials

The dental formulations utilized in this study were based on
the standard used bisphenyl
glycidylmethacrylate (Bis-GMA) and triethylenglycoldimethacrylate (TEGDMA)
acrylate resin system. In all cases, the resin composition was modified by the
addition of 0.3 wt.% of hyperbranch polyesteramide into the acrylate matrix
polymer. Electrospun nanofibers of a variety of chemistries (PVOH, PLLA,
Nylon6) where chosen as a reinforcing phase for the base resin. Criteria for
choice were as follows:(i)
biocompatibility,(ii)
functional groups available for reaction with the base resin (e.g., OH groups). In each case, the
same hyperbranched polymer-modified acrylate matrix resin was reinforced with
electrospun nanofibers of a given chemistry and given concentration. All
samples were analysed for composite compressive strength (ISO 9917), flexural
strength (ISO 4049), diametral tensile strength, and linear shrinkage.
Nanofibers were incorporated into the resin over a broad weight fraction range
(0.05–1% wt.) to establish the relationship between amount of reinforcement and
composite performance.The materials used in this study are shown in Tables1(a)
and 1(b).Table 1(a)
Description of the matrix resin
ComponentChemical compositionOrganic resin matrixAcrylic mono- or multifunctional monomers and olygomersPhotosensitizers for the light curing processCamphorquinone (CQ) and Ethyl-4-dimethylaminobenzoate (EDB)Initiators for polymerizationTertiary amine and Benzoyl peroxideDendritic modifier componentHyperbranched polyesteramide(b)
Description of electrospun nanofibers
Nanocomponent TypeNanocomponentBrand name, manufacturerDescriptionElectrospun NanofibersPVOHMedical Device Concept Laboratory, Department of Engineering, NJIT, USAElectrospun Polyvinyl Alchohol NanofibersPLLAMedical Device Concept Laboratory, Department of Engineering, NJIT, USAElectrospun Poly-L-Lactic Acid NanofibersPA6Elmarco s.r.o., Czech RepublicNylon 6 (Polyamide 6) Nanofibers


## 2.3. Electrospinning

Electrospun
nanofibers were produced in the Medical Device
Concept Laboratory (Department of Biomedical Engineering, New Jersey Institute
of Technology, USA).
Electrospinning was performed within a closed chamber, fitted to a mild vacuum
to minimize atmospheric changes during spinning. Solution was supplied to a
standard syringe with a Harvard infusion pump and charging of the fluid was
effected through use of a Gamma high-voltage source model RR60-0.3R. The
solvent for PVOH spinning was deionized water; PLLA was spun from methylene
chloride. Takeup was accomplished on a flat aluminum plate. More than 50
repeats of each spinning condition were performed to generate sufficient fiber
for composite fabrication and testing. Fiber diameters were determined by averaging
of multiple SEM micorographs from multiple runs. Typical SEM micrographs are
shown in Figures1(a) and 1(b).(a) SEM image of PVOH nanofibers (250 nm diameter), 200 nm magnification. (b) SEM image of PLLA nanofibers (250 nm diameter), 300 nm magnification.
(a)(b)The nanofibers were either used as
produced or were silane surface treated with a 1 wt.%γ-Methacryloxypropyl-tri-methoxy-silane in 80 : 20 wt by wt ethanol: water solution and dried at 120°C for 30 minutes
prior to their incorporation into the resin matrix.


## 2.4. Testing Methods

The compressive strength was determined
using a Lloyd Testing Machine (Model LR 10K, Lloyd Instruments, serial no.
9211) in accordance with ISO 9917. The crosshead speed was 1.0 mm/min and ten
specimens were prepared for each composite formulation. The two resin parts
(nanofiber-reinforced matrix and crosslinking agent) were stored separately at
4°C prior to preparation. The specimens were prepared by polymerization in a
cylindrical Teflon mold of 4.0±0.1 mm diameter and 8.0±0.1 mm length. After
polymerization, the specimens were taken out of the mold and held at room
temperature for 1 hour and then immersed in water at (37° ± 1)°C for 24 hours
prior to measurement.The flexural strength was measured using
rectangle specimens25×2×2 mm produced in split Teflon molds in accordance with
ISO 4049, 9917. The specimens were stored for an hour at ambient temperature
and then stored in distilled water at 37°C for 24 hours. Testing
was performed using a Lloyd Testing Machine (Model LR 10K, Lloyd Instruments, serial
no. 9211) equipped with a load cell of 10 N and Bencor multi-T testing device
as a flexural test apparatus (described in ISO/TS 11405:2003).
Each specimen was placed under continuous loading at 1.0 mm per minute.Linear shrinkage measurements were performed
using glass capillary tubes (4.15 mm diameter) which were filled with composite
material and self polymerized by a free-radical mechanism. The length (height
of the specimens) was measured by using an optical microscope “WILD Herrbrugg,”
magnification×10, with the intrinsic scale accurate to 0.1 mm. The
measurements were carried out during the polymerization process as a function
of time (5 minutes, 30 minutes, 60 minutes) then immersed in water at (37° ± 1)°C
for 24 hours. The value of linear shrinkage (%) was calculated (l−l0/l0).Atomic force microscope (AFM) was used to
characterize the three-dimensional topography of the various dental composite
systems evaluated.A high-resolution scanning electron
microscope (HRSEM) was employed to characterize the internal structure and
topography of the various dental composite systems (JEOL Ltd., 2004, 2005).All data was statistically analyzed by the analysis
of variance (ANOVA) method to ensure the
statistical significance of
differences noted experimentally.


## 3. Results

The compressive strength (CS), diametral tensile strength
(DTS), flexural strength (FS), and linear shrinkage (LS) of the base resin reinforce with different
concentrations of PVOH, PLLA, and PA6 nanofibers (with different diameters) are
shown in Table2.Table 2
Mechanical properties of QC STD H UF as a function of electrospun nanofibers.
mechanical propertyconcentration [%wt.]PVOH nanofibersPLLA nanofibersPA6 nanofibersΦ250 nmΦ125 nmΦ250 nmΦ250 nmΦ125 nmΦ250 nmCS [Mpa]0184.0184.0184.0184.0184.0184.00.01121.4140.7137.2136.2144.2132.10.05213.9126.4206.3165182.177.80.1138.2100.5145.3143.9133.363.10.389.994.1115.694.0125.067.6P [ANOVA]0.0030.0020.00030.0040.054.35E-9(n=6)(n=10)(n=10)(n=10)(n=10)(n=10)DTS [Mpa]03030303030300.0122.327.528.930.028.631.90.0527.827.629.524.927.620.80.121.321.528.822.422.018.90.323.518.121.918.822.721.6P [ANOVA]0.735.04E-60.00011.12E-41.22E-79.01E-4(n=6)(n=10)(n=10)(n=10)(n=10)(n=9)FS [Mpa]03143143143143143140.01260.3214.7354.4290.2363.7257.50.05171.8226357.3293.4267.9236.60.1225.6145.5317.6317.6307.5249.50.3244.9230.5189.6342.9280.5234.3P [ANOVA]0.0010.021.17E-80.10.020.7(n=6)(n=9)(n=9)(n=10)(n=10)(n=9ï)LS [%]04444440.011.23.12.02.82.63.90.052.02.82.82.42.22.10.13.02.33.02.52.33.20.32.22.42.02.71.82.7P [ANOVA]0.710.090.950.050.470.65(n=6)(n=10)(n=10)(n=10)(n=10)(n=7)Atomic force microscope (AFM) and scanning electron
microscope (SEM) imaging were used to study the resultant nanostructure and
morphology of the nanofiber-containing dental composites. Figure2(a) illustrates
the surface nanotopography of a 1.0 wt.% 250 nm PVOH nanofiber-reinforced
dental composites as observed by AFM.(a) AFM nanotopography of QC STD H UF (standard) + 1.0 wt% PVOH
(250 nm). (b) SEM of QC STD H UF 1.0 wt% PVOH (250 nm) unsilansed, 2 micron
magnification. (c) SEM of QC STD H UF 1.0 wt% PVOH (250 nm) silanised, 2 micron magnification.
(a)(b)(c)SEM studies of composite fracture surfaces are instructive
in understanding the mechanism of property enhancement exhibited by some of the
PVOH nanofiber-reinforced dental composites. An SEM micrograph of a PVOH
nanofiber-reinforced composite, silanised and unsilanised, is shown in Figures2(b) and 2(c).In cases where improvements are noted, good adhesion between
fiber and matrix, as evidenced by resin coating on the fiber (Figure2(b)), is
observed. Conversely, poorer properties are associated with fiber pull-out on
the fracture surfaces (Figure 2(c)). It is
evident from Figure 2(c) that the treated fibers system shows better fibers–matrix
interaction than the untreated ones.


## 4. Discussion

The compressive strength results for all nanoreinforcements
studied are shown in Figure3 as a function of reinforcing fiber concentration.Figure 3
Compressive strength of QC STD H UF as a function of nanofibers concentration
and type.Figure3 illustrates that a significant improvement in
compressive strength is exhibited by the dispersion of t of 0.05 wt.% PVOH
nanofibers to the standard dental composite matrix. This is probably in part
due to the increase in reactive hydroxyl group concentration of the PVOH, but
the fiber morphology within the composite also plays an important role—note
lack of improvement of the 150 nm diameter PVOH fiber-reinforced samples. The
improvement noted for the 0.05 wt.%, 250 nm PVOH nanofiber-reinforced
composite was s manifest in both compressive strength increases and linear
shrinkage decreases. No other nanocomposites tested, including the PLLA
nanofiber (125 nm diameter, 250 nm diameter), the PA6-based nanocomposites, or
the 125 nm PVOH nanofiber-reinforced composites, showed consistent property
improvements. These results suggest that the mechanical properties of these
nanofiber-reinforced acrylate composites are very sensitive to fiber dispersion
within the matrix and fiber matrix bonding.Studies of composite surfaces and fracture indicate that
mechanical performance correlates with fiber adhesion to matrix. The AFM result,
shown in Figure2(a), suggests that the nanostructure of 1% wt. PVOH nanofiber
composite (diameter 250 nm) system
correlates with lowering of mechanical properties from those exhibited by the
matrix alone. As the concentration of the nanofibers increases, increased fiber
pullout is evident from the increase of surface roughness (302.4 nm at 1 wt.% PVOH
nanofibers).The SEM results (Figure2(c)) imply that the finest structure
and strongest matrices-fiber interaction were obtained in the case of the 0.05 wt.% PVOH nanofibers (diameter 250 nm) composite system. This critical
nanostructure correlates with the highest compressive strength of the dental
composites as well as with the lowest linear shrinkage exhibited. The results
show conclusively that
the nanofibers survive composite processing and that property improvement
correlates with fiber adhesion to the matrix.These preliminary results indicate that the introduction of
hydroxyl-rich nanophases with specific composite architecture (need to
differentiate from 150 nm PVOH) into acrylate matrix dental composites can lead
to significant and unexpected improvement in clinically important dental
material performance. In the case of acrylate resin modified by hyperbranched
polyesteramide, the improvement is attributed to crack blunting by the
nanophase inclusion, coupled with increased system cross linking through the
hydroxy-rich chemistry [9]. The addition of the hydroxyl-rich PVOH nanofiber
phase further increases crosslinking density, while improving overall system
toughness by allowing cracks to run and dissipate energy along the fiber-matrix
interface. These results are consistent with the complex mechanisms suggested
for the toughening of polymers, polymer blends, and fiber-reinforced composites
[16]. While a detailed mechanism is beyond the scope of the present work, it is
clear that composite performance is a function of matrix chemistry, fiber
diameter, fiber dispersion, and fiber matrix interaction. In a more general
sense, this work shows that significant improvement to the performance of
simple polymer systems, in this case acrylates, can be effected by the
introduction of small weight
percentage (<1 wt.%) of nanoscale reinforcements. The introduction of
more than one such phase can, as illustrated here by the inclusion of both
hyperbranch (0.3 wt.%) resin modification and PVOH nanofiber (250 nm diameter,
0.05% wt.), can be synergistic and lead to even greater improvement of key
performance properties. Future work will concentrate on the definition of
detailed mechanisms of property enhancement and the extension of these concepts
to new chemistries.



---

*Source: 840254-2008-08-10.xml*