[Show abstract][Hide abstract] ABSTRACT: Self-assembled nanostructures of crystallizable block copolymers can be tuned by controlled crystal thickening during annealing. In this contribution, we present a strategy, based on time- and temperature-dependent DSC, SAXS and WAXS measurements, which enables to study, both, the mechanisms and kinetics of crystal thickening and the respective morphological development, exemplarily discussed for the soft-confined PB-b-PEO block copolymer. Thereby, DSC based PEO crystal thickness distributions yield qualitative information about the mechanisms during annealing. Conclusions on the kinetics and the absolute long-period growth due to crystal thickening can be drawn from the time- and temperature dependent SAXS investigations, by calculating the average long-period and its deviations from the SAXS reflection position and shape, respectively. By this combined study, three annealing regimes were observed. (i) At low annealing temperatures Ta, steady lamellae-thickening was found, due to defect healing of the PEO crystals. (ii) Thermal fractionation was observed at intermediate Ta, due to the exclusion of shorter PEO chains from the crystals. (iii) Annealing close to and above the peak melting temperature, self-nucleation of the molten PEO fractions dominates. The combination of the applied techniques provides deeper insights into the kinetics and ordering mechanisms of the controlled long-period growth by crystal thickening under variable confinements, which enables to tailor the morphology of the block copolymer within several nanometers, without changing the degree of polymerization.
European Polymer Journal 04/2015; 68:10-20. DOI:10.1016/j.eurpolymj.2015.04.010 · 3.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microstructured surfaces mimicking the endothelial cell (EC) morphology is a new approach to improve the blood compatibility of synthetic vascular grafts. The ECs are capable of changing their shapes depending on different shear conditions. However, the quantitative correlation between EC morphology and shear stress has not yet been investigated statistically. The aim of this study was to quantitatively investigate the morphology of ECs in dependence on the shear stress. Blood flow rates in different types of natural blood vessels (carotid, renal, hepatic and iliac arteries) originated from domestic pigs were first measured in vivo to calculate the shear stresses. The EC morphologies were quantitatively characterized ex vivo by imaging with high resolution scanning electron microscopy (SEM) and cross-sectioning of the cells using a state-of-the-art Focused Ion Beam (FIB). The relationships between EC geometrical parameters and shear stress were statistically analyzed and found to be exponential. ECs under high shear stress conditions had a longer length and narrower width, i.e. a higher aspect ratio, while the cell height was smaller compared to low shear conditions. Based on these results, suitable and valid geometrical parameters of microstructures mimicking EC can be derived for various shear conditions in synthetic vascular grafts to optimize blood compatibility.
Tissue and Cell 12/2014; 47(2). DOI:10.1016/j.tice.2014.12.005 · 1.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotubes (CNTs) and their polymer nanocomposites are interesting materials for future applications, for example in optics or electronics. Research faces two major challenges with these outstanding nanofillers: Control over dispersion and spatial arrangement within the nanocomposite, both required to achieve optimal structure and properties of CNT based nanocomposites. We report on novel self-assembled multi-wall CNT (MWCNT)/block copolymer (BCP) nanostructures realized by patterning MWCNTs with amphilphilic diblock copolymer micelles. A high molecular weight Poly(styrene)-b-poly(2-vinylpyridine) BCP which forms large micelles (250 nm) was chosen to facilitate the templating by reducing the bending energy induced in the MWCNTs. We tested the hypothesis, that it is possible to use an amphiphilic BCP as a dispersing agent and its spherical micelles as a template at the same time without modification of the CNTs. In thin films of the MWCNT/BCP micelles, highly separated MWCNTs were repeatedly observed which enveloped the core of the BCP micelles, i.e., the unfunctionalized MWCNTs segregated to the interface between the two BCP phases. Depending on the size of the MWCNTs, ring like (split-ring) or network forming structures were obtained. The MWCNT templating mechanism, i.e. the segregation to the interface, is explained by the interfacial tension within the BCP interface and the chain entropy. The reported new complex nanocomposite has potential to be applied for example as cost-effective split-ring resonators for metamaterials or for conductive polymer films with an extremely low percolation threshold.
[Show abstract][Hide abstract] ABSTRACT: The dynamics of adhesion and growth of bacterial cells on biomaterial surfaces play an important role in the formation of biofilms. The surface properties of biomaterials have a major impact on cell adhesion processes, eg the random/non-cooperative adhesion of bacteria. In the present study, the spatial arrangement of Escherichia coli on different biomaterials is investigated in a time series during the first hours after exposure. The micrographs are analyzed via an image processing routine and the resulting point patterns are evaluated using second order statistics. Two main adhesion mechanisms can be identified: random adhesion and non-random processes. Comparison with an appropriate null-model quantifies the transition between the two processes with statistical significance. The fastest transition to non-random processes was found to occur after adhesion on PTFE for 2-3 h. Additionally, determination of cell and cluster parameters via image processing gives insight into surface influenced differences in bacterial micro-colony formation.
[Show abstract][Hide abstract] ABSTRACT: Injectable, brushite-forming calcium phosphate cements (CPCs) have great potential as bone replacement materials due to enhanced degradability and long-term inclusion in bone remodeling. However, the use of such brushite-forming CPCs in load-bearing areas is limited by their low mechanical strength. One approach to overcome this limitation is the use of reinforcing fibers. Thus, an injectable, biodegradable, brushite-forming CPC based on beta-tricalcium phosphate/phosphoric acid with fiber reinforcement was developed for minimally invasive surgery. The fibers (diameter 25µm; length 0.25, 1 or 2mm) were extruded from poly(l-lactide-co-glycolide) acid (PLGA) and added to the CPC (2.5, 5 or 7.5% (w/w)). Independent of the fiber content, injectability of the CPC was retained up to a fiber length of 1mm. The addition of all PLGA fiber types increased diametral tensile strength, biaxial flexural strength, and flexural strength by up to 25% (p≤0.05 for the diametral tensile strength for the CPC with 5% (w/w) 1mm fibers and the biaxial flexural strength of the CPC with 5% (w/w) 0.25mm fibers). In contrast, the work of fracture strongly and significantly increased (p<0.01) by up to 12.5-fold. At constant fiber content, the mechanical properties of the fiber-reinforced CPC were mostly augmented with increasing fiber length. Also, the addition of PLGA fibers to the brushite-forming CPC (up to 7.5% (w/w)) only transiently delayed cell growth and did not decrease cell viability. Fiber reinforcement of CPCs thus augments their mechanical strength while preserving the injectability and biocompatibility required for their application in modern surgery.
Journal of the Mechanical Behavior of Biomedical Materials 08/2014; 39C:328-338. DOI:10.1016/j.jmbbm.2014.07.028 · 3.42 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Objectives:
Endodontic pathogens can penetrate deep into dentinal tubules and therefore survive the chemo-mechanical disinfection procedures. Bacterial penetration has been mainly studies using sliced infected human teeth which, besides creating artifacts, can hinder the observation of the inner tubules due to the dense and opaque dentin structure. The aim of the present study was to develop a standardized dentin model by using artificial SiO/SiO2-microtubes of different diameters and lengths to test the penetration ability of Enterococcus faecalis.
E. faecalis was grown in Schaedler fluid media for 24h and thereafter cell density was settled to 10(3)cells/ml by addition of fresh media. The bacterial solution was then incubated for 2, 3, 5 and 10 days with the SiO/SiO2-microtubes of different diameters (2-5.5μm) and lengths (100-500μm). The colonization of the tubes was evaluated by phase-contrast microscopy and the amount of colonization was determined by using a colonization index (CI; 0-none, 1-mild, 2-moderate, 3-heavy).
The diameter of the tubes strongly influences the microbial colonization. After 2 days of cultivation the 5.5μm tubes showed a moderate to heavy colonization (CI 2-3). In comparison, the 2 and 3μm tubes were clearly less colonized at the same point in time. In detail: at day 3, only mild to moderate bacteria colonization (CI 1-2) were found in the 3μm tubes and at day 10 penetration of the 2μm tubes just started. The colonization of the 5.5μm tubes was also influenced by their length. In case of the longer microtubes, though, a smaller share of heavily colonized tubes was observed.
Our results show that E. faecalis was able to penetrate and reproduce within the standardized SiO/SiO2-microtubes in a short time. To examine the mechanisms of bacterial adhesion and invasion into tubular structures the 2μm tubes could serve as a model system because the diameters are similar to those of dentinal tubules.
Dental materials: official publication of the Academy of Dental Materials 06/2014; 30(6). DOI:10.1016/j.dental.2014.03.003 · 3.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An effective method for fabrication of long range ordered micro- and nanostructures on surfaces is to control the interactive crystallisation of block copolymers. In this study, the influence of different initial mesophases of a double crystalline polyethylene-block-poly (ethylene oxide) (PE-b-PEO) diblock co-oligomer on the interactive crystallisation process was studied using synchrotron radiation X-ray diffraction (SAXS/WAXD), in situ optical microscopy and differential scanning calorimetric analysis (DSC). According to the applied annealing procedure, different PE-b-PEO initial mesophases, i.e., disordered, cylindrical and spherical, have been induced. In all cases, the subsequent PEO crystallisation disrupted these initial microdomains and transformed them into crystalline lamellar morphologies with the same long periods. However, the different initial mesophases significantly affected the PEO crystallisation kinetics due to different topological confinements. An initial disordered mesophase induced the highest PEO crystallisation rate because PEO nucleation and crystal growth were limited only by chain diffusion. For an initial spherical or cylindrical mesophase, decreased PEO crystallisation rates were observed. Here, the chain diffusion was decreased by the microdomain structure. For an initial cylindrical mesophase, the earlier formed PE crystals act as a template for the subsequent PEO crystallisation and, thus, increased the PEO crystallisation as compared to the spherical mesophase where the PE was amorphous. This study demonstrates that the topological confinement of the block copolymer's initial mesophase strongly influences the crystallisation kinetics and, thus, the structures formed at the surface of drop-casted films.
[Show abstract][Hide abstract] ABSTRACT: Ferrihydrite (Fh) is a widespread poorly crystalline Fe oxide which becomes easily coated by natural organic matter (OM) in the environment. This mineral-bound OM entirely changes the mineral surface properties and therefore the reactivity of the original mineral. Here, we investigated the reactivity of 2-line Fh, Fh with adsorbed OM and Fh coprecipitated with OM towards microbial and abiotic reduction of Fe(III). As a surrogate for dissolved soil OM we used a water extract of a Podzol forest floor. Fh-OM associations with different OM-loadings were reduced either by Geobacter bremensis or abiotically by Na-dithionite. Both types of experiments showed decreasing initial Fe reduction rates and decreasing degrees of reduction with increasing amounts of mineral-bound OM. At similar OM-loadings, coprecipitated Fhs were more reactive than Fhs with adsorbed OM. The difference can be explained by the smaller crystal size and poor crystallinity of such coprecipitates. At small OM loadings this led to even faster Fe reduction rates than found for pure Fh. The amount of mineral-bound OM also affected the formation of secondary minerals: goethite was only found after reduction of OM-free Fh and siderite was only detected when Fhs with relatively low amounts of mineral-bound OM were reduced. We conclude that direct contact of G. bremensis to the Fe oxide mineral surface was inhibited when blocked by OM. Consequently, mineral-bound OM shall be taken into account besides Fe(II) accumulation as a further widespread mechanism to slow down reductive dissolution.
[Show abstract][Hide abstract] ABSTRACT: We report here a facile strategy to fabricate three-dimensional (3D) hydroxyapatite (HA) architectures with well-defined long continuous interconnected pores by using electrospinning and biomimetic mineralization. To this end, a polymeric nanofiber (NF) scaffold with well-defined architecture was fabricated by electrospinning, and bone morphogenetic protein 2 (BMP2) was then adsorbed onto the chemically modified NFs through bio-conjugation. The 3D nanoporous HA architecture was finally fabricated by biomimetic mineralization of NF-BMP2 hybrid in simulated body fluids and subsequent dissolution of NFs in hexafluoroisopropanol. The formation of NF-BMP2 hybrid was identified by confocal laser scanning microscopy analysis. The crystal structure of HA crystals formed on NFs was examined by X-ray diffraction. The chemical composition and interconnected porous structure of the created 3D HA architectures were measured by X-ray photoelectron spectroscopy, focused ion beam scanning electron microscopy, and transmission electron microscopy, respectively. This bottom-up strategy based on electrospinning and biomimetic mineralization opens up a new way to prepare diverse porous HA-based hybrid materials and show great potentials in drug delivery, gene transfer and tissue engineering.
[Show abstract][Hide abstract] ABSTRACT: Thin film surface nanostructures of semicrystalline diblock copolymer are promising for the fabrication of photonic crystals and bioanalytical devices because they might be tailorable by controlled crystallization. One approach to systematically control polymer crystallization is a self-nucleation experiment. The self-nucleation experiment for block copolymers has only been reported for the bulk and so far not for thin films. Considering the versatility of a tailorable surface nanostructure, it is promising to apply the controlled crystallization of a bulk self-nucleation experiment to thin films of a diblock copolymer. In the current study we tested the hypothesis that within two self-nucleation experiments, i.e., in the bulk and thin film, the calorimetric bulk properties of a polybutadiene-block-poly(ethylene oxide) can be correlated to the resulting thin film surface nanostructures and to understand as well as predict their formation. The calorimetric bulk properties measured by differential scanning calorimetry in the bulk self-nucleation experiment were correlated to surface nanostructures measured by atomic force microscopy of the thin film self-nucleation experiment samples. In analogy to the bulk self-nucleation experiment, we introduced a crystalline standard for the thin film self-nucleation experiment where the crystalline lamellae consisted of once-folded chains. Annealing the thin film crystalline standard promoted the thickening of crystalline lamellae on the film surface which is explained by the formation of less folded chain crystals that obtain higher melting temperatures. The crystalline lamellae thickness was steplessly variable within the range of 8–16 nm. In analogy to the Hoffman–Weeks and Gibbs–Thomson plots, we derived a function which can be used to predict the lamellae thickness as a function of the annealing temperature. Bulk and thin film self-nucleation experiments were successfully related, since thin film surface nanostructures were consistently correlated to calorimetric results. We established the dual self-nucleation experiment as a powerful tool to predictably tailor thin film nanostructures in the range of several nanometers.
[Show abstract][Hide abstract] ABSTRACT: Various microstructures and phase morphologies of an amphiphilic poly(ethylene oxide)-block-polyethylene (PEO-b-PE) co-oligomer, controlled by topological restriction of PE segments on the tethered PEO chains, were characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), scanning electron microscopy (SEM), and synchrotron radiation wide-angle/small-angle X-ray scattering (WAXS/SAXS) in drop-cast films. The crystallization processes were mediated by two pathways, a one-step crystallization process (I) and a sequential crystallization process (II). Results show that the thermal procedures have great influence on the microstructures and phase morphologies of PEO-b-PE co-oligomer, e.g., negative spherulites with radial stripes were detected in the one-step crystallization process (I), while crystalline texture, which contains a large number of crystals with reduced sizes, formed in the sequential crystallization process (II). Based on our experimental data, the topological restriction effect encountered by PEO chains depends on the hard confinement of PE crystals and the soft confinement of amorphous PE in the two crystallization procedures. The formation mechanisms of the long-range order structures within the co-oligomer were elucidated through morphology models. These nano-patterned structures make the double crystalline block copolymers outstanding candidates for surface modification, micromolding, and optoelectronic devices in nanotechnological and biomedical applications.
[Show abstract][Hide abstract] ABSTRACT: Biomaterials-associated infections are primarily initiated by the adhesion of microorganisms on the biomaterial surfaces and subsequent biofilm formation. Understanding the fundamental microbial adhesion mechanisms and biofilm development is crucial for developing strategies to prevent such infections. Suitable in vitro systems for biofilm cultivation and bacterial adhesion at controllable, constant and reproducible conditions are indispensable. This study aimed (i) to modify the previously described constant-depth film fermenter for the reproducible cultivation of biofilms at non-depth-restricted, constant and low shear conditions and (ii) to use this system to elucidate bacterial adhesion kinetics on different biomaterials, focusing on biomaterials surface nanoroughness and hydrophobicity. Chemostat-grown Escherichia coli were used for biofilm cultivation on titanium oxide and investigating bacterial adhesion over time on titanium oxide, poly(styrene), poly(tetrafluoroethylene) and glass. Using chemostat-grown microbial cells (single-species continuous culture) minimized variations between the biofilms cultivated during different experimental runs. Bacterial adhesion on biomaterials comprised an initial lag-phase I followed by a fast adhesion phase II and a phase of saturation III. With increasing biomaterials surface nanoroughness and increasing hydrophobicity, adhesion rates increased during phases I and II. The influence of materials surface hydrophobicity seemed to exceed that of nanoroughness during the lag-phase I, whereas it was vice versa during adhesion phase II. This study introduces the non-constant-depth film fermenter in combination with a chemostat culture to allow for a controlled approach to reproducibly cultivate biofilms and to investigate bacterial adhesion kinetics at constant and low shear conditions. The findings will support developing and adequate testing of biomaterials surface modifications eventually preventing biomaterial-associated infections.
PLoS ONE 01/2014; 9(1):e84837. DOI:10.1371/journal.pone.0084837 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background:
Certain coatings such as titanium may improve the biocompatibility of hernia meshes. The coating with biopolymers such as polyethylenimine (PEI) can also improve the material characteristics of implants. This approach has, however, not yet been explored. Thus, it was the aim of the present work to clarify if and how hernia meshes with their three-dimensional structure can be successfully coated with PEI and with which technique this coating can be best analysed.
Commercially available meshes made from polypropylene, polyester and ePTFE have been coated with PEI. The coating was analysed via cell proliferation test (mouse fibroblasts), electron microscopy, X-ray photoelectron spectroscopy (XPS) and fluorescence microscopy. Cell viability and cytotoxicity were tested by the MTT test.
With the PEI surface modification, mouse fibroblasts grow faster and in greater numbers on the mesh surface. XPS as well as fluorescence microscopy show weaknesses in their applicability and meaningfulness because of the three-dimensional mesh structure while XPS showed overall better results. Optical proof in the electron microscope after cell fixation was not unambiguously accomplished with the techniques used here. In the MTT test, no cellular damage from the PEI coating was detected after 24 hours.
The present results show for the first time that PEI coating of hernia meshes is possible and effective. The PEI coating can be achieved in a fast and cost-efficient way. Further investigations are necessary with respect to coating quality and cytotoxicity before such a coating may be used in the clinical routine. In conclusion, PEI is a promising polymer that warrants further research as a coating for medical implants.
Zentralblatt für Chirurgie 12/2013; 140(2). DOI:10.1055/s-0033-1351003 · 1.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quantitative characterization of the complexation between proteins and electroneutral polymers is of great importance in biology and medicine science. However, the investigation of it has long been hindered due to the difficulties in characterization. And the few existing analysis models have various problems. In view of this, we focused on the complexation between proteins and electroneutral polymers in this work, and came up with a novel model analysis method to quantitatively characterize it. The mathematical model, based on the reversible complexing equilibrium among free proteins, free polymers and complexes of proteins with polymers, successfully associates the primary complexing parameters with the variation of the fluorescence intensities of the complexing system, so that one can quantitatively characterize the complexation of proteins with electroneutral water-soluble polymers in aqueous systems in situ, without destroying the dynamic equilibrium, which is more accurate. In this study, the complexation of bovine serum albumin (BSA) with poly(N-isopropylacrylamide) (PNIPAM) is investigated as an example, and with the help of this method, numerous complexing parameters can be calculated accurately and rapidly. And by examining their variation with the increase of the mixing ratio (r(mixing), molar ratio of PNIPAM to BSA), the complex interaction of proteins with electroneutral water-soluble polymers is further illuminated. Compared with traditional analysis methods, this method has the advantages of simplicity, accuracy and extensive application.
[Show abstract][Hide abstract] ABSTRACT: Aligned multi-wall carbon nanotube (MWCNT)/polymer composite films were created in a one-step process. 5 wt% MWCNT/semi-crystalline polymer composite films of approximately 100nm thickness were obtained by melt-drawing. The matrix polymers were isotactic polypropylene (iPP), poly(1-butene) (PB-1) and high density polyethylene (HDPE). Transmission electron microscopy (TEM) investigations revealed an exceptionally high degree of local MWCNT alignment with an angular deviation of <10°<10° (HDPE) and <20°<20° (iPP and PB-1) parallel to the films’ drawing direction for a broad range of drawing velocities. For HDPE, the lamellar polymer-crystals at the interface between the MWCNT and the polymer film were identified as the nano-hybrid shish-kebab morphology by selected area electron diffraction. Based on the direct visualization of the MWCNT disentanglement process in the TEM, a polymer physics-based model for the unraveling of MWCNT entanglements, a source of aligned MWCNTs, is proposed that explains differences in MWCNT alignment encountered for different matrix polymers. The melt-drawing mediated MWCNT alignment provides both an innovative approach for the fabrication of applicable MWCNT containing films and a versatile tool for studying the interface in MWCNT/polymer composites.
[Show abstract][Hide abstract] ABSTRACT: The surface topography is of great importance for the biological performance of titanium based implants
since it may influence the initial adsorption of proteins, cell response, as well as microbial adhesion.
A recently described technique for the preparation of titanium thin films with an adjustable surface
roughness on the nanometer scale is the physical vapor deposition (PVD). The aims of this study were
to statistically evaluate the reproducibility of nanorough titanium thin films prepared by PVD using an
atomic force microscopy (AFM) based approach, to test the microbial adhesion in dependence of the
nanoscale surface roughness and to critically discuss the parameters used for the characterization of the
titanium surfaces with respect to AFM microscope settings. No statistically significant differences were
found between the surface nanoroughnesses of the PVD prepared titanium thin films. With increasing
surface nanoroughness, the coverage by Escherichia coli decreased and the microbial cells were increasingly patchy distributed. The calculated roughness values significantly increased with increasing AFM
scan size, while image resolution and pixel density had no influence on this effect. Our study shows that
PVD is a suitable tool to reproducibly prepare titanium thin films with a well-defined surface topography
on the nanometer scale. These surfaces are, thus, a suitable 2D model system for studies addressing the
interaction between surface nanoroughness and the biological system. First results show that surface
roughness even on the very low nanometer scale has an influence on bacterial adhesion behavior. These
findings give new momentum to biomaterials research and will support the development of biomaterials
surfaces with anti-infectious surface properties.
[Show abstract][Hide abstract] ABSTRACT: Objective:
Bacteria persisting in the root canal system may thwart endodontic therapy. It is therefore interesting to know whether clinically available root canal sealers have an antimicrobial effect. The objective of the present in vitro study was to investigate the antibacterial effect of various sealers on the endodontologically detectable species Enterococcus faecalis (E. faecalis), Fusobacterium nucleatum (F. nucleatum) and Porphyromonas gingivalis (P. gingivalis).
The antibacterial effectiveness of the sealers was tested by means of the agar diffusion test (ADT) and the direct contact test (DCT). Eight different sealers (AH Plus(®), Hermetic(®), RoekoSeal(®), Sealapex(®), Apexit Plus(®), 2Seal(®), EndoREZ(®) and ProRoot MTA(®)) and two temporary sealers (Calxyl(®) and Gangraena Merz(®)) were tested. At first, 100μl of bacterial suspension (BS) of each individual micro-organism (optical density (OD) 0.5) was applied separately to Schaedler agar plates for the ADT. Subsequently, freshly mixed and set sealer was applied. After 48h of incubation, the inhibition zones were measured. Further, 18mg of sealer were put into each well of 48-well cell culture plates and overlaid with 400μl of Schaedler liquid medium and 100μl of BS (OD 0.5) in monoculture. Bacterial growth was determined by the DCT from the optical density of the liquid by photospectrometry after 2, 4, 6, 8, 10, 12 and 24h.
For the application, the sealer Hermetic(®), a significant suppression of the species E. faecalis, F. nucleatum and P. gingivalis was detected in both the ADT and the DCT. AH Plus(®) showed a suppressive effect on E. faecalis and F. nucleatum in the DCT. With all other sealers tested, E. faecalis was not suppressible. RoekoSeal(®), Calxyl(®) and Gangraena-Merz(®) showed no antibacterial effect on the tested species whatsoever.
We have shown in both ADT and DCT that some root canal sealers suppress the growth of E. faecalis in vitro.
Dental materials: official publication of the Academy of Dental Materials 03/2013; 29(5). DOI:10.1016/j.dental.2013.02.007 · 3.77 Impact Factor