Derrick Dean | Alabama State University (original) (raw)

Papers by Derrick Dean

Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 1999

ABSTRACT

Journal of magnetism and magnetic materials, 2010

The delivery of noscapine therapies directly to the site of the tumor would ultimately allow high... more The delivery of noscapine therapies directly to the site of the tumor would ultimately allow higher concentrations of the drug to be delivered, and prolong circulation time in vivo to enhance the therapeutic outcome of this drug. Therefore, we sought to design magnetic based polymeric nanoparticles for the site directed delivery of noscapine to invasive tumors. We synthesized Fe 3 O 4 nanoparticles with an average size of 10 ± 2.5 nm. These Fe 3 O 4 NPs were used to prepare noscapine loaded magnetic polymeric nanoparticles (NMNP) with an average size of 252 ± 6.3 nm. Fourier transform infrared (FT-IR) spectroscopy showed the encapsulation of noscapine on the surface of the polymer matrix. The encapsulation of the Fe 3 O 4 NPs on the surface of the polymer was confirmed by elemental analysis. We studied the drug loading efficiency of polylactide acid (PLLA) and poly (L-lactide acid-co-gylocolide) (PLGA) polymeric systems of various molecular weights. Our findings revealed that the molecular weight of the polymer plays a crucial role in the capacity of the drug loading on the polymer surface. Using a constant amount of polymer and Fe 3 O 4 NPs, both PLLA and PLGA at lower molecule weights showed higher loading efficiencies for the drug on their surfaces.

The next revolutionary leap forward for polymer nano-'composites' necessitates the development of... more The next revolutionary leap forward for polymer nano-'composites' necessitates the development of tools to transform the currently random or ill-defined nanoscale morphologies into compositionally and spatially engineered hierarchal structures, paralleling that underpinning conventional continuous fiber reinforced composites and enabling experimental verification of morphology-mechanical property correlations. Here in, utilization of AC electric fields of modest strength (1-10 V/micron) is demonstrated as a general approach to align organically modified layered silicates (OLS) parallel to the electric field - enhancing CTE, modulus and optical clarity in the reinforced direction. Interfacial polarization arising from electrophoric motion of the organic-modifier on the layer surface induces a dipole parallel to the plane of the layer, which couples to the external field. Comparably, application of a static magnetic field (0.5-2 T) induces alignment of the layers parallel or perpendicular to the field, depending on the chemical composition of the OLS. The impact of field magnitude, field frequency, dielectric permittivity and magnetic permeability of the system is discussed to elucidate the molecular characteristics of induced dipole formation and establish the limits of the process.

Biofabrication, 2009

Nanofibrous electrospun poly (epsilon-caprolactone) (ePCL) scaffolds have inherent structural adv... more Nanofibrous electrospun poly (epsilon-caprolactone) (ePCL) scaffolds have inherent structural advantages, but lack of bioactivity has limited their usefulness in biomedical applications. Thus, here we report the development of a hybrid, nanostructured, extracellular matrix (ECM) mimicking scaffold by a combination of ePCL nanofibers and self-assembled peptide amphiphile (PA) nanofibers. The PAs have ECM mimicking characteristics including a cell adhesive ligand (RGDS) and matrix metalloproteinase-2 (MMP-2) mediated degradable sites. Transmission electron microscope imaging verified successful PA self-assembly into nanofibers (diameters of 8-10 nm) using a solvent evaporation method. This evaporation method was then used to successfully coat PAs onto ePCL nanofibers (diameters of 300-400 nm), to develop hybrid, bioactive scaffolds. Scanning electron microscope characterization showed that the PA coatings did not interfere with the porous ePCL nanofiber network. Human mesenchymal stem cells (hMSCs) were seeded onto the hybrid scaffolds to evaluate their bioactivity. Significantly greater attachment and spreading of hMSCs were observed on ePCL nanofibers coated with PA-RGDS as compared to ePCL nanofibers coated with PA-S (no cell adhesive ligand) and uncoated ePCL nanofibers. Overall, this novel strategy presents a new solution to overcome the current bioactivity challenges of electrospun scaffolds and combines the unique characteristics of ePCL nanofibers and self-assembled PA nanofibers to provide an ECM mimicking environment. This has great potential to be applied to many different electrospun scaffolds for various biomedical applications.

Polymer, 2010

The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nan... more The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites processed with a magnetic field have been studied. Samples were prepared by dispersing the nanotube in the epoxy and curing under an applied magnetic field. The nanocomposite morphology was characterized with Raman spectroscopy and wide angle X-ray scattering, and correlated with thermo-mechanical properties. The modulus parallel to the alignment direction, as measured by dynamic mechanical analysis, showed significant anisotropy, with a 72% increase over the neat resin, and a 24% increase over the sample tested perpendicular to the alignment direction. A modest enhancement in the coefficient of thermal expansion (CTE) parallel to the alignment direction was also observed. These enhancements were achieved even though the nanotubes were not fully aligned, as determined by Raman spectroscopy. The partial nanotube alignment is attributed to resin a gel time that is faster than the nanotube orientation dynamics. Thermal conductivity results are also presented.

Acta Biomaterialia, 2011

Current cardiovascular therapies are limited by the loss of endothelium, restenosis and thrombosi... more Current cardiovascular therapies are limited by the loss of endothelium, restenosis and thrombosis. The goal of this study was to develop a biomimetic hybrid nanomatrix that combined the unique properties of electrospun polycaprolactone (ePCL) nanofibers with self-assembled peptide amphiphiles (PAs). ePCL nanofibers have interconnected nanoporous structures, but are hampered by a lack of surface bioactivity to control cellular behavior. It has been hypothesized that PAs could self-assemble onto the surface of ePCL nanofibers and endow them with the characteristic properties of native endothelium. The PAs, which comprised hydrophobic alkyl tails attached to functional hydrophilic peptide sequences, contained enzyme-mediated degradable sites coupled to either endothelial cell-adhesive ligands (YIGSR) or polylysine (KKKKK) nitric oxide (NO) donors. Two different PAs (PA–YIGSR and PA–KKKKK) were successfully synthesized and mixed in a 90:10 (YK) ratio to obtain PA–YK. PA–YK was reacted with pure NO to develop PA–YK–NO, which was then self-assembled onto ePCL nanofibers to generate a hybrid nanomatrix, ePCL–PA–YK–NO. Uniform coating of self-assembled PA nanofibers on ePCL was confirmed by transmission electron microscopy. Successful NO release from ePCL–PA–YK–NO was observed. ePCL–YK and ePCL–PA–YK–NO showed significantly increased adhesion of human umbilical vein endothelial cells (HUVECs). ePCL–PA–YK–NO also showed significantly increased proliferation of HUVECs and reduced smooth muscle cell proliferation. ePCL–PA–YK–NO also displayed significantly reduced platelet adhesion compared with ePCL, ePCL–PA–YK and a collagen control. These results indicate that this hybrid nanomatrix has great potential application in cardiovascular implants.

Biomaterials, 2011

A limiting factor of traditional electrospinning is that the electrospun scaffolds consist entire... more A limiting factor of traditional electrospinning is that the electrospun scaffolds consist entirely of tightly packed nanofiber layers that only provide a superficial porous structure due to the sheet-like assembly process. This unavoidable characteristic hinders cell infiltration and growth throughout the nanofibrous scaffolds. Numerous strategies have been tried to overcome this challenge, including the incorporation of nanoparticles, using larger microfibers, or removing embedded salt or water-soluble fibers to increase porosity. However, these methods still produce sheet-like nanofibrous scaffolds, failing to create a porous three-dimensional scaffold with good structural integrity. Thus, we have developed a three-dimensional cotton ball-like electrospun scaffold that consists of an accumulation of nanofibers in a low density and uncompressed manner. Instead of a traditional flat-plate collector, a grounded spherical dish and an array of needle-like probes were used to create a Focused, Low density, Uncompressed nanoFiber (FLUF) mesh scaffold. Scanning electron microscopy showed that the cotton ball-like scaffold consisted of electrospun nanofibers with a similar diameter but larger pores and less-dense structure compared to the traditional electrospun scaffolds. In addition, laser confocal microscopy demonstrated an open porosity and loosely packed structure throughout the depth of the cotton ball-like scaffold, contrasting the superficially porous and tightly packed structure of the traditional electrospun scaffold. Cells seeded on the cotton ball-like scaffold infiltrated into the scaffold after 7 days of growth, compared to no penetrating growth for the traditional electrospun scaffold. Quantitative analysis showed approximately a 40% higher growth rate for cells on the cotton ball-like scaffold over a 7 day period, possibly due to the increased space for in-growth within the three-dimensional scaffolds. Overall, this method assembles a nanofibrous scaffold that is more advantageous for highly porous interconnectivity and demonstrates great potential for tackling current challenges of electrospun scaffolds.

Journal of Polymer Science Part B: Polymer Physics, 1998

The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been exa... more The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been examined using tensile, dynamic mechanical, and creep experiments. This amorphous polymer was shown to have a tensile modulus of 1-1.5 Msi, nearly twice that of most common engineering thermoplastics. The relaxation behavior, which is similar to that of common thermoplastics, can be described by the WLF equation. Outstanding creep resistance was observed at low temperatures, with rubbery-like behavior being exhibited as the temperature approached T g . Physical aging was shown to interact with long-term creep, rendering time-temperature superposition invalid for predicting the long-term properties. The effect of physical aging on the creep behavior was characterized by the shift rate .

Journal of Magnetism and Magnetic Materials, 2010

The delivery of noscapine therapies directly to the site of the tumor would ultimately allow high... more The delivery of noscapine therapies directly to the site of the tumor would ultimately allow higher concentrations of the drug to be delivered, and prolong circulation time in vivo to enhance the therapeutic outcome of this drug. Therefore, we sought to design magnetic based polymeric nanoparticles for the site directed delivery of noscapine to invasive tumors. We synthesized Fe 3 O 4 nanoparticles with an average size of 10 ± 2.5 nm. These Fe 3 O 4 NPs were used to prepare noscapine loaded magnetic polymeric nanoparticles (NMNP) with an average size of 252 ± 6.3 nm. Fourier transform infrared (FT-IR) spectroscopy showed the encapsulation of noscapine on the surface of the polymer matrix. The encapsulation of the Fe 3 O 4 NPs on the surface of the polymer was confirmed by elemental analysis. We studied the drug loading efficiency of polylactide acid (PLLA) and poly (L-lactide acid-co-gylocolide) (PLGA) polymeric systems of various molecular weights. Our findings revealed that the molecular weight of the polymer plays a crucial role in the capacity of the drug loading on the polymer surface. Using a constant amount of polymer and Fe 3 O 4 NPs, both PLLA and PLGA at lower molecule weights showed higher loading efficiencies for the drug on their surfaces.

Composites Science and Technology, 2006

Multiscale fiber-reinforced nanocomposites have been manufactured using a vacuum assisted resin i... more Multiscale fiber-reinforced nanocomposites have been manufactured using a vacuum assisted resin infusion molding (VARIM) process. The nanocomposites prepared were epoxy resin prepolymers dispersed with layered silicates. The effect of silicate loading on the flow, isothermal cure behavior and solid state properties was studied. The addition of silicate results in an increase in resin viscosity by a factor of 10, although it is still within the range of processability by VARIM. A slight decrease in resin gel times is also observed. X-ray diffraction studies of Epoxy 815C nanocomposites indicated an exfoliated morphology for the 2% silicate and an intercalated morphology for the 4% and 6% specimens. Dynamic mechanical analyses were conducted to establish the Tg of the specimens. An improvement of 31% in flexural modulus and 24% in flexural ultimate strength for the 2% silicate fiber-reinforced nanocomposites was achieved. Interlaminar shear and fracture toughness studies were also conducted, however no enhancement was observed.

Biomacromolecules, 2007

Nanofibrous biocomposite scaffolds of type I collagen and nanohydroxyapatite (nanoHA) of varying ... more Nanofibrous biocomposite scaffolds of type I collagen and nanohydroxyapatite (nanoHA) of varying compositions (wt %) were prepared by electrostatic cospinning. The scaffolds were characterized for structure and morphology by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The scaffolds have a porous nanofibrous morphology with random fibers in the range of 500-700 nm diameters, depending on the composition. FT-IR and XRD showed the presence of nanoHA in the fibers. The surface roughness and diameter of the fibers increased with the presence of nanoHA in biocomposite fiber as evident from AFM images. Tensile testing and nanoindendation were used for the mechanical characterization. The pure collagen fibrous matrix (without nanoHA) showed a tensile strength of 1.68 +/- 0.10 MPa and a modulus of 6.21 +/- 0.8 MPa with a strain to failure value of 55 +/- 10%. As the nanoHA content in the randomly oriented collagen nanofibers increased to 10%, the ultimate strength increased to 5 +/- 0.5 MPa and the modulus increased to 230 +/- 30 MPa. The increase in tensile modulus may be attributed to an increase in rigidity over the pure polymer when the hydroxyapatite is added and/or the resulting strong adhesion between the two materials. The vapor phase chemical crosslinking of collagens using glutaraldehyde further increased the mechanical properties as evident from nanoindentation results. A combination of nanofibrous collagen and nanohydroxyapatite that mimics the nanoscale features of the extra cellular matrix could be promising for application as scaffolds for hard tissue regeneration, especially in low or nonload bearing areas.

Polymer, 2006

Drug delivery vectors for sustained release include a variety of polymeric constituents, both nat... more Drug delivery vectors for sustained release include a variety of polymeric constituents, both natural and synthetic. Among synthetic polymers several linear block copolymer systems have been explored for use as drug delivery vectors. Release of the pharmaceutical agent is affected by the degradation characteristics and/or by the swelling of the polymer. The goal of this study is to evaluate the degradation behavior of branched polyethylene oxide polylactide polyether ester as a drug delivery vector. Three samples of a star polyethylene oxide/polylactide copolymer with differing polylactide chain lengths were evaluated by characterizing the thermal properties of the neat polymer and in vitro degradation behavior.The thermal and morphological properties were examined by DSC, TGA and XRD. The in vitro polymeric micelle samples were observed over time by UV–vis, TEM and fluorescence. The four star PEO–PLA polymers have exceptional amphiphilic characteristics, which enable their use for a variety of applications. The polymers are thermally stable at biological conditions. In addition, the star polymers have shorter degradation times as compared to previously reported linear PLA and PEG–PLA copolymers, suggesting use as a short-term drug release agent. The four star PEO/PLA copolymer may be an excellent candidate for drug delivery applications.

Journal of Biomaterials Science-polymer Edition, 2006

Mechanical and morphological studies of aligned nanofibrous meshes of poly(epsilon-caprolactone) ... more Mechanical and morphological studies of aligned nanofibrous meshes of poly(epsilon-caprolactone) (PCL) fabricated by electrospinning at different collector rotation speeds (0, 3000 and 6000 rpm) for application as bone tissue scaffolds are reported. SEM, XRD and DSC analyses were used for the morphological characterization of the nanofibers. Scaffolds have a nanofibrous morphology with fibers (majority) having a diameter in the range of 550-350 nm (depending on fiber uptake rates) and an interconnected pore structure. With the increase of collector rotation speed, the nanofibers become more aligned and oriented perpendicular to the axis of rotation. Deposition of fibers at higher fiber collection speeds has a profound effect on the morphology and mechanical properties of individual fibers and also the bulk fibrous meshes. Nanoindentation was used for the measurement of nanoscopic mechanical properties of individual fibers of the scaffolds. The hardness and Young's modulus of aligned fibers measured by nanoindentation decreased with collector rotation speeds. This reveals the difference in the local microscopic structure of the fibers deposited at higher speeds. The sequence of nanoscopic mechanical properties (hardness and modulus) of three fibers is PCL at 0 rpm > PCL at 3000 rpm > PCL at 6000 rpm. This may be explained due to the decrease in crystallinity of fibers at higher uptake rates. However, uni-axial tensile properties of (bulk) scaffolds (tensile strength and modulus) increased with increasing collector rotation speed. The average ultimate tensile strength of scaffolds (along the fiber alignment) increased from 2.21 +/- 0.23 MPa for PCL at uptake rate of zero rpm, to a value of 4.21 +/- 0.35 MPa for PCL at uptake rate of 3000 rpm and finally to 9.58 +/- 0.71 MPa for PCL at 6000 rpm. Similarly, the tensile modulus increased gradually from 6.12 +/- 0.8 MPa for PCL at uptake rate of zero rpm, to 11.93 +/- 1.22 MPa for PCL at uptake rate of 3000 rpm and to 33.20 +/- 1.98 MPa for PCL at 6000 rpm. The sequence of macroscopic mechanical properties (tensile strength and modulus) of three fibers, from highest to lowest, is PCL at 0 rpm < PCL at 3000 rpm < PCL at 6000 rpm. This is attributed to the increased fiber alignment and packing and decrease in inter-fiber pore size at higher uptake rates.

Composites Part A-applied Science and Manufacturing, 2009

Vacuum assisted resin infusion molding (VARIM) was used to produce multiscale fiber reinforced co... more Vacuum assisted resin infusion molding (VARIM) was used to produce multiscale fiber reinforced composites (M-FRCs) based on carbon nanofibers dispersed in an epoxy resin. Flexural, interlaminar shear strength (ILSS) and thermomechanical tests are presented for the 0.1 wt% and 1 wt% M-FRCs and compared with the neat fiber reinforced composites (FRCs). Flexural strength and modulus increased (16–20%) and (23–26%), respectively for the 0.1 wt% and 1 wt% M-FRCs when compared to the neat FRCs. ILSS properties increased (6% and 25%) for the 0.1 wt% and 1 wt% M-FRCs, respectively when compared to neat FRCs. The glass transition temperatures (Tg) of both M-FRC samples were 25 °C higher than the neat FRC. Coefficients of thermal expansion (CTE) of the M-FRC samples improved compared to the neat FRC. The improved Tg and CTE properties in the M-FRC samples are attributed to synergistic interactions between the CNF/PNC matrix and glass fibers.

Polymer, 2008

The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing b... more The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing behavior of diglycidyl ether of bisphenol A (DGEBA) epoxy resin was studied using differential scanning calorimetry (DSC), rheology and infrared spectroscopy (IR). Activation energy (Ea) and rate constants (k) obtained from isothermal DSC were the same for the neat resin and fluorinated MWCNT system (47.7 and 47.5 kJ/mol, respectively) whereas samples containing carboxylated MWCNTs exhibited a higher activation energy (61.7 kJ/mol) and lower rate constant. Comparison of the activation energies, rate constants, gelation behavior and vitrification times for all of the samples suggests that the cure mechanisms of the neat resin and fluorinated sample are similar but different from the carboxylated sample. This can be explained by the difference in how the fluorinated nanotubes react with the epoxy resin compared to the carboxylated nanotubes. Although the two systems have different reaction mechanisms, both systems have similar degrees of conversion as calculated from the infrared spectroscopic data, glass transition temperature (Tg), and predictions based on DSC data. This difference in reaction mechanism may be attributed to differences in nanotube dispersion; the fluorinated MWCNT system is more uniformly dispersed in the matrix whereas the more heterogeneously dispersed carboxylated MWCNTs can hinder mobility of the reactive species and disrupt the reaction stoichiometry on the local scale.

Polymer, 2007

Aligned nanofibrous nanocomposites of Nylon 6 and surface-modified multiwalled carbon nanotubes (... more Aligned nanofibrous nanocomposites of Nylon 6 and surface-modified multiwalled carbon nanotubes (MWNTs) were successfully synthesized via electrospinning, using a rotating mandrel. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM) and dynamic mechanical analysis (DMA) were done to characterize the morphology and properties of the nanofibrous mats. DSC and XRD observations suggested the presence of MWNTs and the high speed take-up facilitated the transformation of Nylon 6 from γ phase crystals to a mixture of α and γ phase crystals. TEM and WAXD were used to characterize the nanotube and molecular orientations, respectively. The storage modulus of the fibers increased significantly although the concentration of MWNTs was relatively low (0.1 and 1.0 wt%). Thus the combination of carbon nanotubes and nanoscale processing results in structural and mechanical enhancements of Nylon 6.

Polymer, 2003

Cyanate ester resins are among the most important engineering thermosetting polymers and have rec... more Cyanate ester resins are among the most important engineering thermosetting polymers and have received attention because of their outstanding physical properties such as low water absorptivity and low outgassing. However, like most thermosets their main drawback is brittleness. Nanocomposites of cyanate esters were prepared by dispersing organically modified layered silicates (OLS) into the resin. Inclusion of only 2.5% by weight of OLS led to a marked improvement in physical and thermal properties (Coefficient of thermal expansion, Tg and effective thermal stability). Most impressively, a 30% increase in both the modulus and toughness was obtained.

Polymer, 2005

The effect of the organoclay nanoparticles on the rheology and development of the morphology and ... more The effect of the organoclay nanoparticles on the rheology and development of the morphology and properties for epoxy/organoclay nanocomposites has been studied. The interlayer spacing increases with the temperature of cure resulting in intercalated morphologies with varying degrees of interlayer expansion, depending on the cure temperature used. Rheological studies of the curing process indicate that intergallery diffusion before curing is essential for exfoliation, before the morphology is frozen in by gelation and vitrification. The maximum increase in modulus was observed for the 2 wt% clay loading. Viscoelastic behavior and mechanical properties of the cured samples were correlated with the morphological and rheological study.

Chemistry of Materials, 2005

The next step forward, transforming polymer nanocomposites from filled-polymer replacements to de... more The next step forward, transforming polymer nanocomposites from filled-polymer replacements to designed and engineered materials, necessitates the development of techniques to transform the random or ill-defined nanoscale morphologies into compositionally and spatially engineered hierarchal structures, paralleling that which underpins conventional continuous-fiber-reinforced composites. By exploiting an orthogonal magnetic susceptibility of montmorillonites (MMTs) from different deposits, a three-dimensional morphology composed of orthogonal alignment of aluminosilicate layers is generated from a mixture of montmorillonites and a uniaxial external magnetic field. Depending on the source, MMTs exhibit remnant magnetization arising from antiferro-and ferrimagnetic impurities and align with layers parallel or perpendicular to the field. Within a few minutes, application of static magnetic fields (1.2 or 11.7 T) induces stable alignment of organically modified MMT within an epoxy resin at room temperature. Structural relaxation is orders of magnitude slower, enabling the alignment to be captured during the subsequent cure. Thermal mechanical measurements demonstrate that morphology manipulation impacts the coefficient of thermal expansion (CTE), decreasing CTE the most in the direction of maximum MMT alignment. Knowledge of the detailed mechanism that leads to a change of the magnetic easy axis within layered silicates opens up opportunities to design novel synthetic layered silicates with unusual magnetic properties.

Polymer, 2007

We report on our attempts to understand the link between the nature of the CNT surface modificati... more We report on our attempts to understand the link between the nature of the CNT surface modification, dispersion in an epoxy resin and the resulting properties. Carboxylated and fluorinated nanotubes were used to synthesize nanocomposites by dispersing them separately in an epoxy resin. Dynamic mechanical analysis, using torsional deformation, was applied both parallel and perpendicular to the long axis of the multiwall nanotubes (MWNTs). Interestingly, for epoxy/MWNT (1 wt%) nanocomposites, the shear moduli in the glassy state were higher for the nanocomposites, and it's highest for the nanocomposites in which the nanotubes are parallel to the direction of applied torque. These nanocomposites also exhibited higher Tgs than the neat resin. In addition, the rubbery plateau modulus (between 150–200 °C) was higher by a factor of three for the nanocomposites. Master curves constructed using time–temperature superposition allowed us to probe low frequency dynamic moduli and further discern differences in the relaxation behavior. Samples containing fluorinated nanotubes exhibited the highest Tgs, longest relaxation times and highest activation energies relative to the carboxylated nanotube samples and the neat resin, indicative of stronger interactions. SEM and TEM studies confirmed the nanotube dispersion and alignment.

Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 1999

ABSTRACT

Journal of magnetism and magnetic materials, 2010

The delivery of noscapine therapies directly to the site of the tumor would ultimately allow high... more The delivery of noscapine therapies directly to the site of the tumor would ultimately allow higher concentrations of the drug to be delivered, and prolong circulation time in vivo to enhance the therapeutic outcome of this drug. Therefore, we sought to design magnetic based polymeric nanoparticles for the site directed delivery of noscapine to invasive tumors. We synthesized Fe 3 O 4 nanoparticles with an average size of 10 ± 2.5 nm. These Fe 3 O 4 NPs were used to prepare noscapine loaded magnetic polymeric nanoparticles (NMNP) with an average size of 252 ± 6.3 nm. Fourier transform infrared (FT-IR) spectroscopy showed the encapsulation of noscapine on the surface of the polymer matrix. The encapsulation of the Fe 3 O 4 NPs on the surface of the polymer was confirmed by elemental analysis. We studied the drug loading efficiency of polylactide acid (PLLA) and poly (L-lactide acid-co-gylocolide) (PLGA) polymeric systems of various molecular weights. Our findings revealed that the molecular weight of the polymer plays a crucial role in the capacity of the drug loading on the polymer surface. Using a constant amount of polymer and Fe 3 O 4 NPs, both PLLA and PLGA at lower molecule weights showed higher loading efficiencies for the drug on their surfaces.

The next revolutionary leap forward for polymer nano-'composites' necessitates the development of... more The next revolutionary leap forward for polymer nano-'composites' necessitates the development of tools to transform the currently random or ill-defined nanoscale morphologies into compositionally and spatially engineered hierarchal structures, paralleling that underpinning conventional continuous fiber reinforced composites and enabling experimental verification of morphology-mechanical property correlations. Here in, utilization of AC electric fields of modest strength (1-10 V/micron) is demonstrated as a general approach to align organically modified layered silicates (OLS) parallel to the electric field - enhancing CTE, modulus and optical clarity in the reinforced direction. Interfacial polarization arising from electrophoric motion of the organic-modifier on the layer surface induces a dipole parallel to the plane of the layer, which couples to the external field. Comparably, application of a static magnetic field (0.5-2 T) induces alignment of the layers parallel or perpendicular to the field, depending on the chemical composition of the OLS. The impact of field magnitude, field frequency, dielectric permittivity and magnetic permeability of the system is discussed to elucidate the molecular characteristics of induced dipole formation and establish the limits of the process.

Biofabrication, 2009

Nanofibrous electrospun poly (epsilon-caprolactone) (ePCL) scaffolds have inherent structural adv... more Nanofibrous electrospun poly (epsilon-caprolactone) (ePCL) scaffolds have inherent structural advantages, but lack of bioactivity has limited their usefulness in biomedical applications. Thus, here we report the development of a hybrid, nanostructured, extracellular matrix (ECM) mimicking scaffold by a combination of ePCL nanofibers and self-assembled peptide amphiphile (PA) nanofibers. The PAs have ECM mimicking characteristics including a cell adhesive ligand (RGDS) and matrix metalloproteinase-2 (MMP-2) mediated degradable sites. Transmission electron microscope imaging verified successful PA self-assembly into nanofibers (diameters of 8-10 nm) using a solvent evaporation method. This evaporation method was then used to successfully coat PAs onto ePCL nanofibers (diameters of 300-400 nm), to develop hybrid, bioactive scaffolds. Scanning electron microscope characterization showed that the PA coatings did not interfere with the porous ePCL nanofiber network. Human mesenchymal stem cells (hMSCs) were seeded onto the hybrid scaffolds to evaluate their bioactivity. Significantly greater attachment and spreading of hMSCs were observed on ePCL nanofibers coated with PA-RGDS as compared to ePCL nanofibers coated with PA-S (no cell adhesive ligand) and uncoated ePCL nanofibers. Overall, this novel strategy presents a new solution to overcome the current bioactivity challenges of electrospun scaffolds and combines the unique characteristics of ePCL nanofibers and self-assembled PA nanofibers to provide an ECM mimicking environment. This has great potential to be applied to many different electrospun scaffolds for various biomedical applications.

Polymer, 2010

The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nan... more The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites processed with a magnetic field have been studied. Samples were prepared by dispersing the nanotube in the epoxy and curing under an applied magnetic field. The nanocomposite morphology was characterized with Raman spectroscopy and wide angle X-ray scattering, and correlated with thermo-mechanical properties. The modulus parallel to the alignment direction, as measured by dynamic mechanical analysis, showed significant anisotropy, with a 72% increase over the neat resin, and a 24% increase over the sample tested perpendicular to the alignment direction. A modest enhancement in the coefficient of thermal expansion (CTE) parallel to the alignment direction was also observed. These enhancements were achieved even though the nanotubes were not fully aligned, as determined by Raman spectroscopy. The partial nanotube alignment is attributed to resin a gel time that is faster than the nanotube orientation dynamics. Thermal conductivity results are also presented.

Acta Biomaterialia, 2011

Current cardiovascular therapies are limited by the loss of endothelium, restenosis and thrombosi... more Current cardiovascular therapies are limited by the loss of endothelium, restenosis and thrombosis. The goal of this study was to develop a biomimetic hybrid nanomatrix that combined the unique properties of electrospun polycaprolactone (ePCL) nanofibers with self-assembled peptide amphiphiles (PAs). ePCL nanofibers have interconnected nanoporous structures, but are hampered by a lack of surface bioactivity to control cellular behavior. It has been hypothesized that PAs could self-assemble onto the surface of ePCL nanofibers and endow them with the characteristic properties of native endothelium. The PAs, which comprised hydrophobic alkyl tails attached to functional hydrophilic peptide sequences, contained enzyme-mediated degradable sites coupled to either endothelial cell-adhesive ligands (YIGSR) or polylysine (KKKKK) nitric oxide (NO) donors. Two different PAs (PA–YIGSR and PA–KKKKK) were successfully synthesized and mixed in a 90:10 (YK) ratio to obtain PA–YK. PA–YK was reacted with pure NO to develop PA–YK–NO, which was then self-assembled onto ePCL nanofibers to generate a hybrid nanomatrix, ePCL–PA–YK–NO. Uniform coating of self-assembled PA nanofibers on ePCL was confirmed by transmission electron microscopy. Successful NO release from ePCL–PA–YK–NO was observed. ePCL–YK and ePCL–PA–YK–NO showed significantly increased adhesion of human umbilical vein endothelial cells (HUVECs). ePCL–PA–YK–NO also showed significantly increased proliferation of HUVECs and reduced smooth muscle cell proliferation. ePCL–PA–YK–NO also displayed significantly reduced platelet adhesion compared with ePCL, ePCL–PA–YK and a collagen control. These results indicate that this hybrid nanomatrix has great potential application in cardiovascular implants.

Biomaterials, 2011

A limiting factor of traditional electrospinning is that the electrospun scaffolds consist entire... more A limiting factor of traditional electrospinning is that the electrospun scaffolds consist entirely of tightly packed nanofiber layers that only provide a superficial porous structure due to the sheet-like assembly process. This unavoidable characteristic hinders cell infiltration and growth throughout the nanofibrous scaffolds. Numerous strategies have been tried to overcome this challenge, including the incorporation of nanoparticles, using larger microfibers, or removing embedded salt or water-soluble fibers to increase porosity. However, these methods still produce sheet-like nanofibrous scaffolds, failing to create a porous three-dimensional scaffold with good structural integrity. Thus, we have developed a three-dimensional cotton ball-like electrospun scaffold that consists of an accumulation of nanofibers in a low density and uncompressed manner. Instead of a traditional flat-plate collector, a grounded spherical dish and an array of needle-like probes were used to create a Focused, Low density, Uncompressed nanoFiber (FLUF) mesh scaffold. Scanning electron microscopy showed that the cotton ball-like scaffold consisted of electrospun nanofibers with a similar diameter but larger pores and less-dense structure compared to the traditional electrospun scaffolds. In addition, laser confocal microscopy demonstrated an open porosity and loosely packed structure throughout the depth of the cotton ball-like scaffold, contrasting the superficially porous and tightly packed structure of the traditional electrospun scaffold. Cells seeded on the cotton ball-like scaffold infiltrated into the scaffold after 7 days of growth, compared to no penetrating growth for the traditional electrospun scaffold. Quantitative analysis showed approximately a 40% higher growth rate for cells on the cotton ball-like scaffold over a 7 day period, possibly due to the increased space for in-growth within the three-dimensional scaffolds. Overall, this method assembles a nanofibrous scaffold that is more advantageous for highly porous interconnectivity and demonstrates great potential for tackling current challenges of electrospun scaffolds.

Journal of Polymer Science Part B: Polymer Physics, 1998

The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been exa... more The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been examined using tensile, dynamic mechanical, and creep experiments. This amorphous polymer was shown to have a tensile modulus of 1-1.5 Msi, nearly twice that of most common engineering thermoplastics. The relaxation behavior, which is similar to that of common thermoplastics, can be described by the WLF equation. Outstanding creep resistance was observed at low temperatures, with rubbery-like behavior being exhibited as the temperature approached T g . Physical aging was shown to interact with long-term creep, rendering time-temperature superposition invalid for predicting the long-term properties. The effect of physical aging on the creep behavior was characterized by the shift rate .

Journal of Magnetism and Magnetic Materials, 2010

The delivery of noscapine therapies directly to the site of the tumor would ultimately allow high... more The delivery of noscapine therapies directly to the site of the tumor would ultimately allow higher concentrations of the drug to be delivered, and prolong circulation time in vivo to enhance the therapeutic outcome of this drug. Therefore, we sought to design magnetic based polymeric nanoparticles for the site directed delivery of noscapine to invasive tumors. We synthesized Fe 3 O 4 nanoparticles with an average size of 10 ± 2.5 nm. These Fe 3 O 4 NPs were used to prepare noscapine loaded magnetic polymeric nanoparticles (NMNP) with an average size of 252 ± 6.3 nm. Fourier transform infrared (FT-IR) spectroscopy showed the encapsulation of noscapine on the surface of the polymer matrix. The encapsulation of the Fe 3 O 4 NPs on the surface of the polymer was confirmed by elemental analysis. We studied the drug loading efficiency of polylactide acid (PLLA) and poly (L-lactide acid-co-gylocolide) (PLGA) polymeric systems of various molecular weights. Our findings revealed that the molecular weight of the polymer plays a crucial role in the capacity of the drug loading on the polymer surface. Using a constant amount of polymer and Fe 3 O 4 NPs, both PLLA and PLGA at lower molecule weights showed higher loading efficiencies for the drug on their surfaces.

Composites Science and Technology, 2006

Multiscale fiber-reinforced nanocomposites have been manufactured using a vacuum assisted resin i... more Multiscale fiber-reinforced nanocomposites have been manufactured using a vacuum assisted resin infusion molding (VARIM) process. The nanocomposites prepared were epoxy resin prepolymers dispersed with layered silicates. The effect of silicate loading on the flow, isothermal cure behavior and solid state properties was studied. The addition of silicate results in an increase in resin viscosity by a factor of 10, although it is still within the range of processability by VARIM. A slight decrease in resin gel times is also observed. X-ray diffraction studies of Epoxy 815C nanocomposites indicated an exfoliated morphology for the 2% silicate and an intercalated morphology for the 4% and 6% specimens. Dynamic mechanical analyses were conducted to establish the Tg of the specimens. An improvement of 31% in flexural modulus and 24% in flexural ultimate strength for the 2% silicate fiber-reinforced nanocomposites was achieved. Interlaminar shear and fracture toughness studies were also conducted, however no enhancement was observed.

Biomacromolecules, 2007

Nanofibrous biocomposite scaffolds of type I collagen and nanohydroxyapatite (nanoHA) of varying ... more Nanofibrous biocomposite scaffolds of type I collagen and nanohydroxyapatite (nanoHA) of varying compositions (wt %) were prepared by electrostatic cospinning. The scaffolds were characterized for structure and morphology by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The scaffolds have a porous nanofibrous morphology with random fibers in the range of 500-700 nm diameters, depending on the composition. FT-IR and XRD showed the presence of nanoHA in the fibers. The surface roughness and diameter of the fibers increased with the presence of nanoHA in biocomposite fiber as evident from AFM images. Tensile testing and nanoindendation were used for the mechanical characterization. The pure collagen fibrous matrix (without nanoHA) showed a tensile strength of 1.68 +/- 0.10 MPa and a modulus of 6.21 +/- 0.8 MPa with a strain to failure value of 55 +/- 10%. As the nanoHA content in the randomly oriented collagen nanofibers increased to 10%, the ultimate strength increased to 5 +/- 0.5 MPa and the modulus increased to 230 +/- 30 MPa. The increase in tensile modulus may be attributed to an increase in rigidity over the pure polymer when the hydroxyapatite is added and/or the resulting strong adhesion between the two materials. The vapor phase chemical crosslinking of collagens using glutaraldehyde further increased the mechanical properties as evident from nanoindentation results. A combination of nanofibrous collagen and nanohydroxyapatite that mimics the nanoscale features of the extra cellular matrix could be promising for application as scaffolds for hard tissue regeneration, especially in low or nonload bearing areas.

Polymer, 2006

Drug delivery vectors for sustained release include a variety of polymeric constituents, both nat... more Drug delivery vectors for sustained release include a variety of polymeric constituents, both natural and synthetic. Among synthetic polymers several linear block copolymer systems have been explored for use as drug delivery vectors. Release of the pharmaceutical agent is affected by the degradation characteristics and/or by the swelling of the polymer. The goal of this study is to evaluate the degradation behavior of branched polyethylene oxide polylactide polyether ester as a drug delivery vector. Three samples of a star polyethylene oxide/polylactide copolymer with differing polylactide chain lengths were evaluated by characterizing the thermal properties of the neat polymer and in vitro degradation behavior.The thermal and morphological properties were examined by DSC, TGA and XRD. The in vitro polymeric micelle samples were observed over time by UV–vis, TEM and fluorescence. The four star PEO–PLA polymers have exceptional amphiphilic characteristics, which enable their use for a variety of applications. The polymers are thermally stable at biological conditions. In addition, the star polymers have shorter degradation times as compared to previously reported linear PLA and PEG–PLA copolymers, suggesting use as a short-term drug release agent. The four star PEO/PLA copolymer may be an excellent candidate for drug delivery applications.

Journal of Biomaterials Science-polymer Edition, 2006

Mechanical and morphological studies of aligned nanofibrous meshes of poly(epsilon-caprolactone) ... more Mechanical and morphological studies of aligned nanofibrous meshes of poly(epsilon-caprolactone) (PCL) fabricated by electrospinning at different collector rotation speeds (0, 3000 and 6000 rpm) for application as bone tissue scaffolds are reported. SEM, XRD and DSC analyses were used for the morphological characterization of the nanofibers. Scaffolds have a nanofibrous morphology with fibers (majority) having a diameter in the range of 550-350 nm (depending on fiber uptake rates) and an interconnected pore structure. With the increase of collector rotation speed, the nanofibers become more aligned and oriented perpendicular to the axis of rotation. Deposition of fibers at higher fiber collection speeds has a profound effect on the morphology and mechanical properties of individual fibers and also the bulk fibrous meshes. Nanoindentation was used for the measurement of nanoscopic mechanical properties of individual fibers of the scaffolds. The hardness and Young's modulus of aligned fibers measured by nanoindentation decreased with collector rotation speeds. This reveals the difference in the local microscopic structure of the fibers deposited at higher speeds. The sequence of nanoscopic mechanical properties (hardness and modulus) of three fibers is PCL at 0 rpm > PCL at 3000 rpm > PCL at 6000 rpm. This may be explained due to the decrease in crystallinity of fibers at higher uptake rates. However, uni-axial tensile properties of (bulk) scaffolds (tensile strength and modulus) increased with increasing collector rotation speed. The average ultimate tensile strength of scaffolds (along the fiber alignment) increased from 2.21 +/- 0.23 MPa for PCL at uptake rate of zero rpm, to a value of 4.21 +/- 0.35 MPa for PCL at uptake rate of 3000 rpm and finally to 9.58 +/- 0.71 MPa for PCL at 6000 rpm. Similarly, the tensile modulus increased gradually from 6.12 +/- 0.8 MPa for PCL at uptake rate of zero rpm, to 11.93 +/- 1.22 MPa for PCL at uptake rate of 3000 rpm and to 33.20 +/- 1.98 MPa for PCL at 6000 rpm. The sequence of macroscopic mechanical properties (tensile strength and modulus) of three fibers, from highest to lowest, is PCL at 0 rpm < PCL at 3000 rpm < PCL at 6000 rpm. This is attributed to the increased fiber alignment and packing and decrease in inter-fiber pore size at higher uptake rates.

Composites Part A-applied Science and Manufacturing, 2009

Vacuum assisted resin infusion molding (VARIM) was used to produce multiscale fiber reinforced co... more Vacuum assisted resin infusion molding (VARIM) was used to produce multiscale fiber reinforced composites (M-FRCs) based on carbon nanofibers dispersed in an epoxy resin. Flexural, interlaminar shear strength (ILSS) and thermomechanical tests are presented for the 0.1 wt% and 1 wt% M-FRCs and compared with the neat fiber reinforced composites (FRCs). Flexural strength and modulus increased (16–20%) and (23–26%), respectively for the 0.1 wt% and 1 wt% M-FRCs when compared to the neat FRCs. ILSS properties increased (6% and 25%) for the 0.1 wt% and 1 wt% M-FRCs, respectively when compared to neat FRCs. The glass transition temperatures (Tg) of both M-FRC samples were 25 °C higher than the neat FRC. Coefficients of thermal expansion (CTE) of the M-FRC samples improved compared to the neat FRC. The improved Tg and CTE properties in the M-FRC samples are attributed to synergistic interactions between the CNF/PNC matrix and glass fibers.

Polymer, 2008

The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing b... more The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing behavior of diglycidyl ether of bisphenol A (DGEBA) epoxy resin was studied using differential scanning calorimetry (DSC), rheology and infrared spectroscopy (IR). Activation energy (Ea) and rate constants (k) obtained from isothermal DSC were the same for the neat resin and fluorinated MWCNT system (47.7 and 47.5 kJ/mol, respectively) whereas samples containing carboxylated MWCNTs exhibited a higher activation energy (61.7 kJ/mol) and lower rate constant. Comparison of the activation energies, rate constants, gelation behavior and vitrification times for all of the samples suggests that the cure mechanisms of the neat resin and fluorinated sample are similar but different from the carboxylated sample. This can be explained by the difference in how the fluorinated nanotubes react with the epoxy resin compared to the carboxylated nanotubes. Although the two systems have different reaction mechanisms, both systems have similar degrees of conversion as calculated from the infrared spectroscopic data, glass transition temperature (Tg), and predictions based on DSC data. This difference in reaction mechanism may be attributed to differences in nanotube dispersion; the fluorinated MWCNT system is more uniformly dispersed in the matrix whereas the more heterogeneously dispersed carboxylated MWCNTs can hinder mobility of the reactive species and disrupt the reaction stoichiometry on the local scale.

Polymer, 2007

Aligned nanofibrous nanocomposites of Nylon 6 and surface-modified multiwalled carbon nanotubes (... more Aligned nanofibrous nanocomposites of Nylon 6 and surface-modified multiwalled carbon nanotubes (MWNTs) were successfully synthesized via electrospinning, using a rotating mandrel. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM) and dynamic mechanical analysis (DMA) were done to characterize the morphology and properties of the nanofibrous mats. DSC and XRD observations suggested the presence of MWNTs and the high speed take-up facilitated the transformation of Nylon 6 from γ phase crystals to a mixture of α and γ phase crystals. TEM and WAXD were used to characterize the nanotube and molecular orientations, respectively. The storage modulus of the fibers increased significantly although the concentration of MWNTs was relatively low (0.1 and 1.0 wt%). Thus the combination of carbon nanotubes and nanoscale processing results in structural and mechanical enhancements of Nylon 6.

Polymer, 2003

Cyanate ester resins are among the most important engineering thermosetting polymers and have rec... more Cyanate ester resins are among the most important engineering thermosetting polymers and have received attention because of their outstanding physical properties such as low water absorptivity and low outgassing. However, like most thermosets their main drawback is brittleness. Nanocomposites of cyanate esters were prepared by dispersing organically modified layered silicates (OLS) into the resin. Inclusion of only 2.5% by weight of OLS led to a marked improvement in physical and thermal properties (Coefficient of thermal expansion, Tg and effective thermal stability). Most impressively, a 30% increase in both the modulus and toughness was obtained.

Polymer, 2005

The effect of the organoclay nanoparticles on the rheology and development of the morphology and ... more The effect of the organoclay nanoparticles on the rheology and development of the morphology and properties for epoxy/organoclay nanocomposites has been studied. The interlayer spacing increases with the temperature of cure resulting in intercalated morphologies with varying degrees of interlayer expansion, depending on the cure temperature used. Rheological studies of the curing process indicate that intergallery diffusion before curing is essential for exfoliation, before the morphology is frozen in by gelation and vitrification. The maximum increase in modulus was observed for the 2 wt% clay loading. Viscoelastic behavior and mechanical properties of the cured samples were correlated with the morphological and rheological study.

Chemistry of Materials, 2005

The next step forward, transforming polymer nanocomposites from filled-polymer replacements to de... more The next step forward, transforming polymer nanocomposites from filled-polymer replacements to designed and engineered materials, necessitates the development of techniques to transform the random or ill-defined nanoscale morphologies into compositionally and spatially engineered hierarchal structures, paralleling that which underpins conventional continuous-fiber-reinforced composites. By exploiting an orthogonal magnetic susceptibility of montmorillonites (MMTs) from different deposits, a three-dimensional morphology composed of orthogonal alignment of aluminosilicate layers is generated from a mixture of montmorillonites and a uniaxial external magnetic field. Depending on the source, MMTs exhibit remnant magnetization arising from antiferro-and ferrimagnetic impurities and align with layers parallel or perpendicular to the field. Within a few minutes, application of static magnetic fields (1.2 or 11.7 T) induces stable alignment of organically modified MMT within an epoxy resin at room temperature. Structural relaxation is orders of magnitude slower, enabling the alignment to be captured during the subsequent cure. Thermal mechanical measurements demonstrate that morphology manipulation impacts the coefficient of thermal expansion (CTE), decreasing CTE the most in the direction of maximum MMT alignment. Knowledge of the detailed mechanism that leads to a change of the magnetic easy axis within layered silicates opens up opportunities to design novel synthetic layered silicates with unusual magnetic properties.

Polymer, 2007

We report on our attempts to understand the link between the nature of the CNT surface modificati... more We report on our attempts to understand the link between the nature of the CNT surface modification, dispersion in an epoxy resin and the resulting properties. Carboxylated and fluorinated nanotubes were used to synthesize nanocomposites by dispersing them separately in an epoxy resin. Dynamic mechanical analysis, using torsional deformation, was applied both parallel and perpendicular to the long axis of the multiwall nanotubes (MWNTs). Interestingly, for epoxy/MWNT (1 wt%) nanocomposites, the shear moduli in the glassy state were higher for the nanocomposites, and it's highest for the nanocomposites in which the nanotubes are parallel to the direction of applied torque. These nanocomposites also exhibited higher Tgs than the neat resin. In addition, the rubbery plateau modulus (between 150–200 °C) was higher by a factor of three for the nanocomposites. Master curves constructed using time–temperature superposition allowed us to probe low frequency dynamic moduli and further discern differences in the relaxation behavior. Samples containing fluorinated nanotubes exhibited the highest Tgs, longest relaxation times and highest activation energies relative to the carboxylated nanotube samples and the neat resin, indicative of stronger interactions. SEM and TEM studies confirmed the nanotube dispersion and alignment.