Gary E. Wnek

Release of tetracycline hydrochloride from electrospun poly(ethylene-co-vinylacetate), poly(lactic acid), and a blend.

Electrospun fiber mats are explored as drug delivery vehicles using tetracycline hydrochloride as a model drug. The mats were made either from poly(lactic acid) (PLA), poly(ethylene-co-vinyl acetate) (PEVA), or from a 50:50 blend of the two. The fibers were electrospun from chloroform solutions containing a small amount of methanol to solubilize the drug. The release of the tetracycline hydrochloride from these new drug delivery systems was followed by UV-VIS spectroscopy. Release profiles from the electrospun mats were compared to a commercially available drug delivery system, Actisite (Alza Corporation, Palo Alto, CA), as well as to cast films of the various formulations.

TAILORING TISSUE ENGINEERING SCAFFOLDS USING ELECTROSTATIC PROCESSING TECHNIQUES: A STUDY OF POLY(GLYCOLIC ACID) ELECTROSPINNING

Poly(glycolic acid) (PGA) has long been a popular polymer in the tissue engineering field. PGA possesses many favorable properties such as biocompatibility, bioabsorbability, and tensile strength. The traditional fiber formation techniques of melt extrusion and cold-drawing are generally limited to fibers of 10–12 μm in diameter. Electrostatic spinning, or electrospinning, is an attractive approach for the production of much smaller diameter fibers which are of interest as tissue engineering scaffolds. We demonstrate the ability to control the fiber diameter of PGA as a function of solution concentration and fiber orientation, as well as show a correlation between the fiber orientation, elastic modulu, and strain to failure of PGA in a uniaxial model.

Electrospinning of poly (ethylene-co-vinyl alcohol) fibers

Solutions of poly(ethylene-co-vinyl alcohol) or EVOH, ranging in composition from 56 to 71 wt% vinyl alcohol, can be readily electrospun at room temperature from solutions in 70% 2-propanol/water (rubbing alcohol). The solutions are prepared at 80 degrees C and allowed to cool to room temperature. Interestingly, the solutions are not stable at room temperature and eventually the polymer precipitates after several hours. However, prior to precipitation, electrospinning is extensive and rapid, allowing coverage of fibers on various substrates, including a grounded metal plate, dielectrics interposed between the charged jet and the metal ground, and on the human body. Fiber diameters of ca. 0.2-8.0 microm were obtained depending upon the solution concentration, an attractive range for tissue engineering, wound healing, and related applications. Electrospun EVOH mats have been shown to support the culturing of smooth muscle cells and fibroblasts.

Electrospinning of Collagen Type II: A Feasibility Study

Collagen is the natural scaffolding found in all tissues and has been explored extensively for use as a tissue engineering scaffold with limited success. In this feasibility study, the electrospinning of collagen type II and subsequent chondrocyte seeding was investigated for potential use in cartilage tissue engineering. The electrospinning process utilized lyophilized, chicken sternal cartilage collagen type II suspended in 1,1,1,3,3,3 hexaflouro-2-propanol and demonstrated that collagen type II could be electrospun to form nonwoven fibrous mats composed of type II fibers that ranged from 110 nm to 1.8μm in diameter. The fiber diameter was dependant on the type II concentration in solution with a higher concentration producing the larger diameters. The preliminary chondrocyte seeding study demonstrated that electrospun collagen type II scaffolds support cell growth and are readily infiltrated. In conclusion, the feasibility of collagen type II electrospinning has been demonstrated and the novel scaffolds produced are composed of nano- to micron-scale fiber diameters that have been shown to be compatible with chondrocytes.

Electrospinning of collagen nanofiber scaffolds from benign solvents.

Nanofiber scaffolds of collagen have been fabricated via electrospinning using benign solvent systems as a replacement for 1,1,1,3,3,3 hexafluoro-2-propanol. Simple binary mixtures of phosphate-buffered saline and ethanol have been found to be highly effective for electrospinning. FTIR spectra suggest that the triple helical structure of collagen was conserved after dissolution and electrospinning. Crosslinking of the electrospun collagen scaffolds was achieved with standard methods.

Arterial smooth muscle cell proliferation on a novel biomimicking, biodegradable vascular graft scaffold

JOEL D. STITZEL,1 KRISTIN J. PAWLOWSKI,1 GARY E. WNEK,2 DAVID G. SIMPSON3 AND GARY L. BOWLIN1,* 1Dept. of Biomedical Engineering, Virginia Commonwealth University, P.O. Box 980694, Richmond, VA 23298-0694 2Dept. of Chemical Engineering, Virginia Commonwealth University, P.O. Box 843028, Richmond, VA 23298-3028 3Dept. of Anatomy, Virginia Commonwealth University, P.O. Box 980709, Richmond, VA 23298-0709

Electrically conductive polymer composites: polymerization of acetylene in polyethylene

Abstract Composites of low density polyethylene, LDPE, and polyacetylene, (CH)x, were prepared by polymerization of acetylene in LDPE films impregnated with a Ti(OBu) 4 Et 3 Al Ziegler-Natta catalyst. LDPE films were immersed in a toluene solution of this catalyst at 70°C to accomplish impregnation. Polymerization of acetylene in the LDPE films was carried out at 100°–110°C. The resulting composite films remain flexible and tough upon prolonged air exposure and those containing

A proposal for the mechanism of conduction in polyaniline

Abstract Polyaniline is known to become a respectable conductor of electricity (σ ∼ 10 S/cm) upon treatment with simple Bronsted acids such as HCl. A mechanism is proposed which aims to explain the conductivity increase upon acid exposure in terms of well-established criteria for conduction in ‘organic metals’, namely intermolecular π-overlap and fractional oxidation (mixed-valence). It is suggested that the principal mode of conduction involves the intermediacy of a semiquinone radical cation. The credibility of this mechanism with respect to observations in the literature is discussed.

Conduction mechanisms in polyaniline (emeraldine salt)

Abstract The conductive emeraldine salt form of polyaniline features a partially filled valence band. Conduction parameters can therefore be evaluated from linear sweep voltammetry data on the further assumption that charge transport occurs via a hopping mechanism. From cyclic voltammograms the density of states at the Fermi level is estimated at 0.6 states/(eV polaron). Similarly the mobility is of the order of 0.2 m 2 V −1 s −1 . This low value indicates that the electronic states are localized, consistent with the assumption of a hopping mechanism. This localization may be the result of structural and dynamic disorder inherent in the polyaniline system. Proton exchange with absorbed moisture and proton tautomerism represent forms of dynamic disorder affecting conductivity.

The use of progenitor cell/biodegradable MMP2-PLGA polymer constructs to enhance cellular integration and retinal repopulation.

The inability of the adult mammalian retina to regenerate can be partly attributed to the expression of injury-induced inhibitory extracellular matrix (ECM) and cell adhesion molecules. In particular, photoreceptor degeneration stimulates deposition of the inhibitory ECM proteins neurocan and CD44 at the outer limits of the dystrophic retina, where they act as a barrier against cellular migration and axonal extension. We have previously shown that degradation of these molecules, via induction of MMP2, promotes host-donor integration and retinal repopulation following transplantation. Here we present a biodegradable/biocompatible polymer scaffold that has the ability to deliver MMP2, in conjunction with retinal progenitor cells, directly to the site of retinal injury in an attempt to enhance cellular integration and promote retinal repopulation. Pre-activated MMP2, loaded into a PLGA polymer, maintained its activity throughout polymer fabrication and hydrolysis. Following delivery, significant degradation of CD44 and neurocan from the outer limits of the dystrophic retina, without further disruption of retinal architecture, was observed. As a result, the number of retinal progenitor cells that migrated beyond the glial barrier into the degenerating host increased significantly. These cells took up residence in the retinal outer nuclear layer, adopted appropriate photoreceptor morphology and expressed the mature photoreceptor markers recoverin and rhodopsin. Thus, we have created a cell delivery platform that upon transplantation provides controlled release of active-MMP2 directly to the site of retinal injury, stimulating inhibitory ECM barrier removal and enhancement of stem cell integration and retinal repopulation.