Elastic/viscoelastic polymer bilayers: a model-based approach to stretch-responsive constructs.

The use of polymers in the fabrication of bilayers for stimuli-responsive systems is well-known, yet viscoelasticity and viscoelastic models representing bilayer behavior have received surprisingly little attention. Of particular recent interest to us are simple polymeric bilayers in which one material, such as styrene-ethylene-propylene-styrene (SEPS) or styrene-isobutylene-styrene (SIBS), shows typical rubbery elastic response upon extension and retraction, and the other, an unvulcanized, low-Tg polymer such as butyl rubber (butyl), exhibits a viscoelastic response. When such a bilayer strip is extended to a fixed strain and held for several seconds followed by sudden release of this strain, rapid curling is observed, achieving a maximum curvature within 1 second, with a gradual uncurling, typically taking 300-600 seconds to eventually return to a flat strip. Attention has been directed to modeling the observed bilayer behavior. We compare predicted curvature and relaxation time constants from finite element analysis (FEA) simulations using Maxwell, Zener, Generalized Maxwell, and Parallel Rheological Framework (PRF) viscoelastic models to the experimentally measured values. We find that the Generalized Maxwell model predicts curvature over time with the lowest overall mean absolute scaled error (MASE) of 0.519, corresponding to a 4.9% difference from the second lowest error model and a 76.8% difference from the highest error model. Building upon an understanding of the material mechanics in simple bilayer strips, more complex bilayer systems can be designed. Samples of cross and weave geometries were fabricated from bilayer films and initial testing demonstrates how these materials can be used in potential applications.

Polymeric Nanofiber/Antifungal Formulations Using a Novel Co-extrusion Approach.

We report the successful implementation of a novel melt co-extrusion process to fabricate ca. 1 µm diameter fibers of poly(caprolactone) (PCL) containing the antifungal compound clotrimazole in concentrations between 4 and 8 wt%. The process involves co-extrusion of a clotrimazole-loaded PCL along with poly(ethylene oxide) (PEO) as a co-feed, with subsequent removal of PEO to isolate PCL-clotrimazole fibers. In vitro tests of the clotrimazole-containing fibers against the fungus Aspergillus fumigatus, Candida albicans, and Trichophyton mentagrophytes strains demonstrated good antifungal activity which was maintained for more than 3 weeks. An in vivo study using a mouse model showed the lowest tissue fungal burden for PCL-clotrimazole when compared to a PCL-only patch and untreated controls. Comparative studies were conducted with clotrimazole-containing PCL fibers fabricated by electrospinning. Our data showed that the co-extruded, clotrimazole-containing fibers maintain activity for longer times vs. electrospun samples. This, coupled with the much higher throughput of the co-extrusion process vs. electrospinning, renders this new approach very attractive for the fabrication of drug-releasing polymer fibers.

Bio-Mimicking, Electrical Excitability Phenomena Associated With Synthetic Macromolecular Systems: A Brief Review With Connections to the Cytoskeleton and Membraneless Organelles.

Electrical excitability of cells, tissues and organs is a fundamental phenomenon in biology and physiology. Signatures of excitability include transient currents resulting from a constant or varying voltage gradient across compartments. Interestingly, such signatures can be observed with non-biologically-derived, macromolecular systems. Initial key literature, dating to roughly the late 1960's into the early 1990's, is reviewed here. We suggest that excitability in response to electrical stimulation is a material phenomenon that is exploited by living organisms, but that is not exclusive to living systems. Furthermore, given the ubiquity of biological hydrogels, we also speculate that excitability in protocells of primordial organisms might have shared some of the same molecular mechanisms seen in non-biological macromolecular systems, and that vestigial traces of such mechanisms may still play important roles in modern organisms' biological hydrogels. Finally, we also speculate that bio-mimicking excitability of synthetic macromolecular systems might have practical biomedical applications.

Oriented Tapes of Incompatible Polymers Using a Novel Multiplication Co-Extrusion Process.

Continuous tapes of polypropylene (PP) and high-density polyethylene (HDPE) were produced using a novel multiplication co-extrusion process. The structure of the PP/HDPE tapes consists of co-continuous PP and HDPE domains aligned in the extrusion direction, forming a fiber-like composite structure with individual domain thicknesses of 200-500 nm. This unique structure created a significantly large contact interface between the polymer domains. AFM images suggest strong interfacial interactions between incompatible PP and HDPE domains. Orientation at 130 °C was possible due to the enhanced adhesion arising from epitaxial crystallization and the large interfacial area. The modulus, tensile strength, and orientation factor of the oriented composite tapes increased as the draw ratio increased. The existence of two independent shish kabab-like morphologies in the oriented tapes at different draw ratios was indicated by the appearance of two melting peaks for each material. After one-step orientation at 130 °C to a draw ratio of 25, the moduli of the oriented tapes increased to approximately 10 GPa, and the tensile strength increased to approximately 540 MPa. These oriented tapes are stiffer and stronger than commercial tapes and do not fibrillate during the orientation process indicating some interfacial interaction between the domains.

Critical Salt Loading in Flexible Poly(vinyl alcohol) Sensors Fabricated by an Inkjet Printing and Plasma Reduction Method.

We report a low-temperature inkjet printing and plasma treatment method using silver nitrate ink that allows the fabrication of conductive silver traces on poly(vinyl alcohol) (PVA) film with good fidelity and without degrading the polymer substrate. In doing so, we also identify a critical salt loading in the film that is necessary to prevent the polymer from reacting with the silver nitrate-based ink, which improves the resolution of the silver trace while simultaneously lowering its sheet resistance. Silver lines printed on PVA film using this method have sheet resistances of around 0.2 Ω/□ under wet/dry and stretched/unstretched conditions, while PVA films without prior treatment double in sheet resistance upon wetting or stretching the substrate. This low resistance of printed lines on salt-treated films can be preserved under multiple bending cycles of 0-90° and stretching cycles of 0-6% strain if the polymer is prestretched prior to inkjet printing.

Microcapillary Reactors via Coaxial Electrospinning: Fabrication of Small Poly(Acrylic Acid) Gel Beads and Thin Threads of Biological Cell Dimensions.

Poly(acrylic acid) (PAA) bulk gels and threads, typically derived via free-radical polymerization, are of interest as anionic polyelectrolyte mimics of cellular cytosol and as models for early protocells. The thread dimensions have been limited by the diameters of readily-available glass or plastic capillaries, and threads with diameters of less than 50 µm have been difficult to achieve. Here, we report a useful approach for achieving crosslinked, partially neutralized PAA, namely poly(acrylate), gel threads with diameters of a few microns when dry. This technique utilizes coaxial electrospinning to effectively produce capillaries (shells) of polystyrene loaded with a gel-forming precursor mixture composed of 3 M acrylic acid, methylene-bisacrylamide, potassium persulfate and 2.2 M NaOH in the core, followed by thermally-induced polymerization and then the removal of the polystyrene shell. Relatively long (up to 5 mm), continuous PAA threads with thicknesses of 5-15 µm are readily obtained, along with a multitude of PAA gel particles, which result from the occasional break-up of the fluid core prior to gel formation during the electrospinning process. The threads and beads are of the sizes of interest to model ancient protocells, certain functional aspects of excitable cells, such as myocytes and neurons, and various membraneless organelles.

Physically-cross-linked poly(vinyl alcohol) cell culture plate coatings facilitate preservation of cell-cell interactions, spheroid formation, and stemness.

We employed aqueous solutions of highly-hydrolyzed (>99+%) poly(vinyl alcohol), PVA, to coat plastic dishes as a method to efficiently induce three-dimensional (3D) culturing of cells. The coatings were prepared by simple evaporation of 3 wt/vol% solutions of PVA in water and require no additional processing steps after air drying under sterile conditions. The coating allows spheroids to form in solution. Spheroid formation is usually preferable to two-dimensional (2D) culturing as it creates a more realistic ex vivo model of some human tissues and tumors. Using PVA-coated cell culture plates, we demonstrated that we can grow reproducibly sized spheroids using several human glioma cell lines, including LN229, U87 MG, and Gli36, and the embryonic kidney cell line, 293T. Spheroids formed on PVA-coated plates grow as well as on other commercially-available, low-attachment plates, and have excellent optical imaging properties. As spheroids, LN229 cells express markers of cancer stem cells. Finally, we confirmed that spheroids generated on PVA-coated plates are sensitive to molecular perturbations, as increased expression of the cell adhesion molecule PTPµ significantly increased the size of spheroids. The PVA hydrogel layer is an effective tool for creating a more realistic ex vivo culture system than traditional 2D culture and can be used to generate cell spheroids for potential application in drug screening and personalized medicine for diseases such as cancer.

Intumescent, Epoxy-Based Flame-Retardant Coatings Based on Poly(acrylic acid) Compositions.

Intumescent coatings expand upon exposure to a flame to create a protective char layer between the flame and underlying substrate. Widely used commercially, these coatings are applied notably to steel load-bearing beams, significantly extending their time to failure. Boric acid has proved to be a particularly effective additive in the formulation in these coatings, although regulatory concerns are driving an urgent need for more environmentally friendly additives. We report here the characterization of poly(acrylic acid) (PAA) for its use as a novel material in flame-retardant and intumescent coatings. Thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC) were performed on the novel flame-retardant additives to evaluate individual degradation mechanisms and heat release rates. Promising compositions were immobilized in an epoxy binder and formulated with other intumescent additives such as ammonium polyphosphate (APP) and melamine (MEL) to evaluate performance in a coating system. These formulations were then evaluated via quantitative cone calorimetry. Particular PAA-containing formulations show peak heat release rates (PHRR) and total heat release (THR) of 283 kW/m2 and 50.5 MJ/m2, respectively, which compare favorably to data for BA-containing systems, specifically PHRR = 229 kW/m2 and THR = 43.1 MJ/m2. Results showed promise and need for further investigation into PAA as a multifunctional additive for use in flame-retardant and intumescent coatings.

Antimicrobial Activity of Silver Containing Crosslinked Poly(Acrylic Acid) Fibers.

: Bacterial and fungal pathogens have caused serious problems to the human health. This is particularly true for untreatable infectious diseases and clinical situations where there is no reliable treatment for infected patients. To increase the antimicrobial activity of materials, we introduce silver nanoparticle (NP) patches in which the NPs are incorporated to the surface of smooth and uniform poly(acrylic acid) (PAA) nanofibers. The PAA nanofibers were thermally crosslinked with ethylene glycol via heat treatment through a mild method. The characterization of the resulting PAA-silver NP patches was done using scanning electron microscopy (SEM), UV spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). To demonstrate the antimicrobial activity of PAA, we incorporated the patches containing the silver NPs into strains of fungi such as Candida albicans (C. albican) and bacteria such as Methicillin-resistant Staphylococcus aureus (MRSA). The PAA-silver fibers achieved zones of inhibition against C. albicans and MRSA indicating their antimicrobial activity against both fungi and bacteria. We conclude that silver NP patches exhibited multiple inhibitory actions for the interruption and blockage of activity fungal and bacterial strains, which has the potential as an antimicrobial agent in infectious diseases. Moreover, the proposed material has the potential to be used in antimicrobial textile fabrics, food packaging films, and wound dressings.

Mechanical properties of electrospun fibrinogen structures.

Fibrin and fibrinogen have a well-established track record in tissue engineering due to their innate ability to induce improved cellular interaction and subsequent scaffold remodeling compared to synthetic scaffolds. Use of fibrinogen as a primary scaffold component, however, has been limited by traditional processing techniques that render scaffolds with insufficient mechanical properties. The goal of this study was to demonstrate, based on mechanical properties, that electrospun fibrinogen overcomes these limitations and can be successful as a tissue engineering scaffold or wound dressing. Electrospun fibrinogen scaffolds were characterized for fiber diameter and pore area and subsequently tested for uniaxial mechanical properties while dry and hydrated. In addition, uniaxial mechanical testing was conducted on scaffolds treated to regulate scaffold degradation in serum-containing media by supplementing the media with aprotinin or cross-linking the scaffolds with glutaraldehyde vapor. A linear relationship between electrospinning solution concentration and measured fiber diameter was seen; fiber diameters ranged from 120 to 610 nm over electrospinning concentrations of 80 to 140 mg/ml fibrinogen, respectively. Pore areas ranged from 1.3 microm(2) to 13 microm(2) over the same fibrinogen concentrations. Aprotinin in the culture media inhibited scaffold degradation in a predictable fashion, but glutaraldehyde vapor fixation produced less reliable results as evidenced by mechanical property testing. In conclusion, the mechanical characteristics of electrospun fibrinogen strongly support its potential use as a tissue engineering scaffold or wound dressing.

Artificial Polymeric Scaffolds as Extracellular Matrix Substitutes for Autologous Conjunctival Goblet Cell Expansion.

PURPOSE: We fabricated and investigated polymeric scaffolds that can substitute for the conjunctival extracellular matrix to provide a substrate for autologous expansion of human conjunctival goblet cells in culture. METHODS: We fabricated two hydrogels and two silk films: (1) recombinant human collagen (RHC) hydrogel, (2) recombinant human collagen 2-methacryloylxyethyl phosphorylcholine (RHC-MPC) hydrogel, (3) arginine-glycine-aspartic acid (RGD) modified silk, and (4) poly-D-lysine (PDL) coated silk, and four electrospun scaffolds: (1) collagen, (2) poly(acrylic acid) (PAA), (3) poly(caprolactone) (PCL), and (4) poly(vinyl alcohol) (PVA). Coverslips and polyethylene terephthalate (PET) were used for comparison. Human conjunctival explants were cultured on scaffolds for 9 to 15 days. Cell viability, outgrowth area, and the percentage of cells expressing markers for stratified squamous epithelial cells (cytokeratin 4) and goblet cells (cytokeratin 7) were determined. RESULTS: Most of cells grown on all scaffolds were viable except for PCL in which only 3.6 ± 2.2% of the cells were viable. No cells attached to PVA scaffold. The outgrowth was greatest on PDL-silk and PET. Outgrowth was smallest on PCL. All cells were CK7-positive on RHC-MPC while 84.7 ± 6.9% of cells expressed CK7 on PDL-silk. For PCL, 87.10 ± 3.17% of cells were CK7-positive compared to PET where 67.10 ± 12.08% of cells were CK7-positive cells. CONCLUSIONS: Biopolymer substrates in the form of hydrogels and silk films provided for better adherence, proliferation, and differentiation than the electrospun scaffolds and could be used for conjunctival goblet cell expansion for eventual transplantation once undifferentiated and stratified squamous cells are included. Useful polymer scaffold design characteristics have emerged from this study.

Electrospinning polydioxanone for biomedical applications.

Polydioxanone (PDS) is a colorless, crystalline, bioabsorbable polymer that was first developed specifically for wound closure sutures. The compatibility, degradation rate, and mechanical properties (including shape memory) of PDS are of interest when considering the design of tissue engineering scaffolds. This research presents the electrospinning of PDS to fabricate unique nanofibrous structures for a variety of biomedical applications. Electrospinning is a polymer processing technique that utilizes an electric field to form fibers from a polymer solution or melt and allows the fabrication of nanofibrous non-woven structures. Results demonstrate the ability to control the fiber diameter of PDS as a function of solution concentrations and the fiber orientation with our prototype electrospinning apparatus. The results also show dependence between the fiber orientation and the elastic modulus, peak stress, and strain to failure of PDS in a uniaxial model.

Electrospinning collagen and elastin: preliminary vascular tissue engineering.

Significant challenges must be overcome before the true benefit and economic impact of vascular tissue engineering can be fully realized. Toward that end, we have pioneered the electrospinning of micro- and nano-fibrous scaffoldings from the natural polymers collagen and elastin and applied these to development of biomimicking vascular tissue engineered constructs. The vascular wall composition and structure is highly intricate and imparts unique biomechanical properties that challenge the development of a living tissue engineered vascular replacement that can withstand the high pressure and pulsatile environment of the bloodstream. The potential of the novel scaffold presented here for the development of a viable vascular prosthetic meets these stringent requirements in that it can replicate the complex architecture of the blood vessel wall. This replication potential creates an "ideal" environment for subsequent in vitro development of a vascular replacement. The research presented herein provides preliminary data toward the development of electrospun collagen and elastin tissue engineering scaffolds for the development of a three layer vascular construct.

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.

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.

Utilizing acid pretreatment and electrospinning to improve biocompatibility of poly(glycolic acid) for tissue engineering.

Poly(glycolic acid) (PGA) has a long history as a bioresorbable polymer. Its biocompatibility is widely accepted, yet PGA is often rejected as a soft-tissue scaffold because of fibrous encapsulation. The goal of this study was to improve the soft-tissue biocompatibility of PGA by producing scaffolds composed of small-diameter fibers through electrospinning and subjecting these scaffolds to a concentrated hydrochloric acid (HCL) pretreatment. The theory is that small-diameter fibers will elicit a reduced immune response and HCl treatment will improve cellular interactions. Scaffolds were characterized in terms of fiber diameter and pore area via image-analysis software. Biocompatibility was assessed through a WST-1 cell-proliferation assay (in vitro) with the use of rat cardiac fibroblasts and rat intramuscular implantations (in vivo). Fibers produced ranged in diameter from 0.22 to 0.88 microm with pore areas from 1.84 to 13.22 microm(2). The untreated scaffold composed of 0.88-microm fibers was encapsulated in vivo and supported the lowest rates of cell proliferation. On the contrary, the acid pretreated scaffold with 0.22-microm fibers was incorporated into the surrounding tissue and exhibited proliferation rates that exceeded the control populations on tissue-culture plastic. In conclusion, this study has shown the ability to improve the biocompatibility of PGA through acid pretreatment of scaffolds comprised of submicron fiber diameters.

Electrospun DOXY-h loaded-poly(acrylic acid) nanofiber mats: in vitro drug release and antibacterial properties investigation.

Electrospun DOXY-h loaded-poly(acrylic acid) (PAA) nanofiber mats (PAA/DOXY-h nanofiber mats) were prepared by the electrospinning technique and post-spinning sorption method at various doses: PAA/DOXY-h125, PAA/DOXY-h250, PAA/DOXY-h500, and PAA/DOXY-h1000. The morphology, drug content, release characteristics, and antibacterial activities of the PAA/DOXY-h nanofiber mats were investigated with scanning electron microscopy, UV-vis spectrophotometry, and disc diffusion methodology. The PAA/DOXY-h nanofiber mats had a diameter range of 285-340 nm, and a smooth surface without beads. Adsorption isotherms of DOXY-h could be described well with the Freundlich model. The amounts of DOXY-h, after the post-spinning sorption process, in the PAA/DOXY-h nanofiber mats ranged between 27.57 and 101.71 mg/g. All of the PAA/DOXY-h nanofiber mats exhibited an initial burst release characteristic with cumulative releasing percentages between 37.14 and 45.97%, which followed the Fickian diffusion mechanism. Based on the antibacterial investigation, the tested gram-positive bacteria, Staphylococcus aureus and Streptococcus agalactiae, seemed to be more sensitive to PAA/DOXY-h nanofiber mats than the tested gram-negative bacteria, Pseudomonas aeruginosa. These PAA/DOXY-h nanofiber mats could be used as an antibacterial wound dressing.

Electrospun collagen and its applications in regenerative medicine.

In recent years, electrospinning has increased in popularity as a processing technique for obtaining nanometer-to-micron diameter polymer fibers collected to form a nonwoven scaffold. It possesses the ability to process collagen into nanofibrous scaffolds which have been used for a number of applications, such as artificial vascular grafts and for wound repair. This paper offers a review of some of the basic yet essential aspects of producing nanofibrous scaffolds of collagen by electrospinning. A primer to collagen structure, cross-linking techniques, and electrospinning principles is provided, along with some of the many applications of these unique materials.