Increasing ionic conductivity within thermoplastics via commercial additives results in a dramatic decrease in fiber diameter from melt electrospinning.

Polyethylene melt conductivity was increased by adding a commercial anti-static agent, which resulted in a 20× decrease in electrospun fiber diameter and formation of a significant fraction of sub-micron diameter fibers. Two polyethylene formulations and varying additive concentrations were utilized to span the parameter space of conductivity and viscosity. The key role of conductivity in determining the jet radius (which sets the upper limit on the fiber size) is discussed in the context of fluid mechanics theory and previous simulations. Parameters which affect the conversion of the liquid jet to a solid fiber and the pertinent theory are outlined. An "unconfined" experimental configuration is utilized to both avoid potential needle clogging and enable direct observation of important characteristic length scales related to the interaction of the fluid and the applied electric field. In this approach, the fluid spontaneously forms an array of cone perturbations which act as stationary "nozzles" through which the mobile fluid flows to form the jet. The experimental data and theory considerations allow for a holistic discussion of the interaction between flow rate, viscosity, conductivity, and the resultant jet and fiber size. Information about the fluid viscosity and conductivity gained by observing the electrospinning process is highlighted. Schemes for theoretically predicting the cone-jet density, cone size, and flow rate are compared to experimental results.

Nanoparticle-based photothermal heating to drive chemical reactions within a solid: using inhomogeneous polymer degradation to manipulate mechanical properties and segregate carbonaceous by-products.

Photothermal heating via metal nanoparticles is utilized to degrade polyethylcyanoacrylate (PECA), which undergoes a thermally-driven depolymerization process, resulting in (i) monomer loss from the sample, (ii) repolymerization to form shorter chains (oligomer), and (iii) formation of carbonaceous by-products which are graphene-like and luminescent. These unique PECA properties are used to demonstrate the heterogeneous temperature distribution present during photothermal processing and the results are compared to degradation via conventional methods where a uniform temperature is present. Photothermal heating results in formation of pockets of depolymerized material around each nanoscale heating site. The characteristic size of these photothermally-generated mechanical defects is determined from changes in the material's tensile strength. Changes in mass loss and molecular weight are utilized to determine the fraction of the sample that has depolymerized: distributing this volume equally to each heating site (based on the nanoparticle concentration) results in a volume that matches the defect size from independent mechanical measurements. In this way, macroscopic measurements elucidate the mesoscopic pattern of photothermal degradation. Sample morphology on scales from millimeters to nanometers is assessed via optical and electron microscopy. The carbonaceous by-products of degradation form in the hot region around each nanoparticle during photothermal heating, as revealed by transmission electron microscopy studies. Heterogeneous heating is also evident from optical images where starch granules, employed as an inert dilute additive to enhance PECA mechanical properties, also become luminescent due to degradation in "hot spots" created by the overlap of warm regions from nearby nanoparticle sites. Beyond the fundamental knowledge gained by these studies, the results demonstrate the ability to manipulate the connection between mechanical properties and chemical degradation which is important for developing new strategies for management of polymeric waste.

Photothermally-driven thermo-oxidative degradation of low density polyethylene: heterogeneous heating plus a complex reaction leads to homogeneous chemistry.

Photothermal heating from embedded nanoparticles, a process whereby visible light is converted into heat resulting in a high temperature in each particle's immediate vicinity, was utilized to degrade low density polyethylene (LDPE) via thermo-oxidation. The spatially-varying steady-state photothermal temperature field is a potential mechanism by which ambient light (e.g. sunlight) could be used to drive chemical reactions within solid materials and may result in a non-uniform pattern of products, an advantage or disadvantage depending on application. Novel approaches to control polymer degradation are of interest because of the goal of remediating plastic waste, including autonomous means to minimize its effect when unconfined in the environment. For thermoplastic auto-oxidation, heterogeneous degradation would likely enhance deleterious micro-fragmentation however, the multi-step, multi-site nature of the reaction mitigated the temperature non-uniformity. A photothermally-heated LDPE nanocomposite with silver nanoparticle and cobalt-stearate additives showed degradation, characterized by ultraviolet-visible and Fourier-transform infrared absorption spectroscopy, electron microscopy, and mechanical testing, nearly identical to that resulting from uniform conventional treatment at the same average temperature.

Nanoscale steady-state temperature gradients within polymer nanocomposites undergoing continuous-wave photothermal heating from gold nanorods.

Anisotropically-shaped metal nanoparticles act as nanoscale heaters via excitation of a localized surface plasmon resonance, utilizing a photothermal effect which converts the optical energy into local heat. Steady-state temperatures within a polymer matrix embedded with gold nanorods undergoing photothermal heating using continuous-wave excitation are measured in the immediate spatial vicinity of the nanoparticle (referred to as the local temperature) from observing the rate of physical rotation of the asymmetric nanoparticles within the locally created polymer melt. Average temperatures across the entire (mostly solid) sample (referred to as the global temperature) are simultaneously observed using a fluorescence method from randomly dispersed molecular emitters. Comparing these two independent measurements in films having varying concentrations of nanorods reveals the interplay between the local and global temperatures, clearly demonstrating the capability of these material samples to sustain large steady-state spatial temperature gradients when experiencing continuous-wave excitation photothermal heating. These results are discussed quantitatively. Illustrative imaging studies of nanofibers under photothermal heating also support the presence of a large temperature gradient. Photothermal heating in this manner has potential utility in creating unique thermal processing conditions for outcomes such as driving chemical reactions, inducing crystallinity changes, or enhancing degradation processes in a manner unachievable by conventional heating methods.

Spatial temperature mapping within polymer nanocomposites undergoing ultrafast photothermal heating via gold nanorods.

Heat emanates from gold nanorods (GNRs) under ultrafast optical excitation of the localized surface plasmon resonance. The steady state nanoscale temperature distribution formed within a polymer matrix embedded with GNRs undergoing pulsed femtosecond photothermal heating is determined experimentally using two independent ensemble optical techniques. Physical rotation of the nanorods reveals the average local temperature of the polymer melt in the immediate spatial volume surrounding each rod while fluorescence of homogeneously-distributed perylene molecules monitors temperature over sample regions at larger distances from the GNRs. Polarization-sensitive fluorescence measurements of the perylene probes provide an estimate of the average size of the quasi-molten region surrounding each nanorod (that is, the boundary between softened polymer and solid material as the temperature decreases radially away from each particle) and distinguishes the steady state temperature in the solid and melt regions. Combining these separate methods enables nanoscale spatial mapping of the average steady state temperature distribution caused by ultrafast excitation of the GNRs. These observations definitively demonstrate the presence of a steady-state temperature gradient and indicate that localized heating via the photothermal effect within materials enables nanoscale thermal manipulations without significantly altering the bulk sample temperature in these systems. These quantitative results are further verified by re-orienting nanorods within a solid polymer nanofiber without inducing any morphological changes to the highly temperature-sensitive nanofiber surface. Temperature differences of 70-90 °C were observed over a distances of ∼ 100 nm.

Application of low-frequency alternating current electric fields via interdigitated electrodes: effects on cellular viability, cytoplasmic calcium, and osteogenic differentiation of human adipose-derived stem cells.

Electric stimulation is known to initiate signaling pathways and provides a technique to enhance osteogenic differentiation of stem and/or progenitor cells. There are a variety of in vitro stimulation devices to apply electric fields to such cells. Herein, we describe and highlight the use of interdigitated electrodes to characterize signaling pathways and the effect of electric fields on the proliferation and osteogenic differentiation of human adipose-derived stem cells (hASCs). The advantage of the interdigitated electrode configuration is that cells can be easily imaged during short-term (acute) stimulation, and this identical configuration can be utilized for long-term (chronic) studies. Acute exposure of hASCs to alternating current (AC) sinusoidal electric fields of 1 Hz induced a dose-dependent increase in cytoplasmic calcium in response to electric field magnitude, as observed by fluorescence microscopy. hASCs that were chronically exposed to AC electric field treatment of 1 V/cm (4 h/day for 14 days, cultured in the osteogenic differentiation medium containing dexamethasone, ascorbic acid, and ß-glycerol phosphate) displayed a significant increase in mineral deposition relative to unstimulated controls. This is the first study to evaluate the effects of sinusoidal AC electric fields on hASCs and to demonstrate that acute and chronic electric field exposure can significantly increase intracellular calcium signaling and the deposition of accreted calcium under osteogenic stimulation, respectively.

Nanofibrous composites for tissue engineering applications.

Development of artificial matrices for tissue engineering is a crucial area of research in the field of regenerative medicine. Successful tissue scaffolds, in analogy with the natural mammalian extracellular matrix (ECM), are multi-component, fibrous, and on the nanoscale. In addition, to this key morphology, artificial scaffolds must have mechanical, chemical, surface, and electrical properties that match the ECM or basement membrane of the specific tissue desired. In particular, these material properties may vary significantly for the four primary tissues in the body: nerve, muscle, epithelial, and connective. In order to address this complex array of attributes with a polymeric material, a nanocomposite approach, employing a blend of materials, addition of a particle to enhance particular properties, or a surface treatment, is likely to be required. In this review, we examine nanocomposite approaches to address these diverse needs as a function of tissue type. The review is intended as a bridge between material scientists and biomedical researchers to give basic background information on tissue biology to the former, and on material processing approaches to the latter, in a general manner, and specifically review fibrous nanocomposite materials that have previously been used for cell studies, either in vivo or in vitro.

Structure of submonolayer oleic acid coverages on inorganic aerosol particles: evidence of island formation.

A series of submonolayer deposition studies of oleic acid on both hydrophobic and hydrophilic surfaces has shown that oleic acid self-associates into islands rather than uniformly covering the surfaces. The studies were performed by vapor deposition on 1.6 mum diameter polystyrene aerosol particles as well as on polystyrene and silica surfaces. The surfaces were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), ellipsometry and contact-angle goniometry. After timescales of minutes to hours of vapor deposition at 70 degrees C, the oleic acid arranged itself in the form of islands with diameters of about 100 nm. Many of the islands are 25-30 A high, suggesting that the oleic acid sits vertically on the surface. The surface structure of oleic acid on particles is expected to impact on several atmospherically relevant properties such as the reactivity of the oleic acid and the hygroscopicity of the particles.

Fabrication and characterization of electrospun chitosan nanofibers formed via templating with polyethylene oxide.

Chitosan is an abundantly common, naturally occurring, polysaccharide biopolymer. Its biocompatible, biodegradable, and antimicrobial properties have led to significant research toward biological applications such as drug delivery, artificial tissue scaffolds for functional tissue engineering, and wound-healing dressings. For applications such as tissue scaffolding, formation of highly porous mats of nanometer-sized fibers, such as those fabricated via electrospinning, may be quite important. Previously, strong acidic solvents and blending with synthetic polymers have been used to achieve electrospun nanofibers containing chitosan. As an alternative approach, in this work, polyethylene oxide (PEO) has been used as a template to fabricate chitosan nanofibers by electrospinning in a core-sheath geometry, with the PEO sheath serving as a template for the chitosan core. Solutions of 3 wt % chitosan (in acetic acid) and 4 wt % PEO (in water) were found to have matching rheological properties that enabled efficient core-sheath fiber formation. After removing the PEO sheath by washing with deionized water, chitosan nanofibers were obtained. Electron microscopy confirmed nanofibers of approximately 250 nm diameter with a clear core-sheath geometry before sheath removal, and chitosan nanofibers of approximately 100 nm diameter after washing. The resultant fibers were characterized with IR spectroscopy and X-ray diffraction, and the mechanical and electrical properties were evaluated.

Dynamics within alkylsiloxane self-assembled monolayers studied by sensitive dielectric spectroscopy.

Self-assembled monolayers are a ubiquitous laboratory tool and have been the subject of many experimental investigations which have primarily focused on static properties of full coverage monolayers, with the maximum density and ordering possible. In this work, dynamics within low density, planar siloxane self-assembled monolayers are studied utilizing highly sensitive dielectric spectroscopy. Dilute, disordered films were intentionally fabricated in order to study the widest range of possible motions. At low coverage, an interacting relaxation is observed, which has similar dynamics to polyethylene-like glass transitions observed in phase-segregated side-chain polymers, despite the rigidity of the substrate and the constraint of ethyl groups in relatively short chains. As density is increased, a second local relaxation, previously observed in three-dimensional SAMs and associated with rotation within a small segment of the alkyl chain, is also observed.

Characterization of electrospun nanocomposite scaffolds and biocompatibility with adipose-derived human mesenchymal stem cells.

Electrospun nanocomposite scaffolds were fabricated by encapsulating multi-walled carbon nanotubes (MWNT) in poly (lactic acid) (PLA) nanofibers. Scanning electron microscopy (SEM) confirmed the fabrication of nanofibers, and transmission electron microscopy identified the alignment and dispersion of MWNT along the axis of the fibers. Tensile testing showed an increase in the tensile modulus for a MWNT loading of 0.25 wt% compared with electrospun nanofibrous mats without MWNT reinforcement. Conductivity measurements indicated that the confined geometry of the fibrous system requires only minute doping to obtain significant enhancements at 0.32 wt%. Adipose-derived human mesenchymal stem cells (hMSCs) were seeded on electrospun scaffolds containing 1 wt% MWNT and 0 wt% MWNT, to determine the efficacy of the scaffolds for cell growth, and the effect of MWNT on hMSC viability and proliferation over two weeks in culture. Staining for live and dead cells and DNA quantification indicated that the hMSCs were alive and proliferating through day 14. SEM images of hMSCs at 14 days showed morphological differences, with hMSCs on PLA well spread and hMSCs on PLA with 1% MWNT closely packed and longitudinally aligned.