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15 Feb 2012

Volume 111, Issue 4, Articles (04xxxx)

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J. Appl. Phys. 111, 043501 (2012); http://dx.doi.org/10.1063/1.3680881 (8 pages)

Gregory J. McGraw and Stephen R. Forrest
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back to top Applied Biophysics

Charge generation, charge transport, and residual charge in the electrospinning of polymers: A review of issues and complications

George Collins, John Federici, Yuki Imura, and Luiz H. Catalani

J. Appl. Phys. 111, 044701 (2012); http://dx.doi.org/10.1063/1.3682464 (18 pages) | Cited 2 times

Online Publication Date: 24 February 2012

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Electrospinning has become a widely implemented technique for the generation of nonwoven mats that are useful in tissue engineering and filter applications. The overriding factor that has contributed to the popularity of this method is the ease with which fibers with submicron diameters can be produced. Fibers on that size scale are comparable to protein filaments that are observed in the extracellular matrix. The apparatus and procedures for conducting electrospinning experiments are ostensibly simple. While it is rarely reported in the literature on this topic, any experience with this method of fiber spinning reveals substantial ambiguities in how the process can be controlled to generate reproducible results. The simplicity of the procedure belies the complexity of the physical processes that determine the electrospinning process dynamics. In this article, three process domains and the physical domain of charge interaction are identified as important in electrospinning: (a) creation of charge carriers, (b) charge transport, (c) residual charge. The initial event that enables electrospinning is the generation of region of excess charge in the fluid that is to be electrospun. The electrostatic forces that develop on this region of charged fluid in the presence of a high potential result in the ejection of a fluid jet that solidifies into the resulting fiber. The transport of charge from the charge solution to the grounded collection device produces some of the current which is observed. That transport can occur by the fluid jet and through the atmosphere surrounding the electrospinning apparatus. Charges that are created in the fluid that are not dissipated remain in the solidified fiber as residual charges. The physics of each of these domains in the electrospinning process is summarized in terms of the current understanding, and possible sources of ambiguity in the implementation of this technique are indicated. Directions for future research to further articulate the behavior of the electrospinning process are suggested.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
87.85.jj Biocompatibility
87.85.Lf Tissue engineering

Direct observation of potassium ions in HeLa cell with ion-selective nano-pipette probe

Tomohide Takami, Futoshi Iwata, Koji Yamazaki, Jong Wan Son, Joo-Kyung Lee, Bae Ho Park, and Tomoji Kawai

J. Appl. Phys. 111, 044702 (2012); http://dx.doi.org/10.1063/1.3688770 (5 pages)

Online Publication Date: 28 February 2012

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The local concentration of potassium ion in a single HeLa cell was observed with an ion-selective nano-pipette probe. Ion selectivity was achieved by using a polyvinyl chloride film with selected ionophores placed within the nano-pipette. Both alternating and constant bias voltages were applied to the counter electrode for the observation of local ion concentrations with a response time of less than 0.1 s. These measurements were enabled by a low-current detection system prepared specifically for this study. The difference in local potassium concentrations between inside a living HeLa cell and the surrounding solution was approximately 100 mM, while no difference in potassium ion concentration was observed between the interior of dead cells and the surrounding solution.
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87.17.-d Cell processes
82.45.-h Electrochemistry and electrophoresis

Roles of silica and lignin in horsetail (Equisetum hyemale), with special reference to mechanical properties

Shigeru Yamanaka, Kanna Sato, Fuyu Ito, Satoshi Komatsubara, Hiroshi Ohata, and Katsumi Yoshino

J. Appl. Phys. 111, 044703 (2012); http://dx.doi.org/10.1063/1.3688253 (6 pages) | Cited 1 time

Online Publication Date: 29 February 2012

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This research deals with detailed analyses of silica and lignin distribution in horsetail with special reference to mechanical strength. Scanning electron images of a cross-section of an internode showed silica deposited densely only around the outer epidermis. Detailed histochemical analyses of lignin showed no lignin deposition in the silica-rich outer internodes of horsetail, while a characteristic lignin deposition was noticed in the vascular bundle in inner side of internodes. To analyze the structure of horsetail from a mechanical viewpoint, we calculated the response of a model structure of horsetail to a mechanical force applied perpendicularly to the long axis by a finite element method. We found that silica distributed in the outer epidermis may play the major structural role, with lignin’s role being limited ensuring that the vascular bundle keep waterproof. These results were in contrast to more modern tall trees like gymnosperms, for which lignin provides mechanical strength. Lignin has the advantage of sticking to cellulose, hemicellulose, and other materials. Such properties make it possible for plants containing lignin to branch. Branching of tree stems aids in competing for light and other atmospheric resources. This type of branching was impossible for ancient horsetails, which relied on the physical properties of silica. From the evolutional view points, over millennia in trees with high lignin content, true branching, and many chlorophyll-containing leaves developed.
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87.19.R- Mechanical and electrical properties of tissues and organs
87.85.jc Electrical, thermal, and mechanical properties of biological matter
87.23.Kg Dynamics of evolution
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