Electrical excitation properties

One of the most exciting properties of some materials is superconductivity. Some complex metal oxides have the ability to conduct electricity free of any resistance, and thus free of power loss. Many materials are superconducting at very low temperatures (close to absolute zero), but recent work has moved the so-called transition temperature (where superconducting properties appear) to higher and higher values. There are still no superconductors that can operate at room temperature, but this goal is actively pursued. As more current is passed through... 

Since 2004 [183], graphene research has evolved from a heavily theoretical and fundamental field into a variety of research areas [301]. Its electrical, magnetic, physical-mechanical, and chemical properties position it as the most promising material for molecular electronic and optoelectronic applications, possibly replacing the currently used silicon and metal oxide based devices. Nonetheless, further research is essential in order to control easily such properties and construct devices with specific and novel architectures to explore in depth all of these exciting properties, as well as to achieve the synthesis of large-scale, size- and layer-count controlled graphene. 

In addition, their ability to stabilize electrically excitable membranes (i.e., quinidine-like properties) through inhibition of fast sodium channels is the action most likely responsible for their most important adverse effects cardiotoxicity and neurotoxicity (410). 

Qualitatively, the most transparent type of model, as ever, would be a one-electron model that is capable of rendering both the ground state and, to a high degree, its excitation properties. However, in the present case, accommodations are called for, on both aspects, that are not trivial. These we will try to pursue and represent within the present one-electron-type framework as closely as possible. In seeking to develop the present model, we base it as firmly as possible on the available data, optical, photoemission, electrical, structural, etc. Much of this data is still open to interpretation, and many of the interpretations to follow are made in the light of experience gained with transition metal compounds (2). 

An operational definition of thin-layer electrochemistry is that area of electrochemical endeavor in which special advantage is taken of restricting the diffii-sional field of electroactive species and products. Typically, the solution under study is confined to a well-defined layer, less than 0.2 mm thick, trapped between an electrode and an inert barrier, between two electrodes, or between two inert barriers with an electrode between. Diffusion under this restricted condition has been described in Chapter 2 (Sec. II.C). Solution trapped in a porous-bed electrode will have qualitatively similar electrochemical properties however, geometric complexities make this configuration less useful for analytical purposes. The variety of electrical excitation signals applicable to thin-layer electrochemical work is large. Three reviews of the subject have appeared [28-30]. 

Because of cocaine s toxicity and addictive properties, a search began for synthetic substitutes for cocaine. In 1905, procaine was synthesized and became the prototypic local anesthetic for half a century. Newer derivatives include mepivacaine and tetracaine (Figure 13.1). Briefly, the SAR of local anesthetics revolves around their hydrophobicity. Association of the drug at hydrophobic sites, such as the sodium channel, is believed to prevent the generation and conductance of a nerve impulse by interfering with sodium permeability (i.e., elevating the threshold for electrical excitability). 

If the time consumption is acceptable and the image drift is negligible, a scan line can be scanned twice to separate topography and electrical properties. In this case, a first scan in contact or better in a dynamic mode without an electrical excitation is performed. The tip is lifted and for the following second scan the z-piezo is controlled in a way that the tip follows the same topography as for the first scan (constant tip-sample distance or interleave scan). During this second line scan, one of the above-mentioned measurements of electrical properties can be performed [396]. 

It was shown recently that disordered porous media can been adequately described by the fractal concept, where the self-similar fractal geometry of the porous matrix and the corresponding paths of electric excitation govern the scaling properties of the DCF P(t) (see relationship (22)) [154,209]. In this regard we will use the model of electronic energy transfer dynamics developed by Klafter, Blumen, and Shlesinger [210,211], where a transfer of the excitation... 

The thickness-shear mode (TSM) resonator, widely referred to as a quartz crystal microbalance (QCM), typically consists of a thin disk of AT-cut quartz with circular electrodes patterned on both sides, as shown in Figure 3.2. Due to the piezoelectric properties and crystalline orientation of the quartz, the application of a voltage between these electrodes results in a shear deformation of the crystal. The crystal can be electrically excited in a number of resonant thickness-shear modes. 

In the previous section we considered the conditions under which mechanical resonances would occur in a TSM resonator. In considering only the mechanical properties of the crystal, however, we neglected consideration of how these resonances would actually be excited or detected. The device uses a piezoelectric substrate material in which the electric field generated between electrodes couples to mechanical displacement. This allows electrical excitation and detection of mechanical resonances. In constructing a practical sensor, changes in resonant frequency of the device are measured electrically. The electrical characteristics of the resonator can be described in terms of an equivalent-circuit model that describes the impedance (ratio of applied voltage to current) or admittance (reciprocal of impedance) over a range of frequencies near resonance. 

Neuron cells Neuron cells are generally electrically excitable, and their electrophysiological property changes upon physical and chemical stimulation. For this reason neuron cells are widely used as the sensing element in cell-based biosensors [42], Primary rat pup astrocytes are also used in co-culture with endothelial cells for the in vitro mimic of blood-brain barrier (BBB) [43],... 

In this chapter, we will describe as well as review briefly the work on conventional BLMs, s-BLMs, and closely related systems. As such, planar BLMs are realistic models of biomembranes they have been used to study the molecular basis of ion selectivity, membrane transport, energy transduction, electrical excitability, and redox reactions. However, one drawback with conventional BLMs is their mechanical instability. This major obstacle has now been overcome with s-BLMs which possess the requisite mechanical stability and other desired properties for biosensor development. 

The changes produced by drugs on other electrical properties of the heart may also be evaluated. For example, an alkaloid may decrease the electrical excitability of the auricles. Thus, a greater intensity of current would then be required to provoke an extra systole. The time of application of this electric shock must be precisely determined. Following each systole, the heart has a refractory period, during which time it is highly resistant to external stimuli thus, it insures the necessary rest period for the heart. Measurements of the auricular refractory period are commonly made with antifibrillatory alkaloids. If the Thomas Lewis-Mines circus movement theory of auricular fibrillation is to be believed, then any sub-... 

Thanks to the studies of Hodgkin Huxley, which culminated in 1952 with the publication of a series of articles, of which the last was of theoretical nature, the physicochemical bases of neuronal excitability giving rise to the action potential were elucidated. Soon after, Huxley (1959) showed how a nerve cell can generate a train of action potentials in a periodic manner (see also Connor, Walter McKown, 1977 Aihara Matsumoto, 1982 Rinzel Ermentrout, 1989). Even if the properties of the ionic channels involved have not yet been fully elucidated, cardiac oscillations originate in a similar manner from the pacemaker properties of the specialized, electrically excitable tissues of the heart (Noble, 1979,1984 Noble Powell, 1987 Noble, DiFrancesco Denyer, 1989 DiFrancesco, 1993). These examples remained the only biological rhythms whose molecular mechanism was known to some extent, until the discovery of biochemical oscillations. 

The development of the concept of ionic channel started with the realisation by Bernstein that cellular excitability was a property of the membrane. The starting point at the experimental level was the observation by Cole and Curtis that, concomitant with a propagated electrical impulse (manifestation of cellular electrical excitability) in the squid giant nerve fibre, a decrease in the electrical resistance took place with no detectable change in the membrane capacitance. This result lent strong support to Bernstein s concept and clearly indicated that the most plastic components of the axolemma, the proteins, underwent structural transitions leading to a transient increase in ionic fluxes. 

---