Pulse clamping

In AC impedance, a titanium electrode exhibits the same double-layer capacitance per area as that of platinum. However, titanium suffers from irreversible buildup of a high-impedance oxide layer, which prevents sustained charge-injection usage. The irreversible oxidation can be observed using the pulse-clamp technique. 

Fig. 17 Pulse-clamp data of titanium, platinum, and iridium electrodes...

Bonner M, Daroux M, Ciish T et al. (1993) The pulse-clamp method for analyzing the electrochemistry on neural stimulating electrodes. J Electrochem Soc 140 2740 Hung A, Zhou D, Greenberg R et al. (2007) Pulse-clamp technique for characterizing neural-stimulating electrodes. J Electrochem Soc 154(9) 479-486... 

Acoustic Measurements. Measurement of the propagation of ultrasonic acoustic waves has been found useful for determining the viscoelastic properties of thin films of adhesives. In this method, the specimen is clamped between transmitting and receiving transducers. The change in pulse shape between successive reverberation of the pulse is dependent on the viscoelastic properties of the transmitting material. Modulus values can be calculated (267,268). 

Figure 4.5 Influence of oxidant stress on action potentials recorded In an isolated rabbit ventricular myocyte, (a) Control action potential, (b) Action potential recorded 3 min after exposure to oxidant stress induced by the photoactivation of rose bengal (50 nu). (c) Spontaneous and repetitive action potential discharges induced 6.5 min after exposure to rose bengal. Action potentials were recorded via a 2.5 MQ suction electrode and a current-clamp amplifier. The cell was stimulated at 0.1 Hz with a 2 ms suprathreshold current pulse and, when the cell showed automaticity (after 6 min), stimulation was stopped. Redrawn from Matsuura and Shattock (1991b).

Fig. 21.2. Two-microelectrode current-clamp technique used to observe, in single Ascaris body muscle cells in a body-flap preparation, the response to a controlled pulsed application of levamisole. One micropipette, to measure membrane potential, and another micropipette, to inject current, are inserted inside the area of the muscle cell known as the bag region. Levamisole is applied in a time- and pressure-controlled manner from a microcatheter placed over the bag region of the muscle. A second microcatheter is used to apply additional chemical agents (Martin, 1982).

Fig. 21.3. Two-micropipette current-clamp recording and effect of maintained application of 30 pM levamisole, which produces a 15 mV depolarization (upward movement of trace). The downward transients are the result of injected current pulses used to measure membrane conductance. The trace gets narrower as the input conductance increases from 2.35 pS to 4.35 pS as the levamisole ion channels open. The peak amplitude of the membrane potential response and change in input conductance are used as an assay of the number and activity of the levamisole ion channel receptors present in the muscle cell membrane. The response was fully reversible on washing (not shown).

One can use either a symmetrical or an asymmetrical protocol. First both cells are clamped to —40 mV holding potential in order to inactivate the sodium current. Thereafter, one cell is clamped to potential ranging from —90 to +10 mV for 200 ms (pulse duration asymmetrical protocol). Thereby, a transcellular voltage difference of + 50 mV can be applied... 

For a symmetrical protocol both cells are clamped to the common holding potential of —40 mV. Thereafter, both cells are pulsed equally but with opposite polarity. Thus, cell 1 is clamped to —50 mV and cell 2 to —30 mV, then to —60 and —20 mV, respectively, and so forth. 

In a record obtained by the patch clamp technique, the channel is closed for much of the time (i.e. no current flows across the patch of membrane that contains it), but at irregular intervals the channel opens for a short time, producing a pulse of current. Successive current pulses are always of much the same size in any one experiment, suggesting that the channel is either open or closed, and not half open (there are exceptions to this rule). The durations of the pulses, however, and the intervals between them, vary in an apparently random fashion from one pulse to the next. Hence the openings and closings of channels are stochastic events. This means that, as with many other molecular processes, we can predict when they will occur only in terms of statistical probabilities. But one of the most useful features of the patch clamp method is that it allows observation of these stochastic changes in single ion channels as they actually happen individual protein molecules can be observed in action. 

Film welding requires special precautions in view of the small thickness. When the two overlapping ends are heated by an external heat source, the heat is supplied at the wrong side of the film so that the films are heavily distorted. With very short electric pulses good welds can be made, though only with thin films (up to 0.2 mm), and with special precautions to avoid sticking of the film to the clamping device. 

A. Becker, N. Akozbek, K. Vijayalakshmi, E. Oral, C. M. Bowden, S. L. Chin, Intensity clamping and re-focusing of intense femtosecond laser pulses in nitrogen molecular gas, Applied Physics B 73, 287 (2001). 

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