Inchworming

Figure 16.n Photographs of inchworm motion of the droplet in solution. See text for details. Ebias = 0-5 V. Eoffiet = —300 mV (a), —340 mV (b) and —300 mV (c). (d) Trace of the multi-step inchworm motions of the droplet. Six photographs were superimposed. 

Figure 2.25 Schematic representation of the STM head and electrochemical assembly. (I) Inchworm motor, (2) Inch worm, (3) Faraday cage around tube scanner, (4) Teflon electrochemical cell, (5) working electrode (i.e. sample), (6) stainless steel plates, (7) halved rubber O rings, (8) elasticated ropes attatched to baseplate. The counter and reference electrodes and the various electrical connections arc not shown for clarity. From Christensen (1992).

Velankar, S. S., Soultanas, P, DUlmgham, M. S., Subramanya, H. S. and Wigley, D. B. (1999). Crystal structures of complexes of PcrA DNA hehcase with a DNA substrate indicate an inchworm mechanism. Cell 97, 75-84. 

Translational motions of very small steps can be utilized by a commercial device, the Inchworm ." Figure 12.8 shows the principle of the walking mechanism of the Inchworm . An alumina shaft is slid inside a PZT sleeve, which has three sections. The three sections of the PZT sleeve are numbered 1 through 3, respectively. By sequentially activating the three sections, axial motions in small steps are made. 

Fig. 12.8. The Inchworm . It consists of an alumina shaft, slid inside a piezoelectric tube, which consists of three sections. Top, a photograph of the Microlnchworm Motor, actual size. Right, walking mechanism (1) Clamp the left-hand side by applying a voltage to section 1. (2) Extend section 2. (3)...

Inchworm i.s a registered trademark of Burleigh instrutnents, Inc., Burleigh Park, Fishers, New York 14453. 

The outer two PZT elements, numbered 1 and 3, act as clamps. When a voltage is applied to one of them, its diameter shrinks to grip the shaft tightly. The center section does not contact with the shaft, but the length can change according to the applied voltage. The function of the Inchworm is illustrated in Fig. 12.8. 

Fig. 12.9. An STM with an Inchworm as coarse positioner. (Courtesy of Burleigh InstrumenLs, Inc.)...

Various types of the so-called magnetostrictive inchworm type of motor have been proposed and built. A two-leg travelling machine using a magnetostrictive bimorph actuator with 7.5 /zm-thick polyimtde is shown in fig. 86b (Honda et al. 1994 Arai and Honda 1996). In an alternating magnetic field, it can travel in one direction. The maximum speed of approximately 5 mm/s was obtained around the mechanical resonant frequency... 

Still not fully understood is the processive action of kinesin.221 224 It is often assumed that this protein moves in a hand-over-hand fashion with the two heads binding alternatively to the microtubule. Some substantial reorganization of the peptide chain in the hinge region at the end of the neck is presumably involved.173 An alternative "inchworm" mechanism has been suggested.2203... 

Although this picture seems clear and simple, many uncertainties remain. Transcription does not proceed evenly but by pauses and spurts. This has suggested the possibility of an "inchworm" type of movement of RNA polymerase.72-75 However, the observations may also be explained by variations in the sequence. There are both pausing or stalling sites76 and terminator sequences. The concentrations of the needed ribonucleotide triphosphate precursors will also affect the kinetics. In addition, defects in the... 

The movement of the tip is usually carried out by drivers based on piezoelectric elements, similar to those used in STM, as described in Chapter 2. Typically, inchworm drivers (Burleigh Instruments, Fishers, NY) are used, since they can move larger distances than simple piezoelectric tube scanners. However, where higher resolution is needed, piezoelectric pushers can be added, so that the inchworms provide coarse drives and the pushers nm-resolution drives. Generally the direction normal to the substrate is taken as the z direction, while x and y are those in the plane of the substrate. 

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