Alternative Methods for Applying Strain

Magnets are used to provide a high degree of control in devices capable of stretching a substrate, and such magnetostrictive actuators have been used in order to generate up to 22,000 ^e on particular substrates. The electromagnetic elements placed at either end of the substrate are attached to clamps mounted on a guide plate that ensures unidirectional travel. It is an aspect of these devices that they generate powerful electromagnetic fields that may directly affect cell behavior, and shielding from these fields should always be used.

Fig. 1. Schematic diagram of the custom-built apparatus used for straining cells by subjecting plastic strips to four-point bending. As the eccentric cam rotates it presses the loading platen onto the vertical edges of the plastic strips and causes their deformation (to the dotted position) in an arc around the paired fulcra underlying the strips (white). The platen is returned to the unloaded position by the spring. The rotation of the cam also acts to rock the chamber into which strips to be subjected to flow "control" are placed. For dose-response experiments, peak strain magnitudes can be altered by vertical displacement of the base unit by using a range of washers (with different thickness, placed between base and upper units; see Fig. 4), and frequency altered with rheostat control of cam revolution rate. This unfortunately affects the waveform produced. However, a novel device has recently been designed (yet untried), in which the platen's vertical displacement is governed by a screw mechanism that is computer controlled. This offers greater flexibility in the waveforms that can be generated, such that on/off dwell times and the "on" as well as "off" strain rates can be varied independently (see Fig. 5; Stromberg et al., personal communication). In addition this model utilizes commercially available cell culture strips (of equivalent size to microscope slides) that do not possess the end walls.

As an alternative, piezoelectric extension may be used to provide the displacement. This allows accurate control, however, like magnetostrictive actuators, generates high electrical fields. Such actuators have been used in vitro to apply strains of controlled magnitude (200-40,000 ^e), frequency (up to 100 Hz), and waveform, which adequately cover the range experienced by bone cells in vivo. The cells are mechanically strained by moving a plunger, connected to the center of an actuator, both ends of which are inserted into grooves on an acrylic resin frame, thus generating maximum actuator displacement. Cultured cells are seeded on, or in, a collagen gel block between plungers made of a non-conductive acrylic resin material, and the gel block anchored by stainless-steel wire meshes (lattice size; 0.4 x 0.4 mm) fixed to the plunger-ends (49).

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