Apparatus for contour-clamped homogeneous electric fields

These notes and diagrams describe the construction of a simple hexagonal electrode array contour-clamped homogeneous electric field (CHEF) apparatus (1, 2). Included are a number of improvements on the original design. The circuit is shown in modules so that the investigator can upgrade an existing device as desired. The modules represent modifications of a second-generation device with vertical electrodes (2). One module replaces the original mechanical relay with an electronic switching circuit, which is (blissfully) silent and more reliable and permits faster switch times. The other module inserts transistor circuits to provide more accurate voltage clamping. However, it should be noted that the clamping is still not as precise as it is in commercial devices available from Bio-Rad or BRL. We have also developed a new system with electrodes in a square array that produces precise voltage clamping with operational amplifiers and is capable of arbitrary reorientation angles. That device is described elsewhere (3). Nevertheless, the circuits shown here are inexpensive, are easy to construct, are adaptable to older generation devices, and give good results for most applications. The circuits can be followed by anyone with a rudimentary training in electronics.