Design for a Simple and Inexpensive Cylinder-within-a-Cylinder Gradient Maker for the Undergraduate Biochemistry Laboratory

Abstract

A design for a simple and inexpensive gradient maker is described. The gradient maker is assembled by (i) cutting the tops off two plastic bottles of differing diameters to produce two cylinders with intact bottoms; (ii) drilling a small hole toward the bottom of the smaller diameter cylinder and plugging the hole with a size 00 cork stopper; and (iii) placing the smaller diameter cylinder within the larger diameter cylinder and holding in place with epoxy. Removal of the cork stopper connects the inner and outer chambers, and two lengths of glass tubes, joined by a length of flexible plastic tubing, connect the inner chamber and the environment. The resulting cylinder-within-a-cylinder design is compact and can easily fit on a small magnetic stirrer, and conductivity tests showed that the gradient produced is linear. The cost is low enough that many units can be assembled and used in the undergraduate laboratory for less than the price of a single commercial unit.