The diffusion capsule, a novel device for the addition of a solute at a constant rate to a liquid medium. Its application to metabolic regulation.

The diffusion capsule consists of a cylindrical container that can be completely filled with a solution and sealed with a small semi-permeable membrane at one end. In use, the capsule is immersed in an agitated liquid. Experiments on concentrated solutions in the capsule showed that, contrary to diffusion theory, the rate of diffusion of solute (sugars or amino acids) out of the capsule remained virtually constant until about 65% of the solute had diffused out of the capsule. Thus the device has been used to maintain constant material feed rates for periods exceeding 30h. The capsule is a simple and compact substitute for a pump and is superior to a pump for small feed rates in many applications. The capsule greatly extends the scope of the shake-flask culture technique for micro-organisms in that substrate-limited growth, possibly the aspect of greatest interest, is readily achieved simply by dropping in the flask a capsule containing the substrate. Diffusion feed should facilitate study of the metabolism of toxic substrates: also it is likely to provide an improved means for supplying a pulse of tracer to a culture.