Micro-nucleation in supersaturated oxygen solution injection

We present here an example of an important biomedical technique in which it is critical to understand and avoid bubble nucleation in supersaturated aqueous solutions of oxygen. By doing so it is possible to inject highly supersaturated oxygen solutions through a small capillary without the formation of significant gas bubbles. The potential medical benefits of a successful technique of this kind are substantial and multi-faceted. Deprivation of oxygen even for brief periods of time such as occur during heart attacks or strokes results in cell damage or death and is a primary cause of permanent physiological damage. Consequently rapid therapeutic oxygen delivery systems could substantially enhance the treatment, for example, of acute myocardial infarction or acute cerebral stroke. The strategy discussed here has been described previously (Brereton et al. 1998). It involves the preparation of a highly concentrated solution of oxygen in an aqueous solution under very high pressure and the injection of this liquid into the bloodstream through a small capillary tube or tubes. Recent experiments performed at TherOx, Inc., in Irvine, California, have clearly demonstrated that the nucleation, when it does occur, results from heterogeneous nucleation (Brennen 1995) on the interior surface of the distal end of the capillary and that treatment of this interior surface of the capillary can be remarkably successful in suppressing nucleation. Brereton et al. (1998) suggested that the potential nucleation was homogeneous. On the contrary, the present experiments clearly show that heterogeneous nucleation is the dominant phenomenon and a recent heterogeneous nucleation theory by Creech et al. (2002) yields results consistent with the observations. Two basic strategies mitigate for success. The first of these is to prepare and treat the interior surface of the capillary in a way that minimizes the occurrence of nucleation sites. The second of the strategies is that of high fluid velocity. Inside the capillary tube this leads to a high longitudinal pressure gradient which implies a short distal length of tube for which the fluid pressure is below the saturation pressure. Minimizing the interior surface area below the saturation pressure minimizes the chance of a nucleation site being activated. A second benefit of high fluid velocity is that it maximizes the rate of mixing in the jet external to the capillary. Nevertheless, given the uncertainities naturally associated with the occurrence of micron and sub-micron nucleation sites on the interior surface it is inevitable that some of the capillaries will happen to have one or more nucleation sites close enough to the distal end to produce bubbles. A well-regulated testing procedure has been developed by TherOx to eliminate these defective capillaries. However, the fact that this testing procedure can achieve acceptable success rates is one of the most remarkable and promising aspects of this research.