The Science of Defoaming

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11

Medical Applications

of Defoaming

11.1 INTRODUCTION

There are two main medical applications of defoaming. The rst concerns the treat-

ment of symptoms associated with excessive gas in the gastrointestinal tract. The

second concerns the treatment of blood during cardiopulmonary bypass surgery.

Formation of excessive gas in the gastrointestinal tract is often associated with

hyperacidity and can produce chronic pain. The prevalence of this problem derives

from the formation of extremely stable mucous foam where gas is introduced either

through swallowing during food ingestion or through gas generation during digestion.

Not only does this foam produce unpleasant symptoms but it also hinders diagnosis of

various conditions using techniques such as endoscopy. These problems of excessive

foam usually succumb to either oral or enteric administration of PDMS–hydrophobed

silica antifoam, which is often given the generic name “simethicone” in this context.

Here, we review the issues associated with the application of simethicone.

Procedures such as open heart surgery and heart transplants have been practiced

for several decades with progressively increasing survival rates. Death rates after

such cardiac surgery in the United States, for example, have recently been reported

as now less than 1% [1]. This has been made possible by the development of ever

more efcient cardiopulmonary bypass (heart–lung) machines. A key function of

such machines concerns the oxygenator where venous blood is supplied with oxygen

and carbon dioxide is removed. Early designs of oxygenators involved gas bubblers

where excess gas is removed using defoamer/lters [2]. These defoamers usually

consisted of porous matrices impregnated with PDMS–hydrophobed silica antifoam.

Although use of such devices reduces the risk of gas emboli, the antifoam is slowly

dispersed in the blood supply. This can also result in a serious incidence of emboli

due to the entrapment of antifoam drops in blood capillaries [3–5]. Such problems

are alleviated by use of membrane oxygenators where gas exchange with venous

blood occurs by molecular diffusion across permeable membranes. Defoamers are

still used with such devices, however, to minimize the risk of gas emboli due to inad-

vertent introduction of air bubbles. This is a particular problem when “cardiotomy”

blood from the “surgical eld,” containing both detritus from surgery and a risk of

the presence of air bubbles, is reunited with venous blood for recirculation.

Difculty of decontamination of either type of oxygenator after use means that

they have to be affordably disposable. Even 15 years ago, it was estimated that more

than 1 million disposable oxygenators were used globally where about 85–90% were