Gas Bubble Oxygenators and Use of Antifoams

Foamy arterial blood from bubble oxygenator 

The mode of action of PDMS-hydrophobed silica antifoams in aqueous surfactant solutions has been extensively stndied by Denkov et al. [53] and reviewed in detail in Chapter 4. Essentially the hydrophobed silica particles rupture the so-called air-water-oil pseudoemulsion film, thereby enabling the oil to emerge into the air-water surface. It is known that once they emerge into the air-water surface, drops of PDMS oils usually initially spread over that snrface, exhibiting either complete wetting or pseudo-partial wetting behavior (see Section 3.6.2). This means that the oil spreads as either a thick duplex layer or spreads and breaks up into lenses in equilibrium with a thin oil layer. Since such behavior is ubiquitous with aqueous surfactant solntions, it is reasonable to expect similar behavior when PDMS oil drops are introduced into the gas-blood surface. It is not, however, known whether complete or pseudo-partial wetting behavior is to be expected. 

FIGURE 11.3 Schematic illustration of probable configuration formed by foam babbles in direct contact with PDMS-hydrophobed silica antifoam trapped in pores of a hydrophobic polyurethane sponge typical of those nsed in cardiotomy reservoirs. 

Finally, it is worth noting that, before application, the defoamer consists of a hydrophobic porous medium containing a hydrophobic fluid trapped in the smaller pores with the remainder of the pore space occupied by air. Before exposure to blood flow, it is usual to prime the defoamer with a suitable fluid to remove air. However, we should remember that use of a fluid to displace another fluid (air in this case) with which it is not miscible in a porous medium is not usually complete as those familiar with crude oil production will attest. Perhaps the best approach to this problem is to first flush out the defoamer with CO2 followed by an aqueous priming fluid. 

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