All of the following are types of membrane oxygenators except for:

In the context of membrane oxygenators used in cardiopulmonary bypass, it is essential to understand the different types and their characteristics. Microporous, hollow fiber, and depth types are recognized as actual membrane oxygenator designs.

Microporous oxygenators utilize a fine pore structure that allows gas exchange while filtering out unwanted materials. Hollow fiber oxygenators consist of thousands of small fibers that facilitate gas exchange, providing a large surface area in a compact configuration for efficient oxygenation. Depth oxygenators, however, typically refer to a design where gas exchange happens within a thicker matrix, which, while it can serve similar purposes, is not classified under the typical membrane designs like microporous or hollow fiber.

On the other hand, polypropylene is a material rather than a type of oxygenator. It is often used in the construction of medical devices due to its favorable chemical properties and viability for biocompatibility, but it does not represent a design or category of membrane oxygenators.

Therefore, recognizing that depth is not categorized as a membrane oxygenator type clarifies why it is the correct choice in this scenario.