Pore Generators for Membranes, Tissues and Food

Recently I encountered the concept of a "porogen". It is an approach to forming pores in a solid medium that has been applied in a variety of areas, including membrane fabrication, tissue engineering and food technology.

Everyday physical objects often consist of a solid matrix in which pores are embedded. The porous nature of these materials can be important for how they function. A sponge can absorb a large volume of water because of its porosity. The pores in wine corks allow them to be compressed in the neck of a wine bottle, wherein the cork expands to form a seal. Many foods, like cakes, breads and meringues, partly owe their characteristic texture to the presence of thousands of tiny pores.

In a paper in the Journal of Chemical Education a simple method was proposed to fabricate membranes. Membranes are semi-permeable — they allow the passage of some particles but not others — due to the presence of pores of a specific size. Membrane technology is used in a vast number of applications, including the cleaning of water and the clarification of beer. To create a membrane the scientist used a polymer solution that formed a gel (cellulose acetate and glycerol). Insoluble particles — the porogens — were embedded in the gel. Once the gel was formed the porogens could be dissolved and leached out of the gel, leaving pores in the space that they vacated. The porogens were made of calcium carbonate, which is soluble in acid, so acid could be used to selectively remove the porogens.

The method was developed for undergraduate teaching. Materials like calcium carbonate and cellulose acetate are readily available in laboratory cabinets, making this a convenient and cost-effective approach. When I last purchased laboratory filtration cartridges they each cost about 2000 euro.

While the author did not use the term "porogen leaching", such a process is typically described this way in other fields, specifically tissue engineering. Biological cells can be grown into specific biological forms to repair or replace damaged tissues. Engineering these tissues typically requires a scaffold on which the cells can grow. Often a base material, such as a polymer, is mixed with a porogen that acts as a template for generating pores in the solidified polymer. The same approach can be used in the preparation of cultured meat.

Egg white gels into a solid when heated — an irreversible process. Gelatin solutions, however, form reversible gels when cooled that melt close to body temperature. One study used these common kitchen ingredients to explore the use of porogen technology in food applications. The researchers started by mixing egg white and gelatin. Heating the mixture solidified the egg and cooling solidified the gelatin. When this gel was immersed in warm water the gelatin leached into the water. After the gel was dried it was found to be highly porous. There are many potential food applications for such a material. It could hold a large volume of liquid vegetable oil in an effectively solid state, serving as a replacement for solid animal fat in a diet low in saturated fatty acids.

I'm not certain about the origin of the term "porogen" but based on how a porogen functions I assume the word is a blend of pore and generator. in the literature, articles used as porogens are frequently referred to as being "sacrificial"; presumably this relates to how they are sacrificed (to waste) in order to generate pores. They are also sometimes referred to as "fugitive particles", perhaps because a particle escapes the confines of the solid matrix to leave behind an empty cell, like an escaped prisoner.

It may not be immediately obvious how membrane fabrication, tissue engineering and cultured meats are connected, but one connecting factor is the potential usefulness of porogens.