Pore Generators for Membranes, Tissues and Food

Recently I encountered the concept of a "porogen" for the first time. It is an approach to forming pores in a solid medium that I have since seen applied in a variety of areas, including membrane fabrication, tissue engineering and food science.

Everyday physical objects often consist of a solid matrix in which pores are embedded. The porous nature of these materials can have a big impact on how well they function. A sponge can be filled with water because of its network of pores. The porosity of wine corks allows them to be compressed into a narrow glass channel where the cork expands to form a seal. Many foods, like cakes, breads and meringues, owe some of their characteristic texture to the presence of thousands of tiny pores. It's useful then to know how to make such porous materials in a variety of contexts.

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 — by virtue of their pores. Membrane technology is used in a vast number of applications, including the cleaning of water. To create a membrane the scientist used a polymer solution that formed a gel (cellulose acetate and glycerol) and some insoluble particles that were embedded in the gel. Once the gel was formed the embedded particles could be dissolved and leached out of the gel, leaving only pores behind. The particles were made of calcium carbonate, which is soluble in acid, so washing the gel with acid was sufficient to create the porous membrane. The method was developed for an undergraduate teaching laboratory and it definitely sounds like a cost-effective approach, as pre-fabricated membranes can be expensive (from experience).

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 usually 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. Meat is itself a biological tissue and meat can also benefit from a porous scaffold on which to grow.

Using porogen technology does not necessarily require sophisticated materials. One study used egg white and gelatin, commonly found in kitchens. 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. 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. Glancing at the literature in tissue engineering reveals some other interesting language choices. Particles 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, just like an escaped prisoner? I'm less sure about that one.

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