A hydrogel is formed when a solution of hydrophilic polymer chains is cross-linked and thus traps the water present in the solution in a soft semi-solid state. This happens when you cook and egg or make Jello, both examples of natural hydrogels. One of the most widely used hydrogels is another natural material, calcium alginate, made from the sodium salt of the acidic carbohydrate alginic acid (sodium alginate or SA) produced by various seaweed species. Hydrogels can also be made from synthetic polymers such as PEG acrylates, acrylamide, hydroxyethyl methacrylate (HEMA) and polyvinyl alcohol (PVA). Such synthetic polymer materials are used to make soft contact lenses for instance.
Many of the projects pursued over the last 5 or 6 years in the Lab have made use of hydrogels, mostly for encapsulating bacteria and other microbes. Encapsulation of microbes that are to be used in settings like a bioreactor or wastewater treatment plant generally simplifies their handling in these engineered settings. The active organisms can easily be strained out of the liquid medium and reused, or they are easily retained in a continuous flow-through process by allowing them to settle. This is quite advantageous when slow- growing microbes are used in flow-through processes as the slow-growing organisms are otherwise easily washed out and lost. The hydrogel matrix also affords some protection for the microbes against adverse conditions in much the same way that a biofilm does. Microbes that do not normally associate in biofilms can be cultured and then combined in a hydrogel particle to take advantage of their complementary metabolisms, such as when organism #1 converts a substrate into a product that organism #2 then uses to make another product which organism #1 alone can not make. Combining the two organisms together in close proximity in gel particles makes this shared metabolism more efficient by keeping the local concentration of the intermediate product/substrate high. The use of hydrogel encapsulation allows microbial combinations to be made which are otherwise difficult to produce.
We have used hydrogels described in the scientific literature in some projects and have developed custom formulations with specific properties for other projects. One property that any gel used for encapsulating live microbial cells must have is low toxicity to the cells. In addition, the process of forming the gel from the soluble precursor molecule or monomer must not be too stressful to the cells. Some cells are more sensitive than others to the cross-linking chemistry or process used to form gels. This means some experimentation is often required to arrive at a practical approach for a given experiment. Some gels are easy to make and the process is minimally stressful to cells, but the gel may be relatively weak and thus not suitable for some uses. We have developed and used many different gels for different purposes ranging from short-term laboratory experiments to long-term industrial pilot process testing to animal ingestion experiments.