Algae play crucial roles in providing nutrients and food resources for the higher trophic levels in addition to serving as a promising biofuel source and an environmental hazard (in the form of harmful algal blooms, HABs). Current methods employed to evaluate algal growth, viability, and migration rely on batch culturing in shaker flasks that require large volumes of reagents, extended incubation times to achieve necessary biophase concentrations, and external counting and assessment. These flasks are also limited to uniform doses and concentrations of nutrients that do not accurately reconstitute the heterogeneous, dynamic environment in which the algae reside. Advantages of using a microfluidic approach over current methods include (i) the direct, real-time visualization of single algal species, (ii) a significant reduction in experimental time and reagent volume, and (iii) the ability to create heterogeneous or homogeneous cellular microenvironments.
This presentation highlights the development of a microfluidic gradient generator, fabricated from different materials, capable of both short- and long-term experimentation on algal growth and migration. One challenge in the design of a microfluidic device compatible with algae is the elimination of direct flow while allowing for the formation of a nutrient gradient in the culturing channels as algae are non-adherent cells. This was achieved in a device comprised of three separate fluidic channels fabricated into PDMS layered on top of an agarose slab. The two outer channels provided continuous flow of nutrients and, if necessary, a chemical gradient, while the center, ‘flow-free’, channel served as the experimental culture channel. To achieve a fluid tight seal in the channel, the PDMS/agarose device was enclosed in a custom built Plexiglas chamber. To overcome the need for this external chamber, we explored an alternative polymeric material using thiol-ene chemistry to generate the top and bottom components of the device. By tuning the polymerization chemistry of the thiol-acrylate, we were able to fabricate a rigid, PDMS-like top layer that could be chemically bound to a porous, hydrogel-like bottom layer to allow for ‘flow-free’ diffusion into the center channel. Nutrient diffusion and the stability of the chemical gradients of both materials were evaluated using both fluorescent microscopy and by COMSOL simulation. Finally, both devices fabricated from both materials were used to assess both the long-term growth dynamics and chemotactic response of the model algal species Chlamydomonas reinhardtii under varying nitrogen and phosphorus concentration. Ultimately, this technology allows for greater control and reduced experimental time and cost compared to current methods to assess algal growth and viability.