Some of us are on the search for new renewable materials that can make our lifestyle a bit more sustainable. Although a new resource may be renewable, that doesn’t mean it makes sense to use. For example, many people went crazy over corn-based ethanol because it would eliminate (or at least reduce) America’s dependence on foreign oil. America is the world’s largest producer of corn, but that’s for a reason; we use it for a lot of things already. By diverting a percentage of corn production towards ethanol, we shook up our corn economy, and not necessarily for the better. Yet we find ourselves looking to corn again, but this time as an alternate material for microfluidics. Gang Logan Liu et al. have presented us with a new method for corn-based microfabrication in "Green microfluidic devices made of corn proteins". The group from the University of Illinois Urbana-Champagne uses the protein zein, which is actually a byproduct of ethanol production. An existing product previously treated as waste now has the promise of new value; this is exactly what we’re looking for as a green resource.
Not only would this corn-based device be renewable, its disposal would also be more environmentally friendly. The authors stress that this is important in sectors that perform a high number of tests. This could be particularly important for agriculture, which needs to constantly test for pollutants and diseases (it would also give a nice ‘full circle’ type feeling). I’m normally more interested in medical applications for microfluidics, but there are a ton of areas that run chemical tests that can be optimized.
This is not the first time Zein has been examined as a viable resource; it has been studied for use in coatings, adhesives, food packaging, drug and functional food delivery systems. Zein is a good candidate for microfluidics because it resembles a rod, and self-organizes with other rods into a two-dimensional film. The authors employed a soft lithography fabrication process using PDMS as a master. Once a single layer was molded, the authors examined binding processes with glass slides as well as other zein layers. While it is conceivable that someone would want to use a glass-zein or zein-zein device, it is also necessary to test it with glass because zein is opaque and prevents us from visually evaluating how well it functions.
Two methods for attachment were considered: solvent and vapor deposition. Both methods involve heating the surfaces and coating with ethanol before mating the sides. In solvent deposition, the zein side is placed on an ethanol-coated surface to pick up a thin layer before sealing. In vapor deposition, the zein side is exposed to ethanol vapor before sealing. The process of heating and exposing to ethanol allows the zein to become more mobile as well as crosslink with either the opposing zein surface or the micro voids in the glass.
The authors tested the zein-structures in a couple different ways, but they basically boiled down to how well the channels were replicated, sealed and able to allow for normal microfluidics operations. The resulting channels deviated slightly from the master, which could be rectified by reducing the elasticity of the PDMS or changing the zein film thickness. When comparing the solvent and vapor depositions, reliable results were obtained using the vapor. When the solvent was deposited, evidently extra ethanol was transferred to the zein film, distorting the geometry of the channels. While the authors demonstrated that zein sealed well, they did note that it is a somewhat permeable material, and this could be manipulated in some controlled way. Also of note is the fact that zein autofluoresces. Zein is excited using wavelengths from 450 to 490 nm, resulting in excitation emissions at 550 nm to 580 nm. While this doesn’t line up with the properties of Green Fluorescent Protein, it might align with others, making fluorescent detection a bit harder.
I think that the use of zein is promising. It utilizes a renewable material in a new way that gives value to a pre-existing process. Not only is zein renewable, but it is biodegradable, which will answer one of the problems that will surely arise when microfluidic devices have worked their way into mass production. I think we’ll have to see what complicated structures can be created with zein and what solvents really shouldn’t be used.
Luecha, J., Hsiao, A., Brodsky, S., Liu, G., & Kokini, J. (2011). Green microfluidic devices made of corn proteins Lab on a Chip, 11 (20) DOI: 10.1039/C1LC20726A