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Biomimetics. I love that word. Well, probably not as much as microfluidics, but it’s a close second. If you’re unfamiliar with the word, it basically refers to design that mimics biology. Biological systems have evolved into finely tuned machines, why not mimic them in order to synthesize what we need? Biomimetics isn’t new, it’s been around in one form or another for a long time (my favorite instance is Velcro), but our capabilities are broadening as we are able to manufacture at smaller, micro and nano levels. If you want to learn more about this topic, you should check out the Biomimetic Microsystems Platform at the Wyss Institute. Today I’d like to share biomimetic microfluidic research that mimics the silk-spinning process of spiders from Korea University.
Mimicking Spider Silk
Sang-Hoon Lee et al.’s paper “Digitally tunable physiochemical coding of material composition and topography in continuous microfibers” showcases the process of finely tuned silk-spinning and its benefits in Nature Materials. There has been interest in using microfibers as a scaffold in tissue engineering, but there has been little success in achieving microfibers with varying chemical and morphological properties. The researchers have developed a process to continuously spin microfibers while varying their properties spatiotemporally. With a digitally controlled microfluidic chip, they are able to produce microfibers with varying chemical compositions, structures, gas bubble size and type, live cells and morphologies. The inspiration for this project comes from spiders. They can produce seven different types of silk that originate from different glands within the spider. As the need for one type of silk arises, fluid from that specific gland is used and the produced silk’s composition changes.
Lee’s spider-chip has several pneumatically controlled inlets that meet at a central point. The resulting flow is focused by a coaxial, sheath flow. In this case, the researchers chose to create alginate microfibers, although other materials such as chitosan and polyethylene glycol can be used. Since alginate requires Ca+ to gel, the sheath contains CaCl2 to convert the stream into a fiber. In addition to focusing the resulting stream, the sheath also acts as a lubricant during the gelation process. Multiple microfibers can be produced continuously in at once and twisted together, which allows for parallel and serial encryption. This encoding could be used in biosensing, high-throughput screening and cell positioning for tissue engineering, but the researchers also wanted to tackle specific structures produced by spiders.
Have you ever seen a spider web in the morning, laden with dew and wondered why the drops of water are spaced out instead of drenching the whole strand? Apparently there are puffs along the silk that attract the water along the silk. These puffs were mimicked by the spider-chip by alternating two different streams, one of which contained the salt CaCl2 . After the salt was leached from the structures, they had a higher surface energy than the rest of the microfiber, drawing in the water. The amount of water collected is dependent on the size of the puff, which is tunable and the researchers suggest that this approach could be used in water collection and purification.
Lending a Helping Hand to Neurons
Spider eggs are encapsulated with grooved silk, which was also reproduced in the spider-card with grooved channels. When embryonic neurons were placed on the grooved silk, their axons and dendrites ended up more aligned than those found on smooth microfibers. This might be a vital component in tissue regeneration after spinal cord or nerve damage. The researchers also demonstrated their ability to encapsulate gas bubbles within the fibers at different frequencies and bubble sizes. Gas bubbles within the microfiber might provide a temporary source of oxygen for cells encapsulated cells within the microfiber. Finally, Lee et al. encapsulated fibroblasts and hepatocytes and measured their viability. Unfortunately they only investigated cell viability for 5 days, which isn’t long enough to determine its practicality. They also encoded a microfiber with a chemoattractant and put it in a cell culture. The migration of cells toward the chemoattractant is evident, showing that a spatially encoded fiber could be useful in cell culture organization.
Expanding greatly on the current uses for microfibers, this advancement will hopefully push the use of microfibers forward. The ability to fine tune the properties of segments within a microfiber has its obvious benefits. But this process is also capable of rapid changes in the microfiber in real time. I’m not sure we have the same needs to quickly change the type of silk produced like a spider. But, I wonder what type of system would need to quickly respond to its environment with different microfiber properties. Any ideas?
Kang, E., Jeong, G., Choi, Y., Lee, K., Khademhosseini, A., & Lee, S. (2011). Digitally tunable physicochemical coding of material composition and topography in continuous microfibres Nature Materials, 10 (11), 877-883 DOI: 10.1038/nmat3108