A new, inexpensive technology can limit the buildup of algae on the walls of photobioreactors. Reducing this fouling avoids costly cleanouts and allows more photosynthesis to happen within tanks. Image: Jose-Luis Olivares, MI
by David L. Chandler, MIT News Office
Most algae grown commercially are cultivated in shallow ponds, while others are grown in transparent tubes called photobioreactors. The tubes can produce seven to 10 times greater yields than ponds for a given amount of land, but they face a major problem: The algae tend to build up on the transparent surfaces, requiring frequent shutdowns of the whole production system for cleaning, which can take as long as the productive part of the cycle, thus cutting overall output in half and adding to operational costs.
The fouling also limits the design of the system. The tubes can’t be too small because the fouling would begin to block the flow of water through the bioreactor and require higher pumping rates.
To address this drawback, MIT researchers have developed a simple and inexpensive technology that could substantially limit fouling, potentially allowing for a much more efficient and economical way of converting the greenhouse gas into useful products.
The key is to coat the transparent containers with a material that can hold an electrostatic charge, and then apply a very small voltage to that layer. The system has worked well in lab-scale tests, and with further development might be applied to commercial production within a few years.
The findings are being reported in the journal Advanced Functional Materials, in a paper by recent MIT graduate Victor Leon PhD ’23, professor of mechanical engineering Kripa Varanasi, former postdoc Baptiste Blanc, and undergraduate student Sophia Sonnert.
Dr. Varanasi and his team decided to use a natural characteristic of the algae cells to defend against fouling. Because the cells naturally carry a small negative electric charge on their membrane surface, the team figured that electrostatic repulsion could be used to push them away.
The idea was to create a negative charge on the vessel walls, such that the electric field forces the algae cells away from the walls. To create such an electric field requires a high-performance dielectric material, which is an electrical insulator with a high “permittivity” that can produce a large change in surface charge with a smaller voltage.
“What people have done before with applying voltage [to bioreactors] has been with conductive surfaces,” Dr. Leon explains, “but what we’re doing here is specifically with nonconductive surfaces.”
“Our study basically solves this major problem of biofouling, which has been a bottleneck for photobioreactors.”
“If it’s conductive, then you pass current and you’re kind of shocking the cells,” he said. “What we’re trying to do is pure electrostatic repulsion, so the surface would be negative, and the cell is negative, so you get repulsion. Another way to describe it is like a force field, whereas before the cells were touching the surface and getting shocked.”
The team worked with two different dielectric materials, silicon dioxide — essentially glass — and hafnia (hafnium oxide), both of which turned out to be far more efficient at minimizing fouling than conventional plastics used to make photobioreactors. The material can be applied in a coating that is vanishingly thin, just 10 to 20 nanometers (billionths of a meter) thick, so very little would be needed to coat a full photobioreactor system.
“What we are excited about here is that we are able to show that purely from electrostatic interactions, we are able to control cell adhesion,” Dr. Varanasi says. “It’s almost like an on-off switch, to be able to do this.”
Additionally, Dr. Leon says, “Since we’re using this electrostatic force, we don’t really expect it to be cell-specific, and we think there’s potential for applying it with other cells than just algae. In future work, we’d like to try using it with mammalian cells, bacteria, yeast, and so on.”
The same system could be used to either repel or attract cells by just reversing the voltage, depending on the application. Instead of algae, a similar setup might be used with human cells to produce artificial organs by producing a scaffold that could be charged to attract the cells into the right configuration, Varanasi suggests.
“Our study basically solves this major problem of biofouling, which has been a bottleneck for photobioreactors,” he says. “With this technology, we can now really achieve the full potential” of such systems, although further development will be needed to scale up to practical, commercial systems.
As for how soon this could be ready for widespread deployment, he says, “I don’t see why not in three years’ timeframe, if we get the right resources to be able to take this work forward.”
The study was supported by energy company Eni S.p.A., through the MIT Energy Initiative.
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