Team:TU Delft/Demonstrate

iGEM TU Delft

Demonstration a functional proof of concept

Testing our biological microlenses on solar panels

Improving the way we capture light

The ultimate goal of our research was to be able to capture light more light in a new way. We used synthetic biology to produce biological microlenses. Once these microlenses are placed in a 2D-matrix, which we call a microlens array, we can use them for applications where capturing light is important. Capturing light is vital in many applications, including microscopy, photography, or solar cells.

Although the focus of our project is mainly improving microscopy, we also saw a new and exciting opportunity to also improve solar cells with our biological microlenses. In less than an hour, the theoretical potential of the sun represents more energy striking the earth’s surface than worldwide energy consumption in one year (Crabtree, 2006). However, the efficiency of solar panels is still very low nowadays and has to be increased to make them profitable. One promising finding is the use of microlens arrays. It is already proven that the use of a microlens array as an encapsulation layer for the solar panels results in 20% to 50% increase of the efficiency (Jutteau et al., 2015; Nam et al., 2013). However, the production of these microlens arrays is both expensive and environmentally unfriendly. Therefore, this technology is not used yet in day-to-day life.

Production of biological microlenses

In our project, we believe to have found a solution for the costly and environmentally unfriendly microlens arrays. The goal of our project was to produce biological microlenses to improve microscopy, but these can also be used for more efficient solar panels. In our study we have successfully produced biological microlenses by expression of the silicatein gene in Escherichia coli. Silicatein is a protein that is able to convert the molecule silicic acid to polysilicate, a kind of biological glass. By fusion of the protein silicatein to the membrane protein OmpA we were able to express the enzyme of the membrane and therefore coat the cells in a layer of polysilicate. Once the cells are coated in this layer of glass, they are able to function as microlenses.

Microlenses
Figure 1: By coating cells in polysilicate using the enzyme silicatein, the bacteria might function as a lens.

Also, we have demonstrated that we are able to produce spherical microlenses of only 1 µm in diameter. This is revolutionary, since typical microlenses are usually in the 10-100 µm range, since current techniques cannot produce smaller lenses (Krupenkin, Yang, & Mach, 2003).

Microlenses
Figure 2: SEM images of (A) regular E. coli and (B) our spherical microlenses.

Testing our cells under real-world conditions

The improvement of solar panels using microlens arrays have already been proven (Jutteau et al., 2015). Our models have shown that our microlenses have a defined focal point at around 1 µm of the lens, so the lenses are suitable for focusing light on a surface, such as a solar cell. Unfortunately, we are not yet able to produce a well-defined microlens array as we have seen in literature, since we were not able to position the cells in a 2D array. Therefore, we cannot yet give a definitive conclusion on whether our cells can improve solar cells. However, there were other real-world conditions that we could test already.

The main difference between the structure of our microlenses and conventional microlenses is that our biological microlenses contain a core of live bacteria. This has two consequences for our project. First of all, we couldn’t take our bacteria out of the lab to put them on solar cells due to the antibiotic resistance our cells carry. In order to solve this problem, we developed two different protocols to sterilize the cells without harming the shape of the lens. One method was based on autoclaving and the other on UV-sterilization. The protocol based on UV-sterilization has shown to be successful: we have successfully sterilized our cells without harming the integrity of the lens.

Microlenses
Figure 3: SEM images of UV-sterilized E. coli Biolenses. UV-sterilization does not impact the shape of the cells.

The second challenge we face was that the core of bacteria in our lens, either dead or alive, could mean our cells had different optical properties compared to conventional microlenses. The best way to test this was to test our cells under real-world conditions, by applying the cells onto an actual solar cell and testing them on a solar simulator. A solar simulator is a light source that reproduces the light emitted by the sun. First of all, we measured the absorption of our cells. Because our lenses contain cells on the inside, they could absorb the light instead of focus it. We tested this feature with spectroscopy. Our results show that our cells did not absorb significantly more light than the glass layer that is usually applied on solar cells. Furthermore, testing our cells on solar cells under the solar simulator showed no detrimental effects of our cells on the efficiency of solar cells. Therefore, we can conclude that our cells do not have a negative impact on the solar cells, which is a promising outlook for our biological microlens arrays.

Solar cells
Figure 4: Testing the cells in the solar simulator

Outlook

The most important follow-up experiment would be to construct an actual microlens array with our microlenses. Now our microlenses were positioned randomly on the solar cells. This way, the light is not focused as efficiently as in a microlens array, so we expect to see increases in solar cells once we have a well-structured microlens array. Furthermore, there is still a variation in the sizes and shapes of our microlenses, even though we are already able to control the cell shape. An interesting future study would be sorting the cells in size using FACS, so we can produce a homogeneous layer of microlenses.

References

  1. Berezin, M. Y., & Achilefu, S. (2010). Fluorescence lifetime measurements and biological imaging. Chemical reviews, 110(5), 2641-2684.
  2. Crabtree, J. T. N. L. G. (2006). Solar FAQs Retrieved from http://www.sandia.gov/~jytsao/Solar%20FAQs.pdf
  3. Einstein, A. (1917). Zur quantentheorie der strahlung. Physikalische Zeitschrift, 18.
  4. Gather, Malte C., and Seok Hyun Yun. "Single-cell biological lasers." Nature Photonics 5.7 (2011): 406-410.
  5. Jonáš, Alexandr, et al. "In vitro and in vivo biolasing of fluorescent proteins suspended in liquid microdroplet cavities." Lab on a Chip 14.16 (2014): 3093-3100.
  6. Jutteau, S., Paire, M., Proise, F., Lombez, L., & Guillemoles, J. F. (2015, 14-19 June 2015). Micro solar concentrators: Design and fabrication for microcells arrays. Paper presented at the Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd
  7. Krupenkin, T., Yang, S., & Mach, P. (2003). Tunable liquid microlens. Applied Physics Letters, 82(3), 316-318.
  8. Nam, M., Kim, K., Lee, J., Yang, S. S., & Lee, K.-K. (2013). Solar cell packaged by a microlens array and its long-term optical efficiency enhancement. Paper presented at the Proc. of SPIE Vol.
  9. Shaner, N. C., Patterson, G. H., & Davidson, M. W. (2007). Advances in fluorescent protein technology. Journal of cell science, 120(24), 4247-4260.
  10. Svelto, Orazio. Principles of lasers. Ed. David C. Hanna. London, New York, Rheine: Heyden, 1976.
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