Bioprinting uses living cells and molecular signals to produce biological tissues and even whole organs, building them layer by layer in the lab. To keep the bioink, or blend of cells and materials that power this printing, correctly aligned to grow a target tissue, researchers use electricity. However, this typically requires bulky electronic interfaces and invasive wiring. A team led by Penn State engineers has cut the cables, illuminating the first steps towards cell alignment control using miniature solar panels.
In a paper published in Advanced Functional Materials, the team described dotting tiny solar cells in various patterns across computer chips. They placed samples of fibroblasts - a common cell type found in connective tissue around the body - on top of each chip and shone different intensities of light on the system. The researchers reported successfully controlling cell alignment and achieving desired shapes by adjusting the patterning and diameter of the solar, or photovoltaic, cells.
"We can pattern cells precisely when we first print, but after this initial period, they're free to naturally move around, migrating and spreading," said corresponding author Ibrahim T. Ozbolat, professor of engineering science and mechanics, of biomedical engineering, of neurosurgery and Huck Institutes of the Life Sciences Chair in 3D Bioprinting and Regenerative Medicine. "Traditionally, researchers have influenced alignment with tethered devices, particularly electrodes that have a wired connection into the system. In this work, we demonstrate that photovoltaic cells can wirelessly facilitate this interaction just using light."
Ozbolat said scientists have known for years that exposing cells to a weak electrical current can influence their alignment, but this has required bulky systems that require electrodes and invasive wiring to interface with the cells. Previous work has shown that photovoltaic cells - the same material that make up solar panels - can offer a wireless approach to electrically influencing biological cell alignment by capturing light energy and converting it into electricity, but it was unclear if manipulating the layout of a photovoltaic cell array could help align larger clusters of cells into desired shapes, Ozbolat explained.
To test the method, the team layered various arrangements of photovoltaic cells on different silicon wafers, the fundamental base used to build a computer chip, testing two different strategies of adjusting the layouts. The first changed solar cells' diameters on each chip to span from just smaller than a millimeter to a mere 80 micrometers - less than the width of a human hair. The second kept the diameters the same, but adjusted the overall shape of how the cells were patterned on the chip.
"By changing the configuration of the solar cells in the array - their size, their shape, the spacing between the individual cells - we can alter the electric field they generate," said Yifan Tao, engineering science and mechanics doctoral candidate and first author on the paper. "This, in turn, allows us to more actively influence and observe how the layout of the array influences cell alignment."
Commercially sourced lung and dermal fibroblasts were then placed on the top of the chips and were given seven days to grow. Over this time, the chips were blasted with various intensities of light, which was captured and converted into an electrical current. At the end of the experiment, cell alignment was compared to fibroblasts grown on chips that had not received the illumination.
The result was striking, Tao said. In the chips that had not been exposed to light, cell alignment was largely random. With light, however, the cell alignment very clearly correlated with the patterning of the photovoltaic cells on the chip. What's more, Tao explained, testing showed that the intensity of the illumination influenced the photovoltaic cells' activity, meaning that researchers can influence cell alignment not just by changing the pattern of the solar cells on a chip, but by shining a brighter light or bringing the light source closer to the sample.
