It sounds like a plot from a science fiction movie, but quite incredibly scientists have managed to implant tiny lasers into living cells. In the quest to track cells as they move about and interact, the researchers have created miniature lasers that when internalized by the cell can be used to follow cells for weeks at a time. The study is published in Nano Letters.
For the “biointegrated” laser to work, like other conventional lasers, it requires three main components: some sort of material that will emit light when stimulated, known as the “gain medium,” a resonator that confines the light by total internal reflection, and a “pump source,” or a way of transferring energy from an external source to the gain medium.
The researchers, from the University of St. Andrews, achieved this by making what they call a “whispering gallery mode microsphere resonator” out of a particular plastic called polystyrene divinylbenzene. They were able to make these resonators with a radius of just 5-10 µm (0.005-0.01 mm), or small enough to be able to fit inside a living cell.
Previously, gain mediums such as vitamins and naturally produced fluorescent proteins had been used, but these needed resonator cavities much larger than typical cells, and so had limited use. For this study, the scientists instead turned to a green fluorescent dye inserted into the microsphere resonators. The pump source was provided by nanosecond pulsed output from an “optical parametric oscillator laser system.”
They then tested how well four different cells types engulfed the microspheres, using human macrophages (found in the immune system), mouse fibroblasts (that help give tissue structure), mouse microglia cells (found in the brain), and human embryonic kidney cells. They found that the cells were able to internalize the miniature lasers, and that the macrophages then continued to move, dragging the tiny tech as they go.
Once the lasers were stimulated and started emitting their own light, the scientists then followed the cells for 19 hours, and found no significant difference in the amount of light they were releasing over the time period. They also managed to show that the macrophages were able to live normally and survive for up to four weeks with the laser still embedded.
There are quite a few advantages of using a tiny laser to track cells over more traditional techniques, such as fluorescent proteins. The range in different light frequencies emitted by the microspheres, determined by their diameter, coupled with the ability to use around 30 different dyes to stain them, means that scientists could theoretically uniquely tag up to 100,000 individual cells. The technique also allows them to follow cells in 3D structures, and is less complicated to carry out then other tagging methods.
They hope that this new method will allow better imaging of cell cultures in the lab, but also the tracking of macrophages in their immune response, dendritic cells in lymph nodes, or even map circulating tumor cells.