Seeing Cells Through Silicon

A new type of near-infrared microscopy can image cells through a silicon wafer, providing more information about diseased or infected cells flowing through microfluidic devices.

“This has the potential to merge research in cellular visualization with all the exciting things you can do on a silicon wafer,” said Ishan Barman, a former postdoc in MIT’s Laser Biomedical Research Center (LBRC) and a lead author of a paper on the technology.

Silicon, the workhorse of the microelectronics industry, is commonly used to build lab-on a-chip microfluidic devices, which can sort and analyze cells based on their molecular properties. Such devices have many potential applications in research and diagnostics, but they could be even more useful if scientists could image the cells inside the devices, said Barman, who is now an assistant professor of mechanical engineering at Johns Hopkins University.

Schematic of the near-infrared quantitative phase microscope (NIR-QPM) setup using a near-infrared illumination source (Ti:sapphire laser) to acquire images of biological samples through an opaque silicon substrate. CDL = condenser lens; CLL = collimating lens; MO = microscopic objective; BS1 and BS2 = Beamsplitters; M1, M2 and M3 = mirrors; FL = focusing lens; CMOS = complementary metal oxide semiconductor. Scientific Reports 3, Article number: 2822 (doi:10.1038/srep02822).

Such devices "are revolutionizing high-throughput analysis of cells and molecules for disease diagnosis and screening of drug effects. However, very little progress has been made in the optical characterization of samples in these systems," said Bipin Joshi, a recent University of Texas at Austin (UTA) graduate and another lead author on the paper. "The technology we've developed is well suited to meet this need."

Taking advantage of the fact that silicon is transparent at infrared and near-infrared wavelengths, Barman and colleagues at MIT and UTA assistant physics professor Samarendra Mohanty and students adapted a microscopy technique known as quantitative phase imaging. The technique works by sending a laser beam through a sample, then splitting the beam in two. By recombining the beams and comparing the information carried by each, the researchers can determine the sample’s height and its refractive index.

Traditional quantitative phase imaging uses a helium neon (HeNe) laser, which produces visible light, but the new system uses a Ti:sapphire laser that can be tuned to IR and near-IR wavelengths, with a wavelength of 980 nm optimal for this study.

Schematic of the lab-on-a-chip system for the study of mechanical, chemical and electric perturbation of various types of cells on silicon-based microfluidic and multielectrode array platform. Quantitative phase image of a human embryonic kidney cell, and RBC imaged through silicon is shown on the top. Scientific Reports 3, Article number: 2822 (doi:10.1038/srep02822).

Using the system, the researchers measured changes in the height of red blood cells, with nanoscale sensitivity, through a silicon wafer similar to those used in most electronics labs.

As red blood cells flow through the body, they often have to squeeze through very narrow vessels. When these cells are infected with malaria, they lose this ability to deform, and form clogs in tiny vessels. The new microscopy technique could help scientists study how this happens, former MIT postdoc Narahara Chari Dingari said; it could also be used to study the dynamics of the malformed blood cells that cause sickle-cell anemia.

The new system also was used to monitor human embryonic kidney cells as pure water was added to their environment — a shock that forces the cells to absorb water and swell up. The researchers measured how much the cells distended and calculated the change in their index of refraction.

“Nobody has shown this kind of microscopy of cellular structures before through a silicon substrate,” said Mohanty, senior author of the paper.

“This is an exciting new direction that is likely to open up enormous opportunities for quantitative phase imaging,” said Gabriel Popescu, an assistant professor of electrical engineering and computer science at the University of Illinois at Urbana-Champaign, who was not part of the research team.

“The possibilities are endless: From micro- and nanofluidic devices to structured substrates, the devices could target applications ranging from molecular sensing to whole-cell characterization and drug screening in cell populations,” he added.

Mohanty’s lab is using the system to study how neurons grown on a silicon wafer communicate with each other. His group recently combined the near-infrared quantitative phase imaging with near-infrared optical tweezers for tomographic imaging of cells [N. Cardenas and S.K. Mohanty (2013). Optical tweezers assisted quantitative phase imaging led to thickness mapping of red blood cells. Applied Physics Letters, Vol. 103, p. 013703.]

The work, funded by the National Institute of Biomedical Imaging and Bioengineering and Nanoscope Technologies LLC, appears in the Oct. 2 issue of Scientific Reports. In the paper, the researchers used silicon wafers that were about 150 to 200 µm thick, but they have since shown that thicker silicon can be used if the wavelength of light is increased into the infrared. They also are working on modifying the system so that it can image in three dimensions, similar to a CT scan.

For more information, visit: www.uta.edu