Seeing cells through silicon

A new type of near-IR microscopy images 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 Dr. 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.

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.”

Schematic of the near-IR quantitative phase microscope setup using a near-IR 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).

Taking advantage of the fact that silicon is transparent at IR and near-IR wavelengths, UTA assistant physics professor Dr. Samarendra Mohanty and students, and Barman and colleagues at MIT 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 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.

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

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 (red blood cells) imaged through silicon are shown on the top.

The 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 to swell. 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 Dr. 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.

The work appears in Scientific Reports. In the paper, the researchers reported using 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 IR. They also are working on modifying the system so that it can image in 3-D, similar to a CT scan.