Phase imaging shows tension in red blood cells

03 Nov 2006

An optical technique that allows researchers to observe the membranes of living cells in real time could offer a better understanding of diseases such as sickle-cell anemia, malaria and cancer.

The interferometry-based technique, developed by US researchers at MIT's Spectroscopy Laboratory, can be used to study the mechanical properties of cell membranes on a nanometer scale.

In an upcoming issue of Physical Review Letters, the researchers describe how the technique -- a new variant of quantitative phase imaging (QPI) -- can be used to measure the mechanics of red blood cells and, in particular, the elasticity of their membranes. The technique could offer new insight into diseases that cause deformed red blood cells, such as sickle-cell anemia.

"We have developed several new techniques of quantitative phase imaging to obtain the refractive index of live cells, and to study highly dynamic phenomena such as cell motility, cell growth, and membrane dynamics," said lead author, Gabriel Popescu.

All forms of phase microscopy are based on the idea that different regions of the cell have different refractive indices, which causes light interacting with the cell to experience different phase changes. QPI takes this principle a step further by precisely quantifying the phase changes with optical interferometry. Here, a light beam probing the cell is compared with a reference beam to obtain an interference pattern that delivers nanometre- and millisecond-scale real-time data on the cell's structure and topography.

In their study, the researchers at MIT used a new variant of QPI, called stabilized Hilbert Phase Microscopy (sHPM), to determine the frequency of thermally-induced vibrations or "flickering" that are characteristic of red blood cells.

From this the team quantified the elasticity or tension of the cell membrane. Elasticity is a vital property of the cell membrane as it allows oxygen-carrying cells to squeeze through narrow capillaries in the body.

"By quantifying the nanometer-level fluctuations of red blood cells with QPI we have, for the first time, measured the membrane tension in a completely non-perturbing way," said Popescu.

Unlike alternative imaging techniques such as scanning electron microscopy (SEM), QPI does not require the target cell to be dehydrated, frozen or treated in other ways. However, QPI does not yet offer the same resolution as SEM, and the system must also be mounted in an isolated environment to optimize its resolution because the inteferometry device is highly sensitive to vibrations.

The MIT group is now using QPI to study how the mechanics of red blood cells are affected by diseases such as sickle-cell anemia and malaria. According to Popescu, QPI has the potential for widespread clinical use.

"We anticipate that QPI can become a clinical tool for studying the progression of these diseases and the effects of various drugs on membrane properties. With improved automation and packaging, this type of instrument will be suitable for commercialization."

Meanwhile, another group in MIT's Spectroscopy Lab is using QPI to study signal propagation in neurons. The project, led by Chris Fang-Yen, is based on the idea that neuronal membranes undergo deformation as electrical currents (in the form of action potentials) travel along their length.