A new laser-based analysis method can detect optical pressure differences among classes of blood cells, and it doesn't rely on prior knowledge, antibodies or fluorescent labels for discrimination. Scientists at the US Naval Research Laboratory wanted to find an alternative to biological analysis systems that rely on labels, as they can be costly and labor-intensive. So the investigators tested the efficacy of optical pressure in identifying the various components of whole blood. When a laser beam impinges on a biological particle, a force is generated due to the scattering and refraction of photons. The resulting force is called optical pressure and can be used to physically move a biological cell, suspended in water, several millimeters. Microscope image showing a trapped cell in the optical chromatography setup. Cells are brought into the channel by the pressure-driven flow that runs from the top left to the bottom right of the image. As cells enter the channel, the laser shutter is opened and the beam pushes the cell from right to left. Flow is adjusted until the cell rests at the measurement location, at which point the flow rate is recorded and the cell is released. (Image: US Naval Research Laboratory) Using this laser technique, scientists can exploit the inherent differences in optical pressure, which arise from variations in particle size, shape, refractive index or morphology, as a means of separating and characterizing particles. As an initial step toward developing a system for label-free sorting and characterization of blood components, the optical pressures of purified human blood components — including lymphocytes, monocytes, granulocytes and erythrocytes — have been determined. Significant differences exist among the cell types, indicating the potential for separations based on these optical pressures. “While additional research is required, this is an important step toward the development of a system for the label-free optical fractionation of blood cells and components based on intrinsic characteristics,” said Sean J. Hart, who worked alongside Colin G. Hebert and Alex Terray. In general, the throughput for optical-based sorting has been relatively low, on the order of tens of cells per second. However, with an increase in both fluid flow and laser power, the throughput could be increased significantly, exceeding 100 particles per second in some favorable cases. Such a system could be used in the future for antibody-free detection of blood-borne pathogens for the prevention of sepsis and other diseases as well as for the detection of biological threat agents. For more information, visit: www.nrl.navy.mil