Viscoelastic Response of Red Blood Cells in Linear Optical Traps

Abstract

The mechanical properties of biological cells can be used as a marker for individual cell's health, information not accessible by bulk measurements. For example malaria parasites are known to significantly stiffen the red blood cell (RBC) membrane, allowing identification of single infected cells based on their deformability. We have developed a non-contact method employing non-invasive optical forces, efficiently elongating RBCs within microfluidic channels to determine cell elastic properties. In this, the anisotropic beam of a single laser diode bar is used to create a line-shaped optical trap, capturing and elongating specimens along the laser axis. We perform static measurements on single RBCs to demonstrate the utility of this method. Simulations employing ray-tracing methods illustrate how the refraction of the asymmetric laser profile inherently creates antipodal stretching forces. Applying the membrane theory of thin shells enables us to determine the expected deformation based on our simulations and compare results to measured data.As the behavior of a viscoelastic material, such as the RBC's membrane, strongly depends on the timescales of applied forces, we perform frequency-dependent measurements with modulated external stimulus to determine the RBC's complex elastic moduli, properties not accessible by comparable techniques. Laser intensity, and therefore the forces responsible for cell deformation, is modulated at frequencies varying over three orders of magnitude. Cell response is recorded using a high-speed camera system, allowing us to correlate the applied external load to the phase-shifted viscoelastic behavior of the RBC and to obtain the frequency-dependent dissipation of energy during one cycle of oscillation.Employing this new technique, cells can be deformed while streaming along the line-shaped trap in flowing environment, allowing application in high-throughput systems. Use of comparably simple optics and inexpensive laser sources facilitates potential implementation in small platforms and hand-held devices.