Single Cell Refractive Index Measurements for Optical Biomechanics

Abstract

Accurate measurements of the optical properties of living cells are essential for both therapeutic and diagnostic applications of biomedical optics. Changes in the index of refraction of red blood cells, for example, have been used to chart the progress of the malaria parasite (1). The cellular index of refraction is also of critical importance in cellular biomechanics measurements. In the optical stretcher, a dual-beam optical trap, small variations in the index of refraction can lead to substantial errors in the assessment of single-cell elasticity. To determine the cell-to-cell variation of the index of refraction, we have integrated a Mach-Zehnder interferometer with a conventional transmitted light microscope to obtain high-resolution interferograms of individual cells. Quantitative phase images are obtained by performing a Hilbert transform and using Goldstein's algorithm to unwrap the wrapped phase image. The decoupling technique of Rappaz et al. was used to obtain cell height and index of refraction maps (2). By employing several laser sources, the cellular index of refraction can be measured as function of wavelength. Measurements of multiple cell lines at several wavelengths indicate that the refractive index can, in fact, vary significantly from cell-to-cell. Along with these results we will discuss the implications for accurate single-cell biomechanical measurements made with the optical stretcher. 1. Park et al. Proc. Natl. Acad. Sci. USA 105, 1730 (2008) 2. Rappaz et al. Opt. Express 13, 9361 (2005) Supported by NIH P20 RR16469 from the National Center for Research Resources (NCRR) and NIH R15 GM085776