Optical Systems for Cellular Imaging

Abstract

Optical imaging and manipulation of cells, associated with computer-aided cell recognition algorithm, are essential measures to execute cell identification and classification in modern biological and medical engineering. Generally, implementation of physically classification of cells/bio-particles involve real-time recognition of cells upon acquisition of cellular image, followed up with optical cell manipulation such as branching (directly push off to another microfluidic channel) by an optical force using laser beams, or immediate trapping and translation by an optical tweezer to a designated position. Contactless and nondestructive nature of optical manipulation, where focused laser beams are used for cell trapping, translation/rotation, and 3D arrangement, has brought about numerous applications in biotechnology, DNA nanotechnology, and cell processing in microfluidics. Following a brief introduction to working principle of generic optical tweezers, this chapter discusses the manipulation of biological particles using a single beam in LP₂₁ mode, a low-order fiber optic transmission mode. With an intrinsic four-lobed intensity distribution and high coherence, we demonstrate that an LP₂₁ mode beam can be focused to form an optical chuck, allowing the capture and reorganization of biological particles inside clusters, as well as both translation and rotation of the particle by simply rotating a segment of fiber in the optical train. The force that the optical chuck exerts on target bio-particles in the process of rotation and translation was analyzed using a theoretical model based on ray optics, with a good agreement between the simulated model and the experimental measurements. An optical tweezer system formed by a focused beam of LP₂₁ mode in fiber was demonstrated to be a simple and efficient method in the manipulation of bioparticles, including cell pairing, separation or re-grouping by selective translation of captured cells, and the rotation of cell clusters. Translational was estimated to be 0.84 pN experimentally, in comparison with 1.2 pN modeled by using geometric optics (RO model) and a Gaussian approximation of beam lobes. This all-fiber single-probe optical chuck has the advantages of (1) being a robust, easy-to-implement single fiber probe; (2) peak irradiance on target particles are significantly lower than multi-beam tweezers using focused Gaussian beams, thus reducing the risk of target damage from high intensity light; (3) LP₂₁ mode can be generated across a wide range of wavelength; (4) LP₂₁ mode fiber optical chuck are capable of performing manipulation with translation, curved motion, and rotation. Such a system has many applications in individual cell sorting/filtering, and orientation control in various biomedical testing and diagnostic systems.