Bing Shao

Parallel transport of biological cells using individually addressable VCSEL arrays as optical tweezers

We have demonstrated the use of vertical cavity surface emitting lasers (VCSELs) for optical trapping and active manipulation of live biological cells and microspheres. We have experimentally verified that the Laguerre‐Gaussian laser mode output from the VCSEL functions just as well as the traditional Gaussian fundamental laser mode for optically trapping biological cells and may be preferable since the highest intensity of the Laguerre‐Gaussian mode is located at the outer ring of the optical aperture, which allows for stronger optical confinement to be obtained for a lower total power. Another advantage that VCSELs have over conventional gas and diode lasers is their ability to be manufactured in an array form. Using a 2 � 2 array of VCSELs, the simultaneous and independent transport of four human red blood cells is demonstrated indicating that much larger two-dimensional VCSEL arrays can be used as individually addressable optical tweezers in biological chips and systems. This parallel transport capability will have a significant impact in currently developing biochip array and assay technologies through the facilitation of the selection, relocation, and precision placement of cells. # 2002 Elsevier Science B.V. All rights reserved.

Angular resolved light scattering for discriminating among marine picoplankton: modeling and experimental measurements

In order to assess the capability to optically identify small marine microbes, both simulations and experiments of angular resolved light scattering (ARLS) were performed. After calibration with 30-nm vesicles characterized by a nearly constant scattering distribution for vertically polarized light (azimuthal angle=90 degrees ), ARLS from suspensions of three types of marine picoplankton (two prokaryotes and one eukaryote) in seawater was measured with a scattering device that consisted of an elliptical mirror, a rotating aperture, and a PMT. Scattered light was recorded with adequate signal-to-noise in the 40-140 degrees . Simulations modeled the cells as prolate spheroids with independently measured dimensions. For the prokaryotes, approximated as homogeneous spheroids, simulations were performed using the RM (Rayleigh-Mie) - I method, a hybrid of the Rayleigh-Debye approximation and the generalized Lorentz-Mie theory. For the picoeukaryote, an extended RM - I method was developed for a coated spheroid with different shell thickness distributions. The picoeukaryote was then modeled as a coated sphere with a spherical core. Good overall agreements were obtained between simulations and experiments. The distinctive scattering patterns of the different species hold promise for an identification system based on ARLS.

Dynamically adjustable annular laser trapping based on axicons

To study the chemotactic response of sperm to an egg and to characterize sperm motility, an annular laser trap based on axicons is designed, simulated with the ray-tracing tool, and implemented. The diameter of the trapping ring can be adjusted dynamically for a range of over 400 microm by simply translating one axicon along the optical axis. Trapping experiments with microspheres and dog sperm demonstrate the feasibility of the system, and the power requirement agrees with theoretical expectation. This new type of laser trapping could provide a prototype of a parallel, objective, and quantitative tool for animal fertility and biotropism study.

Size tunable three-dimensional annular laser trap based on axicons

A three-dimensional (3D) ring-shaped laser trap has been built using axicons. The diameter of this laser trap ranges from 70 to 140 mum and is adjusted by simply changing the position of one axicon in the optical path. Parallel 3D trapping of 5 mum silica microspheres and 3D confinement of cells along the ring are demonstrated. In this system the special optical properties of axicons are used to create a continuous annular trap with high power efficiency and a constant numerical aperture. This new approach, without any mechanical scanning, offers significant potential for applications in cell motility analysis and biotropism studies.

Automated motile cell capture and analysis with optical traps.

Laser trapping in the near infrared regime is a noninvasive and microfluidic-compatible biomedical tool. This chapter examines the use of optical trapping as a quantitative measure of sperm motility. The single point gradient trap is used to directly measure the swimming forces of sperm from several different species. These forces could provide useful information about the overall sperm motility and semen quality. The swimming force is measured by trapping sperm and subsequently decreasing laser power until the sperm is capable of escaping the trap. Swimming trajectories were calculated by custom built software, an automatic sperm tracking algorithm called the single sperm tracking algorithm or SSTA. A real-time automated tracking and trapping system, or RATTS, which operates at video rate, was developed to perform experiments with minimal human involvement. After the experimenter initially identifies and clicks the computer mouse on the sperm-of-interest, RATTS performs all further tracking and trapping functions without human intervention. Additionally, an annular laser trap which is potentially useful for high-throughput sperm sorting based on motility and chemotaxis was developed. This low power trap offers a more gentle way for studying the effects of laser radiation, optical force, and external obstacles on sperm swimming pattern.

Microscope-integrated micromanipulator based on multiple VCSEL traps

The compactness of VCSELs (Vertical Cavity Surface Emitting Lasers) provides them the ability to meet the demands of current biochip technologies. In earlier research, optical trapping of live biological cells and microspheres based on VCSEL array has been realized in the form of parallel static traps on a translation stage. In microfluidic systems (lab-on-a-chip devices), the background flow introduces complexity and uncertainty in velocity and force analysis on target microparticles, making the capability of transporting biological objects without moving sample highly desirable. Moreover independently controllable traps offer more flexibility in microparticle manipulation. In this paper, a microscope-integrated VCSEL trapping system capable of independent control and batch processing of microparticles is devised and demonstrated. Optical design considerations for keeping stable trapping performance while multiplexing are addressed. Both a single optical trap and a trap array can be controlled independently by tilting mirror while their relative depth can be adjusted without power loses in optical system. In the micromanipulator, a single VCSEL trap serves as a collector and distributor, while a VCSEL array provides the carrier for synchronous processing and small range shift. Potential improvement based on two independently controlled VCSEL arrays is discussed and related applications are investigated.

Axicon-based annular laser trap for studies on sperm activity

As a powerful and noninvasive tool, laser trapping has been widely applied for the confinement and physiological study of biological cells and organelles. Researchers have used the single spot laser trap to hold individual sperm and quantitatively evaluated the motile force generated by a sperm. Early studies revealed the relationship between sperm motility and swimming behavior and helped the investigations in medical aspects of sperm activity. As sperm chemotaxis draws more and more interest in fertilization research, the studies on sperm-egg communication may help to explain male or female infertility and provide exciting new approaches to contraception. However, single spot laser trapping can only be used to investigate an individual target, which has limits in efficiency and throughput. To study the chemotactic response of sperm to eggs and to characterize sperm motility, an annular laser trap with a diameter of several hundred microns is designed, simulated with ray tracing tool, and implemented. An axicon transforms the wavefront such that the laser beam is incident on the microscope objective from all directions while filling the back aperture completely for high efficiency trapping. A trapping experiment with microspheres is carried out to evaluate the system performance. The power requirement for annular sperm trapping is determined experimentally and compared with theoretical calculations. With a chemo-attractant located in the center and sperm approaching from all directions, the annular laser trapping could serve as a speed bump for sperm so that motility characterization and fertility sorting can be performed efficiently.

Real-Time Sperm Tracking and Ring Trapping System

An automatic microscope system is designed to study sperm response to annular laser trap. The sperm velocity, microscope stage movement and laser power at each image frame is saved at video rate.

Using optical forces for the characterization of biological cell activities

Optical forces successfully manipulate biological cells and measure small forces such as molecular binding forces. This paper focuses on the use of optical forces for the direct characterization of various activities of biological cells