Aaron L. Birkbeck

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.

VCSEL Arrays as Micromanipulators in Chip-Based Biosystems

The potential use of vertical cavity surface emitting laser (VCSEL) arrays for applications in cell analysis and tissue engineering is investigated by means of parallel optical trapping and active manipulation of biological cells on microfluidic chips. The simultaneous and independent transport of nine cells using a 3×3 array of VCSELs has been demonstrated experimentally; indicating that larger 2-dimensional array transport using individually addressable tweezers is achievable with VCSEL array devices. The transport properties of VCSEL tweezers have been investigated for various types of cells including 3T3 Murine fibroblasts, yeast, rat primary hepatocytes and human red blood cells. Due to the low relative index of refraction between the biological cell and surrounding medium and the relatively low optical power available with present VCSELs, the Laguerre-Gaussian laser mode output of the VCSEL is more favorable to use in an optical tweezer since the highest intensity is located at the outer ring of the optical aperture, producing stronger optical confinement at lower power levels. For larger biological cells or cells with a lower relative index of refraction, the power limitations of a single VCSEL were overcome through the binning of several VCSELs together by combining the outputs of a sub-array of VCSELs into a collective optical tweezer. A comprehensive analysis and simulation of how the VCSELs’ pitch and output beam divergence influence the operation of the resultant optical tweezer array is presented along with our experimental data. Employing the methods of parallel array transport and the binning of multiple VCSEL outputs, allows for the manipulation and spatial arrangement of different types of cells in a co-culture so as to facilitate the formation of engineered tissues.

Laser-tweezer-controlled solid immersion lens for high-resolution imaging in microfluidic and biological samples

A novel technique is presented which integrates the capacity of a laser tweezer to optically trap and manipulate objects in three-dimensions with the resolution-enhanced imaging capabilities of a solid immersion lens (SIL). Up to now, solid immersion lens imaging systems have relied upon cantilever-mounted SILs that are difficult to integrate into microfluidic systems and require an extra alignment step with external optics. As an alternative to the current state-of-art, we introduce a device that consists of a free-floating SIL and a laser optical tweezer. In our design, the optical tweezer, created by focusing a laser beam through high numerical aperture microscope objective, acts in a two-fold manner: both as a trapping beam for the positioning and alignment of the SIL and as an near-field scanning beam to image the sample through the SIL. Combining the alignment, positioning, and imaging functions into a single device allows for the direct integration of a high resolution imaging system into microfluidic and biological environments.

Laser-tweezer-controlled solid immersion microscopy in microfluidic systems.

We describe the creation and implementation of a near-field scanning solid immersion microscope that is specifically tailored for use in microfluidic systems. The microscope comprises a newly fabricated Weierstrass solid immersion lens (SIL), which is detached from its substrate and is free floating in the fluid, and a laser optical tweezer, which serves both as a trapping beam for alignment and positioning of the SIL and as a near-field scanning beam that images the sample through the SIL. A discussion of the SILs fabrication method is presented along with experimental results that demonstrate the effectiveness of our microscope design.

Optical Manipulation of Objects in Microfluidic Devices

In this paper, we present object manipulation methodologies in microfluidic devices based on object-photon interactions. Devices were fabricated by polydimethylsiloxane (PDMS) elastomer molding of channel structures over photolithographically defined patterns using a thick negative photoresist. Inorganic objects including polystyrene spheres and organic objects including live cells were transferred into fluidic channels using a syringe pump. The objects were trapped and manipulated within the fluidic channels using optical tweezers formed by VCSEL arrays, with only a few mW of optical power. We have also shown that it is possible to manipulate multiple objects as a whole assemble by using an optically-trapped particle as a handle, or an “optical handle”. Optical manipulation will have applications in biomedical devices for drug discovery, cytometry and cell biology research.

Optically Written Conical Lenses for Resonant Structures and Detector Arrays

We demonstrate a novel method of fabricating conical lens arrays in photopolymers which offers one-time alignment advantages. The technique may become useful for coupling into detector/modulator arrays as well as optical fibers.

Two-dimensional electrophoretic microlens alignment

An adaptive alignment technique is presented that provides precise control and active positioning of sub-millimeter-sized spherical lenses in two-dimensions through the application of electrophoretic forces in a microfluidic well. The device is comprised of a lithographically patterned microfluidic well and electrodes that can be addressed to position or align the spherical microlens to the corresponding beam source. The motion of the microlens is controlled using CMOS compatible voltages (3V - 1 (mu) A) that are applied to opposite electrodes in the microfluidic well, creating an electrical field in the solution. By applying voltages to opposite electrode pairs, we have demonstrated the movement of spherical microlenses with sizes ranging from 0.87 micrometers to 40 micrometers in directions parallel to the electrode surface. Under a bias of 3 volts, the microspheres had an experimentally measured electrophoretic velocities ranging from 13 to 16 micrometers /s. Optical alignment of the spherical or ball microlens can be accomplished using feedback from a photodetector to position the lens for maximum efficiency. Using this device, it is possible to actively align microlenses to optical fibers, VCSELs, LEDs, photodetectors, etc.

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

Application of VCSEL's to a range of bioengineering assays

Vertical cavity surface emitting lasers (VCSEL) have been used for variety of applications including data storage, data readout, detection, and optical interconnects. Here, we present the application of VCSELs for manipulation and sorting of live cells.