Because of their unique characteristics, colloids have been used to investigate the fundamental physics of soft materials including both equilibrium phase behavior and kinetic processes. Unlike atoms, colloidal sizes can be conveniently tailored and are typically large enough to be probed individually with interaction strengths and effective ranges that can be modulated over several orders of magnitude. Despite these significant advantages, only relatively simple colloidal models such as spheres have been created; such systems in turn assemble into crystals of fairly limited symmetry, precluding the study of problems associated with complex structure. To push towards the synthesis of more complicated colloidal molecules, we use combined applied magnetic and anisotropic optical fields to fabricate colloidal chains. By integrating these induced forces within microfluidic channels and in flow, we grow colloidal chains one particle at one time, mimicking step-growth polymerization. The key advantage of this method is the ability to precisely control chain length and sequence, both essential for studies of self-assembly. In this, chain length is determined by a balance between the hydrodynamic shear stress, applied magnetic field, and the optical forces applied. Once a desired chain length is achieved, we fix the assembly in situ via the use of thiol-functionalized magnetic beads and a functionalized polyethylene glycol crosslinker. With the ability to perform directed assembly and irreversible fixation in flow, a route to the high-throughput synthesis of colloidal molecules can be achieved.
It has recently been demonstrated that diode laser bars can be used to not only optically trap red blood cells in flowing
microfluidic systems but also, stretch, bend, and rotate them. To predict the complex cell behavior at different locations
along a linear trap, 3D optical force characterization is required. The driving force for cells or colloidal particles within
an optical trap is the thermal Brownian force where particle fluctuations can be considered a stochastic process. For
optical force quantification, we combine diode laser bar optical trapping with Gabor digital holography imaging to
perform subpixel resolution measurements of micron-sized particles positions along the laser bar. Here, diffraction
patterns produced by trapped particles illuminated by a He-Ne laser are recorded with a CMOS sensor at 1000 fps where
particle beam position reconstruction is performed using the angular spectrum method and centroid position detection.
3D optical forces are then calculated by three calibration methods: the equipartition theorem, Boltzmann probability
distribution, and power spectral density analysis for each particle in the trap. This simple approach for 3D tracking and
optical control can be implemented on any transmission microscope by adding a laser beam as the illumination source
instead of a white light source.
We report red blood cell (RBC) stretching using a Zeiss Axioplan microscope, modified for phase contrast and optical
trapping using a 808 nm diode laser bar, as a tool to characterize RBC dynamics along a linear optical trap. Phase
contrast offers a convenient method of converting small variations of refractive index into corresponding amplitude
changes, differentially enhancing the contrast near cell edges. We have investigated the behavior of RBCs within both
static and dynamic microfluidic environments with a linear optical stretcher. Studies within static systems allow
characterization of cell interactions with the line optical force field without the complicating forces associated with
hydrodynamics. In flowing, dynamic systems, cells stretch along the optical trap down microfluidic channels and are
eventually released to recover their original shape. We record the dynamic cell response with a CMOS camera at 250 fps
and extract cell contours with sub-pixel accuracy using derivative operators. To quantify cell deformability, we measure
the major and minor axes of individual cells both within and outside of the trap, which also allows measurement of cell
relaxation. In these studies, we observe that cell rotation, stretching, and bending along the linear optical trap, are tightly
coupled to the modulation of optical power and cell speed inside our microfluidic systems.
The measurement of cell elastic parameters using optical forces has great potential as a reagent-free method for cell classification, identification of phenotype, and detection of disease; however, the low throughput associated with the sequential isolation and probing of individual cells has significantly limited its utility and application. We demonstrate a single-beam, high-throughput method where optical forces are applied anisotropically to stretch swollen erythrocytes in microfluidic flow. We also present numerical simulations of model spherical elastic cells subjected to optical forces and show that dual, opposing optical traps are not required and that even a single linear trap can induce cell stretching, greatly simplifying experimental implementation. Last, we demonstrate how the elastic modulus of the cell can be determined from experimental measurements of the equilibrium deformation. This new optical approach has the potential to be readily integrated with other cytometric technologies and, with the capability of measuring cell populations, enabling true mechanical-property-based cell cytometry.
Many proposed applications of microfluidics involve the manipulation of complex fluid mixtures such as blood or bacterial suspensions. To sort and handle the constituent particles within these suspensions, we have developed a miniaturized automated cell sorter using optical traps. This microfluidic cell sorter offers the potential to perform chip-top microbiology more rapidly and with less associated hardware and preparation time than other techniques currently available. To realize the potential of this technology in practical clinical and consumer lab-on-a-chip devices however, microscale control of not only particulates but also the fluid phase must be achieved. To address this, we have developed a mechanical fluid control scheme that integrates well with our optical separations approach. We demonstrate here a combined technique, one that employs both mechanical actuation and optical trapping for the precise control of complex suspensions. This approach enables both cell and particle separations as well as the subsequent fluid control required for the completion of complex analyses.
Conference Committee Involvement (2)
Optical Trapping and Optical Micromanipulation II
31 July 2005 | San Diego, California, United States