Michael P. MacDonald

Microfluidic sorting in an optical lattice

The response of a microscopic dielectric object to an applied light field can profoundly affect its kinetic motion. A classic example of this is an optical trap, which can hold a particle in a tightly focused light beam. Optical fields can also be used to arrange, guide or deflect particles in appropriate light-field geometries. Here we demonstrate an optical sorter for microscopic particles that exploits the interaction of particles—biological or otherwise—with an extended, interlinked, dynamically reconfigurable, three-dimensional optical lattice. The strength of this interaction with the lattice sites depends on the optical polarizability of the particles, giving tunable selection criteria. We demonstrate both sorting by size (of protein microcapsule drug delivery agents) and sorting by refractive index (of other colloidal particle streams). The sorting efficiency of this method approaches 100%, with values of 96% or more observed even for concentrated solutions with throughputs exceeding those reported for fluorescence-activated cell sorting. This powerful, non-invasive technique is suited to sorting and fractionation within integrated (‘lab-on-a-chip’) microfluidic systems, and can be applied in colloidal, molecular and biological research.

Optical tweezers: the next generation

JOHANNES Kepler is famous for discovering the laws of planetary motion, but he is less well known for writing what may have been the first science-fiction story to involve space travel. During his observations, the German astronomer noticed that tails of comets always point away from the Sun, which suggested that the Sun was exerting a sort of radiant pressure. This led him in 1609 – the year in which he published the first of his laws – to propose sailing from the Earth to the Moon on light itself. Of course, that was and still is the stuff of science fiction, but 400 years later Keplers initial ideas about moving matter with light are very much a reality.

Trapping and manipulation of low-index particles in a two-dimensional interferometric optical trap

We demonstrate optical trapping and manipulation of low-index spheres in two dimensions, using the pattern produced by two interfering plane waves. This technique shows, for what is believed to be the first time, alignment of an array of hollow spheres and simultaneous manipulation of high- and low-index particles in the horizontal plane. Furthermore, rodlike particles (up to 30microm in length) are manipulated simultaneously with the low-index particles. This technique offers a practical method for manipulating bubbles, low-index droplets, or rodlike biological samples.

Fractionation of polydisperse colloid with acousto-optically generated potential energy landscapes

The motion of colloidal particles on a periodic optical potential energy landscape in the presence of an external driving force may result in particle separation. In contrast to recent methods of holographic or interferometric generation of such landscapes, we use an acousto-optic deflector to create two-dimensional landscapes. We present what is believed to be the first experimental realization of fractionation with simultaneous sorting of four different sizes of colloidal microparticle into laterally separated parallel laminar streams.

Revolving interference patterns for the rotation of optically trapped particles

Abstract Optically trapped objects are rotated controllably in the interference pattern between a Laguerre–Gaussian (LG) beam and a Gaussian beam. In this work the interference pattern is analysed and its properties as it propagates are modelled, showing the important role played by the Guoy-phase of the two interfering beams. An analysis of producing controlled rotation of the interference pattern using a glass plate is presented demonstrating the ease with which the rotation can be controlled.

Myosin-II-mediated cell shape changes and cell intercalation contribute to primitive streak formation

Primitive streak formation in the chick embryo involves large-scale highly coordinated flows of more than 100,000 cells in the epiblast. These large-scale tissue flows and deformations can be correlated with specific anisotropic cell behaviours in the forming mesendoderm through a combination of light-sheet microscopy and computational analysis. Relevant behaviours include apical contraction, elongation along the apical–basal axis followed by ingression, and asynchronous directional cell intercalation of small groups of mesendoderm cells. Cell intercalation is associated with sequential, directional contraction of apical junctions, the onset, localization and direction of which correlate strongly with the appearance of active myosin II cables in aligned apical junctions in neighbouring cells. Use of class specific myosin inhibitors and gene-specific knockdown shows that apical contraction and intercalation are myosin II dependent and also reveal critical roles for myosin I and myosin V family members in the assembly of junctional myosin II cables.

Cellular and Colloidal Separation Using Optical Forces

The separation or sorting of cellular and colloidal particles is currently a central topics of research. In this chapter, we give an overview of the range of optical methods for cell sorting. We begin with an overview of fluorescence and magnetically activated cell sorting. We progress to describing methods at the microfluidic scale level particularly those exploiting optical forces. We distinguish between what we term passive and active schemes for sorting. Optical forces pertinent to the sorting schemes are described, notably the gradient force and the optical radiation pressure (or scattering force). We discuss some of the most recent advances. This includes techniques without fluid flow where we have either stationary or moving light patterns to initiate separation. Further methods have shown how using an externally driven flow either counter-propagating against a light field (optical chromatography) or over a periodic light pattern (an optical potential energy landscape) may result in the selection of particles and cells based on physical attributes such as size and refractive index. We contrast these schemes with the field of dielectrophoresis where electric field gradients may separate cells and also briefly mention the upcoming area of light-induced dielectrophoresis which marries the reconfigurability of optical fields with the power of dielectrophoresis.

Colloidal sorting in dynamic optical lattices

Passive microfluidic sorting techniques based upon the interaction of particles with an optically defined potential energy landscape have possible advantages over active sorting techniques such as microfluorescence activated cell sorting (FACS), including ease of integration into lab-on-a-chip systems, reconfigurability, and scalability. Rather than analysing and deflecting a single-file stream of particles one by one, a passive approach intrinsically aimed at parallel processing may, ultimately, offer greater potential for high throughput. However attempts to sort many particles simultaneously in high density suspensions are inevitably limited by particle–particle interactions, which lead to a reduction in the efficiency of the sorting. In this paper we describe two different approaches aimed at reducing colloidal traffic flow problems. We find that continuous translation of the sorting lattice helps to reduce nearest neighbour particle spacing, providing promise for efficiency improvements in future high throughput applications, and that a flashing lattice yields a reduction in unwanted pile-up and spillover effects which otherwise limit the efficiency of sorting.

Optical Separation of Cells on Potential Energy Landscapes: Enhancement With Dielectric Tagging

We review the emergent techniques of microfluidic sorting of colloidal and cellular samples using optical forces. We distinguish between what we term as passive and active forms of particle sorting where we can sort either with the use of a fluorescent marker (active) or based on physical attributes alone (passive). We then examine cell sorting with optical potential landscapes such as a Bessel light beam and a multibeam interference pattern. For both forms of optical potential energy landscape, we further present the possibility of enhancing the optical sorting process by tagging dielectric microspheres onto the cells. The results suggest that the methodology of tagging can enhance the sorting of cells as they subsequently respond more strongly to an applied optical field or potential energy landscape. This technique presents a simple method to enhance the sorting process.

Moving interference patterns created using the angular Doppler-effect

We use the angular Doppler-effect to obtain stable frequency shifts from below one Hertz to hundreds of Hertz in the optical domain, constituting a control of 1 part in 1014. For the first time, we use these very small frequency shifts to create continuous motion in interference patterns including the scanning of linear fringe patterns and the rotation of the interference pattern formed from a Laguerre-Gaussian beam. This enables controlled lateral and rotational movement of trapped particles.