David W. M. Marr

Design of a scanning laser optical trap for multiparticle manipulation

In recent years, single-beam optical traps have been used to manipulate individual colloids and biological objects such as cells. We have implemented a rapidly scanning laser optical trap with rates as high as 1200 Hz where a single laser beam is used to trap multiple colloids simultaneously. The optics are optimized to achieve a small laser focus size and a large scanning pattern in the sample. This approach provides great pattern flexibility and, because of the use of piezoelectrics, small particles (1 μm in diameter) in low-viscosity solvents, such as water, can be readily manipulated.

Microfluidic sorting system based on optical waveguide integration and diode laser bar trapping

Effective methods for manipulating, isolating and sorting cells and particles are essential for the development of microfluidic-based life science research and diagnostic platforms. We demonstrate an integrated optical platform for cell and particle sorting in microfluidic structures. Fluorescent-dyed particles are excited using an integrated optical waveguide network within micro-channels. A diode-bar optical trapping scheme guides the particles across the waveguide/micro-channel structures and selectively sorts particles based upon their fluorescent signature. This integrated detection and separation approach streamlines microfluidic cell sorting and minimizes the optical and feedback complexity commonly associated with extant platforms.

Optical trapping, manipulation, and sorting of cells and colloids in microfluidic systems with diode laser bars.

We demonstrate a new technique for trapping, sorting, and manipulating cells and micrometer-sized particles within microfluidic systems, using a diode laser bar.

Fabrication of linear colloidal structures for microfluidic applications

In this letter, an optical microfabrication and actuation method for the creation of microfluidic structures is described. In this approach, an optical trap is used to position and polymerize colloidal microspheres into linear structures to create particle or cell directing devices within microfluidic channel networks. To demonstrate the utility of these structures, two microscale particulate valves are shown, a passive design that restricts particulate flow in one direction and another design that directs particulate flow to one of two exit channels.

On the solid–fluid interface of adhesive spheres

The adhesive‐sphere interaction potential provides a good model system to study the influence of the attractive well depth on phase behavior and interfacial phenomena. We investigate the solid–fluid phase behavior of adhesive spheres with the modified weighted density approximation (MWDA) of Denton and Ashcroft. We then apply a planar‐averaged density functional approach (PWDA) to determine interfacial properties. We find both a narrowing of the interface between fluid and coexisting fcc solid and an increase in the interfacial energy with increasing attractive interaction strength in accord with the empirical relation γ≊ 0.47ΔHρ2/3s. In addition, we investigate metastable solid nucleation through calculation of metastable bcc solid–fluid interfacial tensions and find results suggesting the possibility of such a route to stable solid formation.

Laminar‐Flow‐Based Separations at the Microscale

The natural separation maintained by microfluidic flows is employed as the basis of a particle/cell sorting device. This method of separating particulate suspensions exploits the inherent laminar nature of microscale fluid dynamics and incorporates applied fields and image cytometry to enable sorting based upon any visually identifiable difference between colloid‐sized cells or particles. This technique may be used to easily isolate, separate, sort, or enrich virtually any suspension of microscale biological or colloidal particles within a microfluidic system. The entire footprint of the device described here is less than 0.01 mm2, allowing it to be readily incorporated within highly integrated micro total analysis systems (μTAS).

Flow control for capillary-pumped microfluidic systems

Advantages of performing analytical and diagnostic tasks in microfluidic-based systems include small sample volume requirements, rapid transport times and the promise of compact, portable instrumentation. The application of such systems in home and point-of-care situations has been limited, however, because these devices typically require significant associated hardware to initiate and control fluid flow. Capillary-based pumping can address many of these deficiencies by taking advantage of surface tension to pull fluid through devices. The development of practical instrumentation however will rely upon the development of precision control schemes to complement capillary pumping. Here, we introduce a straightforward, robust approach that allows for reconfigurable fluid guidance through otherwise fixed capillary networks. This technique is based on the opening and closing of microfluidic channels cast in a flexible elastomer via automated or even manual mechanical actuation. This straightforward approach can completely and precisely control flows such as samples of complex fluids, including whole blood, at very high resolutions according to real-time user feedback. These results demonstrate the suitability of this technique for portable, microfluidic instruments in laboratory, field or clinical diagnostic applications.

Flow resistance for microfluidic logic operations

Control of relative flow resistance is used for the actuation of both one- and two-input microfluidic “logical gates”. By taking advantage of system nonlinearities and despite the linear response of laminar flows associated with these length scales, a number of operators including the NOT, AND, OR, XOR, NOR, and NAND are demonstrated. Because these gates can be actuated simultaneously they can be combined to form more complicated devices such as a half adder. This approach is therefore flexible and illustrates that any macro- or microscale technique that can alter flow resistance can be used as the basis of a fluid-based logical micro-operator.

Cell deformation cytometry using diode-bar optical stretchers

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.

In situ assembly of linked geometrically coupled microdevices

Complex systems require their distinct components to function in a dynamic, integrated, and cooperative fashion. To accomplish this in current microfluidic networks, individual valves are often switched and pumps separately powered by using macroscopic methods such as applied external pressure. Direct manipulation and control at the single-device level, however, limits scalability, restricts portability, and hinders the development of massively parallel architectures that would take best advantage of microscale systems. In this article, we demonstrate that local geometry combined with a simple global field can not only reversibly drive component assembly but also power distinct devices in a parallel, locally uncoupled, and integrated fashion. By employing this single approach, we assemble and demonstrate the operation of check valves, mixers, and pistons within specially designed microfluidic environments. In addition, we show that by linking these individual components together, more complex devices such as pumps can be both fabricated and powered in situ.