Soft lithography is an alternative to silicon-based micromachining that uses replica molding of nontraditional elastomeric materials to fabricate stamps and microfluidic channels. We describe here an extension to the soft lithography paradigm, multilayer soft lithography, with which devices consisting of multiple layers may be fabricated from soft materials. We used this technique to build active microfluidic systems containing on-off valves, switching valves, and pumps entirely out of elastomer. The softness of these materials allows the device areas to be reduced by more than two orders of magnitude compared with silicon-based devices. The other advantages of soft lithography, such as rapid prototyping, ease of fabrication, and biocompatibility, are retained.

http://science.sciencemag.org/content/sci/288/5463/113.full.pdf

Monolithic microfabricated valves and pumps by multilayer soft lithography

High-density microfluidic chips contain plumbing networks with thousands of micromechanical valves and hundreds of individually addressable chambers. These fluidic devices are analogous to electronic integrated circuits fabricated using large scale integration (LSI). A component of these networks is the fluidic multiplexor, which is a combinatorial array of binary valve patterns that exponentially increases the processing power of a network by allowing complex fluid manipulations with a minimal number of inputs. These integrated microfluidic networks can be used to construct a variety of highly complex microfluidic devices, for example the microfluidic analog of a comparator array, and a microfluidic memory storage device resembling electronic random access memories.

https://science.sciencemag.org/content/sci/298/5593/580.full.pdf

Microfluidic large scale integration

Microsystems create new opportunities for the spatial and temporal control of cell growth and stimuli by combining surfaces that mimic complex biochemistries and geometries of the extracellular matrix with microfluidic channels that regulate transport of fluids and soluble factors. Further integration with bioanalytic microsystems results in multifunctional platforms for basic biological insights into cells and tissues, as well as for cell-based sensors with biochemical, biomedical and environmental functions. Highly integrated microdevices show great promise for basic biomedical and pharmaceutical research, and robust and portable point-of-care devices could be used in clinical settings, in both the developed and the developing world.

Cells on chips

The ability to manipulate biological cells and micrometre-scale particles plays an important role in many biological and colloidal science applications. However, conventional manipulation techniques--including optical tweezers, electrokinetic forces (electrophoresis, dielectrophoresis, travelling-wave dielectrophoresis), magnetic tweezers, acoustic traps and hydrodynamic flows--cannot achieve high resolution and high throughput at the same time. Optical tweezers offer high resolution for trapping single particles, but have a limited manipulation area owing to tight focusing requirements; on the other hand, electrokinetic forces and other mechanisms provide high throughput, but lack the flexibility or the spatial resolution necessary for controlling individual cells. Here we present an optical image-driven dielectrophoresis technique that permits high-resolution patterning of electric fields on a photoconductive surface for manipulating single particles. It requires 100,000 times less optical intensity than optical tweezers. Using an incoherent light source (a light-emitting diode or a halogen lamp) and a digital micromirror spatial light modulator, we have demonstrated parallel manipulation of 15,000 particle traps on a 1.3 x 1.0 mm2 area. With direct optical imaging control, multiple manipulation functions are combined to achieve complex, multi-step manipulation protocols.

Massively parallel manipulation of single cells and microparticles using optical images.

Ac electrokinetics is concerned with the study of the movement and behaviour of particles in suspension when they are subjected to ac electrical fields. The development of new microfabricated electrode structures has meant that particles down to the size of macromolecules have been manipulated, but on this scale forces other than electrokinetic affect particles behaviour. The high electrical fields, which are required to produce sufficient force to move a particle, result in heat dissipation in the medium. This in turn produces thermal gradients, which may give rise to fluid motion through buoyancy, and electrothermal forces. In this paper, the frequency dependency and magnitude of electrothermally induced fluid flow are discussed. A new type of fluid flow is identified for low frequencies (up to 500 kHz). Our preliminary observations indicate that it has its origin in the action of a tangential electrical field on the diffuse double layer of the microfabricated electrodes. The effects of Brownian motion, diffusion and the buoyancy force are discussed in the context of the controlled manipulation of sub-micrometre particles. The orders of magnitude of the various forces experienced by a sub-micrometre latex particle in a model electrode structure are calculated. The results are compared with experiment and the relative influence of each type of force on the overall behaviour of particles is described.

/pdf/ac-electrokinetics-a-review-of-forces-in-microelectrode-529ym3z4cu.pdf

Ac electrokinetics: a review of forces in microelectrode structures

Electrorotation measurements were used to demonstrate that the dielectric properties of the metastatic human breast cancer cell line MDA231 were significantly different from those of erythrocytes and T lymphocytes. These dielectric differences were exploited to separate the cancer cells from normal blood cells by appropriately balancing the hydrodynamic and dielectrophoretic forces acting on the cells within a dielectric affinity column containing a microelectrode array. The operational criteria for successful particle separation in such a column are analyzed and our findings indicate that the dielectric affinity technique may prove useful in a wide variety of cell separation and characterization applications.

/pdf/separation-of-human-breast-cancer-cells-from-blood-by-491v8k59k5.pdf

Separation of Human Breast Cancer Cells From Blood by Differential Dielectric Affinity

Applications of dielectrophoresis in biotechnology

A simple and practical procedure is described for the design of electrodes that produce well defined non-uniform electric fields for the study and application of negative dielectrophoresis. Such electrode systems can be used to direct and assemble colloidal particles away from electrode edges, and this is demonstrated for the case of yeast cells.

https://iopscience.iop.org/article/10.1088/0957-0233/2/12/005/pdf

Electrode design for negative dielectrophoresis

A method was developed for studying the dielectrophoretic properties of both homogeneous and mixed populations of mammalian cells. Computerized image analysis was employed to quantify the rate of motion of cells suspended in low conductivity medium as they moved under the influence of a non-uniform alternating electric field produced by an interdigitated electrode array. As expected for dielectrophoresis, cells collected at highly inhomogeneous electric field regions of the array when the electrical polarizability of cells exceeded that of the suspending medium or away from such regions when their polarizability was less than that of their medium. These two types of behaviour are classified as positive or negative dielectrophoresis respectively, and the cell collection patterns agreed well with those computed for the electrode configuration employed. The dielectrophoretic characteristics of normal, leukaemic, and differentiation-induced leukaemic mouse erythrocytes were measured as a function of frequency in the range 5*102-105 Hz and were shown to be significantly different.

https://iopscience.iop.org/article/10.1088/0957-0233/3/5/001/pdf

Dielectrophoretic separation of mammalian cells studied by computerized image analysis

In a method of manipulating particles suspended in a liquid medium, a moving standing wave ultrasonic vibration and an electrical field capable of generating a dielectrophoretic force on the particles are applied. The ultrasonic vibration may be applied to move the particles from a first suspending liquid to a second suspending liquid, or to move the particles into proximity with electrodes to apply the dielectrophoretic force, or to move the particles into the centre of the liquid medium. Alternatively, the ultrasonic vibration and the electrical field may be applied simultaneously.

Ronald Pethig

Papers

Separation of Human Breast Cancer Cells From Blood by Differential Dielectric Affinity

Applications of dielectrophoresis in biotechnology

Electrode design for negative dielectrophoresis

Dielectrophoretic separation of mammalian cells studied by computerized image analysis

Manipulation of particles in liquid media