Research Details

Employing a novel three-dimensional alternating-current electro-osmotic (3D ACEO) pump, a group of MIT researchers led by Prof. Martin Z. Bazant may have just taken a substantial leap forward in the development of microfluidics, a leap that could enable portable, or even implantable, biomedical lab-on-a-chip devices.

Existing ACEO pumps move fluids over a planar microelectrode array applying an AC voltage. “However,” says Prof. Bazant, “this design is inefficient, since it involves opposing surface flows, where one direction slightly ‘wins’ over the other due unequal electrode widths.” Prof. Bazant theoretically predicted that dramatic increases in flow rate could be achieved – by more than order of magnitude – by using electrodes with raised steps. His 3D design actually turns what was previously considered a drawback of ACEO into a means of increasing flow speed. “The basic idea,” says Bazant, “is to recess the reverse flows to form ‘rollers’ for the raised pumping flow, thus forming a ‘fluid conveyor belt.’”

Bazant’s collaborators in the lab of Prof. Todd Thorsen have demonstrated that the concept really works. Using the new 3D design, the team has fabricated, by far, the fastest low voltage (<10 Volt) AC electrokinetic pumps. They expect to achieve mm/second velocities after design optimization. To reach similar flow speeds, standard DC electro-osmotic pumps require a high voltage power supply (> 100 Volt) and produce undesirable electrochemical reactions. By working at battery voltages with low power consumption (< 10 milliWatt), the team foresees the possibility of portable lab-on-a-chip devices, where fluids are pumped and mixed at the micron scale by 3D ACEO and other phenomena of induced-charge electro-osmosis (ICEO), also springing from Bazant’s theoretical work.

Research in ICEO flows is ongoing, with the goal of being able to manipulate common electrolytes, such as blood and other biological fluids, through microchannels. Current pressure-driven systems, many of which require substantial external plumbing, are not very sensitive to the fluid, but they offer little local flow control and require extraordinary – even unfeasible – pressures when channel sizes approach the micron scale. ICEO can generate fast, tunable flows in microchannels, but seems limited to relatively dilute electrolytes (< 10 mM), for reasons currently under investigation. Nevertheless, Jeremy Levitan (Bazant's postdoc) has successfully demonstrated rapid DNA hydridization microarrays and ELISA immunoassays using ICEO flows in 10x diluted buffer solutions. These are important milestones for Prof. Thorsen’s vision of a portable ICEO-based microfluidic device for early detection of exposure to toxic warfare agents. Similar devices could also be used for point-of-care diagnostics in civilian medical applications.

A paper on Bazant’s theoretical work (with former postdoc, Yuxing Ben) is published online and in press at the journal Lab on a Chip. A related paper on the team’s experimental progress will appear in an upcoming edition of Applied Physics Letters (coauthored by graduate student, J. P. Urbanski, postdoc, Jeremy Levitan, and Profs. Thorsen and Bazant). A company, ICEO, Inc., has been created to commercialize the technology.

The new pumping devices are being developed as part of a broader effort to increase the safety of Soldiers and first responders at MIT's Institute for Soldier Nanotechnologies.

Note for Microfluidics
Microfluidics deals with the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter, scale. It is a multidisciplinary field intersecting engineering, physics, chemistry, microtechnology and biotechnology, with practical applications to the design of systems in which such small volumes of fluids will be used. Microfluidics has emerged in the beginning of the 1980s and is used in the development of inkjet printheads, DNA chips, lab-on-a-chip technology, micro-propulsion, and micro-thermal technologies.

The behavior of fluids at the microscale can differ from 'macrofluidic' behavior in that factors such as surface tension, energy dissipation, and fluidic resistance start to dominate the system. Microfluidics studies how these behaviors change, and how they can be worked around, or exploited for new uses.

At small scales (channel diameters of around 100 nanometers to several hundred micrometers) some interesting and unintuitive properties appear. The Reynolds number, which characterizes the presence of turbulent flow, is extremely low, thus the flow will remain laminar. Thus, two fluids joining will not mix readily via turbulence, so diffusion alone must cause the two fluids to mingle.

Microfluidic structures include on one hand micropneumatic systems, i.e. microsystems for the handling of off-chip fluids (liquid pumps, gas valves, etc), and on the other hands microfluidic structures for the on-chip handling of nano- and picolitre volumes. The commercially most successful application today is the inkjet printhead.

Advances in microfluidics technology are revolutionizing molecular biology procedures for enzymatic analysis (e.g., glucose and lactate assays), DNA analysis (e.g., polymerase chain reaction and high-throughput sequencing), and proteomics. The basic idea of microfluidic biochips is to integrate assay operations such as detection, as well as sample pre-treatment and sample preparation on one chip.

An emerging application area for biochips is clinical pathology, especially the immediate point-of-care diagnosis of diseases. In addition, microfluidics-based devices, capable of continuous sampling and real-time testing of air/water samples for biochemical toxins and other dangerous pathogens, can serve as an always-on "bio-smoke alarm" for early warning.

Note for Electroosmotic Flow
Electroosmotic flow is the motion of ions in a solvent environment through very narrow channels, where an applied potential across the channels cause the ion migration. Electroosmotic flow is an essential component in chemical separation techniques, notably capillary electrophoresis. Electroosmotic flow can occur in natural unfiltered water, as well as buffered solutions.

The cause of electroosmotic flow is an electrical double layer that forms at the stationary/solution interface. In capillary electrophoresis, the narrow channels are made up of silica, and silanol groups form the inner surface of the capillary column. These silanol groups are ionized above pH3. Thus, the inner surface of the channel is negatively charged. In solutions containing ions, the cations will migrate to the negatively charged wall. This forms the electric double layer. When an electrical potential is applied to the column, with an anode at one end of the column and a cathode at another, the cations will migrate towards the cathode. Since these cations are solvated and clustered at the walls of the channel, they drag the rest of the solution with them, even the anions. This results in an electroosmotic flow, not to be confused with the electrophoretic migration.

Electroosmotic flow was first reported in 1809 by F.F. Reuss in the Proceedings of the Imperial Society of Naturalists of Moscow. He showed that water could be made to flow through a plug of clay by applying an electric voltage. Clay is composed of closely packed particles of sand and other minerals, and water flows through the narrow spaces between particles just as it would through a narrow glass tube. Any combination of an electrolyte (a fluid containing dissolved ions) and insulating solid would generate electro-osmotic flow, though for water/silica (that is what glass or sand is, chemically) the effect is particularly large. Even so, flow speeds are typically only a few millimeters per second.

An early application of electroosmotic flow was in drying or decontaminating soil. There has been a great deal of interest and research in electroosmotic flow in the last decade, since it was realized that it provides a very efficient way to generate fluid flows in microfluidic devices, including electroosmotic pumps that can generate flow rates as large as a few milliliters per minute, and pressures as large as hundreds of atmospheres. Another reason for the increased interest in electroosmotic flow is its effect on capillary electrophoresis, where the flow tends to counteract the electric field used to drag the DNA molecule.

Note for Conveyor Belt
A belt conveyor consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called the idler. There are two main industrial classes of belt conveyors; Those in general material handling such as those moving boxes along inside a factory and bulk material handling such as those used to transport industrial and agricultural materials, such as grain, coal, ores, etc. generally in outdoor locations. Generally companies providing general material handling type belt conveyors do not provide the conveyors for bulk material handling. In addition there are a number of commercial applications of belt conveyors such as those in grocery stores.

The belt consists of one or more layers of material they can be made out of rubber. Many belts in general material handling have two layers. An under layer of material to provide linear strength and shape called a carcass and an over layer called the cover. The carcass is often a cotton or plastic web or mesh. The cover is often various rubber or plastic compounds specified by use of the belt. Covers can be made from more exotic materials for unusual applications such as silicone for heat or gum rubber when traction is essential.

Material flowing over the belt may be weighed in transit using a beltweigher. Belts with regularly spaced partitions, known as elevator belts, are used for transporting loose materials up steep inclines. Belt Conveyors are used in self-unloading bulk freighters and in live bottom trucks. Conveyor technology is also used in conveyor transport such as moving sidewalks or escalators, as well as on many manufacturing assembly lines. Stores often have conveyor belts at the check-out counter to move shopping items. Ski areas also use conveyor belts to transport skiers up the hill. A wide variety of related conveying machines are available, different as regards principle of operation, means and direction of conveyance, including screw conveyors, vibrating conveyors, pneumatic conveyors, the moving floor system, which uses reciprocating slats to move cargo, and roller conveyor system, which uses a series of powered rollers to convey boxes or pallets.

The longest belt [conveyor system] in the world is in Western Sahara. It is 100 km long, from the phosphate mines of Bu Craa to the coast south of El-Aaiun. The longest single belt conveyor runs from Meghalaya in India to Sylhet in Bangladesh. It is 17 miles long and conveys limestone and shale. The Conveyor belt was manufactured in about 300 meter lengths and was joined together and installed on the conveyor at site. The job was carried out by NILOS India Pvt. Ltd. in Chennai India.The Idlers or Rollers for this very special Conveyor was produced and supplied by Kali BMH Systems (P) Ltd, Kumbakonam, India. The Idler Rollers were unique for the project that they were designed to accommodate both Horizontal and Vertial Curves along the terrain.

Conveyors are used as components in automated distribution and warehousing. In combination with computer controlled pallet handling equipment this allows for more efficient retail, wholesale, and manufacturing distribution. It is considered a labor saving system that allows large volumes to move rapidly through a process, allowing companies to ship or receive higher volumes with smaller storage space and with less labor expense.

In figure, A scanning electron microscope image shows the stepped microelectrodes in an experimental realization of Prof. Martin Bazant's 3D ACEO array. This new design greatly increases the flow rate per voltage compared to traditional planar ACEO arrays.