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THE
microchip
revolution made possible todays miniaturized electronics industry.
In like manner, the microchip is changing laboratory instruments
that analyze fluids. Large and costly instruments are being replaced
by microchip-based systems known as microfluidic devices. These
miniature systems move fluids through a maze of microscopic channels
and chambers that have been fabricated with the same lithographic
techniques used for microelectronics.
Microfluidic devices are
fashioned from silicon, glass, plastics, and ceramics into 2- or
3-square-centimeter slices with cover plates. In them, red blood
cells, bacteria, biological macromolecules (such as proteins and
DNA), polystyrene beads (that bond to targeted macromolecules),
and other materials can be manipulated in channels with characteristic
length scales on the order of 100 micrometers. The devices integrate
sensors, actuators, and other electromechanical components to dispense
with myriad moving parts and the people required to operate and
service them.
Microscale instruments and
processing are the future of medical research and the chemical and
pharmaceutical industries. Microfluidic devices hold the promise
of a small analytical laboratory on a chip to identify, separate,
and purify cells, biomolecules, toxins, and other materials. They
would perform these tasks with greater speed, sensitivity, efficiency,
and affordability than standard instruments.
They might also be used in
the future for detecting chemical and biological warfare agents,
delivering precise amounts of prescription drugs, keeping tabs on
blood parameters for hospital patients, and monitoring air and water
quality.
For more than a decade, Lawrence
Livermore researchers have been working on several aspects of microfluidic
devices. The Laboratorys Center for Microtechnology has more
than 30 experts in electronics, biology, optics, and engineering
who are developing microfluidic components for transporting, sensing,
separating, mixing, and storing fluids and their constituents. (See
S&TR, July/August
1997, The Microtechnology
Center: When Smaller Is Better.) Current Livermore projects
include the design and prototyping of devices for the human genome
program, chemical and biological warfare agent detection,
and medical analysis.
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In
actual size, this microfluidic device designed by Livermore
engineer Peter Krulevitch is barely larger than a postage stamp. |
First
Complete Model Designed
To help guide the design
of microfluidic devices at the Center for Microtechnology and elsewhere,
a team of Livermore researchers is developing a complex, three-dimensional
simulation tool. The team consists of chemical engineers David Clague
and Elizabeth Wheeler, postdoctoral mechanical engineer Todd Weisgraber,
and University of California (UC) at Berkeley student Gary Hon.
In this work, they collaborate with other Livermore researchers
from several disciplines as well as colleagues at universities.
The team has been funded for the past three years by the Laboratory
Directed Research and Development (LDRD) Program through Livermores
Center for Computational Engineering and, more recently, by the
Defense Advanced Research Projects Agency (DARPA) of the Department
of Defense.
The teams computer
code has drawn increasing interest because it provides an accurate
representation of the behavior of suspended particles, especially
polystyrene beads and biological macromolecules, as they travel
inside a microfluidic device. The simulation capability incorporates
into a single numerical code complex channel geometries and such
parameters as fluid flow rates, particle interactions, and external
forces. We want to predict the complex interplay of the forces
involved in microfluids to give designers a way to accurately predict
how beads, cells, and macromolecules will behave, says team
leader Clague.
Clague notes that suspended
particles traveling within microscopic channels are subject to a
number of physical forces that influence their transport and separation
from each other and the channel walls. The forces, such as subtle
electrical attractions and repulsions, can be used to achieve the
movement and manipulation of suspended particles in ways that would
not work in traditional bench-scale laboratory instruments.
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Simulations
can accurately reflect a host of physical forces that act on
suspended particles flowing in a microfluidic device that typically
measures 100 micrometers long, wide, and high. These forces,
such as subtle electrical attractions and repulsions, are typically
of much less importance in traditional bench-scale laboratory
instruments. |
Synchrotron
Is Key
Joe Wong has been
performing experiments with synchrotron radiation to examine materials
for the past two decades. He and others helped to develop the experimental
facility at the Stanford Synchrotron Radiation Laboratory back in
1977.
Synchrotron radiation is
a particularly intense form of electromagnetic radiation. Highly
energetic charged particles traveling at almost the speed of light
and deflected in a magnetic field emit synchrotron radiation. This
intense, highly collimated radiationmillions of times more
powerful than that from a conventional x-ray tubecan probe
the atomic structure and electronic states of matter. Experiments
that would have taken hours with an x-ray tube source take milliseconds
instead.
Synchrotron
radiation spans the electromagnetic spectrum from infrared to hard
x rays. X rays are ideal for probing matter because the wavelength
of x-radiation is about the same size as an atom. Thus, with synchrotron
x rays, the team can make direct observations of phase transformations
in welds, watching microstructural changes as they evolve.
Synchrotron radiation sources
at Stanford and elsewhere around the world are used by scientists
working in many fieldsby materials scientists like Elmer and
Wong to study the dynamic properties of solid and amorphous materials,
by biomedical researchers to study proteins and other large biomolecules,
by medical workers for coronary angiography and other forms of imaging,
and by geologists for structure characterizations and trace-element
analyses of minerals.
The Livermore team is using
x rays from the 31-pole x-ray wiggler at Stanford Synchrotron
Radiation Laboratory for their experiments. In this device, an x-ray
beam wiggles between an array of 31 magnetic poles, gathering intensity
along the way. By carefully directing this small, intense synchrotron
beam at a given location in a weld, they can obtain an x-ray diffraction
pattern to identify the phases present in the material at that location
during the welding process. The x-ray diffraction pattern depends
on the atomic structure of the material. The diffraction pattern
is the fingerprint of a materials crystal structure,
says Wong. Liquid is chaotic with no long-range order,
he continues, so there is no diffraction.
The Livermore simulation
capability provides a new tool to assist microfluidic device designers
who want to engineer systems that will reliably move, separate,
concentrate, and identify suspended particles of interest. With
effective simulation, the designers can see the effects of design
decisions before they build a prototype. For example, a designer
may want to position selected biological macromolecules in the central
region of a microchannel for capture by an electric field and therefore
must determine what field strength will be required. Or a designer
may want to see how restricting a channel with a tiny post might
affect the fluid flow rate and the mixing behavior of particles
as they are forced to slalom around it.
The program uses a form of
the Boltzmann transport equation called the lattice Boltzmann equation
(LBE) to represent the behavior of fluids and suspended particles
within microfluidic devices. (Ludwig Boltzmann was an Austrian physicist
whose greatest achievement was the development of statistical mechanics,
which explains how the microscopic constituents of matteratoms
and their properties determine macroscopic properties such
as thermal conductivity or viscosity.) In recent years, the LBE
method has gained popularity and usefulness in simulating the flow
of complex gases and liquids. It is based on a statistical description
of the fluid on a cubic lattice in which each lattice site represents
up to several thousand individual fluid molecules.
In the teams numerical
model, spheres represent polystyrene beads and biological macromolecules
within the lattice. The spheres can be assigned different sizes,
densities, and electrical properties. Because of their size, the
spheres can occupy several lattice sites. The code tracks the spheres
as they move on the lattice and calculates the extent to which the
spheres interact with each other, the channel walls, the fluid,
and external forces that may be applied to manipulate them. The
simulation tracks the time evolution of both the fluid and suspended
spheres. The algorithms (mathematical routines) used by the program
tend to be readily applied, allowing calculations in a straightforward
manner and making it easy to incorporate new forces.
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Simulations
using the lattice Boltzmann equation method are based on a cubic
lattice, here with dimensions of 40 by 100 by 100 micrometers.
Spheres (in this example, measuring 5 micrometers in diameter)
represent polystyrene beads and biological macromolecules within
the lattice. The simulations track the spheres as they move
on the lattice and calculate the extent to which they interact
with each other, the channel walls, the fluid, and the external
forces that are used to manipulate them. |
A
Natural for Parallel Computing
Because the LBE method is
naturally suited for parallel computing, the simulation capability
is designed for large computers, preferably supercomputers that
use tens to hundreds of microprocessors together. Simulations representing
time scales on the order of tens of seconds of continuous suspension
require a few days of computer time. The team uses several Livermore
machines for their simulations, including the Compass Cluster and
two massively parallel supercomputers: Blue, the 740-gigaops unclassified
portion of Blue Pacific, one of the Department of Energys
Accelerated Strategic Computing Initiative supercomputers, and the
680-gigaops TeraCluster2000. (See S&TR, October
2001, Sharing
the Power of Supercomputers.) The TeraCluster2000 is the preferred
computing platform; simulations on it use up to 50 microprocessors
working simultaneously.
One important advantage of
the code is its flexibility. The simulated suspended particles can
be assigned different physical and electrical attributes, including
electrostatic forces that cause fluids containing biological macromolecules
to act far less predictably than ideal species, which would consist
of hard, inert spheres. External forces such as gravity, alternating
current, or direct current can be simulated. These forces can be
turned on and off to isolate their specific effects on particle
behavior. Livermore engineer Peter Krulevitch, a microfluidic device
project leader, says that until now, no program was capable of simulating
all the forces acting on fluids containing particles. The
problem has just been too complex, he says.
The
LBE method contrasts with traditional fluid modeling based on finite-element
analysis and boundary- element methods, which typically deal with
pure fluids. Results from the Livermore code, however, can be handed
off to larger-scale computer-aided design simulation tools that
use standard finite-element analysis.
Mike Pocha, a Center for
Microtechnology section leader, notes that device designers can
build prototype devicesa long and painstaking processand
determine their capabilities or, preferably, simulate them first
and then build a prototype guided by the simulation results. Going
from concept to manufacturing a prototype is increasingly more time-consuming
and expensive as microfluidic devices get more complex, says Clague.
With a more comprehensive simulation tool, researchers will
be better able to predict what will happen to the suspended species
in these complex microenvironments. Ultimately, such a capability
will speed the design effort and reduce costs.
The physics involved with
the operation of microfluidic devices is complex and varies, depending
on the fluid, the molecules suspended in the fluid, and the extent,
if any, of external fields. In building the code, the team has steadily
added capabilities that more completely represent the physical forces
at work in microfluidic devices. After every addition of a new feature,
the team makes sure the results are in excellent agreement with
existing theory and, where possible, with published alternative
numerical methods.
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The
Livermore simulation work is part of the Simbiosys (Simulation
of Biomolecular Microsystems) program administered by the Defense
Advanced Research Projects Agency. The program funds the development
of advanced computational tools for the BioFluidic Chips design
effort. |
LDRD
Laid the Groundwork
One of the teams first
accomplishments under LDRD funding was simulating hydrodynamic forces
acting on a stationary sphere. These forces are dependent on the
velocity of the suspending fluid and the proximity of the suspended
particles to channel walls. The LBE method naturally takes into
account the entire spectrum of fluid and particle behavior, including
inertial effects and hydrodynamic interactions between suspended
particles. In other words, the simulations account for the minute
disturbances propagated within a fluid by the particles that feel
each others presence and, as a result, change their trajectories
and the properties of the fluid.
The hydrodynamic forces,
including inertial effects, are particularly well captured. The
first is the drag force, which is a result of the fluid exerting
a force on a suspended particle because of differences in fluid
and particle velocities. The second force is a lift force, which
is caused by small inertial effects and gradients in fluid velocity.
The lift force is exerted perpendicular to the flow, causing the
species to migrate to the center of the channel. Also coming into
play is a particles density, which affects its buoyancy within
a fluid and the extent to which it can be lifted.
Fluids
normally flow through microfluidic channels without turbulence so
that suspended particles typically mix only by diffusion. One of
the key parameters used to characterize fluid flow is the Reynolds
number, which defines flow conditions and measures the relative
importance of inertial effects to viscous effects. Most fluid flow
in small channels occurs at a low (but finite) Reynolds number.
However, even at small Reynolds numbers, researchers have found
that there are small lift effects. The Livermore simulation capability
takes into account these inertial effects for predicting the extent
of lift as a function of Reynolds numbers.
The code also simulates the
effects on particles that are near channel walls. Much like the
effect of a boats wake, the motions of molecules cause disturbances
in the fluid that bounce off the channel walls and reflect back
on the particles. Close to the walls, particles experience forces
retarding their motion, and even closer to the walls, they experience
large resistive forces known as lubricating forces.
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(a)
The simulation capability can be used to predict the extent
of inertial lift as a function of the fluids Reynolds
number. The lift force acts to push suspended particles up or
down toward the center of the channel. (b) Dielectrophoresis
(DEP) is an efficient method for capturing selected particles
in microfluidic devices. DEP electrodes (rectangles) generate
nonuniform alternating current electric fields that induce electrical
polarization in biological macromolecules. The DEP forces overcome
inertial lift forces to cause a selected particle to move toward
the electrodes and to remain there. |
Adding
Real Effects
If the simulation is to be
accurate, it must also account for non-Newtonian characteristics
that are exhibited by biofluids containing human cells, bacteria,
and biological macromolecules such as proteins and DNA. These materials
do not behave like electrically neutral and perfectly round spheres.
Instead, they have widely varying shapes, densities, and often electrical
charges that are asymmetrically distributed.
More importantly, these materials
tend to have elastic character, which gives rise to unexpected effects.
Strands of DNA, for example, can be long and gangly with a preferred,
three-dimensional shape that orients itself in a particular manner
to its neighbors. If forced to travel through a narrow channel,
the strands deform but then exert a small force in an attempt to
recover their favored configuration, much like a compressed spring
reverts to its normal shape. If there is a sufficient concentration
of such strands, this restoring force can have a profound effect
on fluid behavior.
Depending on their concentration,
particles interact with each other and with the channel walls. Under
certain conditions, they can coagulate with each other or stick
to walls because of van der Waals and electrostatic forces (electrical
attraction and repulsion forces between species). The simulation
team is incorporating these and other forces associated with biological
macromolecules into the models, including hydrophobic (water hating)
and hydrophilic (water loving) interactions. Clague explains that
some proteins have hydrophobic regions that cause the proteins to
aggregate when they are in close proximity to other proteins; therefore,
these unique forces must be taken into account.
Last August, the team began
work for DARPA, the advanced research arm of the Department of Defense
and a major backer of microfluidic technology. One of DARPAs
goals is to develop devices called BioFluidic Chips (BioFlips) that
will identify biological macromolecules and microbes based on certain
electrical or chemical properties. Soldiers would use BioFlips devices
both to detect chemical and biological agents and to monitor their
own general health. (See the box below.) As part of the microfluidic
development effort, a program called Simulation of Biomolecular
Microsystems (Simbiosys) is funding the development of advanced
computational tools for the BioFlips design effort. The Livermore
teams simulation work is part of the Simbiosys program.
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The
Laboratory team is collaborating with University of California
researchers at Berkeley and Davis to simulate the transport
of suspended particles in microneedles. These simulations are
helping to obtain a better understanding of why particles can
stick together and plug microneedles, as shown. (Photos courtesy
of Professor Dorian Liepmann of the University of California
at Berkeley.) |
Focus
on Dielectrophoresis
The teams work for
DARPA builds upon LDRD research, particularly with regard to simulating
the coupling of hydrodynamic and dielectrophoretic forces. Dielectrophoresis
(DEP) is an efficient and increasingly popular method for separating
molecules in microflows. DEP electrodes generate nonuniform, alternating
current electric fields that induce electrical polarization in target
species. On an absolute scale, the force is quite small, but in
microfluids, the force can be quite effective in manipulating and
positioning biological macromolecules with electrodes using less
than 10 volts. The degree of induced polarization is dependent on
the electrical properties of the molecule, the surrounding fluid,
and the magnitude and frequency of the applied electric field.
Different species typically
have their own unique dielectric response fingerprint that can be
exploited by DEP, says Clague. As a result, DEP can be used
to select from among a number of different particles suspended in
the same fluid. The selected particle will either be drawn toward
or repelled from the region of high field intensity (toward or away
from the DEP electrode located within a channel wall). The first
instance is referred to as positive DEP, and the second is referred
to as negative DEP.
DEP forces can be switched
on and off to selectively capture cells, bacteria, spores, polystyrene
beads, DNA, proteins, and other matter. Once captured, the molecules
can be held in place or, with the removal of the force, sent on
their way to a different location for analysis. For example, DEP
can be used to selectively capture a suspected pathogen. The pathogen
would then be shuttled to a different area where its DNA would be
extracted and analyzed.
The
DEP simulation work involves close collaboration with pathologist
Peter Gascoyne at the University of Texas M.D. Anderson Cancer Center
in Houston, Texas. Gascoyne and his colleagues, in a project sponsored
by DARPA, are developing an instrument that uses DEP to separate
cells and identify them based on their dielectric properties. A
prototype has been used on whole blood samples to separate malignant
cells from normal cells.
An
important group of simulations is focused on examining the interplay
of suspended particle concentration, flow rates (and inertial lift
effects), and DEP forces with the effects from different kinds of
suspended particles. Preliminary simulations show that the hydrodynamic
interactions between particles can screen and thwart DEP forces;
therefore, concentration effects become very important. The suspended
particles that are not screened encounter a positive DEP force and
are pulled to the electrode surface, where they are held motionless.
Monitoring
the Health of Soldiers
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(a)
The Defense Advanced Research Projects Agency is developing
BioFluidic Chips (BioFlips) that are small enough to be
worn on an earlobe and can identify biological macromolecules
based on certain electrical or chemical properties. (b)
A BioFlip uses an array of microneedles for continuous
blood monitoring. (c) View of a microneedle tip and (d)
an array of microneedles. (Photo and figures courtesy
of Professor Rosemary Smith of the University of California
at Davis.) |
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The
BioFluidic Chips (BioFlips) program of the Defense Advanced
Research Projects Agency (DARPA) is developing a clinical
lab on a chip. BioFlips would offer all the advantages
of microfluidic devices: miniaturized channels and reservoirs
for increased speed of reaction, increased sensitivity,
reduced cost of reagents, and reduced power consumption.
The devices would be capable of rapid detection of infections
and chemical and biological warfare agents, making possible
potentially rapid treatment. BioFlips would be worn directly
on the skin, perhaps on the earlobe for continuous blood
monitoring through microneedles.
BioFlips would
provide real-time, unobtrusive monitoring to directly
assess the health of defense personnel. A commander could
continuously monitor the status of troopswhether
they are fatigued or have been exposed to biological threats,
including bacteria, viruses, and toxins. The devices could
monitor such entities as white blood cells, antibodies,
blood pH, and blood glucose.
BioFlips promise
fast health assessment, from seconds to minutes, in contrast
to laboratory blood cultures using traditional methods
that take hours or even days to process. If successful,
the technology could perhaps be extended to improve national
health care by unobtrusive and continuous monitoring of
high-risk patients.
BioFlips designers
need powerful computational tools to guide and speed their
efforts. Hence, DARPA is sponsoring an allied DARPA program
called Simulation of Biomolecular Microsystems (Simbiosys).
The Simbiosys program recognizes that engineers have limited
understanding of biological molecules and biochemical
reactions and, furthermore, that biologists do not generally
have knowledge about key biochemical reaction rates and
little knowledge about the behavior of biological molecules
in microscopic channels. The goal is the creation of what
DARPA managers are terming the first interface between
biology and engineering. Effective simulation models
will enable greater understanding of the transport of
biological materials at the micrometer scale to enable
better control and efficiency of the devices. |
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The team is continuing to
enhance the numerical model to investigate the forces influencing
DEP manipulation of molecules suspended in flowing fluids. One research
avenue they are taking is to give biological macromolecules more
realistic characteristics. For example, the team has explored replacing
the simulated spheres with more accurate bead-and-spring representations
of long-chain polymers such as DNA fragments. Also under development
are representations of cell properties unique to organelles and
membranes, that can significantly influence the response. Finally,
the team is working on the inclusion of electrostatic and van der
Waals forces as well as hydrophobic and hydrophilic interactions.
The team has collaborated
with UC Berkeley researchers on developing arrays of 50-micrometer-diameter
needles. The goal is to deliver drugs more efficiently, but interactions
between particles cause the microneedles to become clogged. The
Livermore teams simulation work is targeted at obtaining a
better understanding of the problem. This work complements a DARPA-funded
project at UC Davis, where researchers are developing microneedle
arrays for drawing body fluids painlessly to monitor soldiers
health on the battlefield.
Clague expects the simulation
program to become increasingly useful as applications for microfluidic
devices expand. By providing a tool that allows microfluidic device
designers to turn the variety of physical forces at play on and
off, the team hopes to make possible the discovery of new ways to
manipulate suspended particles. Such detailed and accurate simulations
speed the design and development of novel microfluidic devices.
As a result, the simulation effort may well have an important role
in saving soldiers lives and in developing new medical devices
that could help drive down national health care costs.
—Arnie Heller
Key Words:
BioFluidic Chips (BioFlips), Center for Microtechnology, Defense
Advanced Research Projects Agency (DARPA), dielectrophoresis (DEP),
lattice Boltzmann equation (LBE), microfluidic devices, Reynolds
number, Simulation of Biomolecular Microsystems (Simbiosys).
For further
information contact David Clague (925) 424-9770 (clague1@llnl.gov).
About
the Scientist
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DAVID
CLAGUE is a staff engineer in the Electronics Engineering
Technologies Division of the Engineering Directorate. He joined
the Laboratory in 1998, after a year as a postdoctoral researcher
at the Los Alamos National Laboratory Center for Nonlinear
Studies. Clague received a B.S. in chemical engineering from
the University of California at Santa Barbara in 1987, an
M.S. in engineering in 1993, and a Ph.D. in chemical engineering
in 1997, both from the University of California at Davis.
His research specialties are in transport phenomena, complex
fluids, microfluidics, and numerical methods. At Livermore
and previously at Los Alamos, he has developed three-dimensional
simulation methods for modeling particulate behavior. This
work has been published in a number of refereed journals.
Additionally, Clague has experience in industry, working for
four years as a research and development engineer at Space
Systems Loral to provide engineering and technical support
related to polymeric composite materials and adhesives.
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