mems final paper
TRANSCRIPT
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Macro, Micro, and
NanoLab-on-a-Chip Technology UnderDiffering Flow Regimes
Anthony Salvagno, Brittany Branch, Darin Leonhardt, Martin Donovan
12/17/2008
There are major differences when one compares fluid flow at different scales. Here we take acloser look at many of the various differences of each regime: macroscale (larger than 100 um),
microscale (100 um to 100 nm), and the nanoscale(fewer than 100 nm). We will also present
applications of each scale in order to demonstrate the usefulness of developing Lab-on-a Chip
technologies.
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Introduction
The need for analysis of biological specimens utilizing as little sample as
possible has created an offshoot branch of MEMS (aptly named BioMEMS)
utilizing liquid flow on the very small scale. The research performed in this
field has created many advances in the technology and has allowed for the
creation of laboratory-on-a-chip (LOC) type setups. In the early stages of
development, these devices were constructed to handle chemical analysis on
the order of a lab. This was the creation of the first Micro Total Analysis
Systems (TAS).
The improvementof fluidics technologies allowed researchers to do so much
more on a single chip than just chemical sensing and analysis. Shortly after,
creation and development of pumps, valves, mixers, motors, and anything
else that could be used in a fluidics lab was miniaturized to the microscopic
scale and further enhanced fluidics [1-4]. People began to realize that this
could be applied to more than just chemistry, and looked to the ever
advancing field of genomics and microbiology for new applications.
During typical biological experiments (on the macroscale) large sample sizes
are used on the order of millions or even billions of cells, proteins, amino
acids, etc. On the smaller scales of micrometers and nanometers,
experiments may be carried out where single molecules can be analyzed and
characterized. In the field of genomics, for instance, this is particularly
useful because there is a gap in understanding how molecular interactions
affect gene expression [10].
The need to reduce cost and improve speed and efficiency were initially the
driving forces behind LOC technology. Eventually it became obvious that
higher resolution could be obtained because now one could analyze (down
to) individual molecule behavior. Reducing the need for such sample sizes,
as in the case of micro-/nanofluidic experiments, provides a much more cost
and effort efficient process.
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The design of these tools even allows for high throughput processing, simply
because the entire chip is completely automated. Fluids can be moved,
separated, mixed, screened, etc. all on a single chip without ever needing tocontact human hands. The control gained from this automation is quite
impressive and may range from spatial and temporal characterizations on
the subcellular level [5] or organismal level [6], patterning of molecules and
cells [7], and passive and active cell handling and environment control [8] up
to the cellular level [9].
While most of the previously mentioned research has been done on the
microscopic level, there is as potentially impactful work going on at the
nanoscopic level too. The benefits here are similar to microfluidics in that
reduced cost of reagents, parallelization of experiment, and high resolution
(and sensitive) detection are all available technologies and could even be
more useful on the smaller nanoscale. The smaller structures of the
nanochannels (thus entering a completely different regime of physics) could
also present interesting interactions between the fabricated devices and the
molecules/particles to be studied.
It is pretty clear that these technologies haveits usefulness in the biologicalspectrum of science, but still we have not seen the explosion of technology
that LOC brings. Currently there is a slight gap between the biologists who
wish to employ the use of LOC technologies and the engineers who fabricate
them. This divide is ever narrowing as science becomes more open and
researchers themselves are pursuing interdisciplinary study. This
collaborative effort is rapidly changing the future of study for both fields and
will assuredly enable high-impact research.
Here, we present some of the technologies that have been and are being
developed in the fields of microfluidics and nanofluidics. Each regime is
dictated by its own set of parameters and will be discussed. We will provide
a compare/contrast of both and discuss the limitations that come with each
technology. We will also detail some of the technologies that were previously
(and still are) utilized in a biological setting, the uses for each, and the
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limitations they provide. By discussing the current technologies, we hope to
demonstrate the usefulness of further developing both microfluidics and
nanofluidics to the point the LOC may be pushed to the forefront of current
research.
Background
Investigations into the complex and dynamic pathways governing the
function of organisms require the isolation and purification of specific
molecular populations from the myriad inhabiting the intracellular milieu. In
the biological sciences, a broad spectrum of separation technologies are
employed to both isolate individual components from the diverse andextensive mixtures comprising the cell, and to dissect the genome into
manageable segments for analysis or manipulation. Indeed, the field of
biology in general tends to makes its greatest advancements shortly
following the development of novel and powerful separation and isolation
techniques, e.g. electrophoresis, polymerase chain reaction (PCR), and
microarray technologies. Within the cell, the primary molecules to be
separated and analyzed are the deoxyribonucleic acids (DNAs), ribonucleic
acids (RNAs), and proteins; additional populations include segments of the
lipid bilayersof the plasma membrane, or carbohydrate sugars integral to the
cell signaling pathways that malfunction during pathological conditions.
Furthermore, entire cell populations are often also separated from one
another in tissues, e.g. immune response cells from the bone marrow, or
surfactant secreting cells from the lung. The following provides a brief
review of the more commonly employed separation techniques in the
biological sciences to separate and analyze DNA and proteins. It should be
noted that these separations are merely the means to an end, as once the
desired bio-molecule population has been isolated, further investigations
elucidating their structure, morphology, and function will be performed.
However, as these analytic techniques fall outside the scope of this paper,they will not be mentioned further.
Nucleic Acid Separation
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Analytical techniques take advantage of various molecular properties
including size, shape, density, electrical charge, chemical structure or
combinations thereof, to isolate a small population of identical molecules
from all of the other types of molecules comprising the cell. One of the more
ubiquitous separation techniques found in molecular biology is gel
electrophoresis, which employs a combination of molecule size, shape and
electrical charge to separate a heterogeneous solution of nucleic acids into a
ladder of fragments sorted according to size. Due to their abundance of
phosphate groups comprising the backbone of the double helix, nucleic acids
possess an overall negative charge, and accordingly in an electric field they
will head toward the positive pole. By applying an electric field and allowing
the nucleic acids to migrate through a gel matrix comprised of an inert, jello-
like porous material, smaller-sized molecules will travel a greater distance
than their larger counterparts, and DNA fragments of the same size willcluster together in bands on the gel, hence exploiting the electrical charge of
these molecules to separate them according to size. By comparing the size
fragments of the sample DNA to the known size fragments of a control DNA
strand (referred to as the DNA ladder), the lengths, in base-pairs, of the
sample fragments may be determined. This technique is applicable to both
DNA and RNA molecules, although as RNA molecules are single stranded and
possess a tendency to assume a variety of three-dimensional configurations
via intramolecularhydrogen bonds, RNA is usually treated with a detergent
prior to separation, ensuring that only the length of the molecule, and not its
morphology, will factor into its migration rate through the gel. Typical length
scales over which the gel electrophoresis separations are performed range
from 10 20 cm, although resolution of many similar sized DNA fragments
increases with distance.
Protein Separation
In contrast to nucleic acids, which share an identical helical structure,
making them distinct only by their precise nucleotide (base-pair) sequence,
proteins possess discreet properties that make their separation different
from polypeptide to polypeptide. Protein separation techniques exploit
differences in protein size, morphology, charge and in many instances,
function. The primary method for protein separation is column
chromatography, where protein fractions are passed through long ( > 25 cm)
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glass columns containing modified acrylamide or agarosebeads that separate
the proteins on the basis of different properties.
Ion exchange chromatography is a form of the aforementioned column
chromatography, where proteins are separated from a heterogeneousmixture on the basis of their surface ionic charge. The beads in the column
will be modified with chemical groups such that they will possess either an
overall positive of negative charge. Proteins containing ionicallycharged
amino acids will either be repelled or attracted to the column matrix, and as
each protein typically contains numerous acidic and basic amino acids,
multiple interactions will occur simultaneously within the same protein. On
the basis of the strength of these interactions, protein populations will elute
with distinct temporal profiles.
Another variation on column separation is gel filtration chromatography, also
known as size-exclusion chromatography, which exploits the size and shape
of proteins to separate similar molecular populations. As opposed to
possessing a chemical modification, the bead matrix for gel filtration
chromatography is fashioned to contain a variety of different sized pore
throughout their surface. When the heterogeneous protein mixture is passed
over this porous matrix, smaller sized proteins will enter the pores far more
often than their larger counterparts, thereby impeding their flow through the
column. Consequently, larger proteins will elute faster than smaller ones,again separating populations into distinct temporal profiles. An additional
column chromatography method employed with lesser frequency than the
previous two examples is metal binding chromatography. This capitalizes on
the ability of metal ions such as nickel, to bind with strong affinity to specific
amino acids, such as histidine. However, for this technique to be worthwhile,
the protein of interest would have to contain a significant amount of
histidineamino acids exposed on the protein surface, which is not typically
encountered.
While column separations are relatively simple and inexpensive methods of
protein fractionation, they lack adequate resolution such that a single
passage through one type of column will suffice for producing a
homogeneous population. Typically, a protein solution is passed through a
column multiple times, refining its yield, and then the sample solution (e.g.
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the fraction containing the target protein population) is passed through a
different column, for further refinement. An example of this would be the
separation of a large, positively charged protein. The sample would first be
passed through a size exclusion column a few times, isolating a protein
population within the size range of the target. This mixture would then be
filtered through an ion exchange column containing a negatively charged
matrix. Thus, while a heterogeneous protein mixture may contain many
molecules that are similar in size to the protein of interest, the likelihood that
it contains multiple protein types of similar size and charge density is
minimal. However, multiple passages through multiple columns quickly
become time intensive, and therefore higher resolution separation
techniques are employed frequently for protein separations.
The problems of regular column chromatography are overcome using affinity
chromatography, commonly known as Immunoprecipitation, where the
surfaces of the beads composing the column matrix are coated with an
antibody specific for the protein of interest. With this technique, only the
desired protein will bind strongly, effectively separating them from the
homogeneous cell extract. This differs from the previous chromatographic
separation methods in that the specific binding targets of the protein must
be known ahead of time, which may not be possible when separating a novel
protein. Additionally, while highly specific, coating the beads with an
antibody can be expensive, and once coated the matrix may only be used for
one protein population.
In addition to the techniques that separate nucleic acids and proteins, there
also exist separation techniques that isolate multiple populations of
molecules to assay in vivoassociation and function. One technique employed
extensively in molecular biology is the co-immunoprecipitationassay, which
is used to investigate which proteins bind to, or are localized with, the
protein of interest in the cell. Based on affinity chromatography, a cellcolony is treated with formaldehyde to crosslink any associated proteins with
each other. The cell extract is then passed through beads coated with
antibodies for a protein, and additional proteins bound to it will also be
retained. Interaction between DNA and proteins are investigated with
chromatin Immunoprecipitation (ChIP). This is identical to co-
immunoprecipitation, except the protein that is targeted is typically a DNA
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binding protein, and therefore has a segment of DNA crosslinkedto it. This
technique is commonly employed to determine where in the genome a
known DNA-binding protein is located.
While the separation techniques used to investigate intracellular structureand function are quite diverse with regards to the molecular properties
exploited to isolate a specific intracellular population, they all share a
common feature in that the length scales required to achieve an adequate
resolution reside in the macro-world, ranging from a few centimeters for
electrophoresis to over half a meter or more for size exclusion
chromatography. Additionally, macro-scale separations are not exclusive to
biology, and many common analytical techniques in the physical sciences,
including gas chromatography, HPLC, and mass spectrometry also require
the analyteto travel significant lengths for effective resolution. Accordingly,
the primary challenge encountered when developing separation techniques
on the micro-scale is overcoming the size requirement, meaning that these
miniaturized separation platforms must achieve comparable resolution to
their macro-scale counterparts while utilizing only a fraction of the distance.
This necessitates the development of novel techniques that exploit the
differences in physicochemical properties between the macro and
microscaleto compensate for the reduction in travel length.
Microfluidics
Flow devices at the micron and nanometer scale provide precise control of
fluids and chemical reactions in the fluid-phase. The high surface to volume
ratio at this small size facilitates rapid heat and mass transfer while taking
advantage of physical phenomena that do not normally have an influence at
the macro domain. An important feature of flow at such small transverse
lengths is that the flow occurs only at low Reynolds numbers due to the
dominant dependence of the fluid viscosity, whereas in larger flow systems
turbulent flow can occur further complicating the effects. With the
development of new fluidic devices there is a need to understand the
fundamental differences of fluid flow at these small transverse lengths. Flow
within a microchannel and nanochannel differ greatly in mechanism and
electrical double layer profiles.
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Pressure driven flow and electro-osmotic driven flow are the two common
mechanisms of pumping liquid in microchannels, each are both beneficial
depending on the application. As the size of the channel decreases into the
nanometer range pressure driven flow is no longer feasible. For instance,
consider a pressure driven flow in a 100 nm channel where the average
velocity is on the order of mm/s. The pressure gradient required to achieve
this flow is as high as 3x109 Pa/m assuming a viscosity of 1x10-3kg/m s [18].
The integration of such a high-pressured pump into a nanoscale device
defeats the purpose of miniaturization, therefore electro-osmotic driven flow
is the primary flow mechanism used in nanoscale devices. Electro-osmotic
driven flow for both micro and nanofluidic devices requires an electrical
double layer at the channel wall. The double layer thickness and profile vary
for the different sized channels.
The first study of electro-kinetic flow in channels focused on the electrical
double layer (EDL) thickness. Two phenomenas occur in electro-kinetic flow,
electrophoresis and electro-osmosis.Electrophoresis describes the motion of
a charged surface submerged in a fluid under the action of an applied
electric field.Electro-osmosis refers to the bulk movement of a liquid past a
stationary solid surface, due to an externally applied electric field. Electro-
osmosis requires the existence of an electrical double layer at the solid-liquid
interface within the channel [19,20]. This charged double layer results from
an attraction between bound surface charges and ions in the passing fluid. It
is described by the Poisson equation:
o
e
=2
where
is the electrical field potential,
e
is the free charge density, and
o
and
are the dielectric constants in the vacuum and medium
respectively. The electrical double layer is illustrated in Figure 1.
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Figure 1: Ionic distribution in an electrical double layer for a channel wall in contact with an aqueous
solution.
Immediately next to the charged wall of the channel is an immobile layer of
ions that are strongly attracted to the surface. This layer is called the
compact layer which is normally several Angstroms thick. The potential
distribution in this area is known as the zeta potential and is determined by
the geometric restrictions that the wall imposes on the ions, along with the
short-range interactions they have. From the compact layer into the neutral
bulk liquid the net charge density reduces to zero. This region is call the
diffuse layer because ions are less affected by the electrostatic charge of the
wall and therefore are mobile in the liquid. The ion and potential distributionin this region is further described by the Poisson-Boltzmann equation in
which the concentration of ions is predicted by the Boltzmann distribution:
2
=2zeno osinhze
kbT
Where z is the valence of the ion, n is the bulk ionic concentration, e is the
charge of a proton, kb is the Boltzmann constant, and T is the absolute
temperature [20]. Solving this equation with appropriate boundary conditions
results in the EDL potential field for a micro-sized channel illustrated in Figure
2.
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The EDL in nano
sized channels
changes dramatically due to the fact that the size of the channel is
comparable to the thickness of the ELD [21]. In addition, the Boltzmann
distribution is no longer accurate since the EDL now affects the
concentration of ions in the bulk fluid. Figure 3 illustrates the overlapped ELD
resulting from the scaled down channel.
The differences in the EDL of micro and nanochannels have a significant
effect on the velocity profiles within the channel. The velocity within a
microchannel can be predicted by the steady state Navier-Stokes equation
for low Reynolds number of incompressible fluid:
2u=- eE
Where is the viscosity of the liquid, u is the velocity distribution,
e
is the
free charge density, and E is the externally applied field. If we assume the
double layer to be very thin compared to the channel width the Stokes
equation can be solved with
e
defined by the Poisson equation resulting in
the Helmholtz-Smoluchowski equation which predicts a plug flow velocity
profile [22].
ueo=ux=- o E
Conversely, the velocity profile for a nanochannel has a parabolic profile. The
velocity near the wall of the channel is slower than the bulk movement of the
liquid. This is due to the overlapping electrical double layer. COMSOL-FEMLAB
was used to model the velocity profile for both micro and nanochannels for
fluidic devices. Figure 4 shows the modeled plug flow velocity profile for
microfluidic channels.
Figure 2: The electrical double layer potential in a microchannel in contact with an aqueous
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Figure 4: The velocity field measured in m/s in a microchannel.
Figure 5 shows the modeled parabolic velocity profile due to the overlapping
EDL for nanofluidic channels.
Fluid flow through small channels has enabled studies of single molecules
and cells. The benefits of this miniaturization include, but are not limited to,
direct manipulation and separation of proteins and nucleic acid along with
single cell analysis on a single chip [23]. Microfluidic devises are becoming
more prevalent with the emergence of biochemical lab-on-chip systems.
There has been successful use of these devices for cell sampling, cell
trapping and sorting, cell treatment, and cell analysis. Microfluidic-based
biological applications including polymerase chain reaction (PCR), DNA
separation, and DNA sequencing have been implemented [24]. Many
techniques in microfluidics have been used for separation including
magnetic, optical, mechanical, and electrical manipulation [25]. Currently
many of the separation techniques employed are the same techniques for
macro separations. There is an initiative to develop separation techniques
unique to microfluidic devices to improve sensitivity [25].
There has been great success in the development of microfluidic devices forseparation and analysis purposes. The envelope is being pushed further to
extend these systems down to the nanoscale. The nanoscale regime offers
unprecedented control of single cell and molecule transport, in manipulation
and separation as well as detection [25].
Nanofluidics
Figure 3: The electrical double layer potential in a nano-sized channel in contact with a aqueous
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Lab-on-a-chip devices have been increasingly used in the last decade for
bioseparation,
detection, and chemicalsynthesis [26-29]. These devices have received growing attention because of
their speed, efficiency, reduced sample consumption, and detection
multiplexing. Many of these advantages led to commercial miniaturized total
analysis systems [30]. Following the first demonstration of such devices for
amino acid [31] and deoxyribonucleic acid (DNA) analysis [32], for instance,
microfluidic devices have emerged as a separation platform. More recently,
however, nanofluidic devices have been explored to achieve greater
efficiency, and lower sample diffusion than those observed in microscale
systems [33]. In particular, nanofluidic systems can provide enhanced
electro-osmotic flow control, compared to microfluidic systems [34], when
the surface charge on channel walls is manipulated in the presence of
significant double layer overlap [35].
This control scheme, where a third potential is applied to a gate electrode
surrounding the channel walls, is very much analogous to that of the field
effect transistors (FETs) used in complementary metal oxide silicon (CMOS)
technology. The fluidic devices that use this control scheme are hence
termed fluidic FETs [36]. Since the typical size of biomolecules, the Debyescreen length of electrolyte solutions containing the biomolecules, and the
size of nanochannels, can be at nearly the same length scale, it is expected
that the nanofluidic FETs provide enhanced manipulation of biomolecular
flow, leading to pronounced separation of biomolecules, while multiple
nanochannels (>104) in an array may simultaneously serve as an integrated
detector with increased sensitivity.
Various transport and electrokinetic phenomena in nanofluidic channels,
such as electrophoresis, electrostatic control, ion transport, and size and
shape effects, have been intensely studied [37,38]. Complementing these
studies, researchers at UNMs Chemical and Nuclear Engineering Department
have demonstrated an in situ analytical approach to monitor the average
flow speed and localization of molecules in nanochannels during separation
[37]. Their approach relies on an experimental platform where nanofluidic
Figure 5: The velocity field measured in m/s in a nanochannel.
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channels are integrated into an infrared waveguide. The platform enables
multiple internal reflectionFourier transform infrared spectroscopy (MIR-FTIR)
to probe the signature vibrational peaks of molecules flowing through the
channels. The multi-purpose nanofluidic device is made by conventional Si
fabrication technology. The device architecture incorporates a heavily doped
Si gate that surrounds the channels to control the surface charge of
SiO2channel bottom and sidewalls. The device has been shown to
successfully control and monitor the flow of electrolyte solutions containing
fluorescent dye molecules by modulating the surface and to analyze the
interaction between the dye molecules and the internal SiO2channel surface,
using MIR-FTIR. The details of the device architecture will be describes
below.
Two main configurations are used with and without a heavily doped gate that
surrounds the nanochannels. The purpose is to compare the effectiveness of
the flow control, depending on the gate contact resistance. The following
fabrication sequence covers the gate based device, and is shown in Figure 6.
Double-side-polished Si (100) wafers are used as substrates to prevent the
scattering and loss of IR beam intensity during multiple internal reflectionin
the MIR-FTIR analysis system. Each wafer is diced into rectangular pieces,whose length, width, and thickness are 50, 10, and 0.7, respectively. The
rectangular samples are first immersed in a Piranha solution to remove
organics and other contaminants on the surface. Piranha is prepared by
mixing H2SO4 (2M) with H2O2 (30wt%) in 4:1 volumetric ratio. Piranha is a
strong oxidant that forms a thin layer of chemical SiO2on Si. The chemical
oxide is subsequently removed in dilute HF (Fig. 6(a)), for which 48 wt % HF
Figure 6: Schematic flow diagram of fabrication steps for integrating nanochannels into a MIR wa
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of the nanochannels. Plastic wells are attached
to the through holes to facilitate the injection of
electrolyte solution containing fluorescent dye
molecules (Fig.
8). Fig. 7(c)
shows an SEM
image of the
Pyrex sealed nanochannel array. Note that
since doped Si surrounds only channel bottom
and sidewalls, the gate bias controls surface
charge only on channel bottom and sidewalls
but not on the channel top Pyrex surface. The
width and depth of each channel are
approximately 100 and 400-500 nm,respectively. Therefore, a 3 mm w x 10 mm l
nanochannel section contains well over 8000
nanochannels, and if desired, this number can
be increased to 105over the 1 cm width of the
MIR crystal. The device is referred to as a
nanofluidic waveguide, since the device serves
simultaneously as a separation matrix and as
an analytical tool for MIR-FTIRS. Fig. 8shows conceptually how the
nanofluidic waveguide is operated. For MIR-FTIRS analysis, a focused IR
beam enters one of the beveled edges of the Si MIR crystal and makes
approximately 35 top reflections before the beam exits the opposite end. A
HgCdTedetector collects the IR signal leaving the second beveled edge. Due
to these multiple reflections, the Si MIR crystal is opaque to IR below 1500
cm-1. Note that the width of the channels is much less than the mid-IR
wavelength (2.5 to 15 m). As the channel width increases to the
micrometer level, the IR beam scatters substantially upon each internal
reflection, and very little IR passes through the MIR crystal.
To monitor the electro-osmotic (EO) flow in nanochannels with MIR-FTIR and
with laser-scanning confocal fluorescence microscopy (LS-CFM), the
nanochannels are first completely filled with a desired buffer solution by
capillary force. The LS-CFM technique is considered a standard optical
technique to monitor the flow of fluorescent dye molecules in nanochannels
and also used to provide a baseline comparison with the MIR-FTIR technique.
Figure 7: Cross-sectional SEM images of nanochannels. (a) Nanochanneplasma etching. (b) SiO2 covered nanochannelsafter thermal oxidatio
The channelwidthis less than 100 nm, and the depth is 400 to 500 nm.
Nanochannels sealed with a Pyrex cover after anodic bonding.
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Since the depth of focus of the LS-CFM technique is limited to 1um, the
optical images are averaged over the entire channel depth. A platinum wire
is then inserted into each well as an electrode. A positive potential is applied
to one well, while grounding the other (Fig. 8). This potential difference
creates a longitudinal electric field (E) along the channels and induces EU
flow. While the EO flow is in motion, fluorescent dye molecules pre-dissolved
in identical buffer solution are injected into the positively biased well. The
gate potential (VG) is then applied to control the surface charge on the
channel walls. The motion of the fluorescent dye molecules is observed
under EO flow and upon modulating VG, using both MIR-FTIR and LS-CFM for
purpose of comparison.
Fig. 8 Experimental setup to monitor flow control and segregation of dye
molecules within nanochannels in response to the gate bias, using MIR-FTIR.
FET flow control experiments are conducted with Rhodamine B (MW = 479),
C28H31ClN2O3, dissolved in a pH 4 buffer solution previously described. After
filling the nanochannels with only buffer, the EO flow is initiated by applying
+6 V (VEO) to one of the wells, while the opposite well is grounded, and the
gate is left floating (Fig.3). In this case, the gate is heavily doped to
minimize contact resistance. Fig. 9shows LS-CFM images of Rhodamine B
under FET control. Note that within the field of view, approximately 5000
channels are present, extending horizontally in Fig. 9, as the dye molecules
traverse farther into the channels. The dimensional uniformity amongst the
channels will have to be improved to achieve a uniform front.
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Fig. 9 LS-CFM images of Rhodamine B in a pH 4 buffer solution under EO flow,
with and without the gate bias. (a)-(b) show the accelerationofEO flow with
negative gate bias. (c)-(d) show the reversed EO flow under positive gate bias.
In fig. 9, the dotted lines mark the
reference point, and the arrows
mark the direction and magnitude
of the dye flow. The observed EO
flow velocity is 2 m s-1. When anegative gate bias (VG = -30 V) is
applied to produce a large
negative z-potential, the flow
velocity is increased to 6 m s-1.
In Fig. 9(a)-(b), RhodamineB
moves from
right to left
at an
accelerated
pace.
Conversely, Fig. 9(c)-(d) shows that Rhodamine B rapidly reverses its flow at
8 m s-1when +30 V is applied to the gate. The flow response to the change
in the gate bias is immediate and repeatable. The observed flow response is
also independent of the position of the dye molecules with respect to the
gate position.
Figure 10: A time series IR absorbance spectra of RhodamineB in D2O taken
FET flow control. The background spectrum is taken after filling the
nanochannelswith D2O buffer solution. The sample spectra are taken eve
min, while Rhodamine B flows into the nanochannels be electroosmosis.
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From the results, the flow control with Rhodamine B shows a limitation when
the flow is reversed (Fig. 9(d)). Most of the Rhodamine B is observed to
reverse its flow direction, marked by the solid arrow, while the left mostoutlineremains at the same position. This observation indicates that
potentially two layers of Rhodamine B exist in the nanochannels because
Rhodamine B is positively charged in the
H range below 6.0, while the surface of
the nanochannels is negatively charged.
Its hypothesized that an inner layer, away
from the channel walls, is controllable,
while an outer layer near the channel
walls in not, due to electrostatic
interaction between the positively
charged dye molecules and negatively
charged channel walls.
The hypothesis is supported by MIR-FTIR
analysis during FET flow control. The IR
background spectrum is recoded after
completely filling thee channels and
inlet/outlet wells with the buffer solutionand injecting Rhodamine B into the
positive well. While the dye molecules
flow into the nanochannels due to EO flow
and respond to the gate bias, the sample
spectra are recorded every 5 min with 2 cm-1 resolution (Fig. 10). Each
spectrum is averaged over 50 scans to achieve good signal-to-noise- ratio,
while maintaining a reasonable time resolution. Fig. 11(a) shows
representative IR absorbance spectra extracted from Fig. 10, taken during EO
for different gate voltages.
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The focus is made on the COO- stretching mode of Rhodamine B at 1590-
1600 cm-1,
which is its
strongest
vibrational
mode [40].
The increase or decrease of the intensity of the COO-peak represents: a
convolution of an increasing amount of Rhodamine B filling the channels
during EO flow and upon applying a negative gate bias; increasing
concentration of Rhodamine B near the channel bottoms and sidewalls as an
electrostatic response to negative gate bias; decreasing amount of
Rhodamine B during reverse flow with a positive gate bias; and decreasing
concentration of Rhodamine B near the channel bottoms and sidewall as an
electrostatic response to positive gate bias.
The device architecture of an array of nanochannels integrated into a MIR
infrared waveguide with FET flow control was described. The device provides
the capability to simultaneously monitor flow control and probe molecule-
wall interactions, using LS-CFM and MIR-FTIRS. Flow control is demonstrated
using Rhodamine B by changing the gate bias to accelerate and/or reverse
the flow in the nanochannels. The implication of this charge-dependent
molecule-wall interaction is that the polarity and magnitude of channel
surface charge significantly influences the mobility of charged molecules in
nanochannels and that this mobility control can be used as an additional
mechanism to separate the molecules. Further investigations of the impact
of molecule-wall interactions, using fluorescent dye molecules of similar
molecular weight with different charges is needed as a function of pH.
Lab-on-a-Chip Integration
Figure 11: MIR-FTIR analysis of flow control, segregation, and adsorption/deso
of Rhodamine B in pH 4 buffer within nanochannels. The dye molecule is
positively charged at pH 4. (a) A representative set of IR spectra taken dur
flow in response to the gate bias. (b) Changes in the integrated absorbanc
COO- peak in response to the EO flow and to the gate bias.
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Standard microfluidics and
nanofluidics experiments utilize
flow through channels (of their
respective sizes) to conduct an
experiments whether it is for
analysis, characterization, force
spectroscopy, or other. With
most cases, only a few (if even
more than one) tests are
performed. With the advent of the integrated circuit, one can conduct
numerous tests on a single chip. This was the precursor to todays LOC.
Low fluid volume consumption means less waste, lower reagent cost, and asmaller required sample volume for investigation. Small volumes also
provide the integration of much functionality, in that a single chip can
incorporate a bunch of different diagnostics. High surface to volume ratios,
small heat capacities, and short diffusion distances also lead to faster
analysis. These factors also allow for real time data tracking which is
conduciveto enhanced process control. Because of the small stature of these
devices (another benefit of the LOC), experiments can be run parallel for
high throughput analysis. The low fabrication costs of such technology also
allow for mass production and disposable chips.
No technology is without its disadvantages, however. Because these
concepts are just the beginning, they are not fully developed and there is still
much room for improvement. The physical and chemical effects that are
more dominant at such small scales also make certain processes more
complex to fabricate (and operate) than would be the case in conventional
lab equipment. Another problem could arise from the scaling down of
features and detection principles, especially at the nanoscale. While there
are always possibilities to work around this, in some cases the effort may notbe enough and could render a less-than-effective or inefficient product.
Figure 12: Concept of lab-on-a-chip technology pertaining to DNA
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Conclusion
Typical biological experiments rely on assays that tend to rely on the quantity
of components used for more of an average value instead of a specific
characteristic per molecule (particle or whatever is being analyzed). Byperforming tests on the microscale or even the nanoscale, there can be a
more efficient use of the materials needed for a typical experiment. To
induce further persuasion, it can be said that each regime has its own
advantages and each can provide the backdrop for applications and
techniques that have yet to be fathomed.
Microfluidics for biology has its advantages because it is a somewhat more
mature domain of study. Techniques useful for today and in the future have
been well documented, fabrication methods can be low cost, and automation
allows for total control of the environment. The size of channels can becreated to accompany small particles or even large cells for complete
analysis, and the difference in flow effects at this scale make this possible.
There is a little more discrepancy on the nanofluidics side. The concepts and
techniques used here are less developed and slightly harder to control.
Since this realm is on a smaller length scale (nanometers vsmicrometers),
there is a completely different world of effects to take into account. This
makes some of the more traditional approaches difficult to employ. In the
same way, however, this may facilitate other processes far better than can
be achieved on the microscale.
As of now, the lab-on-a-chip is a novel concept in its infancy. While the
current designs have their purpose, there is still a lot to be gained by utilizing
the full potential of the subject. By developing microfluidic and nanofluidic
technologies (and even the more macroscopic techniques), we can rapidly
increase theuse, effectiveness, and efficiency of LOCs. This can only be
attained with further interdisciplinary and collaborative efforts between the
biologists who employ the technology and the engineers who fabricate it.
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References
1. H.T.G. Van Lintel, F.C.M. Vandepol and S. Bouwatra, Sens. Actuators,
1988, 15(2), 153-167.
2. S. Shoji, M. Esashi, and T. Matsuo, Sens. Actuators, 1988, 14(2), 101-
107.
3. N.T. Nguyen and Z. Wu,J. Micromech. Microeng, 2005, 14, R1-R16.
4. D.J. Laser and J.G. Santiago,J. Micromech. Microeng, 2004, 14, R35-
R64.
5. A. Sawaro, S. Takayama, M. Matsuda, A. Miyawaki, Dev Cell, 2002, 3(2),
245-257.
6. E.M. Lucchetta, M.S. Munson, Ismagilov, Lab Chip, 2006, 6, 185-190.
7. A. Folch, M Toner,Annu Rev Biomed Eng, 2000, 2, 227-256.
8. G.M. Walker, D.J. Beebe, Lab Chip, 2002, 2, 131-134.
9. C.S. Chen, M. Mrksich, S. Huang, G.M. Whitesides, D.E. Ingber, Science,
1997, 276, 1425-1428.
10. R. Arjun, A. van Oudenaarden, Cell, 2008, 135(2), 216-226.
11. D. Mijatovic, J.C.T. Eijkel, A. van den Berg, Lab Chip, 2005, 5, 492-500.
12. S. Haeberleand, R. Zengerle, Lab Chip, 2007, 7, 1094-1110.
13. A.L. Paguirigan, D.J. Beebe, Bio Essays, 2008, 30, 811-821.
14. G.M. Whitesides, Nature, 2006, 442, 368-373.
15. P.S. Dittrich, A. Manz, Nature Reviews, 2006, 5, 210-218.
16. H. Becker, Med Device Technology, 2008, 19(3), 21-24.
17. M.A. Burns et al., Science, 1998, 282, 484-487.
18.Jay Taylor, C.L. Ren, Microfluidic Nanofluidic, 2005, 1, 356-363.
19.W.B.J. Zimmerman, Microfluidics: History, Theory and Applications,
(2006) 49-100.20. Dongquig Li, Electrokinetics in Microfluidics, (2004) 7-28.
21.J.C.T Eijkel, A.V.D Berg, Nanofluidics: what is it and what can we expect
from it?, Microfluidic Nanofluidic, 1 (2005) 249-267.
-
8/14/2019 Mems Final Paper
24/25
22.T.S. Hug, N.F. Rooij, U. Staufer, Fabrication and Electro-osmotic Flow
Measurements in Microchannel Flow and Nanofluidics Channels, 2
(2006) 117-124.
23. C. Yi, C.W. Li, S. Ji, M. Yang, Microfluidics Technology for Manipulationand Analysis of Biological Cells, Analytica Chimica Acta, 560 (2006) 1-
23.
24. M. Kumemura, D. Collard, C. Yamahata, N. Sakaki, G. Hashiguchi and H.
Fujita, Single DNA Molecule Isolation and Trapping in a Microfluidic
Device, Chem Phys Chem, 8 (2007) 1875-1880.
25. Abgrall, A-M Gue, Lab-on-chip technologies: making a microfluidic
network and coupling it into a complete microsystem, Journal of
Micromechanics and Microengineering, 17 (2007) 15-49.
26. A. Daridon, M. Sequeira, G. Pennarun-Thomas, H. Dirac, J. P. Krog, P.Gravesen, J. Lichtenberg, D. Diamond, E. Verpoorte and N. F. de Rooij,
Sens. Actuators, B, 2001, 76, 235-243
27. A. van den Berg and T. S. J. Lammerink, Top. Curr. Chem., 1998, 194,
21-49.
28. S. Liu, Y. Shi, W. W. Ja and R. A. Mathies,Anal. Chem., 1999, 71, 566-
573.
29. P. C. Simpson, D. Roach, A. T. Wooley, T. Thorsen, R. Johnston, G. F.
Sensabaugh and R. A. Mathias, Proc. Natl. Acad. Sci. U. S. A., 1998, 95,2256-2261.
30. A. Manz, n. Graber and M. H. Widmer, Sens. Actuators, B, 1990, 1, 244-
248.
31. D. J. Harrison, K. Fluri, K. Seiler, Z. Fan, C. S. Effenhauser and A. Manz,
Science, 1993, 261, 895-897.
32. R. A. Mathies, P. C. Simpson and A. T. Wooley, in DNA Analysis with
Capillary Array Electrophoresis Microplates, Micro Total AnalysisSystems, ed. D. J. Harrison and A. van den Berg, Kluwer academic
Publishers, Banff, Canada, 1998, pp. 1-7.
33. G. Grandi, TrendsBiotechnol., 2001, 19, 181-188
34. R. B. M. Schasfoort, S. Schlautmann, L. Hendrikse and A. van den Berg,
Science, 1999, 286, 942-945.
-
8/14/2019 Mems Final Paper
25/25
35. R. Fan, R. Karnik, M. Yue, D. Li, A. Majumdar and P. Yang, Nano Lett.,
2005, 5, 1633-1637.
36. K. Ghowsi and R. J. Gale,J. Chromatogr., 1991, 559, 95-101.
37.Y. J. Oh, T. C. Gamble, D. Leonhardt, C. H. Chung, S. R. J. Brueck, C. F.
ivory, G. P. Lopez, D. N. Petsev, and S. M. Han, Lab Chip, 2008, 8, 251-
258.
38. R. Karnik, K. Castelino, R. Fan, P. Yang and A. Majumdar, Nano Lett., 5,
1638-1642.
39. K. F. Lei, S. Ahsan, N. Budraa, W. J. Li and J. D. Mai, Sens. Actuators, A,
2004, 114, 340-346
40. L. F. Johnson, G. W. Kammlott and K. A. Ingersoll,Appl. Opt., 1978, 17,
1165-1181
41. M. Pospisil, P. Capkova, H. Weissmannova, Z. Klika, M. Trchova, M. M.
Chmielova and Z. Weiss,J. Mol. Model., 2003, 9, 39-46.