Integrated Elastomeric Microfluidic and Optical System Fabrication

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Integrated Elastomeric Microfluidic and Optical
System Fabrication

• Shraddha Avasthy
• John Rogers

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Motivation
Conventional lithography limited in terms of the
materials compatible with the processing
300 mm wafer
Intel P4
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Motivation for Nano CEMMS center
create a viable manufacturing technology and
science base that can fabricate
ultrahigh-density, complex nanostructures
3-D structures
Nano-CEMMS website
Chemical and Biological Sensing
Optoelectronics
Nano-CEMMS website
Combinatorial Chemistry
Nano-CEMMS website
http//www.contech.com/images/ABS_Sensor_Schematic
.gif
Todd Thorsen, ME, MIT
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Motivation
Nano-Toolbit being developed by the Nano CEMMS
center for integrating individual efforts at
nanofabrication
Scheduling
Staging and positioning
Machining tool
Nano Sensing
Fluidic Toolbit
Nano-CEMMS website
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Motivation
Fluidics controller
Scheduling
Elastomeric optical system
Sensing
Microfluidic device
Elastomeric microfluidic system
Staging and Positioning
Nano Fabrication
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Elastomeric pneumatic printing mechanism
Quake et al, Science, Vol. 288, April 2000
Fluid inlet to apply pneumatic pressure on the
flow layer
Channels in PDMS made by soft lithography
PDMS (51) Control layer, 5mm thick
PDMS (201) Flow Layer, 15 microns
Kapton Nozzle layer, 40 microns
Fluid ejected out of nozzle
Cross Sectional view of the device
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Techniques used for nozzle layer fabrication
Laser Ablation
Focused Ion Beam Milling
Top view
Bottom view
30µm holes
20µm holes
PDMS
Kapton
FIB resolution goes down to 7nm!
5µm nozzle
5µm nozzle
Jang Ung Park, John Rogers Group
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PDMS Flow and Control layer channels by Soft
Lithography
Cross sectional views
Spin coat photoresist on Silicon substrate
Master
Pattern the resist using a mask with desired
pattern and develop the pattern in developer
Silanize patterned resist and silicon surface to
prevent the elastomer from sticking to the master
Top view
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Assembling the device
Control layer
Flow layer
Made by spin coating PDMS
UVO exposure and bonding
Bonding to Kapton by RIE
Nozzle drilled in Kapton under the intersection
of flow and control layers by Laser Ablation
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Understanding system behavior
Control layer
Flow layer
Nozzle
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Design principle for flow layer
Flow layer should have semicircular geometry for
efficient valving
Photoresist reflow in Flow Layer Master
Heating at 200C for 30 minutes
Pressure
Pressure
Rectangular geometry by photolithograhy
Semicircular geometry by photoresist reflow
Cross sectional view
Cross sectional view of a single flow channel
Curved profile
100 degC 30min
200 degC 30min
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Controlling the valving system
Pneumatic device
Actuated control layer
90µm
Flow layer
Nozzle
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Back flow in flow layer on actuation of control
layer
Air pressure
Backflow
Fluid following low resistance path
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Aggravation of backflow by nozzle layer deflection
Control layer
Flow layer
Flexible Nozzle layer
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Design parameters for the printing system
w
C
t
F
D deflection of membrane between flow and
control layer

• Larger control layer width easier to actuate
• Lower membrane thickness between flow and control
  layer allows greater deflection

Adv. Mater.,16, No. 23-24, December 2004
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Future design

• Backflow reduced by use of confined fluid pockets
• Wider control layer regions on fluid pockets and
  slimmer elsewhere would confine nozzle layer
  deflection to some extent
• Better control over the volume of deposition

Bottom view
Cross sectional view
Control layer
Confined fluid pockets layer
Kapton nozzle layer
Filling of fluid pockets by suction mechanism
Activate the control layer and place the nozzles
over an ink reservoir and deactivate the control
layer
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Developing elastomeric optical system for
microfluidic device
Idea
PDMS stamp with micro-fluidic channel
Planar embedded waveguides easier to integrate
with microfluidic system
PDMS substrate with planar waveguides of a
material with higher refractive index material
UVO expose and stick
Intensity Analyzer
Can determine the chemical composition of the
fluid in channel by spectroscopic analysis of
waveguided light
Input light
Fluid 1
Fluid 3
Fluid 2
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Waveguides
A waveguide is a structure that causes a wave to
propagate in a chosen direction with some measure
of confinement in planes transverse to the
direction of propagation
n1gt n2
n2
cladding
n1
core
n2
Total internal reflection at the interface if
angle of incidence greater than the critical angle
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Evanescent wave used for sensing
Evanescent wave is a real exponential decay
(imaginary wave number) in one spatial direction
of wave-guided light out of the high refractive
index core
Side View
n3 n1
Top View
Absorbs evanescent waves and internally reflected
wave is attenuated
Fluid
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Some existing integrated systems
Applied Optics, Vol. 40 No. 34, Dec 2001
IEEE photonics technology letters, Vol. 16, No.
6, June 2004
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Materials for core and cladding
Core OE4110 (Transparent optical silicone
elastomer and uses platinum catalyzed
hydrosililation cure)
Cladding PDMS
Refractive index 1.4
Refractive index 1.46-1.47 at room temperature
Phenyl group as the organic substituent on Si
B. Schnyder et al, Surface Science 532-535 (2003)
1067-1071
Collaboration with Dow Corning
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Procedure for making thin polymer samples for
ellipsometry
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Profilometer measurements
PDMS
10µm
OE4110
2.5µm
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Ellipsometer measurements for PDMS and OE4110
OE4110 has a consistently higher refractive index
than PDMS Results still need verification
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PDMS stamp used for making waveguide
SEM of the PDMS stamp with 25 micron channels
Empty channels in PDMS
Rough edges due to transparency masks having low
optical density
Flip the stamp on a substrate to establish
conformal contact with the substrate which
enables reversible bonding with the substrate
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Profilometer measurements of PDMS stamp
25µm
10µm
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Filling channels by capillary action
Capillary forces in channels depend on the
surface energy, contact angles with walls and the
geometry of channel
Direction of filling
However, we can fill only a limited length of
channels at a reasonable speed (1-1.5 cms)
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Enhancing filling rate
PDMS is a highly permeable material which lets us
achieve the following 2 ways of enhancing the
filling length as well as rate
System works faster than pure capillary filling
or filling from one side in vacuum

• Put the stamp in vacuum for 20minutes
• Take out the sample from vacuum
• Fill in the channels from both ends by putting a
  drop of OE4110 on either side of the channel

Filling from both ends!

• Observed parameters affecting the rate of
  filling
• Time for which the PDMS stamp is exposed to
  vacuum
• Thickness of the PDMS stamp

Can fill lengths as long as 6-7cms!
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Procedure for planar waveguides
Side view
Top view
PDMS stamp
Put in vacuum jar
300nm Oxide wafer
Pour some OE4110
Extracted air moves back
After curing for a few hours at 150 deg C
Channels fill in with OE4110
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Straight planar waveguides
No residue due to stable conformal contact of
PDMS stamp to substrate
OE4110
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Set up for testing waveguides
Can couple light into the waveguides!
Mirrors
He-Ne laser
Microscope
Optical fiber
Intensity Analyzer

• Background light
• Rough waveguide interface

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1.Getting rid of background light
2a
Bent waveguides
Cone of light accepted by the waveguide
(Numerical Aperture) sin 2a n2core-n2cladding1/
2/nair Rest is background!
Wave guided light (Photodetector)
Background light
Enabling data collection from waveguide
Input Light (Laser Source)
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Fabrication of bent waveguides
SEM image
Optical image
1mm
90 bends with a radius of curvature of 4mm
Can make different shapes of embedded waveguide
structures to correspond to the network of
microfluidic channels in the printing device
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Procedure for longer waveguides
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Optical images of long waveguides
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Optical images of long waveguides
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2. Getting smooth interfaces to prevent light
scattering at inlet and outlet
Cleaving the wafer along crystallographic
direction with array of OE4110 25 micron lines
Zhao et al., APL, Vol 71, No.8, 1997
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Sensing experiments with the waveguides using a
spectrometer
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He-Ne Laser
Mirror
Spectrometer
Mirror
Fiber optic sensor
Objective
Objective
Sample
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Conformal contact of a flat PDMS stamp on top of
a waveguide spreading on the waveguide surface
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CCD imaging of the waveguides
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CCD imaging set up
Sample stage
Fiber optic white light illumination
40x objective
Computer Monitor
40x objective
CCD camera
Silver mirror
Silver mirror
He-Ne Laser
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CCD images of waveguided light
Light coupling into a single waveguide
Light coupling into two waveguides
Light coupling into three waveguides
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CCD images with waveguides visible
Coupling into different waveguides in the
waveguide array
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FEMLab modeling of waveguides
PMC

• Perfect magnetic conductor (PMC) boundary
  condition for all outer boundaries
• Wavelength used for light 1.55µm

n1.45 (PDMS)
PMC
PMC
n1.47 (OE4110)
n1.00 air
n1.33 water
PMC
Number of degrees of freedom 48178 Mesh
refinement done twice
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Modeling Results
Power distribution along the edge of the waveguide
PDMS in contact with OE4110
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Different modes in 25x10 micron waveguides 5
microns apart with wavelength 1.55µm
Surface plots of Power flow time average norm
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25 micron waveguides 20 microns apart with
water/air channel in contact
PDMS
25µm
OE4110
10µm
Air/ Water
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Effective mode index 1.460882
Line plots for 0.5microns into the channel from
the interface of the waveguide
Effective mode index 1.460926
0
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Integration of microfluidic channels and the
optical waveguides
Waveguide array
Microfluidic channel
Inlet to channel
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Future Work
Summary

• Elastomeric microfluidic printing system and its
  operation
• Elastomeric Optical system for sensing fluidics
• Application of an integrated system combining the
  above in the Nano- CEMMS toolbit

• Spectroscopic analysis of the different fluids
  flowing in the microfluidics integrated with the
  waveguides
• Integrating the system thus developed to the rest
  of Nano-Toolbit

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Acknowledgements

• John A. Rogers
• Julio Soares
• Viktor Malyarchuk
• Jang Ung Park
• Rogers group members
• Laser Lab at MRL
• Microscopy suite at Beckman Institute

Funding
NSF (Nano CEMMS at MIE)