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PDMS Casting
Mateusz L. Hupert, Ph.D.
How to Make It! Workshop
Lawrence, KS
August 7-9, 2019
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Mask based, global patterning:- UV or X-ray lithography,- wet etching,- DRIE,- evaporation techniques.
Tools:- micro-milling bit,- laser beam,- beam of ions,- beam of electrons,- plastic filament extruder (additive)- UV light.
Substrate
Maskless, “tool”-based:- CNC micromilling,- laser writing,- ion beam lithography,- e-beam lithography,- 3D printing (FDM, SL, DLP).
Direct Microstructuring Techniques
Patterning agents:- radiation,- liquid or gas etchants,- metals and dielectrics(additive).
Mask:- metal layer,- photoresist.
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
mold master
molded chip
replication:- hot-embossing,- injection molding,- casting.
Microstructuring via Replication
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Fabrication of molding master via direct
techniques
Molding
Demolding
Replicate
Substrate
Directmicrostructuring
Device
SubstrateReplication
Device
Slow and expensive
Fast and low cost
Multiple devices can be produces from a single master
PDMS Casting
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
PDMS: General Overview
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
PDMS: Poly-Dimethyl Siloxane
Applications:
Industrial lubricants, adhesives, sealants (Rain-X, Silly Putty,
kinetic sand, solar panels)
Food (anti-foaming agent in cooking oil)
Beauty Products (conditioners)
Medical devices
Basic Properties:
Hydrophobic Surface
Chemically resistant
Optically transparent
Non-toxic
Gas permeable
Insulator
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
• High replication fidelity down to nm range,
• Biocompatible,
• Transparent with low fluorescence background:
- high resolution microscopy,
- optical detection for sensing applications,
• Elastomer:
- fabrication of on chip valves and pumps,
- ease of fluidic chip interconnects to external pumping,
• Gas permeable:
- on chip cell culture and growth, organs on-chip,
- filling dead-end channels,
• Two-part system - cross-linker/curing agent and siloxane:
- tuning of material properties,
- easy blending with other materials,
• Easy to bond to itself or glass using oxygen plasma pretreatment,
• Low cost replication setup,
• Perfect for prototyping and proof of concept studies.
PDMS use in Micro/Nano-Fluidics
Attributes:
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
PDMS use in Micro/Nano-Fluidics
Limitations:
• Low surface energy, hydrophobic:
- requires surface modification prior use to improve wettability
and reduce biomolecule adsorption,
- virtually impossible to deposit metals on the surface,
• Unstable surface chemistry
• Absorbs organic molecules (e.g.: solvents, dyes) and may release
organic molecules from incomplete crosslinking,
• Permeable to water vapor – difficult to control evaporation,
• Composition variability of bulk material – requires batch
prescreening for large volume production,
• Relatively expensive for mass production.
Main steps
1. Fabricate mold master
2. Pretreat mold with release agent
3. Mix PDMS and curing agent
4. Degas to remove bubbles
5. Pour PDMS on mold
6. Bake PDMS
7. Cut and peel PDMS off mold, punch out holes
8. Plasma treatment and bonding
PDMS Casting Process Flow
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
Master Mold Pouring and Baking Release of Replica
Punching I/O Holes Plasma Treatment Bonding
PDMS Casting: Fabrication of Mold Master
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
• Typically SU8 photoresist on Si wafer
• Can be made from other materials
- PDMS Master
- Glass
- KOH-Etched Si Wafer
- Molded Thermoplastics
- Machined materials
Su-8 on Si wafer Molded PMMA Master
PDMS Casting: Molding Master Pretreatment
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Hydrophilic surface
after treatment
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
Pretreatment Agents:
• Hexamethyldisilazane (HMDS)
• Trichloro(octadecyl)silane (OTS)
• Trimethylchlorosilane (TMCS)
• Trichloro-(1H,1H,2H,2H-perfluorooctyl)-silane (PFOTS)
• Perfluorodecyltrichlorosilane (FDTS)
• Etc, etc, etc…
Application Methods:
• Vacuum Deposition (heated or room temp)
• Solution Phase Deposition
• Evaporation at STP
• Pretreatment facilitates demolding and prolongs life
of the master
PDMS Casting: Mixing
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication• Ratio base/curing agent (by mass): 10:1 most common
• 40 grams of mixed PDMS will result in ~ 5
mm thick layer for 4” Si wafer master,
• Add more curing agent to increase PDMS
elastic modulus
Most Common PDMS brands:
• Sylgard 184 – used for single layer devices,
mammalian cells, most common, higher
purity,
• RTV-615 – more robust, used for multilayer
chips, lower purity than Sylgard.
After a thorough mixBefore mixingSylgard 184
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
PDMS Casting: Degassing
• Place PDMS mixture in vacuum desiccator
• Degas under vacuum for 30 min - 2 hours
• Larger mixing containers prevent spills
Vacuum pump Desiccator
Degassing Setup
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
PDMS Casting: Pouring
• Place mold into a container and gently pour PDMS on top
• Plastic Petri dishes are common containers
• Pour PDMS close to surface of Si wafer to prevent
formation of new bubbles
• If new bubbles are introduced during pouring repeat
degassing of poured PDMS,
• Thin layer replicas with uniform thickness can be made via
spin-coating of PDMS onto patterned wafer
Spin-coater for thin-layer replicasCasting of thicker replicas inside Petri dish
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
PDMS Casting: Baking
Temp Time
Pouring on mold - 25°C 1.5 hrs
Curing – 25°C 48 hrs
Heat curing – 80°C 2 hrs
Heat curing – 100°C 35 min
Heat curing – 125°C 20 min
Heat curing – 150°C 10 min
• Bake in an atmospheric oven or on hot plate
• Do not heat over ~90°C if using plastic petri dish
• Protect casting setup from environmental
contaminations
Curing times for given oven temperature
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication• Cut cured PDMS from the edge of the wafer with a razor
blade or X-Acto knife,
• Carefully peel the PDMS from the wafer,
• Pulling too hard can result in tearing of PDMS replica or
damaging the master,
• Over-baking the PDMS can make removal difficult,
• Punch out holes for fluidic inlets and outlets – biopsy
punches work the best. Place soft plastic under PDMS
replica to protect sharp edges of the punch.
PDMS Casting: Demolding
Biopsy punches
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
Master Pretreatment
Mix
Degas
Pour
Bake
Demold
Plasma bonding
Master Fabrication
PDMS Casting: Plasma-assisted Bonding
• Bond devices to enclose channels
• PDMS can bond to other PDMS or glass after plasma
activation
• Each part must be free of particles
- clean PDMS with isopropanol if necessary
• Activate PDMS surface by exposing to oxygen plasma
- 60 – 100 W for 1-2 mins
• Plasma treatment transforms Si-CH3 groups of PDMS to
Si-OH and contacting treated PDMS with another
surface with Si-OH groups will create a strong and
permanent Si-O-Si link
• Poor bonding may be due to dust/debris
O2 plasma generators
• Valved, 3D microfluidic, and/or organ-on-a-chip devices
require alignment between layers.
• Alignment can be done under a microscope or using an
alignment tool
• Most multi-layer devices must be irreversibly sealed to
prevent device failure
Aligned Bonding of Multi-layer PDMS Devices
Home-built PDMS aligner at KU
Surface Modification of Assembled PDMS Devices
• UV/ozone and plasma treated PDMS has a temporary
hydrophilic surface that is silica-like.
• Hydrophilicity will revert to hydrophobicity in about 30
minutes.
• Sealed devices can be re-treated a few times using a
UV/ozone corona treater
- Improves wettability of fluidic devices
- Improves EOF in electrophoretic separations
- Can help to minimize analyte adsorption
Electro-Technics Products’ BD-20
“How to Make It and How to Use It”, The University of Kansas, Lawrence, KS, August 2019
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