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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.


mold master

molded chip

replication:- hot-embossing,- injection molding,- casting.

Microstructuring via Replication


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


PDMS: General Overview


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


• 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:


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


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


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


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


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


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


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


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



• 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


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


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


pdms casting - university of kansascbmm.faculty.ku.edu › ... › 06-pdms_casting_2019.pdf · pdms...

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