1. Microfabrication technologies for plastic microfluidics
Angeliki Tserepi
Researcher
IMEL - NCSR “Demokritos”
“Methods in Micro- Nano-technology and Nano-bio-technology” Summer School 08, NCSR-“Demokritos”

2. “Unlike microelectronics, in which the current emphasis is on reducing the size of transistors, microfluidics is focusing on making more complex systems of channels with more sophisticated fluid-handling capabilities, rather than reducing the size of the channels. These systems require the same types of components as larger fluid-handling systems: pumps, valves, mixers, filters, separators, and the like. Although the sizes of channels are large relative to the size of features in microelectronic devices, they are small enough so that flows in them behave quite differently than do the large-scale flows that are familiar from everyday life. The components needed at small scales are therefore often quite different from those used at large scales.”
G. Whitesides, Dept. of Chemistry & Biological Chemistry, Harvard

3. Introduction
Microfluidics: science and technologies for designing/ fabricating devices and processes for handling and control of minute amounts of fluids in a miniaturized system
Yole Développment Report Oct. 2004: “Emerging markets for Microfluidics Applications”

4. Market predictions
Yole Développment Report Nov. 2004: “Emerging markets for Microfluidics Applications”
Yole Développment Report May 2007: “Emerging markets for Microfluidics Applications”

5. Chem Lab-on-a-Chip from Sandia
Outline
History
Miniaturization
Motivation for using plastics/polymers
Fabrication (patterning+ bonding) technologies for plastics/polymers
Examples of plastic microfluidic devices
Conclusions
Chem Lab-on-a-Chip from Sandia
Lab-on-a-Chip (“Bio-analyzer”) from Agilent Technologies

6. History History of μ-fluidics
1975: 1st analytical miniaturized device on Si (GCA, separation in sec, Stanford)
1990: μTAS (Manz), integrated device for sample pretreatment, CE separation, detection, on Si
devices on plastic substrates appeared in patents only
1993: devices on Si and glass for DNA amplification (PCR)
: growing to critical mass, devices for PCR and PCR+CE
1997: 1st miniaturized CE system in PDMS
commercial companies started investing heavily in microfluidics
interest in plastic substrates begun to increase

7. Motivation for miniaturizing applications in Chemistry and the Life Sciences
transport controlled by diffusion: flow regime is laminar
Scaling laws:
- diffusion time-a molecule needs to travel l by diffusive processes
tD ~ l2 when l ↓ from cm to 100 μm, timescale ↓ from hours to sec
- separation efficiency in CE: N/t ~ 1/d2 when d↓ 10 separation speed x100
reduced consumption of reagents and analytes
reduced time of analysis
increased resolution
high throughput (parallel + faster analysis)
Example: Proteomics
1960 : 1 protein/day mole
2002 : proteins/day – mole

8. Plastics/Polymers as structural materials
low cost (disposable systems)
versatility of material properties (mechanical, optical, thermal stability, biocompatibility, surface energy, ...)
ease of fabrication (routine access to clean room environment not required)
ease of sealing
more accessible to chemists/biologists
Advantages:
Disadvantages:
More care to control surface energy
Incompatible with organic solvents
Incompatible with high temperatures

9. Polymers: general properties
macromolecular (10, ,000 Da, >1000 monomeric units)
Tg: glass transition temperature: plastic-viscous, can be molded
melting temperature: highly viscous mass
decomposition temperature: material ceases to function
fabrication of microfluidics
Thermoplastic (PA, PC, PE, PMMA, PP, PS, PEEK, COC)
Elastomers (PDMS)
Duroplastic

10. Plastic/Polymer patterning technologies: Replication methods
Late 1990’s
Injection molding
Hot embossing
Casting techniques
(replica molding)
Common characteristic in all techniques: require use of masters

11. Master fabrication I Rapid prototyping
Master in SU(8) photoresist (UV) on a Si wafer: quite durable, however replication in a hard polymer (polyurethane) can further extend its lifetime (resolution 5 μm)
Master fabricated with x-ray sensitive resists followed by electroplating (LIGA-technique, resolution 0.2 μm)

12. Master fabrication II Other master fabrication technologies include:
-Si micromachining (high resolution: 2 μm however subsequent electroplating is recommended for increased durability)
-laser ablation (resolution 5 μm)
-mechanical micromachining (resolution 50 μm)
Master fabricated by Si micromachining

13. Injection molding
Low viscosity copolymer is injected into a mold insert
Mold by Ni electroform from Si master
Good contact with the mold is required (low viscosity) to result in well resolved feature reproduction
Mold temperature and process time should be adjusted for excellent precision
Mold insert and Ni electroform can be used to produce 100,000’s parts
Materials: PMMA, PC
Production of 3D structures
Inclusion of preformed parts in the molded plastic

14. Hot embossing/imprinting
Low cost mass production technique
Stamps can be Si or metal
Heating of the plastic at its softening temperature at lower pressures or at room-temperature at elevated pressures
Materials: PS, PMMA, PVC, …
Resolution 25 nm
Microplate with 96 CE devices
Metal mechanical micromachined tool
Si stamp Imprinted channel in PMMA

15. Casting/Replica molding
Casting of prepolymer against a master and allowed to cure generating a negative replica easily detachable from the master
Materials: elastomeric polymers, PDMS
Masters not subjected to excessive heat or pressure, therefore usually produced in photoresist but other materials can be used
Resolution and roughness of the master is reproduced
Advantage: easily bonded to most of surfaces (Si, SiO2, other plastic)
facilitates fabrication of multilayered structures
Channels of a miniaturized CE device created by molding PDMS against a lithographic master

16. Soft lithography Soft lithography Group of G. Whitesides Harvard Univ.
Use of elastomeric patterned PDMS stamps to generate structures
Require little capital investment
Ambient laboratory conditions
Able to generate features on curved substrates
Soft lithography μ TM RM
MIMIC CP
Group of G. Whitesides
Harvard Univ.
MicroContact Printing MicroTransfer Molding
MIcroMolding In Capillaries Replica Molding

17. Soft lithography techniques (I)
Replica Molding: allows duplication of 3D topologies in a single step
Use of elastomers facilitates the release of small fragile structures
Harder polymers are then molded against the secondary master
Resolution 30 nm
Master (Au) Replica (PU)

18. Soft lithography techniques (II)
Microtransfer molding:
PDMS stamp is filled with a prepolymer and placed on substrate
Polymer cured and stamp removed
Able to generate multilayer structures
3-layer structure
Resolution 250 nm

19. Soft lithography techniques (III)
Micro-molding in capillaries:
Continuous channels formed upon contact of PDMS stamp with substrate
A polymer precursor fills channels with capillary action
Polymer is cured and stamp is removed
Resolution 1 μm
Membrane of PU

20. Soft lithography techniques (IV)
Micro-contact Printing:
An “ink” is spread on a patterned PDMS stamp
The stamp is then brought into contact with the substrate
The “ink” is then transferred to the substrate where it can act as a resist against etching
“Ink” can be a SAM or a biological sample
A. Fabrication of the stamp B.Transfer of the “ink” to substrate
C. Transfer of the “ink” on curved substrate
Resolution 300 nm

21. Soft lithography Restrictions
High resolution registration is problematic due to distortion of PDMS
Multilevel structures necessitate accurate placement of many layers, only possible with relaxed requirements
Defects: dust particles, bubbles in precursor, residual thin polymer film

22. Comparison of micro-molding technologies    

23. Direct methods (not based on use of a master)
laser photo-ablation (1997)
polymer degradation by UV absorption
direct-write or through mask
roughness formation
materials: PMMA, PVC, PET, PS, cellulose acetate, …
resolution ~1 μm
Channel fabricated by laser ablation of PMMA
optical lithography (deep resists, e.g. SU(8))
stereolithography (with focused laser beams)
micromilling (CNC micromachining, resolution down to 100 μm)
ion milling (Ar+)
plasma etching (under development, appropriate for mass production)

24. Summary Non-photolithographic patterning methods low cost technology
allows patterning of non-planar surfaces
can generate 3D structures
applicable to patterning of a variety of materials
applicable to patterning of functionalities of certain chemistry
These techniques complement photolithography and extend micropatterning into dimensions, materials, and geometries, where photolithography is not practically applicable

25. Surface modification and sealing (I)
few methods of stable chemical modification of plastic channels (e.g. PMMA)
modification by amine functionalities, polyelectrolytes, protein adsorption
PDMS exposure to O2 plasma activation creates SiOH groups on the surface, thus
renders the channels hydrophilic (easily wettable by aqueous solutions)
supports strong EOF in contact with neutral or basic solutions (CE)
PDMS easily sealed on materials: glass, silica, plasma-treated PDMS, pressures psi (irreversibly)
conformal sealing with flat surfaces (Van der Waals), pressures up to 5 psi (reversibly)
Schematics and photo of a “passive” micromixer

26. Sealing (II) Polymer-to-polymer bonding methods include: adhesives
solvent-assisted glueing
thermal bonding (near or above Tg)
ultrasonic/microwave bonding (small area, 1x1 cm)
(disadvantage: possible deformation of substrates and microchannel patterns)
5. laser welding
6. lamination (with foils)
PET/PE foil
J. Rossier et. al., Electrophoresis 23 (2002)

27. Sealing (III)
polymers as adhesive layers (SU(8), PDMS,..)
surface activation (e.g. PDMS/plexiglass after chemical and plasma modification)
21,9μm
22μm
216μm
Βulk PDMS
PMMA
W. Chow et. al., Smart Mater. Struct. 15 (2006) S112-S116
“Bonding method” Κ. Misiakos, Α. Τserepi, Μ.Ε. Vlachopoulou, Patent Appl. No. : /
W. Chow et. al., Smart Mater. Struct. 15 (2006) S112-S116

29. Examples of commercial polymer microfluidic devices
Lilliput® microtiter plate by Steag Microparts (96 reaction wells, 1.8μl each, injection molding from PS, for bacteria identification)
Gyrolab® by Gyros AB (injection molding from polyolefin)

30. Microfluidic devices for μ-analysis (I)
Devices for capillary electrophoresis
Becker and Lacascio, Talanta, 56 (2002)
Microchannel device that couples with ESI/MS
(PMMA, hot embossing)
The 1st widely published application of μTAS was in CE, 1st to be commercially available
materials: thermoplastic polymers (PMMA, PC) and elastomers (PDMS)
fabrication methods: hot embossing, injection molding, laser ablation
detection: LIF, electrochemical detection
results: separation speed and resolution comparable to glass devices

31. Microfluidic devices for μ-analysis (II)
Devices for miniaturized PCR
PCR the most widely used process in biotechnology for DNA fragments amplification
It involves three T’s, only polymers with higher T-stability can be used: PC, COC, PDMS
Polymer devices for continuous-flow PCR
Kopp et. al. Science, 280 (1998)
Kohler et. al. IMRET (1998)

32. Microfluidic devices for μ-analysis (III)
Lab-on-chip system for bacterial detection and identification (on poly-cyclic olefin)
It integrates:
DNA amplification
microfluidic valves
sample injection
separation by CE
detection by LIF
ACLARA BioSciences Inc., Anal. Chem. 2003, 75
PCR sample volume: 29 nl
DNA detection limit: 6 copies of target DNA

33. Epilogue
Challenges for research:
integration of more functions
cheap and mass production processes
on-chip integration of external controls
Long-term success of μ-fluidic devices is assured due to broad range of applications
Key-players in commercialization of (bio) analytical devices:
Agilent: provider of instrumentation +software +services to life sciences and chemical analysis markets
Caliper LifeSciences: liquid handling + lab-on-chip technologies for accurate drug discovery and diagnosis of disease
ACLARA BioSciences: development of advanced tools for drug discovery and development by using assay platform
Affimetrix: development of state-of-the-art technology for acquiring-analyzing genetic information
………………many more….

34. References
G. Whitesides and A.D. Stroock, “Flexible methods for Microfluidics”, Physics Today (
Yole Développment Report: “Emerging Markets for Microfluidics Applications”, (Oct./Nov. 2004, May 2007)
J.C. McDonalds, et. al. “Fabrication of microfluidic systems in PDMS”, Electrophoresis (2000),
E. Verpoorte and N. de Rooij, “Microfluidics meets MEMS”, Review Proc. IEEE (2003)
P. Grodzinski et. al. “Development of plastic microfluidic devices for sample preparation”, Biomedical Microdevices (2001), 3:
H. Becker and L. Locascio, “Polymer microfluidic devices” Review Talanta (2002)
Y. Xia and G. Whitesides “Soft Lithography” Annu. Rev. Mater. Sci. (1998)
A. Gerlach, et. al. “Microfabrication of single-use plastic microfluidic devices for high-throughput screening and DNA analysis” Microsystem Technologies (2002)
R. Linhardt and T. Toida “Ultra-high resolution separation comes of age” Science
M.U. Kopp et. al. “Chemical amplification: Continuous-flow PCR on a chip” Science (1998)
CG Koh et. al. “Integrating polymerase chain reaction, valving, and electrophoresis in a plastic device for bacterial detection” Anal. Chem. (2003)
O. Geschke, H. Klank, P. Telleman, “Microsystem Engineering of Lab-on-a-chip Devices” (2004) Wiley-VCH Verlag
H. Becker, C. Gartner, “Polymer microfabrication methods for microfluidic analytical applications” Electrophoresis (2000) 21, 12-26