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”

“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

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”

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

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

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) 1994-1997: 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

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 -10-3 mole 2002 : 10-100 proteins/day – 10-13 mole

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

Polymers: general properties macromolecular (10,000-100,000 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

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

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)

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

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

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

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

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

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)

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

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

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

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

Comparison of micro-molding technologies     

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)

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

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 30-50 psi (irreversibly) conformal sealing with flat surfaces (Van der Waals), pressures up to 5 psi (reversibly) Schematics and photo of a “passive” micromixer

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) 858-867

Sealing (III) polymers as adhesive layers (SU(8), PDMS,..) 8. 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. : 20060100518/15.9.2006 W. Chow et. al., Smart Mater. Struct. 15 (2006) S112-S116

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)

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

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)

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

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….

References G. Whitesides and A.D. Stroock, “Flexible methods for Microfluidics”, Physics Today (www.physicstoday.org/pt/vol-54/iss-6) 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), 21 27-40 E. Verpoorte and N. de Rooij, “Microfluidics meets MEMS”, Review Proc. IEEE (2003) 91 930-957 P. Grodzinski et. al. “Development of plastic microfluidic devices for sample preparation”, Biomedical Microdevices (2001), 3:4 275-83 H. Becker and L. Locascio, “Polymer microfluidic devices” Review Talanta (2002) 56 267-87 Y. Xia and G. Whitesides “Soft Lithography” Annu. Rev. Mater. Sci. (1998) 28 153-184 A. Gerlach, et. al. “Microfabrication of single-use plastic microfluidic devices for high-throughput screening and DNA analysis” Microsystem Technologies (2002) 7 265-68 R. Linhardt and T. Toida “Ultra-high resolution separation comes of age” Science 298 1441-42 M.U. Kopp et. al. “Chemical amplification: Continuous-flow PCR on a chip” Science (1998) 280 1046-48 CG Koh et. al. “Integrating polymerase chain reaction, valving, and electrophoresis in a plastic device for bacterial detection” Anal. Chem. (2003) 75 4591-98 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