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Fabrication Technologies

Microfluidics represents today a broad range of technologies both in the fluid motion strategy and the choice of materials and fabrication methods. Hybrid approaches are an increasing trend that Fluence is focused on delivering.

Total system solutions and function integration within associated modules are dictating the way forward.

The choice of technologies will be made by:

• End use, related specification & device performance needs
• Method of fluid flow and throughput needed
• Volume of devices required at an acceptable cost
• Combination of functions requiring integration

On the whole all commercially available materials are available for microfluidics. Some of the selection criteria are listed below. Fluence can provide access to all materials options through INTEGRAMplus partners and associates. Fluence own expertise is directed at the use of polymers to provide a versatile solution for microfluidic system fabrication with integration technologies to enable the packaging and interconnection of other materials and devices to achieve the optimum cost and performance balance in the volumes required (multidomain and multifunction approach).

• Silicon: major microfluidic application in ink jet print heads – favoured by electronics companies developing portable healthcare instruments with ambient intelligence or Si/polymer hybrid
• Metal: micro/milli-fluidic process intensified chemical synthesis – heat conduction, robustness, temperature range (cryo and hot)
• Glass: bioanalysis instruments and high throughput chemical synthesis and analysis
• Polymers: versatile alternative to all above for low, medium & high volume disposable consumables use extremes excluded + hybrids with glass or Si
• Ceramics: high temperature applications

A basic channel based microfluidic device is typically fabricated by the techniques shown below. As well as providing access to these approaches Fluence can also supply planar substrates with embedded functions or surfaces modified for use in electrowetting or digital fluidics, subject to IP consents, and in combination with demountable components containing a range of functions and interconnections to peripherals equipment. 


Channel structure

• Etched:
  • Chemical,
  • laser, powder,
  • plasma ions
• Micromachined
• Micromoulded

Holes in lid or base

• Etched as above
• Drilled
• Punched or moulded

Formats

glass slide, credit card, board, tape, film etc

Non-compliant etched substrates are required to have high levels of flatness if fusion or anodic bonding techniques are to be used for lidding. Polymer approaches also require flatness but benefit from some self-compliance. Thus in summary the methods of fabrication of enclosed microfluidic channels are as follows.

• All materials require highly flat surfaces
  • glass + silicon: polished (lapped),
  • metal: diamond machined
  • polymers: flat mould plates or roller
• If not flat a compliant interface is required (gasket). Self compliancy during bonding possible for polymers
• Bond
  • Adhesives
  • Annealing / Fusing e.g anodic bonded Si & glass
  • Physical attachment with clamps, screws, nuts and bolts
  • Thermoplastic welding (ultrasonic, laser, microwave)
  • Partially cured lid and base (curable polymers – PDMS, epoxy & ceramics)

All the techniques use for fabricating channels can be used for making the master tools from which mould tools can be obtained. Other mould tool origination methods include ultraprecision milling, electrodischarge machining (EDM), laser ablation plus electroforming, UV and e-beam lithography plus electroforming.

Machined or electroformed (Ni or Ni alloy) metal tools are generally used for moulding thermoplastic polymers by compression (embossing), transfer or injection moulding or extrusion casting. Metal or polymer mould tools can be used for replication using reaction formable polymers such as thermosets or room temperature curable systems such as polyurethanes, PDMS and UV curable formulations. Direct plasma etching, UV lithography, UV embossing or imprinting and laser ablation are mass production processes that can be undertaken using sheet processing or reel to reel film or foil processing.

Selection of whether a monolithic or a multilayer approach is adopted largely depends on the number of functions being integrated and the cost implication of subsequent product assembly steps. Monolithic approaches are good for simple structures, particularly if unlidded, and include direct machined metal, glass, silicon, polymer and moulded thermoplastics (micromoulding & CD), thermosets, low softening glass. Multilayer approaches are good for multiple function integration and 3D stacks; channels are formed in a coating on a support substrate, foil or laminate and include photochemically etched metals, glasses & polymers, semiconductor style lithographic deposition / subtraction, approaches from the printed circuit board industry and “Plastic or polymer electronics” approaches including reel to reel coating, printing / embossing, laminating. Processes that tend to be considered as prototyping processes such as microstereolithography, direct write and printing are also options for mass production.

Many microfluidic applications require integration of a range of functions and components. The choice of whether a monolithic or multilayer approach is the most cost effective requires consideration of the total system requirements.

The total processing system also needs consideration particularly methods of interconnection for each of the functions and for managing the processes from the molecular to macro system levels. Plug and play approaches using standard modules are expected to develop as the industry matures.

• Systems developed or being developed in metal, polymer, silicon (glass?) for creating microfluidic system architectures analogous to electronics and optics i.e. equivalents of wire, connector, backplane, board and chip, racks of modules
• Interconnection levels
  • Microfluidic channel backplanes or manifolds for series or parallel microfluidic processor chip linking
  • Microfluidic channel switching using microvalves (wax, diaphragm)
  • Microbore tubing standard connectors to microfluidic channels and to various inputs and outputs
  • Electrical, optical, gas interconnects integrated in “boards”

The choice of fabrication method also depends on the method of fluid movement and function actuation.

• Centrifugal
  • Favours circular devices – CD mouldings or wafers
• External pressure (syringe, diaphragm, HPLC, gas etc)
  • Favours devices that can withstand 100 Bar –
    • metal, some polymers, some glass
  • Piezoelectric (on chip diaphragm)
  • Requires diaphragms that are resistant to used chemicals
• Electrokinetic (electrophoretic, EOF)
  • Conductive metals excluded
• Capillary including electrowetting
  • Suited to small volumes. Surface wetting required
    • electrowetting avoids need for channels?
  • Other actuation requires electrical or gas and interconnects for valves, electrodes, magnets etc

The fabrication process used at the proof of concept, prototyping and product development stages may be the same or different. Methods are available through Fluence that avoid expensive mould tool costs before the product design is finalised.

The Fluence team and our partners can provide you with independent advice on the most appropriate fabrication processes at all stages through the product development cycle through to manufacture.

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