Chip materials in microfluidics: a review

Choosing the right material for the development of microfluidic devices is critical and not insignificant, as the chosen material must combine optical qualities close to those of glass (or ideally fused silica) for optimal detection, surface properties avoiding adsorption of biomolecules and electro-osmosis, and the ability to reproduce micrometre-sized structures. Polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), polycarbonate (PC), polystyrene (PS), polyimide and polyethylene (PE) are some of the polymer materials commonly used to manufacture chips. Of these, PDMS is the most popular substrate.

More recently, SU-8 resin has entered the microfabrication market for microfluidic chips. Cyclic olefin copolymer (COC) and polyethylene glycol diacrylate (PEGDA) are also widely used materials for microfluidic chip manufacturing.

Viewing the number of materials used, it can be complicated to make a choice when you are looking to fabricate your chip. This review will help you choosing the right material for your needs.

The most commonly used chip materials in microfluidics

Fabrication in polymers is easy, and their use as materials reduces the time, complexity, and cost of prototyping and manufacturing. In general, microfluidic systems are made by moulding and bonding replicas in elastomers such as PDMS, or thermoplastics such as PMMA or PC. Others are more and more appreciated in the field of microfluidics such as COC or SU-8. These polymers owe their success to four essential properties: biocompatibility, transparency, low cost and, of course, the absence of copyright.

Polydimethylsiloxane or PDMS

A significant proportion of microfluidic devices are manufactured in PDMS. One of the main advantages of this material is its high optical transparency in the visible and ultraviolet range, which makes it suitable for many fluorescence-based applications. Another strength of PDMS is that it is permeable to gases, which allows the culture of cells in microfluidic devices made of this material. Manufacturing is also facilitated with PDMS since chips in this material are simply made from a mould with the negative relief of a desired pattern. The pre-structured pattern can be achieved using standard photolithography techniques, and a minimal clean room infrastructure is also required for the assembly of PDMS chips. PDMS is well suited to prototyping techniques, making PDMS a material of choice in microfluidics.

Beyond all its advantages, there have been some reported disadvantages to using PDMS as a drug absorption device [Regehr KJ et al. 2009 Lab Chip] or other molecules when PDMS is oxidised [McDonald et al. 2000 Electroph.], which has prompted researchers to produce devices in other materials. Other disadvantages encountered with PDMS are, for example, the swelling of organic solvents (which limits the range of microfluidic applications), low mechanical strength (leading to the collapse of high aspect ratio structures in the device) and unstable surface treatments [Paul et al. 2007 Electroph.].

Polymethylmethacrylate or PMMA

PMMA is a transparent thermoplastic which monomer is methyl methacrylate (MMA). This polymer is better known under its first commercial name, Plexiglas. PMMA has many advantages such as its transparency, resistance and the fact that it is amorphous. It therefore allows excellent light transmission, up to 92% of visible light, i.e. more than glass [Good Fellow website].

PMMA is relatively sensitive to scratches (more than conventional mineral lenses) and becomes brittle when in contact with additives. It also has a relatively poor resistance to many chemicals and dissolves in many organic solvents. However, its environmental stability is better than that of most other plastics.

Polycarbonate or PC

PC is a thermoplastic polymer. Compared to other materials used in microfluidics, such as COC (for cyclic olefin copolymer) for example, it is less hydrophobic and the channels therefore have a better filling behaviour. Ideally, it is used for high temperature applications such as PCR.

The real disadvantage of polycarbonate is its relatively high intrinsic fluorescence, compared for example to PMMA. This disadvantage is really prejudicial for fluorescence microscopy and therefore restricts its use.

Cyclic olefin copolymer or COC

Cyclic olefin copolymer (COC) is an amorphous polymer. COC is a relatively new class of polymers compared to commodities such as polypropylene and polyethylene. Cyclic olefin copolymer is a material well suited for microfluidic applications. Indeed, the list of its advantages is long and includes good biocompatibility, low water absorption properties, good chemical resistance and high transparency in the deep UV range.

Due to its many advantages, COC can of course be used for the manufacture of chips in many areas, for example for the analysis of parameters such as blood gas, glucose and lactate concentration or for the determination of haematocrit levels. In addition, as the channel walls are UV-transparent, they allow the use of UV initiators such as benzophenone, which is widely used for Western blotting.

SU-8 resin

SU-8 is a negative photosensitive resin commonly used in the manufacture of microsystems. SU-8 resin is made of epoxy resin, propylene carbonate, triaryl-sulfonium initiator and an organic solvent. Due to its composition, SU-8 is therefore a highly viscous polymer which is spread over a thickness ranging from 1 micrometre to 2 millimetres and generally worked by photolithography. It is increasingly used for the manufacture of microchannels as it has excellent chemical stability against several acids and bases as well as high thermal stability. In addition, SU-8 is mechanically reliable, optically transparent and hydrophilic. These properties therefore make SU-8 a very attractive material for a wide range of applications such as microfluidic applications of course, but also for micro-optics, micromachining, packaging or analytical applications.

However, SU-8 is rarely used as a stand-alone chip but in combination with other materials such as PDMS or silicone.

Polyethylene glycol hydrogels or PEGs

PEGs are hydrogels that offer significant advantages over other biomaterials. Indeed, PEGs are synthetic, allowing absolute control of mechanical properties such as modulus of elasticity and degradation rate, unlike natural biomaterials. This also means that PEG has less batch-to-batch variability compared to materials of natural origin. The chemical composition of these hydrogels make them highly hydrophilic, resistant to protein adsorption, and biocompatible. There are a variety of chemical groups which can be incorporated into PEG macromers to facilitate polymerization and network crosslinking (acrylate, methacrylate, vinyl ether, norbornene, etc.) [Amy H et al. 2013 Chem J].

Conclusion

Material properties that may be of fundamental importance include machinability, surface charge, molecular adsorption, electro-osmotic flow mobility, optical properties, and many others. The choice of substrate is therefore critical to both the manufacturing process and the successful application of the device. It can therefore be very complicated to upgrade one material rather than another. To help you in your choices, Darwin Microfluidics offers a wide range of off-the-shelf microfluidic devices combining different designs and materials compatible with a wide range of applications.

Find the catalogue of chips available on Darwin Microfluidics right here!