Through manipulating fluids using microfabricated channel and chamber structures, microfluidics is a powerful tool to realize high sensitivity, high speed, high throughput, and low-cost analysis. A significant feature of microfluidics is that the material of the device dominates its functions. To realize certain functions, attention should be paid to choosing the suitable material for the device as it endows the inherent property of the device and determines the applicable microfabrication approaches.
Many materials have been developed over the past decades that are appropriate for transformation into microfluidic biomaterials. The suitable techniques for fabricating microfluidic devices often depend on the materials and specific application of the device and include micromachining, soft lithography, embossing, in situ construction, injection molding, and laser ablation.
The first-generation microfluidic device was prepared in silica and glass owing to their resistance to organic solvents, ease in metal depositing, high thermoconductivity, and stable electroosmotic mobility, although many other chip materials have been introduced afterward. Silicon materials often find use when semiconductor characteristics or devices are needed, and glass is the best material for sensitive optical detection or high voltage applications.
The semiconducting properties of silicon make it the dominant material in microfabrication. Silicon is processed with standard photolithography. Extensively characterized surface modification properties based on the silanol group, along with chemical resistance and flexibility in design, make silicon a desirable material for creating microfluidic devices. Silicon-based microfluidic devices have been employed in many applications, such as cell analysis, label-free detection, point-of-care medical diagnostics, and drug toxicity screening.
Glass has excellent performance due to its optical transparency, low fluorescence background, surface stability, chemical resistance, and biological compatibility. Glass is processed with standard photolithography and forms microchannels with laser direct writing (LDW). First-generation glass-based microfluidic devices had microchannels, flow reactors, and capillaries for chromatography or electrophoresis.
Fig.1 An example of a hybrid glass/PDMS device with a well and single channel for embryo culture. (Beebe, 2002)
The vast variety of polymers offers great flexibility in choosing the most suitable materials for different needs. Compared with inorganic materials, polymers are easy to access and inexpensive to microfabricate and therefore have become the most commonly used microchip materials. According to their properties, polymers are classified into elastomers, thermosets, and thermoplastics.
Polydimethylsiloxane (PDMS) is the most used elastomer in early-stage microfluidics research because it is easy to mold, optically transparent, cheap, biocompatible, and suitable for prototyping. Soft lithography is the common way of making PDMS devices, where features are created by casting PDMS on a mold, followed by bonding to another PDMS slab or a different planar material to make a device. PDMS is broadly used in bio-related research, primarily, cell culture, cell screening, and biochemical assays.
With proper bonding methods, microfluidic chips can be fabricated entirely in thermosets. One major advantage of thermosets is for true 3D microfabrication using photopolymerization. Another advantage is their high strength, which allows the fabrication of high-aspect-ratio and free-standing structures.
Thermoplastics can be reshaped multiple times by reheating, which is important for the convenience of their molding and bonding. Typical thermoplastics for microchips are polymethyl methacrylate (PMMA), polycarbonate (PC), polystyrene (PS), polyethylene terephthalate (PET), and polyvinylchloride (PVC). They can be manipulated with fabrication methods such as hot embossing, laser ablation, or precision milling.
Microchannels can be built in the hydrogels for the delivery of solutions, cells, and other substances. They are highly porous with controllable pore sizes, allowing small molecules or even bioparticles to diffuse through. Most hydrogels are gelled at mild conditions in aqueous solutions; thus, they can be molded from masters made of almost any material insoluble in water. In contrast to the ease in molding, the bonding is challenging. Reported bonding strategies include (1) melting a thin layer of the bonding surface by heating or chemicals right before attaching and (2) utilizing a second linking agent at the interface.
Paper is a highly porous matrix made of cellulose, excellent in wicking liquids. The fabrication of paper-based microfluidic devices is simple. In general, any method that generates hydrophobic patterns on paper is feasible. The reported methods can be divided into two groups. Lithographic processes apply polymer solution to a paper and subsequently, remove the formed coating from certain regions where channels are defined. In contrast, the printing (cutting) methods directly generate hydrophobic barriers without pre-exposure of the channel area to reagents.
3DP is a relatively new approach to fabrication, and many 3DP methods have been successful in forming fluidic channels, including fused deposition modeling (FDM), PolyJet (PJ), stereolithography (SLA), and more.
The materials mentioned above can be modified or combined into one hybrid chip to fabricate more powerful devices for specific aims.
Table 1. Applications of microfluidic systems made of different materials. (Ren, 2013)
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