Author: İbrahim H. Erbay, University of Dokuz Eylül, Institute of Izmir BioMedicine and Genomics
Lab-on-a-Chip is an interdisciplinary field that allows controlling low volume fluids and is used in many natural sciences and engineering applications. Recently, industrial applications of the field have been frequent due to their important functions within biosensors and model of tissue/organ applications. Properties of microfluidic chips are determined by the materials and corresponding fabrication technique used, therefore material selection carries utmost importance. Materials used in the fabrication of chip systems are broadly categorized as following; inorganic, polymeric and paper.
This review focuses on frequently used materials and their properties with an emphasis on how it affects the overall performance of the lab-on-a-chip (LOAC) devices.
Inorganic materials for microfluidic devices
In early studies of the microfluidic experiments, glass and silicon were the most frequently used materials. This was due to the technology required to fabricate microchannels was already available for these materials of micro-electro-mechanical systems (MEMS). In addition, glass and silicon provide advantages which include resistance to organic solvents and high thermal conductivity. However, due to silicon’s high elastic modulus, it is inconvenient for applications that include active parts such as pumps or valves. Moreover, being opaque to visible light and the high cost of production, silicon becomes a rather unattractive material for the chips.
Glass possesses attractive properties for chips such as transparency, electroosmotic mobility, low-fluorescence background and resistance to organic solvents. Glass based chips are generally manufactured by photolithography followed by wet-etching methods. Applications of these type of devices include droplet formation, solvent extraction, in situ synthesis and wide flow-chemistry reactions. However, the high elastic modulus of glass limits broad applicability in the field. Moreover, the fabrication cost of the devices is relatively higher and dangerous chemicals are used which require advanced facilities.
In addition, the fabrication process for both silicon and glass requires cleanroom conditions which adds another drawback of initial cost increase and need of expert researchers. However, the performance of glass or silicon-based devices can be enhanced by manufacturing hybrid products using elastomers on glass or silicon.
Polymeric materials for microfluidic devices
Polymeric materials enable rapid prototyping at low cost. For this reason, polymers are the most frequently used materials at mass production of the LOAC devices. Polymers are divided into three groups according to their physical properties: elastomeric, thermosetting, and thermoplastic.
Elastomers can change shape under compressive and tensile loads elastically. Poly-dimethylsiloxane (PDMS) which is an elastomer, is the most commonly used material in the LOAC at mostly research level. PDMS offers advantages including; biocompatibility, high elasticity, the permeability of gas (which is important in cell culture studies) and mid-range cost. In addition, it can provide a nano-scale resolution at low temperatures. Moreover, it is possible to combine multiple fluidic layers with different components and elastic properties which allow the integration of active functional parts on the devices. Finally, its optical and low-autofluorescence properties permit real-time tracking in the microchannels which is important for cell culture studies and other applications.
However, inability to be used with organic solvents and adsorption of small molecules on channel walls is limiting its usage. Furthermore, its surface shows a hydrophobic behavior, however, this can be intervened by plasma exposure thereby hydrophilizing the surface. Adsorption of bio-macromolecules such as proteins is a known issue in addition to swelling in this kind of devices.
Thermosetting polymers are rigid, durable, solvent-resistant and stable at elevated temperatures. Upon heating and exposing to radiation, cross-linking in the structure occurs and it cannot be recycled. Despite their applications for durable devices, usage in the fluidic technology is limited due to their high cost and hardness.
The third group; thermoplastics, soften over the glass transition temperature and therefore are easy to shape. In contrast to thermosets, they are generally linear or branched and therefore they can be recycled and reused. Thermoplastics are most commonly used polymers due to ease of production, cost and flexible properties which can be adjusted for applications. Commonly used thermoplastics in the chip systems are; polymethyl methacrylate (PMMA), polystyrene (PS), polyethylene terephthalate (PET), carbon olefin copolymer (COC) and polyvinyl chloride (PVC). Some thermoplastics are not suitable for long-term cell culture studies due to low gas permeability, but some others can be used. In addition, thermoplastics can be given hydrophilic character by surface treatments such as plasma oxidation, dynamic coating.
Polymers are diverse, easy to use and cost-effective materials which allow various applications. As often as it does in the field, each type of polymer is suitable for some applications. However, since the properties of polymers can be tailored according to the application, they provide rather a wide spectrum of choice.
Paper as material for microfluidic devices
Paper is a cellulose-based material with a porous matrix, which is useful when combined with capillarity of liquids in the devices. It is also possible to impart hydrophobic properties to the paper in order to pass the liquids through the desired regions. Paper-based devices are portable and relatively inexpensive systems for biological analysis. In addition, applications based on liquid absorption does not require any power input. This allows them to be used in bioanalytical medical applications as a portable tool anywhere in the world such as lateral flow assay kits.
Basically, hydrophobic areas on the paper can be produced by any method and can easily be stacked in layers. Moreover, they can serve as a filter to separate a sample. However, paper-based chips also have difficulties with low surface tension liquids and low selectivity for separation. Paper-based sensor kits are one of the widely used solutions of point-of-care applications. Especially, in rural areas where electricity is limited, a paper-based device for instance for blood tests may have a tremendous impact.
Consequently, the properties of the devices are determined by their building blocks. Material selection is the most important process in order to have a functional device. However, the perfect, universal material for a chip suitable for all applications is yet to be discovered. Due to the broad applications in many fields, today, each application requires a different design and material. For instance, glass-based devices provide excellent solvent resistance however due to the high stiffness active parts cannot take a role. Polymers are extraordinary for cell culture but mostly are hydrophobic. Paper-based devices provide a no power input system yet possess low surface tension. The chip technology is progressing in two distinct ways: high-performance micro-size research platforms and inexpensive, portable analysis systems. Each material possesses advantages and disadvantages, therefore, material selection depends on many parameters including function, usability, cost, biocompatibility, and ease of production.
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