In a paper recently published in the journal MaterialsIn this article, the researchers reviewed recent developments in the fabrication of thermoplastic microfluidic devices and highlighted current and emerging methods of releasing chemical and/or physical properties to suit real-world applications.
Stady: Thermoplastic microfluidic fabrication and bonding: a review. Image Credit: HannaTor / Shutterstock.com
Microfluidic systems have periodically reinvented the latest approaches to high-input clinical examination, biochemical assays, liquid automation, and a variety of other disciplines since their inception. Due to their chemical and physical versatility, polydimethylsiloxane (PDMS) and glass have been primary materials choices in microfluidics. However, the extensive use of PDMS and glass devices has been severely hampered by the expensive and time-consuming production method, low scalability, and high irregularity between devices.
On a larger scale, these devices often fail in high-pressure environments, making many post-manufacturing processing procedures impractical. To achieve this goal, thermoplastic microfluidic platforms have received much attention due to their inexpensive costs and superior physical properties compared to PDMS and glass devices. The creation of microfluidic platforms from thermoplastics consists of two steps: (1) device assembly, which includes the formation of microfluidic pathways and linking of the layers, and (2) channel operation for the intended application.
Micro-channel engineering development
Hot etching has been a popular mid-cost method for mass manufacturing of thermoplastic polymers. Microfluidic arrays in the form of polymer cavities are formed by heating a polymer sheet above glass transition temperature (Tg) under hydraulic pressure against a main mold with channel protrusions. Another method for developing microchannels in thermoplastics is injection molding.
This process involves melting the thermoplastic polymer beads followed by injection into the mold cavity. In general, injection molding and heat embossing are well suited for medium cost mass production via replication techniques and can be used to produce complex channel layouts. Moreover, the quality of surface finishing in these processes is better than other techniques.
Photolithography is commonly used to create master molds for thermoplastic polymer fabrication in both injection molding and hot etching. Laser ablation, suitable for several polymers including nitrocellulose, polyethylene terephthalate (PET), and Teflon, is another technique used to produce thermoplastic polymers featuring microfluidic channel cavities. The main disadvantages of this technology are the inability to fabricate complex microchannel layouts and poor surface smoothness.
Due to the short production cycle time, additive manufacturing has received a lot of interest in microfluidics. However, the accuracy of 3D-printed microchannels and the mechanical and optical qualities are not on par with other methods. The ability of 3D printing to create 3D objects in one step with detailed and complex features with lower space requirements using a digital model is an important advantage. Although recognized, it still shows shortcomings when used in microfluidics, such as poor bonding with layers that lack structural integrity. This is because the extruded material hardens instantly, and the adjacent layers do not stick well.
Thermal fusion bonds, chemical bonding, and solvent bonding are among the most documented strategies for permanently incorporating thermoplastic polymers and preparing closed microfluidic devices. Thermal fusion involves heating the polymeric layers on top of their Tgs while compressing the layers together using a vacuum thermal compressor or hydraulic press. This is a quick way to attach the thermoplastic layer. However, higher temperature and pressure requirements limit the possibility of low-cost mass fabrication of microfluidic devices.
Solvent bonding involves diffusion of the solvent on the polymer interface, dissolving the polymer chains and packing them. Compression of the polymer substrates is followed by the evaporation of the solvent. The resulting mobile phone chains intertwine into each other at the interface, forming a strong binding force.
Solvent bonding is usually incompatible with pre-functionalization because the solvent must be applied to both the bottom and top thermoplastic layers that can remove functional groups while denaturing bound biomolecules. On the other hand, chemical bonding enables simultaneous operation of the inner microchannel surface where the reactive chemical groups that are produced to join the two layers can also be incorporated for further bonding of the covalent biomolecule. Another advantage of this approach is the higher bonding strength achieved by the covalent bond.
Ultrasonic vibration can also be used as a heat source, thus greatly speeding up the bonding process. This technology provides faster as well as superior bonding strength, making it an excellent method for rapid prototyping. Laser welding is another method of thermoplastic bonding of a polymer. Pressure-sensitive double-sided adhesives, such as silicone and acrylic tapes, have also been widely used. Furthermore, the use of PDMS as an interface glue could be an option to produce effective binding strength.
To summarize, the researchers reviewed and summarized techniques for incorporating thermoplastic polymers within microfluidic systems. According to the authors, transcription methods such as injection molding or hot etching, combined with rapid prototyping techniques, such as laser ablation and micromilling, present a remarkable opportunity in the production of low-cost microfluidic devices.
Shakry, A.; Khan, S.; locust, NA; Didar, T. F. Fabrication and bonding of thermoplastic microfluids: a review. Materials 2022, 15, 6478. DOI: https://doi.org/10.3390/ma15186478