Fundamental Concepts

Microfluidic technologies harness the unique properties of fluids when manipulated in micro-scale channels. When confined to channel geometries of less than 1000 μm, fluids do not behave according to the conventional laws of fluid dynamics.

When describing the principles governing microfluidics, we must first consider scaling laws (Berthier and Silberzan, 2010). If a system has a defined length of L, then the surface area and volume scale as L² and L³ respectively. To micro-size this system, we decrease the length L to micron dimensions which increases the ratio of surface area to volume, as L^-1, as shown in Equation 1. Volume forces (such as weight and inertia) are proportional to the volume and surface forces (such as viscosity, capillary and interface phenomena) are proportional to the surface area. Therefore, in microsystems, volume forces become less prominent and surface forces instead drive the system.

         (1)

A common and important feature of microfluidics is laminar flow. The flow regime of a fluid can be predicted with the dimensionless Reynolds number (Re) (Gravesen, Jens and Sondergard Jensen, 1993). Re represents the ratio between inertial forces and viscous forces, as a function of density (ρ), dynamic viscosity (μ), velocity (u) and characteristic system length (L), as shown in Equation 2.

       (2)

At high Re values (> ~3500) the flow regime is considered turbulent and at low Re values it is laminar (< ~2000), however the regime transition point depends on the individual system properties (Ong et al., 2008). In microfluidics, channel dimensions are inherently small, therefore the characteristic system length (L) is a low value. When applied to Equation 2 this results in a low Re and corresponds to a laminar flow regime. Laminar flow is characterised by regular streamlines and predictable flow patterns, represented schematically in Figure 1.

In a laminar flow regime lateral mixing is dominated by diffusion (Hessel, Löwe and Schönfeld, 2005). This leads to a highly predictable and stable system. Limited turbulence means reagents can be delivered with precise temporal and spatial control, allowing high resolution gradients to be developed. Computational fluid dynamics software can simulate flow patterns in microfluidic devices improving predictability of the system and allowing for precise applications to be modelled and verified.

How do we fabricate microfluidic chips?

A microfluidic chip is a set of microchannels etched or moulded within a material. The microchannels are connected forming a network that responds to a specific application or desired function. Every microfluidic chips exhibit inlets and outlets through which fluids can be directly perfused and manipulated (Figure 2).

Figure 2. A microfluidic chip

 A variety of methods exist to manufacture microfluidic devices. Most of them involve the use of a master mold fabricated by photolithography. The mold exhibits precise patterns that will serve as a frame for the elastomeric casting. The process entails a photoresist coated on top of a silicon wafer, on which UV light selectively enlightens the material through a photomask, inducing its solidification (Cha, Piraino and Khademhosseini, 2014) (Figure 3C).

Soft lithography refers to an extension of the previous technique involving elastomeric “soft” materials such as polydimethylsiloxane (PDMS). The master mold is filled with the molten polymer, degassed in vacuum and cured. Once solidified, the PDMS is peeled off and bonded to a surface creating the microfluidic channels (Figure 3D). Soft lithography benefits from the low costs of elastomers and allows multiple replications once the mould is provided.

As soft lithography, Hot embossing and Injection moulding require the use of replicate moulds. Injection molding is a simple process where a thermoplastic is melted and pushed into the mold. The thermoplastic fills the cavities of the mold and cools down until it can be handled without risks of deformations, keeping the desired pattern after the mold removal (Figure 3A) (Cha, Piraino and Khademhosseini, 2014; Gale et al., 2018). Similarly, Hot embossing enables manufacturing of thermoplastic. It generally requires flat sheets that can be easily patterned when heated near its glass transition temperature and specific pressures. A master mold exhibiting the positive replica is then pressed against the thermoplastic sheet allowing for the transfer of the features onto the substrate (Figure 3B) (Rezai, Wu and Selvaganapathy, 2012).

The techniques detailed above are all based on “subtractive” methods. Additive manufacturing, also called 3D printing, has several assets over traditional chip production techniques. These include significant time and cost savings since it enables a direct printing from raw material and rapid prototyping (Cha, Piraino and Khademhosseini, 2014).

Compared with elastomers which have interesting elastic properties, thermoplastics are materials that soften under when heated and harden on cooling in a reversible manner. The increasing demand for flexible, cheap and adapted for mass production of microfluidic devices have led to the emergence of thermoplastic elastomers (TPE) (Drobny, 2014). In that regard, Eden Tech has developed a biomaterial named Flexdym^TM combining the benefits of both thermoplastics and elastomers.

Biomedical Applications of Microfluidics 

The first applications of microfluidics emerged in the second half of the 20th century, predominantly in the fields of molecular chemistry and biology analysis, microelectronics and biodefence (Whitesides, 2006). The ability of microfluidics to miniaturise analytical set-ups lead to the coining of the term ‘laboratory-on-a-chip’. A reduced experimental set-up means reduced reagent and power requirements, saving on cost and space. Developments in materials engineering and microfabrication techniques has enabled rapid prototyping and the production of high-resolution constructs, broadening the prospective fields for microfluidic applications.

One such field is that of biotechnology, specifically ‘organ-on-chip’ devices. Microfluidic channels are of physiologically relevant dimensions, pointing towards cell culture as a logical application. Traditional monolayer culture methods fall short in recapitulating the full complexity of in vivo tissue. 3D cell culture methods have emerged to provide a more accurate model of tissue microenvironments (Charwat & Egger, 2018). Microfluidics technology is a principle method for this application. Known as organ-on-chip devices, these bioreactors recapitulate 3D tissue structure to provide microscale biomimetic in vitro models. By considering fluid shear force, concentration gradients, mechanical stress, cell patterning (Wu et al., 2020) and channel surface properties (Ong et al., 2008) devices can be engineered according to the specific culture requirements of the tissue of interest.

During our projects we will use Eden Tech’s novel FlexDym™ microfluidic material, fabrication techniques to develop a vascularised muscle-on-chip model. The device will aid the study of muscle-specific processes by RENOIR consortium members – such as endothelial-mesenchymal transition, cell population interactions and response to therapeutic compounds.

References

Berthier, J. and Silberzan, P. (2010) Microfluidics for Biotechnology. 2nd edn. Artech House.

Cha, C., Piraino, F. and Khademhosseini, A. (2014) Microfabrication Technology in Tissue Engineering. Second Edi, Tissue Engineering: Second Edition. Second Edi. Elsevier Inc. doi: 10.1016/B978-0-12-420145-3.00009-2.

Charwat, V., & Egger, D. (2018). The Third Dimension in Cell Culture: From 2D to 3D Culture Formats. 75–90. https://doi.org/10.1007/978-3-319-74854-2_5

Drobny, J. G. (2014) ‘Brief History of Thermoplastic Elastomers’, Handbook of Thermoplastic Elastomers, pp. 13–15. doi: 10.1016/b978-0-323-22136-8.00002-8.

Gale, B. K. et al. (2018) ‘A review of current methods in microfluidic device fabrication and future commercialization prospects’, Inventions, 3(3). doi: 10.3390/inventions3030060.

Gravesen, P., Jens, B. and Sondergard Jensen, O. (1993) ‘Microfluidics – a review’, Journal of Micromechanics and Microengineering, 3(4), pp. 168–182.

Hessel, V., Löwe, H. and Schönfeld, F. (2005) ‘Micromixers – A review on passive and active mixing principles’, Chemical Engineering Science, 60(8-9 SPEC. ISS.), pp. 2479–2501. doi: 10.1016/j.ces.2004.11.033.

Ong, S.-E. et al. (2008) ‘Fundamental principles and applications of microfluidic systems’, Frontiers in Bioscience, 13(7), pp. 2757–2773.

Rezai, P., Wu, W.-I. and Selvaganapathy, P. R. (2012) ‘Microfabrication of polymers for bioMEMS’, MEMS for Biomedical Applications, pp. 3–45. doi: 10.1533/9780857096272.1.3.

Whitesides, G. M. (2006) ‘The origins and the future of microfluidics’, Nature, 442(7101), pp. 368–373. doi: 10.1038/nature05058.

Wu, Q. et al. (2020) ‘Organ-on-a-chip: Recent breakthroughs and future prospects’, BioMedical Engineering Online, 19(1), pp. 1–19. doi: 10.1186/s12938-020-0752-0.

Pictures :

Cover Image: courtesy of Eden Tech @Copyright 2021

Making Microfluidics:Embossing as an alternative to PDMS and injection molding for microfluidic devices. 2021. Making Microfluidics:Embossing as an alternative to PDMS and injection molding for microfluidic devices. [online] Available at: <https://www.edge-embossing.com/blog/making-microfluidics-embossing-as-an-alternative-to-pdms-and-injection> [Accessed 29 July 2021].

Bhatia SN, Ingber DE. Microfluidic organs-on-chips. Nat Biotechnol. 2014 Aug;32(8):760-72. doi: 10.1038/nbt.2989. PMID: 25093883.