Circular channel fabrication in microfluidic devices

Written by Oore-ofe Olumuyiwa Akeredolu
Published September 28th 2020
Contact: partnership@elvesys.com, Elvesys SAS, 172 Rue de Charonne 75011 Paris

Circular channels in microfluidic devices are complex structures and their fabrication requires specific techniques not commonly implemented in mass production. Therefore, it is today nearly impossible to find microfluidic devices with circular channels on the market. In this review we will present the fabrication of circular microfluidic channels using a metal wire and the example of an Artery-on-Chip model.

Introduction to circular channels and their applications in microfluidics

Only few representative models of vasculature and specifically, carotid arteries have been developed so far, and existing models are not fully adapted to study the effects of disease, like the consequences of a stroke, on vascular cells. The most advanced models require the use of laboratory animals or human trials, which raise ethical concerns. The main objective of this study was to develop a novel microfluidic model with circular channels on the chip that combines strategically designed polymeric structures for cell culture to mimic the human vascular system in a reproducible way. A microfluidic platform provides the added advantage of a controlled flow within the designed chip and its channels to assess the effects on cells. Figure 2 shows a schematic representation of a microfluidic Artery-on-Chip model and the use of circular channels within the device.

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Figure 1: Schematic presentation of an Artery-on-Chip model, depicting the cells seeded in the circular microfluidic channel.

Microfabrication of circular channels using soft lithography

Standard microfluidic chips on the market have a rectangular cross-section of their channels due to technical limitations during manufacture. Standard procedures using PDMS use photolithography to fabricate a mold using a negative photoresist liquid or dry film resist such as Ordyl© which is laser etched from a predesigned mask into the desired pattern for the microfluidic chamber. The developed mold is then used as a frame for the liquid silicone elastomer base, PDMS, which then solidifies to form the desired chamber design (Figure 2). Multiple chambers with the same design can be fabricated by simply reusing the mask to produce the desired number of molds and pouring the PDMS to form an equivalent number of chambers. Here, we tried to produce PDMS microfluidic chips with a circular cross section of the channels. Such chips are currently not commercially available but would mimic the human blood vessel more accurately than devices with rectangular channels. A channel structure with a circular cross-section would ensure biophysically representative fluid flow properties and shear forces inside the device. Unfortunately, first attempts to produce microfluidic chips with circular channels showed that PDMS was not suitable for this channel architecture as the way of manufacture is limiting and reserved for channels with rectangular structure.

Figure 2: Representation of the entire microfabrication process for a microfluidic PDMS chip. Image source Bhatia, S.N. and D.E. Ingber, Microfluidic organs-on-chips. Nature Biotechnology, 2014. 32: p. 760. Copyright 2014 Springer Nature.

Microfabrication of a circular channel using hot-embossing

Since PDMS was found to not be suitable for the fabrication of circular channels, we tried and alternative approaches and settled for hot embossing using Flexdym®. An overview of the process of hot embossing is shown in Figure 3. The styrene ethylene-butylene block copolymer thermoplastic Flexdym® that has been developed by Eden Microfluidics has already been successfully used for cell culture. We fabricated circular channels using a metal wire of the desired diameter, which was inserted between two layers of Flexdym® prior to hot-embossing and extracted after the mold had cooled down. Using this technique, we were able to reproducibly fabricate circular channels using a metal wire with the originally proposed lumen diameter of 6.0 mm. The results are shown in Figure 4.

Figure 3: Schematic representation of a typical micro hot embossing process including four major steps: heating, molding, cooling and demolding. Image source M. Worgull, J. F. Hétu, K. K. Kabanemi, M. Heckele, Hot embossing of microstructures: characterization of friction during demolding, Microsystem Technologies, 2008, 14:767–773.

Figure 4: Images of circular microfluidic channels fabricated by placing a 6 mm wire between two Flexdym layers, applying pressure at a temperature of 150°C. A) Cross-section of the fabricated layer showing a circular channel. B) Top-view of the fabricated device showing a continuous uninterrupted channel. C) Top view of the channel inlet. D) Top view of the outlet.

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Microfluidic cell-culture in a circular channel

The microfluidic chip fabricated in Flexdym® was first assessed visually by using dyed water to demonstrate uninterrupted laminar flow prior to seeding of cells within the channel (Figure 5 A&D). Following validation, HEK 293 cells were manually seeded at a concentration of 100 x 105 cells/mL into the device by pipetting. After 2 and 3 hours of incubation, the seeded cells were confirmed to have successfully attached to the walls of the device (Figures 5B and C). DMEM growth media supplemented with CS (10%), Penicillin/Streptomycin was passed in through the chip twice every 4 hours by manual pipetting to sustain the cells for up to 3 days. These results were demonstrated to be reproducible with three similar fabricated chips.

Figure 5: Images of dye-filled and seeded channels demonstrating desirable fluidic profiles within the fabricated chip: laminar flow front/rear and sustainable perfusion of seeded cells. A) Perfusion of the channel with blue-dyed water. B) HEK 293 cells attached within the fabricated channel and perfused with media for 2 hours after seeding. D) Growth media front showing a laminar profile.

Conclusions and perspectives

Having a microfluidic chip with circular channels is a prerequisite for an Artery-on-Chip model close to the natural human physiology. This first attempt to manufacture circular channels in a microfluidic chip showed that it is possible to have a defined channel with the desired diameter. Further, it was shown that cells adhere successfully to the channel walls and that perfusion of the microfluidic chip is successfully maintaining the cells in culture. As a further step, the chip could be connected to an automated microfluidic perfusion system to reduce manual manipulations. In addition, the introduction of branches to the channel would be a challenging further development.