Materials for chip microfabrication

PDMS (polydimethylsiloxane)

PDMS is the material of choice for microfluidics. It combines many advantages to ease the microfabrication process such as its elastomeric properties and the ability to be bonded permanently with glass or itself, and also allows the observation of dynamics into microfluidic systems thanks to its optical transparency. Also, its biocompatibility and the gas permeability makes PDMS adapted to cell culture, and its very low autofluorescence makes it ideal for fluorescence microscopy purposes.

Here, PDMS-microfluidic devices are used to observe and quantify the dynamics of cellular behaviors in real time in a well-controlled micro-environment without disturbing cell viability or functions. For example:

(i) Wolfgang Keil’s lab (QDevBio, Institut Curie) built a PDMS-based microfluidic chip to follow in real time the long-term C.elegans development at high spatio-temporal resolution, from hatching to adulthood (Fig. 1A) [1] (Instagram post).

(ii) Jacques Fattaccioli’s lab (NBMS, ENS-PSL) in collaboration with a J.C. Palauqui from INRA, fabricated a microfluidic platform for highthroughput protoplast trapping and combined it with a custom-made microscopy to follow the development of immobilized protoblasts (Fig. 1B) [2].

(iii) Matthieu Piel’s lab (Bio6, Institut Curie) use PDMS-based chip to study how 3D micro-environments of different sizes and shapes, within which immune or cancer cells circulate, can impact the cell migration (Fig. 1C) [3].

(iv) Catherine Villard’s team (MMBM, Institut Curie) study how topography of different microenvironment affect the migration of mice explants interneurons (Fig. 1D) [4].

Figure 1: (A) C. Elegans in a microfluidic cage [1] - From QDevBio lab. (B) Moss plant on a microlfuidic trap. Scale bar: 50 µm [2] - From NBMS lab. (C) Cancer cell confined in a chamber. Scale bar: 10 µm [3] - From Bio6 lab. (D) Neuron network between microfluidic pillars [4] - From MMBM lab.

[1] W. Keil et al., Long-Term High-Resolution Imaging of Developing C.elegans Larvae with Microfluidics, Dev. Cell, 40(2):202-214 (2015)

[2] K. Sakai et al.,  Design of a comprehensive microfluidic and microscopic toolbox for the ultra-wide spatio-temporal study of plant protoplasts development and physiology, Methodology, 15(79) (2019)

[3] A. J. Lomakin et al., The nucleus acts as a ruler tailoring cell responses to spatial constraints, Science, 16(370) (2020)

[4] C. Leclech et al., Topographical cues control the morphology and dynamics of migrating cortical interneurons, Biomaterials, 214, 119194 (2019)

Cyclo-olefin copolymer (COC)

COC is an amorphous polymer composed of multiple kinds of monomers. Some of COC present a biological inertness, are biocompatible and are, therefore, suitable for cell culture [5]. Unlike PDMS, COC is not porous and does not afford the oxygen transport and prevent any undesirable liquid/gaz adsorption, which makes chip highly suitable for oxygen control or for the creation of highly controlled microreactor. It is easy-to-microfabricate for both rapid prototyping and low‐cost mass production of microfluidic devices. It is the most promising tool for injection molding technique for large scale industrialization and one-use-chip for medicine purposes. COC is highly hydrophobic hence requires a strong surface treatment to make it hydrophilic. One of the challenge is finding a long-term hydrophilic treatment for day-experiments.

Biological purposes:

Stephanie Descroix and Jean-Louis Viovy team (MMBM, Institut Curie) created a 3D microfluidic system for circulating-cell capture and analysis. In this system, COC prevents air bubbles issues while heating to perform biological assays (proximity ligation assay), and is also used for its good optical properties (transparency) for imaging. The hot-embossing of cyclo olefin copolymer allows the development of a low-cost and robust device (Fig. 2) [6].

Figure 2: 3D microfluidic channel allowing the capture and analysis of circulating cells [6] - From MMBM lab.

Chemical purposes:

(i) To improve the long-term use of such thermoplastic, Anne Varrenne’s lab (SEISAD, Chimie ParisTech-PSL) achieved a 14‐day stability of hydrophilic properties for a COC‐embedded microchannel (PDMS loses its hydrophilicity after a few hours). They proposed in a novel controllable and fine protocol based on the COC silica coating process by plasma‐enhanced chemical vapor deposition at atmospheric pressure (Fig. 3) [7].

(ii) Michael Tatoulian’s lab (2PM, Chimie ParisTech-PSL) produced COC-based microreactors to synthesize benzaldehyde from benzyl alcohol in water solution. They immobilized gold and zeolite nanoparticles on the microreactors walls and revealed a selectivity of 99% for the benzaldehyde. This microreactor is highly controlled since COC prevents every gaz or pollution exchange with the outside environment  [8].

Figure 3: Microscopy images of a COC microfluidic channel before (top) and after (bottom) silica coating via plasma-enhanced chemical deposition [7] - From SEISAD lab.

[5] B. L. Johansson et al.,  Characterization of air plasma‐treated polymer surfaces by ESCA and contact angle measurements for optimization of surface stability and cell growth. Journal of applied polymer science86(10), 2618-2625 (2002)

[6] G. Mottet et al., A three dimensional thermoplastic microfluidic chip for robust cell capture and high resolution imaging. Biomicrofluidics8(2), 024109 (2014)

[7] S. Bourg et al., Surface functionalization of cyclic olefin copolymer by plasma‐enhanced chemical vapor deposition using atmospheric pressure plasma jet for microfluidic applications, Plasma Processes and Polymers, 16(6) (2019)

[8] X. Rao et al., Synthesis of benzaldehyde with high selectivity using immobilized AuNPs and AuNPs@zeolite in a catalytic microfluidic system, Lab-on-chip, 19, 2866-2873 (2019)

Paper-based microfluidics

Paper-based microfluidics is a recent technology for low cost applications. One method consists in microfabricating culture microreactors onto paper by modifying the surface properties to enhance cellular adhesion. Such developments are particularly promising for diagnostic purposes [9].

For example, Patrick Tabeling’s lab (MMN, ESPCI-PSL) created a paper-based microfluidic devices to detect the Ebola virus on several human sample RNA extracts in Guinea. They performed isothermal reverse transcription and Recombinase Polymerase Amplification (RT-RPA) of synthetic the virus RNA directly on paper microfluidics devices, which bind with complementary sample of Ebola virus. This methods minimizes facilities as it is a carry-on detection device and uses freeze-dried reagents on paper (Fig. 4) [10].

Figure 4: Scheme of Paper-based device workflow for RNA detection and analysis of Ebola virus [10] - From MMN lab.

[9] E. Corradini, P. S. Curti, A. B. Meniqueti, A. F. Martins, A. F., Rubira, and E. C. Muniz, Recent advances in food-packing, pharmaceutical and biomedical applications of zein and zein-based materials. International journal of molecular sciences15(12), 22438-22470 (2014)

[10] L. Magro, B. Jacquelin, C. Escadafal, P. Garneret, A. Kwasiborski, J.-C. Manuguerra, F. Monti, A. Sakuntabhai, J. Vanhomwegen, P. Lafaye, and P. Tabeling, Paper-based RNA detection and multiplexed analysis for Ebola virus diagnostics, Scientific Reports, 7, 1347 (2017)