Introduction

The diffusion processes are slow, and the inertial effects are negligible on micro-scale with low Reynolds number ^{[Angelescu2011]}. Hence microfluidics enables complex control of the intercellular microenvironment. Microfluidic experiments in conjunction with live fluorescence microscopy were designed and performed to verify and characterize the signal transmission mechanism in the developed IOD band system.

Soft-lithography

Fabricated microfluidic devices (three PDMS molds bonded to single glass slide)

Microfluidic channels were formed using PDMS soft-lithography technology, which has proven biocompatible and readily applicable to available laminar flow cabinets ^[Fikar2015]. Photomask and silicon master fabrication was outsourced. Fabrication of microfluidic devices was divided into two subsequent steps. In the first step, silicon masters were used for the PDMS molding. In the second step, PDMS molds were bonded to the glass substrates to form encapsulated microfluidic devices using air plasma technology.

PDMS molding

PDMS molding workflow

Two-part silicone elastomer Sylgard 184 was used to produce PDMS. The base part was mixed with sufficient amount of curing agent (10:1 ratio). The mixture was centrifuged to remove air bubbles introduced by the mixing. The silicon master was placed in an aluminum foil container and the mixture was poured over. The remaining air bubbles were removed from the PDMS by the sharp tip of a needle. The poured PDMS was maintained in a perfectly horizontal position to assure good planarity and was cured in an oven for 2 hours at 80°C. The PDMS edges were cut off with a sharp tool and the PDMS was peeled off of the silicon master. The PDMS mold was sliced into sections containing individual devices. Inlets and outlets were drilled carefully by biopsy punch of the appropriate diameter at the desired locations of the PDMS replica. Detailed experimental protocol is provided here.

Bonding of PDMS to the glass substrate

Bonding process workflow

Prepared PDMS replicas with imprinted microstructures were cleaned properly with a scotch tape. The PDMS and glass substrate were treated by air plasma for 2.5 minutes. The air plasma affects the PDMS backbone and forms reactive silanol functional groups (Si-OH) enabling formation of permanent irreversible covalent bond of the PDMS to the glass substrate ^[Wong2009]. In addition, the PDMS treatment with air plasma is beneficial as it avoids non-specific adsorption, decreases cell clogging, and turns the PDMS hydrophilic. The hydrophilicity of PDMS facilitates the future microchannel wetting ^[Kalio2006]. Immediately after the air plasma treatment, the glass substrate was brought into contact with the PDMS replica and placed in the oven for 5 minutes at 80°C. The bonded devices were stored at room temperature. Inlets and outlets were sealed with scotch tape to avoid contamination. Detailed experimental protocol is provided here.

Experimental setup

Experimental setup

Microfluidic experiments were conducted on a platform already established by the Georgiev lab. The laboratory is equipped with a precise microfluidic syringe system (neMESYS Low Pressure Syringe Pumps) and a microscopic station enabling fluorescence imaging and live cell microscopy (Olympus IX83, CellSens software).

Microfluidic experiments

On-chip signal transmission characterization

Example filling of microfluidic channel.

We developed a new on-chip fluorescence reporter assay system, which allows dynamic detection of GPCR signaling in yeast using live fluorescence microscopy. First, fluorescent reporter proteins are connected to specific functional products signaling activation of the specific G protein–coupled receptor (GPCR). Details on the synthesis and function of reporter plasmids are provided by Module 1. Synthesized reporter plasmids were transformed into specific yeast strains to enable characterization of yeast signaling using the set of orthogonal signals designed and synthesized by Module 2.

Fabricated microfluidic devices were used to characterize the signal transmission between selected yeast strains. Two cell types were selected for each experiment. One cell type transmits the signal in the form of specific pheromone molecules. And the other cell type produces the green fluorescent protein (GFP) when the signal is received and if the pheromone molecules fit the specific receptor. Both cell types are introduced to the microfluidic device in parallel through separated inlets. Due to the laminar nature of the flow, the individual cell types remain spatially separated inside the microfluidic device. The spatial separation is visible well during the filling of the microfluidic device. The flow is stopped when the filling of the microfluidic device is finished and the rest of the experiment is performed statically. Subsequently, time-lapse fluorescence microscopy is used to dynamically measure levels of GFP expression in the cells that receive the signal.

Level of fluorescence and its dependence on the distance from transmitting cells. Performed on S. cerevisiae wildtype pheromone signaling molecules and receptors.

All experiments were conducted with Saccharomyces cerevisiae cells. Signal transmission was tested between the following types:

• POSITIVE CONTROL: Wildtype MATx ↝ MATa: STE2 receptor from Sacharomyces cerevisiae
• POSITIVE CONTROL: MATa expressing "Candida parapsilosis" alpha-factor; \(\Delta\)Bar ↝ MATa: STE2 receptor from Candida parapsilosis
• NEGATIVE CONTROL: MATa expressing "Candida parapsilosis" alpha-factor; \(\Delta\)Bar ↝ MATa: STE2 receptor from Sacharomyces cerevisiae

Results of the experiment show...

Time-lapse fluorescence microscopy showing the evolution of GFP expression in time and its dependence on the distance from signaling cells. Performed on S. cerevisiae wildtype pheromone signaling molecules and receptors.

Time lapse fluorescence microscopy showing the evolution of GFP expression in time and its dependence on the distance from signaling cells. Performed on Candida parapsilosis pheromone signaling molecules and S. cerevisiae wildtype receptors.

Personnel

• Martin Cienciala - Microfluidic experiments
• Vaclav Pelisek - Fabrication of microfluidic devices
• Pavel Fikar - Scientific advisor

References

1. ↑ Dan E. Angelescu (2011). Highly Integrated Microfluidic Design. Artech House, Norwood.
2. ↑ Ieong Wong (2009). Surface molecular property modifications for poly(dimethylsiloxane) (pdms) based microfluidic devices. Microfluid Nanofluid, 7:291–306.
3. ↑ Johana Kuncova-Kallio (2006). Pdms and its suitability for analytical microfluidic devices. In Proceedings of the 28th IEEE EMBS Annual International Conference, New York City, USA, 9, IEEE.
4. ↑ Sung Hwan Choi (2009). Microinjection molded disposable microfluidic lab-on-a-chip for efficient detection of agglutination. Microsystem Technologies, 15(2):309-316.
5. ↑ P. Fikar (2015). SU8 microchannels for live cell dielectrophoresis improvements. In Proceedings of the Design, Test, Integration and Packaging of MEMS/MOEMS Symposium, Montpellier, FR.