Team:BIOTEC Dresden/Methods Vesicles

Vesicles - Methods

In order to make vesicles we use a microfluidic chamber incorporated with pumping system, controlled by the software. The experiment is observed by means of Biozero fluorescent microscope.

Microfluidic chamber

The chamber consist of two parts: glass coverslide and a PDMS layer with the microstructure printed on it. Glass does not require any special preparation but plasma oven treatment for 30 seconds at 15 mBars.

PDMS (Polydimethyloxane) is an organic, silicon based polymer. [SiO(CH3)2] is the monomer (Fig. 1). PDMS is chemically inert, transparent, stable at room temperature, non-toxic and non-flammable and thus can be used as a material for the chamber.

In order to prepare PDMS layer one needs a silicon wafer with litographically etched microstructures of required configuration and PDMS mixed with silicon elastomer curing agent in proportion 10:1. For two wafers that we have (Fig.2) we used 12g of PDMS and 1.2g of curing agent. Those two substances have to be well-mixed, degassed and placed on the clean wafer. Polymerization takes about 30 minutes at 150C. After the heating drops of PDMS 2-3 mm high should be placed on top of the structure in the places where the inlets are supposed to be. The polymerized PDMS should be carefully removed from the wafer and the holes for the inlets should be made. There are two ways how one can do those holes: by means of 0.8 mm needle or using a laser cutter. Both methods have advantages and disadvantages. Using needle requires a lot of training and most of samples get cracks around the holes, which means that the chamber will leak. Laser cutting is more accurate but it's difficult to place the PDMS precisely.

When PDMS layer with microstructure printed on it is ready one cuts out the needed size for a chamber and treats it in plasma oven for 30 seconds (together with cover glass). This treatment makes the surface hydrophilic. After that polymer layer is placed onto the glass and the chamber is baked at 60C for 6 hours.

In order to check if the chamber is working one tests in under the light microscope with mineral oil. 1 mm tube connected with the syringe with oil is inserted in lower inlet and oil is to fill the whole chamber. When the camber is washed with oil one should check every upper inlet in the similar way. This allows to check if the chamber is not leaking at the inlets and if it is working properly. Also, this makes the surface more hydrophobic which facilitates vesicles formation.

The next step is to connect the chamber with the pumping system.

Pumping system

Our setup allows simultaneous control of pumping rate of 4 syringes driven by the software.

Measurement

The chamber should be placed on the microscope sample holder, connected with the syringes (inlet 1 – oil, inlet 2 water and DNA plasmid, 3 – water and an expression kit). The microscope is focused on the spot where channels bringing oil and water meet. In order to make the vesicles one should play around with the flow rates. Usually the ratio of oil/water flow rate is about 1/0.75. The expression of GFP in the vesicles is proved by fluorescent microscopy.