1       Material and methods

MITOMI Device Fabrication by Multilayer Soft Lithography

1. PDMS resin: Heat curable silicone elastomer (Dow Corning Sylgard 184).
2. Trimethylchlorosilane (TMCS) (Sigma-Aldrich).
3. Mixing and degassing of PDMS: Thinky Mixer ARE-250 equipped with adaptor for 100 ml disposable PP beakers (C3 Prozess- und Analysentechnik GmbH).
4. Degassing of PDMS control layer: Vacuum desiccator (Fisher Scientific AG).
5. Spin coating of PDMS flow layer: Programmable spin coater SCS P6700 (Specialty Coating Systems Inc.).
6. Stereomicroscope, SMZ1500 (Nikon AG).
7. Manual hole punching machine and pin vises, 21 gauge (0.04’’ OD) (Technical Innovations, Inc.)

Method:

1. The control layer mold is placed in a glass Petri dish lined with aluminum foil to facilitate easy removal. Care must be taken that the aluminum foil lining does not contain any holes.
2. To generate a hydrophobic surface, both flow and control mold are exposed to vapor deposits of TMCS for 10 min by placing them into a sealable plastic container with 1 ml TMCS filled into a plastic cap.
3. For the control layer, 30 g (60 g) of a 5:1 Sylgard mixture (25 g Part A:5 g Part B) (50 g Part A:10 g Part B) is prepared, mixed for 1 min at 2,000 rpm (~400 × g) and degassed for 2 min at 2,200 rpm (~440 × g) in a centrifugal mixer.
4. The mixture is poured onto the control layer mold and degassed in a vacuum desiccator for 30-50 min.
5. For the flow layer, 20 g (10.5 g) of a 20:1 Sylgard mixture (20 g Part A:1 g Part B) (10 g Part A:0.5 g Part B) is prepared, mixed for 1 min at 2,000 rpm (~400 × g) and degassed for 2 min at 2,200 rpm (~440 × g) in a centrifugal mixer.
6. The mixture is spin coated onto the flow layer with a 15 s ramp and a 35 s spin at 2,200 rpm. 
7. After removing the control layer mold from the vacuum chamber any residual surface bubbles are destroyed by blowing on top of the PDMS layer. Any visible particles on top of the control channel grid are carefully removed using a toothpick.
8. Both layers are cured in an oven for 30 min at 80°C.
9. Following polymerization, both molds are taken from the oven and allowed to cool for 5 min.
10. The control layer is then diced with a scalpel and holes (1–8 and B, S, C, O in Fig. 1a) are punched at the control input side using a hole puncher or a 21 gauge luer stub.
11. The channel side of the control layer is thoroughly cleaned with Scotch Magic Tape.
12. The cleaned control layer is then aligned to the flow layer on the stereomicroscope.
13. The device is bonded for 90 min at 80°C in an oven.
14. Bonded devices are removed from the oven and allowed to cool for 5 min.
15. Following the outline of the control layer each individual device is cut with a scalpel and peeled off the flow layer. Holes are punched for the sample inlet and outlet (S1–S7 and O in Fig. 1a) using a hole puncher.
16. Holes are punched for the sample inlet and outlet (S1–S7 and O in Fig. 2a) using a hole puncher.
17. The flow channel side is cleaned thoroughly with tape before aligning the device to a spotted glass slide.
18. The flow mold is cleaned of any residual polymerized PDMS either by peeling off the thin layer of PDMS using a pair of tweezers or by an additional PDMS layer. For the latter, 11 g of a 10:1 Sylgard mixture (10 g Part A:1 g Part B) is mixed for 1 min at 2,000 rpm (~400 × g), degassed for 2 min at 2,200 rpm (~440 × g), poured on the flow mold cured in the oven for 30 min at 80°C, and peeled off after cooling down to room temperature. The control mold is cleaned with a nitrogen air gun of any PDMS debris.

Glass slide preparation:

 Cleaning procedure:

1. All glassware is prepared by rinsing with Milli-Q water.
2. 750 ml Milli-Q water and 150 ml ammonium hydroxide are heated to 80°C in a staining bath.
3. 150 ml hydrogen peroxide is carefully poured to the ammonium solution.
4. Glass slides are added into the staining bath and incubated for 30 min.
5. After removal from the staining bath, the glass slides are allowed to cool for 5 min.
6. Glass slides are then rinsed with Milli-Q water in the staining bath.
7. Clean glass slides are dried with nitrogen and stored in a dust free box.

Epoxysilane deposition:

1. Before epoxysilane deposition, all glassware is rinsed with acetone and dried at 80°C.
2. Cleaned glass slides are incubated for 20 min in 891 ml toluene with 9 ml 3-GPS.
3. After rinsing with fresh toluene to remove unbound 3-GPS, the glass slides are dried with nitrogen.
4. Glass slides are baked at 120°C for 30 min.
5. Following sonication in toluene for 15 min, glass slides are rinsed with fresh IPA.
6. Coated glass slides are dried with nitrogen and stored in a dust-free box under oxygen free conditions until usage.
7. In case of systematic PDMS chip delamination: Prior to DNA spotting, glass slides are rinsed with toluene and dried with nitrogen.

Adapted from Rockel, S., Geertz, M., & Maerkl, S. J. (2012). MITOMI: A Microfluidic Platform for In Vitro Characterization of Transcription Factor–DNA Interaction. In Gene Regulatory Networks (pp. 97-114). Humana Press.

1      Material and methods

-768 MITOMI chip

-Lysis buffer (30mM NaOH + 12% w/v SDS)

-LB with chloramphenicol + arabinose (0.2% w/v)

-Microfluidic material

-Scanner

-Micropscope

-GenePix software for data analysis

The aim of the experience was to test the expression of sfGFP (BBa_I746908) induced by arabinose (pBAD promoter) and constitutive GFP (BBa_K584001). Each plasmid was transformed individualy in E.Coli bacteria. After an overnight culture of these bacteria in LB medium containing chloramphenicol and arabinose, they were flown in the chip and subsequently scanned.

2      Results

2.1     Data

The picture shows the chambers of the chip containing the bacteria. Fluorescence is only visible in the chambers because lysis buffer was flown in the channels, thus removing all signal comming from the bacteria outisde the closed chambers. First line contains the sfGFP, second line the const. GFP and this is repeated over the whole chip by pair of lines.

2.2     Interpretation

Great signal of both superfolded GFP and constitutive GFP was obtained. In theory superfolded GFP should be more intense than conventional GFP. A slight difference can be seen on the scan, however as the cell density could not be checked precisely this could be due to different concentrations of bacteria in the chambers.

1       Material and methods

1.1 Material

-768 MITOMI chip

-Lysis buffer (30mM NaOH + 12% w/v SDS)

-LB with chloramphenicol

-Microfluidic material

-Scanner

-Micropscope

-GenePix software for data analysis

1.2 Methods

The aim of this experience was to test the expression of RFP (BBa_K339007) induced by the CpxR responsive promoter. The plasmid was transformed in E.Coli bacteria. After an overnight culture of these bacteria in LB medium containing chloramphenicol they were flown in the chip and subsequently scanned after stressing the bacteria.

CpxR is synthetised upon membrane stress (see our Cpx pathway description for more informations). The idea here was to stress bacteria by applying pressure on them in order to make the bacteria produce their Cpxr protein and thus inducing RFP synthesis. In the biobrick we used (BBa_K339007), there is a CpxR protein sensor upstream of the RFP coding sequence. This sensor should trigger the RFP synthesis once a CpxR protein binds the sensor.

Even though The MITOMI chip isn't oprimised for applying mechanical pressure on bacteria, this experience was performed as follows.

Cells were flown into the chip afer an overnight culture in LB + chloramphenicol. Then the buttons of the chip were activated, thus applying pressure on the upper part of the chambers. As most of the bacteria sit in the lower part of the chambers, pressur affected only a small amount of them.

Scans were taken at regular time intervals.

2       Results

2.1 Data

The aim of this experience was to induce CpxR expression by arabinose. The Cpxr is fused to a GFP reporter (BBa_K1486002) allowing us to determine if the protein is really expressed. The experiment was previously done on wet bench and this one repeats it on a microfluidic chip.

1      Material and methods

1.1 Material

-768 MITOMI chip

-Lysis buffer (30mM NaOH + 12% w/v SDS)

-LB with chloramphenicol

-LB with chloramphenicol and arabinose (5mM)

-Microfluidic material

-Scanner

-Microscope

-GenePix software for data analysis

-Plasmid with pBAD promoter followed by cpxR linked with GFP on the N-terminal (Bba_k???)

1.2 Method

Cells were inoculated and grown overnight in 3 ml LB medium with chloramphenicol.

The next morning the cells were loaded on chip and the upper half of the chip had a flow of LB with arabinose and chloramphenicol whereas the lower half of the chip had a flow of LB with only chloramphenicol.

The cycle was the following :

-take a scan

-close chamber valves and flow lysis for 10min

-close M2 and flow LB with arabinose and chloramphenicol for 5min

-open M2, close M1 and flow LB with only chloramphenicol for 5min

-open M1 and close the outlet and main inlet valve

-culture time of 40min

Repeat 6x for a total of 3h.

This is a schematic representation of how the bacteria were divided on the chip. M1 and M2 stands for multiplex 1 and 2 and are valves that allow to block the flow of bacteria (or medium). Thanks to these multiplexes up to 16 different types of cells/bacteria can be flown in distinct rows. In this experiment we only used a separation in two region (ara+ and ara-). 

2      Results

2.1     Data

Figure 1. Scan of the microfluidic chip at t = 0min. No signal is detected

Figure 2. Scan of the microfluidic chip at t = 300min. The upper half of the chip has medium with arabinose and the lower half doesn't. Expression is detected on the upper half

After GenePix analysis, where we calculated the intensity of each chamber and also the intensity of the area next to the chamber (to subtract as background value) we calculated an average of fluorescence expression. Figure 3 shows the results.

Figure 3. Evolution of CpxR-GFP fluorescence over time

2.2     Interpretation

A great GFP signal was detected which confirms the expression of our portein of interest Cpxr.

The increasing standard deviation for the cells with arabinose can be explained as some chambers did not have a lot of cells and so there was a low intensity thus increasing the deviation. As it can be seen in the following picture :

Figure 4. These are chambers with arabinose in the medium, you can see that there are different cell density and thus different intensity in the chambers. Inducing a high standard deviation