TITLE: Microfluidics 5: PDMS Microchannel Bonding on Glass/PDMS
AUTHORS: Serhat Sevli,  C. Yunus Sahan

[DESCRIPTION]
Microfluidic chips, made of PDMS, are one-side open when fabricated. Another layer of glass, PDMS, or etc is needed. Liquid seal is provided by complete covalent bonding between layers or by external forces like threads, magnets or similar factors. This protocol describes the covalent bonding of PDMS on glass by air plasma technique.


[BEFORE_START]
This protocol is derived from "soft lithography" literature and can be modified by user freely.

[STEPS]
SECTION: Oxygen Plasma Exposure
1. 1.0.1. Cured PDMS manufactured at previous steps using lithography or 3D printed molds are cut or removed from the mold and put inside a clean petri dish. Since PDMS is vulnerable to surface adsorption of dust, each must be clean and performed inside cleanroom facilities.

1.0.2. NehirBT's air plasma generator is a homemade setup of microwave instrument in which there is a glass vacuum desiccator. 

 

 

 

 

SECTION: Oxygen Plasma Exposure
1.1. 1.1.1. PDMS material is cleaned inside a fume hood by isopropanol and dried by nitrogen gas spraying. 

1.1.2. Glass slide is cleaned inside a fume hood by isopropanol and dried by nitrogen gas spraying. 
SECTION: Oxygen Plasma Exposure
1.2. 1.2.1. PDMS microchannels can be bonded on a glass microscope slide, glass petri dish, or another PDMS layer according to the necessities of the application. 

1.2.2. The glass and PDMS materials are placed inside the desiccator inside the plasma generator system without touching each other. Glass petri dishes and/or glass cell flasks can also be used for PDMS bonding when their sizes fit inside the desiccator.

1.2.3. The lid of the desiccator is closed. The door of the instrument is closed well.

1.2.4. The vacuum pump is run for 5-6 minutes to remove the air inside the desiccator and to secure a low vacuum level. 

1.2.5. Then the microwave is kept turned on till purple oxygen plasma waves are generated and observed. It is kept on for 5-6 seconds more with plasma generated inside the desiccator.
SECTION: Final Heating and Ready
3. 3.0.1. The final PDMS on Glass/PDMS chip is heated at 85oC for 5-10 minutes on a heating plate or for 10 minutes in a heater oven.
 

 

SECTION: Stock of PDMS chips
4. 4.0.1. The prepared chips can be stored at room temperature inside a closed clean box or petri dish after sealing the top and down of channels with scotch tape and they can be used anytime.

 

 
SECTION: Visual Quality Check
5. 5.0.1. Visual check of bonding PDMS on glass layer is performed under a digital microscope,  and these parameters are checked mainly;

5.0.2. Poor adhesion or poor bonding, Debris (Trace), air bubbles (active areas), Stains-Voids, Scratch marks, Broken PDMS, Uneven PDMS, Wrong Orientation (PDMS), Crooked trace, Chipped or Broken Glass, and Wrong Alignment of PDMS on Glass.
SECTION: Bonding After Exposure
2. 2.0.1. After exposure bonding and alignment is critical and a few minutes is present to keep plasma effect on materials. 
SECTION: Bonding After Exposure
2.1. 2.1.1. When the alignment is important between layers, a special jig may be used. In this case, place the jig between layers perform the further steps.  
SECTION: Bonding After Exposure
2.2. 2.2.1. The plasma exposed PDMS layer is put on the other plasma exposed layer which may be glass or PDMS layer and both will bond permanently with covalent bonding.

2.2.2. The exposed PDMS and bottom layer may not be enforced for bonding, this behavior may cause over bonding of microchannels inside. 

2.2.3. After bonding if there is any air present between layers, a plastic tip may be used to remove air by manual smooth pressure. 

2.2.4. The duration of bonding may take a few minutes at room temperature.