PDMS is one of the most popular materials for microfluidic lab-on-a-chip. Bite-size information about PDMS is here.
Poly-DiMethyl-Siloxane, aka PDMS, Dimethicone, E900, anti-foaming agent, Sylgard184 / RTV615 as commercial names, or CH3[Si(CH3)2O]nSi(CH3)3 (for chemists).
PDMS microfluidic devices are made by the mixture of base and curing agent (mostly preferred the %10 ratio of cross-linker in monomer) without air droplets inside, and a mold of microchannels or microstructures are used to replicate design on PDMS chips. Heating enhances the cross-linking reaction.
Here, in the tabs on the right, the relevant physical and chemical information about using PDMS in lab-on-a-chip applications is given as a list to ease the reading.
|Plasma Treatment in vacuum||Necessary for the bonding of the PDMS layer onto glass or another PDMS layer. After plasma treatment inside a vacuum, Si-O-Si binding occurs and materials bind covalently. The power and duration of plasma treatment strictly depend on user and lab, but mostly less than 1 minute is enough. Over and under exposure to plasma results in poor bonding.|
|UV Treatment||UV oxidizes the surface of the PDMS. After UV treatment, the surface becomes hydrophilic but at longer exposures eventually a crust of SiO2 on the surface of the PDMS with cracks in between exposing some of the other reaction products which may include aldehydes and ketones.|
|Spin Coating||Thin layers of PDMS is possible on molds by spin coating.
The speed of 500rpm at 30s results 220um of PDMS layer. The speed of 1000rpm at 30s results 77um of PDMS layer. no of 2000rpm at 30s results 36um of PDMS layer. The speed of 4000rpm at 30s or 60s results 19-20um of PDMS layer. The speed of 8000rpm at 30s results 8um of PDMS layer.
|Degassing||Mixing the base and curing agent of PDMS inevitably incorporates air bubbles into the mixture. These air bubbles are removed from PDMS by letting the mixture for 2 hours in room temperature.
To accelerate the fabrication process, a vacuum usually removes air bubbles but in 30 minutes or more. A typical laboratory centrifuge to degas PDMS is also a quicker solution in minutes. 500g in 5 minutes or 1300g in 2 minutes is enough.
|Sterilization Technique||In principle, autoclave steam sterilization, UV/EtOH sterilization, gamma-radiation, Ethylene Oxide sterilization, all possible with PDMS chips, but all have drawbacks. After autoclaving, PDMS chips are filled with water and change the color, becoming more opaque. Steam sterilized chips must be heated to evaporate the water. UV causes modification of the surface and EtOH is also immersed into PDMS at higher duration. The time of UV/EtOH treatment must be optimized regarding the microfluidic chip geometry and application. Gamma radiation has no negative effects on the chip itself but having a radiation sterilization device is costly.|
|Curing Temperature||Cross-Linking Temperature is important for its effect on gas permeability of the PDMS itself. The permeability of 02 and CO2 is necessary for cell culture studies inside microchannels. The optimum gas permeability is obtained by the curing temperature of 75°C.|
|Aspect Ratio||Aspect ratio is the microstructure dimension ratio which is Z/X. The height of the channel over the width of the structure (Z/X), can be as high as 20:1 with some mold fabrication technique. PDMS is accepted as a high aspect ratio applicable material.|
|Reference 1||Researchgate conversation|
|Reference 2||Zhang et.al. (2004)|
|Reference 3||Chips and Tip, Christopher N. LaFratta|
|Reference 4||Berean et.al. (2014)|
|Biocompatibility||Nonirritating to the skin, no adverse effect on rabbits and mice, only mild inflammatory reaction when implanted|
|Hydrophobicity||Highly hydrophobic, contact angle 90-120°
It adsorbs hydrophobic molecules and can release some molecules from a bad cross-linking into the liquid.
|Mass density||0.97 kg/m3|
|Young's modulus||360-870 KPa|
|Tensile or fracture strength||2.24 MPa|
|Specific heat||1.46 kJ/kg K|
|Thermal conductivity||0.15 W/m K|
|Index of refraction||1.4|
|Electrical conductivity||4x1013 Ωm|
|Magnetic permeability||0.6x106 cm3/g|
|Autofluorescence||The autofluorescence of PDMS is generally negligible for most wavelengths but The fluorescence of TRITC, FITC, and Cy3 overlaps with the autofluorescence of PDMS|
|Transparency||PDMS is optically transparent about 90% at visual wavelengths (400-700nm). At lower wavelengths below 400, the optical transparency starts to decrease. It is around 50% at 300nm and almost 0% at 200nm and below.|
|Air Permeability||PDMS is one of the best permeable elastomers. It is highly permeable to some of the gases but not at the same value for all gases. Also, please remember that the gas permeability of PDMS layers of microfluidic chips depends on the curing agent ratio, cross-linking temperature, the thickness of the PDMS layer, and the additional composition of the material.
Reference-3 has a good guideline to start.
|Deformability||Although this term is something like a negative, deformability of PDMS chips provide ease of forming inlet/outlet ports on top of the chip at the end of microchannels to manipulate sample fluids.|
|Reference 1||MIT 6.777J/2.751J Material Property Database|
|Reference 2||Lee etl.al (2013)|
|Reference 3||Merkel et.al (2000)|
|tetrabutylammonium fluoride (C16H36FN) + n-methyl-2-pyrrolidinone (C5H9NO) 3:1||Wet etching method|
|CF4+O2||Plasma etching method|
|Acetic Acid, Glacial||B-Good|
|Borax (Sodium Borate)||B-Good|
|Carbolic Acid (Phenol)||D-Severe Effect|
|Carbon Dioxide (wet)||B-Good|
|Chlorine Water||D-Severe Effect|
|Hydrochloric Acid 20%-100%||D-Severe Effect|
|Hydrofluoric Acid 20%-100%||D-Severe Effect|
|Hydrogen Peroxide 100%||B-Good|
|Hydrogen Peroxide 10%||A-Excellent|
|Jet Fuel (JP3, JP4, JP5, JP6)||D-Severe Effect|
|Magnesium Sulfate (Epsom Salts)||A-Excellent|
|Methyl Butyl Ketone||D-Severe Effect|
|Methyl Ethyl Ketone||D-Severe Effect|
|Nitric Acid (Concentrated)||D-Severe Effect|
|Nitric Acid (5-10%)||C-Fair|
|Propane (liquefied)||D-Severe Effect|
|Sodium Acetate||D-Severe Effect|
|Sodium Borate (Borax)||A-Excellent|
|Sodium Hypochlorite (100%)||B-Good|
|Sodium Peroxide||D-Severe Effect|
|Sulfuric Acid (10-100%)||D-Severe Effect|
|Sulfuric Acid (less than 10%)||C-Fair|
|Reference 1||MIT 6.777J/2.751J Material Property Database|
|Reference 2||PermSelect PDMS compatibility list is longer|