Our Trip to the PDMS uFluidic Chips

Posted by Nazım Yılmaz on

PDMS is one of the most popular materials for microfluidic lab-on-a-chip. Bite-size information about PDMS is here.

 Our Trip to the PDMS uFluidic Chips

 

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.

 

Property Value
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.
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Reference 1 Researchgate conversation
Reference 2 Zhang et.al. (2004)
Reference 3 Chips and Tip, Christopher N. LaFratta
Reference 4 Berean et.al. (2014)

 

 

Property Value
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
Dielectric constant 2.3-2.8
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.
Flexibility /Elasticity High
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.
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Reference 1 MIT 6.777J/2.751J Material Property Database
Reference 2 Lee etl.al (2013)
Reference 3 Merkel et.al (2000)

 

Chemical Compatibility Value
tetrabutylammonium fluoride (C16H36FN) + n-methyl-2-pyrrolidinone (C5H9NO) 3:1 Wet etching method
CF4+O2 Plasma etching method
Ammonia, Gas A-Excellent
Acetaldehyde A-Excellent
Acetic Acid, Glacial B-Good
Acetone D-Severe Effect
Alcohols:Ethyl B-Good
Alcohols:Isopropyl A-Excellent
Alcohols:Methyl A-Excellent
Ammonia, anhydrous C-Fair
Benzaldehyde D-Severe Effect
Benzene D-Severe Effect
Borax (Sodium Borate) B-Good
Boric Acid A-Excellent
Buttermilk A-Excellent
Calcium Carbonate A-Excellent
Calcium Chloride A-Excellent
Calcium Hypochlorite B-Good
Carbolic Acid (Phenol) D-Severe Effect
Carbon Dioxide (wet) B-Good
Carbon Monoxide A-Excellent
Chlorine Water D-Severe Effect
Chloroform D-Severe Effect
Citric Acid A-Excellent
Cyclohexane D-Severe Effect
Ether D-Severe Effect
Ethyl Acetate B-Good
Fatty Acids C-Fair
Formaldehyde 100% B-Good
Formic Acid B-Good
Gelatin A-Excellent
Glucose A-Excellent
Glycerin A-Excellent
Glycol A-Excellent
Hydrochloric Acid 20%-100% D-Severe Effect
Hydrofluoric Acid 20%-100% D-Severe Effect
Hydrogen Gas C-Fair
Hydrogen Peroxide 100% B-Good
Hydrogen Peroxide 10% A-Excellent
Jet Fuel (JP3, JP4, JP5, JP6) D-Severe Effect
Lactic Acid A-Excellent
Magnesium Sulfate (Epsom Salts) A-Excellent
Malic Acid B-Good
Methane D-Severe Effect
Methyl Butyl Ketone D-Severe Effect
Methyl Ethyl Ketone D-Severe Effect
Milk A-Excellent
Naphthalene D-Severe Effect
Natural Gas A-Excellent
Nitric Acid (Concentrated) D-Severe Effect
Nitric Acid (5-10%) C-Fair
Nitrogen, Gas A-Excellent
Oils:Mineral C-Fair
Oils:Olive D-Severe Effect
Oils:Silicone C-Fair
Potassium Bicarbonate A-Excellent
Potassium Chloride A-Excellent
Potassium Hydroxide C-Fair
Potassium Sulfate A-Excellent
Potassium Sulfide A-Excellent
Propane (liquefied) D-Severe Effect
Silicone C-Fair
Soap Solutions A-Excellent
Sodium Acetate D-Severe Effect
Sodium Bicarbonate A-Excellent
Sodium Borate (Borax) A-Excellent
Sodium Carbonate A-Excellent
Sodium Hydroxide 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
Tartaric Acid A-Excellent
Urea B-Good
Water, Distilled C-Fair
Water, Sea A-Excellent
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Reference 1 MIT 6.777J/2.751J Material Property Database
Reference 2 PermSelect PDMS compatibility list is longer

 

 

 

 

 


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