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Specific Process Knowledge/Back-end processing/PDMS Process Line and Microfluidics

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PDMS Process line at DTU Nanolab

Polydimethylsiloxane (PDMS), also known as silicone, has become a fundamental material for fabricating microfluidic devices with applications in health, chemistry, biomedical sciences and more.

DTU Nanolab has a dedicated laboratory for the processing of PDMS and fabrication of microfluidics located in building 347. The PDMS Lab is part of the PolyFabLab and has been equipped with brand-new instruments funded by the Novo Nordisk Foundation, alongside selected equipment and processes from previous facilities.

The laboratory has 4 different areas:

1. Non-cured PDMS: All mixtures of PDMS resin and curing agent are prepared on this area. Cellophan film is provided and must be placed on the table ALWAYS when working with uncured PDMS. Gloves, a high precision scale, a vacuum desiccator and a ventilated oven are available in this area.

2. Cured PDMS: Only cured PDMS can be handled in this area. A stereoscope, cutting mat, light screen, tweezers, blades, scalpels, punchers, broken glass waste and other tools for handling cured PDMS are located in this area.

3. Fume hood: safe area for handling volatile chemicals that are hazardous for human and environment health. The spin-coating machine and the hazardous waste are also located in this area.

4. Plasma area: an oxygen plasma equipment to condition the surfaces for bonding of parts in microfluidic devices and a press for pressing the devices to be bonded after plasma treatment are provided in this area.

5. Storage area: to store PDMS resin and curing agent, solvents (acetone, ethanol, isopropanol), other chemicals, samples, sample holders, petri dishes and wafer holders of different sizes, pipettes, spoons, spatulas, cups, aluminum foil, bags, and other tools and materials.

6. Personal protective equipment and emergency shower equipment: googles, lab coats, emergency shower and eyewasher are provided for the safety of lab users

Lab coat and googles must be used at all times inside the lab. Personal items must be stored in the lockers at the Support lab.

Cleanliness requirements

Process flow

Mixing PDMS resin and degassing

Sylgard 184 silicone base, Sylgard curing agent,

Soft lithography micro molding

Mold surface conditioning

Molding by pouring

Molding by spin coating

Baking – Curing

Photograph of a PDMS microfluidic wafer after curing and demolding

The PDMS can cure without heating in ~24 hours. Curing the PDMS at higher temperatures in the oven can decrease the time spent curing. The temperatures between 60°C to 100°C can be used, and the choice must be optimized for the specific device design, and the desired mechanical and surface properties. The curing time depends on the temperature and the thickness of PDMS. As a standard procedure to cure PDMS, place the casted PDMS on the mold in the oven for at least 2 hours at ~80°C.

Demolding

After curing, the PDMS microfluidic chip is a solid and chemically inert material. Once cooled, the PDMS is easily peeled off from the mold. The demolding and further processes of cured PDMS must be done in the designated “cured PDMS”. If the demolding does not occur readily, wetting it with isopropanol or ethanol might be helpful. Otherwise, the mold must be conditioned properly before casting the PDMS.

3D printing

The process development for 3D printing of PDMS is currently underway for this line.

Cutting PDMS

Cutting of PDMS to create access ports, also known as througholes, or for separating the individual chips from a microfluidic PDMS wafer are common steps in the fabrication of PDMS microfluidics. The PDMS lab thrives on providing the optimal tools and processes for researchers and users. Common tools that are provided in the lab for manual cutting are tweezers, blades, cutting mats, light screen and press.

Alternative methods for high-thruput fabrication like using wafer-size punching dies, laser ablation, machining/milling and air/water jet machining are under investigation to meet the demands of the industry and large interdisciplinary projects.

Punching

Create access ports, also known as through-holes, using manual commercial punchers available for biopsy or general use. Punch the devices on top of the green cutting board to extend the lifetime of the puncher blade and to avoid marking the table. Solvents like ethanol, isopropanol and acetone can be used as lubricants to facilitate the cutting and result in a cleaner surface. All solvents must be always used inside the fume hood. The resulting punched holes in PDMS tend to be slightly smaller than the puncher diameter, so, the final dimensions of the holes must be confirmed with metrology methods.

Clean well the place before leaving. Dispose all cellophane liner used. Be sure no residues of PDMS are left in the work areas, and place all PDMS residues in the C-waste container.

Scanning Electron Microscopy (SEM) micrographs for comparison of methods to fabricate through-holes in PDMS
Example of a 20 mm commercial puncher used to cut individual chips out of a PDMS microfluidic wafer















Laser ablation

Machining/milling

Air/water jet machining

(left) PDMS microfluidic chips and glass placed inside plasma system. (right) Plasma generated inside the system to activate PDMS and glass surfaces
Bonding of PDMS and glass for fabrication of microfluidic device

Bonding

Standard conditions of the plasma treatment to bond glass and PDMS


PDMS/PDMS and PDMS/glass bonding

The oxygen plasma treatment must be optimized for the specific surfaces to be bonded (Bhattacharya et al., 2005). If the exposure is too short, not enough Si-OH sites might be created for good bonding. If the exposure is too long, too many Si-OH and roughness might result in a non-sticking silica layer. The standard conditions for bonding PDMS and glass in the system at the PDMS lab have been found to be:

  • Pressure: 0.35 mTorr
  • Oxygen flow: 100% (5-6sccm)
  • Power: 100%
  • Time: 30 s
Pictures of contact angle for water drop on PDMS and borosilicate glass after plasma treatment in the PDMS lab at DTU for different time
Press used to improve the bonding of PDMS and glass in a microfluidic device


PDMS/PMMA bonding

Bonding of cured PDMS and PMMA parts using uncured PDMS as glue

References

Bhattacharya, S., Datta, A., Berg, J. M., & Gangopadhyay, S. (2005). Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength. Journal of Microelectromechanical Systems, 14(3), 590–597. https://doi.org/10.1109/JMEMS.2005.844746

PDMS line ‒ Center of MicroNanoTechnology CMi ‐ EPFL. (n.d.). Retrieved April 20, 2026, from https://www.epfl.ch/research/facilities/cmi/equipment/packaging-miscellaneous/pdms-line/


For further information contact Stephan Sylvest Keller suke@dtu.dk or Claudia Chaves Villarreal cchvi@dtu.dk

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