Specific Process Knowledge/Polymer Processing/PDMS Casting

DTU Nanolab has a dedicated laboratory for PDMS processing and microfluidics fabrication located in building 347. The PDMS Lab is part of the PolyFabLab, which opened its doors in 2026. PolyFabLab has been equipped with brand-new instruments funded by the Novo Nordisk Foundation, alongside selected equipment and processes from previous facilities.

The process line for PDMS at DTU Nanolab is described on this website. The reader is referred to literature reviews for a deeper description of PDMS processing and applications. (Lin & Chung, 2021; Morbioli et al., 2020; Tony et. al., 2023)

PDMS Casting

Example of PDMS lab application: Compartmentalized microfluidic device for reconstruction of dopamine pathways between brain organoids (Sozzi et al., 2025)

Polydimethylsiloxane (PDMS), is a popular silicone elastomer for fabricating microfluidic devices and micro-total analytical systems (μTAS) with applications in life sciences, physics, chemistry, and more. The microfluidic devices integrate channels, chambers, porous membranes, valves and actuators, electrodes, optical paths, and more; and are often interfaced with living cells to biomimic organ and tissue functions. Rapid prototyping of PDMS enables fast, low-cost iteration for the development of microfluidic structures.

Working areas in the PDMS lab

The laboratory has 6 different working areas:

Photograph of the different work areas in the PDMS Laboratory at DTU Nanolab

• 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, parafilm, lint-free towels, a vacuum desiccator and a ventilated oven are available in this area.

• Fume hood: Safe area for handling volatile chemicals that are hazardous for human and environment health. The spin-coating machine and the ultrasonic bath are also located in this area. A compressed air gun supplying clean air is available in the fume hood, as well as a fixed compressed air source for users that require to dry large amounts of samples.

• Plasma treatment: In this area, is located the plasma equipment to condition the surfaces for bonding and a press for further improving the bond.

• Cured PDMS: In this area the user can find magnifying lamp, stereoscope with camera, cutting mat, light screen; assortment of tweezers, blades, scalpels, punchers; broken glass waste, thermometer and pH indicator paper; and other tools for further processing and inspecting cured PDMS and microfluidic devices.

• Storage: Three cabinets are located in the lab. The ventilated cabinet is used for storing PDMS resin and curing agent, the solvents (acetone, ethanol, isopropanol), other chemicals, as well as the solid and liquid C-waste in the bottom. A second cabinet is used to store small sample holders, glass and plastic petri dishes, and wafer holders of 2", 4" and 6" sizes; pipettes, micropipettes for viscous liquids, spoons, spatulas, swabs, thickness gauges; an assortment of cups for mixing, aluminum foil, glass slides; among other. A third rack is available for the users to keep their samples. Transparent plastic boxes are available for the user to store their samples with proper labelling, for the matter of order and control.

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

Process flow

Process for PDMS in DTU Nanolab

Mixing the PDMS elastomer kit

Photograph of PDMS mixing in fume hood and degassing in vacuum dissecator chamber

In the PDMS lab, all handling of uncured PDMS must be handled in the UNCURED PDMS AREA only. A lining of cellophan must be placed on the table before start working. The commercial Sylgard 184 silicone base kit including the prepolymer and cross-linker, a.k.a. curing agent (Dow Corning Corporation, Midland, USA) is available in the storage cabinet of the lab.

The resin and the curing agent are usually mixed in a 10:1 weight ratio, but lower ratios down to 5:1 can be used if it is required to have a less viscous PDMS for casting of intrincate geometries. A high precision balance, and different size containers, pipettes and spatulas are available for the lab users.

The mixing of the PDMS and the resin must be conducted thoroughly wih the spatula to ensure a homogenous and well-cured polymer. A more homogeneous mixture of the resin and the suring agent can be achieved by sonicating, which ensures consistent optical properties for application where homogeneity is key, such as optical waveguides. All mixing must be done inside the ventilated fume hood, to protect users and the environment from the health hazards of the curing agent.

Once the PDMS and curing agent have been properly mixed, they are degassed in vacuum chamber, until no more bubbles are observed. Care must be taken to not let the PDMS overflow out of the container when degassing. Degassing may take 20 min to 1 hr, depending on the volume of the resin and the container used. Alternating between venting and degassing may help to accelerate the removal of bubbles. The degassing can be done to the resin in the mixing container, or directly in the PDMS once casted on the mold. Some bubbles are always introduced when casting the PDMS on the mold, so it may be necessary to repeat the degassing after castin in the mold.

Please read with great attention the Safety Data Sheets (SDS) and the Standard Operation Procedures (SOP) of products mentioned above at Kemibrug.dk.

Soft lithography micro molding

One of the most popular methods for fabrication of PDMS microfluidic devices is mold replica methods. The process and tools available at DTU Nanolab for mold fabrication of PDMS are described in this section.

Fabrication of the micromold by soft-litography of SU8

SU-8 negative micropattern on silicon wafer fabricated by soft lithography in the clean room for PDMS micromolding

The microfabrication, i.e. photolithography and micromachining, of glass or oxidized silicon (Si/SiO2) substrates, is time consuming, resource-intensive and cost prohibitive in many cases. Soft-lithography was developed in the late 1990s by the Whitesides group at Harvard University, as the first method to rapid-prototype microfluidic devices using PDMS soft-elastomers (Duffy et al., 1998).

The micropatterns to be replicated in the PDMS are often fabricated with SU-8 photolitography and wet etching on silicon wafers in the clean room. The PDMS lab is strategically located across the hallway of the PolyFabLab clean room, facilitating a continuous processing and minimizing contamination sources. Other methods for polymer processing like 3D printing and imprinting are also available for fabricating features with nano to macroscopic dimensions on a variety of substrates to be replicated in PDMS.

Fabrication of the micromold by lithography and etching of silicon

A mold containing micro-features can also be fabricated directly by lithography and etching of the silicon wafer. This process is under development at DTU Nanolab.

Fabrication of the casting mold

(Top) Process flow diagram for the development of the mold for casting PDMS (Bottom) Mold development process. (Left) CAD design of the mold. (Center) 3D printed mold. (Right) Wafer with SU8 micropatterns inside casting mold

PDMS casting in molds by (left) pouring into open mold or (right) injecting into mold inlet

Pneumatic injection system for PDMS

The wafer containing the micropatterns fabricated with SU8 photolitography is often placed on a casting mold, which can be as simple as a frame, or be a more specialized mold fabricated with 3D printing or machining. Transparent materials like polycarbonate are preferable for molds because they are transparent and allow to monitor the complete degassing of the PDMS once casted. DTU Nanolab offers a variety of 3D printers and a machine shop that the users can access to fabricate their casting molds. For the machine shop at DTU Nanolab contact Søren Petersen. Simple rectangular frames of 28 x 46 mm, and cricular frame for 6 in wafers are available in the lab for the users.

Conditioning of the mold surface

The mold for casting PDMS might require a surface modification to ensure propoer demolding of the PDMS after curing. The mold material surface might strongly adhere to PDMS or hinder tha crosslinking reactions. Therefore, silanization might be necessary to condition the surface. When the design has large surface-to-volume ratio, or if very small features are included, the mold conditioning might be necessary.

Molding by casting and injecting

Once the PDMS pre-polymer and crosslinking agent are properly mixed and degassed, and the mold is cleaned and, if necessary, conditioned, the PDMS can be casted. The casting can be done by simply pouring the PDMS into the open mold, or even injected, if the design has complex features that require a special mold. Bubbles may be formed when pouring the PDMS in open mold, and additional degassing may be necessary. When injecting into a closed mold, further formation of bubbles is unlikely. Uncured PDMS must be handled in the UNCURED PDMS AREA only.

An injection system is available in the laboratory, which uses compressed air to push the PDMS in a syringe into a mold inlet. This alternative is useful to fill in molds that contain large aspect ratio features that would be otherwise difficult to fill, and to scale up fabrication projects in a semiautomatic manner. The pressure applied for injection and the time can be tuned to match an specific design.

Molding by spin coating

PDMS thin films can be produced by spincoating in the lab. The process is under development. The spincoating requires to login in LabManager, after proper training by Nanolab staff.

Curing PDMS

(Left) Photo of PDMS in mold curing in oven (Right) Testing for leveling of mold inside oven before curing.

The PDMS can cure without heating in 48 hours. Curing the PDMS at higher temperatures by cinvective heating in the oven can reduce the curing time. 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, optical, and surface properties. The curing time depends on the temperature and the thickness of PDMS. As a standard procedure, cure the PDMS in the oven for at least 2 hours at ~80°C. The oven requires to login in LabManager, after proper training by Nanolab staff.

PDMS typically shrinks up to 10% depending on curing temperature, which may affect the compliance of dimensions to the design or application. Overnight curing at 45 °C results in ∼0.3 % shrinkage which is within tolerable limits for integration. (Madsen et al, 2014)

The thermal curing of PDMS is one of the slowest steps in the fabrication of microfluidic devices. Alternatively, the curing of PDMS can be done in microwave oven, which is under investigation at DTU Nanolab. (Katare et. al. 2020)

Demolding

Photograph of PDMS demolding (left) and PDMS microfluidic chip demolded (right)

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. Pressurized air can also be used to help the demolding step. Otherwise, the mold must be conditioned for anti-sticking properties before casting the PDMS.

When the thickness of the PDMS film is less than ~500 μm, its demolding from SU8/silicon molds may result in rupture of the film. A flexible polymer film can be use as transfer susbtrate to provide mechanical support to the PDMS while demolding. (Estlack et. al., 2022)

Cutting PDMS

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

Scanning Electron Microscopy (SEM) micrographs for comparison of methods to fabricate through-holes in PDMS

Alternative methods for high-throughput fabrication are under investigation at Nanolab to meet the demands of the industry and large interdisciplinary projects. The cutting methods being researched include the development of wafer-size punching, laser ablation, mechanical and air/water jet machining.

Punching

Example of a 20 mm commercial puncher used to cut individual chips out of a PDMS microfluidic wafer

The access ports of microfludic devices, also known as through-holes, can be created using manual punchers commercially available for biopsy or general use. Always 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, smo, the final dimensions of the holes must be confirmed with metrology methods.

Laser ablation

The ultra short pulsed laser (pisecond to femtosecond) can be used as a "cold machining" approach to cut transparent dielectrics like PDMS. (Feit et al., 2004; Liu et al., 1997; Stuart et al., 1996)

At DTU Nanolab, the Laser Micromachining Tool can be used for processing PDMS. The process development for laser ablation of PDMS is currently underway for this line.

Machining/milling

Air/water jet machining

3D printing of PDMS

Soft-litography micromolding and cutting protocols pose limitations for complex designs, require long processing times, and are labour intensive. 3D printing of PDMS provides the automation and versatility features to meet the demands for future technologies. The 3D printing of PDMS is currently under development at Nanolab. (Tony et al., 2023)

PDMS cleaning

Ultrasonic bath for cleaning the PDMS devices

The processing of PDMS may contaminate its surface yielding it inadequate for its final application, or affecting its surface for further bonding. Cutting with sharp tools like puncher and milling may leave reasidues of PDMS particles and lubricants. Laser ablation may leave silicone powder, carbon, ashes, and further by-products of the thermal decomposition. The manipulation may also contaminate it with grease or particles from the environment.

PDMS surface modification

Different physical and chemical treatments can modify the wettability, refractive index, and light propagation of PDMS, due to both chemical bond rearrangements and changes in surface roughness.

Chemical treatment

Laser treatment

Bonding of PDMS

In many applications, including microfluidics, it is necessary to bond the PDMS microfluidic chips to glass, PMMA, other PDMS parts and more. In this section we describe the methods so far used for creating stron, long-lasting bonds.

Bonding of PDMS to glass and other PDMS parts

Oxygen plasma treatment of PDMS and glass surfaces can be used for bonding. The plasma induces activation of Si-O-H groups in the surface of glass and PDMS, which induces a strong bond when the surfaces are shortly brought into contact. The plasma equipment requires to login in LabManager, after proper training by Nanolab staff.

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

The modified surface will fastly recover its hydrophobic structure. Therefore, the treated surfaces must be brought into contact immediately after the plasma had been applied.

Bonding of oxygen plasma-treated PDMS and glass for fabrication of microfluidic device

A press is available in the lab to hold parts under pressure after brought into contact to reinforce the bonding.

Press used to improve the bonding of PDMS and glass in a microfluidic device

The oxygen plasma treatment to apply 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 at DTU Nanolab have been found to be:

• Pressure: 0.35 mTorr

• Oxygen flow: 100% (5-6sccm)

• Power: 100%

• Time: 30 s

The plasma treatment above described does not induce any permanent chemical modification to the glass or PDMS, as demonstrated by XPS analysis.

Standard conditions of the plasma treatment to bond glass and PDMS

Pictures of contact angle for water drop on PDMS and borosilicate glass after plasma treatment in the PDMS lab at DTU for different time

Graphs of contact angle for water on PDMS and borosilicate glass vs oxygen plasma treatment time

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

Bonding of PDMS to PMMA

PDMS can be bonded to other polymers, like PMMA, for interfacing the microstructured patterns with tubes, vials and other macroscopic parts. The Sylgard 184 elastomer kit can be used as a "glue" between PMMA and cured PDMS, provided that an extended curing time of 24 hours at 70°C is applied to create a robust bond between the two surfaces. Roughening the PMMA surface beforehand may result in stronger adhesion.

Cleanliness requirements

The precursors and elastomer PDMS materials are extremely difficult to see and to clean. The cross-linker from the elastomer kit is also hazardous for human health and the environment. The experience shows that a laboratory for PDMS processing can easily become very contaminated, and fastly turn into a non-reliable space for working if the users are not careful. Therefore, extreme measures of cleanliness are required to work in the PDMS laboratory.

• 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.

• When handling uncured PDMS, double gloves must be used, and the user must be constantly inspecting the gloves for any contamination with PDMS precursors, especially if handling the elastomer kit bottles. The gloves must be inmediatly discarded in the C-waste container whenever the slightest evidence of contamination is observed.

• The uncured PDMS must be handled exclusively in the designated area and the table must be protected with cellophan lining. Aluminum foil lining must be used also when mixing the PDMS pre-polymer and the cross-linking agent are mixed inside the fume hood. All the materials contaminated with uncured PDMS must be carefully discarded in the C-waste container.

• Any spill of uncured PDMS, including inside vacuum chamber and oven, must be completely cleaned by the responsible user.

• When demolding and cutting PDMS clean well the area 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.

• The user is responsible to maintain all table, equipment and tools surfaces free of uncured PDMS. Preventive cleaning of all the surfaces with isopropanol is recommended before and after usage, including the chemical bottles.

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

Duffy, D. C., McDonald, J. C., Schueller, O. J. A., & Whitesides, G. M. (1998). Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane). Analytical Chemistry, 70(23), 4974–4984. https://doi.org/10.1021/AC980656Z

Estlack, Z., Compton, B., Razu, M. E., & Kim, J. (2022). A simple and reliable microfabrication process for a programmable microvalve array. MethodsX, 9, 101860. https://doi.org/10.1016/J.MEX.2022.101860

Feit, M. D., Komashko, A. M., & Rubenchik, A. M. (2004). Ultra-short pulse laser interaction with transparent dielectrics. Applied Physics A 2004 79:7, 79(7), 1657–1661. https://doi.org/10.1007/S00339-004-2683-1

Katare, P., & Gorthi, S. S. (2020). Microwave irradiation-based rapid curing of PDMS for microfluidic applications. Microfluidics and Nanofluidics 2020 24:7, 24(7), 46-. https://doi.org/10.1007/S10404-020-02348-0

Lin, L., & Chung, C. K. (2021). PDMS Microfabrication and Design for Microfluidics and Sustainable Energy Application: Review. Micromachines 2021, Vol. 12, Page 1350, 12(11), 1350. https://doi.org/10.3390/MI12111350

Liu, X., Du, D., & Mourou, G. (1997). Laser ablation and micromachining with ultrashort laser pulses. IEEE Journal of Quantum Electronics, 33(10), 1706–1716. https://doi.org/10.1109/3.631270

Madsen, M. H., Feidenhans’l, N. A., Hansen, P. E., Garnæs, J., & Dirscherl, K. (2014). Accounting for PDMS shrinkage when replicating structures. Journal of Micromechanics and Microengineering, 24(12), 127002. https://doi.org/10.1088/0960-1317/24/12/127002

Morbioli, G. G., Speller, N. C., & Stockton, A. M. (2020). A practical guide to rapid-prototyping of PDMS-based microfluidic devices: A tutorial. Analytica Chimica Acta, 1135, 150–174. https://doi.org/10.1016/J.ACA.2020.09.013

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/

Sozzi, E., Corsi, S., Bruzelius, A., Scordo, G., Maraschin, S. T. da S., Thongkorn, S., Heiskanen, A., Ramos-Passarello, G., Kajtez, J., Emnéus, J., & Parmar, M. (2025). Reconstruction of the human nigrostriatal pathway in vitro reveals target-dependent dopamine neuron maturation. BioRxiv, 2025.12.23.696164. https://doi.org/10.64898/2025.12.23.696164

Stuart, B., Feit, M., Herman, S., Rubenchik, A., Shore, B., & Perry, M. (1996). Nanosecond-to-femtosecond laser-induced breakdown in dielectrics. Physical Review B, 53(4), 1749. https://doi.org/10.1103/PhysRevB.53.1749

Tony, A., Badea, I., Yang, C., Liu, Y., Wells, G., Wang, K., Yin, R., Zhang, H., & Zhang, W. (2023). The Additive Manufacturing Approach to Polydimethylsiloxane (PDMS) Microfluidic Devices: Review and Future Directions. Polymers 2023, Vol. 15, Page 1926, 15(8), 1926. https://doi.org/10.3390/POLYM15081926

Contact and access

For further information contact:

• Stephan Sylvest Keller suke@dtu.dk
• Meena Dhankhar meenadh@dtu.dk
• Claudia Chaves Villarreal cchvi@dtu.dk