Polydimethylsiloxane (PDMS) in Microfluidics : Rise of Smart PDMS

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Microfluidic reviews

Polydimethylsiloxane (PDMS) in Microfluidics: Properties and Emerging Smart Functionalities

What is Polydimethylsiloxane (PDMS)

PDMS reticulated poly di methyl siloxane

Polydimethylsiloxane, called PDMS or dimethicone, is a polymer widely used for the fabrication and prototyping of microfluidic chips.

It is a mineral-organic polymer (a structure containing carbon and silicon) of the siloxane family (word derived from silicon, oxygen and alkane). Apart from microfluidics, it is used as a food additive (E900), in shampoos, and as an anti-foaming agent in beverages or in lubricating oils.

For the fabrication of PDMS microfluidic devices, Polydimethylsiloxane (liquid) mixed with a cross-linking agent is poured into a microstructured mold and heated to obtain an elastomeric replica of the mold (cross-linked).

PDMS Chemical Structure

PDMS formula

The chemical structure will help us better understand the advantages and drawbacks of PDMS for microfluidic applications.

The Polydimethylsiloxane empirical formula is (C2H6OSi)n and its fragmented formula is CH3[Si(CH3)2O]nSi(CH3)3, n being the number of monomers repetitions.

Depending on the size of the monomer chain, the non-cross-linked PDMS may be almost liquid (low n) or semi-solid (high n). The siloxane bonds result in a flexible polymer chain with a high level of viscoelasticity.

After PDMS “cross-linking”

PDMS, once cured, becomes a hydrophobic elastomer. Due to its low surface energy, polar solvents, such as water, struggle to wet the PDMS causing droplets to bead rather than spread and this leads to the adsorption of hydrophobic components on the material’s surface.

PDMS Oxidation

PDMS oxidation using plasma changes the surface chemistry, and produces silanol terminations (SiOH) on its surface. This helps make the material hydrophilic. It takes a few hours to one day to go back to normal hydrophobicity. This process also makes the surface resistant to the adsorption of hydrophobic and negatively charged molecules. In addition, its plasma oxidation is used to functionalize the surface with trichlorosilane for example or to covalently bond polydimethylsiloxane (at the atomic scale) on an oxidized glass surface by the creation of Si-O-Si bonds.

PDMS in microfluidics

PDMS is one of the most employed materials to mold microfluidic devices, along with paper. to mold microfluidic devices [1]. It involves a master mold which is used to create a negative of the design of the microfluidic chip on a substrate, using for example the soft lithography technique. Once the mold is created, it can be re-used and allows high replication rate of microfluidic chips from a single mold.

(1) The molding step allows mass-production of microfluidic chips from a mold, the first step is mixing the PDMS and degassing.

(2) A mixture of polydimethylsiloxane (liquid) and crosslinking agent (to cure it) is poured into the mold and heated at high temperature.

(3) Once it has hardened, it can be taken off the mold. We obtain a replica of the micro-channels on the block.

Microfluidic device completion:

(4) To allow the injection of fluids for future experiments, the inputs and outputs of the microfluidic device are punched with a PDMS puncher the size of the future connection tubes.

(5) Finally, the face of the block of PDMS with micro-channels and the glass slide are treated with plasma.

(6) The plasma treatment allows PDMS and glass bonding to close the microfluidic chip.

The chip is now ready to be connected to microfluidic reservoirs and pumps using microfluidic tubing. Tygon tubing and Teflon tubing are the most commonly used tubings on microfluidic setups.

If you are unsure about choosing the appropriate tubing for your setup, see our dedicated tutorial pages : Basics About Microfluidic Tubings & Sleeves and How to choose microfluidic Tubing?

What makes PDMS the ideal material for Microfluidic Device Fabrication

PDMS was chosen to make microfluidic chips primarily for those reasons:

Biology-inspired-microfluidic-alveolar-model-Elveflow — *Biology-inspired microfluidic alveolar models applying mechanical stretch* [2]

Transparent: At optical wavelength(240 nm– 1100 nm), which facilitates the observation of contents in micro-channels visually or through a microscope. [3]
Biocompatible: allowing cell culture assays in a controlled environment (with some restrictions).
Gas permeability: It enables cell culture by controlling the amount of gas through the material or dead-end channels filling (residual air bubbles under liquid pressure may escape through PDMS to balance atmospheric pressure).
Cross-linking: PDMS can be coated with a controlled thickness on a substrate using a simple spincoater. This allows the fabrication of multilayer devices and the integration of micro valves.
Easy to bond: The PDMS bonds easily with glass or another PDMS layer with a simple plasma treatment. This allows the production of multilayer PDMS microfluidic devices. With the use of glass substrates, possibilities such as the use of metal deposition, oxide deposition or surface functionalization.
Deformability: which allows the integration of microfluidic valves using the deformation of PDMS micro-channels, the easy connection of leak-proof fluidic connections and can be used to study its use to detect very low forces like biomechanical interactions from cells.
Inexpensive: compared to previously used materials (e.g. glass).
Easy to mold: Even when mixed with the cross-linking agent, it remains liquid at room temperature for many hours. The PDMS can mold structures at high resolutions. With some optimization, it is possible to mold structures of a few nanometers [4].

Some of the challenges for Microfluidic Applications

Electrodes deposited on glass to be integrated in the ’PDMS microfluidic chip [5]

Metal and dielectric deposition on PDMS is difficult, limiting the direct integration of electrodes and resistors. However, this can be overcome by spin-coating a thin layer of uncured PDMS onto a metalized surface, which then bonds well with the main PDMS layer after curing. While plasma bonding to glass is common, this method offers a simple workaround for bonding PDMS to metal.
PDMS ages, therefore after a few years the mechanical properties of this material can change.
PDMS can be autofluorescent
It adsorbs hydrophobic molecules and can release some molecules into the liquid and this can be a problem for some biological studies in PDMS microfluidic devices.
This material is permeable to water vapor which makes evaporation in such devices hard to control.
It is also sensitive to the exposure to some chemicals (see below).

Chemical resistance of PDMS

You will find below an immersion study of microstructured PDMS (h: 11µm, L: 45µm) in a variety of chemicals [4], this study was performed with PDMS Sylgard 184. Find more information about chemical resistance of different materials for microfluidics.

Table 2: Chemical and PDMS2 substrate characterization after immersion

Chemical	Concentration (wt.%)	Time (min)	No	Mild	Medium	Total	Weight gain (%)
Buffered hydrofluoric acid	49.0	10	x				−0.97
Hydrochloric acid	37.9	10	x				−0.34
Potassium hydroxide^a	86.9	10	x				−0.27
Hydrogen peroxide (H₂O₂)	31.8	10	x				2.01
Piranha	3(H₂O₂) : 1(H₂SO₄)	10, 30	x				0.64
Photoresist developer^b	100.0	10	x				−0.97
Water (deionized)	100.0	10, 30	x				−0.33
Hexane	99.9	10	x				4.67
Toluene	99.9	10	x				45.51
Acetone	99.7	10, 30	x				2.78
Methanol	100.0	10	x				−0.34
Isopropanol	100.0	10	x				−0.97
Potassium hydroxide^a	86.9	60		x			0.75
Buffered hydrofluoric acid	49.0	30			x		1.39
Nitric acid	69.8	10			x		4.13
Hydrofluoric acid	49.0	10				x	−1.38
Sulfuric acid (H₂SO₄)	96.0	10				x	−14.00
Potassium hydroxide^a	86.9	1620				x	−0.90

(Legend: No: no effect on microstructures, Total: complete destruction of microstructures)

The Evolving Role of PDMS in Microfluidics

Polydimethylsiloxane (PDMS) is a go-to material in microfluidics for various reasons. Its flexibility, transparency and biocompatibility make it the best material for fabricating microfluidic devices like single layer,bilayer and micro-imprint stamps. Researchers mainly use two types of PDMS: PDMS RTV-615 and PDMS Sylgard 184. The exact composition of these two products is kept secret; Both have their own particularities and uses, depending on the objective being pursued. This allows researchers to choose the most suitable PDMS for a specific application [6]:

PDMS RTV-615
This PDMS is popular for its robust nature and ability to bond bilayer microfluidic devices. This is even favoured by the co-inventor of the microfluidic valve, S.Quake.
Despite all these factors, it has the reputation of being dirty (filled with impurities). It is known to vary from batch to batch in plasma bond strength. It is then necessary to adjust the bonding parameters with each purchase.
PDMS Sylgard 184 (Dow Corning)
On the other hand, The PDMS Sylgard is known for being cleaner and is most commonly preferred for applications like mammalian cell culture. It’s less suited to multilayer chips, as bonding can be more difficult and less reliable which can result in ineffective device fabrications.

Rise of Smart PDMS: Innovations in Microfluidics

The rapid advancement in materials science have not only expanded the use of PDMS in microfluidics but have also transformed it into a ‘smart’ material. With new functionalities like self-report, self-heal, and self-clean. Smart PDMS is now more effective, durable, and reliable in challenging environments, making it ideal for a variety of applications.

Self-Cleaning PDMS

This feature is ideal for biomedical applications, as these are restricted to little to no biofouling and hydrophobic recovery over a long period. This limits a normal PDMS because of instability over time. To overcome this, researchers are developing modified PDMS surfaces that aim to enhance their performance by integrating antifouling properties and pH sensitivity.

PDMS Surface coating

A common method consists of chemically grafting poly(GMA-co-SBMA-co-DMAEMA), a zwitterionic and pH-sensitive polymer, onto a PDMS surface after it has been activated by UV and ozone. By decreasing fibrinogen adsorption and blood cell attachment, this improves PDMS’s antifouling properties. When exposed to basic pH conditions, the amount of attached E. coli bacteria is remarkably reduced by 98%. [7].

Micro/Nano structured PDMS

One of the big innovations in PDMS is the development of superhydrophobic micro/nano dual-scale (MNDS) films using a streamlined, wafer-level fabrication process. This method involves direct replication from an ultralow-surface-energy silicon substrate at high temperatures, eliminating the need for surfactant coatings. An improved deep reactive ion etching (DRIE) technique enhances surface passivation. It results in hierarchical structures with excellent self-cleaning properties. These MNDS PDMS films are especially promising for micro/nanofluidic applications and advanced light-absorbing coatings. [8].

SELF-HEALING PDMS

PDMS often undergoes complex stresses that can lead to gradual or sudden failure, impacting device performance.Preventing and managing such damage is key to ensuring long-term stability. Following are some of the popular approaches used to implement this.

High-modulus PDMS

One of the exciting breakthroughs is the work around high-modulus, self-healing elastomers. Researchers achieve this by incorporating acylsemicarbazide (ASC) groups with varying polarities, boosting both the stiffness and robustness of the material. The highlight of using this method is that the enhanced PDMS still retains its transparency. This opens up new possibilities in areas like reversible adhesives and rugged microfluidic systems. It’s a promising leap forward for applications where standard PDMS just isn’t enough. [9].

PEG PDMS

While PDMS remains a staple in microfluidic and lab-on-a-chip applications, its hydrophobic nature often leads to challenges, like unwanted protein and drug molecule adsorption. To overcome this, researchers have successfully modified PDMS with polyethylene glycol (PEG), considerably improving its hydrophilicity.

Treating the PDMS surface and getting a contact angle down to 23.6° ± 1° decreases the wettability of the material. Even more impressively, this modification is effective for up to 20 months, ensuring long-term performance. Thanks to reduced unwanted adsorption and enhanced compatibility, PEG-PDMS is proving to be especially useful in sensitive biological models, like liver-on-a-chip systems and other advanced organ-on-a-chip platforms. [10].

Self Reporting PDMS

This modification into polymeric materials like polydimethylsiloxane (PDMS) is accumulating significant attention. As this enables traditional polymer matrices to communicate the mechanical stress or damage, hence Self-Reporting. This is crucial for maintaining structural integrity. Following approaches can help reach this.

Microcapsules embedded PDMS

When microcapsules are incorporated into PDMS matrices, it allows the release of charge transfer precursors upon mechanical failure. In this method ,1,2,4,5‐tetracyanobenzene are used as an electron acceptor and two types of electron donors, forming charge transfer complexes (CTCs). This would reveal any damage through visible color transformations. These microcapsules can be synthesized via in-situ polymerization of an oil-in-water emulsion. This could be also adopted for dual functionality, if enhanced by self-healing agents like hexamethylene‐diisocyanate. This would allow autonomous self-reporting and healing processes, making the matrix suitable for diverse applications (like anti-corrosion performance or crack penetration depth visualization in PDMS chips) [11].

Mechanophores embedded PDMS

Another method concerns mechanophores (MPs), embedded within elastomeric matrices such as spiropyran-functionalized PDMS. These mechanophores are molecular units that identify the mechanical force and exhibit fluorescent properties. With the help of confocal microscopy, we can observe the fluorescence intensity that correlates with uniaxial strain. Researchers have found a linear relationship between mechanical stress and fluorescence output through finite element analysis. This method enhances the ability to quantitatively gauge local stresses, paving the way for refined damage-sensing applications [12].

Mechanochemiluminescent PDMS

This method is related to the advancements in mechanochemiluminescent (MCL) sensitivity. This can be achieved by synthesizing PDMS polymers that incorporate difluoroboron β-diketonate dye and 1,2-dioxetane as co-crosslinkers. The combination of these materials enhances the energy transfer within the polymer network. It also minimizes aggregation that hinders fluorescence, essential for effective stress reporting. The resulting materials have increased MCL intensity and can provide a more sensitive diagnostic tool for mechanical integrity over both thermal and mechanical disruptions [13].

Conclusion

In conclusion, polydimethylsiloxane (PDMS) has been playing an essential role in advancing microfluidic research, due to its unique properties like flexibility, biocompatibility, and transparency. Today, it’s even more powerful, thanks to the significant advances that have been made. Functionalities such as self-reporting, self-healing and self-cleaning have been added to the PDMS with which we are all familiar. Each technique used represents cutting-edge techniques aimed at improving the performance and versatility of PDMS in a variety of applications.

Elveflow, with its full microfabrication stations and its microfluidic instruments, continues to lead the way in enabling researchers to explore new discoveries and applications in PDMS-based microfluidics. Elveflow is a trusted partner in scientific innovation with its range of instruments created by researchers for researchers. For additional insights on microfluidics, you can check out: Microfluidics Reviews.

The photos in this article come from the Elveflow® data bank or elsewhere if specified. Article written by Guilhem Velvé Casquillas and Timothée Houssin, revised by Louise Fournier

For more tutorials about microfluidics, please visit our other tutorials here: «Microfluidics tutorials». The photos in this article come from the Elveflow® data bankor elsewhere if specified. Article written by Guilhem Velvé Casquillas and Timothée Houssin, revised by Louise Fournier

References

[1] Y. Xia and G. M. Whitesides, “Soft lithography,” Angewandte Chemie International Edition, vol. 37, pp. 550–575, 1998.

[2] S. Zhang, K. Devloo-Casier, P. Wuytens, K. Baert, H. Bender, S. de Gendt, and A. Delabie, “Chemical stability of PDMS-based materials for microfluidics in analytical chemistry,” ACS Applied Materials & Interfaces, vol. 16, no. 2, pp. 1931–1942, 2024.

[3] S. Zhang, K. Devloo-Casier, P. Wuytens, K. Baert, H. Bender, S. de Gendt, and A. Delabie, “Chemical stability of PDMS-based materials for microfluidics in analytical chemistry,” ACS Applied Materials & Interfaces, vol. 16, no. 2, pp. 1931–1942, 2024.

[4] F. Hua et al., “Polymer imprint lithography with molecular-scale resolution,” Nano Letters, vol. 4, pp. 2467–2471, 2004.

[5] S. Bunthawin, J. Kongklaew, A. Tuantranont, K. Jaruwongrangsri, and T. Maturos, “Microchip electrode development for traveling wave dielectrophoresis of non-spherical cell suspensions,” Engineering, vol. 5, pp. 88–93, 2012.

[6] J. M. Spotts, Microfluidics Course, Institute for Systems Biology, Nov. 17, 2008.

[7] J. S. De Vera, A. Venault, Y. Chou, L. L. Tayo, H. Chiang, P. Aimar, and Y. Chang, “Self-cleaning interfaces of polydimethylsiloxane grafted with pH-responsive zwitterionic copolymers,” Langmuir, American Chemical Society, 2018, doi: 10.1021/acs.langmuir.8b01569.

[8] A. Mata, A. J. Fleischman, and S. Roy, “Characterization of polydimethylsiloxane (PDMS) properties for biomedical micro/nanosystems,” Biomedical Microdevices, vol. 7, pp. 281–293, 2005.

[9] X. Zhang, F. Zhu, M. Han, X. Sun, X. Peng, and H. Zhang, “Self-cleaning poly(dimethylsiloxane) film with functional micro/nano hierarchical structures,” Langmuir, American Chemical Society, 2013, doi: 10.1021/la4023745.

[10] W. Ma et al., “A self-healing polydimethylsiloxane elastomer with high strength and high modulus,” Polymer, Elsevier, 2023, doi: 10.1016/j.polymer.2023.126164.

[11] A. Gökaltun, Y. B. Kang, M. L. Yarmush, O. B. Usta, and A. Asatekin, “Simple surface modification of poly(dimethylsiloxane) via surface segregating smart polymers for biomicrofluidics,” Scientific Reports, Springer, 2019, doi: 10.1038/s41598-019-43625-5.

[12] Y. Chen, F. Li, H. Cang, S. Chen, and G. Zhang, “Early detection of polymer deformation via poly(urea‐formaldehyde) microcapsules encapsulated with charge transfer precursors,” Journal of Applied Polymer Science, Wiley, 2025, doi: 10.1002/app.57120.

[13] M. L. Rencheck, B. T. Mackey, Y. Hu, C. Chang, M. D. Sangid, and C. S. Davis, “Identifying internal stresses during mechanophore activation,” Advanced Engineering Materials, Wiley, 2021, doi: 10.1002/adem.202101080.

[14] Y. Yuan, B. Di, and Y. Chen, “Mechanically induced bright luminescence from 1,2‐dioxetane containing PDMS boosted by fluoroboron complex as an in‐chain fluorophore,” Macromolecular Rapid Communications, Wiley, 2020, doi: 10.1002/marc.202000575.