Introduction about PDMS soft-lithography and polymer molding for microfluidics

 What is PDMS soft lithography?

Soft lithography can be viewed as a complementary extension of photolithography. Originally, standard photolithography was mainly developed to deal with semiconductors used in the microelectronics industry. Photolithography is inherently well adapted to process photoresists. Thereby, most microfluidic devices still rely on photolithography for fabricating SU-8 masters (see Figure 1a-d).

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PDMS soft lithography process scheme

Figure 1: Fabrication of a PDMS layer incorporating microstructures. Drawings (a) – (d) correspond to the fabrication of a rigid master via photolithography. Drawings (e) and (f) can be considered as part of the soft lithography process (image from [1])

Soft lithography, however, extends the possibilities of conventional photolithography. Unlike photolithography, soft lithography can process a wide range of elastomeric materials, i.e. mechanically soft materials. This is why the term “soft” is used. For instance, soft lithography is well suited for polymers, gels, and organic monolayers. PDMS, however, has been the most widely used material for the applications of soft lithography because of its useful properties including low cost, biocompatibility, low toxicity, chemical inertness, versatile surface chemistry insulating, as well as mechanical flexibility and durability,the PDMS can also be easily manipulated and doing PDMS device can require only few equipments [2].

It should be noted that the terminology soft lithography does not refer to a unique fabrication technique. Soft lithography actually encompasses a collection of fabrication methods that are all based on using a patterned layer of PDMS similar to the one exposed in Figure 1f. In order to provide a brief overview, the next sections of this tutorial will introduce some of the core techniques related to soft lithography.

PDMS Soft lithography for microfluidics

A first essential feature of soft lithography is the possibility to obtain a sealed microfluidic device. Typically, the microchannels imprinted in the PDMS layer are simply closed with a glass slide (see Figure 2). Alternatively, another piece of PDMS can be used. PDMS bonding is required to obtain a strong permanent bonding between the layer of PDMS and the glass slide. This bonding is usually made by plasma bonding. When the microchannels are properly sealed, fluids can be pumped at pressures as high as ~350 kPa without failure [1].

 

PDMS soft lithography microfluidics bonding Figure 2: Fabrication of a microfluidic chip: a PDMS layer incorporating microchannels is sealed to a glass slide (image from [1])

PDMS Soft lithography: Replica molding

In replica molding, a patterned layer of PDMS is used as a soft mold where a polymer is poured. After curing, the polymer is separated from the PDMS mold. Similarly to a rigid master used in photolithography, the initial patterns of the PDMS mold are imprinted in the surface of the polymer. Replica molding, however, permits to pattern a wider range of materials. For instance, biocompatible polymers such as agar or agarose can be patterned with replica molding. Moreover, replica molding can duplicate 3D structures in a single step and the same PDMS mold can be reused many times.

PDMS soft lithography replica molding

Figure 3: Illustration of replica molding (image from [1])

PDMS Soft lithography: Capillary molding

Capillary molding is a second technique where a patterned PDMS is used as a mold. The patterns of the PDMS layer must first be brought into contact with a substrate (e.g. a glass slide). Capillary molding is then intended to fill the patterns of the PDMS mold with a liquid polymer. As the name of the technique implies, capillarity is exploited to progressively fill the patterns. As an alternative, suction can be used. After having cured the polymer, the PDMS can be gently removed, leaving solid microstructures at the surface of the substrate.

PDMS soft lithography micromolding in capillaries

Figure 4: Illustration of capillary molding (image from [1])

PDMS Soft lithography: Microcontact printing

In microcontact printing, a PDMS layer is used as a stamp. The PDMS layer is first soaked in a molecular “ink” and then brought into contact with a substrate in order to transfer the ink onto the substrate surface. In microcontact printing, only the ink from the raised surface of the PDMS stamp is transferred to the substrate. Various inks, including small biomolecules, proteins or suspension of cells can be used.

Figure 5: Illustration of microcontact printing (image from [1])

PDMS Soft lithography: Microtransfer molding

In microtransfer molding the patterned surface of a PDMS layer is filled with a liquid polymer. When the excess polymer has been removed, the PDMS layer is inverted and brought into contact with a substrate. After the curing of polymer, the PDMS layer is cautiously peeled away, leaving a solid structure with a feature size down to 1µm on the surface of the substrate. Similarly to replica molding, the same PDMS layer can be refilled many times.

PDMS soft lithography microtransfer molding

Figure 6: Illustration of microtransfer molding (image from [1])

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Tutorials : Introduction about soft lithography

Definition Microfluidic

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Tutorials : How to choose your soft lithography instrument

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Tutorials : How to get the best soft lithography  process

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In soft lithography, the fabrication of a mold, often made in SU-8, is required for replicating PDMS microfluidic structures ...
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Soft Lithography: SU-8 baking

Replicating PDMS-based structures first requires the fabrication of a SU-8 master mold that will serve as a patterned template for PDMS casting ...
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SU-8 photolithography: Spin-coating

How do you do to have a successful spin coating? Here you will find the tips and tricks to do it ...
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SU-8 photolithography: Baking

How do you do to have a successful photoresist baking? Here you will find the tips and tricks to do it ...
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How do you do to have a successful SU-8 exposure? Here you will find the tips and tricks to do it ...
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Soft lithography: Glass/PDMS bonding

How do you do to have a successful PDMS bonding with a plasma cleaner? Here you will find the tips and tricks to do it ...
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Tutorials : Microfluidic devices fabrication

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The photolithography mask is an important tool in soft photolithography processes, we explain here how they are made ...
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It exist different technics to fabricate microfluidic devices but the main can be resume by Etching, Thermoforming, Polymer ablation and Polymer casting ...
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The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if precised. Article written by Guilhem Velvé Casquillas and Timothée Houssin.

References:

[1] D. B. Weibel, W.R. DiLuzo and G.M. Whitesides, Microfabrication meets microbiology, Nature Rev. Microbiol. (2007), Vol. 5, No.3, pp. 208-218.

[2] G. M Whitesides and A.D Stroock, Flexible methods for microfluidics, Phys. Today (2001), Vol. 54, pp. 42-46.