The role of PDMS in the field of microfluidics

The role of PDMS in the field of microfluidics

When it comes to microfluidics or micromachining, almost everyone thinks of PDMS or polydimethylsiloxane. However, since George Whitesides first introduced PDMS to microfluidic technology in 1998, PDMS has become the primary material of choice and plays a vital role in microfluidic technology. This article reviews some of the advantages of PDMS and the part of PDMS in microfluidic technology.

1. PDMS material characteristics

PDMS belongs to the silicone family and has some unique functions. These functions have made PDMS the ideal material in the field of microfluidics. Transparency, biocompatibility, flexibility (Young’s modulus of elasticity is about 1-3 MPa), high gas permeability, low dielectric constant, low surface tension, and low solubility are some of the main characteristics PDMS.

2. PDMS sealing method

The advantage of PDMS comes from its ability in soft lithography. First, the PDMS base monomer and its curing agent are thoroughly mixed, then the prepolymer is degassed to remove all bubbles, and finally, it is poured into the mold. The mold can be manufactured by a conventional method such as photolithography or a new technology of 3D printing. Regardless of the mold type, PDMS can replicate the features on the mold from macro to nano. After curing and peeling the PDMS replica from the mold, the PDMS part should be sealed with a flat surface. Here, another advantage of PDMS manufacturing comes into play. There are various techniques for sealing PDMS replicas, such as conformal contact, physical bonding, vacuum bonding, oxygen plasma bonding, corona surface activation, flame bonding, wet-bonding, adhesive bonding. Some of these methods are reversible and irreversible and can be selected according to the application. In most of these technologies, there is no need to use chemicals or solvents to achieve sealing. The chemical or solvent-free bonding of PDMS to PDMS substrates or other substrates eliminates any chance of chemical contamination of test samples.

3. PDMS surface modification treatment method

PDMS soft lithography technology enables researchers to fabricate devices with multiple layers of PDMS. This process is called a “sandwich,” which means that several layers of PDMS replicas can be stacked on top of each other to build more complex geometric shapes. Other components such as membranes (porous or non-porous) can be added between the layers to create the desired device in a sandwich. There are many ways to bond these films to PDMS: (1) First, the film can be treated with silane molecules (such as 3-aminopropyl triethoxy silane), and then both the treated film and PDMS can be exposed to oxygen plasma (2) If you do not seek high pressure, you can use some adhesive (double-sided tape or adhesive) to attach the film to the PDMS, and (3) Silica (SiO 2 can be sputtered It is coated on the film and bonded to PDMS using oxygen plasma.

There are many strategies to adjust the volume or surface properties, allowing researchers to modify their PDMS equipment as needed. The following methods can be used to adjust the overall performance of PDMS:
First, change the ratio of base material and curing agent.
Second, modify curing conditions (temperature and time).
Finally, add other molecules to PDMS.
1.And Fillers such as SiO 2 are added to PDMS.
Adjusting these parameters will affect the overall performance of PDMS, such as elasticity, transparency, photothermal effect, refractive index, electrical conductivity, and so on. A review article written by P. Wolf et al. provides a comprehensive review of all the different techniques for batch or surface modification of PDMS. In most cases, the surface modification of PDMS is an area of ​​concern in the field of microfluidics. The most common method is to use oxygen or air plasma, corona discharge, and ultraviolet or ozone exposure to introduce silanol (Si-OH) groups onto the PDMS surface (Si-CH 3) at the expense of methyl groups. It will form a thin layer of a few nanometers (which may also contain some cracks) on the surface and reducing the water contact angle to below 5°. When a durable surface modification strategy is required, the character of PDMS can be customized by polydopamine (PDA), various polyethylene glycol (PEG) derivatives, or silane-based molecules. Using one of these methods, researchers can achieve the hydrophobicity or hydrophilicity of the PDMS surface. There are various strategies and techniques in the literature to adjust the performance of PDMS, which can be considered as one of the advantages of PDMS over other materials used for micromachining.

4. PDMS performance adjustment usage

Adjusting the overall performance of PDMS to improve its conductivity has become a research topic in flexible and stretchable electronics. Different fillers (such as carbon nanotubes, graphite, silver particles, nanowires, and gold nanotubes) can be added to PDMS to achieve higher conductivity. Since PDA is conductive, it has been coated on PDMS to integrate electrochemical sensors for various applications.

5. PDMS potential in organ chips

PDMS has attracted more attention from researchers in other fields with the origin of organ chips and microfluidic cell culture platforms. The surface of DMS has been coated with PDA, gelatin, collagen, or fibronectin to enhance the adhesion, proliferation, and growth of the cells inside the chip. In addition, the surface of PDMS can first be modified with PEG or silane molecules or PDA as a linker. Then bioactive molecules can be connected to the linker to achieve specific applications, such as cell differentiation or detection of secreted biological molecules.

PDMS can permeate gases such as oxygen and carbon dioxide, and this makes the origin of microfluidic blood oxygenators that required gas exchange between blood and air. Spin-coating liquid PDMS can easily manufacture the PDMS film on a flat substrate (such as a wafer). The performance of the PDMS film can be adjusted by changing the speed of the spinner, curing agent ratio, and curing temperature. Without observing pinhole defects, a PDMS film with a thickness of about 1μm can be produced. Reachers can increase the thickness of PDMS by more than 1 mm to avoid permeability.

In short, PDMS has shown its potential in realizing so many applications in microfluidics. For example, microfabrication using PDMS allows us to manufacture devices with simple designs into devices with very complex functions. Furthermore, PDMS has been extensively researched, which provides a wealth of resources for everyone to optimize PDMS attributes according to their needs.

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