In the realms of modern biomedical research, point-of-care testing (POCT), and drug screening, a technology known as "Lab-on-a-Chip" is quietly revolutionizing scientific experimentation paradigms. This technology integrates several meters of tubing, reactors, separation columns, and detection units from traditional laboratories onto a chip just a few square centimeters in size, enabling automated, high-throughput, low-cost analysis of minute samples. In this miniaturization revolution, polydimethylsiloxane (PDMS)—a transparent, flexible, and easily processed form of silicone rubber—has become the most popular material for constructing microfluidic chips, earning it the title of the "transparent cornerstone of flexible laboratories."

Why PDMS? Five Core Advantages:

Excellent Optical Transparency:

PDMS has high light transmittance (>90%) in the visible to near-ultraviolet range (approximately 280–900 nm), making it ideal for integrating microscopic observation, fluorescence detection, or spectroscopic analysis. Researchers can monitor processes such as cell migration, droplet generation, or chemical reactions in real-time, an advantage not offered by opaque materials like PMMA or PC.

Superior Elasticity and Airtightness:

With a Young's modulus of around 0.5–2 MPa, PDMS can be stretched, bent, or even folded without breaking. This elasticity allows it to bond irreversibly with glass, silicon wafers, or other PDMS layers to form sealed microchannels. Moreover, its flexibility enables the integration of microvalves and micropumps: applying air pressure in control channels causes membrane deformation, opening/closing main channels, thereby achieving fluid manipulation without mechanical moving parts.

Biocompatibility and Surface Modifiability:

Medical-grade PDMS is non-toxic and non-allergenic, suitable for long-term biological experiments such as cell culture and organ-on-a-chip applications. While its surface is naturally hydrophobic, oxygen plasma treatment can instantly convert it to hydrophilic, lasting from hours to days, facilitating water solution filling. Additionally, surfaces can be further modified with antibodies, peptides, or polymers for specific molecular capture.

Rapid Prototyping via Soft Lithography:

The greatest process advantage of PDMS lies in soft lithography: pouring liquid PDMS prepolymer onto a silicon template fabricated by photolithography, curing at 60–80°C, and then peeling off creates micron-scale channel structures (precision up to 1–10 μm). This entire process does not require a clean room, is cost-effective, and allows for rapid iteration from design to testing within hours, significantly accelerating scientific innovation.

Gas Permeability Supporting Cellular Respiration:

PDMS exhibits high permeability to oxygen and carbon dioxide (with an O₂ diffusion coefficient of about 2,300 mm²/s), far exceeding that of glass or plastic. This characteristic allows living cells encapsulated within PDMS chips to maintain physiological activity through continuous gas exchange, critical for building functional lung-on-a-chip or liver-on-a-chip devices.

Typical Application Scenarios:

Single Cell Analysis: Using microcavity arrays to capture individual cells, combined with fluorescent labeling to study heterogeneity.

Digital PCR: Splitting samples into thousands of nanoliter-sized droplets for ultra-sensitive nucleic acid detection.

Organ-on-a-Chip: Simulating vascular endothelial shear stress, alveolar stretching, or intestinal peristalsis for drug toxicity assessment.

Droplet Microfluidics: Generating picoliter-sized oil-in-water droplets as miniature reactors for high-throughput screening.

Point-of-Care Diagnostic Devices: Integrating sample preprocessing, amplification, and detection for use in field or primary healthcare settings.

Challenges and Improvement Directions:

Despite its advantages, PDMS has limitations:

Hydrophobicity Leading to Small Molecule Adsorption: Certain drugs or fluorescent dyes are easily absorbed by PDMS, affecting quantitative accuracy. Solutions include surface coatings (such as Parylene, Pluronic F-127) or using fluorinated silicones.

Swelling Issues: Channel expansion upon contact with organic solvents (like ethanol, acetone). Researchers have developed higher cross-linking or fluorinated side-chain modified PDMS.

Difficulty in Mass Production: Soft lithography suits small-batch laboratory production but struggles with industrial-scale manufacturing. Thermoplastic materials (such as COC) are gradually being used for mass production, yet PDMS remains irreplaceable in high-end research.

Future Prospects: From Research to Clinical Applications:

As microfluidic technology moves from the lab to practical applications, PDMS continues to evolve:

Multi-material Integration: Combining PDMS with paper-based substrates, hydrogels, or electronic sensors to expand functionality.

3D Microstructure Printing: Utilizing two-photon polymerization or direct ink writing (DIW) techniques to construct complex three-dimensional vascular networks.

Standardization and Packaging: Developing plug-and-play PDMS chip modules to lower usage barriers.

Conclusion:

While PDMS may lack the strength of metals, the precision of silicon wafers, or the affordability of plastics, its unique characteristics of flexibility, transparency, permeability, and vitality provide an irreplaceable platform for microfluidic technology. It allows scientists to simulate life, explore diseases, and accelerate new drug development within a tiny space, potentially turning future family doctors into users of palm-sized transparent chips. In this microscopic revolution, silicone rubber is not just a material but a carrier of imagination—transparent and flexible, yet bearing the immense power to transform medicine.


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