Bio MEMS

Answer: (ii) PDMS

PDMS (Polydimethylsiloxane) is the most widely used polymer in soft lithography for microfluidics because it is transparent, flexible, biocompatible, gas-permeable, and easy to mold into microchannels.

Answer: (iii) Laminar

Microfluidic flows generally have low Reynolds numbers because channel dimensions and flow velocities are small. Therefore, viscous forces dominate inertial forces and the flow is predominantly laminar, with little turbulent mixing.

Motivation for Microfluidic Systems and Surface-to-Volume Ratio

Microfluidics deals with controlling very small volumes of fluids, typically in channels with dimensions from micrometres to hundreds of micrometres.

Motivation

1. Very small sample and reagent consumption.
2. Low operating cost.
3. Rapid analysis and reaction times.
4. High-throughput parallel processing.
5. Integration of sample preparation, reaction and detection.
6. Portable point-of-care diagnostic systems.
7. Precise control of fluid transport and cellular environments.

Surface-to-volume ratio

For a characteristic length $L$, surface area scales approximately as $L^2$ while volume scales as $L^3$. Therefore, surface-to-volume ratio scales as $1/L$ and increases strongly as dimensions decrease.

Consequences include:

• surface forces and adhesion become important,
• heat transfer becomes rapid,
• diffusion can dominate mass transport,
• capillary forces become significant,
• wall interactions strongly influence fluid behavior.

Thus, the high surface-to-volume ratio is one of the main reasons microfluidic systems behave differently from conventional fluidic systems.

Types of Microfluidic Channels and Working Principles

Microfluidic channels can be classified according to geometry and function.

1. Straight channels

A simple rectangular or circular passage transports fluid from inlet to outlet. At low Reynolds number, flow is mainly laminar and pressure-driven.

2. Serpentine channels

The channel contains repeated bends. The longer path increases residence time and can enhance mixing through repeated deformation of fluid streams.

3. T-junction channels

Two streams meet at a T-shaped junction. They are commonly used for controlled mixing and droplet generation.

4. Y-junction channels

Two inlet streams merge at an angle into one channel. They are useful for controlled co-flow and diffusion-based mixing.

5. Flow-focusing channels

A central stream is squeezed by side streams, allowing controlled formation of droplets or particles.

6. Microchambers

Expanded regions connected to channels provide space for reactions, cell trapping, sensing or incubation.

Working principle

Fluid movement is produced by pressure-driven flow, electroosmotic flow or other microscale actuation methods. Because flow is generally laminar, mixing is often achieved by diffusion or deliberate channel geometry rather than turbulence.

Biocompatible Polymers and PDMS in BioMEMS

Biocompatible polymers are polymeric materials that can perform their intended function in contact with biological tissues, cells or fluids without causing unacceptable adverse biological responses.

Important properties of PDMS

• Biocompatibility for many laboratory and diagnostic applications.
• Optical transparency, useful for microscopy.
• Flexible and elastomeric.
• Easy replication by soft lithography.
• Low cost and rapid prototyping.
• Good gas permeability, especially oxygen and carbon dioxide.
• Low surface energy and easy molding.
• Can be bonded to glass or other substrates after surface treatment.

Applications

1. Microfluidic channels.
2. Cell culture and cell manipulation platforms.
3. Lab-on-Chip devices.
4. Drug delivery research.
5. Micropumps and valves.
6. Diagnostic and biological analysis devices.

Limitation

PDMS can absorb some small hydrophobic molecules and its surface properties can change with treatment, so material selection must match the biomedical application.