PART 7 • MICROFLUIDICS DEVICE DESIGN

Fluids at the scale of cells

A red blood cell is about 6-8 µm across. So is a typical MEMS structure.

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That match in scale is what makes microfluidics possible — channels, electrodes, and sound waves small enough to sort, move, and study individual cells.

Key idea: When your device and your target are the same size, you can interact with it directly instead of just observing it from a distance.

WHY MICROFLUIDICS?

Built to match biology's scale

Red & white blood cells, platelets 6 – 8 µm in diameter

Neuron cells 4 – 100 µm

Typical microelectrode ~100 µm in diameter

Because MEMS devices sit at the same scale as cells, they're a natural fit for interfacing with and interrogating them — with minimal damage.

BIOMEMS TOOLBOX

Six ways to move a cell

Electroosmosis

Fluid flow driven by an electric field

Electrophoresis

Charged particles driven by an electric field

Dielectrophoresis

Particles polarized and moved by a non-uniform field

Acoustophoresis

Particles moved by acoustic waves

Capillary action

Fluid drawn through channels by surface tension

Electromagnetics

Particles moved by magnetic or electromagnetic fields

Beyond silicon, BioMEMS chips are also built on plastics, glass, and carbon substrates.

CASE STUDY

Sorting cells with sound

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Acoustophoresis uses sound — not chemicals, not filters — to separate cells by size.

This part walks through a real acoustophoresis cell-separation chip, from the physics that make it work to the fabrication steps that build it.

A piezoelectric substrate, a pair of electrodes, and a plastic channel — that's the entire device.

THE CORE IDEA

Standing waves sort by size

Two surface acoustic waves (SAWs), traveling toward each other across a piezoelectric substrate, combine to form a standing surface acoustic wave (SSAW) — alternating pressure nodes and antinodes.

antinode | node | antinode | node | antinode

Larger particles get funneled into the pressure node at the center of the channel. Smaller particles stay out near the antinodes.

ANATOMY OF THE DEVICE

Three parts, one chip

Piezoelectric substrate Lithium niobate — generates the acoustic wave

Interdigitated transducer (IDT) Comb-shaped electrodes that launch the SAW

PDMS microchannel Polymer channel bonded on top, carrying the cells

The wave is generated in the substrate below; the cells flow through the channel above. The two only need to line up.

UNIT 25 STUDY COMPLETE

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Ready for the Fab Challenges?

You've covered Microfluidics & Sorting Cells with Sound.