THE RECIPE

Cast, peel, bond

Soft lithography copies the master into soft silicone rubber. It needs no cleanroom after the master is made, so a new chip can be ready in a day.

Soft lithography turned chip-making into something a biology lab can do.

MIX AND DEGAS

No bubbles allowed

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Mix Base and curing agent, usually 10 to 1 by weight

Degas Put the mix in a vacuum chamber until the trapped air bubbles rise out

Pour Over the master to the thickness you want, often several millimetres

Why Bubbles left in the PDMS become holes and weak spots

Careful mixing and degassing make clear, defect-free chips.

CURE AND PEEL

From liquid to rubber

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Cure

Bake around 65 to 80 °C for one to a few hours (or wait about two days at room temperature)

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Peel

Cut around the pattern and peel the rubber off: the ridges are now channels

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Release layer

A silane coating on the master keeps the PDMS from sticking

The PDMS is a perfect negative copy, down to features below a micrometre.

PORTS

Getting liquid in and out

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A biopsy punch cores inlet and outlet holes through the PDMS before bonding. Tubing pushed into a slightly smaller hole seals by squeezing the rubber. Clean holes, with no rubber crumbs, prevent clogs later.

Ports are where the outside world meets the chip, and where many leaks start.

THE MASTER AND THE MATERIAL

Strengths and limits

Stacking two PDMS layers, one with flow channels and one with control channels, creates valves. The same casting can make thin membranes and posts. But for products, PDMS gives way to plastics.

PDMS is unbeatable for prototypes and often swapped out for products.

UNIT 8 STUDY COMPLETE

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Ready for the Fab Challenge?

You've followed a PDMS chip from mixing and degassing through curing, peeling and punching ports.