Washing and post-curing

Two steps, in this order, and the order is not negotiable: wash completely, then cure. Curing first traps whatever is still inside, permanently.

Washing

The goal is every trace of uncured resin gone — from the outside, and much more importantly from inside the channels.

Solvent

Isopropanol for standard resins. 91 % or better; the water in 70 % leaves residue.

Water for water-washable resins. Convenient, and it avoids managing isopropanol volumes at bench scale. Do not put water-washable rinse water down the drain — it contains dissolved uncured resin, which is an aquatic toxicant. Cure it in a shallow tray under UV until it solidifies and dispose of it as solid waste.

Procedure

Drain by gravity first. Ten minutes standing so channels run downhill towards a port. Costs nothing.
Flush the channels with a syringe. Solvent in one port, out the other. Steady pressure. Repeat until it runs clean.
Wash the outside. Bath, ultrasonic bath, or wash station.
Flush again. The outside wash puts dissolved resin back into the ports.
Second solvent, if you can. A first bath does the bulk and gets dirty; a second clean bath does the last of it. This is what makes the difference on a part with internal features.
Dry. Compressed air through each port, then air-dry. Trapped solvent in a channel will cloud the surface during cure.

How long

Two to five minutes of agitation is plenty for the outside. The channels are the part that takes patience — keep flushing until it is genuinely clean, not until you are bored.

Do not soak for long. Isopropanol swells cured photopolymer. Extended soaking softens the part, and on a thin chip that means warping. Under ten minutes total immersion.

Checking before you cure

This is the irreversible step, so check properly.

Hold the chip up to a strong light and look down each channel. Resin inside looks glossier and denser than an empty channel, and the meniscus moves if you tilt the part. Anything ambiguous gets flushed again.

On a monolithic chip, check the volume against what the CAD panel reported as the channel void. If the chip held 22 mm³ and you have flushed out something like 22 µL of resin and solvent, you are probably clear. If you got a tenth of that, you are not.

See Draining uncured resin.

Post-curing

Polymerisation finishes under UV. Until it does, the part is soft, tacky and leaching heavily.

Use a UV cure box. 385–405 nm, matched to the resin. Sunlight is slow, uneven, and weather-dependent, and it does not rotate the part.

Time. 5–15 minutes for a chip this thin. Follow the resin’s data sheet.

Rotate. Every face needs exposure. Turntable boxes do it for you; otherwise turn the part halfway.

Do not over-cure. Beyond the recommended time the part continues to embrittle and to yellow. A brittle chip cracks when you tighten a fitting.

Curing under water

Submerging the part in water while curing excludes oxygen, which inhibits surface polymerisation. It gives a less tacky, harder surface.

For a microfluidic chip, be careful: water in the channels during cure can leave deposits. If you do it, flush and dry thoroughly afterwards.

Biocompatibility

Cured photopolymer is not inert. It leaches unreacted monomer and photoinitiator, both cytotoxic to varying degrees.

If cells are involved:

Post-cure thoroughly. More complete polymerisation means less to leach.
Extract. Soak the finished chip in the medium you will use, or in ethanol, for 24–48 hours, changing it once. This pulls out the mobile fraction.
Test. Run the chip with medium only and check viability against a control.

There are resins with biocompatibility claims and they are worth the money if you need one. Do not assume a standard resin is acceptable because the chip looks clean. See Choosing a printer and resin.

Sterilising

Autoclaving deforms it. Most resins soften well below 121 °C.
Ethanol is usually fine, and is the practical choice.
UV works on surfaces you can illuminate, which does not include the inside of a channel.
Ethylene oxide works if you have access.