Tubing, fittings, and connecting to a syringe pump
The chip is the easy part. Most first-run problems are in the plumbing.
The two routes
Section titled “The two routes”Via luer. Print a luer port, fit a syringe or a male-luer adapter. Standard, cheap, and everything in a lab already speaks it.
Direct to tubing. Print a PVC tubing socket and push the tubing straight in. Fewer connections means fewer leaks and less dead volume — but you commit to one tubing size at design time.
Tubing
Section titled “Tubing”| Type | Notes |
|---|---|
| PVC | Cheap, flexible, easy to push onto a barb. What the catalogue socket is sized for. Not for organic solvents. |
| PTFE | Chemically inert, rigid, low dead volume. Needs a proper fitting; will not push-fit. |
| Silicone | Very flexible, gas-permeable — which is useful for cell work and a problem for anything you want sealed. |
| FEP | Inert and transparent. Best for optical work along the line. |
Sizes. The catalogue socket is cut for 0.125“ ID × 0.188“ OD (3.18 / 4.76 mm) PVC, which is a standard laboratory size and widely available.
Keep it short. Every millimetre of tubing is dead volume and compliance. A long compliant line turns a step change at the pump into a slow ramp at the chip, and at microfluidic flow rates that can be minutes.
Connecting
Section titled “Connecting”Luer slip
Section titled “Luer slip”Push on and twist slightly. Friction alone holds it.
Relieve strain before the tubing reaches the chip. Tape or clip the line to the bench a few centimetres back. The most common way a printed port fails is someone catching the tubing and levering the boss off.
Luer lock
Section titled “Luer lock”Push on and screw the collar down. Use it whenever the line is pressurised or will run unattended.
Remember the printed thread is a single-start approximation of the two-start
ISO form, with its pitch set to the real fitting’s lead. It winds on; it is not
the standard profile. Test-fit before relying on it. See
thread-support.
Tubing socket
Section titled “Tubing socket”Push the tube in until it butts against the socket floor. The socket is 4.70 mm against a 4.76 mm tube, so the PVC compresses about 0.06 mm and grips.
If it will not start, warm the tube — a few seconds in hot water is enough. Do not force it; you will split the boss.
If it is loose, the socket printed oversize. Open it by 0.05 mm at a time on the next print, or shim with PTFE tape.
Syringe pumps
Section titled “Syringe pumps”Syringe size sets your resolution. A pump moves the plunger at a set linear rate, so a smaller barrel gives finer flow control and lower pulsation. For µL/min work use the smallest syringe that holds enough for your run — the consumption estimate tells you how much that is.
Check the pressure rating. The maximum pressure in the Fluid Dynamics summary strip is what the pump has to deliver. Compare before you find out on the bench.
Expect pulsation. Stepper-driven pumps ripple at the step frequency. It shows up as periodic variation in droplet size and as noise in a concentration gradient. A short length of compliant tubing acts as a low-pass filter; a pressure-driven controller avoids it entirely.
Priming, and bubbles
Section titled “Priming, and bubbles”Bubbles are the most common practical problem and the most annoying.
At this scale a bubble does not float past. Surface tension makes it a plug that has to be pushed out, and while it sits in a channel it blocks flow completely and compresses under pressure — which makes your flow rate meaningless.
To avoid them:
- Fill the syringe and tubing first, with the chip disconnected. Expel every bubble from the line.
- Connect wet to wet. A drop standing on the port, a drop at the tubing end, join them. Air cannot get in.
- Prime slowly. Fast filling entrains air at every junction.
- Degas if it matters. Sonicate or vacuum the fluid beforehand. Dissolved gas comes out of solution at low pressure and forms bubbles inside the chip.
- Tilt so ports are highest while priming, so air leaves rather than being pushed round a corner.
To clear one: increase the flow briefly to push it to an outlet. If it is stuck at a junction, back-flush.
A run checklist
Section titled “A run checklist”- Chip washed, cured, bonded, pressure-tested.
- Tubing cut short, fittings dry-fitted.
- Syringes filled and purged of air.
- Lines primed, connected wet-to-wet.
- Chip strain-relieved to the bench.
- Outlet routed to waste, below the chip.
- Flow started slowly, ramped up.