UPDATE1:
We added a test-protocol that shows the behaviour in comparison to a commercial PEEP valve. Tests have shown that a 1-2mm thick rubber or silicone square flap gives better results than the printed TPU seal. See protocol for details.
At the moment the best performing setup is square rubber/silicone seal flap and bike hub spring mounted on the retrofit kit.

Team PANDA [Zurich, Switzerland]:
We propose a new 3D printed spring loaded and adjustable inline PEEP valve. These types of inline valves are needed in differential multiventilation setups for treating covid19.
See this link for more information on differential multiventilation:
https://www.differentialmultivent.org

We designed these to be printable even on low-cost FDM 3D printers. The sealing areas can be printed against the glass bed and result in a mirror-like finish. Yes, the whole setup could be designed more compact for usage on SLA printers. And it could be adapted for plastic injection molding. But we wanted these to be printable pretty much anywhere in the world under subpar conditions.

It uses conical bicycle hub springs (thanks to Enrico Bassi for pointing these springs out to us!) which seem to be quite the same dimensions in most countries. You can adjust the desired PEEP value by turning the head. Additionally it can be used with printed springs or lasercut metal springs. Use the overview picture to choose a spring setup:

• Plastic (PET) spring has creep and deforms in < 48h.
• Lasercut metal triple-helix spring is relatively untested but should have the best overall characteristics (use 0.3mm or thinner spring metal steel).
• Bike hub spring works if the dimensions match the ones in the picture. (tested in US and Europe)

PARTS:
For all versions you need the base, the cover and the TPU_seal.
Depending on the version you want choose the other files (plastic, steel or bikeSpring in the name).

INSTRUCTIONS:

• Print orientations according to Cura picture. Really! We mean it! You won't get a good seal if you print it wrongly.
• Start with base and make sure it matches the 22 mm connectors.
• Make a test print to check O-ring clearance and thread match. Scale cover in X,Y-direction only.
• You need a 3 mm O-ring with ID < 37 mm (35 mm)
• You can also take an O-ring with larger ID and cut it diagonally (see picture). This will seal it as well.
• Use TPU for TPU_seal
• Use PET for the spring. After 48h or less the spring will deform due to creep. Keep this in mind.
• You can also use a square (ca. 27 x 27 mm, trimmed corners) piece of rubber/silicone as a seal.
• If the start of the thread on the base is slightly wider due to print bed adherence, use a cutter to trimm it.
• Bike hub spring dimensions are in the pictures.

This is NOT a medically approved device and is not intended for medical use.

This might be interesting for teams that are trying to build ventilator prototypes and do not have an inline PEEP valve at hand. We also attached all STEP files should you have to change something for your setup. Please upload as derivate should it be helpful for other teams.

WARNING/DISCLAIMER:
We have tested these on our PANDA ventilator test bed but we can’t compare it to a medical PEEP valve since we don’t have one. Someone that has a medical PEEP valve should print ours and compare/benchmark it against the medical one. Please post relevant findings in the comments. These are not medically approved and should not be used on humans. Use at your own risk and liability.
The spring is magnetic. Under no circumstances bring this device near an MRI.

PRINTING:
Print without supports as shown on the Cura picture. That's important to get a good seal!! If the thread is too narrow, scale cover in X,Y-direction only.

TESTS:
We were able to adjust the PEEP pressure in the necessary range of 5 - 20 cmH2O. The O-ring keeps the setup completely airtight. Tests are ongoing.

Other PEEP valves in our line-up:

Gen2:
https://www.thingiverse.com/thing:4250354

Gen3:
https://www.thingiverse.com/thing:4278384

PANDAcase:
https://www.thingiverse.com/thing:4314931

PANDAflow:
https://www.thingiverse.com/thing:4289425