Battlefield Business Card

In transport medicine, medics leave syringes attached to the IV line so they don’t get lost. Titrating a drug means giving a small dose, watching the response, then giving more, so the stopcock stays open as well to keep the next dose quick. Then both get forgotten. During a move, a strap, a hand or a body brushes the plunger, and the casualty gets a dose by accident.

A lot of the times people leave a syringe on and the stopcock open to be able to titrate and then forget to close both systems. Someone accidentally brushes against it and bam your pt gets a dose.

Caspar, combat medic in Ukraine

He asked for “a little clamp that attaches to the plunger so that it can’t be pushed on.” So I built one. It’s a 3D printed lock that stops a syringe from being pushed by accident while it is still connected to a casualty’s line. Version 1.0 prints on a consumer printer in PETG, assembles by hand with no tools, and works on a standard 10 mL Luer-lock syringe.

What It Had to Do

I turned Caspar’s description into requirements before drawing anything. Each one ruled out an easier design.

Fit a syringe that is already connected. Nothing can pass over the Luer tip, so every part the syringe sits in has to be open on one side.

Lock at any fill level. The thumb press moves about 6 mm per mL on a 10 mL syringe, so a single fixed stop can’t work.

Release with one hand, on purpose only. The next titrated dose has to be quick, and the lock must never let go by itself.

Hold the syringe in. It can’t pop out while the casualty is being moved.

Print anywhere, assemble with no tools. It has to survive the variation between consumer printers without anyone drilling or filing a part to fit.

Nothing that can be lost. No loose clips or caps in the field.

Adjustable for other syringe brands. Dimensions vary between manufacturers.

How It Works

Seven printed parts: a body with a toothed rail, a sliding carriage with a fork, a release lever, a hinged gate, and three pins. The model below is the real geometry, the same files that go to the printer, so you can set a fill level and try to push the plunger through the lock.

Body and rail. The finger flange drops into a slot and the barrel snaps into a collar. A toothed rail runs back alongside the plunger.

Carriage. It slides along the rail with a fork sitting in front of the thumb press. Slide it back until it touches and it clicks into the nearest tooth, and the plunger can no longer travel forward.

Release lever. A pawl holds the carriage in the teeth. Squeeze the lever toward the syringe and the pawl drops clear; let go and it locks again at the new position. Thumb off, squeeze, push a little, let go: that’s the titration loop.

Self-locking teeth. They’re angled at 30 degrees, so pushing on the plunger pulls the pawl deeper into the rail instead of out of it. It cannot be released under load, which forces the medic to take their thumb off first and stops a sudden dose if the lever is squeezed mid-push.

Gate. It closes over the finger flange and clicks shut so the syringe can’t lift out. The hinge pin keeps it attached to the body, so there is nothing to drop in the dark.

Spring leaf. A thin printed leaf pushes the pawl back into the teeth. It’s the one part designed to flex, which makes it the one part designed to wear.

Design Decisions and Iterations

Most of the useful changes came from checking the geometry automatically, and from real test prints.

From a fixed clamp to a ratchet. A simple stop behind the thumb press failed the fill-level requirement, because the thumb press moves with every volume. A ratchet that locks wherever the carriage is placed solved it.

Keeping the flex away from the load. Printed plastic cracks when it flexes over and over, so I gave all the flexing to a thin separate spring leaf and kept every load-bearing part rigid. The pawl carries the load in tension, not bending.

A hinged gate instead of a sliding one. A sliding gate needs room to slide clear, and on this body that space runs straight into the medic’s fingers. A pinned hinge stays attached and closes with one thumb.

A test sweep that caught real mistakes. I wrote a script that rebuilds the assembly at 40 syringe sizes and checks each one: no collisions, the syringe drops in and lifts out, the fork blocks the thumb press, the pawl stays engaged, and the gate swings clear and holds. It caught a hinge that blocked the flange on insertion and a gate that could hit a wide barrel, both before anything was printed.

Printing lessons. The first slice showed a lever pivot hole with 0.05 mm of plastic around it. I added a solid boss around the hole, widened the tooth tips so they print cleanly, and enlarged the hinge.

Pins that fit on any printer. The first press-fit pins wouldn’t go in, because printers make holes smaller than they are drawn. Every gripping hole is now oversize with three thin crush ribs that deform as the pin is pushed through, and each pin got a push head and a chamfered tip. Assembly needs nothing but a thumb.

Deeper teeth and firmer clips. On the first test print the pawl only just caught, so the teeth went from 1.0 to 1.5 mm deep, with the pawl now sitting 1.4 mm in, and the lever was reshaped to still swing clear. The gate latch groove went to 1.1 mm deep with a bump that reaches 0.8 mm into the wall: a gentle slope to close it, a steep face to resist vibration.

Print orientation, tried both ways. I redesigned the body and carriage to print upright with no supports. That version didn’t print correctly, so I went back to printing both on their sides with tree supports. That is the version that printed and worked.

Built to Adapt

The model lives in two forms: a parametric OpenSCAD file, and a Fusion 360 script that generates the full parametric timeline. Every sketch point is dimensioned from seven syringe measurements, barrel diameter, flange length, height and thickness, rod width, thumb press diameter and maximum stroke, plus printer tolerances. Moving to another syringe brand means measuring it and editing those values. Everything else follows.

Version 1.0, Printed and Working

Printed in PETG on a Bambu Lab printer, assembled by hand, and tested on a 10 mL syringe. The lock held, the release worked, and the gate and pins stayed put.

The Battlefield Business Card

The lock is free, with the Sagemill Labs name engraved on its underside.

The goal is to get our name directly into medics’ hands in a form they actually use. Anything we can give them for free, we will, so that when Sagemill Labs turns up with something bigger, they already know our motive.

The first sets go to Caspar to spread among other medics. Each one ships with a README in English and Ukrainian with printing and assembly instructions, a two-sided field card with the fit, lock and release steps, a clear statement that this is an evaluation prototype and not a certified medical device, and a feedback address so medics can tell me what worked and what failed. If you have a printer, you can download the whole thing for free below and make your own.