The Plastic Blood Bag

PVC film is joined by heat, not adhesive, and the join is stronger than glue because it is the same material fused through. Cut two rectangles of flexible PVC film. Lay a strip of release paper where you want the ports, so the seal skips those zones. Weld the perimeter with a heat sealer or a temperature-controlled iron run against a straightedge over baking parchment, at a temperature that softens the film without charring it — find it on scrap first, because the window between a weak seal and a burnt hole is perhaps fifteen degrees. Seal in the tubing stubs at the port zones by sandwiching the tube between the two films and welding around it. This is the join that fails, so make it wide and make three test samples before you make the real one. A weld that looks perfect can peel; test by inflating each sample with water until it bursts and check that the burst is in the film, not along the seam. Leave one edge open, fill with your anticoagulant analogue, expel the air completely, then weld the last edge. Air in the bag is not just untidy — it makes the centrifuge separation unreliable, because the bubble migrates during the spin and disturbs the interface.

Make a surrogate that separates visibly. A suspension of fine dense particles in a viscous liquid works: sieved silt or ceramic powder in glycerol solution, tuned so it takes a few minutes to settle under gravity and seconds under spin. Aim for two clearly different particle sizes so you can hunt for the soft-spin and hard-spin behaviours the notebook predicted. Fill your welded primary bag, expel all air, seal, and spin it in a balanced rotor. Balance matters — spin an unbalanced rotor and you will destroy the centrifuge and possibly yourself. Weigh the counterbalance to within a gram. Express the top layer by laying the bag between two flat plates and squeezing evenly, watching the interface move up the tubing towards the satellite. Stop before the interface reaches the port. This is the skill in the whole process: stop too late and you contaminate the top fraction with the bottom one, and there is no way to undo it inside a closed system. Seal the tubing between the bags by clamping and welding, then separate. Weigh each bag. Your recovery is the mass in each fraction against the mass you started with, and it will not be 100 percent because some volume stays in the tubing — the dead volume, which is why collection sets are designed with the shortest tubing that still lets the bags reach the centrifuge and the press.

A blood bag spends its life at between one and six degrees, and PVC gets stiffer and more brittle as it cools. A weld that passes at room temperature can fail in the cold room, so test where it lives. Fill three bags, chill them to four degrees overnight, and then drop each from waist height onto a hard floor while still cold. This is the actual industry drop test and it is not a joke — bags are dropped in real handling. Then flex a chilled bag repeatedly at the seam, fifty cycles, and inspect the weld under the microscope for crazing. Compare against three identical bags tested warm. Any difference in failure rate is your material telling you about its glass transition, and it is the reason plasticiser content is specified rather than left to the supplier. Where this stops, and it stops absolutely. You have made flexible welded containers, run a closed transfer, and separated a suspension by density. That is real packaging and separation engineering with uses far beyond medicine — it is how a great many sterile fluids are handled. It is not a blood collection system, and there is no maker path to one. Human blood carries transmissible infection, collection from a donor is a clinical procedure with real risks to that donor, the anticoagulant formulation and its ratio to volume are pharmaceutical specifications, every component is tracked from donor to recipient, and a transfusion given to the wrong person is among the most lethal errors in medicine. Handle no human or animal blood in this build. Use silt and glycerol, and take from it the closed-system principle, which is the genuinely portable idea.