Bioprocess tubing

Tubing is the most underestimated polymer contact in a bioprocess. Everything in the fluid touches it, and a surprising amount crosses it: oxygen and carbon dioxide permeate through, water vapor escapes, hydrophobic molecules partition in, and tube material itself sheds into the product. The physics of each direction is well understood; the engineering is choosing which one to tolerate.

Permeability is structural

Silicone is a rubbery polymer with a very flexible backbone and a lot of free volume between chains, and small nonpolar molecules move through that network by diffusion without any pores at all. Oxygen and carbon dioxide cross readily — which is deliberately exploited where gas exchange through a tube wall is wanted — and water vapor escapes slowly in the same way, which is why a fluid held in silicone lines for weeks concentrates slightly. Volatile solutes escape with the water. The common thermoplastic alternatives are orders of magnitude less permeable, so the choice of tube material is literally a choice of permeability: silicone where the wall must breathe, thermoplastic elastomer where it must not. There is no material that does both.

Sorption: the loss you cannot see

The same rubbery, hydrophobic network dissolves hydrophobic solutes. Nonionic surfactants — the polysorbates that protect proteins and cells from interfacial stress — partition into the tube wall along the whole run, and their concentration falls without any visible change. Hydrophobic vitamins, fragrances and actives go the same route. The traffic runs the other way too: siloxane oligomers migrate out of the silicone into the fluid, where they may sit on product or on downstream surfaces. Sorption is worst at low concentrations, long contact and warm temperatures — precisely the conditions of a slow feed or a long hold. The consequence is process-level: what leaves the mixing vessel is not what arrives at the bioreactor, and the difference is chemistry nobody planned.

The peristaltic constraint

What finally limits material choice is usually not chemistry but the pump. A peristaltic pump works by repeatedly compressing the tube against a track, and a tubing line may take millions of such cycles in a campaign. The material must survive them in three specific ways: it must flex without cracking; it must recover its lumen after each compression, because a tube that takes a permanent set flattens and the delivered flow drifts downward while the pump runs at the same speed; and it must not shed particles, because each compression grinds the inner surfaces against each other and any spalled fragment goes straight into a sterile fluid. Platinum-cured silicone is the traditional compromise; some thermoplastic elastomers are engineered for longer flex life and lower spallation, at the price of the permeability described above. The pump’s accuracy claim is in fact the tube’s recovery claim — calibration of a peristaltic rate is a property of the specific tube lot and its fatigue history, which is why feed rates are re-verified on the tube actually installed.

The single-use assemblies page describes migration as the central polymer problem of disposables; tubing is that problem compounded by mechanical service — the material ages chemically and mechanically at once, and the two clocks are independent.