Lyophilisation: difference between revisions
Diff·revision 28 → 29·07:43, 20 Jan 2025
Difference between revision 28 and revision 29 of Lyophilisation. 9 lines changed; the page grew by 1,620 bytes.
| Revision 28 — 11:45, 10 Jan 2025 CouplingCillian (talk) expand §Cake attributes and their interpretation 15,095 bytes +1,601 | Revision 29 — 07:43, 20 Jan 2025 CouplingCillian (talk) fix significant figures — source gives fewer 16,715 bytes +1,620 | ||
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| 87 | A 2.0 mL fill therefore requires approximately 12.5 hours of primary drying under these conditions, before any safety margin is added. Doubling the shelf-to-product temperature difference halves that figure, which is why the choice of critical temperature — and how close the cycle is permitted to approach it — dominates cycle economics.{{r|tang2004,rambhatla2003}} | 87 | A 2.0 mL fill therefore requires approximately 12.5 hours of primary drying under these conditions, before any safety margin is added. Doubling the shelf-to-product temperature difference halves that figure, which is why the choice of critical temperature — and how close the cycle is permitted to approach it — dominates cycle economics.{{r|tang2004,rambhatla2003}} |
| 88 | 88 | ||
| + | 89 | Vials do not all behave alike. Those at the edge of a shelf receive additional radiant heat from chamber walls and door, sublime faster, and reach higher product temperatures; this edge-vial effect can amount to several degrees and is the usual reason a cycle validated on a laboratory dryer fails at production scale.{{r|rambhatla2003}} | |
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| 89 | === Collapse and eutectic melting === | 91 | === Collapse and eutectic melting === |
| 90 | Two distinct structural failures bound primary drying. In a system whose freeze-concentrate crystallises, exceeding the eutectic melting temperature produces liquid, and the vial dries as a shrunken residue or a glassy shell — usually described on inspection as melt-back. In an amorphous system, exceeding a temperature a few degrees above Tg′ allows viscous flow of the freeze-concentrated matrix into the pores that the sublimed ice has vacated. The dried layer loses its structure, and the result is a shrunken, sometimes glassy or foamed cake. The temperature at which this becomes observable by freeze-drying microscopy is the collapse temperature, Tc, conventionally 1–3 °C above Tg′ for small-molecule and peptide systems.{{r|tang2004}} | 92 | Two distinct structural failures bound primary drying. In a system whose freeze-concentrate crystallises, exceeding the eutectic melting temperature produces liquid, and the vial dries as a shrunken residue or a glassy shell — usually described on inspection as melt-back. In an amorphous system, exceeding a temperature a few degrees above Tg′ allows viscous flow of the freeze-concentrated matrix into the pores that the sublimed ice has vacated. The dried layer loses its structure, and the result is a shrunken, sometimes glassy or foamed cake. The temperature at which this becomes observable by freeze-drying microscopy is the collapse temperature, Tc, conventionally 1–3 °C above Tg′ for small-molecule and peptide systems.{{r|tang2004}} |
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| 98 | 100 | ||
| 99 | The practical difficulty is that the last vials to finish are not the ones being monitored. Endpoint criteria are therefore normally applied with a hold period appended, and the batch is held under primary-drying conditions for a defined time after the signal is observed.{{r|patel2010}} | 101 | The practical difficulty is that the last vials to finish are not the ones being monitored. Endpoint criteria are therefore normally applied with a hold period appended, and the batch is held under primary-drying conditions for a defined time after the signal is observed.{{r|patel2010}} |
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| + | 103 | == Secondary drying == | |
| + | 104 | Once ice has gone, water remains sorbed to the solid — hydrogen-bonded to the peptide and to any amorphous excipient. Removing it requires desorption rather than sublimation, and desorption is driven by raising the product temperature well above the primary-drying value, typically to between 20 and 40 °C, while holding chamber pressure low.{{r|nail2002}} | |
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| + | 106 | Desorption kinetics are fast at first and then slow markedly, so the last fraction of a percent of moisture is disproportionately expensive in time. Secondary drying is consequently the step most often truncated, and truncation is a common explanation for a cake that meets appearance specifications but exceeds its moisture specification.{{r|wang2000}} | |
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| + | 108 | The heating ramp matters because the glass transition temperature of the partially dried solid rises as water leaves it. Early in secondary drying the solid is plasticised by residual water and its glass transition may be only slightly above the product temperature; heating too quickly can carry the product above the moving glass transition and cause the same collapse phenomenon seen in primary drying, at a much higher temperature. Conventional practice ramps shelf temperature at 0.1–0.5 °C per minute for this reason.{{r|tang2004}} | |
| 100 | 109 | ||
| 101 | == References == | 110 | == References == |