Lyophilisation (revision 18)

Lyophilisation, also called freeze-drying, is a dehydration process in which a solution is frozen and the ice is then removed by sublimation at a pressure below the triple point of water, leaving a porous solid whose volume approximates that of the original fill. It is the standard final step in the isolation of synthetic peptides after preparative purification, and the reason a research peptide is distributed as a dry cake in a sealed vial rather than as a solution.^[1]

The process is used because peptides in aqueous solution degrade by hydrolytic routes — deamidation of asparagine and glutamine, backbone cleavage, aggregation — whose rates depend strongly on molecular mobility and on the availability of water. Removing water and holding the residue in an amorphous glass suppresses those routes by orders of magnitude, converting a product with a shelf life of days at ambient temperature into one with a shelf life measured in years.^[2][3]

A lyophilisation cycle has three phases: freezing, primary drying in which ice is sublimed, and secondary drying in which water sorbed to the solid is desorbed. Each phase is bounded by a physical constraint. Freezing determines the ice-crystal structure and therefore the resistance the vapour must overcome later; primary drying must be conducted below the temperature at which the freeze-concentrated matrix loses rigidity; secondary drying must reach a residual moisture low enough for stability without over-drying a solid that may require a small amount of water for conformational integrity.^[4][5]

The finished cake is characterised by appearance, residual moisture, reconstitution time and the chemical purity of the reconstituted solution. None of these is visible from a purity figure alone, which is one reason a certificate of analysis that reports only area percent purity leaves the physical quality of the vial undocumented.^[6]

Water can pass directly from solid to vapour only at pressures below its triple point, which lies at 611.657 Pa and 0.01 °C. Above that pressure, warming ice produces liquid; below it, warming ice produces vapour. Every lyophilisation cycle therefore operates at a chamber pressure well beneath 611 Pa, typically between 5 and 20 Pa, so that heat supplied to the product drives sublimation rather than melting.^[1]

The driving force for sublimation is the difference between the vapour pressure of ice at the product temperature and the partial pressure of water vapour in the chamber. Because the vapour pressure of ice falls steeply with temperature, small changes in product temperature produce large changes in drying rate.

Values follow the standard formulations reviewed by Murphy and Koop.^[7] The table explains a practical asymmetry of the process: a cycle run with the product at −40 °C rather than −30 °C loses roughly two-thirds of its driving force, and a cycle run 5 °C warmer than intended may dry substantially faster while risking the structural failure described below.

Sublimation is strongly endothermic. The enthalpy of sublimation of ice is approximately 2.83 MJ·kg⁻¹, about 13% greater than the enthalpy of vaporisation of liquid water, so the shelf must supply a large heat flux for a modest mass flow. That heat must cross a gas gap between shelf and vial base, and the efficiency of that transfer — conventionally expressed as a vial heat transfer coefficient — is itself a function of chamber pressure, because conduction through rarefied gas falls as pressure falls.^[8]

Two consequences follow and recur throughout the article. First, product temperature is not a set point but a result: it emerges from the balance between heat supplied and heat consumed by sublimation, and it is lower than the shelf temperature throughout primary drying. Second, anything that changes the resistance of the dried layer to vapour flow changes the product temperature, which is why the freezing step — which sets that resistance — governs the behaviour of the two steps that follow.^[9]

Cooling an aqueous solution below 0 °C does not immediately produce ice. Nucleation requires supercooling, and in clean pharmaceutical solutions supercooling of 10–20 °C below the equilibrium freezing point is routine. The temperature at which nucleation happens to occur determines the number and size of the ice crystals formed: nucleation close to 0 °C produces few, large crystals and a coarse, low-resistance dried structure, whereas deep supercooling produces many small crystals and a fine structure that impedes vapour escape.^[9][10]

Because nucleation is stochastic, vials within one batch nucleate at different temperatures and therefore dry at different rates. This is a principal source of vial-to-vial variability in residual moisture and cake appearance, and it is the reason controlled-nucleation techniques — depressurisation, ice fog, or brief pressure cycling — have been developed to force all vials to nucleate within a narrow window.^[10]

As ice forms, solutes are excluded from the crystal lattice and concentrate in the remaining liquid. This freeze concentration continues until the residual phase either crystallises at a eutectic temperature or becomes so viscous that it vitrifies. The temperature at which the maximally freeze-concentrated amorphous phase undergoes its glass transition is written Tg′ and is a formulation property, not a process setting.