The CryoSurge phenomenonThe nucleation-induced crystallisation exotherm of a freezing formulation

01Supercooling: liquid below its freezing point

Water-based formulations rarely freeze at their equilibrium freezing point. Cooled in clean containers, they stay liquid well below it — supercooled — until ice nucleates, and nucleation is stochastic: identical containers on the same machine nucleate at different temperatures and times (Searles et al. 2001, PMID 11458335). Heterogeneous supercooling becomes heterogeneous drying, which is why the freezing step governs so much of what follows (Assegehegn et al. 2019, PMID 30529167; Kasper and Friess 2011, PMID 21426937).

02The exotherm: ice announces itself with heat

Freezing releases the latent heat of fusion, about 334 joules for every gram of water that becomes ice (Wikipedia, Enthalpy of fusion). When a supercooled formulation nucleates, ice spreads through the liquid and that heat lifts the product towards the equilibrium freezing point of the freeze-concentrated solution. Searles and colleagues described freezing after more than 5 °C of supercooling as three steps: primary nucleation, a rapid secondary nucleation through the whole liquid, and final solidification (PMID 11458335). The upward excursion of that process — the nucleation-induced crystallisation exotherm — is what Panacea Bio Chem calls CryoSurge™.

03Three landmarks

CryoSurge Onset, T_CS

The product temperature at detected nucleation onset.

CryoSurge Peak, T_CP

The maximum credible product temperature during the natural crystallisation exotherm.

CryoSurge Return, T_CR

The subsequent return towards the onset temperature.

“Credible” is a deliberate word. A probe can be knocked, can sit in a holder that has not nucleated, or can lag the product it touches; S3Pulse records a landmark only from a probe whose reading is believable at that moment.

04Same temperature, different state

Before nucleation the product is a supercooled liquid. After it, ice and freeze-concentrated liquid coexist. The same numerical temperature therefore describes two different physical states, one before the CryoSurge and one after it — and a controller that reads only the number cannot know which one it is looking at. S3Pulse keeps the event in its record so that every later decision knows which side of the CryoSurge the product is on.

05Why the onset matters

The temperature at which ice nucleates sets the size of the ice crystals, and the ice crystals become the pores the vapour must leave through in primary drying. Searles and colleagues found the ice nucleation temperature to be the primary determinant of the primary drying rate (PMID 11458335). Controlling nucleation through chamber pressure, Konstantinidis and colleagues roughly doubled the effective pore radius of a 5 % mannitol cake, from 13 to 27 µm, and shortened primary drying by 41 % in optimised cycles (PMID 21465488); with an ice-fog method, Rambhatla and colleagues found that lower nucleation temperatures gave a higher specific surface area and a higher product resistance (PMID 15760055). Raising and controlling the nucleation temperature is now a field of its own (Geidobler and Winter 2013, PMID 23643793).

06How S3Pulse sees it

On the Lyochrysalis every product holder carries its own immersed temperature probe, and S3Pulse watches each one separately. A CryoSurge is recognised when a probe that has been cooling through the supercooled zone rebounds sharply within a short window; its onset, peak and return are then recorded for that holder alone. The holders of one load nucleate at their own moments, so S3Pulse never assumes that what happened in one holder happened in the others. The instruments →

07Glossary

Supercooling

A liquid held below its equilibrium freezing point without freezing (Wikipedia, Supercooling).

Nucleation

The formation of the first stable ice crystals, from which ice then grows through the liquid.

Nucleation-induced crystallisation exotherm

The release of latent heat when nucleation sets crystallisation going, seen as a rise in product temperature. Panacea Bio Chem’s name for the observed event is CryoSurge™.

Recalescence

The metallurgists’ word for a temperature rise caused by latent heat released during solidification; listed here for readers who know the term from that field.

Freeze-concentrate

The liquid left between the ice crystals, enriched in every solute as water leaves it to form ice.

Annealing

Holding a frozen product at a raised sub-freezing temperature to let the ice and the freeze-concentrate restructure (Searles et al. 2001, PMID 11458336).

Ostwald ripening and glassy-state relaxation

The two processes of annealing: large ice crystals growing at the expense of small ones, and water trapped in the glassy freeze-concentrate crystallising; they run at different rates (Kharatyan et al. 2022, PMID 35745749).

For the physics in more depth — the energy ledger of the freeze and the places where catching the exotherm matters — read the Cryosurge Point essay.

References

1. Searles JA, Carpenter JF, Randolph TW. The ice nucleation temperature determines the primary drying rate of lyophilization for samples frozen on a temperature-controlled shelf. J Pharm Sci 2001;90(7):860-71. PMID 11458335
2. Searles JA, Carpenter JF, Randolph TW. Annealing to optimize the primary drying rate, reduce freezing-induced drying rate heterogeneity, and determine T(g)' in pharmaceutical lyophilization. J Pharm Sci 2001;90(7):872-87. PMID 11458336
3. Konstantinidis AK, Kuu W, Otten L, Nail SL, Sever RR. Controlled nucleation in freeze-drying: effects on pore size in the dried product layer, mass transfer resistance, and primary drying rate. J Pharm Sci 2011;100(8):3453-3470. PMID 21465488
4. Rambhatla S, Ramot R, Bhugra C, Pikal MJ. Heat and mass transfer scale-up issues during freeze drying: II. Control and characterization of the degree of supercooling. AAPS PharmSciTech 2004;5(4):e58. PMID 15760055
5. Geidobler R, Winter G. Controlled ice nucleation in the field of freeze-drying: fundamentals and technology review. Eur J Pharm Biopharm 2013;85(2):214-22. PMID 23643793
6. Kharatyan T, Gopireddy SR, Ogawa T, Kodama T, Nishimoto N, Osada S, Scherließ R, Urbanetz NA. Quantitative Analysis of Glassy State Relaxation and Ostwald Ripening during Annealing Using Freeze-Drying Microscopy. Pharmaceutics 2022;14(6):1176. PMID 35745749
7. Kasper JC, Friess W. The freezing step in lyophilization: physico-chemical fundamentals, freezing methods and consequences on process performance and quality attributes of biopharmaceuticals. Eur J Pharm Biopharm 2011;78(2):248-63. PMID 21426937
8. Assegehegn G, Brito-de la Fuente E, Franco JM, Gallegos C. The Importance of Understanding the Freezing Step and Its Impact on Freeze-Drying Process Performance. J Pharm Sci 2019;108(4):1378-1395. PMID 30529167
9. Supercooling, Wikipedia; Enthalpy of fusion, Wikipedia.