Cryopreservation vs. Lyophilisation: Decoding the glass transition phenomenon

In the realm of preserving delicate biological substances and pharmaceuticals, two prominent methods take centre stage: cryopreservation and lyophilisation. Cryopreservation involves either controlled or rapid freezing of biological materials, while lyophilisation focuses on removing water through freeze-drying. At the heart of these techniques lies a crucial phenomenon known as the glass transition – a pivotal factor influencing the stability and longevity of products.

The glass transition concept plays a critical role in both methods, ensuring optimal freezing rates, storage temperatures and structural preservation. Understanding Tg and Tg' empowers scientists to safeguard product integrity and longevity in the demanding realms of cryopreservation and lyophilisation.

Understanding the glass transition

Before looking at the different techniques and their applications it is important to understand the role that Tg’ plays in ensuring material does not degrade or degenerate over a specific time. In essence the Tg’ is the temperature where mobility change occurs making the rigid material start to become flexible (rubbery). When this occurs, changes in structure and mobility can have an effect on the stability of the material, so fundamentally this becomes our zero temperature to measure safety margins for temperature effects on the stored material.

Measurement of glass transition:

How do we measure Tg’? The common process would be differential scanning calorimetry (DSC) which will show changes in heat flow of material against a reference product going through various temperature changes. However, the heat flow associated with the glass transition is typically very small and in the presence of ice, can be difficult to detect, even with a good DSC.  Other thermal methods are used, such as Impedance analysis, particularly in the pharmaceutical industry, since it allows for visibility of more discrete mobility changes. Dynamic mechanical analysis (DMA) can also be used to determine the Tg’ by measuring the stiffness change under varying temperatures.

Lyophilisation uses the critical frozen state Tg’ measurement during the freeze drying process, which provides a method of removing as much moisture from the material as possible, leaving a product which typically can be preserved (stored) at high temperatures prior to reconstitution. Key elements to secure a high dry state Tg within the product is the knowledge of excipients together with the effect of the freeze-drying process on the material. A large majority of pharmaceutical material uses Lyophilisation to provide a stable product which can be safely stored and transported at above ambient temperatures.

Cryopreservation on the other hand, as the name implies, brings the material down to ultra cold temperatures below the natural Tg’ of the material to ensure its viability. Materials which need to maintain their structure cannot typically be lyophilised and therefore cryopreservation is the safest way to maintain long term stability of the material. Cryopreservation is typically employed to store whole cells and tissues which by nature contain a large water content therefore using the Tg of water provides a safe guideline to the material itself.

In conclusion, the glass transition phenomenon is a critical factor in both cryopreservation and lyophilisation. By measuring the glass transition temperature through techniques like DSC and DMA, researchers ensure the stability and integrity of biological and pharmaceutical products, ushering in a new era of innovative preservation techniques that reshape the landscape of scientific preservation endeavours.

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Key areas in Lyophilisation.

• Freezing temperature and cooling rate
• Start temperature for Primary drying.
• Start temperature and warming rate for secondary drying.
• Strength of the Lyophilised material

Key areas in Cryopreservation.

• Freezing temperature and cooling rate
• Storage temperature
• Storage conditions (time?)
• Transient storage temperatures
• Shipping

When looking at these different techniques for preserving material there are key areas for both applications and we will look at freezing rates in our next article.

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