How to use temperature to assess if primary freeze drying is finished

The freeze-drying process is infamous for how much time the process consumes. Rather than having the time effect deter you from performing lyophilization, I’ve tried to give you some ideas on how to speed up the entire process. This time around, I’d like to zoom in on the most time-consuming part of freeze drying, the primary drying step. This post presents an effective approach for endpoint determination that shortens the time the intensive primary drying step requires.

A couple of weekends ago, we were having some friends over for a dinner party. We have one of those robot vacuum cleaners and we decided that it would be helpful to let the machine clean while we were grocery shopping. But once we came back, we found that the robot had shut himself off in the kitchen and had run out of battery power cleaning the same area repeatedly.

Well, I was not so amused. The robot had to be re-charged, so we couldn’t really use it anymore. So the other cleaning robot, me, had to step in and get the job quickly done with our regular vacuum cleaner. I couldn’t help but think of how much more economical and time-efficient it would be if the robot could turn itself off once it has vacuumed a certain area, even it couldn’t reach its charging station.

Well, automatic determination of an endpoint can greatly improve not only your robot vacuum at home, but also your freeze dryer in the lab.

I’ve already discussed ways to speed up lyophilization, ranging from shell freezing to heatable shelves to pressure gradients. Now I’d like to focus on one particular part of the freeze-drying process.

Think about it, primary drying is by far the longest step of the lyophilization process. It would undeniably bring great benefits if there was a way to decrease the amount of time this step requires, while ensuring the process is not terminated too early.

If my vacuum robot stops cleaning before he’s covered the whole floor, there would still be dirt spots resulting in a low-quality performance. Similarly, if we proceed to secondary drying before all the ice is removed from the product, we will likely end up with product defects such as collapse or eutectic melt.

The time it takes to clean the floors varies, as it is influenced by the amount of furniture, objects and dirt lying around. Similarly, the time required for the primary drying step during freeze drying varies. This time is affected by several parameters such as sample concentration, sample size and sample container.

An automatic stop to the cleaning process would save battery life, electricity and prevent loss of time. Similarly, an automated process to detect the endpoint of primary drying during each run would save time and resources.

Lucky for us, several analytical methods exist to assess the endpoint of primary drying.

The most basic and frequently used method of automatic endpoint determination involves measuring product temperature using thermocouples.

After this measurement, we compare the product temperature to the set temperature of the shelf on which the product sits. The product temperature is colder than the shelf temperature during sublimation, since heat from the shelf is needed for the phase change to occur. When sublimation of the ice is complete, the product temperature will increase and approach the value of the shelf temperature. Once product temperature equals shelf temperature within 1°C difference, primary drying is considered complete.

Importantly, the vial containing the thermocouple is not representative of the entire batch. As the wire conducts more heat, the sample in contact with the sensor will typically dry faster than the rest of the batch. In bulk drying, the area around the thermocouple will similarly dry more quickly than other areas of the tray. We should then add some additional drying time  after the thermocouple temperature matches the shelf temperature. The additional drying time ensures that the ice in the entire batch of product has been completely removed.

Typically, between several minutes to a few hours of extra time need to be added, depending on sample characteristics.

Thermocouples should always be placed in the center and at the very bottom of the container (see figure below). This is because the product dries from the top down. If drying in vials, you should consider placing the vial containing the thermocouple in the middle of the shelf. This avoids edge vial effects which cause the product in the vial to dry quicker than the rest of the batch.

freeze-drying, endpoint determination, temperature sensor, thermocouple, dried sample,

But what happens if you go ahead and perfrom the lyophilization run without endpoint determination? Let us look at some comparison experiments.

We ran two experiments using solutions of 10 wt% Trehalose. The method we set-up in the freeze dryer software involved a primary drying phase of 72 hours (see table below). The method was run with and without active endpoint determination. For the active endpoint determination, we programmed the software to start checking if the requirement of ΔT≤ 1 °C was met one hour after the start of the primary drying. The test passed if the difference of temperature of 1 °C or less between the shelf and the sample stayed constant for at least 20 minutes. Once this occurred, the software was programmed to automatically switch to secondary drying.

Parameters for a freeze-drying process with and without endpoint determination

PhasePrimary DryingPrimary DryingSecondary DryingSecondary Drying
Duration [hh:mm]05:0072:0001:0001:00
Shelf Temp. [°C]20.020.025.025.0
Shelf Temp. gradient [°C/min]0.170.000.080.00
Pressure [mbar]0.1000.1000.1000.050

You can find the temperature profile of the process with endpoint determination (green line) and without endpoint determination (red line) in the graph below. After 20 hours, both sample and shelf had reached the sample temperature and the primary drying phase was completed. In the case where no endpoint determination was programmed, the machine kept running for the remaining 57 hours of the method before switching over to secondary drying. In contrast, when we programmed active endpoint determination  in the freeze dryer, the freeze dryer automatically switched to secondary drying after 22 hours, once the sample and the shelf were at a similar temperature (ΔT≤ 1 °C for more than 20 minutes).

freeze drying, lyophilization, primary drying, secondary drying, endpoint determination, temperature profile

The results clearly demonstrate that

endpoint determination via temperature can optimize the freeze-drying process so that it runs for the smallest possible amount of time, helping increase productivity and save time and resources.

It might be then wise to invest in a freeze dryer with automatic endpoint determination of the primary drying using temperature measurement that can be directly programmed on the method. In such cases, the system measures temperature differences throughout the process. Once the difference is below a previously defined set point (normally around 1°C), the freeze dryer automatically switches to the next phase of the freeze drying process. I would also recommend adapting safety software features that reduce time and help prevent premature switching of phases if sample compositions vary slightly.

Although temperature is the most common method for endpoint determination of the primary drying step, other approaches are available. I will discuss another one of these with you soon. I hope you have re-charged with ideas after this post and keep coming back for more.

Till next time,

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