The concept of the F0 value was explained in the previous article F0 Value in Steam Sterilization. Let’s explore the further practical application of F0 value and sterilization cycle development. The automation industry has exploded in recent years and sterilization, which was then carried out manually now being practiced through complete automation.
Variations and uncertainties about the dynamics of SAL values involved challenges in the manual sterilization processes. The temperature control across the system during sterilization was the key aspect. Also, concerns about audit findings regarding the sterility of the finished drugs raised questions on its integrity.
While in the changing times, because of top-end controllers and the much-developed automation industry; it became possible to control the Sterilization cycles through pre-written logic in PLC for effective control mechanism either based on time or F0 value. Let us see through an example, the requirements and development of the automated sterilization cycle further below.
At present, various pharma industries use the following types of sterilizations based on their requirement and suitability.
Sterilization Based on Categories
- Sterilization Out of Place e.g. Autoclave (loose components or pieces that can’t be sterilized at their designated place)
- Sterilization in Place (SIP) e.g. SS Process Vessel (systems that aren’t mobile)
Ultimately, they deal with only one thing, and that is “Reduction of micro-organisms to their predetermined acceptable levels“. To achieve this, systems under sterilization must maintain uniform temperature distribution and proper control over Sterilization Hold Times.
That assures the drug makers about their product’s sterility, safety, and quality. Let us select the second category for illustration, Sterilization In Place (SIP). This will allow us to understand the need and mechanism of sterilization cycle development.
Consider the Stainless Steel (SS) Vessel under sterilization with multiple temperature sensors installed in the system. For simple and overall understanding we’ll consider two, one in the vessel shell (nearest) and one in the drain line (farthest). The figure below depicts the sterilization using a controller. The primary aim of developing an automated sequence deals with the fact that the sterilization cycle is about to end based on certain factors.
Developing Sterilization Cycle
To control the termination of the sterilization cycle automatically, the following conditions exist;
1. Time-based (Cycle ends once the sterilization timer ends)
2. F0 value-based (Cycle ends based on achieved F0 value)
First, we’ll understand Time Based and later the next one.
Time Based Sterilization Cycle
Running a moist heat sterilization cycle typically introduces a recipe from HMI. The example below shows about the different recipe parameters:
As seen above, the user can edit recipe parameter set points. Users can choose from the range as appropriate. As per the recipe, there are three steps of the sterilization cycle viz. Heating, Sterilization, and Cooling explained below;
The steam inlet valve opens for introducing the steam into the system. Heating 1 heats the system to a certain temperature typically up to 100°C. This is to ensure that proper temperature distribution across all the installed temperature sensors in the vessel only. When it reaches the Heating 1 set point, the exhaust evacuation set point timer starts. This step eliminates air pockets by opening exhaust line valves. Valves get closed when the timer is complete. The system is pre-ready for sterilization now.
Post completion of a Heating phase, the Sterilization phase begins with the Heating 2 set point, which is 121.1°C. Once the lowest temperature sensor achieves the Temperature – (minus) oscillation set point, the controller proceeds for the next step, i.e. stabilization.
Stabilization ensures constant temperature distribution across the system. During stabilization, the steam inlet valve pulse (ON/OFF) for temperature control in the system.
Note that, for stabilization and sterilization, the coldest (worst) temperature sensor in the system drives the transition steps. This means once the coldest temperature sensor enters the acceptable range, the stabilization and sterilization timer activates. The sensors update in real-time based on steam flow inside the chamber and in the drain line. The programmer writes logic accordingly.
The steam inlet valve during Stabilization and Sterilization hold times controls the temperature as per the above-mentioned philosophy. Once the Stabilization timer ends, the controller triggers the next sub-step i.e. Sterilization.
As a recipe setpoint, Sterilization Hold Time entered as 30 minutes. Allowed Oscillation entered as 1°C. This means the temperature range considered acceptable when the temperature is between 120.1°C to 122.1°C. When all the temperature sensors enter the acceptable range, the sterilization phase starts and completes after 30 mins.
However, if any of the sensors during sterile hold goes beyond the acceptable range, the sterilization hold timer pauses. PLC then triggers “Low Temperature Monitoring Timer” as per the recipe setpoint. This timer runs for a set time; simultaneously monitoring particular temperature sensor/s enter acceptable range or not.
If temperature sensor/s achieve set temperature again, sterilization hold timer resumes and sterilization then ends after remaining hold time. Sometimes, instead of resuming the timer, some manufacturers prefer to restart the timer to assure themselves about once-through steam penetration for complete hold time without any interruption.
During this monitoring timer, steam inlet valves kept open for allowing steam inside the system. In our recipe, the Low-Temperature Monitoring time is 3 min. During this running timer, suppose at 1 min 30 sec, sensor/s regain the temperature. This skips the remaining timer and the sterilization cycle resumes from the previously held Sterile Hold Timer.
Suppose a monitoring timer has ended, and none of the sensors has achieved the required temperature. Here, sterilization ends followed by cooling. This requires the user to restart the sterilization cycle as the cycle has failed.
Meanwhile, when the temperature during Sterilization hits the Abort Cycle temperature setpoint, the sterilization cycle ends followed by Cooling. This means the Sterilization Cycle has failed.
Now that the sterilization has either completed or aborted, the sterilization cycle further progresses to the next step “Cooling”. As a general practice, there are two sub-steps to perform a Cooling operation.
- Cooling the system by introducing Process Grade air
- Jacket cooling with chilled brine or water (optional).
Cooling via Air flushing serves the following purpose:
- Reduces the temperature of the system
- Helps to remove the condensate in the system effectively
To perform Cooling, Process Grade air introduced through the same path used for sterilization (steam path). This ensures the removal of complete condensate accumulated in the steam path. The system should not over pressurize while performing cooling via air. PLC logic can take Pressure Transmitter feedback with a defined preset value and maintain pressure by actuating or closing the air supply valve to restrict the system from exceeding the set value.
Sometimes, hot (~80°C) Process Grade air also considered for Cooling to remove the condensate effectively. Again, this depends on the pure steam quality at POUs (Point of Use) of individual industry.
Jacket Cooling (optional) used when the system needed to bring down to more low temperatures. This cooling method reduces the temperature along with the reduced pressure, which can create a vacuum in the system. This is because instead of pressure, this operation performed via jacket with the help of suitable chilling media such as brine. Pressure must maintain to avoid vacuum formation. PLC suffice this requirement by actuating the pressure supply valve as and when required.
Calculating and Printing F0 Value on Sterilization Report of Time-Based Sequence
Though the sterilization is time-based, the F0 value calculated and printed via appropriate written logic. Reports of the executed sterilization cycle commonly developed through SQL (Standardized Query Language) servers.
SQL receives data from the PLC database and logs that data in a meaningful format as per user-set print intervals. Note that, this data processed simultaneously till the cycle completes. Even though the cycle ends, this data can help in making valid interpretations such as performing calculations and testing success rates of the cycle.
A controller can do computations on the F0 value through pre-written formula logic. The F0 value may get digitally printed on the report. Despite time-based steam sterilization, the calculated F0 value can help in future investigations and analysis.
So far we have seen the Time Based Sterilization Cycle. Now let’s understand what fresh approach considered in F0 Based Sterilization Cycle below.
F0 Based Sterilization Cycle
Consider the same Recipe parameters shown above replacing Sterilization Hold Time with F0 value. Also, the abort conditions mentioned in the recipe screen becomes invalid here. This is because the temperature fluctuations automatically get adjusted for lethal doses during sterilization and associated F0 value calculations. For this cycle, consider the F0 value as a setpoint i.e. 30 minutes.
Remember, proper calculations become crucial to control the sterilization cycle based on the F0 value over the specified time. F0 calculation can either start from Heating 1 or from Heating 2 till Sterilization ends. Some developers consider only Heating as a single-step rather than dividing it into Heating 1 and Heating 2 based on their preference and system suitability.
There are various ways by which PLC can perform F0 value calculations.
- Upon completion of the sterilization cycle, PLC calculates the average F0 value from each probe. Therefore, bracketing between sterilization start and end times is practiced to avoid impractical approaches.
- Calculate the individual lethal rate for each time interval for each temperature probe and then calculate the individual F0 value for that time interval. Likewise, follow the same pattern for other probes. Once all the probes have their final F0 value, then averaging them would result in the final F0 value.
- Calculate the lethal rate for each probe. Average it. And then calculate the F0 value.
Either way, we bound to witness slight variations in the final F0 value. To keep things simple, we’ll stick with the 1st way.
As seen in above recipe parameter screen, “Allowed Oscillation” set as 1°C.
- If the F0 value calculated from 1°C lower than the coldest (worst) temperature sensor entering the Sterilization set temperature, the lethal doses accounted for the F0 (minutes) setpoint, i.e. only sterilization phase.
- If F0 calculated from 100°C for the coldest (worst) temperature sensor entering the Sterilization set temperature, the lethal doses accounted for Heating 1 + Sterilization + Cooling phase.
These doses are taken into account for determining the successful termination of the Sterilization phase. Professional practitioners consider the 1st case as a sound practice for sterilization of hard or porous material. Though, 2nd case recommended for heat-sensitive materials.
Note: To give F0 value a biological meaning, steam must be in a saturated condition. Not super-heated and not sub-cooled. Remember, the right moisture does the right job.
Controlling Sterilization Cycle Using F0 Value
To calculate the F0 value, it is important to take all the temperature sensors into account individually. For that, the PLC/Controller accumulates the values of each temperature probe every second or less in the back-end. This data then used to compute the F0 value and control the sterilization cycle accordingly.
Though Fedegari also explains this approach systematically and in-depth on a theoretical level, let us break down what a PLC may perform in this scenario:
- For every second or less, the PLC/Controller takes values from each temperature probe (once the temperature reaches the sterilization temperature setpoint). In our case, two probes.
- It then averages them for each probe.
- It uses these values to calculate the imperfect F0 value for that probe.
- It then adds that value to the already gathered F0 value for that probe i.e. expected F0 value.
PLC/Controller performs these calculations based on pre-written logic as per provided formulae.
Briefing the above part,
Sterilization begins when the lowest (worst) temperature probe reaches the sterilization temperature set point. Sterilization end once the lowest temperature probe has hit the F0 value setpoint (in our case 30 min).
On an industrial scale, power back-up systems are in place to avoid any loss or damage that may occur to product quality. When power failure (3-phase) occurs and the sterilization cycle is in progress, PLC/Controller remains active on UPS (Single Phase). F0 value in background calculated and cross-checked with the preset value.
Note: “3-phase supply failure doesn’t affect sterilization cycle. This is because of no 3-phase related movements”.
We are here so far, considering sterilization in-place cycles (remember? we considered 2nd case for easy understanding). For Sterilization Out of Place such as Autoclaves, the control philosophy remains the same with some additional hooks like vacuum pumps and so on.
Either SIP or Autoclaves, all of them require system validation prior to conducting commercial runs. We’ll discuss this validation approach sometime later. For now, let us review the key highlights to better conclude our above understanding.
- F0 Based Sterilization cycle is more meaningful than Time based Sterilization because of its more practical approach. This is true for less heat-sensitive material under the sterilization.
- F0 value calculation used either for analysis purpose or controlling the sterilization cycle as per requirement. The ultimate aim, however, lies in achieving sterile product containers or accessories.
- PLC must be capable to take the temperature values for every second or less for proper F0 value calculations in real-time.
- Failure conditions are accounted for the development with control measures. Interlocks need to be in place to avoid cross-mixing of different utilities (such as Process air and Steam).
- The PLC takes care of all the required data and calculations. Though it is necessary to double-check the physical readiness of the system before initiating sterilization.
- We must consider sound and practical recipe management for the sterilization cycle as per the user requirements right at the trial stages. Remember, the ultimate aim lies in eliminating the micro-organisms.