Issue link: https://www.e-digitaleditions.com/i/1248960
Pharmaceutical Technology BIOLOGICS AND STERILE DRUG MANUFACTURING 2020 19 the process connector's flexible isolating diaphragm provides a sterile barrier to keep the pressure sensor separate from the process media, which eliminates the chance of contamination, while still providing the performance required in these types of applications. The example instrument shows a simple connector design, which allows designers to integrate the combi- nation into a large variety of existing installations, and also into new single-use bioreactor bags. A connector with this design can withstand 7.5 psi when a transmit- ter is not installed, and 75 psi when the sensor is installed, which means for some applications, the sensor can be removed and replaced as needed during production. DO. As with pressure sensors, DO sensors are a mature technology and usually must have some level of contact with the process media. And simi- lar to SUT-capable pressure sensors, it is possible to use traditional sensing methods if they are applied in conjunction with a suitable connector. An example of this would be a connector that is de- signed with an oxygen-permeable membrane window, which isolates the sensor from the process media. This isolation means the sensor can be removed and cali- brated, even during a process run, because the mem- brane can withstand pressures higher than typical bag ratings of 5 to 7 psi. For example, a connector (Rose- mount 550DW Single-Use Dissolved Oxygen Sensor Adapter, Emerson) can withstand pressures up to 10 psi. Even though the membrane introduces a small lag time into the measurement, this concern is insignificant due to the slowly changing rate of DO for these types of applications. Stringent vendor testing of a DO sensor in this service is needed to ensure it will not be de-rated for accuracy or linearity in any way. Because of the special disposable single-use connector, the traditional DO sen- sor can be reused for many batches without the need to be sterilized. Furthermore, the sensor can be removed briefly at any time for calibration. pH. A more challenging analytical measurement for single-use processing is pH. The glass bulb sensing technology for pH must contact the process media, which means for SUT applications, the sensor must be installed into a compatible connector. One chal- lenge for pH sensors is shelf life. Another challenge is calibration. If sensors are stored dry to extend shelf life, they must be wetted with buffer and standardized solutions and then checked for calibration before use. Sensors that have low drift allow long run times with- out recalibration. While running without recalibration is suitable for many applications, it is also possible to per- form longer runs by performing one-point standardiza- tions against an offline sample to adjust for sensor drift. Conclusion Instruments designed specifically for single use maintain aseptic conditions in as economical a manner as possible and provide accuracy and re- sponse to allow users to realize the advantages of single-use production. PT Figure 1: Some pressure transmitters for single-use service (e.g., Rosemount 550PT Pressure Transmitter for single-use bioprocessing applications with a Rosemount 551 Single-Use Process Connector, Emerson) are specifically designed with a process connector incorporating a flexible diaphragm as a sterile barrier. FIGURE IS COURTESY OF EMERSON.