The pressure sensors use an in novative microelectromechanical (MEM) chips. MEM technology integrates mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology (1). These chips are manufactured using a silicon piezoresistive sensing element in a Wheatstone bridge circuit, through which an applied pressure gives a proportional output voltage. Before installation into finished devices, the chips undergo an accuracy test and integrity testing that includes a pressure stress test to >100 psi.

The plastic material used to mold the sensor body is either polycarbonate or polysulphone that, along with the other fluid contact materials used in the sensor, meets USP Class VI requirements. The sensors are manufactured in a clean room at an ISO13485-certified, FDA-registered facility. Each individual device undergoes several tests to determine electrical integrity, to confirm the absence of leaks, and to ensure proper calibration within a tight specification range. These sensors are qualified for use ≤75 psi, with burst testing conducted up to 150 psi. Gamma irradiation has been qualified for ≤50 kGy, so the sensors can be preassembled with ready-to-use tubing, filter, and bag assemblies. The accuracy specification of 30 psi (2 bar), +/−3% is sufficient for most clarification process operations (2). All these factors ensure that PendoTECH pressure sensors provide highly accurate pressure monitoring.

Each pressure sensor chip circuit requires a narrow range of applied voltage. The circuit voltage output directly proportional to pressure is not a traditional field output signal such as 4–20 mA or 0–10 V, which gives a higher resolution for analog-to-digital conversions. The PressureMAT system serves as a voltage source and processes the output signal from the sensor into a pressure reading. It is therefore required as an intermediate device to integrate the sensors into a control system for building a feedback control loop. Once the sensors are inserted into a flow path, data will be transmitted to the PressureMAT system at intervals as frequent as 1 data point per second. Pressure data can be viewed on the PressureMAT monitor display. Outputs (4–20 mA) from the PressureMAT transmitter can be brought into a data handling system, which facilitates data recording and processing.

A Case Study

We clarified cell culture harvest from a 1,000-L bioreactor using an LAPX404 disc-stack centrifuge from Alfa Laval (www.alfalaval.com), followed by depth filtration using Pod A1HC filters and 0.2-µm filtration with Express SHC filters, both from Millipore Corporation (www.millipore.com). The cell culture in defined media was harvested after 14 days, showing a viable cell density of 2.96 cells/mL and cell viability of 68%. This process was continuous, with no break tanks in between the steps (Figure 1). Flow rate throughout the clarification train remained constant at 5 L/min. We collected filtrate in 200-L collection containers, each with a 0.2-µm filter attached. As each collection bag was replaced during culture clarification, the 0.2-µm filter was also replaced to reduce the number of interruptions in the overall process and reduce potential bioburden or endotoxin contamination of the final filtrate.

Before obtaining the sensors, we set up stainless steel pressure gauges in front of both filters (Figure 2). We recorded pressure readings manually and calculated the pressure differential as the difference between readings from those two gauges. The stability of the pressure differential at a constant flow rate indicates almost no change on the depth filter's performance throughout the process.

To test the pressure sensors and PressureMAT system, we installed half-inch sensors into the flexible tubing directly upstream of both the depth filter and the 0.2-µm filter without tee connectors. Cell culture was clarified from a 1,000-L bioreactor using the same process train described above. A similar (but not identical) feed stream was used in this particular case study.

Cell culture in defined media was harvested after 16 days, showing a viable cell density of 3.56 cells/mL and viability of 63%. Pressure data were collected using the PendoTECH single-use pressure sensors and PressureMAT system (Figure 3). We took pressure readings (Figure 4), every 30 seconds during processing. The “valleys” in those pressure readings (about every 50 minutes) represent replacement of a 0.2-µm filter and a 200-L filtrate collection bag. Small pressure decreases seen at ~7-minute intervals are due to the discharge of solids from the centrifuge bowl. Each time the centrifuge enters a discharge period, the feed flow is stopped until that discharge period ends, so the pressure differential dropped to zero at those times.