The evolution of single-use bioreactor (SUB) systems in biologics manufacturing has been rapid and influential. Reliance on efficient and flexible bioreactor technology will be critical for reducing scale-up costs and facility capital expenses as well as aggressively growing biotherapeutic and monoclonal antibody production. The biopharmaceutical industry has seen considerable growth in the proliferation of single-use bioreactor platforms as well as manufacturing areas where these systems are applied.

Need for Improved Single-Use Sensors

As disposable technologies are widely accepted and established, focus on fully integrated SUB platforms in GMP manufacturing turns toward compatibility of related technologies. One potential adoption-limiting factor is the availability of reliable, accurate, and low-cost sensors that meet “disposable” requirements, interface seamlessly with existing SUB platforms, and control systems without breaking sterility. Early implementation of SUB systems has relied on integrating traditional sensors through complex interfaces while sacrificing effectiveness and cost. In general, single-use sensors (e.g., those for pH, dissolved oxygen, temperature, pressure, and possibly for cell density) have not tracked the development path for the SUB suite of options. Ideal sensors would be preinserted into SUBs and compatible with gamma irradiation, thereby allowing both container and sensor to be sterilized and delivered as a single unit. Another opportunity is to develop sensors that can be precalibrated or that are self calibrating, thereby minimizing operator interaction and accelerating time to operation. Collaborations among suppliers of disposable containers and developers of sensor technology could lead to single integrated, qualified systems with full support for installation, training, validation, certification for all components, and service and maintenance programs. Another benefit would be to extend such technologies to downstream filtration applications, with support provided for optimizing a complete end-to-end approach.

Sensor Evolution in Progress

Traditional glass pH sensors can be used with SUBs. However, many systems do not solve some of the known challenges of traditional glass pH sensors such as their drift from calibration over the duration of typical bioreactor runs, breakage, and general handling problems related to wet storage and calibration. Currently the gaps in disposable sensor technology for single-use applications focus on the ability to measure cell-based parameters such as cell density and viability.

Alternatives to traditional glass probes are available, particularly for SUB applications. Much focus has been given to optical sensors for monitoring both pH and dissolved oxygen. For example, PreSens (www.prens.de) sensors measure phase fluorimtery using a combination of organo-metallic dyes with measured fluorescence intensity changes over time with pH changes. Such detectors can be provided in very simple spot formats of various sizes. Inserted into clear vessels, they communicate through a fiberoptic sensor located outside the vessel. Similar chemistries have also been integrated into a disposable sheath that can be inserted into a single-use bag and sterilized in situ. That requires use of an optical reader and transmitter. Such probes are distributed by PoleStar Technologies, Inc. and Finesse. Although their systems come precalibrated to NIST-traceable standards, they do require users to “prime” the system with the precalibration information that is shipped along with individual probes. There is also the possibility of implementing one-point standardization or two-point calibration. Further, these systems are sensitive to a solution’s ionic strength. All fluorescence-based probes undergo photobleaching, which limits the number of interrogations, attenuates the signal, and requires that the systems be kept in the dark. Finally, these systems operate best over a narrow pH range, restricting them to upstream applications.

Sensorin developed a solid-state sensor based on amperometric measurement for pH sensing and for other analytes. The system has a proprietary reusable electronic component that interfaces with existing pH meters and controllers and disposable probes. The flat format of the probe allows seamless introduction into single-use vessels. Two working silicon-based electrodes work in tandem: One is a pH-sensitive measurement electrode and the other a pH-insensitive reference electrode. They are incorporated onto the surface of the probe and thus experience the same environment within a reaction vessel. Signal from the pH-insensitive electrode remains constant within a solution, whereas signal from the pH-sensitive electrode varies as a function of proton concentration. The potential difference between the two electrodes is converted to a pH measurement. Because measurements are made simultaneously within each electrode at each measurement point the system is inherently self-calibrating. Similarly any factor that may typically cause drift (e.g., temperature or pressure) will affect both electrodes similarly, so the potential difference remains constant and the output is stable.

Continued adoption of SUB technology will lead to adaptation to larger volumes to match standard large-scale, stainless-steel reactor systems and to further maturation of the technology into more GMP manufacturing facilities. Similarly the optimization of single-use systems for downstream applications allowing end-to-end use of single-use systems is causing a major shift in the way pharmaceuticals are produced. Continued development and refinement of single-use sensor technologies will complement vessel development and speed adoption of single-use production technologies.

About the Author

Author Details
Jim Wilkins is chief technology officer at Sensorin, Inc., 863A Mitten Road, Burlingame, CA 94010; 1-650-552-9324, fax 1-650-552-9327.