introduction
As the biopharmaceutical industry increasingly demands higher production efficiency and flexibility, traditional fed-batch culture methods are facing bottlenecks in certain applications. Perfusion culture, by continuously adding fresh culture medium while simultaneously removing it, maintains cells in a high-density, high-activity state for extended periods, significantly increasing product yield per unit time and volume. This technology is particularly suitable for producing unstable proteins, products requiring complex post-translational modifications, and continuous biomanufacturing platforms designed to reduce facility footprint. In recent years, with the maturation of cell retention technologies and automated control strategies, perfusion culture is steadily progressing from laboratory and small-scale production to clinical and commercial manufacturing.
I. Innovation and Selection Strategies of Cell Retention Technology
Cell retention is the foundation for the stable operation of the perfusion process, and its reliability and efficiency directly determine the success or failure of the process.
1.1 Tangential Flow Filtering (TFF)
Transmembrane flow filtration (TFF) is the oldest retention technology. Its principle involves tangential flow of the culture medium at the membrane surface, using the pressure difference to allow small molecule metabolites and products to permeate the membrane, while cells are retained. Recent advancements focus on improving membrane materials (such as hydrophilic modified polymers) and modular designs (such as hollow fibers and flat membrane packs) to reduce membrane fogging and cell damage. Precise control of periodic backflushing and transmembrane pressure (TMP) is crucial for maintaining long-term TFF operation. Studies have shown that optimized TFF systems can achieve stable perfusion operation for over 60 days, maintaining cell viability above 90%.
1.2 Alternating Tangential Flow Filter (ATF)
The ATF system operates through a hollow fiber membrane module and a reciprocating diaphragm pump, enabling the culture medium to flow alternately in both directions inside and outside the membrane. This design effectively flushes the membrane surface, reduces cell deposition and concentration polarization, and significantly extends membrane lifespan. ATF has become one of the mainstream retention solutions in current perfusion processes, and its scale has expanded to 2000-liter bioreactors, suitable for all levels of production from clinical to commercial applications.
1.3 Centrifugal sedimentation device
This device utilizes gentle centrifugal force to separate cells from the culture medium. A representative technology, such as the Centritech™ system, achieves closed-loop aseptic operation through disposable centrifuge bags. Its greatest advantage lies in almost completely avoiding cell damage and membrane clogging caused by shear forces, making it particularly suitable for shear-sensitive cell lines. However, its processing capacity typically has a certain upper limit, making it more suitable for pilot-scale or certain-scale commercial production.
1.4 Acoustic cell retention
Acoustic sedimentation technology utilizes standing sound fields to cause cells to aggregate and settle at nodes, achieving contactless and shear-free cell retention. This technology completely avoids the use of membranes, fundamentally eliminating the risk of clogging, and is easily scaled up linearly. Although continuous optimization is still needed in terms of ultra-high flow rate processing capabilities, it represents an important future direction for cell retention technology.
Choosing a strategy requires comprehensive consideration. Factors considered include: cell line characteristics (shear sensitivity, aggregation tendency), target production scale, process duration, cost, and familiarity with the technology platform. Parallel evaluations are typically required early in process development to determine the most suitable retention scheme.
II. Key Parameters for Injection Process Development and Optimization
A robust infusion process requires precise control of several interrelated parameters.
2.1 Perfusion rate and cell-specific growth rate
The perfusion rate (typically expressed as the number of exchanges per day in the reactor working volume, VVD) is a core control parameter. Its setting must be matched to the specific growth rate (μ) and metabolic consumption rate of the cells. A fixed perfusion rate strategy is simple to operate but may result in culture medium waste or nutrient limitation. The current trend is towards dynamic control strategies based on process parameters, such as:
Based on live cell density (VCD) Feedback adjustment is performed based on the required culture medium volume (pL/cell/day) per unit cell density.
Based on metabolite concentration By monitoring the concentration of key nutrients (such as glucose and glutamine) or metabolic waste (such as lactic acid and ammonia) online or offline, the perfusion rate can be dynamically adjusted to maintain the culture environment at its optimal window.
2.2 Decoupling of cell growth and product production modes
A major advantage of perfusion technology is its ability to decouple the cell growth phase from the product production phase. Typically, the target high cell density (e.g., 50-150 x 10^6 cells/mL) is achieved first at a high growth rate, and then the cells are maintained at a lower growth rate or even in a quiescent phase by adjusting culture conditions (e.g., lowering the temperature, using production-enhancing media), allowing more cellular metabolic resources to be devoted to the synthesis of the target product. This strategy can significantly increase the yield per unit cell (Qp).
2.3 Culture medium design and waste management
Perfusion media require specific optimization, and their composition typically differs from fed-batch media. Because waste is continuously removed, critical nutrient concentrations can be maintained at low, but non-limiting, levels. This is not only more economical but also reduces the inhibitory effects or adverse modifications that could be caused by high concentrations of metabolites. Lactic acid metabolism is a key focus; optimized processes often achieve net lactate consumption, thereby maintaining a more suitable pH environment.
III. Challenges and Strategies from Process Development to Scale-Up
Successfully transferring laboratory-scale infusion technology to production scale presents a series of engineering and operational challenges.
3.1 Linear Amplification Principle
Scale-up of the infusion process must ensure the consistency of key parameters, including:
Power input per unit volume (P/V) It affects mixing and oxygen mass transfer, and needs to be kept within a reasonable range to avoid shear damage or uneven mixing.
Oxygen mass transfer coefficient (kLa) The oxygen demand of high-density cells must be met, which is usually achieved by adjusting the ventilation strategy (such as bubbling, membrane ventilation) and the stirring speed.
Scale-up of cell retention device It is necessary to ensure that the retention efficiency, cell residence time, and shear stress environment are consistent with those at the small scale. Scale-up is usually achieved by increasing the membrane area (for TFF/ATF) or the number of treatment channels (for acoustic devices), and the performance is then rigorously verified.
3.2 Process Monitoring and Automation
Large-scale perfusion operations have long cycles (typically 30-60 days), demanding extremely high robustness and automation in process monitoring. The calibration and maintenance of online sensors (such as pH, DO, and live cell density probes) are crucial. Furthermore, a robust Process Analysis Technology (PAT) solution is needed, combining offline detection (metabolites, product titers, quality properties) with advanced data analysis (such as multivariate analysis) to assess process status in real time and intervene promptly. Automated management of culture medium connection, sampling, and retention devices is key to ensuring asepticity and reducing operator workload.
3.3 Equipment and Plant Design
The use of single-use bioreactors (SUBs) and single-use retention devices greatly simplifies the scaling up of the perfusion process, reduces the burden of cleaning validation, and improves production flexibility. Plant design must consider the continuous supply and storage of materials such as culture media and buffer solutions during long-term operation, as well as the continuous harvesting of products and the connection with downstream processing. This brings a new paradigm to plant layout and logistics management.
IV. Economic Considerations and Product Quality
Although the equipment investment and initial development costs of perfusion culture may be higher, its economic benefits are multifaceted: volumetric yield can be 5-10 times higher than fed-batch processes, significantly reducing the size of the core production reactor; product quality is often more uniform, with potentially lower impurity profiles (such as host cell proteins and aggregates); and the process offers high flexibility, suitable for multi-product co-production. Regulatory agencies (such as the FDA and EMA) have issued guidelines encouraging continuous manufacturing, providing a policy framework for the application of perfusion technology. Sufficient data must be provided during the application process to demonstrate the stability of the process, the consistency of product quality, and the adequate control of leaching/precipitate risks from the retrieval device over long-term operation.
in conclusion：Key Advances and Future Trends of Perfusion Culture in Biomanufacturing
Perfusion culture technology has become a key enabling technology for improving capacity and flexibility in upstream processes of bioreactors. With the continuous improvement of the reliability of retention devices, the increasing intelligence of process control strategies, and the industry's growing acceptance of continuous biomanufacturing, perfusion technology is expected to play an even more central role in future biopharmaceutical production. Future development will focus on higher levels of integration, automation, and digitalization to achieve end-to-end continuous upstream production from seed revival to final harvest, and combine this with downstream continuous purification technologies to ultimately build a complete continuous biomanufacturing platform.
