Rapid advances have been made in the adoption of single-use technologies over the last decade in many small-scale biopharmaceutical processes. However, it is still technically challenging to utilize disposable consumables such as bioreactors, bags, filtration systems, and accessories in large-scale (>6,000 liter) production processes.
The design of new large-scale single-use filter systems has now enabled the adoption of disposable technology in larger scale processes. Economic modeling of these systems has shown the benefits over traditional stainless steel systems, and it is expected that adoption of this technology will accelerate over the next several years.
In 2009, 3M Purification launched the Zeta Plus™ Encapsulated System (EZP), a single-use depth-filtration system that is capable of processing up to 2,000 liter cell harvest streams per system assembly. In bioprocessing applications, depth filters are often employed at the primary recovery step to remove cells and cell debris from cell culture media.
The Zeta Plus product line is composed of charge-modified depth filters produced from cellulose, filter aids such as diatomaceous earth and perlite, and cationic polymer surface modifiers. The product line has a range of product formats and sizes to ensure scalability from the lab to large-scale production. In 2011, a large-scale single-use depth-filtration system, the Automated EZP System, was developed by 3M Purification. The Automated EZP System is a further advancement of the technology that enables the economic application of depth filtration in processes at a scale of 6,000 liters and above.
The Automated EZP System
A key design feature of the Automated EZP System is the capability to be pivoted from the horizontal position where loading and unloading of capsules takes place, to the vertical position where venting, conditioning, and process filtration is performed. The design is flexible to accommodate three to five EZP racks. A rack is defined as a stack of disposable capsules up to seven units high.
Figure 1A shows a 5-around Automated EZP System that holds 5 EZP racks and 35 EZP capsules. This configuration can offer a media surface area of 56.0 m2 for a double-layer design or 87.5 m2 for a single-layer design.
The transition from horizontal to vertical position is powered by an electric motor that is controlled through a programmable logic controller (PLC) interface. Each rack is indexed with an individual electric motor installed at the end of each rack. Pivoting of the entire 5-around carousel to the vertical operating position is controlled through a single electric motor. After loading seven capsules into the rack, a specially designed disposable manifold equipped with tubing and pressure sensors is then installed on each rack.
All mechanical indexing of the racks and carousel, along with pressure sensors, are controlled by the PLC. The PLC, motor starters, relays, and terminals are housed in a stainless steel enclosure mounted to the framework of the system. An operator can control and monitor the entire operation through a 10” touchscreen.
A large-scale system requires reliable scale-up tools for filter sizing, trouble shooting, cost analysis, and process and system optimization. 3M Purification is launching three Zeta Plus scale-up capsules with effective surface areas of 170 cm2, 340 cm2, and 1,020 cm2 in early 2012 (Figure 1B).
Compared to a conventional stainless steel system, a single-use system eliminates clean in place (CIP) and the associated cleaning validation. This results in considerable savings in process time and production costs. These savings will diminish as batch size increases, since the extra costs that an end-user will spend on disposable units over conventional systems will increase faster than the savings on CIP.
To better understand the costs and benefits an Automated EZP System can offer, 3M Purification has developed an economic model based on applications knowledge and the field trial experience of our scientific application support services (SASS) staff.
A total filtration cost analysis was conducted to compare the filtration costs of using a 5-around Automated EZP System and an equivalent conventional stainless steel system. This analysis was based on the batch size of 10,000 L, a throughput of 225 L per m2 of filter media, and a safety factor of 1.25. Other assumptions included various raw material costs such as WFI at $2 per liter, buffer at $2.5 per liter, and labor at $90 per hour.
The capital investment and the associated depreciation costs were not included in the analysis. Several intangibles such as reduction in cross contamination, reduction in system footprint, and cleanliness of the process were not taken into consideration in the economic model.
As shown in Figure 2, the filter cost of the single-use capsules for a 10,000 liter batch is higher than the cost of the filter cartridges in a conventional stainless steel system. However, the savings in CIP, buffer, WFI, and labor relative to a conventional system results in an estimated 7% reduction in total operational cost.
The majority of the savings results from the elimination of CIP and the associated validation work. The CIP calculations in the model include reductions in the use of sodium hydroxide, water, and cleaning indicators such as riboflavin. Additional savings are derived from the reduction in labor associated with the CIP process. The model does not take into account additional benefits such as discharge of waste liquid to the environment, which may require additional treatment.
In some cases Automated EZP Systems can be installed to replace centrifuges, resulting in additional reductions in operational costs.
Double-Layer vs. Single-Layer Depth Filters
An Automated EZP System can accommodate both double-layer and single-layer depth filters. In total, 19 double-layer Zeta Plus EXT Series grades are available, and each capsule has 1.6 m2 of media. Each Zeta Plus EXT Series grade consists of an open-grade filter media over a tighter grade filter media. The open-grade media functions as a prefilter to extend the throughput of the tighter grade.
In most applications, double-layer depth filters are expected to outperform single-layer depth filters. There are 17 single-layer construction Zeta Plus grades available in the EZP large capsule format. Each capsule has 2.5 m2 of media.
A Zeta Plus filter relies on two major mechanisms to remove contaminants. The first is mechanical sieving, which relies on the pore size, depth, and tortuous path to remove contaminants through mechanical interception of contaminants that are too large to penetrate the physical structure of the media. The second is electro-kinetic adsorption, which is a result of the surface modification of the media.
In electro-kinetic adsorption, a depth filter takes advantage of its positively charged surfaces to remove contaminants having a negative charge. In the applications where electro-kinetic adsorption is desired, such as in the removal of the negatively charged viruses, DNA, endotoxins, and lysed cell debris, the double-layer Zeta Plus EXT grades often outperform the single-layer grades. The added depth in the EXT grades increases particle residence time, thus enhancing the electro-kinetic adsorption effect by allowing more time for mass transfer of contaminants to the internal surfaces of the media.
In applications where mechanical sieving plays a dominant role—such as in the first stage of cell culture clarification, where depth filtration is used primarily to remove whole cells—a single-layer Zeta Plus media may perform better than a double-layer EXT grade due to the higher surface area per capsule.
Factors such as cell density and cell viability must be considered when designing a depth-filtration train in cell culture applications, and often a two-stage system utilizing single-layer, higher area capsules, followed by double-layer finer grade media capsules results in better process economics. It is generally recommended that filtration testing be conducted during process development to optimize the depth-filtration train.
The Automated EZP System is a viable solution for implementing large-scale disposable depth-filter technology at process scales greater than 6,000 liters. Modeling of the operating costs at the 10,000 liter scale has predicted benefits that suggest consideration of large-scale depth-filtration systems when designing new bioprocessing facilities.
These systems can accommodate both single- and double-layer media design in disposable capsules, which provide flexibility in the selection of an optimal depth filter train in cell clarification applications. Scale up tools are available in disposable formats to assist the process engineers in the media selection process using small scale trials.