It is no secret that the transition to single-use technology for biopharmaceutical applications is often disruptive. Yet, as the industry continues to see the proven benefits of integrating these technologies, and in some cases fully transitioning to them, the paradigm shift that happened in the upstream is making its way into the downstream.

In the specific case of low volume mixing in the downstream, the go-to solution has always been to use traditional stainless steel mixers. Now that is changing, and although the technology transfer from traditional mixers can be complicated, the transition offers a new opportunity and assurance of integrity that is unmatched by stainless steel mixers. The following discussion investigates how single-use technologies are advancing into the downstream and also investigates the different innovations the market has to offer.

A Paradigm Shift

Merely a decade ago, single-use technology was considered unachievable; but, over time, it flourished in the biopharmaceutical production world. Initially used to replace traditional stainless steel equipment for media storage and make-up, single-use bioreactors have become the go-to solution for cell culture. Yet, downstream purification has long been the one part of the biomanufacturing chain that has not seen the benefits of single-use—until now.

Once fermentation is completed, downstream processing for biopharm manufacturing typically involves sampling, purification, and then resampling to prove the purification has been successful. Mixing is required to create homogeneous mixtures that provide representative samples, and prove their efficacy. However, the smaller volumes and more sensitive materials involved in downstream processes introduce additional challenges to achieve successful mixing.

In the Mix of Things

Most single-use mixing systems work best in the 30- to 500-liter range, and even work with up to 1,000 liters. While this is good for common processes such as buffer preparation, the solutions for mixing just a couple of liters are more limited. It took many years to design away to achieve gentle and effective mixing at low volumes in a stainless steel mixer. When it came time to convert this into a single-use format, the challenge was amplified.

Materials need to be mixed as they drain from the tank during, for example, a final fill operation. Mixing is also a key part of downstream vaccine manufacturing. Adjuvants are often added to the biological component to increase its potency, and because these are rarely water-soluble, continuous mixing is required during the final fill to ensure the correct composition of the final vaccine product.

Effective mixing is also essential for viral inactivation in the downstream. This is a critical step in the manufacture of biologics, as the cells used in cell culture processes can contain viruses that must be removed before the final fill. This is accomplished either by filtering them out, or chemically inactivating them. Mixing is particularly important in the pH inactivation step, as it is vital that every single drop of the product is exposed to the inactivation process. In addition to effective mixing, to prevent portions of the product mixture from remaining activated or having contaminants introduced, a dead leg free drain is necessary.

At least a dozen types of mixers and mixing technologies are available in single-use formats. Recirculation mixers are the most common, particularly for the final fill. A recirculation loop leaves the bottom of the tank, passing through a peristaltic pump and returns the contents back to the top of the tank. This constant motion of the materials results in gentle mixing, and is particularly common in vaccine manufacturing.

However, more efficient mixing is often required for many purification process steps and is usually achieved with some type of agitator. In the stainless steel world, top-mounted shaft mixer technology is common. In the single-use world, magnetic or levitating mixer technology is more common—with an impeller at the bottom of the tank. However, as the volumes drop the impeller functions far less effectively, hence the heavy reliance on recirculation.

Wand mixers provide another alternative and are effective right down to volumes below one liter. This type of technology was invented specifically for single-use systems where a three-dimensional bag sits inside a plastic tank (which gives it a degree of rigidity), and a wand descends into the liquid from the top of the bag. The wand – essentially a bent stainless steel rod enclosed within a silicone tube – swirls gently around in the tank, stirring the solution like a finger. This moves the product around effectively and gently enough to ensure the products are kept homogeneous and in solution, but is not robust enough to mix powders with liquids.

Single-Use Overcomes Risk

The main advantage of single-use systems over stainless steel systems is the fact that the system vessels are disposable. This is a key benefit as a biological product becomes more concentrated in the downstream, rendering it much more difficult to remove residual product from surfaces. In the simplest of terms, the technology removes the possibility of cross-contamination from one batch to the next.

This is especially useful since the closer a product is to the final dosage form, the more critical the processes become and the more sensitive it is to contamination from sterility breaches, particulates, or leachables from the plastics. Because of the increasing value and importance of the product, it is imperative that no additional contaminants are introduced during biopharmaceutical purification.

Of the contamination risks, particulates are especially notable. It is necessary to ensure that bags, tubing and any other single-use components are reliable and do not have any risk of contamination or breach of security (i.e.: holes, tears, or ill-fitting connections). Additionally, it is important to consider and monitor any particulate risk that may arise during mixing.

As noted before, the biggest challenge comes from handling the low volumes towards the end of a biomanufacturing process. Biological products are notoriously sensitive to shear, and much gentler mixing is needed for downstream processes.

Looking Ahead

As single-use technologies continue to advance, more innovation is sure to come. Larger-scale mixers designed to monitor critical parameters such as conductivity and pH are already available. The increasing value of the product as it moves downstream is paving the way for the next generation of technologies that will excel in low shear, low volume downstream single-use mixing.