Disposable Plastic Bioreactors & Single Use Devices, The “E & L Challenges”

By Impact Analytical

Historically pharmaceutical companies would generate their monoclonal antibodies using huge stainless-steel vats, instruments that required immense amounts of cleaning and preparation between each use. These vats could be found in every factory, producing varying products each week.  This was until a new process was discovered, the process of using large disposable plastic bags.

It seems unusual to consider plastic bags as being a better way to grow cultures and produce drugs, but they have some natural advantages, they are sterile, tightly sealed and they cut down on the potential for microbial contamination during production.  From a fiscal perspective disposing of plastic bags is not only cheaper than cleaning and preparing a reactor but it is better for the environment as less resources are used to prepare the reactors.

Single-use plastic systems, the types used for drug delivery and manufacturing, can vary in size from 1 liter to 2,500 liters and although they can cost in the thousands of dollars they are still considered cheaper than the traditional cleaning route for the steel reactors; which is estimated to cost between 25-30% more per run.

The application of single use devices is not a fad, in a study conducted by the firm BioPlan associates they found more than 90% of respondents use some sort of single-use component, with more than 70% of respondents using single-use bioreactor bags.

But single-use systems aren’t without their challenges. From a materials science standpoint, the biggest of these challenges is dealing with so-called extractables and leachables—chemicals that migrate out of the plastic and, in the case of bioreactors, into the cell culture inside the bag. Another challenge is the lack of standardization among suppliers of disposable systems and components.

A typical single-use bioreactor is a plastic bag made of a multilayered polymer film. The three main layers are the inside layer, which is in contact with the cell culture; a barrier layer, which prevents gas diffusion into and out of the bag; and an outer layer, which provides mechanical stability. Those layers are “glued” together with tie layers that bridge their disparate chemical properties.

Graphic showing typical layer structure of polymer films used in single-use systems.

PEELING BACK THE LAYERS

Single-use bioreactors are made of polymer films with multiple layers that serve different functions. The most common polymer in the films is polyethylene.

SOURCES: GE Global Research and other companies

The most common material used in making these reactors is polyethylene, which is a well understood polymer, but for it to work in this application it needs to have antioxidants added during the manufacturing process.

Extractables and Leachables are among the biggest concerns of manufacturers today.  These are compounds that under the right conditions might be extracted from the bags.  Extractables are pulled out by using harsh solvents and elevated temperatures, typically considered the worst-case scenario, whereas leachables migrate under normal operating conditions.

In both cases, the compounds are either part of the plastic formulation or are by-products that form when a single-use product is made or sterilized.

One extractable—bis(2,4-di-tert-butylphenyl)phosphate (bDtBPP)—has a particularly bad reputation. bDtBPP is a degradation product of the antioxidant additive tris(2,4-di-tert-butylphenyl)phosphite (TBPP). It typically forms as a result of a combination of the extrusion and sterilization processes.

(Notice the bis(2,4-di-tert-butylphenyl)phosphate (bDtBPP) peak #16)

It is difficult for manufacturers to obtain consistent extractables and leachables testing. Data can come from many suppliers and in many different formats which makes comparison very confusing. It is likely to result in manufacturers pushing a standardized testing package requirement up the chain to their raw material suppliers. These standards are needed because drug manufacturers have in a sense outsourced a large part of their quality control to vendors and the companies that supply raw materials for single-use systems.

Previously manufacturers didn’t care where their raw materials were sourced, but with batches coming from different suppliers and quality variations being inherent the need to better understand and standardize is becoming more important. By working with suppliers and teaching them cGMP requirements a level of control is gained over the quality of the product. This now means, with single-use plastics, biopharma is relying on suppliers to help with quality control.

In order to ensure accurate data, the samples need to be extracted in a range of solvents to cover all possibilities. This might mean a polar, neutral and a non-polar solvent for the extraction process.  Once extracted the samples are analyzed using either GC/MS, LC/MS, or both.  The chromatogram above shows the complexity of the potential contaminants present in the sample.

In order to identify these peaks Impact uses a database with over 1000 known chemicals typically found in the polymer processing industry.  This software allows us to quickly and accurately identify the fragments and thus parent compounds likely to be present.  It is this technique that helps our clients present their products to the FDA faster, enabling them to get to market in a more timely manner.