Jim Simmons is the Senior Director of Education at SpectrumLabs.com. He has over 30 years experience in membrane separations and spent 15 years as the Regional Sales Manager for the central United States. Jim now conducts the Spectrum Lab’s Training and Education Program.
In this installment, we address different membrane chemistries (sometimes called membrane media) commonly used in Tangential Flow Filtration (TFF) for bioseparations.
When characterizing a TFF process, often the first task is to select the membrane media. The two most common classes of membrane media used in bioseparations are cellulosic and synthetic membrane. Most solutes used in biotech applications are aqueous, and when considering media selection, one must begin with a consideration of the relative hydrophobicity or hydrophilicity of the membrane media.
Cellulosic Media is generally hydrophilic – aqueous solutes pass freely.
- Regenerated Cellulose (RC), commonly found in dialysis and flat TFF membranes.
- Mixed Ester (ME), commonly found in hollow fiber microfiltration membranes.
Cellulosic membranes are characterized by a more narrow range of pH and chemical compatibility.
Synthetic membrane is often characterized by a wide pH tolerance and chemical compatibility, high rigidity, strength and stability, high thermal and oxidative stability (can tolerate ~ 126°C), and compatibility with gamma irradiation. Commonly found in all membrane designs, synthetic media is widely used in TFF.
Polysulfone (PS) is hydrophobic. It is necessary to ‘temporarily convert’ the membrane surface from hydrophobic to hydrophilic in order to use the membrane with aqueous solutions. This is often accomplished by rinsing the membrane with alcohol, such as IPA. If this step is not performed, polysulfone membrane performance will be greatly diminished.
Another characteristic of hydrophobic media is that proteins/antibodies tend to bind to the membrane surface. Bound proteins form a ‘boundary (gel) layer’ on the membrane surface that generally results in the pore size becoming tighter and the process becoming much slower. Proteins bound to the membrane surface cannot be recovered, resulting in lost yield. Typically, polysulfone media binds proteins very tightly, particularly if wetting agents are not used. Use of wetting agents greatly improves binding issues with PS membrane.
- Binding mechanisms include
- Electrical static charge
- PS typical binding ~ 3 – 10 µg/cm2 with wetting agents and ~100ug without wetting agents
- Binding is irreversible
Polyethersulfone (PES) and Modified Polyethersulfone (mPES) are hydrophilic. Since hydrophilic surfaces pass aqueous solutions freely, hydrophilic membranes are easily rinsed with water and do not require a special wetting step. In sterile applications, irradiated mPES hollow fiber modules can be used without an initial rinse, enhancing options for disposability.
Another characteristic of hydrophilic media is that proteins/antibodies do not tend to bind to the membrane surface.
- mPES typical binding < 3 µg/cm2
Solute passage is defined as the relative efficiency or inefficiency with which the soluble species passes through a membrane. Solute passage has a great impact on the TFF process. As solutes pass through different media materials with different efficiencies it is necessary to select a membrane material that will pass the soluble species as freely as possible. If less-than-ideal solute passage is unavoidable, consider increasing membrane surface area to allow for enough solute passage for acceptable processing times.
- Most hydrophilic media will freely pass aqueous solutes, but it is prudent to understand other constituent species that may be present in the solute may inhibit passage. Surfactants and detergents can ‘coat’ membrane surfaces, resulting in reduced solute passage, so if possible, avoid the use of these materials. This will be discussed in greater detail in the next installment.
- How effectively solute passes through the membrane determines the number of buffer volumes (diavolumes) required to complete the wash. This greatly impacts processing time and overall efficiency.
In the next installment, we will discuss the general categories of TFF applications, and begin learning the glossary of terms used in designing a TFF process.