Sterilisation by Membrane Filtration

Can you do terminal sterilisation by membrane filtration?

No, because the product cannot be sterilised while sealed in its final container.

Is sterilisation by membrane filtration an aseptic process?

Yes, because it is not a form of terminal sterilisation, and therefore needs to be conducted in a cleanroom under ISO 5 conditions. Aseptic transfer techniques must also be used.

What are the four steps involved in sterilisation by membrane filtration?

1. Filtration of the solution through a suitable, sterile, bacteria-retaining filter.

2. Aseptic distribution of the filtered solution into previously sterilised containers. This step includes a measurement phase.

3. Aseptic closure of the containers.

4. Testing of the batch for sterility.

What is the purpose of prefiltration?

Prefiltration extends the working life of final containers.

What is a depth filter, and what is it used for?

• Pads containing compacted polypropylene, cellulose or glass fibres

• Retention efficiency depends on the nominal pore rating and the selective adsorption of bacteria to the filter material

• Can retain a larger number of particles than a screen filter

• The pores are much larger than the particles removed, but the channels are of uneven diameter and change direction frequently within the filter bed

• The maximum pore diameter and retention efficiency of the filter determine the size of particles retained

• High dirt-holding capacity

What type of pressure needs to be used when using a depth filter?

• Filtration may be carried out under vacuum or positive pressure

• Uniform pressure is important

• Pressure differential should not exceed 35-70kPa because this would cause compression of the mat, decreasing the flow rate

What are the disadvantages of depth filters?

• A significant volume of the liquid may be lost by retention in the filter pad

• Components of the liquid (e.g. proteins) may be adsorbed to the filter, reducing their concentration in the filtrate

• Filter fibres may be shed into the filtrate

• Penetration of microorganisms depends on the time required for filtration, and some may eventually pass through, especially if pressure differentials increase

How can we minimise the effect of depth filter fibres passing into the filtrate?

Reject the first portion of filtrate that passes through the filter, or use pre-flushing.

What are screen filters?

• Also known as membrane filters

• Microporous films with specific pore size ratings

• Membranes retain particles or microorganisms larger than their pore size, primarily by surface capture

What is the sterilising filter in a filtration process?

Where multiple filters are used to increase the efficacy of the filtration process, the filter closest to the filling point in the final container is the sterilising filter.

What is the pore size required for sterilisation by membrane filtration?

Not greater than 0.22 microns.

What are some disadvantages associated with screen filters?

• Low dirt-handling capacity

• Easily clogged by particles slightly larger than the pore diameter

• “Air locking” of filters

What are some advantages of screen filters?

• All particles larger than the pore diameter are retained, and bacteria cannot pass through

• The volume of liquid retained in the membrane is small, and can be expelled into the filtrate without breaking sterility

• The quality of the filtrate is usually not altered by adsorption of solutes or contamination with foreign material (e.g. shedding of filter fibres)

What are four types of screen filters?

• Hydrophilic filters

• Surfactant-free filters

• Low protein binding filters

• Hydrophobic filters

What are hydrophilic screen filters typically made of?

Cellulose esters, such as acetate and nitrate.

What are hydrophobic filters typically made of?

• Polytetrafluoroethylene (PTFE)

• Polyvinyl chloride (PVC)

• Polyvinylidene difluoride (PVDF)

What are hydrophobic filters typically used for?

Vacuum line protection, sterilising gases, venting sterile containers, and sterilising or clarifying organic solutions.

What is a hydrophobic edge membrane?

A hydrophilic filter with a hydrophobic rim. This type of filter means that the rim of the filter will not get wet, which is an advantage in sterility testing.

What are vacuum filter holders and pumps?

A vacuum operated unit in which all components have to be sterile. The product is placed at the top, and the filtrate is collected at the bottom.

What are pressure filter holders?

A pressure-operated unit with a sealed tank, increasing the flow rate of the product through the filter. The filtrate is collected in a sterile vessel. The supply side is closed to provide pressure, and the delivery side is open.

What is an integrity test for an assembled sterilising filter?

A verification test performed before and after use. Types of integrity tests include bubble-point, pressure hold, or diffusion rate tests.

What is the bubble point of a filter?

The bubble point is the pressure required to force an air bubble through the pore of a screen filter for a given fluid and pore size. This pressure is inversely proportional to the size of the hole.

Is the bubble-point test a destructive test?

No

What is the procedure of a bubble-point test?

1. Wet the filter with an appropriate fluid. This is typically water for hydrophilic membranes, or an alcohol/water mixture for hydrophobic membranes.

2. Pressurise the system to about 80% of the expected bubble point pressure.

3. Slowly increase the pressure until rapid, continuous bubbling is observed at the outlet.

What are some of the possible reasons when the bubble point value is lower than the specification given by the manufacturer?

• A fluid with a different surface tension was used, compared to the recommended test fluid

• Integral filter, but the wrong pore size was chosen

• Higher temperature than the recommended conditions

• Incompletely wetted membrane prior to conducting the test

• Non-integral membrane or seal (broken filter)