Filtration allows for the exclusion of organisms based on size. There are many types of filtration techniques, but when sterilizing a system membrane filtration is used. To obtain a sterile filtrate it is necessary that the filter and all connecting parts likely to come into contact with the filtrate must be sterile. Contaminants are traps on the membrane filter due to larger size than the pore size of the membrane. It is useful for antibiotic solutions, sera, carbohydrate solutions etc. This method does not destroy but removes the microorganisms. It is used for both the clarification and sterilization of liquids and gases as it is capable of preventing the passage of both viable and non-viable particles.
The major mechanisms of filtration are sieving, adsorption,
and trapping within the matrix of the filter material. Based on the pore size
of the membrane filter, various types of microbes are trapped. Like, 0.01 µM is
used to trap the smallest viruses, 0.025 µM is used to trap larger viruses,
0.45 µM is used to trap the largest bacteria, 1.2 µM is used to trap protozoa
and the smallest unicellular algae, etc.
Some observations are essential for successful
sterilization by filtration:
1. The whole of the apparatus must be sterile.
2. An aseptic technique should be followed in order to
minimize the risk of contamination.
3. The assembly of the filtration unit should be so adjusted
that there is minimum exposure of filtrate to the atmosphere.
4. The filter selected must be fine enough to obstruct the
passage of all bacteria.
Four steps for sterilization by filtration:
1. Filtration of the solution through one of the
bacteria-proof filters.
2. Aseptic distribution of the filtered solution into the
previously sterilized final containers.
3. Aseptic closure of the containers.
4. Performing the sterility test.
There are various types of filtrations used for various
purposes viz. HEPA filters, sintered glass filters, colloidal or membranous
filtrates, earthenware candles etc. The various types of bacteria-proof filters
used are as follows.
1. Ceramic filters: These are also known as filter
candles, made of porcelain or kieselguhr and are available in a range of pore
sizes. Kieselguhr filters are usually softer than the porcelain type. The
candle is placed in the solution to be sterilized and its opening is attached
to the vacuum system. When the vacuum is applied the pressure inside the candle
is decreased. Due to the difference in pressure between the outside and inside
of the candle, the solution moves into the candle. The filtrate is collected in
a sterile container. The main disadvantage of ceramic filters is their tendency
to absorb materials from aqueous solutions.
2. Seitz filter: It consists of two parts. The lower
part holds a perforated disc and the upper part is a compressed asbestos sheet.
Two parts are joined together with the help of nuts. There is a valve on the
upper part through which a vacuum is applied. Due to the fibrous nature of
asbestos pads, they may shed fibres into the filtrate and also absorb drugs
from the solution. Hence, a few ml of filtrate should always be rejected and the
sintered glass disc may also be fixed in the filtration unit immediately after the
seitz filter.
3. Sintered glass filters: They are made from
borosilicate glass. The glass is finely powdered and particles of the required
sizes are separated and are then packed into disc moulds. These discs are fused
to funnels of suitable shape and size. Sintered glass filters are available in
different pore sizes and are numbered accordingly. For bacteria-proof
filtration number 5 or 3 is used. The filtration is carried out under a
reduction pressure. Sintered glass filters do not absorb the medicaments from
the solution. These filters are made from borosilicate glass, so a change in the
pH of the solution does not occur.
4. Sintered metal filters: They are the metallic
counterpart of sintered glass filters. These are usually made from stainless
steel. They have the advantage of having greater mechanical strength.
5. Membrane filters: These are made of cellulose
acetate or cellulose nitrate. These are fixed in metallic holders similar to
those used with asbestos pads. The pore size in the membranes lies in the range
of 100-150 µ. They are also called millipores filters. They are suitable for
sterilizing aqueous and oily solutions but are not suitable for organic
solvents like alcohol, ketones, esters or chloroform.
6. Air filter: (HEPA filter): It is high-efficiency
particulate air originally called High-Efficiency Particulate Absorber (HEPA).
It is used to describe filters that are able to trap 99.97 per cent of
particles that are 0.3 microns (Fig). Air particles are circulated through a HEPA
filter in four directions viz.
(a) Direct Impaction: Large contaminants, such as
certain types of dust, mould, and pollen, travel in a straight path, collide
with fibre, and stick to it.
(b) Sieving: The air stream carries a particle
between two fibres, but the particle is larger than the gap, so it becomes
ensured.
(c) Interception: Airflow is nimble enough to reroute
around fibres, but, thanks to inertia, particles continue on their path and
stick to the sides of fibres.
(d) Diffusion: Small, ultrafine particles move more
erratically than larger ones, so they are more likely to hit and stick to
fibres (Fig).
Advantages:
• The method is suitable for sterilization of thermolabile
medicaments, such as blood products, insulin and enzymes.
• All types of bacteria i.e., living as well as dead, are
removed from the preparation.
• Both clarification and sterilization are done side by
side.
• It is an excellent method for the rapid supply of a small
volume of a parenteral solution in an emergency.
Disadvantages:
• The method is not a reliable one and therefore a sterility
test is necessary.
• The suspension and oily preparations cannot be sterilized
by this method.
• There are chances of absorption of medicaments from a
solution by the filter.
• Defects in the media are not immediately detectable.
• An aseptic technique is necessary.
• Highly trained staff is required.
• The process is only suitable for medicaments which are in
solution form.
Fig: HEPA filter
Applications:
This filtration method is useful for sterilization of
parenteral solutions containing thermolabile medicaments without any
decomposition e.g. insulin, blood serum and other products containing protein
matters, heat sensitive injections, biological products etc.