Sterilization 2

Question 1

Gaseous sterilisation generally reserved for temperature-sensitive items such as:

Answer 1

Medical devices e.g. endoscopes
Pharmaceutical products
Electrical equipment
Infusion giving sets, syringes, plastic containers
Some thermolabile powders

Question 2

Gases used for sterilization

Answer 2

Ethylene oxide
Formaldehyde
Vapour phase hydrogen peroxide
Gas plasma

Question 3

Ethylene oxide overview

Answer 3

Colourless gas
Slightly sweet aromatic odour
Used for sterilisation- biocidal activity

Question 4

Ethylene oxide mechanisms of action

Answer 4

Effective through alkylation (an alkylating agent)
Reacts with amino, sulphdryl, carboxyl and hydroxyl groups, converting them to hydroxyethyl adducts
Results in cross linking within/between proteins and nucleic acids, inhibiting vital functions, leading to cell death

Question 5

Problems with ethylene oxide

Answer 5

Toxicity- acute, mutagenic, carcinogenic
Safe working level in air is 5ppm
Desorption- diffuses readily into many packaging materials including rubber, plastics, fabric and paper
Explosive- in as little as 3% air

Question 6

Ethylene oxide microbicidal activity

Answer 6

Slow process even at high concentrations

As concentration increases, so does aeration time

Question 7

First stage of EO sterilisation

Answer 7

Conditioning- Air can limit EO penetration, therefore draw a vacuum, low pressure, control humidity, temperature
EO is injected
Forced gas circulation used to minimise variations in conditions throughout the chamber

Question 8

Second stage of EO sterilisation

Answer 8

Sterilization
At desired concentration of EO, humidity and temperature, load is held under these conditions (at sub-atmospheric pressure)
EO concentration generally 400-1200 mg/L

Question 9

Third stage of EO sterilisation

Answer 9

Aeration
Gases are evacuated either directly to the outside atmosphere or through a special catalytic exhaust system
Desorption EO from chamber/load by a series of vacuum and steam pulses, followed by vacuum and pulses of sterile air to cool and dry the load

Question 10

EO equipment design

Answer 10

Leak/explosion proof steel chamber
Can be surrounded by a hot water jacket
Normally 100-300 litre capacity
Flammability risks reduced with non-flammable blends of EO

Question 11

EO sterilisation advantages and disadvantages

Answer 11

Advantages: high penetrability, sterilisation of temperature sensitive materials, not degrading to plastic, metals etc., rapidly degraded in the environment
Disadvantages: toxicity, flammability and explosion risks, long cycle times, cannot be used to sterilize liquids

Question 12

Radiation sterilisation overview

Answer 12

Liquids: disinfectants, water, serum, proteins and enzymes
Foods: fruits and vegetables, meats, pre-packaged meals
Devices: pacemakers, implants, needles, syringes
Other materials: plasticware, gloves and gowns, bandages

Question 13

Types of radiation used for sterilization

Answer 13

Gamma rays
Accelerated electron (beta rays)
UV light
Infrared radiation
Advantages of gamma/beta rays: cold process, no pre-conditioning for heat/humidity, no aeration phase required to make products residue/chemical free

Question 14

Mechanisms of radiation sterilization

Answer 14

Ionizing radiation: strip off electrons from the atoms of the material through which it passes
UV: causes excitation of atoms i.e. alteration of electrons within their orbits, does not posses enough energy to eject electrons and create an ion
Microwave: converted to heat when absorbed with solids or liquids
Infrared: transmit energy directly to exposed surfaces or articles

Question 15

Mechanisms of action- ionizing radiations

Answer 15

Energy from this radiation results in ionization, yielding highly active electrons and highly reactive free radicals
Radiolysis of water leads to breakdown into assorted chemicals, electrons and free radicals
Free radical are responsible for structural damage in microbial DNA, unless repaired it will inhibit DNA synthesis or cause errors in protein synthesis

Question 16

Gray and Rad

Answer 16

Gray: unit of absorbed dose, 1 gray is the absorption of 1 joule of energy/kg of material
Rad: 1 gray =100 rads

Question 17

Microbial sensitivity to irradiation

Answer 17

Sensitive (0.2kGy) e.g. gram negative bacteria
Moderately resistant (0.5-0.6kGy) e.g. moulds and yeasts
Resistant (22-3.3 kGy) e.g. most viruses
Highly resistant (4.5-5.0 kGy) e.g. foot and mouth virus, prions (BSE)

Question 18

What are high energy gamma rays used on?

Answer 18

Industrial sterilization of heat sensitive products
Generally applied to articles in the dried state@ surgical instruments, sutures, prostheses, unit-dose ointments, plastic syringes and dry pharmaceutical products
Process commonly performed as a continuous-duty process- products on a conveyor belt are passed through the rays/beam
Dose depends on exposure time (typically 20 hours)

Question 19

Process: high energy gamma rays

Answer 19

Derived from isotopes, spontaneously decay from a high to a low energy over time
Half life of 5.25 years
Emits radiation at two energy levels of 1.33 and 1.17 MeV (higher number = more powerful source)
Gamma rays bombard material resulting in emission of lower energy photons and electrons, emitted electrons cause further ionisation

Question 20

Problems of high energy gamma rays

Answer 20

Some product materials may be affected by the radiation
Destructive process may continue after sterilization finished
Discolouration of some glasses and plastics
Liberation of gases
Hardness and brittleness properties of metals may change
Cost (safety considerations)

Question 21

Filtration sterilization overview

Answer 21

Filtration: not a true biocidal process, removes rather than destroys micro-organisms
Liquid or gas passed through filters, retard the passage of contaminants based on size
Aseptic process- filter and its housing, and all downstream equipment must have been pre-sterilized

Question 22

Screen/sieve filtration

Answer 22

Involves filtration across a membrane, all articles greater than a given size are excluded from the filtrate, have an absolute pore size rating
Made from natural polymers e.g. cellulose acetate; also some plastics e.g. polyethersulfone, can be sterilized by moist heat or ethylene oxide

Question 23

Depth filtration

Answer 23

Filters composed of thick layers of material or fibres, can be made from glass fibre etc.
Traps particles in the torturous paths created throughout, usually sterilized by moist heat, high dirt-handling capacity

Question 24

Problems of depth filtration

Answer 24

Potential hazard of microbial multiplication within a depth filter and subsequent contamination of the filtrate
Shedding of filter components
Fluid retention
Metal fibres susceptible to corrosion
Solute adsorption

Question 25

Factors influencing performance fo the filters

Answer 25

Pore size
Depth of the membrane
Charge
Tortuosity of the channels

Question 26

Liquid filtration

Answer 26

Generation of water for steam production
Critical applications: water for injection, rinsing water for medical devices
Temperature sensitive materials- antibiotics, vaccines, ophthalmic solutions
Regulatory authorities state importantly that the filter material must not shed fibres or leach undesirable material into solution being sterilized

Question 27

To increase the filtration area:

Answer 27

Several discs can be used in parallel in multiple-plate filtration systems
Membrane filters can be fabricated into cylinders and installed in cartridges
Filters may be pleated

Question 28

Gas filtration

Answer 28

Air decontamination- treatment of air supplied to aseptic areas e.g. clean rooms, isolators
Parts of venting systems on fermenters, centrifuges, autoclaves and freeze-dryers
Medicinal (sterile) gas

Question 29

HEPA filters

Answer 29

Most widely used air filters, dirt filter (vacuum cleaner)
Can remove up to 99.997% of particles greater than 0.3 micrometres in diameter
Pre-filters are used to remove larger particles to extend the lifetime of the HEPA filter

Question 30

Advantages of filtration sterilization

Answer 30

Low cost and rapid disinfection/sterilization
Used for heat labile materials
Can be used to remove chemical contaminants
Impregnated with biocide combining capture and kill

Question 31

Disadvantages of filtration sterilization

Answer 31

Not generally a cidal process
Shedding of filter components
Fluid retention (principally depth filters)
May become blocked easily