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suddenly at the end of the exposure time and it should be stable in storage, cheap and, if possible, sensitive only to moist heat…..”
Chemical Indicator formats. The simplest form of chemical indicator is an ink printed on a substrate. The substrate may be paper or plastic based and used to make an adhesive label which can be fixed onto the outside of surgical instrument sets and sterile packs. The ink might be printed directly onto sterile packaging materials such as paper bags or pouches. The ink might also be applied to adhesive tapes to make indicator tape which can be used to secure instrument sets wrapped in flexible wrapping materials.8
There are also special kinds of chemical indicators, called “moving front” CIs.11 These CIs have a more complex construc- tion. They consist of a pellet of dye which is placed within an indentation on a foil base. A paper wick is then placed in contact with the dye pellet and the whole assembly then sandwiched between a semi permeable membrane and the foil base. An adhesive label is then usually placed over the semi permeable membrane with informational material printed on it. Figure 1, below, shows the construction of a typical moving front chemical indicator.
In recent times a new kind of CI has been introduced in which the indicator ink is printed on a substrate covered with an impervious polymer sheet with gaps cut along its length. Beyond the “accept” line printed on top of the CI, the overlying sheet totally encloses the ink creating a small gap down which the sterilant must penetrate to affect the color change.12
Chemical Indicators have also been sup-
plied in other formats in which they are incorporated into some form of test device which is then used in a special test to ensure
the sterilizer is functioning correctly, for example the Bowie and Dick test.13
Chemical indicator inks. The inks used for making printed CIs are usually made from chemical substances which react together when exposed to the process variables of a sterilization process. For example, in steam sterilization the pres- ence of moisture enables the reaction to rap- idly take place and the time and temperature of exposure enable the reaction to proceed at an appropriate speed so that the color change occurs by the end of the process. The chemical reactions which take place usually give rise to a color change which can then be interpreted by the observer. As an example, two reagents may be included in the ink which give a cream starting color but during the reaction they combine to give a black colored by-product.13
These inks
often contain inorganic chemical reagents. Alter-
natively, the color change may be due to the two reacting chemicals giving rise to a change which cause an additional reagent included in the ink to change color. For example, the ink may contain a pH indicator which changes color due to the release of an acidic by-product from the reaction between the two principal reagents. These inks often contain organic chemical reagents.15
A third
category of ink is one in which the moisture present in the sterilization process causes the ink to change color. In this instance the chemical reagent is reacting with the moisture in the steam to change from one
paper/film top cover steam penetration rate controlling
clear plastic display window
SELF-STUDY SERIES hydrated state to a second,16 this giving rise
to the color change. Figure 2 shows examples of the reactions which are used. It is impor- tant to recognise that CIs are designed for use in specific sterilization processes because the chemical reagents included react to the process variables associated with the steril- ization process. Thus, a steam CI should not be used in an ethylene oxide sterilization process and vice versa because the EO indi- cator will have reagents which are designed to react with the EO present in the process.
Figure 2: Some exampls of chemical reactions used in chemical indicators
For moving front chemical indicators, the color change is due to a physical change - rather than a chemical reaction. When exposed to the process variables the dye pellet enclosed within the sandwiched layer of foil base and semipermeable membrane (see figure 1) melts and begins to migrate along the attached wick due to capillary action. The speed of migration is governed by the temperature of exposure and by the presence of moisture passing through the semipermeable membrane.11, 17
paper wick strip bottom cover
steam and temperature sensitive checmical pellet
Figure 1:
The construction of a moving-front chemical indicator showing an aluminum platen (bottom), and indicator dye pellet (indicated), a paper wick (indicated), a semipermeable membrane (middle) and a paper label showing graphics (top).
The nature of the color change. The nature of the color change depends on the reactions taking place within the ink. For some color change indicators, the change from the starting color to the end color is gradual. Figure 3 shows a plot of the depth of color (optical density) occurring as the time of exposure in a steam sterilization process progresses. It can be seen from the shape of this curve that as the exposure time increases the speed at which the color changes slow down until it reaches a plateau. For other printed inks the color change may be quite rapid at a specific point in the reaction. Thus, the starting color is maintained until a pre- defined exposure time is reached at which point the color changes to a second color over
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Self-Study Test Answers: 1. B, 2. A, 3. D, 4. C, 5. D, 6. A, 7. D, 8. B, 9. C, 10. A
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