REUSABLE MEDICAL DEVICES
cleaning efficacy is not to be compromised, this will require an increase in the other process variables, as indicated by the Sinner Circle (see figure 2).
Cycle variables – chemical action
The chemical action variable is often considered to be use of a detergent, but of course must be used in combination with water. While the other cycle variables also need water – as the heat transfer agent for temperature and as the mechanical action
medium, there is more of a chemical consequence of water quality or purity combined with the detergent. Water can be described as the universal solvent, and indeed, water is an excellent medium for dissolving and dispersing many substances. Water is a polar molecule, which gives it some very unique properties; we don’t often think about these properties as we live with them every day, but much of what is unique about water is due to its unique properties - polarity and hydrogen bonding. As water is
Time 25%
Mechanical action 25%
Temperature 25%
Chemical action 25%
Figure 1 – An example of the Sinner Circle Time 11%
Temperature 26%
Mechanical action 26%
Chemical action 37%
not always the perfect solvent, we sometimes need to modify or improve its properties; the most obvious example is the addition of detergents to water. Technically, detergents are known as surfactants (or surface active agents); they allow bonding between a polar solvent such as water, and a non-polar molecule such as a lipid, helping to allow dissolution of hydrophobic molecules into the water. They can also allow wetting of the surface by lowering the surface tension of the water; this can allow the cleaning solution to ‘wet’ or penetrate microscopic areas of the medical devices to be cleaned. One of the bigger variables with cleaning in practice can be variation in water quality; pure water is a much better solvent, but can in itself be corrosive. But perhaps the biggest issue with water is the consequence of dissolved substances within it. High sodium ion content can cause excessive frothing and foaming, which in turn can impede impingement and the resulting decrease in washer-disinfector efficacy. Also, certain conditions can cause deposition of dissolved or suspended species, such as the coating of dissolved metals on instrument surfaces, which can cause staining. Chlorine can cause pitting corrosion on stainless steel surfaces, and is typically present as metal salts in the water supply, e.g. NaCl, MgCl2 CaCl2
, etc.
Finally, detergents can cause excessive foaming if they are not formulated properly or not dosed correctly. As discussed above under mechanical action, water pump cavitation can be exacerbated by detergents with a high propensity to foam.
Optimising the washing process using the Sinner Circle
Figure 2 - An example of the Sinner Circle, showing how an optimisation of time increases the need for other variables
Time 7%
Temperature 20%
Mechanical action 35%
Chemical action 38%
Figure 3 – The Sinner Circle, showing an optimisation of cleaning performance 72 I
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The Sinner Circle can be used to optimise a washer process; this optimisation is typically focused, in practice, on two significant outcomes – cleaning efficacy, and process time. We should clearly not compromise on the level of cleaning that we need to achieve, but equally, we need to clean as time-efficiently as possible. When looking scientifically at the entire process, and utilising the Sinner Circle, the key areas we have an opportunity to manipulate are, to a small degree, temperature, and to a much greater extent, the mechanical and chemical action. When considering the chemical action i.e. the detergent, we must bear in mind that water quality can have a pronounced effect on the performance of the detergent. For a given washing performance, increases in mechanical and chemical action can significantly reduce cycle time (see figure 3). Whilst there are significant advantages in focusing on both mechanical action and chemical action, there are also some factors that must be considered. As briefly mentioned above, increases in mechanical action can cause problems with cavitation, and increases in detergent concentration may contribute to excessive foaming, which in turn may result in loss
AUGUST 2019
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