Richard Bancroft, BSc (Hons), senior director scientific, at STERIS, discusses the importance of high-performance cleaning chemistries for cleaning reusable medical devices.
Temperature, time, cleaning chemistries and mechanical action, in the presence of water, are the key variables for successful cleaning. Herbert Sinner described the inter-relationship between these four variables in 1959, and his work gained widespread acceptance as Sinner’s Circle. The key aspect of Dr Sinner’s concept is that all of these variables are inter-related; a reduction in one of the variables will necessitate an increase in one or more of the other variables. It is possible to optimise the Sinner Circle variables to allow a reduction in process time, but necessitating an increase in the other variables
Temperature
Increasing temperature can improve the cleaning outcome; however, water that is too hot can coagulate or denature proteins when not in the presence of wetting agents, resulting in a marked reduction in cleaning efficacy.
Mechanical action
In a washer-disinfector, mechanical action is delivered by the force of water in the mechanical spray arms. Increasing the pump pressure to give greater impingement on the load can cause cavitation within the pump and associated reduction in pressure. The type of cleaning chemistry and increasing temperature can create or exacerbate cavitation. In addition to the efficiency reduction and noise increase, cavitation can cause equipment damage, too.
Time
Longer time can result in greater cleaning; however, we are always looking to increase efficiency in processing time. There are often unintended consequences that affect time. For example, increasing the temperature will increase total time, due to the time needed to reach the higher operating temperature.
Chemical action
A cleaning chemistry is used in combination with water. While the other cycle variables also need water, which can serve as the heat transfer agent for temperature and as the mechanical action medium. There is also a consideration of water quality or purity, when combined with a cleaning chemistry. Cleaning chemistries may contain surfactants (surface active agents) which allow bonding between a polar solvent (water), and a non-polar molecule (lipid), helping to allow dissolution.
They also enhance wetting of the surface by lowering the surface tension of the water, allowing the cleaning solution to ‘wet’ or penetrate microscopic areas of the medical devices to be cleaned. Excessive foaming can be caused by extreme hard or soft water, large amounts of soil, or if the cleaning chemistries are dosed incorrectly. This excessive foam can, in turn, impede mechanical action or cause pump/equipment damage. Also, certain conditions can cause deposition of dissolved metal ions on instrument surfaces, causing staining. Cleaning chemistries should be formulated to handle a spectrum of types/ qualities of water. This optimisation of a washing process is focused on three outcomes – cleaning efficacy, process time and device protection. We don’t compromise on the level of cleaning needed, but equally, we need to clean as quickly as possible. Increases in mechanical and chemical action can significantly reduce cycle time but recognise that increases in mechanical and chemical action may cause foaming and problems with cavitation. The cleaning chemistry may not be the only contributor to foaming, which is often caused by dissolved sodium ions in the water supply, or the presence of proteins from the load. The cleaning process should also be designed in such a way as to not reduce the useful life of devices being processed.
The cleaning chemistries used in medical device decontamination are complex formulations of a wide range of chemicals, designed to achieve the desired cleaning properties and chemistry shelf life. As well as one or more surfactants, sequestrants can be added to addresses the negative effects of hard water or aid in cleaning of proteins. Other inert ingredients are added to significantly enhance performance and give the formulation their classification name - enzymatic, alkaline and neutral.
Cleaning chemistry types
Enzymatic formulations also contain enzymes. Enzymes are bio-organic molecules designed to break down larger molecules into smaller, more water-soluble ones. They are very selective and only target specific soil components and will have negligible effects on other molecules. The formulation is typically neutral pH in order to prevent breakdown to these enzyme proteins. Enzymes are typically named after the substances they help to break down; proteases break down protein, amylases break down starch, lipases break down fats/lipids and cellulases break down cellulose. Only proteases are considered useful in medical device cleaning formulations due to protein being prevalent in clinical soils. Different proteases can be combined to broaden pH or temperature effectiveness, but using a higher concentration of the same enzyme will not necessarily increase efficacy. Alkaline formulations also contain alkalis. They form a solution in water greater than pH 10; they typically contain sodium hydroxide (NaOH) or potassium hydroxide (KOH). The alkali causes hydrolysis of proteins and lipids, breaking them into smaller water soluble pieces. They are more effective at higher temperatures (greater than 60˚C), but high pH and high temperature can have material compatibility issues (i.e. alkaline corrosion), dependent upon the concentration used. Alkaline detergents have been shown to be capable of the inactivation of prion proteins.
Neutral formulations form a solution in water that is between 6 to 8. They can be used as a two-part clean – an enzymatic detergent is used, followed by a neutral detergent. They are used as an alternative to alkaline detergents where material compatibility could be an issue; as well as being less aggressive than alkaline detergents, there can be an increased cycle time and a need for a higher temperature for equivalent cleaning efficacy.
Sequestrants and chelating agents can be used to address the negative effects of hard water metal ions. These chemicals are known as sequestrants, having a chemical structure that allows them to bond with metal ions. A specific form of sequestrant is known as a chelating agent; these agents have a claw-like structure which can bind the free metal ions thus enhancing the effects of the detergent. Chelates prevent chemical deposition onto instruments and can also inhibit scale formation and remove scale deposits. Soils typically encountered on reusable medical devices after use may be organic, inorganic or both in nature. Organic soils may be comprised of muscle, skin, connective tissue, fat, grease, proteins and carbohydrates. Inorganic soils may consist of rust, hard water deposits and residues from cleaners and medical solutions (iodine, saline, skin preparations). Combination (organic and inorganic) soils may originate from substances such as bone.
Protection of medical devices by the cleaning chemistry formulation can prevent rust or pitting corrosion sometimes found on medical devices containing iron (which includes stainless steel). Soft metals, such as aluminium, may also exhibit corrosion due to water and or harsh conditions. Cleaning chemistries can be formulated with corrosion inhibitors to prevent this corrosion.
Choosing a cleaning chemistry
There is much more to a cleaning chemistry than just a surfactant in aqueous solution, with the additional components in the formulation making a significant difference to its cleaning efficacy. Selecting the highest performing cleaning chemistry requires an appreciation of all of these attributes. Through a process of scientific studies and washer validation, a number of high-level conclusions can be drawn:
- Alkaline cleaning chemistries are generally better at rendering surfaces non-infectious than enzymatic detergents
- Alkaline and enzymatic cleaning can be effective at inactivating transmissible spongiform encephalopathies (TSEs)
- The precise formulation may be more important than whether the cleaning chemistry is classified as alkaline or enzymatic
- The suitability and effectiveness of a given cleaning chemistry depends on water quality
- Cleaning chemistries should be formulated to be low foaming and have the ability to defoam soils
- Have high performance against clinically relevant soils such as proteins
- Protect surgical instruments from corrosion and staining
- Reduce maintenance costs by minimising pump cavitation
- Be biodegradable, non-toxic to aquatic life, free from environment-damaging chemicals and be provided in minimal packaging by concentrating the formulation.
STERIS
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