Designing for infection control

Materials World magazine
1 Apr 2010
The over-bed table and bedside locker developed by Geoff Hollington

UK Designer Geoff Hollington describes the difficulties of materials specification for infection control.

Healthcare-associated infections (HCIs) cause distress to patients and their families. In the worst cases they kill, with several thousand deaths each year in the UK attributed to HCIs and those who survive may spend weeks, months or years recovering.

The financial cost to public and private healthcare providers is huge – an estimated £1bln of the UK’s National Health Service (NHS) budget is accounted for in unplanned care for victims, diverted staff time, facility closure and disinfection, expensive drugs, and litigation and compensation. The average cost of an HCI case is around £4,500. Add to this the psychological costs of low morale and frustration among healthcare workers and a pervasive loss of confidence in the healthcare system among the public.

The infections themselves are caused mainly by bacteria such as MRSA (strains of Staphylococcus aureus) and, in the case of Clostridium difficile, by airborne spores. These pathogens are highly infectious, they have evolved and continue to develop strains that are resistant to antibiotics. Clostridium difficile is also resistant to most cleaning agents. While from 20-30% of patients’ infections occur in surgery and other specialised procedures, the majority appear to result from routine interaction with the hospital environment and with other individuals. Material surfaces are the intermediary in many cases of cross-infection.

Design Bugs Out

As one of the winning designers in the NHS-sponsored Design Bugs Out competition, run by the UK’s Design Council in 2008-09 (see also Materials World, July 2009, pp17-18), I came face to face with the problems of materials specification where infection control is an important criterion. Our team, which included UK-based manufacturer-partner Herman Miller International and materials and colour specialists Barron Gould, proposed alternatives to the conventional patient bedside locker and over-bed table.

Studies of cross-infection vectors in hospitals, experience and common sense suggest that frequent and thorough cleaning of surfaces is the single most effective means of infection control, making ‘cleanability’ a primary design goal. Speaking with cleaners and nurses, we learned that sharp corners, dark internal spaces and heavy, difficult-to-move structures make frequent, thorough cleaning hard. So more curvaceous, lightweight structures were called for, which pointed to plastics as the primary materials. While bacterial colonisation of all equipment surfaces needs to be kept under control by frequent cleaning, the places that users – patients, carers, cleaners or visitors – regularly touch require special consideration. These ‘touch points’ should be light in colour so grime is visible, smooth, and wear- and scratch-proof.

Materials and microbes
Specifying materials in product design can be problematic, when performance, appearance and costs interact in complex ways. This is certainly true for infection control where, in addition to the usual considerations of tooling investment, process suitability, mechanical performance, appearance and so on, there may be additional questions relating to performance –
• Can the material resist attack from cleaning agents and disinfectants?
Parts will have to maintain surface and substrate integrity over a 15-year lifetime.
• Can the part be tooled with a smooth surface?
It is easier to clean smooth surfaces than those that are rough and textured. Even the fine level of grain that creates a matte appearance can seriously inhibit cleaning.

Both coccus (spherical) and bacillus (rod-shaped) bacteria have cross-section diameters in the range of 0.5-1.0µm (MRSA is 0.6µm), so a surface with a roughness average in the same range can trap these microbes. A surface with Ra of one has a ‘smooth matte’ appearance, so it follows that if bacteria are to be effectively lifted from a surface by hand cleaning, the Ra should be <0.2µm, or a ‘high gloss’.
• What is the surface impact, wear and scratch performance, at micrometre scale?
Following on from the smoothness argument above, there is little point moulding a super smooth, glossy surface only to have it wear away or scratch quickly and easily. A fine scratch measuring one micrometre wide and one deep is almost invisible to the naked eye, but large enough to trap MRSA. Scratch resistance is proportional to surface hardness and so softer polymers, though mouldable with glossy surfaces and in cleanable shapes, may quickly develop scratches that reduce cleanability.

In the Design Bugs Out products, we felt able to compromise on surface hardness in parts subject to a low incidence of multiple user touching – employing solid and glass-filled polypropylene – but for the high-touch high-wear table top parts, a much harder material was needed.

The polymer material that covered most requirements was compression-moulded melamine thermoset. It is hard enough to resist scratches from glass and ceramic wares and steel cutlery, mouldable, pigmentable in light colours, and affordable in terms of parts and tooling costs. This was a controversial choice – an ‘old-fashioned’ material with problematic end-of-life characteristics – but given the design criteria, it is a good fit.

Looking ahead, it is possible that silica-based coatings can be applied to plastic part surfaces to improve wear and scratch resistance, but the polymer substrate must still be hard enough to resist impact and heavy scratching, otherwise the system will break down.

• What is the degree of surface hydrophobicity?
First thoughts may suggest that a super-hydrophobic surface would be desirable in an infection-control environment, the pathogens will not stick and cleaning will be easy – but it is not that simple. Although a water-shedding surface may be easy to wipe clean and dry, any water-soluble
bactericidal agent in the cleaning system may be unable to function – it will be shed from the surface before it can act. And, although some bacteria will slide off a hydrophobic surface, many others stick to it.

Bacteria can be divided into two classes – Gram-positive and Gram-negative (after a stain test developed by Hans Christian Gram) – according to their cell wall structure. Gram-positive bacteria stick more easily to hydrophobic surfaces than Gram-negative. MRSA is Gram-positive but infection carriers such as E. coli are Gram-negative. Given this polar split in affinities, it appears prudent to specify surfaces that are fairly neutral in hydrophobic terms.

• Is an antimicrobial additive or coating available and, if so, is its use advisable?
Products that enable surfaces in the environment to automatically kill bacteria would appear to be desirable. Many antimicrobial additives and coatings are available and their application in durable products, textiles and consumables is widespread, particularly in consumer markets. While working with the health services we heard cases for and against antimicrobial technologies (see box above) currently on the market.

For effective infection control in healthcare environments, the most important design criterion for products is that they should be easy to clean, often and well. Polymer materials enable designers to achieve the seamless, curvaceous forms that facilitate this. When taking into account the details of bacterial microbiology, polymer materials and process selection must be made with great care.

For and against antimicrobials
• The surest way to control cross-infection in hospitals is through regular and effective cleaning. Labelling surfaces in the environment as antimicrobial will add confusion to the cleaning process, with the risk of falling standards.
• Unlike the hard material touch points close to patients, textile products such as curtains and bed linen cannot be cleaned several times a day. Treating these textiles with antimicrobials reduces the risk of infection.
• Bacteria are developing resistance faster than we can create new antimicrobials. Increasing the exposure of these organisms to antimicrobials through pervasive application in the environment will speed up the development of resistant strains.
• It is impossible to clean touch points often enough to prevent cross-infection. Treating those points with antimicrobials has a real chance of controlling infection.
• Bacteria do not necessarily settle directly on a surface in order to be killed by additives. They are more likely to inhabit sticky grime and may be remote from any surface antimicrobials’ effectiveness.
• It may be possible for the additive agent in a surface to attack organisms embedded in grime using a cascade process.
• Antimicrobial additives in polymers have a finite functional life, with a fall off in effectiveness during that period of service. The useful life of a product may be considerably longer than that of the antimicrobial agents in its parts.