Background
In terms of reliability engineering, long-lasting insecticidal nets (LLINs) should be capable of functioning under field conditions for 3 or more years, and resist failure during normal use by remaining in good physical condition [
1]. However, it is well known that the actual service life of LLINs can fall markedly short of 3 years, depending on prevailing circumstances. Whilst there have been numerous long-term studies monitoring the physical integrity and durability of LLINs using various methodologies [
2‐
8], much less attention has been paid to the design of the LLIN product itself, and the inherent ability of products to resist damage. Therefore, at present, there is no single metric that reliably defines the inherent resistance to damage of a new LLIN before it is used.
Based on detailed laboratory analyses of used LLINs [
9], distinct mechanisms of structural damage take place in LLINs that are common across different geographic regions and brands. Mechanical damage is the primary contributor to LLIN deterioration in normal use, both in terms of hole frequency and area. Of these mechanical damage mechanisms, snagging is responsible for the initiation of the largest proportion of holes. Although such holes may be initially small in dimensions, they can potentially enlarge to form significantly larger holes over time. Collectively, tearing, abrasion and seam failure are also responsible for a large proportion of the hole area, and their underlying mechanisms of damage would be practically difficult to avoid during normal use of a LLIN product. Tears usually form as a result of the net first being snagged on a solid object, such as wooden mattress material, and then when force is applied to pull it free, a tear is created. Abrasion occurs when two surfaces are rubbed against each other e.g. during washing or when the LLINs are tucked between the mattress and bedframe. Furthermore, forms of mechanical damage are recurrent across different geographical settings and are found in all knitted LLINs regardless of whether they are made of polyester (PET) or polyethylene (PE) [
10,
11].
Studies have subsequently focused on developing textile testing methods for LLINs that relate to the actual damage sustained in the field [
12]. Test method selection was based on identifying suitable test methods that accurately reflect the physical damage observed in LLINs analysed after use in the field [
13]. Overall, four textile test methods based on existing or slightly modified ISO standards, have been proposed for LLINs to reflect actual modes of damage observed in the field. One of the proposed methods, i.e. the bursting test, is already used in the specification of LLIN products [
14]. The other three methods are snag strength, hole enlargement and abrasion resistance [
12]. This suite of four laboratory textile test methods currently yields four separate quantitative values to represent the inherent resistance to damage of LLINs.
The aim of the present study was to develop a single quantitative metric to define the resistance to damage (RD) of a LLIN based on the same laboratory test data, but which also considered the question ‘how strong is strong enough?’. This formed the basis to develop a new algorithm for calculating RD for new LLINs that should assist with future innovation and the development of better performing products.
Discussion
Resisting the major sources of damage LLINs are exposed to during normal use is essential if products are going to remain in good physical condition for many years. For years, the vector control community has relied upon measuring the bursting strength of LLINs in the laboratory to characterize the ‘physical strength’ of different products. However, these measurements are clearly insufficient given the root causes of hole formation [
13]. In practice, there are multiple mechanisms of damage, but representing all in one meaningful resistance to damage (RD) metric would be more reliable. The approach described in the present work is distinct from other attempts to develop a single composite metric in that no field study evaluations are needed [
30], and rapid assessment of any LLIN product is, therefore, possible prior to distribution. The RD approach focuses on measurements conducted under controlled laboratory conditions based on ISO procedures, and has the advantage that the inherent variations in field study evaluations are completely obviated.
In the RD methodology, the concept of aspirational targets has been introduced, taking into account the magnitude of forces LLINs are likely to encounter during normal use. Of course, the setting of these aspirational targets is open to contention, but at the very least, it focuses attention on what is actually needed in terms of functional performance. Based on an analysis of the real forces generated by human adults, a pragmatic approach was adopted for defining upper values for resistance to damage in terms of: bursting strength (700 kPa), snag strength (200 N), abrasion resistance (400 rubs) and hole enlargement (residual hole size < 5 mm corresponding to a score of 100 without laddering, unravelling or tearing). Note that the setting of aspirational targets needs to be considered not just in terms of product performance, but also in terms of economics. Generally speaking, engineering of LLINs capable of meeting high aspirational values are likely to improve cost effectiveness if LLINs are more robust for the entire life span.
The RD methodology also accounts for the fact that LLINs could be engineered to exceed aspirational targets in the future. Already, warp knitted nets are manufactured for use in other industries that would meet the targets suggested herein, albeit at significantly higher cost. For example, in sportswear applications, warp knitted fabrics with bursting strength values of > 2000 kPa are produced, albeit at heavier basis weights (> 90 g/m
2) [
31]. In Method 1, the actual proportion of the aspirational target is calculated, such that it is possible to score greater than 100 on the RD scale. This is clearly positive in terms of promoting product innovation. By contrast, in Method 2 the RD value is assigned based on a score matrix. The overall RD value is determined based on tiering of the individual parameter values in equal proportions. In this method it is possible for LLINs with snag strengths of e.g. 40 N to score the same as a net with snag strength of 59 N; similarly, this is possible with bursting strength and abrasion resistance. Therefore, this method does not enable scoring past RD = 100%. It is suggested that Method 1 is employed for future characterization of LLIN’s physical integrity. One could argue that Method 2 might encourage manufacturers to focus on meeting bare minimum testing requirements, rather than targeting higher performance.
The RD results calculated by both Method 1 and Method 2 demonstrate the scope for improvement in the design of LLIN products. In particular, focus is required to improve snag strength (red, Figs.
2 and
4), which is also the most common form of damage observed in the retrieved field nets [
13]. Also, even in LLINs that are considered to be ‘strong’ in terms of their bursting strength, careful attention should be paid to resisting hole enlargement by unravelling or laddering of the fabric structure.
Assuming normal household use of a LLIN, that avoids interaction with rodents and exposure to naked flames, cigarettes, cooking embers or deliberate cutting with a knife, good correlation between PHI values in the field and calculated RD values from the lab has been observed, for both RD Methods 1 and 2. This supports the assertion that LLINs can be improved to better withstand certain types of common and reasonable mechanical damage, and this is unlikely to be resolved without upgrading product specifications. However, progressive improvements in RD scores, combined with behavioural change aimed at taking care of the product, is likely to have a major impact on long-term physical integrity and survivorship [
32]. Although the physical integrity of the nets vary by location, a study carried out by Wheldrake et al. [
13] showed that mechanical damage was the main contributor to hole formation across a range of geographies. Therefore, increases in the RD could improve the lifespan of LLINs across the board. Further field comparisons of RD scores and actual physical integrity are now needed to verify the suggested RD methodology as a valid approach to assess expected field performance of specific LLIN brands.
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