Page 55 - Journal of Special Operations Medicine - Spring 2017

specific mechanism: sunlight, but interactions among and mechanical integrity of plastics exposed to such
different types of exposure may also occur. The med- things as UV radiation is well studied and leads to ac-
ics in the field thought that exposure risked tourniquet celerated aging and embrittlement, but the types and
damage, and the main finding of our study indicates this locations (e.g., surface versus interior) of such material
is true. changes vary by the type of exposure (e.g., humidity,
smog, ozone, or heat). Such changes in material prop-
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The minor finding of this study is that the user effect on erties reflect molecular alterations of the polymer chain
the results was substantial. The user explained a portion (e.g., scission and cross-linking) as a result of photodeg-
of the variance of the statistical model for the parameters radation and can be assessed by measuring changes in
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measured and ranged from a minimum of 9% for both physical traits of the polymer. Environmental stress
total trial time and after-time to a maximum of 50% for cracking of polymers is established as a common cause
blood loss volume. Such a finding of user effects, which of mechanical failure of devices made of polymer com-
can be large, adds to a growing body of knowledge that ponents, and when a fluid such as water enters the
optimal performance in tourniquet use requires optimal cracks during mechanical loading, the fluid acts locally
performance of the tourniquet user. This implies that the in stressed regions, giving rise to small cracks known as
need for user development is a research priority; future crazes and, eventually, if the fluid remains present and
studies may present opportunities to improve outcomes the stress continues, to propagation of existing cracks or
of tourniquet application. even to a catastrophic failure of the structure stressed.
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UV light has been shown to degrade resistance of poly-
Our findings are coherent with established knowledge mers to stress cracking by worsening both the surface
from fields like materials science, which has shown a cracking and the mechanical properties of the polymer.
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specific causal link between ultraviolet (UV) light expo- Such degradation is worsened during high mechanical
sure and degradation of polymers like nylon. Although stress, and photodegradation may be synergistic with
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the preliminary nature of our study does not confirm stress cracking. Environmental stress cracking in mate-
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such a mechanism, a brief review of relevant science may rials like polymers has been associated with device fail-
fill gaps in awareness of such science for the prehospital ure. Crazes can form at the tips of brittle cracks without
medical community. Specific mechanisms of breakdown environmental exposure, but environmental exposure
of the nylon polymer are well known in materials sci- may accelerate the process, leading to crack growth
ence and include photodegradation. To address vul- (in width, length, displacement, or ramification into a
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nerability to light, nylon molecules can be UV hardened, network) during mechanical stress. This may be little
and UV-hardened nylon is common in such things as known among medical caregivers.
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outdoor sporting equipment. Consumers and manu-
facturers, if they know about a need to mitigate risk of The findings of this study are in the context of San Anto-
exposure, can request that the nylon components be UV nio’s environment. Air quality in this location has been
hardened, and such a capability of nylon has been in measured as good on 98% of days, and the studied ex-
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the marketplace for decades. Manufacturers of outdoor posure was similar to those in San Antonio in years past.
sporting equipment, such as the maker of the RMT, are For example, the concentration of ozone was 0.087
often familiar with the need to UV harden components ppm during the exposure period, and this level has been
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such as nylon, whereas medical device makers who rou- San Antonio’s average since 1993. That level of ozone,
tinely make hospital items (i.e., indoor equipment) may a highly corrosive air pollutant, ranks San Antonio as
not be. The RMT maker has tourniquet ratchet sales 39 of 50—among the worst of large cities in America.
but also has their ratchets used with outdoor sporting Ozone exposure can degrade the material strength of
goods, such as for snowboard bindings (M. Moran, per- plastics, nylon, natural rubbers, and elastomers, and
sonal communication). In our experiment, the strength, such materials have been recommended to be avoided
consistency, specificity, temporality, and coherence of in applications where ozone is present, because of the
the exposure-dysfunction association indicate it may be risk of corrosion and cracking. Ozone fades the dyes
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a cause–effect relationship. in nylon. Nitrogen dioxide has a notorious record of
fading dyes in acetate, triacetate, viscose rayon, and cot-
To briefly review relevant science, most goods made of ton materials and has caused yellowing in white blends
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a plastic material are formed by methods such as ex- of acetate and nylon. San Antonio’s clear-sky UV light
trusion, injection molding, or extrusion blowing, which index (0–15 scale) averages 6.755, a high value, whereas
result in heat and high shear within the polymer as it is the US average is 4.9, a moderate value. In 2014, for
made into the desired form. These methods introduce example, monthly averages for UV exposure ranged
impurities and reaction products, which make the goods from 2.94, a low value in December, to 10.63, a very
susceptible to photodegradation and loss of mechani- high value in July; daily values ranged from 0 to 11 in
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cal integrity. The loss of strength, impact resistance, 2014; 11 is an extreme value. The UV values in 2014
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Tourniquet After Environmental Exposure 33