(Nanowerk Spotlight) The outdoor environment has
become an indispensable aspect of our daily lives,
serving as a venue for a plethora of activities, including
recreation, sports, and leisure, as well as work-related
endeavors. However, the increasingly frequent extreme weather
events caused by climate change have made personal comfort in
outdoor spaces a critical consideration for human health and
sustainable development.
During outdoor physical activity , the human body acts as
an internal heat source, exchanging heat and sweat with the
surrounding environment through textiles, which serve as the
second skin. Personal comfort involves three stages: precise
regulation of the microenvironment between the skin and textiles;
the internal section of the textiles; and the macroenvironment
between the textiles and the ambient environment.
Generally, there exist four primary avenues for achieving
personal cooling and drying: radiation, conduction, convection,
and sweat evaporation. These four pathways work in tandem to bring
about optimal personal comfort in real-world situations, albeit with
varying degrees of significance depending on the prevailing outdoor
environmental conditions.
Personal cooling and drying in outdoor sunny or rainy environments
Personal cooling and drying in outdoor sunny or rainy environments.
Advanced functional textiles offer personal thermal and moisture
comfort through various pathways, such as solar reflection,
HBIR emission / transmission, heat conduction, waterproofness,
breathability, and directional water transport. (Reprinted with
permission from Wiley-VCH Verlag)
The human body's most fundamental physiological defense
against overheating during physical exertion in hot outdoor
environments is the evaporation of sweat. However, two
substantial challenges must be contended with to achieve
optimal personal cooling and drying in outdoor settings:
the heat input from exposure to sunlight and the intrusion
of rainwater.
Addressing these challenges necessitates the development of
advanced functional textiles capable of reflecting solar
radiation, facilitating heat dissipation, and resisting
water intrusion, while still permitting the crucial process
of sweat evaporation to take place.
The intricate process of personal cooling and drying involves
the interrelated release and absorption of sweat by the textile,
resulting in evaporative cooling and heat generation,
respectively. The design of advanced functional textiles for
this purpose demands careful consideration of their thermal,
optical, structural, and specialized wetting properties. Unlike
conventional cooling methods, these textiles provide localized
management of body thermal and moisture levels, without the
need for excessive energy consumption to cool and dry the
entire indoor living space.
Advanced functional textiles developed for personal thermal
and moisture comfort Advanced functional textiles developed
for personal thermal and moisture comfort in both research
and industry.
(Reprinted with permission from Wiley-VCH Verlag)
Functional textiles also enhance human comfort in
outdoor activities that require cooling, warming, or
drying, resulting in improved comfort levels and
increased productivity in outdoor work.
Furthermore, there is the potential for functional
textiles to be integrated with IoT devices for a range
of applications, including healthcare, sports, and fashion,
demonstrating how nanotechnology can be used to create smart
textiles that can provide real-time data on various metrics.
The integration of advanced functional textiles with
nanotechnology is currently underway to provide better
personal cooling and drying performance in outdoor
environments, endowing textiles with various high-quality
functions.
There are many examples of this has been achieved in
various ways. For example, infrared radiation is a primary
way the body releases heat and is the focus of a novel
textile technology for a fabric that can automatically
regulate the amount of heat that passes through it depending
on conditions. For example, when conditions are warm and moist,
such as those of a sweating body on a summer day, the fabric
allows infrared radiation (radiant heat) to pass through.
When conditions become cooler and drier, the fabric reduces
the heat that escapes. Infrared radiation is a primary way
the body releases heat and is the focus of this new technology.
researchers have demonstrated smart fabrics that generate
electricity and support temperature regulation through cooling.
It is now also possible to fabricate omniphobic textiles
(which are repellent to oils, water, and dust) that can
protect you from rain, stains, and bacteria, while they
harvest the biomechanical energy of the user to power
textile-based electronics.
In yet another example, researchers have have spun
recycled polyethylene into fibers and yarns designed
to wick away moisture. Woven into silky, lightweight
fabrics the yarns absorb and evaporate water more quickly
than common textiles such as cotton, nylon, and polyester.
A recent review article in Advanced Functional Materials
("Nanoengineered Textiles for Outdoor Personal Cooling
and Drying") discusses recent advancements in the
development of nanoengineered textiles for outdoor
personal cooling and drying.
The article also provides insights into the mechanisms
of heat dissipation and water transport, which work
synergistically to create a comfortable microenvironment
for the human body.
While there are still gaps in practical applications,
this review highlights the potential for these textiles
to improve outdoor living quality and labor productivity
while satisfying increasing demands for health, safety,
and sustainability.
In general, nanoengineering techniques can be used to
enhance the functionality of textiles, such as thermal
conductivity, optical scattering, special wettability,
antibacterial properties, and electrical conductivity.
Textile researchers and industry use three primary methods
to create nanoengineered textiles with nanoporous,
nanofibrous, and nanocomposite structures, which are
discussed in detail in the review:
The first method involves adding nanoporous functional
layers to traditional textiles using nanoporous coating
and laminating, which are simple processes that preserve
textile strength.
The second method is nanofiber spinning, a scalable and
straightforward manufacturing technique that produces
nanofibrous textiles with customizable fiber structures
and functionality.
Finally, nanocomposite coating and embedding are used
to prepare nanocomposite textiles by introducing
nanomaterials onto the fiber surface or into the
fibers themselves. In this case, the textile's
performance depends on the introduced nanomaterials,
such as functional nanoparticles, nanotubes, and nanosheets.
The authors proceed to delve into detailed discussions
concerning nanoengineered textiles designed for two
specific outdoor purposes: personal cooling and personal
drying. Each topic is dedicated its own thorough section.
Personal cooling
The human body regulates its temperature through various
processes such as sweating and dilating skin capillaries.
However, in extreme weather conditions or intense exercise,
it becomes difficult for the body to maintain its thermal
balance. Nanoengineered textiles have been developed as an
external method of regulating body temperature by
dissipating accumulated heat without consuming external
energy. This section provides a systematic review of the
mechanisms of heat dissipation in nanoengineered textiles
and how they are prepared.
Radiative cooling textiles with spectral selectivity:
Developing outdoor clothing that keeps you cool by
reflecting sunlight and radiating body heat is a challenge
for designers. This requires creating materials that reflect
sunlight well but also allow the body to release heat to
the environment. To achieve this, the clothing needs to
have high reflectance in the solar region (0.3-2.5 µm) but
allow for high transmittance and emittance in the body's
heat infrared (HBIR) region (7-14 µm). This design is
challenging because it requires fine-tuning the material's
optical properties across a wide range of wavelengths.
Conductive cooling textiles with high thermal conductivity:
The layer of fabric that touches our skin affects how
efficiently our clothes can cool us down. Traditional
fabrics like cotton, wool, PET, and nylon have low thermal
conductivity, which means they're not very effective at
dissipating heat when our body temperature increases.
This can make us feel hot and uncomfortable. To improve
this, researchers have added thermally conductive fibers
like boron nitride nanosheets, carbon nanotubes, and
graphene to fabrics. These fibers have high thermal
conductivity and can help dissipate heat more effectively,
which can make us feel cooler and more comfortable.
Advanced functional textiles developed for personal
thermal and moisture comfort Nanoengineered textiles
with enhanced thermal conductivity for outdoor personal
conductive cooling. Shown here is the heat dissipation
process of a thermally conductive BNNSs/PVA textile for
personal cooling. (Reprinted with permission from Wiley-
VCH Verlag)Personal drying
Outdoor personal drying refers to how our bodies get rid of
sweat and water vapor efficiently to maintain comfort when
we're sweating. When our body temperature goes up, we sweat
to cool ourselves down. If it's raining outside, it's harder
for the sweat to evaporate and we might feel uncomfortable.
But if it's sunny, we might sweat more than our body can handle,
making us feel uncomfortable as well.
Scientists are looking at how plants and animals repel water
to help them design new fabrics that help us sweat and
evaporate water better. These biomimetic special wettabilities
provides researchers with nature-inspired solutions for
the functional design of nanoengineered textiles. The most
famous are the superhydrophobic lotus leaf effect with a
self-cleaning property and the superhydrophobic legs of
water striders that are covered with microscale bristles
that allow them to stand and walk freely on water surfaces.
Some biological surfaces, such as those found on beetles,
cacti, spider silk, and bird beaks, have unique micro and
nanoscale structures that allow them to transport water in
a specific direction. These structures include anisotropic
wettability, curvature gradient, and surface energy gradients.
These organisms have evolved these structures to survive in
harsh environments, such as deserts, and they can collect water
droplets from fog or capture water using a capillary ratchet
mechanism. These examples from nature could inspire the
development of new materials with directional water
transport capabilities.
What researchers have learned from this is that, when
designing materials that are inspired by nature and have
unique water-repelling or water-attracting properties,
it's important to consider both the material's properties
and its micro and nanostructures. There are two main ways
to do this: by creating structures that are hierarchical
(meaning they have different levels of organization),
which can make the material more effective at repelling or
attracting water, and by arranging the structures in a way
that causes water to move in a particular direction.
The authors then go on to describe examples of waterproof
and breathable textiles with superhydrophobicity and
directional water transport textiles with wettability
gradient. Finally, they describe examples of biomimetic
transpiration textiles that were designed to realize
synergistic personal cooling and drying.
For instance, one example of a biomimetic transpiration
textile for highly efficient personal drying and cooling is
based on the hierarchical and interconnected network of
vascular plants. The researchers found that the underlying
mechanisms of transpiration in vascular plants and water
transport in nanoengineered textiles are the same.
Specifically, the way plants release water from their leaves
and the way nanoengineered fabrics can draw sweat away from our
skin are actually quite similar. Both use a system of nanoscale
tubes or fibers to move water from one place to another.
Plants use this system to pump water and nutrients up from
the ground and cool themselves down by releasing water from
their leaves. Nanoengineered fabrics use a similar system to
move sweat away from our skin and help us stay dry and cool.
In concluding their review, the authors point out four aspects
of nanoengineered textiles that they suggest need to be
further explored to promote their development toward practical
applications.
Nanomaterial safety
Nanoengineered textiles that have great potential for outdoor
cooling and drying potentially also raise concerns about human
health risks and environmental impacts. Different methods of
incorporating nanoparticles into textiles have been tried,
but most suffer from poor adhesion or durability. Embedding
nanoparticles into synthetic fibers during spinning can create
a stable material, but direct-spinning technologies need
reinforcement with traditional fabrics. A careful assessment
of the benefits, risks, and life cycle of nanoengineered textiles
is needed before they can be used practically.
Basic mechanisms
The exchange of heat and sweat through textiles that act as
a second skin to connect the body to the environment is
complex and intertwined. Therefore, nanoengineered textiles
for outdoor cooling and drying need to be carefully designed
based on a comprehensive understanding of the human body,
microenvironment, textiles, and ambient macroenvironment.
To optimize material selection and structural design, a
microstructure model of heat and sweat exchange through
textiles is needed to clarify the synergistic thermal and
moisture management mechanism under various environmental conditions.
Evaluation standards
Scientists and engineers have developed various nanoengineered
textiles for outdoor cooling and drying applications, and have
evaluated their thermal and moisture management performance
using different tests. However, it's unclear whether these
performance parameters are effective for practical applications.
Also, there's no universally adopted test standard for demonstrating
the outdoor cooling and drying effects of nanoengineered textiles.
In practical application, the most direct evaluation standard is
personalized comfort, which includes subjective opinions. Therefore,
more tests involving personal experiences, such as breathing, sweating,
and thermal manikin tests, should be widely used.
Smart thermal and moisture management systems
Scientists are developing smart textiles that use Internet of
Things technologies, nanogenerators, sensors, solar cells,
supercapacitors, and other flexible electronics to improve
personal cooling and drying. These advanced textiles can collect
real-time data from the microenvironment and macroenvironment,
analyze it, and use it to regulate heat and sweat exchange
according to personalized thermal and moisture comfort
requirements in outdoor environments.
This technology can act as an intelligent platform to
automatically improve personal comfort. The goal is to
connect human physiology and psychological activities
with smart textiles, making personal cooling and drying
more efficient and effective. This technology represents a
future direction for the next-generation of advanced textiles.
By Michael Berger – Michael is author of three books by the
Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
Copyright © Nanowerk
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