new technology
Self healing materials
Cut yourself and—if you're lucky—your skin will
heal with no trace in as little as a week. Crash your car into a wall
or scratch its paintwork and you won't be so fortunate; you'll need to drive it to a repair
shop for horribly expensive correction. Skin, bone, and the stuff of
life is truly amazing: it can sense damage, stop it getting any worse
(using cunning mechanisms such as pain), and repair itself
automatically with little or no help from us. It's incredible! If
only metals,
plastics,
composites,
and other everyday materials were half as smart. Soon they could be: in the early 2000s, scientists began developing self-healing
materials that could repair internal damage all by themselves.
Before long, we'll see self-healing paints and coatings—maybe even
self-healing cars, bridges, and
buildings!
So how exactly do these wonder materials actually work? Let's take a closer look!
self healing materials
Self-healing materials are artificial or synthetically-created substances
which have the built-in ability to automatically repair damage to
themselves without any external diagnosis of the problem or human
intervention. Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Cracks and other types of damage on a microscopic level have been shown to change thermal, electrical, and acoustical properties of materials, and the propagation of cracks can lead to eventual failure
of the material. In general, cracks are hard to detect at an early
stage, and manual intervention is required for periodic inspections and
repairs. In contrast, self-healing materials counter degradation through
the initiation of a repair mechanism which responds to the
micro-damage.
Some self-healing materials are classed as smart structures, and can
adapt to various environmental conditions according to their sensing and
actuation properties
Type of self healing materials
Shape-memory materials
Most of us know shape memory materials through
relatively trivial everyday applications such as eyeglasses, made
from alloys like nitinol (nickel-titanium), that flex exactly back to
shape when you bend and then release them. Usually, shape memory
works in a more complex (and interesting) way than this (read all
about it in our detailed article on shape memory); typically
you need to heat (or otherwise supply energy to) a material to make
it snap back to its original, preferred form. Self-healing shape-memory
materials therefore need some sort of mechanism for delivering heat
to the place where damage has occurred.
In practice, that might be an
embedded network of fiber-optic cables similar to the vascular
networks used in other self-healing materials except that, instead of
pumping up a polymer or adhesive, these tubes are used to feed laser
light and heat energy to the point of failure. That causes them to flip back
into ("remember") their preferred shape, effectively reversing the damage.
How do the tubes know where to deliver their light? If the material
cracks, it also cracks the fiber-optic tubes embedded inside it so
the laser light they carry leaks out directly at the point of
failure. Although you might think fiber-optic tubes would weaken a
material, they can actually strengthen it by turning it into a
fiber-reinforced composite (effectively, they serve as the fibers you'd
get in something like fiberglass, or like the steel "rebar" rods in
reinforced concrete).
Systems like this are sometimes known as autonomous adaptive structures
and have been pioneered by materials engineer Henry Sodano.
Reversible polymers
Polymers don't always need sophisticated internal systems, such as
embedded capsules or vascular tubes, to repair internal damage. Some of
them break apart to reveal what we might think of as highly "reactive"
ends or fragments that naturally try to join up again. Energized by
either light or heat, these stray fragments naturally try to rebond
themselves to other nearby molecules, effectively reversing the damage
and repairing the material. Some break to expose electrically charged
ends, which give the broken fragments a built-in electrostatic
attraction. When damage occurs, electrostatic forces pull the fragments
together, enabling the material to self-repair.
Sometimes, all you need to repair damage is a little heat. Plastics come in two main kinds. Some (known as thermoplastics)
are relatively easy to melt down, recycle, and mold into new forms; PVC
(polyvinyl chloride), polyethylene, and polypropylene are typical
examples. Others (known as thermosets or
thermosetting plastics) work a
different way: if you heat them, they degrade before they melt so you
can't heat them to reshape them;
melamine and bakelite are good examples. This suggests that we might be
able to use thermoplastics (but not thermosets) as self-healing
materials. We'd simply need them to melt under stress so the long
polymer chains inside could rearrange themselves back into a strong, new
form.
How would that happen in practice? Thermoplastics can be designed so
that if they're
cracked or damaged, and then heated, the polymers from which they're
made will break down into their monomers (the repeating molecules from
which they're built). When they cool down, the original polymer reforms,
reversing
the damage. This method does rely on a convenient supply of heat—but
sometimes that's readily available.
Materials like this haovides enough energy (a temperature rise in the damaged
area of maybe a couple of hundred degrees) for the polymer to reseal the hole and
completely bind the material together again. It's easy to ive been tested by firing bullets (up to 9mm in diameter)
at them. The localized heat from the impact prmagine invaluable applications in
fighter jets with bullet holes that rapidly seal up and disappear!
Microvascular materials
Embedded healing agents are simple and effective,
but they do have a drawback: interrupting the structure of the
material with capsules can actually weaken it, potentially
increasing the risk of failure—which is the very problem we're
trying to solve! Now the human body doesn't fix damage this way
with makeshift repair materials waiting inside every bit of skin and bone
in case we happen to cut ourselves or fall over. Instead, our body has
an amazingly comprehensive vascular system (a
network of blood vessels of different sizes) that transport blood
and oxygen for energy and repair. If damage occurs, our blood system
simply pumps extra resources to the places where they're needed, but
only when they're needed.
Materials scientists have been trying to design
self-healing materials that work the same way. Some have networks of
extremely thin vascular tubes (around 100 microns thick—a little thicker
than an average human hair) built into them that can pump healing agents (adhesives, or whatever else is
needed) to the point of failure only when they need to do so. The
tubes lead into pressurized reservoirs (think of syringes that are
already pushed in slightly). When a failure occurs, the pressure is
released at one end of the tube causing the healing agent to pump in
to the place where it's needed. Although this method can seal
cracks up to ten times the size that the microcapsule method can manage, it works more slowly
because the repair material has further to travel;
that could pose a problem if a crack is spreading faster than it's
being repaired. But in something like a skyscraper or a bridge, where
a failure might appear and creep (spread slowly) over months
or years, a system of built-in repair tubes could certainly work
well.
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