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Topic Name: An Interdisciplinary Researchers Team has Studied Ground-Breaking Work on a New Type of Polymer that Shows CMA
Category: Polymer Interfaces and Macromolecular Assemblies
Research persons: Case Researchers
Location: Case Western Reserve University, United States
Details
An interdisciplinary team of researchers from the departments
of macromolecular science and engineering and biomedical engineering at the
Case School of
Engineering and the Louis Stokes Cleveland
Department of Veterans Affairs
Medical Center has published ground-breaking work on a new type of polymer
that displays chemoresponsive mechanic adaptability -- meaning the polymer can
change from hard to soft plastic and vice versa in seconds when exposed to
liquid -- in the March 7, 2008, issue of Science, one of the world's most
prestigious scholarly journals covering all aspects of science.
Jeffrey R. Capadona, associate investigator at the
VA's Advanced Platform Technology (APT) Center, graduate student Kadhiravan
Shanmuganathan, and Case Western
Reserve University professors and APT investigators Dustin Tyler (biomedical
engineering), Stuart Rowan (macromolecular science) and Christoph Weder
(macromolecular science) have unveiled a radically new approach for developing
polymer nanocomposites which alter their mechanical properties when exposed to
certain chemical stimuli.
"We can engineer these new polymers to change their
mechanical properties -- in particular stiffness and strength -- in a programmed
fashion when exposed to a specific chemical," says Weder, one of the senior
authors of the paper.
"The materials on which we reported in Science were designed
to change from a hard plastic -- think of a CD case -- to a soft rubber when
brought in contact with water," adds Rowan, who has been Weder's partner on the
project for almost six years.
"Our new materials were tailored to respond specifically to
water and to exhibit minimal swelling, so they don't soak up water like a
sponge," saud Shanmuganathan.
In their new approach, the team used a biomimetic approach --
or mimicking biology -- copying nature's design found in the skin of sea
cucumbers.
"These creatures can reversibly and quickly change the
stiffness of their skin. Normally it is very soft, but, for example, in response
to a threat, the animal can activate its 'body armor' by hardening its skin,"
explains Capadona, who has a sea cucumber in his aquarium. Marine biologists
have shown in earlier studies that the switching effect in the biological tissue
is derived from a distinct nanocomposite structure in which highly rigid
collagen nanofibers are embedded in a soft connective tissue. The stiffness is
mediated by specific chemicals that are secreted by the animal's nervous system
and which control the interactions among the collagen nanofibers. When
connected, the nanofibers form a reinforcing network which increases the overall
stiffness of the material considerably, when compared to the disconnected (soft)
state.
Building on their recent success on the fabrication of
artificial polymer nanocomposites containing rigid cellulose nanofibers, which
earned them the December 2007 cover of Nature Nanotechnology, the team mimicked
the architecture nature 'designed' for the sea cucumbers and created artificial
materials that display similar mechanical morphing characteristics.
The Case Western Reserve/VA team is specifically interested
in using such dynamic mechanical materials in biomedical applications, for
example as adaptive substrates for intracortical microelectrodes. These devices
are being developed as part of 'artificial nervous systems' that have the
potential to help treat patients that suffer from medical conditions such as
Parkinson's disease, stroke or spinal cord injuries, i.e., disorders in which
the body's interface to the brain is compromised. A problem observed in
experimental studies is that the quality of the brain signals recorded by such
microelectrodes usually degrades within a few months after implantation, making
chronic applications challenging. One hypothesis for this failure is that the
high stiffness of these electrodes, which is required for their insertion,
causes damage to the surrounding, very soft brain tissue over time. "We believe
that electrodes that use mechanically adaptive polymer as substrate could
alleviate this problem" explains Dustin Tyler, who specializes in neural
interfacing and functional electrical stimulation. The development and testing
of experimental microelectrodes that involve the new adaptive materials is
currently underway. "That's why we designed our first materials to respond to
water" explains Weder. "This allows the rigid electrodes to become soft when
implanted into the water-rich brain" he adds.
Note for Nanocomposite
Nanocomposites are materials that are created by introducing nanoparticulates
(often referred to as filler) into a macroscopic sample material (often referred
to as the matrix). This is part of the growing field of nanotechnology. After
adding nanoparticulates to the matrix material, the resulting nanocomposite may
exhibit drastically enhanced properties. For example, adding carbon nanotubes
tends to drastically add to the electrical and thermal conductivity. Other kinds
of nanoparticulates may result in enhanced optical properties, dielectric
properties or mechanical properties such as stiffness and strength. In general,
the nanosubstance is dispersed into the matrix during processing. The percentage
by weight (called mass fraction) of the nanoparticulates introduced is able to
remain very low (on the order of 0.5% to 5%) due to the incredibly high surface
area to volume ratio of nanoparticulates. Much research is going into developing
more efficient combinations of matrix and filler materials and into better
controlling the production process.
Note for Biomimetic Material
Biomimetic materials are materials that have been designed such that they elicit
specified cellular responses mediated by interactions with scaffold-tethered
peptides from extracellular matrix (ECM) proteins; essentially, the
incorporation of cell-binding peptides into biomaterials via chemical or
physical modification.
Such peptides include both native long chains of ECM proteins as well as short
peptide sequences derived from intact ECM proteins. The idea is that the
biomimetic material will mimic some of the roles that an extracellular matrix
plays in neural tissue. In addition to promoting cellular growth and
mobilization, the incorporated peptides could also mediate material degradation
by specific protease enzymes or initiate cellular responses not present in a
local native tissue. In the beginning, long chains of ECM proteins including
fibronectin (FN), vitronectin (VN), and laminin (LN) were used, but more
recently the advantages of using short peptides have been discovered. Short
peptides are more advantageous because, unlike the long chains that fold
randomly upon adsorption causing the active protein domains to be sterically
unavailable, short peptides remain stable and do not hide the receptor binding
domains when adsorbed. Another advantage to short peptides is that they can be
replicated more economically due to the smaller size. A bi-functional
cross-linker with a long spacer arm is used to tether peptides to the substrate
surface. If a functional group is not available for attaching the cross-linker,
photochemical immobilization may be used. In addition to modifying the surface,
biomaterials can be modified in bulk, meaning that the cell signaling peptides
and recognition sites are present not just on the surface but also throughout
the bulk of the material. The strength of cell attachment, cell migration rate,
and extent of cytoskeletal organization formation is determined by the receptor
biding to the ligand bound to the material; thus, receptor-ligand affinity, the
density of the ligand, and the spatial distribution of the ligand must be
carefully considered when designing a biomimetic material.
Many studies utilize laminin-1 when designing a biomimetic material. Laminin is
a component of the extracellular matrix that is able to promote neuron
attachment and differentiation, in addition to axon growth guidance. Its primary
functional site for bioactivity is its core protein domain
isoleucine-lysine-valine-alanine-valine (IKVAV), which is located in the α-1
chain of laminin. A recent study by Wu, Zheng et al., synthesized a
self-assembled IKVAV peptide nanofiber and tested its effect on the adhesion of
neuron-like pc12 cells. Early cell adhesion is very important for preventing
cell degeneration; the longer cells are suspended in culture, the more likely
they are to degenerate. The purpose was to develop a biomaterial with good cell
adherence and bioactivity with IKVAV, which is able to inhibit differentiation
and adhesion of glial cells in addition to promoting neuronal cell adhesion and
differentiation. The IKVAV peptide domain is on the surface of the nanofibers so
that it is exposed and accessible for promoting cell contact interactions. The
IKVAV nanofibers promoted stronger cell adherence than the electrostatic
attraction induced by poly-L-lysine, and cell adherence increased with
increasing density of IKVAV until the saturation point was reached. IKVAV does
not exhibit time dependent effects because the adherence was shown to be the
same at 1 hour and at 3 hours.
Note for Nanofiber
Nanofibers are defined as fibers with diameters less than 100 nanometers. They
can be produced by interfacial polymerization and electrospinning. Carbon
nanofibers are graphitized fibers produced by catalytic synthesis.
Applications
In one study, combined neural stem cells with carbon nanofibers triggered neural
tissue regeneration in the brains of rats that had suffered a simulated stroke.
On their own, neither nanofibers nor stem cells could heal the rats.
Napkins with nanofibers contain antibodies against numerous biohazards and
chemicals that signal by changing color (potentially useful in identifying
bacteria in kitchens).
In wound healing nanofibers assemble at the injury site and stay put, drawing
the body's own growth factors to the injury site.
Donaldson develops nanofiber filter media for new air and liquid filtration
applications, such as vacuum cleaners.
Other applications include industrial and high-tech applications for aerospace,
capacitors, transistors, battery separators, energy storage, fuel cells and
information technology.
Note for Parkinson's Disease
Parkinson's disease (also known as Parkinson disease or PD) is a degenerative
disorder of the central nervous system that often impairs the sufferer's motor
skills and speech.
Parkinson's disease belongs to a group of conditions called movement disorders.
It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia)
and, in extreme cases, a loss of physical movement (akinesia). The primary
symptoms are the results of decreased stimulation of the motor cortex by the
basal ganglia, normally caused by the insufficient formation and action of
dopamine, which is produced in the dopaminergic neurons of the brain. Secondary
symptoms may include high level cognitive dysfunction and subtle language
problems. PD is both chronic and progressive.
PD is the most common cause of chronic progressive parkinsonism, a term which
refers to the syndrome of tremor, rigidity, bradykinesia and postural
instability. PD is also called "primary parkinsonism" or "idiopathic PD"
(classically meaning having no known cause although this term is not strictly
true in light of the plethora of newly discovered genetic mutations). While many
forms of parkinsonism are "idiopathic", "secondary" cases may result from
toxicity most notably drugs, head trauma, or other medical disorders.
Parkinson disease affects movement (motor symptoms). Typical other symptoms
include disorders of mood, behavior, thinking, and sensation (non-motor
symptoms). Individual patients' symptoms may be quite dissimilar and progression
of the disease is also distinctly individual.
There are currently no blood or laboratory tests that have been proven to help
in diagnosing PD. Therefore the diagnosis is based on medical history and a
neurological examination. The disease can be difficult to diagnose accurately.
The Unified Parkinson's Disease Rating Scale is the primary clinical tool used
to assist in diagnosis and determine severity of PD. Indeed, only 75% of
clinical diagnoses of PD are confirmed at autopsy. Early signs and symptoms of
PD may sometimes be dismissed as the effects of normal aging. The physician may
need to observe the person for some time until it is apparent that the symptoms
are consistently present. Usually doctors look for shuffling of feet and lack of
swing in the arms. Doctors may sometimes request brain scans or laboratory tests
in order to rule out other diseases. However, CT and MRI brain scans of people
with PD usually appear normal.
Clinical practice guidelines introduced in the UK in 2006 state that the
diagnosis and follow-up of Parkinson's disease should be done by a specialist in
the disease, usually a neurologist with an interest in movement disorders.
In figure, Sea cucumbers inspired the design of chemo-responsive nanocomposite with adaptive mechanical properties
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