Biocompatible material
In surgery, a biocompatible material (sometimes shortened to biomaterial) is
a synthetic or natural material used to replace part of a living system or to
function in intimate contact with living tissue. Biocompatible materials are
intended to interface with biological systems to evaluate, treat, augment or
replace any tissue, organ or function of the body. Biomaterials are usually
non-viable, but may also be viable.
A biocompatible material is different from a biological material such as bone
that is produced by a biological system. Artificial hips, vascular stents,
artificial pacemakers, and catheters are all made from different biomaterials
and comprise different medical devices.
Biomimetic materials are not made by living organisms but have compositions
and properties similar to those made by living organisms. The calcium
hydroxylapatite coating found on many artificial hips is used as a bone
replacement that allows for easier attachment of the implant to the living bone.
Surface functionalization may provide a way to transform a bio-inert material
into a biomimetic or even bio-active material by coupling of protein layers to
the surface, or coating the surface with self-assembling peptide scaffolds to
lend bioactivity and/or cell attachment 3-D matrix.
Different approaches to functionalization of biomaterials exist. Plasma
processing has been successfully applied to chemically inert materials like
polymers or silicon to graft various functional groups to the surface of the
implant. Polyanhydrides are polymers successfully used as a drug delivery
materials.
Rutherford backscattering
Rutherford backscattering (or RBS, for Rutherford Backscattering
Spectrometry) is an analytical technique in materials science. It is named for
Ernest Rutherford who in 1911 first explained Geiger and Marsden's experimental
results for alpha particle scattering from a very thin gold foil in a backward
direction by using the Coulomb electrostatic force between the positively
charged nucleus and the positively charged alpha particle. Rutherford first
correctly described the atom as a tiny positive nucleus surrounded by negatively
charged electrons (essentially the Bohr atom) on the basis of this experiment.
This contradicted J.J. Thomson's "plum pudding model," the popularly accepted
model of the atom at that time. Rutherford famously expressed his surprise at
this experiment: "It was as though one fired a bullet at a piece of paper, and
it bounced back at you!"
A high energy beam ( 2 to 4 MeV ) of low mass ions ( e.g. He++ ) is directed
at a sample. A detector is placed such that particles which scatter from the
sample at close to a 180 degree angle will be collected. The energy of these
ions will depend on their incident energy and on the mass of the sample atom
which they hit, because the amount of energy transferred to the sample atom in
the collision depends on the ratio of masses between the ion and the sample
atom. Thus, measuring the energy of scattered ions indicates the chemical
composition of the sample.
Additionally, in the case that the incident ion doesn't hit any of the atoms
near the surface of the sample, but instead hits an atom deeper in, the incident
ion loses energy gradually as it passes through the solid, and again as it
leaves the solid. This means that RBS can be used as a means to perform a depth
profile of the composition of a sample. This is especially useful in analysis of
thin-film materials. For example, films about half a micrometre in thickness can
be profiled using a 2 MeV He beam, or films up to about 10 micrometres thick can
be profiled with a 2 MeV H beam.
RBS is now a very widely used analytical technique, which has the great
advantage that it is absolute, requiring no standards for quantification (since
the probability of interaction - the cross-section - is given by the Coulomb
potential). It is one of a family of techniques, collectively known as Ion beam
analysis.
Neutron diffraction
Neutron diffraction is a crystallographic method for the determination of the
atomic structure of a material. This is a form of elastic scattering where the
neutrons exiting the experiment have more or less the same energy as the
incident neutrons. The technique is similar to X-ray diffraction but the
different type of radiation gives complementary information. A sample to be
examined is placed in a beam of thermal or cold neutrons and the intensity
pattern around the sample gives information of the structure of the material.
Neutrons are particles found in the atomic nucleus. In a nuclear reactor,
neutrons can be set free when nuclei decay (fission, radioactivity). All quantum
particles can exhibit wave phenomena we typically associate with light or sound.
Diffraction is one of these phenomena; it occurs when waves encounter obstacles
whose size is comparable with the wavelength. If the wavelength of a quantum
particle is short enough, atoms or their nuclei can serve as diffraction
obstacles. When neutrons from a reactor are slowed down and selected properly,
their wavelength lies near one angstrom (0.1 nanometer), the typical separation
between atoms in a solid material.
A neutron diffraction measurement requires a neutron source (e.g. a nuclear
reactor or spallation source), a sample (the material to be studied), and a
detector. At a research reactor other components such as crystal monochromators
or filters may be needed to select the desired neutron wavelength. Some parts of
the setup may also be movable. At a spallation source the time of flight
technique is used to sort the energies of the incident neutrons, so no
monochromator is needed, just a bunch of electronics. (Higher energy neutrons
are faster - v. simple)
Neutrons interact with matter differently than x-rays. X-rays interact
primarily with the electron cloud surrounding each atom. The contribution to the
diffracted x-ray intensity is therefore larger for atoms with a large atomic
number (Z) than it is for atoms with a small Z. On the other hand, neutrons
interact directly with the nucleus of the atom, and the contribution to the
diffracted intensity is different for each isotope; for example, regular
hydrogen and deuterium contribute differently. It is also often the case that
light (low Z) atoms contribute strongly to the diffracted intensity even in the
presence of large Z atoms. Non-magnetic neutron diffraction is directly
sensitive to the positions of the nuclei of the atoms. Although neutrons are
uncharged, they carry a spin, and therefore interact with magnetic moments,
including those arising from the electron cloud around an atom. Neutron
diffraction can therefore reveal the microscopic magnetic structure of a
material.
Neutron diffraction can be used to establish the structure of low atomic
number materials like proteins and surfactants much more easily with lower flux
than at a synchrotron radiation source. This is becuase some low atomic number
materials have a higher cross section for neutron interaction than higher atomic
weight materials.
The first neutron diffraction experiments were carried out in 1945 by Ernest
O. Wollan using the Graphite Reactor at Oak Ridge. He was joined shortly
thereafter by Clifford Shull, and together they established the basic principles
of the technique, and applied it successfully to many different materials,
addressing problems like the structure of ice and the microscopic arrangements
of magnetic moments in materials. For this achievement Shull was awarded one
half of the 1994 Nobel Prize in Physics. Wollan had passed away in the 1990's.
(The other half of the 1994 Nobel Prize for Physics went to Bert Brockhouse for
development of the inelastic scattering technique at the Chalk River facility of
AECL. This also involved the invention of the triple axis spectrometer).
Brockhouse and Shull jointly take the somewhat dubious distinction of the
longest gap between the work being done (1945) and the Nobel Prize being awarded
(1994).
One unusual application of elastic neutron scattering/diffraction is that the
lattice constant of metals can be very accurately measured. Together with an
accurately aligned micropositioner a map of the lattice constant through the
metal can be derived. This can easily be converted to the stress field
experienced by the material. This has been used to analyse stresses in aerospace
and automotive components to give just two examples.
X-ray diffraction
X-ray scattering techniques are a family of non-destructive analytical
techniques which reveal information about the crystallographic structure,
chemical composition, and physical properties of materials and thin films. These
techniques are based on observing the scattered intensity of an x-ray beam
hitting a sample as a function of incident and scattered angle, polarization,
and wavelength or energy.
Materials
Materials are physical substances used as inputs to production or
manufacturing. Raw materials are first extracted or harvested from the earth and
divided into a form that can be easily transported and stored, then processed to
produce "semi-finished materials". These can be input into a new cycle of
production and "finishing processes to create "finished materials", ready for
distribution and consumption.
An example of a raw material is cotton, which can be processed into thread,
and then processed into cloth, a semi-finished material. Sewing and cutting the
fabric turns it into a garment, which is a finished material. Steelmaking is
another example�raw materials are extracted, refined and processed into steel, a
semi-finished material. Steel is then used as an input in many other industries
to make finished products.
Composite Materials
Composite materials (or composites for short) are engineered materials made
from two or more constituent materials with significantly different physical or
chemical properties and which remain separate and distinct within the finished
structure.
Composite materials (or composites for short) are engineering materials made
from two or more constituent materials that remain separate and distinct on a
macroscopic level while forming a single component. There are two categories of
constituent materials: matrix and reinforcement. At least one portion of each
type is required. The matrix material surrounds and supports the reinforcement
materials by maintaining their relative positions. The reinforcements impart
their special mechanical and physical properties to enhance the matrix
properties. A synergism produces material properties unavailable from the
individual constituent materials. Due to the wide variety of matrix and
reinforcement materials available, the design potentials are incredible.
The most primitive composite materials comprised straw and mud in the form of
bricks for building construction. The most advanced examples perform routinely
on spacecraft in demanding environments. The most visible applications pave our
roadways in the form of either steel and aggregate reinforced portland cement or
asphalt concrete. Those composites closest to our personal hygiene form our
shower stalls and bath tubs made of fiberglass. Solid surface, imitation granite
and cultured marble sinks and countertops are widely used to enhance our living
experiences.
There are the so-called natural composites like bone and wood. Both of these
are constructed by the processes of nature and beyond the scope of this text.
Engineered composite materials must be formed to shape. This involves
strategically placing the reinforcements while manipulating the matrix
properties to achieve a melding event at or near the beginning of the component
life cycle. A variety of methods are used according to the end item design
requirements. The principal factors impacting the methodology are the natures of
the chosen matrix and reinforcement materials. Another important factor is the
gross quantity of material to be produced. Large quantities can be used to
justify high capital expenditures for rapid and automated manufacturing
technology. Small production quantities are accommodated with lower capital
expenditures but higher labor and tooling costs at a correspondingly slower
rate.
Most commercially produced composites use a polymer matrix material often
called a resin solution. There are many different polymers available depending
upon the starting raw ingredients. There are several broad categories, each with
numerous variations. The most common are known as polyester, vinyl ester, epoxy,
phenolic, polyimide, polyamide, polypropylene, PEEK, and others. The
reinforcement materials are often fibers but also commonly ground minerals.
The physical properties of composite materials are generally not isotropic in
nature. For instance, the stiffness of a composite panel will often depend upon
the directional orientation of the applied forces and/or moments. In contrast,
an isotropic material has the same stiffness regardless of the directional
orientation of the applied forces and/or moments. The relationship between
forces/moments and strains/curvatures for an isotropic material can be described
with the following material properties: Young's Modulus, the Shear Modulus and
the Poisson's Ratio, in relatively simple mathematical relationships. For the
anisotropic material, it requires the mathematics of a second order tensor and
can require up to 21 material property constants. For the special case of
orthogonal isotropy, there are three different material property constants for
each of Young's Modulus, Shear Modulus and Poisson's Ratio for a total of 9
material property constants to describe the relationship between forces/moments
and strains/curvatures.
Fiber Reinforced Composite Materials can be divided into two main categories
normally referred to as short fiber reinforced materials and continuous fiber
reinforced materials. Continuous reinforced materials will often constitute a
layered or laminated structure.
Shocks, impact, loadings or repeated cyclic stresses can cause the laminate
to separate at the interface between two layers, a condition known as
delamination. Individual fibers can separate from the matrix e.g. fiber
pull-out.
Nuclear reaction analysis
Nuclear reaction analysis (NRA) is a nuclear method in materials science to
obtain concentration vs. depth distributions for certain target chemical
elements in a solid thin film.
If irradiated with select projectile nuclei at kinetic energies Ekin these
target elements can undergo a nuclear reaction under resonance conditions for a
sharply defined resonance energy. The reaction product is usually a nucleus in
an excited state which immediately decays, emitting ionizing radiation.
To obtain depth information the initial kinetic energy of the projectile
nucleus (which has to exceed the resonance energy) and its stopping power
(energy loss per distance travelled) in the sample has to be known. To
contribute to the nuclear reaction the projectile nuclei have to slow down in
the sample to reach the resonance energy. Thus each initial kinetic energy
corresponds to a depth in the sample where the reaction occurs (the higher the
energy, the deeper the reaction).
|
