in nuclear physics at Lawrence Livermore is performed to serve the
Laboratorys national security mission. But along the way,
the research has spun off technologies beneficial to human health.
One example is PEREGRINE, a tool for calculating the dose that patients
receive during radiation therapy for cancer. PEREGRINE combines
Livermores storehouse of data on radiation transport with
Monte Carlo statistical techniques. Modeling of radiation transport
also came into play in developing optical coherence tomography,
a technique that uses infrared light to image through highly scattering
media such as blood and the walls of arteries.
Now, in the Chemistry and
Materials Science Directorate, physicist Mehdi Balooch is using
a new technique he pioneered for examining uranium and other materials
inside aging nuclear weapons and applying it to soft materials in
the human body. Working with researchers at the University of California
at San Francisco (UCSF), he is using this method to learn more about
the strength of human teeth. He has also used it to study both healthy
and damaged human arteries.
Baloochs new applications
make use of a modified atomic force microscope (AFM). Atomic force
microscopy has been in use since the 1990s to produce topographic
maps of nanostructures. In atomic force microscopy, an extremely
sharp tip mounted to a cantilever arm senses the atomic shape of
a sample while a computer records the path of the tip and slowly
builds up a three-dimensional image.
modified AFM makes indentations just 20 trillionths of a meter deep
on the surface of a sample material. Even such a tiny hole100,000
times shallower than the width of a human hairprovides extraordinarily
useful measurements about the sample. When force and displacement
data are combined in various algorithms, the resulting calculations
reveal information on mechanical propertieshardness, stiffness,
or any other reaction to an applied force.
modified AFM is unique in its ability to measure the mechanical
properties of both hard and soft materials. Hard materials are usually
easy to study, but measuring the mechanical properties of soft materials
has been more difficult. Part solid and part fluid, soft materials
include many biological tissues, polymers, and hydrated clays, an
important component of soils. Traditionally, mechanical properties
have been obtained using dried samples. But the modified AFMs
ability to take measurements in liquid allows accurate measurements
of the mechanical properties of soft materials for the first time.
a hard material in liquid has been equally difficult. Teeth, for
example, present a challenge because their normal environment is
in saliva. The modified AFM allows for measurements of fully hydrated
teeth. Two years ago, Baloochs team took some of the first
nanoscale measurements of hardness and elasticity at the junction
between tooth enamel, a hard mineral material, and the dentin just
beneath it. Dentin is a soft materialpart mineral, part protein,
and part fluid.
has been working with dental researchers Bill and Sally Marshall
at UCSF for 11 years, operating under a grant from the National
Institutes of Health. A 5-year extension of the grant for continued
research on the hardness and stiffness of dental material begins
modified AFM is also finding applications beyond weapons and health.
Balooch recently began collaborating with colleagues in the Energy
and Environment Directorate to study the effect of water on the
mechanical properties of clay materials, which cannot be determined
by conventional testing methods. The researchers are making first-ever
measurements of the mechanical properties of single crystals of
clay with water intercalated in the crystal structure. Here, as
in teeth, a hard mineral material coexists with liquid matter. Until
now, it has been impossible to understand how hard mineral and liquid
matter work together on this very small scale.
A traditional atomic force microscope (AFM) traces the atomic
shape of a sample with a tip on a cantilevered arm. The path
of the tip is recorded on a computer and used to build a three-dimensional
image. (b) The modified AFM incorporates the features of a
nanoindenter to make tiny holes on the surface of a sample
material. Instead of a cantilevered tip assemply, this AFM
has a transducer-driven head and tip that can measure the
mechanical properties of a fully hydrated sample.
and a Solution
Baloochs early work with UCSF on teeth, he used a commercially
available nanoindenter, which made indentations 100 to 200 nanometers
deep on the surface of samples. It was useful but not quite what
was needed. For one thing, it could not operate under water or on
hydrated samples. The nanoindenter also could not position an indentation
within less than a few micrometers, which was not precise enough.
Nor was the nanoindenter an imaging device, so it was impossible
to know with certainty where indentations were being made.
To solve these problems,
Balooch, in collaboration with a startup company named Hysitron,
modified an atomic force microscope to incorporate the features
of a nanoindenter. To obtain not only a topographic image but also
mechanical-property measurements, the team replaced the AFM tip
with a transducer-driven head and tip that can operate on a fully
This modified AFM can operate
in either an imaging mode or an indentation mode. In the imaging
mode, the force applied to the cantilever is about 1 micronewton,
a force so small that it does not dislodge a single atom in the
sample. When used in the indentation mode, the force on the arm
is up to 30,000 times as much, though it is still extremely small.
The modified AFM permits
positioning the indentation with an accuracy of a few tens of nanometers
for site-specific studies, which is a thousand times more accurate
than that of a standard nanoindenter with an optical microscope
attachment. It allows measurement of hardness values with indentation
depths of less than 20 nanometers.
Now, says Balooch,
with a coat-on-coat dynamic testing capability, we can get
pixel-by-pixel information on stiffness or elasticity. Plus, we
get topography so we know where weve been.
Schematic diagram of a human tooth, showing enamel, dentin,
and pulp. (b) A scanning electron microscope image of dentin
showing the collagen fibers and tubules that run through dentin.
Getting a Grip on Teeth
is about 50 percent mineral, 30 percent organic material (mostly
collagen), and about 20 percent fluid. Microscopic tubes, called
tubules, run through dentin, from the pulp beneath to the junction
with the enamel above. Dentins structure is not precisely
the same everywhere in a tooth, and both age and disease affect
Collagen, a major component
of dentin, is the most abundant animal protein in mammals, accounting
for about 30 percent of all proteins. It is responsible for the
tissues that hold us together, such as bone, cartilage, tendons,
For the UCSF researchers,
knowing how hard, soft, brittle, or elastic dentin is will help
improve restorative dentistry. Fillings, bridges, crowns, and other
dental repairs must bond to the dentin, or they will fail.
Balooch is assisting the
project by finding or creating the best tools to supply necessary
measurements. His modified AFM may well be the best tool thus far
for determining the mechanical properties of dentin. Because of
the problems inherent in studying soft materials, especially very
small samples of soft materials, there have been large discrepancies
among measurements of the hardness and stiffness of dentin. This
wide variation has made it impossible to establish the baseline
mechanical behavior of dentin or to explore the effects of age,
gender, or disease on tooth strength. The accuracy of the modified
AFM allows evaluations of nanomechanical properties on a highly
site-specific level, for the first time.
With the modified AFM, the
team was the first to reveal major differences in the hardness of
dentin in and around the tubes that traverse it. Hardness is evaluated
using the pressure exerted by the indenter on the contact area.
Using a different tip on the modified AFM, they measured fracture
toughness by inducing cracks, a standard practice in examining the
effects of stress on a material.
A standard atomic force microscope (AFM) topographical image
of a polished dentin specimen. The dark areas are the tubules
that run through dentin. (b) An image with the modified AFM
measures the stiffness of the same site. The peritubular dentin,
the area immediately around the tubules, is stiffer than the
rest of the surface.
At the dentinenamel
junction, hard, brittle tooth enamel overlays soft, ductile dentin.
Macroscopic tensile, compression, or shear tests are difficult at
the junction, an area that represents a small percentage of the
already small human tooth.
Here, things got very
interesting, says Balooch. As shown in the figure below, the
modified AFM exposed in detail the stiffness and hardness differences
between dentin and enamel. The teams results agreed well with
earlier macro- and microscale experiments by others. The smooth
transition across the junction suggests that mineral content there
must gradually change because the mineral component of calcified
tissue is closely related to its mechanical properties.
While creating cracks in
enamel is relatively easy, Baloochs team found that inducing
cracks in the dentinenamel junction is much more difficult.
The strength of the junction suggests that this area is critical
for preserving the physical integrity of the tooth. Because of these
characteristics, the dentinenamel junction may serve as a
model for the linkage of other pairs of highly dissimilar materials,
such as those in artificial hip replacements.
Estimates of the precise
width of the dentinenamel junction have ranged from 12 to
200 micrometers using various micro- and nanoscale methods. Balooch,
ever the tinkerer, tackled this measurement problem by again modifying
an atomic force microscope. He changed it to create nanoscratches
across the junction while simultaneously measuring lateral force.
By measuring changes in friction, his team estimated a width for
the junction of from 1 to 3 micrometers, about 10 times smaller
than the smallest width previously estimated.
An atomic force microscope (AFM) image of indentations across
the dentinenamel junction. (b) Curves show the corresponding
hardness and stiffness (elastic modulus) determined from indentations
with the modified AFM. The smooth increase in stiffness across
the junction suggests that mineral content must gradually change
there because the mineral component of calcified tissue is closely
related to its mechanical properties.
To investigate healthy and
diseased human arteries, Balooch joined forces with Livermore physicist
John Kinney, whose specialty is x-ray tomography.
Heart disease is the leading
cause of death in the U.S. and is most commonly treated by catheter-based
balloon angioplasty to dilate obstructed coronary arteries. While
this procedure results in a 95-percent immediate success rate, 40
percent of all treated sites renarrow within 6 months. This lack
of long-term success has led to considerable interest in how normal
and diseased arteries become deformed. In particular, research focused
on how the artery wall changes from stretching or from changes to
fatty plaque deposits (calcifications).
his modified AFM, Balooch first measured local, in vitro mechanical
properties of femoral artery tissue in saline solution. Then he
moved to diseased calcified arteries. For this work, ultrasound
images of arteries were recorded, and the healthy and calcified
regions were marked. The healthy regions were extracted, and solid
calcified deposits were dissected from the artery wall. More than
40 samples of both types of tissue were then mechanically tested
with the modified AFM. Calcified deposits were found to be many
orders of magnitude stiffer than the healthy artery wall, even as
deposits varied from sample to sample and in their position on the
Kinney took x-ray tomographs
of dissected femoral arteries to make dynamic measurements of the
deformation of plaque and vessel walls during graded stages of balloon
inflation and deflation. He found that fatty plaque deposits were
less elastic than commonly assumed, which could affect the long-term
success of balloon angioplasty.
on a calcified artery. The deposits can be mechanically tested
with the modified atomic force microscope to determine their
stiffness and thus reveal how the artery wall has been changed
Geophysicists Brian Bonner
and Dan Farber and geochemist Brian Viani are using the modified
AFM to address seismological issues. Most of us equate seismology
with earthquakes. But seismology in its broadest sense is simply
the study of Earths dynamic response to any mechanical stimuli.
Seismic research thus encounters not just earthquakes but also problems
related to slope stability, mitigating earthquake hazards, tracking
the movement of pollutants underground, and exploring for oil, gas,
and other hydrocarbons. With its Ground-Based Nuclear Explosion
Monitoring program, Livermore also focuses on forensic seismology,
which is the detection of clandestine explosions.
The project using the modified
AFM addresses a fundamental problem in remote seismic sensing: the
role fluids play in the transmission, modulation, and dissipation
of seismic energy. The importance of water, even at very low
concentrations, was made clear during analysis of seismic events
on our Moon, says Bonner.
When the Apollo astronauts
blasted off, they took along instruments, including seismometers,
to study the Moons properties. The seismograms from the Moon
proved to be totally different from seismograms here on Earth. They
showed that the Moon rang like a bell, says Bonner.
But later, when rock samples
were brought back by the Apollo astronauts, their seismic response
became Earth-like. Conversely, scientists found that when Earth
rocks were baked in high-temperature vacuum ovens, they took on
Moon-like attenuation characteristics. The scientists assumed that
the change in the Moon rocks must have been caused by tiny amounts
of water acquired in Earth-bound laboratories. Even a small amount
of water made a big difference. Now, after more than 30 years, Baloochs
apparatus finally is providing the means to study this effect directly.
Bonner, Farber, and Vianis
experiments are similar to the Moon-rock scenario in that they are
studying the effects of an extremely small amount of water in a
mineral. They use a type of clay, montmorillonite, which can confine
very thin layers of water (0.25 nanometer per layer) between the
sheets of silicate that make up the clay.
An image of the experimental
sample is shown in the figure below. Samples were tested using the
modified AFM in dry nitrogen and in air with about 30-percent relative
humidity. Stiffness decreased dramatically when the sample was hydrated,
as indicated by force modulation tests. Attenuationthe dissipation
of mechanical energyincreases greatly in the hydrated sample.
The measurements show the samples behaving like a classical viscoelastic
material, that is, a viscous material that has some elastic properties.
Now we can say with confidence that a viscoelastic model is
an appropriate one for seismic response in clay-dominated geologies,
even for seismic displacements, says Bonner.
prepared sample of a clay before it is measured with the modified
atomic force microscope. The clay is montmorillonite, which
can confine very thin water layers (0.25 nanometer per layer)
between the sheets of silicate.
These results make perfect
sense on an intuitive level because we know what water does to soil.
But the experiments make this observation quantitative for the first
time. Collecting data on the nanoscale removes the effect of other
interfering phenomena, such as rock porosity. Nanoscale effectswhich
had never before been observedreveal the important mechanisms
of seismic deformation and are consistent with observations on a
The team is now studying
other sheet silicates to observe the effects of additional layers
of water. They are also beginning to use molecular dynamics and
effective-medium modeling to make predictions on a scale that is
useful in the field.
of mechanical properties of clay are important for several other
energy and environmental uses, says geophysicist Pat Berge,
acting division leader for Geophysics and Global Security, who applies
these results to field-scale issues. With them, we can model
larger-scale behavior of clay-bearing soils and rocks.
Geophysicists estimate the
unknown composition of rock and soil underground by making seismic
measurements at environmental cleanup sites, oil fields, or other
regions of interest. Then they use rock-physics theories to interpret
the seismic data and figure out how much fluid and what types and
amounts of minerals are present. The success of these calculations
depends on having good estimates of the seismic velocities of the
pure fluids and minerals that make up the fluid-bearing soil or
rock. Properties of water, quartz, calcite, and other minerals found
in sandstones and sandy soils are readily available. Until Baloochs
new technique came along, geophysicists did not have good estimates
of the properties of clay, particularly at the smallest scales of
individual clay platelets. This made modeling silty sands and shales
difficult, leading to problems in underground imaging for oil reservoirs
and environmental sites. Livermore researchers such as Berge suspected
that the seismic velocity estimates commonly used for clay were
too large by a factor of at least two and possibly a factor of five.
But without laboratory corroboration, it was not possible to change
the minds of the geophysical community. The new measurements by
Farber, Bonner, and Viani show that clay is indeed a very soft material.
recently, Balooch has been working with chemist Wigbert Siekhaus
to modify the AFM further so that stiffness can be imaged directly.
This process, presently being patented, only adds to Livermores
unique ability for nanoscale examination of soft materials, an intermediate
regime that until now has eluded researchers.
Key Words: atomic
force microscope (AFM), dentin, mechanical properties, nanoindenter,
plaque, seismic response, vascular disease.
information contact Mehdi Balooch (925) 422-7311 (firstname.lastname@example.org).