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Fabricating and Characterizing Biodegradable Polymeric Fibers
for Medical Applications: Experiments with Poly(L-Lactic Acid)
New Jersey Governor’s School of Engineering and Technology 2014
Jenna Barton
Hyeri Cho
Sara Lee
Ryan Oakley
Abstract Poly-L-lactic acid (PLLA) is accepted as a
suitable material for bone pins and sutures in the
medical field due to its biodegradable nature and ease
of manipulation. Through this study, many of
PLLA’s notable attributes were tested in order to
further characterize the polymer and encourage
expansion in its biomedical applications. Using X-ray
diffraction, the material’s molecular structure was
determined to be semi-crystalline after being
annealed and amorphous prior to the heating process.
Through differential scanning calorimetry it was
found that PLLA has a glass transition temperature of
46.5˚C, demonstrating that the material will not lose
any of its characteristics when used for biomedical
purposes, as the body’s natural temperature is 37˚C.
Additionally, both the annealed and non-annealed
fibers were characterized using the stress/strain curve
through the tensile strength test. The study
established that the more crystalline annealed fibers
were stiffer and more resistant to strain. Based on the
results, it was concluded that the annealing process
and subsequent more crystalline structure improved
the fibers for their intended purpose of biomedical
aids for bone and muscle injuries.
1 Introduction
With perpetual advancements in the research
and applications of macromolecules, polymers
continue to impact all aspects of the modern
population. These materials are prevalent in a wide
range of industries, including large-scale commercial
production and the biomedical field. This study
specifically revolves around testing poly-L-lactic
acid, PLLA, in order to analyze its characteristics in
respect to the focus of further expanding its
biomedical applications in surgery and patient
treatment.
PLLA is a stereoisomer of poly(lactic acid),
which is a biodegradable polymer produced from
lactic acid found in renewable resources. Focusing on
the biomedical field, this biodegradable characteristic
is highly valuable. Many operations require bone
pins, sutures, screws, wires, and plates, all of which
are often made from non-biodegradable materials.
Subsequently, the use of these materials requires a
second surgery for removal after the patient heals.
Due to PLLA’s biodegradable nature, a second
surgery is not necessary, making it ideal for such
applications. Additionally, if PLLA can be made with
a similar strength to the material it is replacing,
patient recuperation time and comfort could be
improved. During the study, tensile strength, ease of
extrusion, and crystallinity of PLLA fibers were
tested in order to characterize the polymer for future
biomedical applications.
2 Background
2.1 Poly-L-Lactic Acid (PLLA)
A polymer is a chain of repeating units called
monomers that are representative of the original
molecule. The polymer chains can consist of
hundreds of monomer units, often having a molecular
weight ranging in the thousands of atomic mass units
(amu).1
The monomer used in the polymer PLLA is
either lactic acid or lactide, each of which undergoes
a different process to reach PLLA. Lactide is
polymerized through a ‘ring-opening’ process during
which the ring structure of lactide is broken and the
ends are stitched together. Lactic acid is polymerized
through a condensation reaction where the ends of
the lactic acid molecules react with each other to
combine and give off water (see Figure 1).
Polymers have a large scope of interesting
properties, ranging from shape-memory to pH-
sensitivity. Poly-L-lactic acid is studied for its
potential applications in the medical field due to its
biodegradability and tensile strength.2
Additionally, PLLA has a similar strength to
bone and is compatible with human bone cells when
copolymerized, facilitating the healing process.3
2.2 Mechanical Properties of Polymers
The mechanical properties of a polymer are
mainly concerned with how the polymer reacts to
stress and strain. Stress, represented by the symbol σ,
is the pressure applied to the polymer in units of
force per area. Strain, represented by the symbol ε, is
the elongation that the polymer experiences as a
percentile difference from the original length. To test
the tensile strength of a polymer, a machine pulls a
fiber from two ends while taking stress and strain
data. The data is then plotted on a stress-strain curve
which contains multiple vital points of data. The first
point is the tangent modulus (a.k.a. Young’s
Modulus), which is the linear region at the beginning
of the graph that represents the overall stiffness of the
polymer.4 The yield point appears after the tangent
modulus and is the first vertex that the graph shows.
This point represents the transition from elastic to
plastic: to the left of the yield point the polymer will
revert back to its original shape if the applied stress is
removed, where to the right of the yield point the
polymer will be permanently deformed and not revert
back to its original shape when the applied stress is
removed.5 Another important point of data within the
stress-strain curve is the point of rupture, which
shows how long the polymer can be stretched before
it snaps. The study of the mechanical properties of
polymers in relation to biomedical applications is
motivated by the desire to replace biological parts
with the studied polymers such that the replacements
are able to withstand the same stress and strain of the
originals and therefore maintain similar mechanical
properties. The mechanical properties that should be
studied include the polymer’s intrinsic, structural,
and viscoelastic properties. A polymer’s viscoelastic
property is a time-dependent property that relates to
the deformation of the polymer, meaning that it is
dependent on how quickly or slowly the polymer
deforms under a force.6 If the polymer deforms with
a constant stress and the strain increases with time, it
is experiencing creep. If the polymer deforms with a
constant strain and the stress decreases with time, it is
experiencing stress-relaxation. Both creep and stress-
relaxation are viscoelastic properties. All of these
properties must be taken into account when designing
and choosing polymers for biomedical use.
2.3 Calorimetry
Calorimetry is defined as the measurement of
the flow of heat into or out of a sample. A
calorimeter is a piece of equipment that identifies the
thermochemical properties of a sample, such as its
melting point or specific heat. A differential
calorimeter measures the gain or loss in a sample
relative to a fixed reference point and heats the
sample at a constant rate, plots the data, and scans a
whole temperature range.7 This type of calorimeter
clearly communicates the thermochemical properties
of a sample such as glass transition temperature,
crystallization temperature, and melting point by
putting the sample through a heating cycle. This
cycling makes the sample more amorphous and is
necessary if the sample is originally crystalline due to
the fact that it cannot go through a crystallization
process if it is already in a crystal state.8
2.4 Extrusion
Extrusion is the process of melting the
polymer and pushing it through an opening called a
die in order to form a continuous profile in the shape
of the die’s cross-section. This process successfully
produces uniform fibers, pipes, or sheets.9 Extrusion
is a suitable method for polymers that are robust
against heat and results in an amorphous structure
due to rapid cooling. Specifically used in this
experiment, plunger extrusion pushes the polymer
through a die using the pressure produced by the
apparatus and is considered to produce less waste
material than the more common extrusion process,
twin-screw extrusion.10
2.5 X-Ray Diffraction
X-ray diffraction (XRD) is a crucial analytical
technique in determining crystalline structure and
chemical composition of a substance. The
crystallinity of a substance can be determined by
emitting high energy X-ray light on the sample and
analyzing its diffraction pattern (see Figure 2).11
The
degree of crystallinity can be determined by
measuring the width of the peaks on the integrated
graph of diffraction intensity.12
This technique not
only reveals the degree of crystallinity, but it also
identifies the angle of diffraction which can then be
used to determine the crystalline structure through
Bragg’s Law.13
Since many polymer composites used
in the laboratory and industry are semi-crystalline,
XRD is an important tool when analyzing and
characterizing polymers.
Through the integration of collected data, one
can determine the percent crystallinity with the ratios
of integrated intensities calculated from the
diffraction pattern.14
From the XRD data one can also
determine the preferred orientation of the sample.
Using the accompanying software, the integrations
can be manipulated to eliminate background noise
from the air as well as amorphous data in order to
better identify the crystalline structure. Through the
process of X-ray diffraction and the analysis of the
interference patterns, many characteristics of the
molecular structure can be identified and studied.
2.6 Bragg’s Law
Bragg’s law is an optical law used in X-ray
diffraction that is mathematically defined as
, where n is an integer that represents the
number of times the wave oscillates beneath the first
layer, λ is the wavelength of the X-rays, d is the
distance between two layers of the crystal, and θ is
the angle of incidence (see Figure 3).15
In X-ray
diffraction, Bragg’s law is treated as a function of
theta that outputs the d-spacing. Substituting values
into the equation allows for the production of a graph
of intensity vs. incident angle which produces peaks
that can be used as a fingerprint for the material.16
The graph can be used to identify an unknown
sample by comparing it to known materials in the
database. A large intensity is indicative of
constructive interference after the refraction of the X-
rays, and the angle at which it occurs is specific to
certain crystalline compounds.17
Data-wise, this
produces a Bragg peak which can be used to
characterize the sample.
2.7 Annealing
Mechanical characteristics of a polymer can
be altered via the process of annealing. Annealing
polymer involves heating it below its melting point,
resulting in changes of physical properties and
crystalline structure.18
The polymer can undergo
recrystallization depending on the annealing
temperature and cooling temperature. Slow
crystallization after the heating yields a more
crystalline structure while rapid cooling or
inadequate heating will not result in much
recrystallization. In order to achieve greater
crystallinity, the sample has to be cooled down
slowly at a constant rate. Annealing also affects
optical properties of the polymer, such as
transparency, by changing the crystalline structure as
well as increasing hardness and brittleness.19
3 Experimental Design
3.1 Fabricating the Fibers
In order to begin the research, an anhydrous
sample of PLLA was extruded into fibers using
Rosand’s Bench-top RH2000 Capillary Rheometer
(see Figure 4). Although the machine is
conventionally used to measure the viscosity of
materials, the means by which the system runs
worked well to extrude the PLLA into filaments.
Large granules of PLLA were loaded into a
single barrel of the machine through the repetitive
process of pouring a small amount into the barrel and
packing it down with a plunger. This packing
removed any air bubbles that would become
imperfections in the filaments during extrusion. Once
the store of PLLA was heated to over 200˚C and the
pressure had surpassed 2.4 MPa, the sample was
ready for extrusion.
The extrusion process involved the usage of a
system of wheels and a rotating dowel in order to
systemize collection. The PLLA was forced through
a die with a 0.5 mm diameter at a rate of 2 mm/min
by the machine’s plunger. The sample then exited the
machine as a cylindrical strand and hung above the
table. Initially the extruded fiber was thicker because
the polymer was not flowing as fast through the die.
Once the filament was long and thin enough, it was
wound through a series of wheels that kept it on track
towards the mechanized spool. The uneven end of the
fiber was removed so that the entirety of the sample
was ideally free of any irregularities that were caused
by the start of the extrusion process. The fresh end
was then taped to the cardstock spool which was
constructed to fit perfectly around the rotating dowel.
Once the clean end of the filament was secure, the
spooling machine (the CS-194T) was turned on and
the dowel began rotating, effectively collecting the
sample as it was being extruded (see Figure 5).
During this process, the samples were wound
according to differing radial velocities of the spool.
Each sample was extruded at a rate of 2 mm/min by
the RH2000. The fibers had different diameters due
to the different speeds of extrusion. Once an ample
amount of the fiber was collected on the spool, the
CS-194T and the plunger were stopped, the fiber was
cut, and the sample was labeled and placed to the
side. This process produced the five samples used
throughout the experiment.
3.2 Annealing the Fibers
In order to create semi crystalline fibers,
samples of the extruded filaments underwent
recrystallization through the annealing process. The
length and diameter of five different samples were
analyzed using rulers and an Olympus optical
microscope. The samples were then labeled with
tape, placed in a covered glass dish, and heated for
110 minutes in an oven at 145˚C (see Figure 6). After
the heating, the fibers were slowly cooled and then
reanalyzed.
3.3 X-Ray Diffraction (XRD)
The samples of the fibers were tested using
X-ray diffractometers in order to analyze the
molecular structure of PLLA. Sample cards were first
constructed out of cardstock to support the fiber
samples and keep them straight in front of the X-ray
beams. Next, the newly extruded fibers were attached
to the sample cards and labeled. Using XRD and the
General Area Detector Diffraction System (GADDS)
software V4.1.42, the different samples were
analyzed for characteristics indicative of molecular
structure and preference (see Figure 7). Once data
collection for the amorphous fibers was complete, the
annealed fibers were attached to sample cards and
analyzed using X-ray diffraction. With data
collection complete, the different samples, non-
annealed and annealed, were easily compared to one
another in order to determine the effect of the
annealing process on the fibers.
3.4 Testing Tensile Strength
The Sintech 5D Universal Testing Machine
was used to test the stiffness, stress, and strain of the
fibers. Each fiber specimen’s diameter was measured
and strands of the polymer were attached to card
stock tabs in order to ensure consistent sample
lengths and to facilitate setting up the experiment
(see Figure 8). In order for the test to be performed,
the following data was entered into the computer
program: diameter of the sample, grip separation,
initial speed, and length. Due to the common factor
of the card stock frame, the length and grip
separation remained constant at one inch, or 25.4
mm. Likewise, in order to maintain consistency, the
speed, which is the rate of elongation of the sample,
remained constant at 5 mm/min. With this
experiment, the fiber spun at a speed level of 12
(arbitrary) with diameter 90 microns was tested as
well as its annealed counterpart with diameter 95
microns.
In the experiment, the specimen card with the
fiber was placed between the grips of the machine,
and the program was set to the corresponding
parameters. Then the card stock frame was cut to
ensure that the material being tested was truly the
polymer sample. With verification that the specimen
was set, the program was started and the data was
collected on the computer. Once all of the specimens
were recorded, the best results were used in order to
form the specific sample that was used for analysis.
3.5 Differential Scanning Calorimetry
Heat flow was tested using the Mettler Toledo
TSO801RO, a differential scanning calorimeter that
measured heat gained and lost during a heating cycle
with a linear temperature ramp. A small, 9.2 mg
sample of crystalline PLLA went through a heat,
cool, heat, cool cycle over a period of 90 minutes
through a temperature range of 25 to 300˚C. The
machine was set to change at a rate of 10˚C per
minute during both heating cycles, -0˚C during the
first cooling cycle, and -25˚C during the second
cooling cycle. Between each endothermic and
exothermic process the sample was held for sixty
seconds in isothermal conditions before beginning
the next step in the test. Through this experiment the
glass transition temperature and melting temperatures
were identified for the sample in order to determine
possible conditions in which the material can be
utilized.
For data interpretations, only the second
heating cycle was considered due to the nature of the
PLLA. Since the material being tested was already in
crystalline form, the first heating cycle served as a
means to get the PLLA sample into an amorphous
state in order to measure the crystallization in the
second heating cycle.
3.6 Manipulating the Data: X-ray
Diffraction
In order to interpret the XRD data, the raw
calculations collected during the experiments were
mapped into comprehensible graphs and plots. The
following steps were taken to acquire such visuals
using the Jade 7 software. First, the .xy file from the
integrated XRD detector image was read into the
program. Then, the range was clipped such that the 2-
theta value (the x-axis) was between 3 and 32
degrees. Data from another integrated XRD detector
image measuring air diffraction was read into the
program to approximate a background profile and
subtracted from the collective data, subsequently
leaving measurements describing only the sample.
Next, using a nine degree peak, the approximate
amorphous portion was subtracted, leaving behind
the derived pattern. In order to characterize the
crystalline data, ten peaks were found automatically
using the software and two additional peaks were
manually added. These twelve peaks were then
refined into profiles starting with 0.5 degree widths.
From here, the X-ray diffraction data was prepared
for analysis.
4 Results and Discussion
4.1 Dimensions of PLLA Fibers
When analyzing the fiber samples after the
extrusion process, certain data measurements,
including diameter of the fiber and speed of the
spool, were used in order to characterize the fibers.
For this portion of the experiment, the volumetric
flow rate of the extruder can be calculated by
multiplying the cross sectional area of the barrel by
the rate of compression. The diameter of the barrel
was 14.7 mm, and the rate at which the plunger
lowered was 2 mm/min. Based on the information,
the volumetric flow rate constant was calculated to be
5.66×10-9
m3/s.
Referring to Table II in the appendix, the
number of the sample represents its relative speed
that appeared on the dial of the spooling apparatus.
The average diameter of the fiber as well as the
standard deviation of the diameters decreased as the
speed of the spool increased.
The calculated volumetric flow rates from the
sample data showed significant variability. The
calculated volumetric flow rate of sample number 3
showed an extreme positive deviation from the
constant, suggesting unevenness in the thickness of
the fiber. Other calculated volumetric flow rates
exhibited overall negative deviation from the
constant. This negative deviation is most likely a
result of imperfect compaction of PLLA granules in
the barrel of the extruder.
The analysis of the relationship between fiber
diameter and speed of the spool suggests that there is
an inverse square correlation between the diameter
and speed (see Figure 9). The diameter of the fiber
decreased dramatically and appeared to reach an
asymptote as the speed increased. The graph of
diameter plotted against the inverse square of the
speed shows that there is a linear relationship
between the two, indicating an inverse square
relationship between the diameter and speed. The line
of best fit is heavily influenced by the outlier,
calculated based on data obtained on the number 3
fiber. This point is providing significant leverage,
increasing the slope of the overall graph (see Figure
10).
4.2 Annealing and Physical Changes
The annealing process affected the structural
characteristics of the polymer. Although heating the
polymer would not have altered its intrinsic material
properties, the change in its structure due to the
annealing process made significant changes in terms
of its strength, hardness, and even some optical
characteristics. The fibers became milky in color
compared to their original clear appearance and their
dimensions were altered: their diameters increased
and lengths decreased based on measured values
accurate up to ±1 cm (for length) and ±15 μm for
diameter (see Figure 6 for optical change and Table 1
for physical changes).
4.3 Tensile Strength
Using the stress and strain data collected
during the tensile strength test, the Young’s Modulus
was calculated through Hooke’s Law and
successfully used to characterize the polymer. For the
non-annealed samples, there were a total of five
successful specimens that produced a mean modulus
of 2.63×103 MPa with a standard deviation of
1.88×102 MPa (see Table III). When testing the
annealed sample, there were only two specimens that
produced a mean modulus of 3.08×103 MPa with a
standard deviation of 3.19×102 MPa (see Table IV).
Such results are indicative of increased stiffness due
to the crystallization of the polymer during the
annealing process. Additionally it suggests that
depending on the intended application, the polymer
can be modified in order to have varying degrees of
stiffness.
The tensile strength test also revealed the
peak stress and percent strain at the breaking point.
Remaining consistent with the conclusion drawn
based on the modulus of elasticity, the non-annealed
fibers had a lower mean peak stress of 44.0 MPa
(with a standard deviation of 6.3 MPa) when
compared to the annealed fibers’ mean peak stress of
51.3 MPa (with a standard deviation of 11.5 MPa).
Additionally, the mean percent strain at break for the
non-annealed fibers was lower than that of the
annealed, having been measured to be 4.60% (with
standard deviation of 4.18%) in comparison to the
13.76% (with a standard deviation of 9.38%). For a
visual representation of this data, see Figures 11 and
12.
The yield point was also identified on each
graph, further verifying the superior strength of the
annealed fiber. Visually estimated at point Y, the
yield point was determined to fall before 3% strain
for the annealed fiber and 2% strain for the non-
annealed fibers. For a visual representation of this
data, see Figures 11 and 12.
Considering this data, one must take into
account the discrepancy in sample set. When
analyzing the mean values for the non-annealed
fibers, the calculations are taking into account five
specimens, creating a more thorough, though still
small, sample set. In comparison, the annealed fiber
set contained only two fibers, limiting the accuracy
of the measurements. Regardless, the results
remained consistent in showing the improved
strength and stiffness of the annealed fibers.
4.4 Differential Scanning Calorimetry
Figure 13 is the visual representation of the
results of the DSC test. In this graph, the heat flow of
the initial heating is represented by the red curve and
subsequent cooling of the polymer corresponds to the
blue curve. Initial heating and cooling was done to
obtain amorphous structure in the polymer to analyze
its crystal transition. Glass transition temperature of
poly(L-lactic acid) was experimentally determined to
be 46.49 °C, which is approximately 10 degrees
higher than the normal human body temperature.
The literature on the subject puts the glass
transition temperature of PLLA at 58°C, which is
much higher than the experimentally obtained
value.20
This negative deviation of glass transition
temperature might be a result of using a hydrous
sample with too much exposure to atmosphere. High
glass transition temperature suggests that the material
will not undergo structural changes in the body. Such
robustness against body heat makes PLLA a
desirable material for biomedical applications.
The crystallization of the polymer is
represented by the peak at 98.17 °C. The process is
exothermic because the polymer chains reorganize to
obtain minimum energy level. Rapid cooling of the
polymer within this temperature rate will result in
amorphous structure. The melting point of the
polymer is represented by the downward peak at
161.3 °C, supporting its use in the human body.
4.5 X-Ray Diffraction
X-ray diffraction (XRD) was used to
determine the degree of crystallinity of the polymer
and the orientation of the crystals in the sample
PLLA fibers. The diffraction patterns seen as a result
of this test are evidence of crystalline structures in
the polymer complex.
In X-ray diffraction analyses, background
noises may cause some confounding effects. The two
most prevalent background noises are the scattering
of X-rays by air and peaks caused by microcrystalline
cellulose in the cardboard used to hold the specimen.
The most important and influential background
pattern for diffraction analysis is air scattering, due to
the fact that it is used as a control. Air scattering can
be characterized with a broad peak at low 2-theta
values. The X-ray intensity reaches its maximum at
approximately 3 degrees, and then slowly decreases
as 2-theta value increases (see Figure 14). Cardboard
can also cause interference that can be compounded
with the diffraction from the crystal. The crystalline
structure of microcrystalline cellulose shows few
peaks in its diffraction pattern, making it hard to
distinguish the actual diffraction pattern of the
specimen from the cardboard noise. However, unlike
air scattering, the diffraction from the cardboard
holder can be avoided by adjusting the location and
the direction of the specimen before the start of the
test.
Since the diffraction patterns are additive,
subtracting such background noise from the actual
data helps enhance the visibility of various peaks and
determine whether the diffraction was correctly done
on the specimen.
4.6 XRD: Amorphous Fibers
The amorphous fibers had smooth curves in
their diffraction patterns rather than sharp peaks,
indicating that there is no specific structure inside
(Figure 15). Although the graph did not show
patterns at lower 2-theta angles, the shape of the
pattern is very similar to that of air scattering pattern.
Notably, peaks are not present in this graph.
The thicker fibers seem to have more
deviation from the air scatter, but clear peaks are not
present in their graphs either (see Figure 16). Overall,
the graph follows the general air scattering pattern.
4.7 XRD: Annealed Fibers
The annealed fibers showed more crystallinity
compared to their amorphous counterparts, but air
scattering was still an issue when conducting X-ray
diffraction on thinner fibers.
Initially, the diffraction pattern showed little
deviation from the air scatter (see Figure 17). There
are two visible peaks between 2-theta values of 16
degrees and 18 degrees, though the relative intensity
of the peak is much smaller than the peak caused by
the air scattering on the background.
Similarly, the pattern created by the fiber with
a diameter of 75 microns follows the air scatter and
then shows two small peaks around 16 and 18
degrees (see Figure 18). The shape of the diffraction
pattern is almost identical to the diffraction pattern of
fiber with diameter 63 microns, however the intensity
at the first peak was slightly higher (over 400
counts), compared to that of the pattern of the thinner
65 micron fiber (400 counts).
The diffraction pattern of the 100 micron
PLLA fiber shows greater intensity level at the peaks
(see Figure 19). The highest peaks are seen around 18
and 19 degrees. These peaks show much greater
intensity than those of the other two fibers. In
addition, a third peak is visible around 27 degrees.
The thinner fibers were hard to locate in the
X-ray scanner and their diffraction patterns were
heavily influenced by background noise. Such fibers
also did not show extreme peaks in their X-ray
diffraction pattern; however, a comparison of the
three patterns revealed that better X-ray scattering
patterns can be obtained by using a much thicker
fiber.
Although the air scattering is still visible at
low 2-theta values for the thicker fiber, the intensity
of the diffracted X-rays is much higher at the peak
(see Figure 20). Two sharp peaks occur at 16.5
degrees and 18.5 degrees. There are also many
smaller peaks that were not observed in the thinner
fibers. The peak intensity is as high as 3300,
indicating a clear chain-chain structure at the given
theta value.
4.8 Orientation
The orientation of the crystals in the fiber can
also affect the diffraction pattern. When polymer
molecules crystallize, they may prefer certain
orientations over others, resulting in an uneven
diffraction pattern. Any unevenness can be analyzed
using chi-measurements with the X-ray diffraction
data. Chi-measurements calculate the intensity of
diffracted X-ray beams as the specimen rotates.
On the meridian plane the intensity versus chi
plot shows only minor fluctuations, signifying that
there was no preferred orientation (see Figure 21). In
contrast, there are visible peaks on the chi plot on the
axial plane suggesting that the crystals are aligned on
the axial plane and stretch across the length of the
fiber (see Figure 22). The alignment of polymer
chain on the axial plane gives it structural advantage
in enduring vertical strain.
4.9 Peaks and Miller Indices
The plot seen in Figure 23 and described in
Table V shows the 2-theta values and intensity of X-
ray diffraction at the peaks. Using the Jade program,
the amorphous background was subtracted from the
plot in order to isolate the 11 remaining peaks.
The peak with the greatest intensity occurs at
a 2-theta value of 16.36 degrees and has a d-spacing
of 5.308 Å. The angle measure of this peak
corresponds to the index (110). The second most
intense peak corresponds to the index (111) and
occurs at a 2-theta value of 18.6 degrees and has a d-
spacing of 4.664 Å. The third notable peak is at 14.5
degrees and has a d-spacing of 5.984 Å. Although it
is not very intense, this specific peak is significant
because it corresponds to the index (010), which is on
the same plane as the index (110) (See Figure 24).
Using peak (111) is not favorable because it is not
normal to the meridian plane like the other two
planes. Therefore, peak (010) was used instead of
peak (111).
The analysis of peaks (110) and (010)
revealed the structure of the PLLA unit cell. Using
the two d-spacing values as cell parameters, the
planes and their miller indices can be determined.
The cell parameter would be
and , but the angle between the two
axes, , is unknown. Assuming that ,
ideally should equal √ (See Figure 24). According
to this assumption,
√
√ ,
which does not concur with the empirical result. The
difference suggests that either or there was a
significant error in selecting peak centroids.
5 Conclusions PLLA proposes numerous advantages to the
biomedical field due to its biodegradability,
biocompatibility, and strength.21
During the
experiment, PLLA was fabricated and characterized
in order to expand its usage in the medical industry.
First, PLLA was extruded and analyzed using
X-ray diffraction to determine the degree of
crystallinity and orientation of polymer crystals. The
peaks on the collected data displayed that degree of
crystallinity was higher in the annealed sample fibers.
Thus the annealing process and subsequent increase
of crystalline structure could improve the fibers for
their intended purpose of biomedical devices.
In order to verify the biocompatibility of
PLLA with the temperature of the human body, a
sample of non-extruded PLLA was subjected to
differential scanning calorimetry. The study proved
that PLLA has a glass transition temperature of
46.5 , which is significantly higher than the human
body temperature of 37 , verifying that PLLA will
not undergo characteristic changes inside the body.
Finally, a tensile strength test was performed on the
PLLA fibers in order to determine peak stress and
percent strain at the breaking point of various
diameter fibers. It was concluded that the annealing
process increased the tensile strength of PLLA,
which is a necessary aspect in customizing
biomedical devices. The study suggests that PLLA
can be modified to varying degrees of stiffness and
brittleness through annealing to accommodate the
biomedical device needed.
The study of PLLA can be expanded upon
through further research, and it can also potentially
lead to numerous groundbreaking biomedical
advances.22
The copolymerization of PLLA with
other polymers, human tissue, and even minerals in
the human bone is promising in the medical industry.
Additionally, further experimentation of crystallinity
and tensile strength is essential to the improvement of
biomedical devices such as bone pins, sutures,
screws, and wires.23
Future work should also look to
expand the usage of PLLA throughout more
biomedical devices, including stronger devices such
as plates and bone fixtures. Because PLLA has a
variety of favorable characteristics, further testing of
PLLA will considerably broaden its usage in the
biomedical field.
6 Acknowledgements
The authors would like to recognize and
thank everyone who invested time in the creation of
this paper. Everyone involved was a priceless asset,
and the authors want to express their deepest
gratitude. Thank you to mentor Dr. Thomas Emge,
the Chief Crystallography Engineer of Rutgers
University, who provided the smiles and knowledge
that led to this paper. Thank you to mentor Dr.
Sanjeeva Murthy, an Associate Research Professor at
the Rutgers Center for Biomaterials, who shared his
passion and expertise with everyone involved in this
project. Thank you to Edmund Han, the RTA who
gave amazing help and never failed to keep his team
on task.
The authors would also like to thank the
people who made their Governor’s School experience
possible. Thank you Dean Jean Patrick Antoine and
Director Ilene Rosen for providing such an amazing
opportunity for the students attending Governor’s
School. We would also like to thank Sarah Sprawka
and Purac Biomaterials (a Corbion company) for
donating the PLLA used in our experiments. Finally,
thank you to the sponsors of the 2014 New Jersey
Governor’s School of Engineering and Technology:
Rutgers University, The State of New Jersey,
Lockheed Martin, Morgan Stanley, Novo Nordisk,
The Provident Bank Foundation, Silver Line
Windows and Doors, and South Jersey Industries Inc.
References 1. Murthy, Sanjeeva. “Polymers in Biomedical
Devices for use in Regenerative Medicine.” Lecture,
NJCBM from Rutgers University, Piscataway, NJ,
July 2, 2014. 2. Ibid. 3. Ibid.
4. Murthy, Sanjeeva. “Polymer Properties and Future
Directions.” Lecture, NJCBM from Rutgers
University, Piscataway, NJ, July 17, 2014. 5. Ibid. 6. Ibid. 7. Ibid. 8. Ibid. 9. Murthy, Sanjeeva. “Polymers in Biomedical
Devices for use in Regenerative Medicine.” Lecture,
NJCBM from Rutgers University, Piscataway, NJ,
July 2, 2014. 10. Ibid. 11. Emge, Thomas. “X-Ray Diffraction (XRD).”
Lecture, Rutgers University, Piscataway, NJ, July 1,
2014. 12. Ibid. 13. Ibid. 14. Ibid. 15. Ibid. 16. Ibid. 17. Ibid. 18. Kopeliovich, Dmitri. “Annealing of Plastics.”
Substech: Substances and Technology, Knowledge
Source on Material Engineering.
http://www.substech.com/dokuwiki/doku.php?id=ann
ealing_of_plastics (retrieved July 10, 2014). 19. Ibid. 20. Chitoshi N, Shin-ya T. “Glass Transition and
Mechanical Properties of PLLA and PDLLA-PGA
Copolymer Blends.” Journal of Applied Polymer
Science June (2004) 21. Avérous, Luc. “Bioplastics: Biodegradable
Polyesters.” http://www.biodeg.net/bioplastic.html
(retrieved July 19, 2014) 22. Tatsumi A, Kanemitsu N, Nakamura T, Shimizu
Y. “Bioabsorbable poly-L-lactide costal coaptation
pins and their clinical application in thoracotomy.”
The Annals of Thoracic Surgery March (1999). 23. Saito T, Iguchi A, Sakurai M, Tabayashi K.
“Biomechanical study of a poly-L-lactide (PLLA)
sternal pin in sternal closure after cardiothoracic
surgery.” The Annals of Thoracic Surgery February
(2004)
Appendix
Figures
FIGURE 1: THE FORMATION OF PLA
The above image demonstrates the two main routes of synthesis of PLA through a hydration
reaction with lactic acid (top left) or a ring-opening reaction with lactide (top right).
FIGURE 2: THE BASIC LAYOUT OF AN X-RAY DIFFRACTION MACHINE.
This image is the basic layout of an X-ray Diffraction machine. The X-rays travel from the X-ray
source, hit the sample, and then travel to the X-ray detector. The X-ray detector measures the
intensity of the reflected X-rays, and plots the data based on the θ value.
Image courtesy of University of California, Davis.
FIGURE 3: THE PATH OF X-RAYS BASED ON BRAGG’S LAW
The path of x-rays based on Bragg’s Law. The d-spacing is represented by the letter d, and is the
calculated value, since θ is controlled by the machine, λ is constant, and n is constant.
Image courtesy of University of California, Davis
FIGURE 4: IMAGE OF THE EXTRUSION PROCESS
This is an image of the extrusion process. The die is not pictured, but it would be at the top of the
picture from where the fiber seems to originate.
FIGURE 5: IMAGE OF THE SPOOLING PROCESS
This is an image of the spooling process. The fiber travels through the arm in front of the yellow
spool, and the arm moves back and forth to create a uniform spool of fiber.
FIGURE 6: BEFORE AND AFTER IMAGES OF THE FIBERS AFTER THE ANNEALING
PROCESS
These images are of the fibers before and
after the annealing process. The amorphous
fibers before the heating are transparent
whereas the annealed fibers are opaque.
Additionally, the annealed fibers curled up
in the heat, suggesting a change in
crystalline structure.
Amorphous Annealed
Amorphous Annealed
FIGURE 7: IMAGE OF SAMPLE 20A IN CARD STOCK FRAME DURING X-RAY
DIFFRACTION
This is an image of the PLLA fiber 20A in the card stock during the x-ray diffraction process.
The cardstock was used to support the fiber sample and keep it straight in front of the X-ray
beams. The XRD system analyzed characteristics of the annealed fiber with molecular level
detail.
FIGURE 8: SAMPLES FOR TENSILE STRENGTH TEST
The above image is an example of the card stock frames and the fibers before the Tensile
Strength Test. In this case, the sample is of the amorphous fiber 27, our thinnest sample.
FIGURE 9: DIAMETER OF THE FIBER VS. SPEED
The relationship between the speed of fiber in m/s and the diameter in microns is displayed.
Although as speed increases, diameter decreases, the graph does not show a linear relationship
between speed and diameter.
FIGURE 10: DIAMETER OF THE FIBER VS. INVERSE SQUARE OF SPEED
The relationship between the inverse square speed of fiber in m/s and the diameter in microns is
displayed to be linear. Thus, the graph supports the equation.
0.00
100.00
200.00
300.00
400.00
500.00
600.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
DIA
MET
ER (μ
m)
SPEED (m/s)
DIAMETER vs. SPEED
y = 11.971x + 59.68 R² = 0.9981
0.00
100.00
200.00
300.00
400.00
500.00
600.00
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00
DIA
MET
ER (μ
m)
1/SPEED2 (s2/m2)
DIAMETER vs. 1/SPEED2
FIGURE 11: STRESS-STRAIN GRAPH FOR AMORPHOUS FIBERS
The above graph is a combination of all the specimens for Amorphous Fiber 12. Point B marks
the beginning of the linear section, point M marks the end of the linear portion, point Y indicates
the yield point when the fiber transitions from elastic to plastic, and point F indicates the point of
failure.
FIGURE 12: STRESS-STRAIN GRAPH FOR ANNEALED FIBERS
The above graph is a combination of all the specimens for Annealed Fiber 12. Point B marks the
beginning of the linear section, point M marks the end of the linear portion, point Y indicates the
yield point when the fiber transitions from elastic to plastic, and point F indicates the point of
failure.
FIGURE 13: DIFFERENTIAL SCANNING CALORIMETRY OF PLLA
The above image is of the results of the Differential Scanning Calorimetry of Poly-L-Lactic Acid.
The red line represents the first heating cycle, the one that began with the semi-crystalline
sample. The blue line is the first cooling cycle, which occurs at a rate specifically determined to
not re-crystallize the polymer. The green line is the final heating cycle and since it started with
an amorphous sample, the green line gives much better data than the red line.
FIGURE 14: X-RAY SCATTERING DUE TO AIR
This data is the result of an X-ray diffraction test with no sample in the machine. The scattering
is just due to the air, and can be subtracted from other results to eliminate the noise that the air
creates.
FIGURE 15: X-RAY DIFFRACTION OF AMORPHOUS PLLA FIBER –
DIAMETER 100 μm
The above image is of X-Ray Diffraction of the Amorphous PLLA Fiber of diameter 100 μm.
The amorphous PLLA shows no Bragg peaks and looks very similar to the air scattering. Even
though the sample was tested multiple times in multiple locations, the data stayed the same: there
was almost nothing that differed from the air scattering.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0.8 5.8 10.8 15.8 20.8 25.8 30.8
Inte
nsi
ty
2-theta (Degrees)
Air
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70
Inte
nsi
ty
2-Theta (Degrees)
PLLA Fiber - Amorphous, d = 100 µm
FIGURE 16: X-RAY DIFFRACTION OF AMORPHOUS PLLA FIBER –
DIAMETER 600 μm
The amorphous PLLA shows no obvious Bragg peaks, but differs from the air scattering. At
around 33 degrees, there is a tiny deformation that may be a weak peak.
FIGURE 17: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –
DIAMETER 63 μm
The figure shows the X-ray diffraction pattern of annealed 63 micron diameter fiber. The sample
shows two small peaks, but the peak caused by the air scattering is far more intense than the little
peaks.
0
50
100
150
200
250
13 23 33 43 53 63
Inte
nsi
ty
2-Theta (Degrees)
PLLA Fiber - Amorphous, d = 600μm
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0.8 5.8 10.8 15.8 20.8
Inte
nsi
ty
2-Theta (Degrees)
PLLA Fiber - Annealed, d = 63μm
FIGURE 18: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –
DIAMETER 75 μm
The figure shows the X-ray diffraction pattern of annealed 75 micron diameter fiber. The sample
shows two small peaks, yet air scattering dominates the diffraction pattern.
FIGURE 19: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –
DIAMETER 100 μm
The figure shows the X-ray diffraction pattern of annealed 100 micron diameter fiber. The
sample shows two small Bragg peaks.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
1.5 6.5 11.5 16.5 21.5
Inte
nsi
ty
2-Theta (Degrees)
PLLA Fiber - Annealed, d = 75μm
0
100
200
300
400
500
600
700
800
10 15 20 25 30
Inte
nsi
ty
2-theta (Degrees)
PLLA Fiber - Annealed, d = 100 μm
FIGURE 20: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –
DIAMETER 600 μm
The figure shows the X-ray diffraction pattern of annealed 600 micron diameter fiber. The
sample shows very clear Bragg peaks and some amorphous background.
FIGURE 21: X-RAY DIFFRACTION OF ANNEALED FIBER, CHI –
DIAMETER 100 μm, MERIDIAN
The figure shows the intensity plotted against the chi values when the fiber was rotated on the
meridian plane. There is no distinct or periodic pattern, suggesting that there is no specific
orientation that the polymers prefer on the meridian plane.
0
500
1000
1500
2000
2500
3000
3500
1.5 6.5 11.5 16.5 21.5 26.5 31.5
Inte
nsi
ty
2-Theta (Degrees)
PLLA Fiber - Annealed, d = 600μm
0
20
40
60
80
100
120
140
160
-350 -300 -250 -200 -150 -100 -50 0 50 100
Inte
nsi
ty
Chi (Degree)
PLLA Fiber - Annealed, 100 µm - Meridian
FIGURE 22: X-RAY DIFFRACTION OF ANNEALED FIBER, CHI –
DIAMETER 100 μm, AXIAL
The figure shows the intensity plotted against the chi values when the fiber was rotated on the
axial plane. There are two periodic peaks on the plot, indicating that the polymer chains have
some kind of orientation on the axial plane.
FIGURE 23: INTENSITY VERSUS 2-THETA WITHOUT AMORPHOUS BACKGROUND
The graph shows the peaks from the X-ray diffraction pattern of annealed 600 micron diameter
fiber. The peaks were isolated from the background by subtracting the amorphous background
from the diffraction pattern. There are 11 distinct peaks that were isolated.
0
100
200
300
400
500
600
700
800
-350 -300 -250 -200 -150 -100 -50 0 50 100
Inte
nsi
ty
Chi (Degrees)
PLLA Fiber - Annealed, d = 100 µm - Axial
FIGURE 24: INDICES OF PLANES AND CELL PARAMETERS
The diagram represents the cross section of the fiber. Two planes, (110) and (010), correspond to
the peaks at 16.4° and 14.5°. The triangle on the bottom right of the diagram represents the
relative length of a and b, assuming that they form a right angle.
Tables
TABLE I: MEASUREMENTS OF THE FIBERS BEFORE AND AFTER THE ANNEALING
PROCESS (110 MINUTES)
The table compares the length and diameter of amorphous and annealed fiber in order after a 110
minute annealing process . The number of the fiber corresponds to the spool speed
displayed on the dial of the machine.
TABLE II: DIMENSIONS AND VOLUMETRIC FLOW RATE
Number Diameter
(μm)
STDEV
(μm)
Speed
(m/s)
1/Speed2
(s2/m
2)
Radius
(μm)
Cross-Sectional
Area
(m2)
VF Ratec
(m3/s)
%
Error
3 500.00 81.65 0.17 36.73 250.00 1.96E-07 3.24E-08*
472
8 129.14 23.41 0.40 6.22 64.57 1.31E-08 5.25E-09 7.24
12 94.86 16.77 0.56 3.20 47.43 7.06E-09 3.95E-09 25.3
16 88.43 10.08 0.74 1.84 44.21 6.14E-09 4.53E-09 20.0
20 85.43 20.11 0.88 1.30 42.71 5.73E-09 5.02E-09 11.3
27 59.86 8.23 1.12 0.80 29.93 2.81E-09 3.15E-09 44.3
*Outlier
The table shows the diameter, standard deviation of the diameter, spooling speed, and cross
sectional area. It also shows volumetric flow rate calculated based on these values and their
percent error from the constant.
Speed Level
Amorphous Annealed
Diameter
(μm)
Length
(cm)
Diameter
(μm)
Length
(cm)
12 100.00 22.75 100.00 21.50
16 75.00 30.00 82.50 29.25
20 67.50 23.50 75.00 22.50
27 57.50 24.75 62.50 23.50
TABLE III: SAMPLE DATA FOR TENSILE STRENGTH TEST OF AMORPHOUS
FIBER
Specimen
#
Initial
Speed
mm/min
Diameter
mm
Length
mm
Peak Load
N
Peak Stress
MPa
Strain At
Break
%
Modulus
MPa
1 5.0 0.090 0.090 0.254 40.0 1.730 2.702e+003
2 5.0 0.090 0.090 0.288 45.2 5.214 2.662e+003
3 5.0 0.090 0.090 0.316 49.6 2.188 2.527e+003
4 5.0 0.090 0.090 0.224 35.3 11.661 2.391e+003
5 5.0 0.090 0.090 0.316 49.7 2.219 2.889e+003
Mean 5.0 0.090 0.090 0.280 44.0 4.602 2.634e+003
Std. Dev. 0.0 0.000 0.000 0.040 6.3 4.182 1.878e+002
The table shows the data obtained from the tensile strength test of amorphous fiber. It shows the
dimension of the fibers, speed at which the fibers were pulled, the peak load and stress, strain at
break, and Young’s modulus of the fiber determined by the values.
TABLE IV: SAMPLE DATA FOR TENSILE STRENGTH TEST OF ANNEALED FIBER
Specime
n #
Initial
Speed
mm/min
Diameter
mm
Length
mm
Peak Load
N
Peak
Stress
MPa
Strain At
Break
%
Modulus
MPa
1 5.0 0.095 25.400 0.306 43.2 7.128 2.857e+003
2 5.0 0.095 25.400 0.421 59.4 20.389 3.307e+003
Mean 5.0 0.095 25.400 0.363 51.3 13.759 3.082e+003
Std. Dev. 0.0 0.000 0.000 0.081 11.5 9.376 3.188e+002
The table shows the data obtained from the tensile strength test of annealed fiber.
TABLE V: D-SPACING AND RELATIVE INTENSITIES
d(Å) 2 θ (°) Intensity Relative
Intensity
7.093 12.22 72 0.027
5.984 14.50 91 0.034
5.308 16.36 2645 1.000
4.664 18.63 500 0.189
3.986 21.84 123 0.047
3.753 23.20 67 0.025
3.576 24.38 53 0.020
3.269 26.71 47 0.018
3.089 28.30 78 0.029
2.879 30.40 41 0.016
2.810 31.18 117 0.044
The table analyzes the d-spacing values, 2-theta, and intensity of the 11 prominent peaks.
Relative intensity of the peak is the intensity of the peak divided by the intensity of the strongest
peak. Intensity of the peak has no set unit.