Download applications of graphene in orthopedic implants

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APPLICATIONS OF GRAPHENE IN ORTHOPEDIC IMPLANTS
Perebo Altraide, [email protected], Sanchez 5:00, Nick Jacobyansky, nej18, Mena 1:00
Abstract— In the fields of biomedical engineering and
chemical engineering, researchers have started conducting
research on the properties of graphene with hope that it
could be adapted as a possible means of bettering implants
used in orthopedic surgery. The goal of our research is to
find out more about the properties of graphene and how these
properties can potentially improve orthopedic surgery.
Graphene is a carbon allotrope that was discovered in 2004.
It is a one-atom thick layer of graphite whose atoms bond to
form hexagonal rings, which accumulate to form a
honeycomb. This bonding configuration and the unique twodimensional structure of graphene give way to its many
interesting properties such as great strength, low weight, and
an impermeable quality, which make it a very good candidate
to coat orthopedic implants.
These properties of graphene would allow implants coated
with graphene to last longer in the body, while concurrently
being less harmful to the body. Graphene has also been
shown to possess antimicrobial properties, which could boost
the biocompatibility of orthopedic implants. Our research
will focus on the properties of graphene relevant to its use in
orthopedic implants and the process through which coating
of orthopedic implants takes place. These technological
aspects will be looked at in conjunction with the potential
impacts of this technology on all aspects of society, while
keeping sustainability in mind. This new, highly versatile
material could change many aspects of life in the twenty-first
century.
Key Words—Carbon, Coating, Graphene, Graphene Oxide,
Implants, Materials, Orthopedics.
WHAT IS GRAPHENE?
Although Carbon is the most abundant element on the
planet, it is naturally present in two main forms, diamond,
and graphite [1]. In the past three decades, scientists have
discovered other allotropes of carbon with distinct properties
due to their unique structures. One such allotrope is graphene,
a one atom thick layer of graphite whose atoms are covalently
bonded to each other to form a ring structure. Although
graphene was discovered in 2004, its ability to be a universal
material is just now becoming a reality [1]. Graphene’s
structure gives graphene its many interesting and potentially
University of Pittsburgh Swanson School of Engineering 1
Submission Date 03.03. 2017
useful properties that have applications in virtually every
scientific field. Our discussions throughout this paper will
focus on graphene’s use as a coating material in orthopedic
implants.
Obtaining graphene
In order for graphene to be used as a material, it must
first be retrieved. The simplest way of obtaining graphene is
by extracting it from graphite [2]. This process is not efficient
as it is done using a microscope, so it only yields a small
amount of graphene. To mass produce graphene, the method
of chemical vapor deposition (CVD) is employed. CVD is a
common technique for obtaining nanoparticles on a large
scale [2]. It is used with carbon nanotubes, which gave
researchers the idea to implement it in the fabrication of
usable graphene since carbon nanotubes are carefully rolled
sheets of graphene. Generally speaking, CVD produces a thin
film of coating on the surface of a substrate [2]. During the
CVD process, a gas is released inside of a heated reaction
chamber, wherein the gas or components of the gas react with
the substrate to form a thin film.
The film is then exfoliated from the surface of the
substrate once it has been deposited, which allows it to be
used to coat materials. For graphene, the process of CVD is
very specialized, and occurs in two main steps. Graphene
synthesis using CVD requires the addition of catalysts, upon
which the graphene is grown [2]. Without the use of a
catalyst, the vapor deposition would need to occur at 2500°C,
which makes the process very inefficient. Catalysts lower this
reaction temperature to around 1000°C, which is a result of
the catalyst lowering the activation energies of both steps of
the process. The catalyst is present in the reaction on top of
the substrate, with a barrier in between to prevent mixing of
the two substances [2]. In the first step of this deposition
process, carbon atoms dissociate from a hydrocarbon gas (the
“vapor”) on the surface of the catalyst. Depending on the
catalyst’s affinity for carbon, the next step of graphene
formation could occur right on the surface of the catalyst (low
solubility), or the mechanism would need to be cooled (high
solubility), forcing dissolved carbon to precipitate to the
surface of the catalyst as the catalyst’s structure reforms [2].
This carbon arranges itself into a sheet of graphene.
Graphene forms more uniformly when the solubility of the
Perebo Altraide
Nick Jacobyansky
catalyst is low, such as in a copper catalyst. After this
graphene is formed, it must undergo a process to remove it
from the catalyst and place it on the substrate [2]. The
catalyst is usually a conductive metal, so the substrate upon
which the graphene is placed must be an insulator to allow
the graphene to retain its conductive properties. To do this,
the graphene-catalyst-substrate complex is placed in an
etchant, which is a solution of a compound whose cation is
the catalyst. Etchants are used in producing thin films like
graphene to remove the catalyst. This causes the graphene to
rise to the top of the solution. (It is then placed on the
substrate). This allows graphene to exist in a usable form [2].
Another method of obtaining a graphene like substance
is by reducing graphene oxide (GO) to produce reduced
graphene oxide (rGO) [3]. This relative of graphene is
obtained using various methods, most of which involve
solution chemistry or electrochemistry to decrease the
amount of oxygen present in the compound so that it can
mimic the properties of pure graphene. Although this is easier
to obtain, its properties are not as appealing as that of
graphene. Once graphene is obtained, it can be applied to
many fields in science and engineering due to the many
properties it displays [3].
APPLICATIONS OF GRAPHENE
Graphene’s properties allow it to excel as a material in
many different fields of science. Because of this, it can be
used for many different applications. These applications lie in
fields such as electronics, desalination, tissue engineering,
cancer treatments, coatings for orthopedic implants, metal
detection and removal, drug delivery systems and nuclear
waste treatment [5]. These applications in seemingly
unrelated fields can coexist due to the properties that
graphene’s structure yields. Of these potential applications,
the use of graphene as coating for orthopedic implants will be
the focus of discussion throughout this paper.
GRAPHENE IN ORTHOPEDIC SURGERY
Problems with current coating techniques
Orthopedic implants often fail for many reasons. This
could arise as a result of poor osseointegration, the formation
of a direct interface between an implant and bone without
intervening soft tissue, at the implant surface or loosening of
implants caused by the formation of wear debris and
infections after the implants have been inserted [6]. The
surface properties of implants play a vital role in the success
or failure of orthopedic implants. Implant surfaces should
ideally enhance osteoblast functions and inhibit bacterial
colonization. Several surface modification strategies have
been developed and employed to try and stimulate bone
modelling and prevent bacterial inhibition simultaneously [6].
However, there hasn't been a general solution to meet the
requirements for an implant surface. That is, until graphene
was discovered in the early 2000s.
The synthesis and properties of graphene have already
been discussed in previous sections so it is evident that
graphene's properties make it suitable for this purpose.
Studies have also shown that graphene possesses most of the
requirements of an ideal coating material such as good
biocompatibility, excellent mechanical strength, and
tribological characteristics, as well as some antibacterial
properties [6].
Materials used in orthopedic implants have changed
over the last 60 years. They have shifted from being materials
readily available for different industrial applications into
materials being developed with the ability to interact with the
biological environment of the patient and cause specific
biological responses. The problems faced in orthopedics have
not changed over those years but the choice of possible
solutions have been expanded because of new innovations in
materials [6].
Graphene’s structure and properties
Graphene is made up of carbon atoms. Each carbon
atom is singly bonded to two other carbon atoms and double
bonded to one carbon atom [1]. This bonding creates rings of
carbon, which form a honeycomb structure. The covalent
bonds between carbon atoms give rise to graphene’s many
properties. These strong bonds allow graphene to be
incredibly durable; it is hard to break graphene’s structure
since the carbon atoms are strongly bonded and have a strong
affinity for one another [1]. This tight bonding also allows
graphene to stretch to 20-25% of its original length [4]. This
makes it so that graphene can be fitted to most surfaces for
use as a coating. Because graphene is only one atom thick, it
is incredibly light. This lightness allows graphene to protect
materials without changing the weight of the material.
Graphene’s network of carbon atoms allows it to be a very
good conductor of heat and electricity. This is attributed to
the close arrangement of atoms, as well as the fact that there
is little resistance since all the atoms are carbon [4]. Another
important property of graphene is its impermeability. Each
ring of carbon atoms in graphene is 3.35 Angstroms apart,
which is 3.35*10^-10 meters. This very small space between
rings makes graphene act as a quasi-solid net, since most
molecules are much larger than this space [4]. This gives
materials coated with graphene impermeability, which could
be used to prevent corrosion in many different environments.
These main properties of graphene make it seem potentially
usable in various facets of science and engineering [4].
History of implant development
The first generation of implants, which was mainly in
the mid-20th century, consisted mainly of stainless-steel and
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Nick Jacobyansky
cobalt alloys [7]. The goal of this generation was merely to
“achieve a suitable combination of physical properties to
match those of the replaces tissue with a minimal toxic
response of the host” [7].
Later on, researchers started becoming more concerned
with the development of bioactive materials with the ability
to interact with the biological environment to enhance the
response and tissue bonding. They also focused on the
development of bio-absorbable material and their ability to
undergo a progressive degradation while new tissue
regenerates and heals [7]. The metals that were being
researched had no bioactive properties so the idea of coating
was introduced. Earlier forms of coating were done with bioactive ceramics such as Hydroxyapatite (Ca10(PO4)6(OH)2)
(or HA for short) and bioactive glasses (BGs). This
generation lasted from the 1980s to the early 2000s.
The third and final generation of implants, the
generation which we are currently in, is concerned with new
materials which are able to arouse specific cellular responses
on the molecular level [7]. The concepts of bioactivity and
biodegradability of these materials are combined to make bioabsorbable materials bioactive and vice versa. The research in
this generation is focused on nanocrystalline structures,
organic-inorganic composites, three-dimensional ACP
scaffolds, microspheres, and porosity microstructures. Metals
are also being use in the development of these porous
structures for bone tissue engineering [7].
Currently, pure titanium and its alloys are the most
commonly used metals for implants. Nevertheless, because of
their lack of bioactivity, they cause formation of new bone on
the surface of the implant to take a long time (poor
osseointegration) [6]. Also, implants products sometimes fall
off the bone and cause patients to experience excruciating
pain and results in them having to go through retreatment [6].
Corrosion of the metal implant can also cause problems as the
metal ions cause toxicity in the patient. To solve all these
problems the surface of the metal is coated with an enhanced
bioactive layer.
This bioactive layer helps by promoting high
comprehensive bone growth (osteoconductivity) [7].
Normally HA is used for this coating as its bioactivity
improves the integration between implants and bone tissue.
However, because of its inferior wear resistance and fracture
toughness, HA is incapable of matching the mechanical
behavior of natural bone and deters its clinical application in
major load-bearing devices as coating materials [7]. The
performance of coatings in orthopedic implants can be greatly
improved by using a secondary phase reinforcement which
overcomes the weaknesses of the HA coating and provides
better corrosion resistance. This is where graphene is most
useful in orthopedics.
Effectiveness of graphene as a secondary phase coating
A study was carried out in the Department of Chemistry
at Periyar University, India, to see the effectiveness of
graphene as a secondary coating for Titanium [7]. The study
involved mimicking samples of different electrolytes
substituted with graphene oxide reinforced hydroxyapatite
composite coating on titanium through electrodeposition.
Electrodeposition was the chosen method because it produces
bioimplants with better osseointegration capacity and
improved mechanical properties. Electrodeposition involves
the use of electric current to reduce dissolved metal cations to
form metal coatings. Mechanical and biological studies
characterized the coatings [7]. The composite coating on the
titanium was characterized by Fourier Transform Infrared
Spectroscopy, X-ray diffraction, and Field Emission
Scanning Electron Microscopy. The corrosion resistance of
the coatings in the experiment was evaluated using simulated
body fluid [7].
The first step in the experiment was specimen
preparation. In this step, all the different minerals used in the
experiment were dissolved in deionized water to form their
electrolytes. Before the specimen were dissociated, they were
abraded with different grades of Silicon Chloride emery
papers [7]. After this, all the samples were ultrasonically
cleaned in acetone to remove surface residues and then were
given a final rinse in deionized water and left to dry in
flowing air.
The second step of the experiment was the
characterization of the coatings. The samples were
characterized both morphologically and mechanically and
then received an electrochemical evaluation. The evaluation
was carried out in ideal body fluid conditions (7.4 pH and
37°) to make the measurements reliable [7].
The next step in the experiment was to evaluate the
antibacterial activity of each sample. Each sample was tested
against bacterial strains of E. coli using the agar disc
diffusion method. The diffusion was carried out by pouring
the agar into petri dishes to form thick layers and then adding
dense inoculum of the test organisms of bacterial strains to
achieve better growth.
The final step in the procedure involved the use of in
vitro cytotoxicity [7]. For this part of the experiment human
osteosarcoma osteoblast-like cells were purchased and
cultured in a standard culture medium. The medium was
renewed every two days and the cultures were maintained in
a humid atmosphere. When this was concluded, the
percentage cell viability was calculated using the formula:
%Cell viability = [A] Test/[A] Control *100 [7].
After the experiment had been concluded and the data
had been analyzed, the researchers concluded from the data
that the coating gave the titanium enhanced corrosion
resistance, mechanical, biocompatible, and antibacterial
properties. This research shows that graphene is a promising
implant material with multifunctional properties and potential
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Nick Jacobyansky
candidate for use in orthopedic implants as a composite
coating alongside titanium and HA [7].
with graphene. Since the coating of orthopedic implants
would need to be done on a mass scale, spin coating is
preferred, as it is more suited for industrial needs [8].
Spin coating is similar to dip coating in that a solution
of the coating material is used, but spin coating uses a
centrifuge to coat the implant to a desired thickness level [8].
Once this level is achieved, the solvent of the solution is
again evaporated, leaving the graphene coated implant. Spin
coating is more complicated, but it allows for a very uniform
coating. However, spin coating can only be used for flat
surfaces, which would exclude most implants [8]. It can still
be a good technique for other applications of graphene as a
coating material [8].
Graphene’s orthopedic properties
These observed physical properties of implants coated
with graphene come from graphene’s structure. Graphene
allows for implants to have a high level of biocompatibility
for many reasons. Graphene’s impermeability allows
implants to exist in a biological environment without
undergoing corrosion [1]. A naked metal would deposit
debris inside the body since the tissue fluid can react with the
surface of the metal, which damages both the implant as well
as the body [1]. Graphene’s close knit ring structure prevents
any outside molecules from touching the implant, as well as
any loose particles from entering the fluid stream. The one
atom thick layer of carbon molecules acts as a solid
membrane to the fluid environment of the human body. This
impermeability, as well as graphene’s flexibility boosts the
mechanical functionality of the implant [1]. Graphene allows
the implant to move within the body without being affected
by the environment. Its lightness and thinness also help with
this functionality. These properties allow the implant to
mimic the weight of bone well, for it adds virtually no weight
to the implant. By coating titanium implants with graphene
and HA, implants can last longer in the body and lower the
risk of infection [7].
This study also illustrated graphene’s antibacterial
property, as the samples coated with graphene were very
effective in killing bacteria and not allowing it to grow on the
implants. This property of graphene could solve one of the
biggest problems with implants, which is infection [1]. If
graphene coated implants can prevent bacteria from being
present on the implants, the functionality of these implants is
just as important as their durability.
IS IT WORTH IT?
Cost of production
Harnessing the technology of graphene and applying it
to a process which is done about a thousand times a day will
require an optimization of graphene’s production. Currently
the most effective method discovered for synthesizing
graphene is through the use of Chemical Vapor Deposition
(CVD) as it is able to extract graphene in its purest form [2].
This method makes large amounts of pure graphene from
gaseous carbon molecules. However, the equipment required
to make high quality thin sheets of graphene through CVD is
rather expensive and makes this method inconvenient for
producing graphene on a large scale [9]. As this technology
becomes commonplace, the price will decrease as the
efficiency will rise.
Until this happens however, an alternate means of
graphene synthesis must be adopted. Reduction of graphene
oxide is also another means through which graphene can be
obtained [1]. This method is relatively cheaper than that of
CVD, costing only about a few cents per gram [10]. The
reason why extraction through this means is so cheap is
because it is a relatively simple and low-energy treatment
process involving a cheap starting material (graphene) [10].
Although the reduction of graphene oxide is much
cheaper and seems like it should be an easy solution to the
cost problem, it still has its problems. This process produces
reduced graphene oxide, which still has traces of oxygen
from the reduction [1]. Since this related compound is not
made up of solely carbon, its properties are not as ideal as
graphene. For example, reduced graphene does not conduct
electricity as well due to the presence of oxygen. Until CVD
is fully optimized, reduced graphene oxide is a reasonable
substitute for graphene.
Graphene coating process
Once graphene is retrieved, it must be applied to the
surface of the implant. This can be done using numerous
methods. Depending on the target material, graphene can be
deposited onto the material using CVD [8]. However, this is
not always the case, as the implant material could be altered
by CVD. When CVD does not directly coat the implant
material, one of two main methods are carried out: the dip
coating or spin coating. Dip coating is generally used on a
small scale, as it produces variable thicknesses of the coating
[8]. However, this process is much simpler than spin coating,
which makes it seem to be more favorable. Dip coating
involves soaking the desired material, in this case the
implant, in a solution of the coating material for a set amount
of time and pulling the substrate up. This pulling is the
crucial step of the process, as its speed determines the
thickness of the coating layer [8]. After the pulling step, the
implant is then drained of extra solution on its surface and the
solution’s solvent is evaporated, leaving the implant coated
Making it happen
If CVD for graphene becomes a more efficient process,
orthopedic implants coated with a complex of HA and
graphene could become a reality. Implants are currently
coated with HA, which means that graphene would not be
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Nick Jacobyansky
replacing the current technology, but adding on to it [7].
Putting it into practice would not be as difficult as changing
how implants are coated completely. Before any of this can
happen the potential impacts of incorporating graphene into
orthopedics must be considered such as how it will affect the
environment and society.
Effects on the environment
Research done by professors at the University of
California, Riverside, predicted that synthesis of graphene
through the reduction of graphene oxide could lead to
graphene oxide nanoparticles being deposited in lakes or
streams and possibly cause negative environmental impacts
[12]. Having this in mind, the researchers carried out a series
of experiments to see if reduction of graphene oxide did
cause negative environmental impacts. The experiments were
carried out in groundwater and surface water conditions with
the purpose of understanding the stability of graphene oxide
nanoparticles and comparing them between the two water
situations [12].
The experiments carried out in groundwater (which has
a higher degree of hardness and lower concentration of
organic matter) found that the nanoparticles tend to become
less stable and eventually get removed in subsurface
environments. On the other hand, the experiments conducted
in surface water (which has more organic matter and less
hardness) found that the nanoparticles remain stable and
move farther especially in the subsurface layers of the
environment [12]. The results from these experiments showed
that the nanoparticles follow the same theories of stability
and transport as other chemicals that have been used in the
past 30 years, proving that there is no real environmental
concern. This cements graphene as a material that is both
useful and environmentally friendly, which is ideal during a
time when sustainability is the main focus.
IMPACTS OF GRAPHENE IN
ORTHOPEDICS
When solving problems, engineers must look at how their
solutions will affect many aspects of life and society. If a
beneficial technology could potentially be harmful to the
environment, engineers have to decide if that technology is
still worth implanting. In the twenty-first century, engineers
must consider the consequences of their work with
sustainability in mind. Depending on the field of engineering
and the situation, sustainability can take on many meanings.
In most cases, sustainability refers to ensuring that the Earth
continues to support life by having viable resources [11].
However, sustainability is a very general term, so its
definition varies from case to case. In the case of using
graphene to coat orthopedic implants, it is a sustainable
technology in that it promotes longevity and can increase the
quality of life. These qualities allow this application of
graphene to potentially have a lasting impact on the world.
How it affects society
Graphene has the potential to change the way we live by
becoming an everyday material. This paradigm shift could be
as impactful as the adoption of plastic in the last quarter of
the twentieth century. The specific application of graphene as
a coating material in biological implants, however, is more
nuanced, but can potentially change facets of modern surgery.
Research done in the previous sections of this paper show
that implants coated with graphene will reduce corrosion in
implants and as a result, also decrease the toxicity from the
implants. The increase in bioactivity caused by the graphene
coating allows the implants to last longer and causes bone
formation in the implant area to take less time [4]. This
increase in bioactivity also makes it less likely to be rejected
by the body or cause infection. Coating implants with
graphene can increase the lifespan of people who require
implants by making the implants more reliable. By not having
to replace implants as frequently, patients are less likely to
have complications during surgery. This also preserves metal
resources used to make implants, which is sustainability in
the purest sense. These accessory benefits make this
technology even more appealing if it is able to be harnessed.
Implementing graphene for implant coating use will surely
revolutionize the field of orthopedics.
ETHICAL CONCERNS
One of the only concerns about putting graphene coated
implants inside the human body is the theorized toxicity of
graphene to humans [12]. A study at Brown University
regarding the potential harm of graphene to humans showed
that graphene flakes have sharp edges that could pierce cells
and damage them if they are in contact with graphene [13].
This study made the realization that graphene is not usually
found in even sheets, but is uneven and has sharp edges.
However, this study used graphene that was obtained using
exfoliation from graphite, which gives way to the jagged
graphene flakes [13].
If CVD were used to obtain the graphene, which is
ideal, the sheets would be more even and this risk would be a
lot lower [13]. If graphene is being used regularly, there is
also a concern with breathing flakes of it in, where the
graphene is not able to be filtered out of the lungs. If
graphene is used regularly, precautions will be taken to
prevent such respiratory damage [13].
Although this is not a major concern since CVD would
be used if graphene is used to coat orthopedic implants, it still
must be taken into account as it goes against this being a
sustainable technology. This technology is sustainable in the
sense that it improves the quality of life of those requiring
implants by lowering the risk of infection and damage caused
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Nick Jacobyansky
by the implant. However, if the coating of the implant can be
harmful to the human body, this technology is not only
failing in serving one of its main purposes, but is also not
sustainable.
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LOOKING FORWARD
Graphene is a very promising material that has many
practical applications that can help lead to many scientific
discoveries. Graphene is an allotrope of carbon made up of a
single layer of graphite. Its structure gives it a lot of
interesting properties which make it applicable in numerous
fields. The field of orthopedic implants is one of these fields
which can be greatly benefited by graphene. This is in the
form of secondary coatings for titanium and titanium alloy
implants. Graphene has proven to be very effective in
coatings as is seen in research done by various institutions. Its
antimicrobial, biocompatible, and anti-corrosive properties
have a huge part to play in this.
Although graphene might revolutionize the field of
orthopedics, no real work can be started on it because of the
lack of resources to synthesize graphene in bulk. The two
main methods of synthesizing graphene are through Chemical
Vapor Deposition (CVD) and reduction of graphene oxide.
CVD is effective but costly so it can't be used in mass
production, at least until a cheaper way of carrying it out can
be discovered. Reduction of Graphene Oxide on the other
hand, although being a cheaper alternative to CVD, has the
limitation of not being able to produce graphene in its purest
form.
Since graphene has the potential to change many aspects
of life for the better, its sustainability must also be taken into
account. Graphene’s sustainability comes from the fact that it
has an incredibly low impact on the environment which
allows for a clean earth to exist. In many applications,
graphene is able to preserve resources by being a substitute
for materials that are limited, which in turn limits pollution.
In the application of coating orthopedic implants, graphene is
sustainable by improving the quality of life and promoting
longevity, as it makes for more reliable and more
biocompatible implants.
Research on synthesizing graphene has eradicated
environmental and health concerns with using graphene.
These studies have found the process to be non-toxic. As the
processes to make graphene becomes more efficient, expect
to see it being applied to solve a wide range of engineering
problems.
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Perebo Altraide
Nick Jacobyansky
[11] P. Beaumont. “5 Definitions of Sustainability.” The
Green
Dandelion.
6.5.2013.
Accessed
3.29.2017.
http://blogs.rochester.edu/thegreendandelion/2013/06/5definitions-of-sustainability/
[12] “Graphene’s Negative Environmental Impacts.”
05.1.2014. Kurzweil Accelerating Intelligence. Accessed
3.2.2017.
http://www.kurzweilai.net/graphenes-negativeenvironmental-impacts
[13] J. Ayre. “Graphene May Be More Toxic Than
Previously Thought, Research Finds – Graphene Can Enter
Human Cells and Disrupt Cellular Function.” Proceedings of
the National Academy of Sciences. 7.15.2013. Accessed
3.3.2017.
https://cleantechnica.com/2013/07/15/graphenemay-be-more-toxic-than-previously-thought-research-findsgraphene-can-enter-human-cells-and-disrupt-cellularfunction/
ACKNOWLEDGMENTS
We would like to thank our writing instructor Emelyn
Fuhrman and our co-chair Harrison Lawson for giving us
guidance and keeping us on track during the writing process.
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