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Samuel Odlin

1

Factors affecting Protein Adsorption on Titanium Orthopaedic Implants
Within Orthopaedics there are a large selection of materials that can be chosen for implantable
medical devices such as a replacement femoral head or plates used to support diseased or broken
bone
...
Throughout this essay I will focus on the chemical
interactions between titanium and the body that make it such a popular choice within bioengineering
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Context:
“A medical device is defined as implantable if it is either partly or totally introduced, surgically or
medically, into the human body and is intended to remain there after the procedure” (1)
...
Titanium, first discovered in 1791 by William Gregor in Cornwall(3),
is one of the most commonly used metals within orthopaedic and dental surgery alongside stainless
steel
...

Cell proliferation (growth in number and/ or size of cells) and osseointegration (the fusion of bone to
implant that is inseparable without fracture) are influenced by a number of factors, initially controlled
by protein adsorption
...
Protein adsorption will be the main discussion point within this essay as it
can be used to discuss some of the key concepts within A level chemistry
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g
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Therefore it is extremely important for the implant not to
hinder this repair process
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”(4) Such a process is a long and gradual one and though new bone may be found
on the implant surface around 1 week after installation, full bone remodelling may continue
throughout life
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g
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This is important to firstly allow for initial implant stability, and
secondly it causes bleeding that leads to formation of a hematoma, thus speeding up the repair
process
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This contact results in
a cascade of processes: protein deposition, coagulation, inflammation, and tissue formation
...
Said monolayer is directly influenced by the surface properties of the metal and its
chemical interactions with the individual protein subunits
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(5)

Proteins:
Within the A level Chemistry course we learn about the chemical structure of proteins and the
differing types of interactions between them, however, to gain a greater understanding of factors
affecting protein adsorption we need to examine proteins in greater detail
...

Titanium is likely to interact with complex structural proteins that also have a secondary, tertiary and
quaternary structure, controlled by a complex balance of intramolecular forces and bonds including;
hydrogen bonds, hydrophobic interactions and disulfide bonds
...
Figure 3 below illustrates this
...
Despite this “In some cases, a charge reversal on the surface leads to
a decrease in stability when an increase was predicted”(6)
...
M
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make particular reference to this in “Why Are Proteins Charged?
Networks of Charge-”
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Concluding that there are 4 particular reasons for such problems:
1
...

2
...
The apparent pKa value of a particular ionizable group
on a protein may depend significantly on its interactions with other charged and polar groups
3
...

4
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(7)

Protein Adsorption and the Vroman Effect:
Protein adsorption onto a metal surface is roughly divided into 2 phases based upon a species affinity
for the material
...
However it has been postulated by Leo Vroman
that “small and abundant molecules will be the first to coat a surface” and over time, molecules with
higher affinity for that particular surface will replace them
...

The first phase is generally considered irreversible and results in a conformational change to the
proteins
...

The first phase of the adsorption process can be divided into three steps
...
Secondly

Samuel Odlin

4

intermolecular forces begin to have an impact on the proteins
...
“Solvation is the process of reorganizing solvent and solute molecules
into solvation complexes
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Generally solvation forces have a length of about 1nm, depending on the degree of
polarization of the solvent
...
Over much
closer distances Van Der Waals forces are important in the metals interactions with proteins
...
(11)
Fig 4: A diagram representing the reorganisation of charged species as a protein (shown in green)
nears the surface of a metal
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It is generally thought that a hydrophobic surface will adsorb proteins more firmly than a
neutrally charged or hydrophilic surface(12) thus adsorbing a greater volume of proteins, vital to
titanium’s role within orthopaedic implants
...

Protein Structures:
Proteins may sometimes be referred to as “hard” or “soft”(13)
...
Hard proteins adsorb on
hydrophobic surfaces in all conditions of charge interaction
...
Soft proteins lose their conformation more easily and will lead to an
entropy gain when adsorbing
...
“Soft Proteins” may include immunoglobulin and haemoglobin(11)
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pH
We have already learnt that the electrostatic forces between proteins and adsorption surfaces play a
key role in the rate and degree of protein adsorption
...
In a solution where
the pH matches the isoelectric point of a protein, the “pH at which no net migration takes place in an
electric field”(14), the protein will have no overall charge (see fig 5)
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In such cases
protein-protein interactions will only be governed by steric factors
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When pI matches the pH a zwitterion is formed
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5, fibrinogen adsorption onto mica (a group of Silicate minerals(15) is
irreversible for ion concentrations between 3 * 10-4 M and 0
...
4 partial de-adsorption
of fibrinogen is observed for ion concentrations less than 10-2 M
...
As a result, patients with blood
pH’s outside the expected 7
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45 such as those with severe liver disease or at high altitude leading
to alkalosis may experience slower rates of osseointegration
...
Shown below in fig 6:

Samuel Odlin

6

2
...
Protein adsorption
to titanium has some similarities to inorganic kinetics however proteins have the added complexities
of denaturation
...

In many instances protein denaturation has harmful effects e
...
enzymes lose their conformation that
enables them to bind to a substrate and therefore carry out their functions efficiently
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Although this is not true of all cases, for example, the adsorption of the
globular protein; BSA, in a temperature range of 20 to 40C°, “increases with an increase in
temperature, due to the increase of diffusion on adsorbent surfaces with increasing activity of protein”
(19)
...
(20)
Therefore it would be overly simplistic to conclude an increased temperature results in better protein
adsorption, collisions with the implant surface would increase for all surrounding molecules however
relative strength of adsorption may have a greater variation
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Samuel Odlin

7

3
...
In an
ideal situation proteins would adsorb strongly and with maximum coverage on an implant surface,
however, Vörös et al
...
In addition; at higher protein concentrations, Roach et al
...

Increased rearrangement and denaturation of proteins may be beneficial for “strong” adsorption
however the original conformation of the protein is important, as “it determines whether certain
bioactive peptide sequences located within the protein will be available for the incoming cells”
...
(25)
Though careful control of the protein concentration and other surrounding factors may have great
benefit to osseointegration of an implant it is inherently difficult to do so
...
g
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(26)
Kinetics and Rate:
To calculate the spontaneity of a reaction we often refer to the Gibbs free energy equation:
ΔG° = ΔH° - TΔS°
In order for a reaction to be spontaneous (require no additional input of energy to occur) ΔG must be
less than 0
...

Transportation is likely through a combination of diffusion, thermal convection or bulk flow
...
Most simply the
adsorption rate can be modeled on the diffusion rate equation, in a low flow environment with
minimal temperature gradients
...
8)

Though the diffusion rate equation is beneficial to gain a theoretical grasp on the factors involved it
does not fully appreciate all factors involved in protein adsorption onto titanium e
...
large flow rates
and convective diffusion
...


Conclusions:

Proteins, though often broken down into simple components and concepts, are very complex
molecules
...
Within a perioperative environment it may be difficult to control the conditions surrounding
the implant e
...
temperature and pH and the characteristics of the proteins themselves
...
This may include; varying the
surface roughness of the metal, bathing the implant in NaOH pre-insertion or a solution of structural
proteins such as Fibrin
...
However one study did find significant increase in
osseointegration in heat treated implants (up to 3
...
Although
implants were allowed to cool from 280oC to room temperature(27), therefore it may be beneficial to
examine cooling to 37oC, where in theory the implant surface temperature is equivalent to the
surrounding tissues
...

Though protein adsorption is “a complex matter that a full and comprehensive explanation” is still
missing(28), it possesses many features that are relevant to the A level chemist, an illustration of the
real world application, when synthesising topics covered within the course
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Evaluation of sources:

Samuel Odlin

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Throughout this paper every effort has been made to provide reliable and reputable information, citing
appropriate sources
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As this is an A level essay I have made the decision to use a number of A level sources
in order to aid the reader in their understanding of the content in relation to their studies
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References:
1
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3
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5
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7
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9
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11
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13
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15
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17
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19
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21
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23
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25
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27
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https://www
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org/standard/36786
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frontiersin
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3389/fbioe
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00170/full
https://www
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org/periodic-table/element/22/titanium
https://pubmed
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nlm
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gov/28000277/
https://www
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nlm
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gov/pmc/articles/PMC3051116/
https://www
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nlm
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gov/pmc/articles/PMC2144688/
https://datascience
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harvard
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pdf
https://en
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org/wiki/Dielectric
https://en
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org/wiki/Protein_adsorption
https://en
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org/wiki/Solvation
https://webcache
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com/search?q=cache:X3fyNXTFiS0J:https://www
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fr/2015P
A066660
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org/doi/10
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2012
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sciencedirect
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wikipedia
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ncbi
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nih
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ncbi
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nih
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elmhurst
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html
https://core
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uk/download/pdf/185271026
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pdf
https://www
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gov/pmc/articles/PMC3051116/#CIT16
https://www
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com/topics/earth-and-planetary-sciences/osteoblast
https://www
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com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwib2Pmznunv
AhVIhf0HHXgfAEQQFjABegQIDRAD&url=https%3A%2F%2Fwww
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com%2F1422-0067%2F
21%2F12%2F4194%2Fpdf&usg=AOvVaw3EXnjLho5WUKKiGVSDe8MX
https://juniperpublishers
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http://msgallagherlhs
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com/fracture--repair
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https://microbenotes
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https://wou
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https://webcache
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com/search?q=cache:X3fyNXTFiS0J:https://www
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fr/2015P
A066660
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6
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8
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com/academy/lesson/calculating-isoelectric-point-definition-formula
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sciencedirect
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wilsonssyndrome
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wikipedia

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