Surface Modification for
Biomaterials Applications
Topics:
•Protein Adsorption
•Physiochemical Surface
Modification Techniques
•Biological Surface Modification
Techniques
•Surface Patterning Techniques
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Protein Adsorption
Factors affecting adsorption:
•Surface energy (or tension), g
•Surface hydrophobicity
•Surface charge
Definitions:
Hydrophobic: water fearing
Hydrophilic: water loving
Definitions:
Adsorption: adhesion to solid
surface
Absorption: penetration of
molecules into bulk
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Surface Tension
Fgas/liquid
q
Fsolid/liquid
Fsolid/gas
For wetting to occur,
Fs/g > Fs/l + Fg/l cos(q)
Wetting
Non-Wetting
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Adding molecules that
prevent adsorption is called
steric hindrance.
In this example polyethylene
glycol (PEG) attaches to the
surface (hydrophobic) preventing
protein adhesion
Like attracts like
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Physicochemical Surface
Treatments
Covalent and non-covalent
coatings describes how
materials is attached to the
surface
Surface modification with
no overcoat, and laser
methods for surface
modification make surface
locally attractive for
adhesion of desired species
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Methods of surface coating: Plasma
Discharge
Charged particles are
attracted to the sample
surface, which acts as the
cathode.
Particles may be positive
or negative ions, free
radicals, electrons, atoms,
molecules or photons.
Often used to add OH or NH2
groups to surface as a
precursor to further modification
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Plasma Discharge
• Advantages:
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Coatings are conformal
Free of voids/pinhole defects
Easily prepared
Sterile when removed from reactor
Produce low amount of leachable substances
Demonstrate good adhesion to substrate
Allow unique film chemistries to be produced
Easily characterized
• Disadvantages
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Chemistry within reactor may be undefined
Equipment often expensive
Uniform reaction within long, narrow pores may be difficult
Care must be taken in sample preparation to prevent contamination
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Vapor Deposition:
Physical (PVD)
Physical Vapor
Deposition (PVD) may
be from evaporation or
sputtering.
Sometimes a plasma is
used to create high
energy species that
collide with target (right)
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Vapor Deposition:
Chemical (CVD)
In Chemical Vapor
Deposition (CVD) a
reactive gas is passed
over the substrate to be
coated, inside of a
heated, environmentally
controlled reaction
chamber.
In this case (right) CH4
gas is introduced to
create a diamond-like
coating
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Radiation Grafting and Photografting
• Substrate is exposed to a radiation source of
high energy, which forms a reactive species at
the surface to create covalent bonding of the
coating to the underlying material
• Often employed to bind hydrogels to
hydrophobic substrates
• Biomaterial substrate may be placed in a
monomer solution the irradiated by electrons of
gamma rays to form a polymerized coating.
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Self-Assembled Monolayers (SAMs)
SAMs are amphiphilic,
having both hydrophilic
(polar) and hydrophobic
(nonpolar) parts. They are
made up of 3 parts:
•The attachment group
•A long hydrocarbon chain
•The functional (polar) head
group
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In the picture, hydroxyl
groups form a strong
attachment to the
substrate.
A strong exothermic
reaction attaches the
Silane to the OH
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Physiochemical coatings
Physiochemical coatings are used
to coat biomaterials with biologically
active molecules.
These methods include solution
coatings and Langmuir-Blodgett
films (right)
Coatings are amphiphilic, having a
hydrophilic head and hydrophobic tail. This
causes the heads to remain in the water
and the tails to extend above the surface.
The molecules at the head may be tailored
to enable crosslinking with other molecules
or to the biomaterials surface
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Surface Modifying Additives
Surface Modifying Additives (SAMs)
are atoms or molecules that, when
added to the bulk material, will
spontaneously rise to the surface,
producing a coating with characteristics
dictated by the properties of the SMA.
SMAs may be used with metals (e.g. Cr in
steel) to create a corrosion resistant
surface, or in polymers (right). Here the A
copolymer anchors into the material,
leaving the B copolymer exposed, which
provides the desired surface properties.
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Physicochemical Surface
Modifications with no Overcoat
These techniques are designed to modify
existing atoms at the surface, and include:
•Ion beam implantation
•Plasma treatment
Conversion coatings create an
oxide layer at a metal surface, 5 –
500-nm thick, to prevent corrosion
•Conversion Coatings
•Bioactive Glasses
Bioactive glasses come from the
range of compositions depicted in
the phase diagram. These
dissolve and combine with natural
biomaterials depending upon the
ratios of CaO, Na2O, and SiO2 The
IB index is a measurement of the
bioactivity of these materials
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Ion beam implantation
This method can create surfaces
with high hardness, wear, corrosion
resistance and biocompatibility
It can also cause surface damage
in the form of sputtering of surface
atoms, surface roughness and
changes in the crystal structure.
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Biological Surface Modification
Biological surface
Techniques
modification attach
biologically active molecules
to a substrate through a
variety of means that they
then interact with specific
target areas on cells or other
tissue components
Biomolecule attachment has
been successfully achieved
on:
•Soluble polymers
•Solid Polymers
•Porous solid polymers
•Hydrogels
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Methods for the covalent
attachment of biomolecules to
a biomaterial surface. (a-c)
attachment via post fabrication
methods (d-e) attachment
during synthesis. The
biomolecule may be attached
with or without a spacer arm in
any of these methods
Many of these methods can be used to attach
enzymes to solid substrates, and have been
used in many areas, including biosensors,
controlled release devices and protein analysis
Heparin, a
hydrophobic
molecule, may be
attached by (a)
adding a
hydrophobic region
to the heparin or (b)
adsorption of the
heparin (which has
a strong negative
charge) to a
positively charged
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surface
Surface Patterning Techniques
Surface or substrate patterning is used to alter
the surface properties of biomaterials in a controlled
manner, resulting in a geometric design of welldefined regions with very different characteristics. It
may be used on both metals and polymers.
Microcontact printing (right)
creates a “stamp” that is inked
with the desired biomaterial
and printed on the substrate.
This method employs many of
the techniques used in making
integrated circuits
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