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Biomaterials and Protein Adsorption
Examples of Biomaterials
• Medical implants • Contact lenses• Drug delivery
systems• Scaffolding for
tissue regeneration
Proteins are amphiphilic molecules in an aqueous
milieu• Polypeptides are
amphiphilic molecules• BUT -- The human
body is 90% water!• SO : hydrophobic
regions of proteins seek refuge in supramolecular configurations that minimize their exposure to water
Hydrogen Bonding Depends on the Electronegativities of the Donor and
Receptor Groups
H2N CH C
CH2
NH
O
C
NH2
O
CH C
CH2
NH
O
SH
CH C
CH2
OH
O
CH CH3
CH3
• Blue = hydrogen donors
• Red = hydrogen acceptors
• Black = non-hydrogen bonding
Proteins adhere to hydrophobic surfaces
t
•“Foot Model” of protein adhesion•Self-propagating•First step in the humoral response against foreign materials in the body
Design of Biomaterials Surfaces
• Hydrophilicity inhibits protein adsorption, however:
• Some cell adhesion may be desirable
• Compliance is a key consideration
• Solution? Polymers, of course!
Techniques for Coating Biomaterials
• Physisorption– Adhesion to biomaterial
surface is of hydrophobic and/or electrostatic origin
• Chemisorption– Polymer is chemically
attached to the surface, usually via reaction of the surface with a specific end-group on the polymer
– Often referred to as a “self-assembling monolayer” (SAM)
example: an –SH terminated polymer covalently binds to a Au3+ surface
Polymer Brushes
• A “brush” is formed when the spacing d between end-grafted polymers is less than twice the Flory radius, RF, where RF ~ aN3/5 and a is the monomer size
Fundamentals of Protein-Surface Interactions
• Large free energy gain associated with protein adhesion to hydrophobic surfaces
• Attraction due to long-range van der Waals forces, as well as specific and hydrophobic interactions, and the electrostatic double layer (all short-range)
• Repulsion due to steric and osmotic factors (short range)
• Proteins will stick if Ubare(0) < kT
Steric and Osmotic Factors
• Atoms and molecules take up a finite amount of space which cannot be occupied by other elements – i.e. they introduce an excluded volume– Dense packing, rotations, and/or
rearrangements may therefore not be energetically allowed: i.e. steric hindrance
– Crowding leads to an increase in the internal energy and thus the osmotic pressure
The Free Energy Profile of the Brush has Two
Minima
a) brush potential, Ubrush(z)
b) attractive [primarily] van der Waals
potential UvdW(z)
c) net interaction potential
Modes of Protein Adsorption
solid substrate
e.g. human serum albumin7
(IV.)
(I.)
Loend-grafted
polymer brush
s
RP
RP(III.)(II.)
(I.)
adsorbed proteins
(I.) adsorption of proteins to the top boundary of the polymer brush
(II.) local compression of the polymer brush by a strongly adsorbed protein
(III.) protein interpenetration into the brush followed by the non-covalent complexation of the protein and polymer chain
(IV.) adsorption of proteins to the underlying biomaterial surface via interpenetration with little disturbance of the polymer brush
What do the The Primary and Secondary Minima
Correspond to?
Primary minimum: Uin
adsorption at the solid surface
Secondary minimum: Uout
Adsorption at the outer brush surface
Osmotic vs Entropic Forces
The brush thickness, L depends on a balance of forces:
Osmotic Force
2
kT
a 3
L
Na
3
where
So the correspondingforce and free energy per chain:
Elastic Force
2Na
L
kT
f el
31
2
a
Na
L
At Equilibrium
32
23
a
L
Na
34
33
a
kT
a
And the corresponding osmotic pressure:
Monomer volume fraction:
Brush thickness:
Variables:
pressure osmotic
fraction lumemonomer vo
sizemonomer
chainper area
a
osmf
LFosm
2
2
Na
L
kT
Fel
elosm ff 0
L
For
Secondary Adsorption
• Since there is no energy barrier, it is only possible to control Uout thermodynamically
• Uout UvdW(L)• Because penetration of the brush requires chain
compression, large proteins will preferentially undergo secondary adsorption so long as UvdW(L) < -kT
• For a rod-like protein (fibrinogen, e.g.) of radius R and length H, suppression of secondary adsorption may only be achieved if:
Occurs when Uout < -kT
32
313
2
HR212
AL
Where A is the Hamaker constant, A ~ 10-21 J
for proteins interacting with organic materials
Primary Adsorption
When Rp << L :3
PPbrush RVU
LzFosm )(
L
Naz
3
)(
When Rp >> L :
kT
azz
3)()(2 where
There is negligible effect on
Approach to the surface results in compression of the brushand an increase in osmotic pressure
and
Occurs when Uin < -kT
The rate constant for adsorption:
kT
U
L
Dkads
*
exp
Where is the width of the energy barrier and D is the diffusion coefficient
And the free energy barrier, U* for primary adsorption:
33
R
kT
R
kT
U *
Where Uads is the interaction potential of the adsorbed protein at the bare surface
*UUU adsin Finally:
** The presence of an energy barrier enables both thermodynamic and kinetic control
Methods for Counteracting Protein-
Surface Interaction with Polymer Coatings
• Dense polymer coatings (low )
• Long polymer chains (large N)
d
R N
Uout may be manipulated by varying N or Uin is primarily controlled by varying
Poly(ethylene oxide) (PEO)
in Biomaterials• The most extensively used
polymer for biomaterial surface coatings, because:– Completely water-soluble– Creates an extensive H-
bonding network– Helical conformation– Proven to be extremely protein
resistant– Capable of being functionalized
for ligand-receptor specificity
• However: – Poor mechanical stability– Protein adhesioin reported
under certain conditions
O
O O
H HO
O O
OH
HO
H
HO