Download Examples of Biomaterials

Biomaterials and Protein
Adsorption
Examples of Biomaterials
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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
O
H2 N
CH
C
CH2
C
NH2
O
O
N
H
CH
C
O
N
H
CH
CH2
CH2
SH
CH
CH3
C
CH3
OH
• 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!
s
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 “selfassembling 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
(I.) adsorption of proteins to the
top boundary of the polymer brush
e.g. human serum
albumin7
(II.) local compression of the
polymer brush by a strongly
adsorbed protein
RP
adsorbed
proteins
(II.)
(I.)
RP
(III.)
(I.)
end-grafted
polymer brush
Lo
s
solid substrate
(IV.)
(III.) protein interpenetration into
the brush followed by the noncovalent 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?
Secondary minimum: Uout
Adsorption at the outer brush surface
Primary minimum: Uin
adsorption at the solid surface
Osmotic vs Entropic Forces
The brush thickness, L depends on
a balance of forces:
Osmotic Force
a
3
kT
f el
L

kT Na 2
2
3
where
Na

sL
So the corresponding
force and free energy per chain:
f osm  s
Fosm  sL
Elastic Force
Fel
L2

kT Na 2
At Equilibrium
f osm  f el or
F
0
L
Brush thickness:
 a2 
L
 N  
a
s 
1
3
Monomer volume fraction:
Variables:
s  area per chain
a  monomer size
  monomer vo lume fraction
  osmotic pressure
Na 3  a 2 

 
Ls  s 
2
3
And the corresponding osmotic pressure:
a 3
 a3 
 
kT  s 
4
3
Secondary Adsorption
Occurs when Uout < -kT
• 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:
2
 A  3 13 2 3
L
 R H
12
2


Where A is the Hamaker constant, A ~ 10-21 J
for proteins interacting with organic materials
Primary Adsorption
Occurs when Uin < -kT
** The presence of an energy barrier enables both thermodynamic and kinetic control
When Rp << L :
There is negligible effect on 
U brush  VP  RP
When Rp >> L :
3
Approach to the surface results in compression of the brush
and an increase in osmotic pressure
Fosm   ( z )sL
where
 ( z) 
2
The rate constant for adsorption:
And the free energy barrier,
U*
for primary adsorption:
Finally:
 ( z )a 3
and
kT
k ads
Na 3
 ( z) 
Ls
U * 
D


exp 
L
 kT 
U * R 3  R 



kT
kT  s 
U in  U ads  U *
Where  is the width of the energy barrier
and D is the diffusion coefficient
3
Where Uads is the interaction potential of
the adsorbed protein at the bare surface
Methods for Counteracting
Protein-Surface Interaction with
Polymer Coatings
Uout may be manipulated by varying N or s
Uin is primarily controlled by varying s
• Dense polymer
coatings (low s)
• Long polymer chains
(large N)
RN
ds
Poly(ethylene oxide) (PEO)
in Biomaterials
O
H
• The most extensively used
polymer for biomaterial
surface coatings, because:
H
O
O
O
H
O
H
O
H
H
O
O
O
– 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