Biomaterials and Protein Adsorption. Examples of Biomaterials Medical implants Contact lenses Drug delivery systems Scaffolding for tissue regeneration.

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