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 Blue = hydrogen donors Red = hydrogen acceptors Black = non-hydrogen bonding

Proteins adhere to hydrophobic surfaces “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 “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 (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? 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 Elastic Force At Equilibrium or Brush thickness: where Monomer volume fraction: Variables: So the corresponding force and free energy per chain: And the corresponding osmotic pressure:

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: Where A is the Hamaker constant, A ~ 10-21 J for proteins interacting with organic materials

Primary Adsorption Occurs when Uin < -kT When Rp << L : ** The presence of an energy barrier enables both thermodynamic and kinetic control When Rp << L : There is negligible effect on  When Rp >> L : Approach to the surface results in compression of the brush and an increase in osmotic pressure where and Where  is the width of the energy barrier and D is the diffusion coefficient The rate constant for adsorption: And the free energy barrier, U* for primary adsorption: Where Uads is the interaction potential of the adsorbed protein at the bare surface Finally:

Methods for Counteracting Protein-Surface Interaction with Polymer Coatings Dense polymer coatings (low s) Long polymer chains (large N) Uout may be manipulated by varying N or s Uin is primarily controlled by varying s R a N d a s

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