MSE-536

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),

•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

Fsolid/liquid Fsolid/gas

Wetting Non-Wetting

For wetting to occur,

Fs/g > Fs/l + Fg/l cos(

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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:

– 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– 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 coatingsPhysiochemical 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

•Bioactive Glasses

Conversion coatings create an oxide layer at a metal surface, 5 – 500-nm thick, to prevent corrosion

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 implantationThis 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 TechniquesBiological surface 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

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

surface

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

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Surface Patterning TechniquesSurface or substrate patterning is used to alter

the surface properties of biomaterials in a controlled manner, resulting in a geometric design of well-

defined 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