The adhesion of molecules to a solid surface

Affected by surface hydrophobicity (directly proportional) and surface charge (if opposite, adsorption but repulsion if same), roughness (increases protein adsorption by physically trapping proteins on surface 'valleys')

Ex: PEG repels proteins due to large amount of surface-absorbed water by PEG chains; PEG chains are in constant motion, moving too quickly for proteins to adsorb to chains; chains form a barrier-like wall that prevents protein adsorption (steric repulsion); proteins compete w/ water for adsorption

Covalent (grafting) and non-covalent surface coating

Modification of the original surface

Laser methods for surface modification (patterning)

Covalent (grafting) and non-covalent biological coating

Immobilized enzymes

Surface cleaning

Adhesion promotion (creates chemically active functional groups)

Controlling of surface energy (adjusting hydrophobicity)

Improving biocompatibility (grafting molecules to surfaces

Performance enhancing

Amphiphilic

Thermodynamically favorable; exothermic bonding between head group and matl surface

Van der waals and electrostatic forces among alkyl chains

Easy to make (no specialized equipment, room temp., normal atmospheric pressure); very stable

Amphiphilic molecules: hydrophilic head + hydrophobic tail

Advantage: altering surface chemistry similar to SAMs

Disadvantage: instability of coating

Incorporated during matl fabrication

Additives that rise spontaneously to the matl surface, driven by reductions in surface energy

Easily done w/ polymers

Incompatible bulk is outside the matl; compatible bulk is inside

Ex: Chromium added to steel--prevents corrosion

Biolgical surface modifications

RGD (Arginine-Glycine-Aspartic Acid) - Cell recognition site for numerous adhesive proteins in the ECM

Can be introduced covalently or non-covalently

Spacer: greater rotational freedom (improves bioactivity) and release of biological molecules

*Modulus does not change despite surface modifications

1) Starting water w/ layer to be patterned

2) Coat w/ photoresist

3) Bake the photoresist to set its dissolution properties

4) Expose photoresist by shining light through a photomask

5) Immerse exposed wafer in developer solution

6) Etch underlying layer

7) Strip photoresist

A mold of the desired pattern is created using photolithography on a silicon wafer

A silicone rubber material (PDMS) is polymerized in the mold to make a positive stamp

The stamp is inked with the surface-modifying substance by interaction w/ the molecule of interest; the inked stamp is then pressed onto the substrate

Mold is fabricated as in microcontact printing, but is a positive instead of a negative

PDMS is polyermized in the mold to make channels where required; formed PDMS is pressed against a glass slide and plasma-treated to increase inner area hydrophobicity

PDMS form is pressed against the substrate and a small amount of solution containging the molecule of interest is injected or placed near a channel opening; molecule is pulled through by capillary action

Used to provide overall info about surface hydrophobicity; defines surface tension thermodynamically as the work of making a unit area of new surface

Surface tension is measured across 3 interfaces: solid-liquid, solid-vapor, vapor-liquid (summed in +x)

Young's Equation: S-L = S-V + V-L cos (angle between L and V)

Angle = 180- Max S-L (most hydrophobic), 0- min S-L (most hydrophilic), complete surface wetting; L-V = critical surface tension (complete spreading)

Measure how the angle between L-V and S-L determines critical surface tension

Drops of liquid w/ known surface tension are placed onto solids, and the contact angle is measured. Unknown is compared to known values; extrapolated to 0, angle determined to estimate critical surface tension

Binding energy of an electron ejected from the innermost atomic orbital of a surface is measured

Larger binding energy = tighter electron bonding to nucleus

Used to identify elements present at a matl's surface, neighboring atoms

TEM: greater resolving power than optical microscopy (electron wavelength is shorter than white light) but requires thinner images and is used less than SEM

SEM: sample surface is scanned with an electron beam; electrons undergo elastic and inelastic scattering

Atom emits secondary electrons to release excess energy

Primary ions are ejected from an ion gun and strike the sample surface; surface layer is stripped off

Emitted (secondary) ions are analyzed similar to bulk mass spectroscopy

Static SIMS: Uses a low ion dose, inducing minimal surface damage

Dynamic SIMS: Larger ion dose, increase sputtering causes the surface to erode; allows depth profiling of specimens

A small tip is attached to a cantilever; when the tip encounters the matle surface, Vdw and electrostatic interactions create a force profile that attracts the tip to the surface

Used to visualize surface topography; number of interacting atoms increases with tip width, decreasing resolution