Chapter 7 Flashcards
Adsorption
adhesion of molecules to a surface
Absorption
penetration of molecules into the bulk of another material
Surface properties governing protein adsorption
Surface hydrophobicity, Surface Charge, Physical Properties (Steric Concerns, Surface Roughness)
An ideal surface modification technique would have the following characteristics:
Thin layer (to minimize impact on bulk properties) Resistance to delamination Simple and robust (for commercialization) Mode to prevent surface rearrangement
Covalent Surface Coatings
Plasma discharge
Chemical vapor deposition
Physical Vapor Deposition
Self-assembled monolayers
Non-Covalent Surface Coatings
Solution coating
Langmuir-Blodgett films
Surface Modifying Adjectives (SMAs)
No-Overcoat Surface Modifications
Ion beam implantation
Plasma Treatment (etching/ablasion)
Conversion Coatings
Bioactive Glasses
Advantages of plasma treatment:
Conformal (uniform layer formation) Free of void defects Can be easily prepared Sterile when removed from sample holder Produce low amount of leachable substances Good adhesion to substrate (the layer doesn’t leach off easily) Allow unique chemistry to be produced Can be characterized easily
Disadvantages of plasma treatment:
Chemistry within the plasma reactor is ill-defined (you don’t know the sort of free radical reactions that will take place)
Expensive
If the material is porous, the conformal layer may be difficult to achieve
Measures must be taken before or after processing to prevent contamination
Chemical vapor deposition (CVD):
A mixture of gases is exposed to the sample at high temperatures. This promotes reactions to occur between the gases and the sample which result in deposition of decomposed molecules from the gas onto the sample.
CVD requires control of gas source, temperature of the chamber, and waste disposal of gaseous byproducts. Often, plasma can be used to increase reactivity of gases prior to treatment in what is called plasma-assisted CVD.
CVD is often used to fixate pyrolytic carbon coatings on substrates. Hydrocarbon gases undergo pyrolysis (thermal decomposition) and depose on the material.
Physical vapor deposition (PVD):
There are various physical techniques used to achieve PVD, but we focus on sputtering since they can be used to coat nonconductive biomaterials with a thin layer of metals before electron microscopy. (both covalent and non-covalent techniques are possible here)
Sputter deposition (2-step process):
Energized ions or atoms bombard the target material. Transfer of momentum causes ejection of target surface atoms.
Released atoms strike the surface of the biomaterial and condense to form a thin film.
There are also plasma-assisted PVDs in which plasma is used to create high-energy species to collide with target material instead of ions or atoms.
Radiation grafting (Photografting):
Substrate is exposed to high energy radiation which forms reactive species on the surface which react to form covalent bonds with an added coating material.
This method is often used to bind hydrogels to hydrophobic substrates.
This method offers an easy mean of control over surface properties as mixture of monomers or precursors can be used.
Mutual radiation: a biomaterial is dipped into a monomer solution. The entire system is then irradiated by high energy gamma rays to cause polymerization to occur at the material surface. Alternatively, the coating substrate may be irradiated in an inert environment and then it is exposed to the biomaterial to form the coating layer.
Photografting is similar to radiation grafting, except that UV rays or visible rays are used to initiate the polymerization. Certain chemical moieties can be excited by such rays to form free radicals/ reactive species.
Self-assembled monolayers (SAMs):
The coating molecules are designed such that it is thermodynamically favorable for them to align on the surface of the biomaterial and form covalent bonds with the surface.
No specialized equipment is required for this technique. SAMs can easily form and are very stable and can be modified on one end to add a versatile range of properties to the surface of the molecule.
SAMs are often amphiphilic groups that have a hydrophobic and a hydrophilic area. SAMs are subdivided to:
Attachment group (forms covalent bond with surface)
Long alkyl chain (hydrophobic): form strong van der waal interactions that keep them aligned and stabilize the SAM. once the chains are close, they crystallize.
Functional group added on the other end (hydrophilic): can be used to modify the surface properties of the material.
The functional group can be made to be biologically active if needed.
Solution coatings
A substrate is dipped in a solution containing the coating materials. The substrate is then left to dry and as the solvent evaporates, the coating material deposits on the substrate. This coat, however, leaches off easily in vivo.
Langmuir-Blodgett film (LB film):
A langmuir trough is used to deposit amphiphilic molecules onto a substrate. The substrate is placed in an aqueous environment with the amphiphilic polymers. A moving barrier is slowly compressed to move a layer of the polymers right next to the surface of the substrate and the area per molecule is slowly minimized to a critical area. Once this area is reached, all of the molecules align themselves on the surface of the substrate, and the biomaterial is slowly removed from the media with the deposited layer.
The LB film has the same advantages as the SAM layer except that the coat is less stable as it is not covalently bound. Addition of certain moieties allows for cross-linking to overcome this limitation.
