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.
Surface modifying additives (SMAs):
SMAs are added to the material during its fabrication and move towards its surface to minimize tension. The driving force is the difference between tension with and without the SMA coat and the mobility of the SMAs in the bulk material.
SMAs in metals can be other metals that preferentially move towards the outer layer to form a passivation layer to prevent corrosion.
SMAs can’t move in ceramics due to lack of molecular mobility.
SMAs in polymers can be copolymers that have a block that interacts preferentially with the bulk polymer and a block that doesn’t. As such, the copolymer arrange themselves, such that the preferential block acts as a an anchor to the other block which coats the bulk polymer underlying it.
Ion beam implantation:
a high energy beam of ions is focused on the surface of a metal or ceramic. These ions displace the sample atoms on the surface and induce their movement within the lattice structure, which creates interstitial and vacancy defects. Some atoms are spluttered from the surface which changes the surface topology. This technique can be used to create rough or smooth surfaces and integrate defects to improve fatigue resistance, biocompatibility, and hardness.
Plasma treatment:
the previously described setup can be used to etch the surface of the sample using an inert gas medium.
Conversion coating
the surface of a metal is oxidized to generate a passivation layer to resist corrosion. This oxidation may be achieved by acid treatment or anodization (using a the metal surface in a galvanic cell as an anode).
Bioactive glass
bioactive glasses can be fabricated from a combination of CaO, SiO2, and Na2O which can induce a range of possible biological responses such as fibrous encapsulation and material dissolution. These materials are characterized by biological activity index (Ib) which depends on the ratio of each component to the other. For example, with specific combinations of the three molecules, it is possible to induce phosphate ions to bind calcium in vivo to speed up tissue integration and increase Ib.
Patterning techniques
are controllable techniques that alter the surface of a material to produce a regular and well-defined regions of known properties that alternate on the surface of the material.
Contact angle analysis
A system with three interfaced is created: liquid-vapor surface, the solid-liquid surface, and the solid-vapor surface. Water is usually chosen a liquid. Each interfaces causes water droplet to assume a particular shape (different degree of spreading). By measuring angle between drop and solid surface (contact angle theta), surface tension can be calculated using eqn 7.1
Light microscopy (a qualitative assessment)
4 basic components:
-Source-produces white light
-Lenses-glass lenses focus light beam and/or magnify image of sample
-Sample stage-holds sample securely
-Detector(camera or human eye)-views and captures resulting image
Disadvantage: difficult to see thick or hydrated samples
Electron spectroscopy for chemical analysis (ESCA)
X-ray absorption causes removal of an electron from one of the innermost atomic orbitals (not valence shell). The KE of the emmitted electron is used to find binding energy of the electron (equation 7.2). Binding energy gives an idea of how tightly bound the electron is to the nucleus. The closer they are to nucleus or higher the atomic number, the larger their binding energy.
4 components:
-Source- produces x-rays with known wavelength
-Electron analyzer- uses an electrostatic field to separate electrons based on kinetic energy
-Detector- converts impact by separated electrons into an electrical signal
-Processor(computer)- translates the signal from the detector into the appropriate spectrum
ATR-FTIR
When a beam of electromagnetic radiation passes from a medium that is more dense to one that is less dense, reflection occurs. Upon reflection, the beam penetrates a small distance into the less dense medium and this penetrating radiation is called an evanescent wave. Absorption of certain wavelengths causes their attenuation and provides information about the chemical structure
SIMS
Centers on separation of ionic species by mass and involves use of primary and secondary ions. I provides information about the structure and composition of the outermost few A of both inorganic and organic materials.
Atomic Force Microscopy
AFM can provide three-dimensional images of material surfaces with A to nm spatial resolution. The analytical capabilities of AFM are limited to the uppermost atomic layer of a sample because its operation is based on interaction with the electron clouds of atoms at the surface.
4 basic components:
1.Cantilever/tip-bends in response to forces between tip and sample
2.laser/detector-a laser beam is bounced off the cantilever and directed toward a photodiode detector to record the deflection of the cantilever in response to the surface.
3.Sample stage-holds sample securely. Uses a piezoelectric driver to alter position up or down to maintain contact between tip and sample.
4.Computer
Used to visualize surface topography
