Surface Properties of Biomaterials

Surface Tension (gamma)

excess energy caused by regions or atoms at the surface not bonded, unfilled valence shells

adsorption

adhesion of molecules to a solid surface

absorption

penetration of molecules into the bulk of another material (ex: water absorbed into a sponge)

biocompatibility

controlling protein absorption to the surface of biomaterial, high protein adsorption = lower biocompatibility

protein adsorption on surface of biomaterial

can cause: more proteins to attach, inflammatory response, cellular interaction therefore inhibiting proliferation, biofilm formation
- cell adherence, blood clotting, fouling

properties that impact adsorption

1. surface hydrophobicity: more hydrophobic = more protein adsorption
2. surface charge: occurs by the dissociation of ionizable surface groups or through adsorption of ions from a solution (charge of protein on implant can cause ionic bonds)
- steric hinderances: chemical structures can disrupt
- surface roughness can trap proteins
- low entropy is not favored at surface of implant because proteins increase the entropy in biofluid (good)

surface treatments

1. chemical surface modification: covalent bonding to surface, non-covalent bonding
2. biological surface modification techniques: covalent bonding to surface, non-covalent bonding

covalent bonding (plasma treatment)

- plasma environment is created by applying an electrical potential across a gas (temp is 25C or greater under vacuum)
- cathode is the surface to be treated and has a neg potential relative to the anode
- e- collide with molecules in the gaseous environment to form gaseous ions and radicals
- this is expensive, doesn't get into crevices of specific designs

covalent bonding (chemical vapor disposition or CVD)

- mix of gases is exposed to material at a high temp
- this causes decomposition of one or more components of the gas mixture and subsequent deposition
- used for pyrolytic carbon coatings, Tantalum

covalent bonding (physical vapor deposition)

- deposition of atoms generated through physical processes
- ions or atoms bombard source
- strikes the surface and condense to form a thin film
- increase water resistance of metallic implants (orthopedic implants and surgical tools)

covalent bonding (radiation grafting/photografting

- gamma radiation is used to covalently bond an amine-containing molecule to the surface of nanofibers as the basis for further subsequent biological surface modification
- material is exposed to source of high energy, ionizing radiation, e- beam or UV light which forms the reaction at the surface
- typically used for hydrogels to bind hydrophobic substrates

covalent bonding (self-assembled monolayers or SAM)

- no equipment is needed, can be carried in room temp under normal pressure
- amphiphilic: have hydrophilic (polar) and hydrophobic (non-polar) areas
- three key regions: attachment group, long hydrocarbon (alkyl) chain, and functional polar head group

non-covalent interactions (langmuir-blodgett films)

- like SAMs, you have hydrophilic head and phobic tail
- using the langmuir trough, these molecules are transferred to the surface of biomaterial
- reaches a critical area which is a function of the size and type of hydrophobic tail on the molecule

non-covalent interactions (layer-by-layer coatings)

- step 1: positive charge substrate is placed in an anionic solution, which deposits on the surface due to electrostatic attraction and causes the surface to become neg charged
- step 2: coated substrate is then rinsed to remove loosely attached material
- step 3: the now neg charged surface is placed in a cationic solution and adsorption of the pos charged material occurs, making the surface pos charged
- step 4: substrate is rinsed again
- step 5: steps 1-4 are repeated as many times as required to produce a coating of a desired thickness

biological surface coating techniques (covalent biological coatings)

- attaching biologically active molecules to substrates through covalent bonds
- methods: attachment via post-fabrication or attachment during synthesis (the biomolecule may be attached with or without a spacer arm in any of these methods)

biological surface coating techniques (non-covalent biological coatings)

- involves adsorption of the biomolecule into the biomaterial, then crosslinking to improve the coating stability
- heparin: complex carb used to prevent the formation of clots
- interaction of hydrophobic regions of the heparin and the substrate in an aqueous environment will result in the extension of the heparin portion away from the surface, effectively coating the biomaterial
- adsorption of heparin to positively charged surfaces requires no modification of the biomolecule, heparin has a significant neg charge so electrostatic attraction drives the formation of a heparin layer at the surface of pos charged biomaterials

surface characterization

techniques to analyze surface: atomic force microscopy (AFM), scanning e- microscopy (SEM), contact angle analysis

contact angle analysis

- determines hydrophobicity of the surface
- surface tension (gamma)
- liquid-vapor surface
- solid-liquid surface
- solid-vapor surface
- the lower the critical surface tension the more hydrophobic the material

dynamic contact angle measurement: contact angle hysteresis

- adding water slowly to surface with syringe: advancing contact angle is measured
- water is then removed from the surface with syringe and the receding contact angle is measured
- the difference between the two values is the contact angle hysteresis
- this # describes how the surface tension of the material changes before and after it is exposed to an aqueous environment