Surfaces and Interfacial Responses

Question 1

What are the three main response types to biomaterials?

Answer 1

For implants that are physical or chemical irritants, there is inflammation at the implant site.
For implants that are physically and chemically inert, they are encapsulated in a thin layer of collagenous material.
For bioactive implants, there is direct bonding between the implant and tissue.

Question 2

What part of a biomaterial determines the body’s response to it?

Answer 2

The material surface.

Question 3

*List surface properties that affect biological responses to materials.

Answer 3

These are:
* Topography
* Roughness
* Chemistry
* Energy
* Protein adsorption

Question 4

*How does surface topography affect the biological response to a material?

Answer 4

Surface topography has been shown to influence morphogenesis, orientation and the migration of cells, a process referred to as contact guidance. E.g. many cell types have been shown to respond to grooved substrata.
Morphogenesis: the shaping of an organism by embyrological processes of differentiation of cells.

Question 5

*How does surface roughness affect biological responses to a material?

Answer 5

Surface roughness can be considered the random variable in surface texture on the micron/nanometer scale and has been shown to influence proliferation, migration, phenotypic expression and metabolism of cells.
E.g. there have been increases in osteoblast adhesion and proliferation with increasing roughness on both alumina and hydroxyapatite.

Question 6

*How does surface chemistry affect the biological responses to a material?

Answer 6

Many cell types have the ability to differentiate between different surface chemistries, as well as the same surface chemistries with different crystallinities.
Model surfaces have shown that the outermost functional groups of a surface influence cell attachement and proliferation.
Crystallinity: the degree of structural order in the surface. A perfect crystal has an arrangement of atoms/molecules that is consistent and repetitive.

Question 7

*How does surface energy affect the biological response to a material?

Answer 7

Surface energy is the general charge density and net polarity of the charge on a surface. Net positive or negative charge may be hydrophilic, whilst a surface with a net neutral charge may be hydrophobic.
The net effect of surface charge creates a local environment with specific surface tension, surface free energy and energy of adhesion. These factors affect protein and cell interactions with surfaces.

Question 8

*How does protein adsorption affect the biological response to a material?

Answer 8

Nearly all biomaterials adsorb a monolayer or protein within seconds of being placed within the body. The resulting protein ‘conditioning layer’ consists of a mixture of many different proteins. The Vorman effect develops the layer such that low molecular weight, high concentration proteins arrive at surface first, but can be displaced by high molecular weight, low concentration proteins.

Surface chemistry and energy may influence which proteins adhere and subsequent structural reorganisation.

Question 9

*How do we currently exploit the phenomena of surface properties affecting biological responses to materials?

Answer 9

• Altering roughness and topography e.g. endosseous (root) tooth implants and tendon repair
• Applying surface coatings e.g. hydroxyapatite and diamond-like carbon
• Functionalisation with biological components for adhesion promotion or inhibition e.g. tethering albumin (globular protein) to polymer surfaces, incorporation of adhesion molecules onto surfaces, incorporation of peptide sequences onto or into polymer surfaces.

Question 10

*Describe what the material-biological interface is.

Answer 10

Any implanted biomaterial application involves the creation of at least one interface between material and biological system. At these interfaces molecular constituents of both the biological system and biomaterial meet and interact, the effect of which can be observed at the macroscopic scale.

Question 11

What factors can affect the wound healing process (from implants)?

Answer 11

The extent of alteration may be affected by:
• Shape of implant
• Surface roughness and porosity
• Surface charge, hydrophilicty, chemical composition
• Extent of chemical leaching into tissues
• Elastic modulus; mismatch may cause stresses and interfacial movement
E.g. bone cement has an exothermic reaction when setting, which in large masses can cause thermal damage and therefore tissue damage and necrosis.