Biomedical Engineering Reference
In-Depth Information
3.1.4 Surface properties
and surface characterization
of materials
developed methods typically exist to measure these bulk
properties
often these are the classic methodologies of
engineers and materials scientists. Durability, particularly
in a biological environment, is less well understood. Still,
the tests we need to evaluate durability have been de-
veloped over the past 20 years. Biocompatibility repre-
sents a frontier of knowledge in this field, and its study is
often assigned to the biochemist, biologist, and physician.
However, an important question in biocompatibility is
how the device or material ''transduces'' its structural
makeup to direct or influence the response of proteins,
cells, and the organism to it. For devices andmaterials that
do not leach undesirable substances in sufficient quanti-
ties to influence cells and tissues (i.e., that have passed
routine toxicological evaluation), this transduction occurs
through the surface structure - the body ''reads'' the
surface structure and responds. For this reason we must
understand the surface structure of biomaterials.
d
Buddy D. Ratner
Introduction
Consider the atoms that make up the outermost surface of
a biomaterial. As we shall discuss in this section, these
atoms that reside at the surface have a special organization
and reactivity. They require special methods to character-
ize them and novel methods to tailor them, and they drive
many of the biological reactions that occur in response to
the biomaterial (protein adsorption, cell adhesion, cell
growth, blood compatibility, etc.). The importance of sur-
faces for biomaterials science has been appreciated since
the 1960s. Almost every biomaterials meeting will have
sessions addressing surfaces and interfaces. Here we focus
on the special properties of surfaces, definitions of terms,
methods to characterize surfaces, and some implications of
surfaces for bioreaction to biomaterials.
In developing biomedical implant devices and mate-
rials, we are concerned with function, durability, and
biocompatibility. In order to function, the implant must
have appropriate properties such as mechanical strength,
permeability, or elasticity,
General surface considerations
and definitions
This is the appropriate point to highlight general ideas
about surfaces, especially solid surfaces. First, the surface
region of a material is known to be of unique reactivity
( Fig. 3.1.4-1A ). Catalysis (for example, as used in petro-
chemical processing) and microelectronics both capitalize
on special surface reactivity
thus, it would be surprising if
biology did not also use surfaces to do its work. This
d
just to name a few. Well-
A
B
Adsorbed H 2 O
Hydrocarbon
Polar organics
Bulk
Metal oxide
Fig. 3.1.4-1 (A) A two-dimensional representation of a crystal lattice illustrating bonding orbitals (black or crosshatched ovals). For atoms
in the center (bulk) of the crystal (crosshatched ovals), all binding sites are associated. At planar exterior surfaces, one of the bonding sites
is unfulfilled (black oval). At corners, two bonding sites are unfulfilled. The single atom on top of the crystal (an adatom) has three unfulfilled
valencies. Energy is minimized where more of these unfulfilled valencies can interact. (B) In a ''real world'' material (e.g., a block of metal
from an orthopedic device), if we cleave the block (under ultrahigh vacuum to prevent recontamination) we should find hydrocarbon on
the outermost layer (perhaps 3 nm, surface energy w 22 ergs/cm 2 ), polar organic molecules (>1 nm, surface energy w 45 ergs/cm 2 ),
adsorbed water (<1 nm, surface energy w 72 ergs/cm 2 ), metal oxide (approximately 5 nm, surface energy w 200 ergs/cm 2 ), and finally,
the uniform bulk interior (surface energy w 1000 ergs/cm 2 ). The interface between air and material has the lowest interfacial energy
( w 22 ergs/cm 2 ). The layers are not drawn to scale.
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