Biomedical Engineering Reference
In-Depth Information
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distinctive characteristic is their horizontal pattern.
Thus, Class 1 irregularities are associated with form
errors of the substrate surface such as straightness, flat-
ness, roundness, and cylindricity. Class 2 surface features
deal with so-called waviness deviations. Waviness is
considered to occur if the wave spacing is larger than the
wave depth. Class 3, 4, and 5 irregularities all refer to
surface roughness. Roughness is assumed if the space
between two hills is about 5-100 times larger than the
depth. Depending on the manufacturing process used,
roughness can be periodic or random. A periodic surface
roughness is also referred to as surface texture and rep-
resents a regular surface topography with well-defined
dimensions and surface distribution. Further, distinction
has to be made among macro, micro, and nano surface
roughness. Microroughness deals with surface features
sized in cellular and subcellular dimensions. Considering
their appearance and morphological structure, class 3
surface roughness has a groove-type appearance; class 4
roughness deals with score marks, flakes, and pro-
tuberances, for example created by grit-blasting pro-
cedures; and class 5 surface roughness is the result of the
crystal structure of a material.
3.2.15 Textured and porous
materials
John A. Jansen and Andreas F. von Recum
Introduction
Surface irregularities on medical devices, such as grooves/
ridges, hills, pores, and pillars, are expected to guide
many types of cells (including immunological, epithelial,
connective-tissue, neural, and muscle cells) and to aid
tissue repair after injury. With the growing interest in
tissue engineering, porous scaffold reactions
in vitro
and
in vivo
are assuming increasing importance. The final
response to rough or porous materials is reflected in the
organization of the cytoskeleton, the orientation of
ECM components, the amount of produced ECM, and
angiogenesis. Although significant progress has been
made, the exact cellular and molecular events underlying
cellular and matrix orientation are not yet completely
understood.
This section will provide information about how sur-
face roughness is defined, prepared, and measured. In
addition, it will cover the biological effects of surface
irregularities on cells.
Porosity
Besides the surface irregularities as mentioned earlier,
porosity can also be considered as surface irregularity.
Porosity can occur only at the substrate surface or can
completely penetrate throughout a bulk material. It
consists of individual openings and spacings or inter-
connecting pores. Porosity can be created intentionally by
a specific production process, such as sintering of beads,
leaching of salt, sugar, or starch crystals, or knitting and
weaving of fibers. On the other hand, porosity can also
arise as a manufacturing artifact, for example, in casting
procedures.
Definition of surface irregularities
Surface irregularities can be considered as deviations
from a geometrically ideal (flat) surface. They can be
created accidentally by the production process or engi-
neered for specific purposes. Surface irregularities can be
classified according to their dimensions and the way they
are achieved. In view of this, surface irregularities can
be classified into six classes (
Sander, 1991
). The main
