Biomedical Engineering Reference
In-Depth Information
Woven fabrics
Above
this
range,
a
woven
fabric
starts
becoming
mechanically unstable.
Ta b l e 3 . 2 . 4 - 5 offers a list of a number of commercial
woven graft designs with their respective mechanical
properties. As can be seen, many variations in design are
possible, presenting a difficult selection process for the
surgeon. It is interesting to note that the choice of a graft
by a surgeon is often based on the graft's ''ease of handling''
or ''ease of suturing'' rather than on its reported long-term
performance. Plain weaves, in contrast to knits, can be
made very thin ( < 0.004 in.) and have thus become the
material of choice for many endovascular graft designs.
The term ''woven'' is used to describe a textile config-
uration where the primary structural yarns are oriented
at 90 to each other. The machine direction is called the
warp direction and the cross direction is identified as
the filling or weft direction. Because of the orthogonal
relationship between the warp and filling yarns, woven
structures display low elongation and high breaking
strength in both directions. There are many types of
woven constructions including plain, twill, and satin
weaves ( Robinson and Marks, 1967 ). Figure 3.2.4-7 is
a sketch showing several weave designs commonly used
in vascular graft fabrications. Water permeability is one
critical parameter used in the assessment of textile
structures for vascular implants. Water permeability is
a measure of the water flux through a fabric under
controlled conditions and is given in units of ml cm 2
min 1 . It is measured by placing fabric into a test fixture
having a fixed orifice size and applying a pressure of
120 mmHg across the fabric. The water passing through
the fabric is collected and measured over time
and water permeability is calculated (ISO 7198, sec-
tion 8.2.2, Water Permeability). Surgeons use this
parameter as a guide to determine if ''pre-clotting''
of a graft material is necessary prior to implantation.
''Pre-clotting'' is a process where a graft material is
clotted with a patient's blood prior to implantation,
rendering the fabric nonpermeable to blood after im-
plantation. Fabric grafts with water permeability values
less than 50 ml cm 2 min 1 usually do not require pre-
clotting prior to implantation. The water permeability
of the woven graft fabrics can be controlled through the
weaving and finishing process and can range from a low
of 50 ml cm 2 min 1 up to about 350 ml cm 2 min 1 .
Knits
Knitted constructions are made by interloping yarns in
horizontal rows and vertical columns of stitches. They are
softer, more flexible and easily conformable, and have
better handling characteristics than woven graft designs.
Knit fabrics can be built with water permeability values
as high as 5,000 ml cm 2 min 1 and still maintain
structural stability. Currently, highly porous grafts ma-
terials are usually coated or impregnated with collagen or
gelatin so that the surgeon does not have to perform the
time consuming pre-clotting process at the time of sur-
gery. The water permeability values for non-coated
knitted grafts range from about 1200 ml cm 2 min 1 up
to about 3500 ml cm 2 min 1 . When knits are produced,
the fabric is typically very open and requires special
processing to tighten the looped structure and lower its
permeability. This compaction process is usually done
using a chemical shrinking agent such as methylene
chloride or by thermal shrinking. Because of their open
structure, knits are typically easier to suture and have
better handling characteristics; however, in vascular graft
X
4
Y
1
1
4
(A)
(B)
(C)
Plain weave
Twill weave
Satin weave
Fig. 3.2.4-7 Examples of woven graft designs.
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