Biomedical Engineering Reference
In-Depth Information
aligned nanofiber scaffolds were significantly higher than those of the unaligned,
and although the scaffolds degraded in vitro, physiologically relevant mechanical
properties were maintained.
In addition to fiber alignment, the crimped nature of collagen fibrils in ten-
don/ligament is an important feature that should be considered when preparing
nanofiber scaffolds for application in tendon/ligament repair. Fiber crimp leads to
relatively high extension under low loads, providing the characteristic non-linear
mechanical behavior of tendon and ligaments and possibly shielding cells from
high shear stresses. Electrospun nanofibers could be induced to crimp upon re-
moval from a mandrel that rotates at a very high speed (Figure 9.7g). The crimped
morphology could be retained for at least four weeks in phosphate-buffered saline
(PBS) at 37
◦
C (Figure 9.7h). The crimping effect was determined to be a result
of the residual stresses resident in the fibers during the fiber alignment process.
The same group also produced crimp in electrospun nanofibers by using a temper-
ature higher than the glass-transition temperature of the polymer. The resultant
crimped fibers exhibited a long toe region in their stress-strain curve, reproducing
a characteristic of the collagen fibrils in native tendon/ligament [105].
The ease of producing uniaxial alignment represents a major advantage for elec-
trospinning in fabricating scaffolds for tendon/ligament repair. Growing interest
has been focused on generating crimped structures with tension bearing capacity
similar to that of native tendon/ligament. While the shape of the stress-strain curve
for a scaffold made of crimped, electrospun nanofibers resembles that of native
tendon/ligament, the magnitude of the curve for these constructs is much smaller
than that of native tissue. The insufficient mechanical properties for the demand-
ing mechanical physiological conditions of tendons and ligaments may lead to
premature failure of the healing tendon/ligament. Future work should focus on
improving the mechanical properties of aligned and crimped nanofiber scaffolds.
9.5.4
Tendon-to-Bone Insertion Site Repair
The tendon-to-bone insertion (the enthesis) can generally be characterized as either
fibrous or fibrocartilagious [106]. At a fibrous insertion, a tendon attaches to the
bone at an acute angle through collagen fibers that extend directly to the bone.
In contrast, a fibrocartilagious insertion is characterized by a functionally graded
transitional zone of tendon, followed by uncalcified fibrocartilage, mineralized
fibrocartilage, and bone [107]. The transitional zone exhibits a gradual change in
mineral content, spatial organization, cell type, and signaling molecules. While the
tendon tissue is made up of densely packed and well-aligned collagen fibrils, the
bone tissue is made up of less oriented and highly mineralized collagen fibrils. No
sharp boundary exists between the tendon and bone; rather, a functionally graded
architecture connects the very different tissues, mitigating stress concentrations
and enabling the transmission of forces.
Tendon-to-bone insertion repair is a well-known clinical challenge. For example,
surgical repair of the injured rotator cuff usually involves suturing the torn tendon
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