Biomedical Engineering Reference
In-Depth Information
scaffold to the native artery. Therefore it is increasingly used to fabricate vascular grafts and heart
tissue constructs [28,53-56].
Topographically aligned submicron fi bers have similar circumferential orientations to the cells
and fi brils found in the medial layer of the native artery. Because of these similarities between elec-
trospun scaffolds and natural ECM, and because of the large variety of materials that can be used,
cell viability is often superior to other scaffold designs.
The choice of material for designing electrospun scaffolds is critical to cell viability, fl uid
fl ow, and with respect to pulsations. Boland et al. had electrospun submicron collagen and elastin
fi bers in an attempt to mimic the native artery [57]. They discounted poly(α-hydroxyl esters) as
suitable materials for vascular grafting because it degrades to lactic acid and glycolic acid, the
degradation by-products that can cause cell death if their amount becomes too high per volume
tissue. Their concern was that a high pH in a local environment would be toxic to the cells and
induce an infl ammatory response. However, more recently Stitzel et al. discovered that electros-
pun scaffolds produced with poly(α-hydroxyl esters), in particular PLGA blended with collagen, is
biocompatible and without local or systemic toxicity when implanted [53]. Electrospun PLCL can
also be designed to pulsate synchronously through changes to the electrospun wall thickness [56].
Thus, because it is “mechano-active” and degradation products seem not to cause any signifi cant
cell death and foreign body reaction, materials such as PLCL may actually be a suitable material
choice.
There have been many
in vitro
studies to optimize growth, development, and adhesion of cells
on electrospun scaffolds produced for vascular grafts. Xu et al.
[28] attempted to induce a com-
plete monolayer of SMCs over their electrospun vascular grafts prior to implantation. They cul-
tured human coronary artery SMCs on aligned electrospun PLCL (75:25) nanofi bers approximately
500 nm in diameter using polymer fi lms as a control. The SMCs adhered and migrated along the
scaffolds in the direction of fi ber alignment, and the phenotype was expressed as a “spindle-like
contractile.” The distribution of the smooth muscle cytoskeleton proteins within the cells was also
in the direction of alignment, and the cells adhesion and proliferation was superior on the electro-
spun scaffold than the fi lms. This study provided promising evidence that a synthetic electrospun
matrix on the nanoscale dimensions was capable of mimicking
in vivo
vascular structures mak-
ing it a suitable vascular graft. The scaffold may have been superior to polymer fi lms because its
architecture was similar to the coronary artery, and also because the aligned nanofi bers directed
proliferation and adhesion via contact guidance.
Similarly Zong et al. found that the engineering of cardiac tissue and function can be manipu-
lated by the chemistry and geometry of the electrospun scaffold [58]. In their experiment, primary
cardiomyocytes (CMs) were cultured on electrospun PLLA and PLGA scaffolds of approximately
1 µm in diameter and the cellular attachment, structure, and function were examined. However,
they discovered that on porous, nonwoven scaffolds, CMs made use of external cues for isotropic
and anisotropic growth. Therefore, it appears that a desirable scaffold for vascular tissue engineer-
ing should have aligned fi bers to provide contact guidance, but adequate porosity to allow the cells
to respond to external cues.
5.4.4 N
EURAL
T
ISSUE
E
NGINEERING
The central nervous system (CNS) is a complex organ with the cells located within a specifi c region
(the nucleus), which are connected to various points of infl uence via axons. The cell bodies receive
axonal inputs via dendrites from multiple regions. A central tenant in neurology is that the CNS does
not regenerate new neurons or sprout axons following damage. Consequently injury to the brain or
spinal cord by neurodegenerative diseases or trauma is met by limited recovery leading to severe
consequences for the patient. The recent recognition that adult neurogenesis does occur, and the
potential to exploit fetal and embryonic stem cells in cell-based therapies has offered a more posi-
tive outlook. Furthermore, the recognition that adult neurons are prevented from sprouting axons
