91
have degradation profiles ranging from days to months, in order to suit the specific
needs of the engineered vascularized tissue constructs [215]. The polymer backbone
can be cross-linked using a nondegradable cross-linker that provides structural
integrity and/or a degradable cross-linker that allows directed cell migration and
vascular morphogenesis. Hydrolytic degradation by the body fluid can break down
the ester bonds within the polymer backbone, allowing tissue infiltration over time
[214, 215]. MMP-sensitive peptides can also be used to cross-link hydrogels, allowing cell-mediated degradation, leading to a rapid response of vascular growth.
Overall, by adjusting the percentages of nondegradable and degradable cross-linkers, scaffold degradation can be tuned to allow cellular infiltration, lumen formation, and ECM synthesis and distribution.
In order for the intracellular vacuoles to coalesce into a lumen, ECs require adhesive ligands for traction [152] and utilize membrane-type-1 MMPs (MT1-MMPs) to
create physical spaces which facilitate the directed migration of cells to align with
neighboring cells [48, 192, 212]. Therefore, ECs can only invade this synthetic scaffold if the minimal pore size is larger than the cell diameter (e.g., a soft selfassembling peptide) [201] or if the scaffold bears an MMP-degradable sequence
[153]. The Hubbell research group has pioneered this approach by incorporating an
MMP-degradable sequence as a cross-linker into PEG scaffolds to promote vascular
healing and therapeutic angiogenesis [196, 249]. When grafted in vivo, ECs were
able to invade, remodel, and vascularize this MMP-sensitive scaffold [248, 249].
Using concepts from this work, synthetic (HA-based) biomaterials utilized spatial
control of degradation through photopatterning to organize vascular morphogenesis
(Fig. 4.3) [90]. Hence, incorporating MMP-degradable peptides is essential for
directing vascular morphogenesis in 3D synthetic biomaterials.
Physical Orientation of the ECM
The native ECM provides an instructive template for ECs and perivascular cells to
orient, interact, and organize into tubular structures. Studies have demonstrated that
a stable vasculature could be achieved by co-transplantation of ECs and perivascular cells, such as MSCs or SMCs [15, 16, 128, 142, 164]. Recent studies showed that
engineering a stable vascularized tissue construct requires the triculture of ECs,
fibroblasts, and tissue-specific cells, such as cardiac or skeletal muscle cells [31,
142]. Perivascular cells, such as fibroblasts, stabilize the developing vascular tube
through physical support, by differentiating into v-SMCs and wrapping around the
nascent tube [114, 229], and chemical support, by secreting Ang-1, PDGF-BB, and
tissue inhibitor of metalloproteinase-3 (TIMP-3) [95, 97]. These perivascular cells
are also responsible for laying down ECM components in early embryogenesis and
continue to do so throughout adulthood. Many studies using fibroblast-derived
matrices have further revealed the 3D complexity of these ECM networks [206–
208]. A study by Soucy and Romer showed that fibroblast-derived matrix alone is
sufficient to induce HUVECs to undergo vascular morphogenesis, independent of
any angiogenic factors. Further analysis of protein colocalization suggested that
4 Hypoxia and Matrix Manipulation for Vascular Engineering
have degradation profiles ranging from days to months, in order to suit the specific
needs of the engineered vascularized tissue constructs [215]. The polymer backbone
can be cross-linked using a nondegradable cross-linker that provides structural
integrity and/or a degradable cross-linker that allows directed cell migration and
vascular morphogenesis. Hydrolytic degradation by the body fluid can break down
the ester bonds within the polymer backbone, allowing tissue infiltration over time
[214, 215]. MMP-sensitive peptides can also be used to cross-link hydrogels, allowing cell-mediated degradation, leading to a rapid response of vascular growth.
Overall, by adjusting the percentages of nondegradable and degradable cross-linkers, scaffold degradation can be tuned to allow cellular infiltration, lumen formation, and ECM synthesis and distribution.
In order for the intracellular vacuoles to coalesce into a lumen, ECs require adhesive ligands for traction [152] and utilize membrane-type-1 MMPs (MT1-MMPs) to
create physical spaces which facilitate the directed migration of cells to align with
neighboring cells [48, 192, 212]. Therefore, ECs can only invade this synthetic scaffold if the minimal pore size is larger than the cell diameter (e.g., a soft selfassembling peptide) [201] or if the scaffold bears an MMP-degradable sequence
[153]. The Hubbell research group has pioneered this approach by incorporating an
MMP-degradable sequence as a cross-linker into PEG scaffolds to promote vascular
healing and therapeutic angiogenesis [196, 249]. When grafted in vivo, ECs were
able to invade, remodel, and vascularize this MMP-sensitive scaffold [248, 249].
Using concepts from this work, synthetic (HA-based) biomaterials utilized spatial
control of degradation through photopatterning to organize vascular morphogenesis
(Fig. 4.3) [90]. Hence, incorporating MMP-degradable peptides is essential for
directing vascular morphogenesis in 3D synthetic biomaterials.
Physical Orientation of the ECM
The native ECM provides an instructive template for ECs and perivascular cells to
orient, interact, and organize into tubular structures. Studies have demonstrated that
a stable vasculature could be achieved by co-transplantation of ECs and perivascular cells, such as MSCs or SMCs [15, 16, 128, 142, 164]. Recent studies showed that
engineering a stable vascularized tissue construct requires the triculture of ECs,
fibroblasts, and tissue-specific cells, such as cardiac or skeletal muscle cells [31,
142]. Perivascular cells, such as fibroblasts, stabilize the developing vascular tube
through physical support, by differentiating into v-SMCs and wrapping around the
nascent tube [114, 229], and chemical support, by secreting Ang-1, PDGF-BB, and
tissue inhibitor of metalloproteinase-3 (TIMP-3) [95, 97]. These perivascular cells
are also responsible for laying down ECM components in early embryogenesis and
continue to do so throughout adulthood. Many studies using fibroblast-derived
matrices have further revealed the 3D complexity of these ECM networks [206–
208]. A study by Soucy and Romer showed that fibroblast-derived matrix alone is
sufficient to induce HUVECs to undergo vascular morphogenesis, independent of
any angiogenic factors. Further analysis of protein colocalization suggested that
4 Hypoxia and Matrix Manipulation for Vascular Engineering