41
to decrease when boundary limitation is high, corresponding to higher matrix stiffness conditions. In several studies, differential boundary conditions were applied by
constraining the gel to predetermined boundary geometries such as a rectangle, triangle, square, star, or circle (Fig. 2.2c) [50, 52]. Sun et al. demonstrated that vascular networks situated along the boundary of the shapes were significantly denser and
had a shorter mean cord length, compared with the central regions. The local strain
field experienced by microvessels was predicted by computational finite element
models simulating the contraction of gels constrained by various boundary conditions [50]. This boundary effect was then eliminated using a Rho-associated protein
kinase inhibitor, which demonstrated the correlation with cell traction force [50].
These findings demonstrate that boundary conditions and, thus, the effective stiffness of the matrix provide an alternative means of controlling vascular organization
in engineered tissues without modifying matrix chemical properties.
2.3 External Forces
2.3.1 Tensile Forces
During physiological growth, blood vessels remodel and grow in response to tensile
stress and the resulting strain within the vessel wall [34]. In recent years, there have
been growing attempts to include tensile stress, primarily applied by the use of bioreactors, as a mechanical stimulator when engineering blood vessels. Recent experiments show a connection between tensile stress and alignment of forming vessels.
ECs grown on micro-carrier beads, cultured within a fibrin gel containing smooth
muscle cells and subjected to 10% cyclic strain at 0.7 Hz, formed sprouts which
aligned in parallel to the strain direction, whereas the unstrained control sprouted
radially. The plasticity of this alignment was demonstrated, when the aligned
strained vascularized constructs were transferred to static unstrained condition,
which led to random alignment of the vessel sprouts [6]. In a similar work, researchers isolated microvessels, seeded them into a collagen gel, and applied 6% cyclic
strain at 1 Hz. The forming sprouts aligned in parallel to the strain direction [27].
However, contradictory findings reported by Matsumoto et al. noted that ECs seeded
on a dextran micro-carrier surrounded with fibrin gel sprouted outward, at an angle
that was perpendicular to the cyclic strain [37]. Similar behavior was recorded when
human pluripotent stem cell-derived vascular smooth muscle cells (vSMC) were
subjected to uniaxial cyclic strain, which induced their alignment perpendicular to
the stretching direction [58]. ECs seeded on collagen on top of a silicone mold and
subjected to cyclic uniaxial stretch (20%, 1 Hz), also aligned perpendicular to the
stretch direction and formed more vessel sprouts [59]. In a different work, ECs and
fibroblasts co-cultured within Gelfoam 3D scaffolds and subjected to uniaxial cyclic
stretch (10% and 1 Hz) formed vessels that aligned diagonal (30–600) to the stretching direction (Fig. 2.3a) [45]. However, ECs and fibroblasts seeded into a Gelfoam
2 Mechanical Regulation of Vascularization in Three-Dimensional Engineered Tissues
to decrease when boundary limitation is high, corresponding to higher matrix stiffness conditions. In several studies, differential boundary conditions were applied by
constraining the gel to predetermined boundary geometries such as a rectangle, triangle, square, star, or circle (Fig. 2.2c) [50, 52]. Sun et al. demonstrated that vascular networks situated along the boundary of the shapes were significantly denser and
had a shorter mean cord length, compared with the central regions. The local strain
field experienced by microvessels was predicted by computational finite element
models simulating the contraction of gels constrained by various boundary conditions [50]. This boundary effect was then eliminated using a Rho-associated protein
kinase inhibitor, which demonstrated the correlation with cell traction force [50].
These findings demonstrate that boundary conditions and, thus, the effective stiffness of the matrix provide an alternative means of controlling vascular organization
in engineered tissues without modifying matrix chemical properties.
2.3 External Forces
2.3.1 Tensile Forces
During physiological growth, blood vessels remodel and grow in response to tensile
stress and the resulting strain within the vessel wall [34]. In recent years, there have
been growing attempts to include tensile stress, primarily applied by the use of bioreactors, as a mechanical stimulator when engineering blood vessels. Recent experiments show a connection between tensile stress and alignment of forming vessels.
ECs grown on micro-carrier beads, cultured within a fibrin gel containing smooth
muscle cells and subjected to 10% cyclic strain at 0.7 Hz, formed sprouts which
aligned in parallel to the strain direction, whereas the unstrained control sprouted
radially. The plasticity of this alignment was demonstrated, when the aligned
strained vascularized constructs were transferred to static unstrained condition,
which led to random alignment of the vessel sprouts [6]. In a similar work, researchers isolated microvessels, seeded them into a collagen gel, and applied 6% cyclic
strain at 1 Hz. The forming sprouts aligned in parallel to the strain direction [27].
However, contradictory findings reported by Matsumoto et al. noted that ECs seeded
on a dextran micro-carrier surrounded with fibrin gel sprouted outward, at an angle
that was perpendicular to the cyclic strain [37]. Similar behavior was recorded when
human pluripotent stem cell-derived vascular smooth muscle cells (vSMC) were
subjected to uniaxial cyclic strain, which induced their alignment perpendicular to
the stretching direction [58]. ECs seeded on collagen on top of a silicone mold and
subjected to cyclic uniaxial stretch (20%, 1 Hz), also aligned perpendicular to the
stretch direction and formed more vessel sprouts [59]. In a different work, ECs and
fibroblasts co-cultured within Gelfoam 3D scaffolds and subjected to uniaxial cyclic
stretch (10% and 1 Hz) formed vessels that aligned diagonal (30–600) to the stretching direction (Fig. 2.3a) [45]. However, ECs and fibroblasts seeded into a Gelfoam
2 Mechanical Regulation of Vascularization in Three-Dimensional Engineered Tissues