158
pacing. The vessel characterization showed increased CD31 staining, compared to
cardiomyocyte-only control, and anastomosis with host vasculature when implanted
in vivo [111]. Ong et al. developed a 3D printed tri-culture system in which cells are
the matrix. This tri-culture consisted of hiPSC-CMs, hCFs, and HUVECs. The electrophysiological properties consisted of ventricular-like action potentials and conduction velocity near 4.6 cm/s, which appeared to decrease with increased fibroblast
percentage. CD31 staining was found in the histology inside the cell cluster, but
vessel density was not calculated. When the scaffold was implanted in vivo, there
was nascent vasculature development [85].
6.5 Biomaterials
Biomaterials are designed to provide structural support, alignment cues, and bioactive signaling to enhance cardiomyocyte function and maximize contractile and
electrophysiological properties. In addition, incorporation of proangiogenic cues
facilitates the development of dense vasculature [125]. Biomaterial choices for the
development of vascularized cardiac grafts range from natural to synthetic biopolymers. For example, PLLA/PLGA (in a 1:1 ratio) has been used to create a spongelike, biocompatible scaffold. PLGA has a high degradation rate, while PLLA
provides mechanical support to the 3D structure [67]. Alternatively, PEG and
POMaC, which degrades easily through hydrolysis, are the materials to which
hydrogels containing cells will be cultured [50, 51, 79]. These polymers were coated
with Matrigel™ or a combination of Matrigel™ and collagen.
Hydrogels are an attractive material because they are generally made from natural materials and can be modified to tune their properties, encapsulate cells, and
serve as an adhesive to maintain cells in the desired location. The native myocardium is composed mostly of collagen leading to the hypothesis that collagen gels
would facilitate a similar extracellular matrix composition. While hydrogels are
attractive sources for providing a scaffold for development, there are drawbacks.
These include limited mechanical integrity, reduced flexibility in seeding procedures, and isotropy [42]. Various processing strategies have been employed to minimize these limitations. Despite collagen being a major extracellular matrix protein,
of the native myocardium, many multicellular approaches utilize fibrin as a scaffold. Fibrin is composed of fibrinogen and cross-linked with thrombin. Fibrin has
the ability to recruit vasculature because it is proangiogenic [15]. Fibrin is often
used in cardiac patch systems because it has many attractive features and naturally
occurs at wound sites [70, 131]. Among fibrin’s benefits are its ability to promote
angiogenesis, cell survival, and extracellular matrix secretion [117]. This matrix
secretion, in combination with the natural strain stiffening of fibrin, mimics the
mechanics of the cardiac microenvironment [9, 100]. To provide mechanical integrity and facilitate the development of alignment and tension for cardiomyocyte contractility, researchers have developed casting methods like posts for fibrin hydrogels
J. Morrissette-McAlmon et al.
pacing. The vessel characterization showed increased CD31 staining, compared to
cardiomyocyte-only control, and anastomosis with host vasculature when implanted
in vivo [111]. Ong et al. developed a 3D printed tri-culture system in which cells are
the matrix. This tri-culture consisted of hiPSC-CMs, hCFs, and HUVECs. The electrophysiological properties consisted of ventricular-like action potentials and conduction velocity near 4.6 cm/s, which appeared to decrease with increased fibroblast
percentage. CD31 staining was found in the histology inside the cell cluster, but
vessel density was not calculated. When the scaffold was implanted in vivo, there
was nascent vasculature development [85].
6.5 Biomaterials
Biomaterials are designed to provide structural support, alignment cues, and bioactive signaling to enhance cardiomyocyte function and maximize contractile and
electrophysiological properties. In addition, incorporation of proangiogenic cues
facilitates the development of dense vasculature [125]. Biomaterial choices for the
development of vascularized cardiac grafts range from natural to synthetic biopolymers. For example, PLLA/PLGA (in a 1:1 ratio) has been used to create a spongelike, biocompatible scaffold. PLGA has a high degradation rate, while PLLA
provides mechanical support to the 3D structure [67]. Alternatively, PEG and
POMaC, which degrades easily through hydrolysis, are the materials to which
hydrogels containing cells will be cultured [50, 51, 79]. These polymers were coated
with Matrigel™ or a combination of Matrigel™ and collagen.
Hydrogels are an attractive material because they are generally made from natural materials and can be modified to tune their properties, encapsulate cells, and
serve as an adhesive to maintain cells in the desired location. The native myocardium is composed mostly of collagen leading to the hypothesis that collagen gels
would facilitate a similar extracellular matrix composition. While hydrogels are
attractive sources for providing a scaffold for development, there are drawbacks.
These include limited mechanical integrity, reduced flexibility in seeding procedures, and isotropy [42]. Various processing strategies have been employed to minimize these limitations. Despite collagen being a major extracellular matrix protein,
of the native myocardium, many multicellular approaches utilize fibrin as a scaffold. Fibrin is composed of fibrinogen and cross-linked with thrombin. Fibrin has
the ability to recruit vasculature because it is proangiogenic [15]. Fibrin is often
used in cardiac patch systems because it has many attractive features and naturally
occurs at wound sites [70, 131]. Among fibrin’s benefits are its ability to promote
angiogenesis, cell survival, and extracellular matrix secretion [117]. This matrix
secretion, in combination with the natural strain stiffening of fibrin, mimics the
mechanics of the cardiac microenvironment [9, 100]. To provide mechanical integrity and facilitate the development of alignment and tension for cardiomyocyte contractility, researchers have developed casting methods like posts for fibrin hydrogels
J. Morrissette-McAlmon et al.