164
Experiments using electrical stimulation to promote angiogenesis generally utilize
constant electrical fields, as opposed to experiments focusing on cardiomyocyte
development which usually use regular electrical pulses. Thus far, investigations of
cardiomyocyte-endothelial cell co-cultures have opted either to forego electrical stimulation or to use periodic electrical stimulation regimens created for CMs, even though
the effects of periodic electrical stimulation on endothelial cells and angiogenesis are
largely unknown. More directed investigation would be critical to understanding how
electrical stimulation of engineered tissues can be optimized, for both CM maturation
and for vascular development.
In the design of bioreactors with electrical stimulation capabilities, the application of field pulses to a 3D culture is relatively easy to adapt from 2D systems. A
common configuration for such devices involves using two parallel electrodes to
generate an electric field across a substrate [84, 89, 112, 115]. Tandon and colleagues, for example, developed a system that uses carbon rods to induce an electric
field across a chamber that can support monolayers, three-dimensional cardiac tissue
constructs, and micropatterned cellular substrates [115]. In subsequent studies, 1 Hz
stimulation applied using this setup was shown to improve CM elongation and alignment, increase conduction velocity, and strengthen the tissue-level force of contraction [92]. This strategy has proven successful even with more complicated systems
like the Biowire platform, a cell culture system that utilizes both architectural and
electrical signaling to generate more mature cardiomyocytes [84]. The system consists of a collagen matrix, embedded with cardiomyocytes and supporting cell types,
surrounding a single surgical suture, all within a template PDMS channel. This platform has since been expanded to also allow for perfusion of the biowire system during culture, facilitating an array of pharmacologic experiments.
6.6.6 Mechanical Stimulation
Myocardial development depends on a complex series of mechanical forces acting
in concert to promote cardiomyocyte maturation, organization, and electrophysiological function. Animal studies provide strong evidence for the importance of
mechanical stimulation, as they show that altering hemodynamic pressure and contractility in the fetal heart had dramatic effects on myocardial function [55, 80, 114].
It has since been established that imposing static or cyclic strain onto an engineered
tissue has the ability to induce cardiomyocyte hypertrophy, promote maturity in
stem cell-derived CMs, and improve myocardial organization [119, 132]. Cyclic
strain is now understood as particularly important to CM development. Gwak and
colleagues showed that applying 10% strain at 1 Hz to rat ESC-CMs upregulated
cardiomyocyte-specific genes and resulted in a more mature microstructure, with
Z-line formation and more organized myofibrillar bundles [39]. Another study
found that the frequency of strain applications might have important differential
effects on gene expression. Using ESC-CMs, they found that 10% strain applied at
3 Hz for 3 days resulted in a twofold increase in α-cardiac actin expression,
J. Morrissette-McAlmon et al.
Experiments using electrical stimulation to promote angiogenesis generally utilize
constant electrical fields, as opposed to experiments focusing on cardiomyocyte
development which usually use regular electrical pulses. Thus far, investigations of
cardiomyocyte-endothelial cell co-cultures have opted either to forego electrical stimulation or to use periodic electrical stimulation regimens created for CMs, even though
the effects of periodic electrical stimulation on endothelial cells and angiogenesis are
largely unknown. More directed investigation would be critical to understanding how
electrical stimulation of engineered tissues can be optimized, for both CM maturation
and for vascular development.
In the design of bioreactors with electrical stimulation capabilities, the application of field pulses to a 3D culture is relatively easy to adapt from 2D systems. A
common configuration for such devices involves using two parallel electrodes to
generate an electric field across a substrate [84, 89, 112, 115]. Tandon and colleagues, for example, developed a system that uses carbon rods to induce an electric
field across a chamber that can support monolayers, three-dimensional cardiac tissue
constructs, and micropatterned cellular substrates [115]. In subsequent studies, 1 Hz
stimulation applied using this setup was shown to improve CM elongation and alignment, increase conduction velocity, and strengthen the tissue-level force of contraction [92]. This strategy has proven successful even with more complicated systems
like the Biowire platform, a cell culture system that utilizes both architectural and
electrical signaling to generate more mature cardiomyocytes [84]. The system consists of a collagen matrix, embedded with cardiomyocytes and supporting cell types,
surrounding a single surgical suture, all within a template PDMS channel. This platform has since been expanded to also allow for perfusion of the biowire system during culture, facilitating an array of pharmacologic experiments.
6.6.6 Mechanical Stimulation
Myocardial development depends on a complex series of mechanical forces acting
in concert to promote cardiomyocyte maturation, organization, and electrophysiological function. Animal studies provide strong evidence for the importance of
mechanical stimulation, as they show that altering hemodynamic pressure and contractility in the fetal heart had dramatic effects on myocardial function [55, 80, 114].
It has since been established that imposing static or cyclic strain onto an engineered
tissue has the ability to induce cardiomyocyte hypertrophy, promote maturity in
stem cell-derived CMs, and improve myocardial organization [119, 132]. Cyclic
strain is now understood as particularly important to CM development. Gwak and
colleagues showed that applying 10% strain at 1 Hz to rat ESC-CMs upregulated
cardiomyocyte-specific genes and resulted in a more mature microstructure, with
Z-line formation and more organized myofibrillar bundles [39]. Another study
found that the frequency of strain applications might have important differential
effects on gene expression. Using ESC-CMs, they found that 10% strain applied at
3 Hz for 3 days resulted in a twofold increase in α-cardiac actin expression,
J. Morrissette-McAlmon et al.