We describe a protocol for tissue engineering of synchronously contractile cardiac constructs by culturing cardiac cells with the application of pulsatile electrical fields designed to mimic those present in the native heart. Tissue culture is conducted in a customized chamber built to allow for cultivation of (i) engineered three-dimensional (3D) cardiac tissue constructs, (ii) cell monolayers on flat substrates or (iii) cells on patterned substrates. This also allows for analysis of the individual and interactive effects of pulsatile electrical field stimulation and substrate topography on cell differentiation and assembly. The protocol is designed to allow for delivery of predictable electrical field stimuli to cells, monitoring environmental parameters, and assessment of cell and tissue responses. The duration of the protocol is 5 d for two-dimensional cultures and 10 d for 3D cultures.

In contractile tissues such as myocardium, functional properties are directly related to the cellular orientation and elongation. Thus, tissue engineering of functional cardiac patches critically depends on our understanding of the interaction between multiple guidance cues such as topographical, adhesive or electrical. The main objective of this study was to determine the interactive effects of contact guidance and electrical field stimulation on elongation and orientation of fibroblasts and cardiomyocytes, major cell populations of the myocardium. Polyvinyl surfaces were abraded using lapping paper with grain size 1 to 80μm, resulting in V-shaped abrasions with the average abrasion peak-to-peak width in the range from 3 to 13μm, and the average depth in the range from 140nm to 700nm (AFM). The surfaces with abrasions 13μm wide and 700nm deep, exhibited the strongest effect on neonatal rat cardiomyocyte elongation and orientation as well as statistically significant effect on orientation of fibroblasts, thus they were utilized for electrical field stimulation. Electrical field stimulation was performed using a regime of relevance for heart tissue in vivo as well as for cardiac tissue engineering. Stimulation (square pulses, 1ms duration, 1Hz, 2.3V/cm or 4.6V/cm) was initiated 24hr after cell seeding and maintained for additional 72hr. The cover slips were positioned between the carbon rod electrodes so that the abrasions were either parallel or perpendicular to the field lines. Non-abraded surfaces were utilized as controls. Field stimulation did not affect cell viability (live/dead staining). The presence of a well developed contractile apparatus in neonatal rat cardiomyocytes (staining for cardiac Troponin I and actin filaments) was identified in the groups cultivated on abraded surfaces in the presence of field stimulation. Overall we observed that i) fibroblast and cardiomyocyte elongation on non-abraded surfaces was significantly enhanced by electrical field stimulation ii) electrical field stimulation promoted orientation of fibroblasts in the direction perpendicular to the field lines when the abrasions were also placed perpendicular to the field lines and iii) topographical cues were a significantly stronger determinant of cardiomyocyte orientation than the electrical field stimulation. The orientation and elongation response of cardiomyocytes was completely abolished by inhibition of actin polymerization (Cytochalasin D) and only partially by inhibition of phosphatidyl-inositol 3 kinase (PI3K) pathway (LY294002).

In vivo, cardiomyocytes are exposed to multiple biochemical and physical cues including topographical and electrical cues. During prolonged in vitro cultivation in standard tissue culture set-ups, cardiomyocytes are known to de-differentiate due to the lack of appropriate micro-environmental cues. Most currently available cell culture systems provide only a single biophysical cue, thus development of advanced cell cultivation systems incorporating multiple cues is urgently needed. We report here the development of a microfabricated system, incorporating topographical and electrical cues on a single chip, which enables cultivation of differentiated cardiomyocytes. The cell culture chips were created by hot embossing of polystyrene, to create microgrooves and microridges of precisely defined depth, width and periodicity. Substrates consisting of 0.5 microm-wide grooves and 0.5 microm-wide ridges (1 microm period) and those consisting of 3 microm-wide grooves and 1 microm-wide ridges (4 microm period) were investigated, with smooth surfaces used as controls. The depth of the microgrooves was 400 nm. The two gold electrodes were electrodeposited 1 cm apart such that the microgrooves in-between were oriented either parallel or perpendicular to the electrodes, enabling studies of interaction between topographical and electrical cues. Neonatal rat cardiomyocytes cultivated on microgrooved substrates for 7 days were elongated and aligned along the microgrooves forming a well developed contractile apparatus, as evidenced by sarcomeric alpha-actinin staining, with a more pronounced effect on substrates with 1 microm compared to 4 microm periodicity. Importantly, simultaneous application of biphasic electrical pulses and topographical cues resulted in gap junctions confined to the cell-cell end junctions rather than the punctate distribution found in neonatal cells. Electrical field stimulation further enhanced cardiomyocyte elongation when microgrooves were oriented parallel to the electric field. Due to the compatibility of the described cell culture chips with fluorescence and optical microscopy as well as the ability to independently control field stimulation parameters, biochemical and topographical cues on each chip, this system may in the future become a useful tool in drug development and maturation of cardiomyocytes derived from stem cells.