(A) Averaged fractional area change (i.e., contraction amplitude) over 3 days for r-NS spheroids and (B) a characteristic beating profile on day 3 for r-NC, r-NC under stimulation during measurement, and r-NS spheroids. (C) Averaged fractional area change over 3 days for r-WS spheroids and (D) a characteristic beating profile on day 3 for r-WC, r-WC under stimulation during measurement, and r-WS spheroids. (E) Western blot analysis (averaged data of three separate experiments) of protein expression levels relative to GAPDH expression after 7 days with or without electrical stimulation normalized to the r-NC group. (F, G) Immunofluorescent staining of cardiac-specific contractile and conductive proteins for all groups after 7 days. r-NC= rat-neonatal cardiac spheroids, no e-SiNWs, no stimulation; r-NS= rat-neonatal cardiac spheroids, no e-SiNWs, with stimulation; r-WC= rat-neonatal cardiac spheroids, with e-SiNWs, no stimulation; r-WS= rat-neonatal cardiac spheroids, with e-SiNWs, with stimulation. n = 6 spheroids per condition (A-D). Asterisks (*) represent statistical significance with p<0.05; error bars represent standard deviation. Scale bars: (F) = 100 μm; (G) = 20 μm.