The transplantation of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offers a potential treatment for heart failure, but arrhythmogenic automaticity can arise from these transplanted cells. In this study, we investigated the effects of RADA16, a clinically approved self-assembling peptide that forms nanofibers after injection, on the vascularization, myofibril structure, and electrophysiological adaptation of hiPSC-CMs transplanted into rat hearts. RADA16 accelerated the transition of hiPSC-CMs toward adultlike gene expression profiles, enhanced sarcomere organization, and improved vascularization in the transplanted site. Flexible mesh nanoelectronics revealed fibrillation of transplanted hiPSC-CMs within the beating recipient heart, and RADA16 drastically reduced the automaticity of hiPSC-CMs. Our findings demonstrate the potential of self-assembling nanofibers to advance cardiac cell therapy and how flexible mesh nanoelectronics technology could improve safety.