We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress-strain curves for all explored strain rates from 4 × 10(-2) down to 4 × 10(-4)/sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load-unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load-unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the "unattained-as-of-to-date-space" in the material property landscape.
Producing mature and functional cardiomyocytes (CMs) by in vitro differentiation of induced pluripotent stem cells (iPSCs) using only biochemical cues is challenging. To mimic the biophysical and biomechanical complexity of the native in vivo environment during the differentiation and maturation process, polydimethylsiloxane substrates with 3D topography at the micrometer and sub-micrometer levels are developed and used as cell-culture substrates. The results show that while cylindrical patterns on the substrates resembling mature CMs enhance the maturation of iPSC-derived CMs, sub-micrometerlevel topographical features derived by imprinting primary human CMs further accelerate both the differentiation and maturation processes. The resulting CMs exhibit a more-mature phenotype than control groups-as confirmed by quantitative polymerase chain reaction, flow cytometry, and the magnitude of beating signals-and possess the shape and orientation of mature CMs in human myocardium-as revealed by fluorescence microscopy, Ca 2+ flow direction, and mitochondrial distribution. The experiments, combined with a virtual cell model, show that the physico-mechanical cues generated by these 3D-patterned substrates improve the phenotype of the CMs via the reorganization of the cytoskeletal network and the regulation of chromatin conformation.
†Electronic Supplementary Information (ESI) available: In situ SEM videos of flat-punch nanoindentation into a sample of each CNT forest type, videos of stress-strain curves, and CNT TEM images.
Electronic Supplementary Material (ESI) for Nanoscale This journal is
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