We engineered functional cardiac patches by seeding neonatal rat cardiomyocytes onto carbon nanotube (CNT) incorporated photocrosslinkable gelatin methacrylate (GelMA) hydrogel. The resulting cardiac constructs showed excellent mechanical integrity and advanced electrophysiological functions. Specifically, myocardial tissues cultured on 50 μm thick CNT-GelMA showed 3 times higher spontaneous synchronous beating rates and 85% lower excitation threshold, compared to those cultured on pristine GelMA hydrogels. Our results indicate that the electrically conductive and nanofibrous networks formed by CNTs within a porous gelatin framework is the key characteristics of CNT-GelMA leading to improved cardiac cell adhesion, organization, and cell-cell coupling. Centimeter-scale patches were released from glass substrates to form 3D biohybrid actuators, which showed controllable linear cyclic contraction/extension, pumping, and swimming actuations. In addition, we demonstrate for the first time that cardiac tissues cultured on CNT-GelMA resist damage by a model cardiac inhibitor as well as a cytotoxic compound. Therefore, incorporation of CNTs into gelatin, and potentially other biomaterials, could be useful in creating multifunctional cardiac scaffolds for both therapeutic purposes and in vitro studies. These hybrid materials could also be used for neuron and other muscle cells to create tissue constructs with improved organization, electroactivity, and mechanical integrity.

We report a technique to characterize adhesion of monolayered/multilayered graphene sheets on silicon wafer. Nanoparticles trapped at graphene-silicon interface act as point wedges to support axisymmetric blisters. Local adhesion strength is found by measuring the particle height and blister radius using a scanning electron microscope. Adhesion energy of the typical graphene-silicon interface is measured to be 151Ϯ 28 mJ/ m 2 . The proposed method and our measurements provide insights in fabrication and reliability of microelectromechanical/nanoelectromechanical systems. © 2010 American Institute of Physics. ͓doi:10.1063/1.3294960͔Graphene, the monolayer of carbon atoms packed into a two-dimensional honeycomb lattice, has attracted much attention in the scientific community because of its ultrahigh mechanical strength, conductivity with high electron mobility, and optical transparency. Whether graphene is a promising material for transparent or stretchable electronics 1-4 depends predominantly on the nanostructure's mechanical integrity and ability to integrate or to adhere to electronic substrates. There is, therefore, an urgent need for experimental methods to characterize the mechanical properties and adhesion behavior. However, viable techniques remain scarce despite the voluminous literature on theoretical and molecular dynamics simulations. Recently, direct force measurements using atomic force microscope ͑AFM͒ indentations were performed on graphene beams clamped at opposite ends, 5 and also on freestanding graphene sheets suspended over lithographically etched holes. 6 The measured changes in indenter displacement as a function of the applied load allowed researchers to find the elastic modulus in the range of 0.5-1.0 TPa and an ultimate strength of 130 GPa. 5,6 These results are consistent with theoretical computation. 7 On the other hand, measuring graphene adhesion on dissimilar substrates is difficult since the extremely thin sheet is hard to handle and prone to damage by clamps and fixtures as in the conventional peel test. In this paper, we describe a simple method to quantitatively measure local graphene adhesion on silicon surface, and report the value of adhesion energy of the dissimilar interface.Silicon wafer ͑100͒ with a 280-nm-thick oxide layer was chosen as the substrate. 8,9 The wafer was diced into 1 ϫ 1 cm 2 square pieces, cleaned with alcohol, and dried with nitrogen. Gold nanoparticles ͑from BBInternational Ltd., Cardiff, UK͒ in diameter 2R Ϸ 50 nm in an aqueous colloidal suspension with concentration of 4.5 ϫ 10 10 particles/ ml or silver particles with diameter 2R Ϸ 80 nm and 1.1ϫ 10 9 particles/ ml were used in the experiments. They were spread as randomly and evenly as possible on the silicon substrate and then left to completely dry out under ambient conditions. Clustering of the particles was inevitable among the isolated ones. Graphene sheets were mechanically cleaved from the surface of highly oriented pyrolytic graphite ͑HOPG͒ ͑ZYH grade from NT-MDT Co., Moscow, Russia. Note t...

Microfabrication technology provides a highly versatile platform for engineering hydrogels used in biomedical applications with high-resolution control and injectability. Herein, we present a strategy of microfluidics-assisted fabrication photo-cross-linkable gelatin microgels, coupled with providing protective silica hydrogel layer on the microgel surface to ultimately generate gelatin-silica core–shell microgels for applications as in vitro cell culture platform and injectable tissue constructs. A microfluidic device having flow-focusing channel geometry was utilized to generate droplets containing methacrylated gelatin (GelMA), followed by a photo-cross-linking step to synthesize GelMA microgels. The size of the microgels could easily be controlled by varying the ratio of flow rates of aqueous and oil phases. Then, the GelMA microgels were used as in vitro cell culture platform to grow cardiac side population cells on the microgel surface. The cells readily adhered on the microgel surface and proliferated over time while maintaining high viability (∼90%). The cells on the microgels were also able to migrate to their surrounding area. In addition, the microgels eventually degraded over time. These results demonstrate that cell-seeded GelMA microgels have a great potential as injectable tissue constructs. Furthermore, we demonstrated that coating the cells on GelMA microgels with biocompatible and biodegradable silica hydrogels via sol–gel method provided significant protection against oxidative stress which is often encountered during and after injection into host tissues, and detrimental to the cells. Overall, the microfluidic approach to generate cell-adhesive microgel core, coupled with silica hydrogels as a protective shell, will be highly useful as a cell culture platform to generate a wide range of injectable tissue constructs.

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Myocardial microenvironment plays a decisive role in guiding the function and fate of cardiomyocytes, and engineering this extracellular niche holds great promise for cardiac tissue regeneration. Platforms utilizing hybrid hydrogels containing various types of conductive nanoparticles have been a critical tool for constructing engineered cardiac tissues with outstanding mechanical integrity and improved electrophysiological properties. However, there has been no attempt to directly compare the efficacy of these hybrid hydrogels and decipher the mechanisms behind how these platforms differentially regulate cardiomyocyte behavior. Here, we employed gelatin methacryloyl (GelMA) hydrogels containing three different types of carbon-based nanoparticles: carbon nanotubes (CNTs), graphene oxide (GO), and reduced GO (rGO), to investigate the influence of these hybrid scaffolds on the structural organization and functionality of cardiomyocytes. Using immunofluorescent staining for assessing cellular organization and proliferation, we showed that electrically conductive scaffolds (CNT- and rGO-GelMA compared to relatively nonconductive GO-GelMA) played a significant role in promoting desirable morphology of cardiomyocytes and elevated the expression of functional cardiac markers, while maintaining their viability. Electrophysiological analysis revealed that these engineered cardiac tissues showed distinct cardiomyocyte phenotypes and different levels of maturity based on the substrate (CNT-GelMA: ventricular-like, GO-GelMA: atrial-like, and rGO-GelMA: ventricular/atrial mixed phenotypes). Through analysis of gene-expression patterns, we uncovered that the engineered cardiac tissues matured on CNT-GelMA and native cardiac tissues showed comparable expression levels of maturation markers. Furthermore, we demonstrated that engineered cardiac tissues matured on CNT-GelMA have increased functionality through integrin-mediated mechanotransduction (via YAP/TAZ) in contrast to cardiomyocytes cultured on rGO-GelMA.