Three-Dimensional Macroporous Carbon Nanotube Scaffolds for Stem Cell Expansion and Maintenance

Human mesenchymal stem cells (MSCs) show promise for several therapeutic applications including regenerative medicine, drug discovery, cellular therapy and disease modeling. Expansion of stem cells and maintenance of their self-renewal capacity in vitro requires specialized robust cell culture systems. Conventional approaches consisting of animal derived matrices and a cocktail of growth factors have several limitations such as consistency, scalability, and pathogenicity (risk of infection). Furthermore, to achieve high cell densities for practical therapeutic applications, 3D culture systems have been recommended over conventional 2D substrates. To overcome the above limitations, multifunctional 3D porous scaffold, fabricated using synthetic materials that permit stem cell expansion and maintenance in vitro would be a significant advancement. Carbon nanotubes (CNTs) exhibit excellent physiochemical properties such as high mechanical strength, electrical conductivity as well as unique opto-acoustic and electromagnetic response. Therefore, they have been investigated for several therapeutic and diagnostic biomedical applications. Indeed, there has been a growing interest in assembling carbon nanomaterials into various twoand threedimensional architectures for the fabrication of next-generation of energy storage, electronic, super-capacitor, photovoltaic and biomedical devices and implants.[1] For biomedical applications, carbon nanotubes and graphene have been assembled into twodimensional films (using vacuum filtration and chemical vapor deposition (CVD) methods) and 3D foams (using CVD and sacrificial template-transfer methods) and reported as cytocompatible substrates for cellular function (proliferation and differentiation of stem cells for applications in bone, neuron and cardiac tissue engineering).[2-5] These methods have several limitations. CVD method requires very specific substrates capable of withstanding high temperatures and pressure. Vacuum filtration and spray coating methods can produce 2D substrates that may not be suitable for tissue engineering of larger organs that demand three-dimensional scaffolds. Furthermore, a general limitation of these methods is that in the absence of strong chemical bonds between the individual nanomaterials, the structural integrity of architectures assembled relies mainly on weak Van der Wall forces or on physical entanglement of the nanoparticles, leaving them prone to dissociation under physiological shear forces experienced by in vivo biomedical devices and implants. Furthermore, methods such as sacrificial template transfer do not allow control over the porosity of the assembled 3D scaffold, which depends on the template architecture. Therefore, the assembly of carbon nanomaterials into three-dimensional (especially with >1 mm in all three dimensions) macroporous tissue engineering scaffolds with tunable porosity across various length scales (macro, micro and