Printed hydrogel nanocomposites: fine-tuning nanostructure for anisotropic mechanical and conductive properties

Additive manufacturing of composites offers a potential for a new level of control over a material's structure at the microscale. The focus of this work is a 2-hydroxyethyl methacrylate (HEMA)-based gelation system with orderly distributed carbon nanotubes (CNTs). CNTs undergo shear-induced alignment during printing process, and retain their orientation after the polymerisation of HEMA monomers, thereby, forming a nanocomposite with anisotropic mechanical and electrical properties. It is characterised with an intensive programme of mechanical tests including quasistatic uniaxial stretching, and dynamic cyclic loadings, as well as its four-terminal sensing of conductive characteristics. A coupling effect of mechanical and electrical properties is also studied. The experimental findings are discussed in detail and demonstrate that the orientation of CNTs affects both the mechanical and electrical conductive properties of the nanocomposites in terms of its ultimate strength, resistivity, and a piezoresistive coefficient. Understanding of anisotropic electromechanical properties of printed PHEMA-CNT hydrogel nanocomposite will ultimately underpin the development of smart soft materials for diverse applications, such as biomimetic nucleus pulposus or flexible electronics. Summary for the graphical abstract: Shear-induced alignment is applicable for tuning nanofillers at nanoscale, enabling anisotropic mechanical and electrical properties of the nanocomposite.