Nanomaterials and printing techniques for 2D and 3D soft electronics

The merging of electronically conductive elements with soft polymeric materials gave birth to the novel field of soft and stretchable electronics and robotics, in which the key aspect is the maintenance of electrical properties even under severe mechanical deformation. Here, we review the variety of fabrication techniques (dry, wet, and printed) that have been designed, studied, and tested, which leads to a forecast of how soft technologies will have a revolutionary impact on the progress of biomedicine and pre-clinical practice, wearable electronics, environmental monitoring and recognition, smart farming and precision agriculture, and energy harvesting and storage. A particular focus is given to techniques for the printing of 2D and 3D electronics, which allow compliant conductive elements to be coupled to complex three-dimensional objects and platforms. We discuss why it is now necessary to choose between different nanoscale building blocks, nanomaterials, and deposition techniques and to optimize such choices. The watchwords to be prioritized are scalability, versatility, environmental sustainability and biocompatibility, integration, and reduction of the fabrication steps. The target is the design of an eco-friendly and versatile approach for the fully additive manufacture of free-form advanced soft electronic devices (which will eventually be biocompatible and biodegradable) using a multilayer, multimaterial process that can print both active and passive 3D elements on soft polymeric platforms. The sequential combination of dry and wet spray printing is shown to be one of the most promising approaches.