Fully screen printed, stretchable proximity sensors based on carbon nanotube - silicone rubber compounds

The 4th industrial revolution will stimulate a rising demand on novel sensor and actuator systems. Besides machine communication, new systems for human machine communications need to be developed. These technologies are called Human Machine Interface technologies (HMI) and can be basically distinguished by tactile and non-tactile sensors. In order to circumvent the use of rare resources such as gold or silver, many groups have focused their efforts on synthetic nanomaterials such as Carbon Nanotubes (CNT) or Graphene. Within this research CNT silicone rubber compounds are used as an electrode material for screen printed proximity sensors on silicon rubber substrates. The working principle of this sensors is based on capacitive sensing, where the distance of the object is measured by changes in the electrical field that is applied on the CNT silicon rubber electrode when a conductive object approaches (see Figure 1). To make the sensors screen printing is used. It is a highly automated technology that can be employed to process high viscous rubber materials, which are required for this sensor configuration. In addition, this method is used due to its high productivity and reproducibility as well as realizing thick and robust multilayer coatings. Materials such as CNTs are difficult to process with solution processing methods, because of their high aspect ratio that might lead to clogging of nozzles of inkjet printers or dispensers. The high volume of material that is transferred onto the substrate with screen printing makes it possible to process a wide range of material sizes ranging from nano- to micro-particles, including CNTs. As presented by Wei and co-workers, the maximum sensing distance of the proximity sensors based on silver are strongly influences by the layer coverage of the electrode material [1]. By adjusting these parameters, personalized and adjustable sensors, for specific applications can be established. For this reason, the influence of the variation of geometrical and electrical properties of the electrode on the sensing performance will be examined within this work. Thereby, the CNT silicone rubber electrode will be varied in its layer thickness and its area coverage. To characterize the maximum sensing distance, the ""Touch Board"" supplied by Bare Conductive is used.