## 4 Sensing-Oriented Issues and Solutions
### Materials and Manufacturing Technologies  
Simplified processing steps, reduced materials wastage, low fabrication costs, and simple patterning techniques make printing technologies attractive for the cost-effective manufacturing [111]. Such developments are progressing at a fast pace, and demonstrations have been done so far in many areas, including sensors, displays, solar cells, printed batteries, energy harvesters, and capacitors. Above all, additive manufacturing technology [112], substrate materials [113], and conductive materials [114] are three key factors in controlling the cost, chemical, electrical, and mechanical properties for printable sensors. In general, the printing technology can be categorized into contact and non-contact: the contact-based printing technologies comprise of gravure printing, gravure-offset printing, flexographic printing, and roll-to-roll (R2R) printing; the prominent non-contact printing techniques include screen-printing, slot-die coating, and inkjet printing. Critical limitations of each technology have been highlighted and potential solutions or alternatives have been explored [115]. The R2R fabrication provides the ability to deliver cost-effective technical solutions for sensors and other electronic devices [116]. Among which, the inkjet printing technology received more and more attention because of its simplicity, flexibility, precision, accuracy, high speed, and the capacity to process a wide variety of printing materials. In particular, the specific advantage of this technology is its ability to print a controlled amount of ink, down to 1 picolitre, at high frequency, on almost any type of substrate [117]. However, since a low concentration of the conductive ink is jetted on a substrate, it is difficult to avoid coffee ring effect which results in irregular thickness and low density of the electrode pattern after the ink dries out weakening the resulting electrodes. As a result, relatively low conductivity and low mechanical strength usually can be obtained from jet printing [42]. Understanding in droplet generation, surface chemistry, polymer/substrate selection and process scalability should be exploited [118]. Various flexible substrates can be selected for sensor applications: polymer, semiconductor, organic, ceramics, et al. The circuit board's tensile strength, allowable temperature of desired flexible substrates, and thickness are likewise significant factors for R2R processing and transferring techniques [119]. Polyimide (PI), which has a high thermal and chemical resistance, is a most widely used flexible substrate [43]. The benefits of using paper as a substrate were also discussed, reporting a good electrical/dielectric performance for frequency up to 1 GHz [120]. In addition, the evolution towards the first integrated RFID-enabled wireless sensor network infrastructure using inkjet-printed electronics technologies on flexible and paper substrates was first reported in [121]. However, the electrical and mechanical properties of RFID chip joints assembled on a flexible substrate need to be considered [122]. The conductive ink plays a key role for printable antennas. Reference [123] reviewed the basic properties of conductive nanomaterials suitable for printed electronics (metal nanoparticles, carbon nanotubes, and graphene), their stabilization in dispersions, formulations of conductive inks, and obtaining conductive patterns by using various sintering methods. Conductive inks generally contain at least one kind of binder to form a continuous film, however, adding insulating binders such as polymeric or siloxane will reduce the ink conductivity [124]. For example, a graphene oxide (GO) assisted liquid-phase exfoliation process was demonstrated for the preparation of high-quality graphene from graphite, which is a little sacrifice of the conductivity, reported as 6.2 \(\times\) 10\({}^{4}\) S/m) [125].