# Technologies and Fabrication of Intelligent Packaging for Perishable Products
## 3 Printing Techniques
### Inkjet Printing  
The main and popular non-contact printing in fabrication of flexible sensors is inkjet printing. It is a technique that uses a solute dissolved or dispersed in the solvent ink. A nozzle ejects acertain quantity of ink. And the ink dries up after the solvent evaporates. In the past few years, the applications for inkjet printing have grown and it can be used to print onto a wide variety of substrate materials such as conducting polymers [63; 64; 65], metallic layers [66; 67], and bio-materials [68; 69]. Figure 6 indicates a type of inkjet printing: thermal and piezoelectric inkjet system, which applies a pulse that enables the ejection of small droplets of ink. Inkjet printing can also be used to fabricate finely patterned and thin films of conducting polymers. Most printed sensors are fabricated using inkjet printing owing to its simple and digital layout design process.  
[PERSON] et al. fabricated a low-cost chemical sensor array with a ZigBee wireless communication module to detect and analyze armpit odor. For this application, this electronic nose should be wearable. It could measure fresh and native odors instantaneously. CNTs/polymer composites were used as sensing materials. The chemical sensor array was completely fabricated using inkjet-printing [70]. The electronic nose is a rapid detection device for food spoilage, and its installation on packaging is an effective way to monitor food quality [71]. [PERSON] et al. developed a flexible and lightweight chemi-resistor to detect chemically aggressive vapors at parts per billion levels in an air sample at room temperature. The device was made of a thin film, and the film was fabricated using inkjet printing on flexible plastic surfaces [72]. [PERSON] et al. fabricated a sensor for the analysis of ammonia gas using inkjet printing for the deposition of polyaniline nanoparticle films. The sensor was further combined with heater foils for operation at a range of temperatures to monitor leakage of ammonia in a wide range of industrial applications. They also optimized the inkjet printed polyaniline nanoparticle sensor. An aqueous dispersion of the nanoPANI was deposited over a silver interdigitated array using a piezoelectric inkjet printer [73]. [PERSON] et al. recently applied inkjet printing on polyimide foil to fabricate a tin dioxide (SnO\({}_{2}\)) sensor. Gold electrodes and heaters were printed on each side of the substrate. This was the first complete inkjet-printed metal oxide sensor printed on polymeric foil that included a gas-sensitive layer and a heating transducer [74]. [PERSON] et al. presented a simple and low-costnikjet printing technique for fabricating an innovative flexible gas sensor made of a graphene-poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)  
Figure 6: Schematic of the working principles of inkjet printing [53].  
composite film with a high uniformity over a large area. Solution-processed graphene-PEDOT:PSS ink was used to inkjet-print onto a transparent substrate with prefabricated electrodes and used for NH\({}_{3}\) detection at room temperature [49]. [PERSON] et al. fabricated a low-cost and flexible NH\({}_{3}\) gas sensor using thermal ink-jet printing techniques [75]. These aforementioned flexible-printed sensors can also be applied to the intelligent packaging of perishable products. For packaging, [PERSON] et al. applied inkjet printing for the fabrication of a \"smart cap\", which could be used to monitor the quality of liquid foods, such as milk or juice. All electrical components, including resistors, capacitors, and inductors, as well as circuits and passive wireless sensors, were printed [76]. [PERSON] et al. fabricated a fully inkjet-printed RFID sensor tag for humidity monitoring in intelligent packaging, with tests showing that the RFID sensor tag could be fabricated directly onto paper packaging in a single application [77].