# Technologies and Fabrication of Intelligent Packaging for Perishable Products
## 2 Intelligent Packaging Techniques
### RFID Coupling with Sensing Techniques
#### 2.3.3 Passive Tags Coupling with Sensors  
Compared to passive sensors, the communication distance of the sensors with batteries are more extended, which enhances communication. However, the battery size limits the sensor size. The battery is not a flexible component, so sensors with batteries cannot be used as flexible sensors. Moreover, the battery has a limited life and needs to be replaced periodically [35; 36]. These characteristics limit their applications in perishable product packaging [37; 38], single-use  
Figure 4: D2 BH device in the intelligent packaging.  
Figure 5: Main functional blocks of the semi-active tag.  
biopharmaceutical manufacturing [39], and wearable electronics [35; 40]. Thus, passive sensors are often used for the aforementioned applications because they are durable, cheap, easy to manufacture, and are inconspicuous devices [41].  
Passive tags do not require batteries because they are powered by an electromagnetic field inducted by RFID readers. The coupled sensors also obtain their power from the signal sent by the RFID reader. This allows the device to work only when the reader is operational and well-functional, leading to a relatively longer life. However, the limited power generated by induction cannot support complicated sensors, and some sensors cannot function without communication with RFID reader during transportation.  
[PERSON] and [PERSON] developed a passive RFID-based sensor, in which they converted conventional high frequency (13.56 MHz) RFID tags into inductively coupled sensors. They acquired the resonance impedance spectrum of the RFID antenna to obtain digital sensor calibration parameters stored in an integrated circuit (IC) memory chip. They performed a multivariate analysis of the measured resonance impedance spectrum and extracted the relation between the sensor response and the gases of interest [42; 43; 44]. Poly ether urethane (PEUT) was used as the sensing material and was prepared by drop-casting a PEUT solution in dichloromethane, forming a 50-um thick sensing film on a sensing antenna [45]. Later, [PERSON] et al. proposed an innovative approach for modifying conventional RFID tags with chemically sensitive conductive composites by integrating them into the RFID tag circuit. In their design, maleic anhydride was used as a sensing material and the composite was able to detect different biogenic amines associated with food spoilage [46]. Recently, [PERSON] et al. designed a chip less RFID sensor with a tag in a high-frequency structural simulator [47]. The sensor comprised a reader coil that was connected to a readout circuitry, a coil, and an interdigitated capacitor on the flexible tag. The reader generated a magnetic field to make the tags in the field transfer the sensing data to the readout circuitry. [PERSON] et al. developed a device that integrated the sensors in the design of UHF-RFID systems working at a frequency of 868 MHz. A natural polymer was selected for the sensor to monitor carbon dioxide in food packages [48].  
Among these studies, the development of a smart tag that combines RFID and sensors is regarded as a better option for perishable product packaging. In such a design, the tag will use a sensor for measurement, RF for wireless communication, and its circuit for data storage. However, the relatively high costs of manufacturing such tags poses a great challenge for their commercial applications. The use of flexible printing techniques for fabricating these smart tags offers a practical method for reducing costs. The major printing techniques and printed RFID with sensor tags are described in the next section.