## 5 Future Trends and Perspectives
### More Ubiquitous and Adaptable  
As RFID becomes more prevalent, growing economies of scale will enable the integration of environmental sensors with tags reporting on a wide range of conditions. Great efforts are dedicated to the development of RFID chips with integrated sensors where the sensor is powered by the RFID reader signal. This fascinating solution imposes strict constraints on the sensor, which should be both energy efficient and chip-embeddable [162]. Usually, only a few kinds of sensors satisfy such requirements: temperature, light, and pressure sensors are the most common [171; 172; 173].  
The automatic impedance matching (self-tuning) is capable of compensating for the influences of changing objects close to the antenna, thus achieving a constant high reading performance [174]. This can be applied to improve the matching performance of tag antenna and thus to tackle the tradeoff between sensing and communication. For example, a Magnus S Sensor chip supplied by RFMicron can operate at temperatures ranging from \(-40\)\({}^{\circ}\)C to \(+85\)\({}^{\circ}\)C and it consists of a sensor code and on-chip RSSI code [175]. Alternatively, the sensed quantity can be obtained with a self-tuning module which contains a tuning element to compensate for the changed impedance because of tagged object (defect). The digitalized information can reconstruct the RSSI or impedance of tag antenna and consequently alleviate the influence of the channel in the backscattered communication [176]. Therefore, the reliability and measurement uncertainty of passive antenna sensors and systems can be greatly improved.  
Compared with battery-powered sensors, passive antenna sensors have drawbacks in terms of sensing range, lack of time history data storage, and non-real-time data communication [177; 11]. More function materials can be embedded into antenna sensors, to make them smarter, e.g., Shape Memory Alloys (SMA) for memorizing a violation in history [178]. A single event logging functionality by means of direct integration of a printed 1-bit write-once-read-many (WORM) memory into the antenna structure was developed to be a humidity sensor, whose value can be read out at a later occasion since the WORM memory records an event by changing its state [57]. Chemical sintering of silver metal nanoparticles and the deliquescence phenomenon of salts were exploited to monitoring the exceedance of a r.h. threshold without the need of a permanent electric energy supply [179].  
Wearable electronics have received an extensive interest because of the great potential of future wireless body area networks (WBANs) [180], which can be used to monitor the movement [181] or vital signs of human being [182]. In particular, there is a growing demand for cost-effective textile antennas that can endure stretching and moisture for future WBAN and sensing systems [183]. Additive manufacturing provides the foundation for wearable applications, as it has the capacity to integrate with soft and stretchable materials [184; 185; 186; 187; 188]. Electrocardiograms (ECGs) are one of the most common forms of non-invasive diagnostics. The wiring harness connecting a patient to an external ECG monitor poses a significant problem for monitoring ambulatory activities and in long-term monitoring, because of the potential for discomfort and impeded movement. For this reason, a passive wireless multichannel telemetry device capable of transmitting an ECG to an external system was presented [189]. A small-size epidermal RFID thermometer, suitable for the direct placement over the skin, was developed, satisfying the target value for standard thermometers (ear 0.2 \({}^{\circ}\)C, underarm 0.5 \({}^{\circ}\)C) after uniform recalibration [66]. The cost, size and ruggedness advantages shows that passive sensors can offer some potential applications for such devices, e.g., for pills and implanted biomedical sensors.