# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 5 Wireless Technologies
### Wireless Data Communication  
Wireless data communication is a method of transmitting data from one electronic device to another by networking the electronic devices over radio waves [143]. There is a variety of wireless communication protocols, including Bluetooth, NFC, radio-frequency identification (RFID), and Zigbee. Among them, Bluetooth, BLE, and NFC have been used most extensively in wearable healthcare electronics due to their low energy consumption [6] and high compatibility with various mobile electronic devices, such as smartphones. The details of the wireless communication protocols are summarized in Table 1.  
Bluetooth uses radio frequencies between 2.402 and 2.481 GHz [154]. It can communicate up to ~100 m and transfer larger amounts of data than BLE and NFC. To exchange data between two Bluetooth-enabled devices, they must be paired beforehand. BLE operates at the same radio frequencies of 2.402 and 2.481 GHz, but it is designed to provide low power consumption [155]. The Bluetooth and BLE systems are powered by external power modules, such as batteries that are connected directly to them, which limit the miniaturization and flexibility of the devices incorporating them. [PERSON] et al. implemented BLE protocols into a wearable EMG sensor patch to enable the real-time wireless transfer of EMG data to an Android tablet, as shown in Figure 6f [148]. The printed circuit board was integrated with miniaturized functional chips, including a BLE/microcontroller, analog-to-digital \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Frequency** & **Communication Range** & **Power Consumption** & **Data Rate** & **Set-Up Time** & **Network Type** \\ \hline Bluetooth & 2.4–2.5 GHz & \textless{}100 m & \begin{tabular}{c} \textless{}30 mA \\ (read and transmit) \\ \end{tabular} & \textless{}3 Mb/s & \textless{}6 s & WPAN \\ \hline BLE & 2.4–2.5 GHz & \textless{}50 m & \
\begin{tabular}{c} \textless{}15 mA \\ (read and transmit) \\ \end{tabular} & \textless{}2 Mb/s & \textless{}6 ms & WPAN \\ \hline NFC & 13.56 MHz & \textless{}0.2 m &
\begin{tabular}{c} 15 mA \\ (read) \\ \end{tabular} & 424 kb/s & \textless{}0.1 s & Point-to-point \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of characteristics of Bluetooth, BLE, and NFC [152, 153, 6]. converter (ADC), antenna, voltage regulator, and a miniaturized lithium-ion polymer battery. The printed circuit showed comparable Bluetooth performances to that of a rigid circuit up to 15 m. The data transmitted to an Android tablet via Bluetooth were used for wireless human-machine interfaces. NFC operates at 13.56 MHz and has communication ranges up to 20 cm [156]. Its maximum data transmission rate is 424 kb/s, which is much lower than that of Bluetooth and BLE, but the connection between two NFC systems can be acquired within 0.1 s without prior pairing [157]. Their short communication distance and fast connection provide the NFC systems with high security and low power consumption. In addition, passive NFC systems can be powered wirelessly through inductive coupling with active NFC systems available, for instance, in NFC-enabled mobile phones. Thus, the passive NFC systems only require an antenna circuit and an NFC microchip within the devices for powering, which allows them to be more stretchable, flexible, and miniaturized. [PERSON] et al. demonstrated e-tattoos capable of wireless power and body signal transmission using NFC protocols, as shown in Figure 6g [17]. The tattoos consisted of an NFC layer, a functional circuit layer, and a passive electrode/sensor layer. The NFC layer was composed of a stretchable antenna with a double-stranded serpentine design, stretchable copper interconnects, a thermistor, and an NFC transponder integrated circuit (IC). The wireless power transmission via NFC enabled the battery-free system, enabling the ultrathin and ultrasoft features of the devices.