# A Low-Cost Electronic System for Human-Body Communication
## 4 Evaluation Methodology
### Application Scenarios and Consumption Analysis  
This paper presents a low-cost miniaturizable electronic system capable of coupling digital signals to human tissue. Our system has a data transmission rate of up to 11.11 kbps without errors, for distances up to 10 cm. The confinement of signals within the user's body provides security and confidentiality for the transmitted data [15]. In this way, the system is mainly suitable for use in applications that transmit sensitive data. We emphasize its importance in applications such as user authentication [26; 27; 28] and cryptographic symmetric key agreement algorithms [3]. Thus, galvanic communication enables carrying out the key agreement in a secure and plug-and-play mode. The minimum key size of 128 bits is well established to provide security for the key algorithms [25]. Therefore, our system is capable of transmitting an encryption key in 0.0115 s.  
Furthermore, we can consider using the system developed for the direct transmission of clinical data from patients. Data such as temperature, blood pressure, and electrocardiogram, among others, have a measurement rate lower than that achieved by the system [1; 29]. For this purpose, the system is miniaturizable, and could feasibly be applied in implantable sensors. An example is the subcutaneous glucose sensor, which is implanted below the skin (depth of 4 mm) with a less than 10 kbps data rate [30].  
Battery capacity determines the operating lifetime of wearable devices. We rely on a 10.8 V rechargeable lithium-ion battery with 2.8 Ah capacity, resulting in 30,240 mWh [31]. We estimate the wearable device consumes an average of 561.47 mW with a standard deviation of \(\pm\)10.69 mW. In this sense, this battery supports our device for 53.85 h in full-time data transmission.  
In applications with intermittent data transmission, such as user authentication (continuous or non-continuous) and cryptographic key agreement, the system considerably extends the battery life. Besides the use of traditional energy sources, several new technologies allow energy harvesting from the human body, for example, thermoelectric generators (TEGs), which convert thermal energy into electrical energy [32; 33; 34; 35]. A TEG can generate 20 \(\mu\)W/cm\({}^{2}\) of electrical energy density at a walking speed of about 1.1 m/s [35]. Our system transmits a complete 128-bit cryptographic key with an energy cost of 6.47 mW, which would be equivalent to the power generated by a 10 cm\({}^{2}\) TEG during a 32.28 second walk. We can also consider adding alternative energy sources in implantable versions of the device [36; 37]. Thus, it is necessary to use, for example, designed solutions to convert the human-body thermal radiation into energy. In [38], a device capable of generating up to 2.2 \(\mu\)W/cm\({}^{2}\) of power loss is shown. Therefore, considering this technology's addition with an area of 10 cm\({}^{2}\), our solution would need approximately 2.29 seconds to transmit one bit. Table 5 summarizes information regarding energy production from complementary sources that can be attached to the project. It also estimates the time needed to produce enough energy to transmit each bit and a complete cryptographic key.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & & **Energy Production** & **Energy Production** \\
**Source** & **Power (\(\mu\)W/cm\({}^{2}\))** & **Area (cm\({}^{2}\))** & **Time to transmit** & **Time to transmit a** \\  & & & **One Bit (s)** & **Cryptographic Key (s)** \\ \hline Thermoelectric & 20 & 10 & 0.25 & 32.28 \\ Thermal radiation (IR) & 2.2 & 10 & 2.29 & 293.49 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Energy sources and time required for energy production.