# Sensors for Sustainable Smart Cities: A Review
## 3 Results
### Sensors
#### 3.3.1 Sensors for Health Monitoring  
Healthcare has become a prolific area for research in recent years, given that new sensor technology allows real-time monitoring of the patients' state. Smart healthcare  
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**City** & **Health** & **Security** & **Mobility** & **Water** & **Waste** & **Energy** \\ \hline New York [27] & Sensors deployment after 9/11 attacks & Sensors & & & Energy efficiency using LEDs \\ \hline Washington DC [27] & & Bike sharing, smart stations & & & Sensor-based LED \\ \hline Seattle [25] & Flood monitoring, law-enforcement cameras, gushots GPS tracking & Smart traffic lights & Real-time precipitation monitoring & Reduction of CO2 emissions \\ \hline Medellin [30] & & Outdoor electric stairs and air wagons & & \\ \hline Rio de Janeiro [31] & GPS/video monitoring installation in police cars & Traffic monitoring using cameras & & \\ \hline Melbourne [27] & & Smart parking, open urban planning, metro Wi-Fi & Energy efficiency, smart grid, smart lighting \\ \hline Perth [40] & Cyber-security and digital forensics & & & \\ \hline Sydney [32] & & ICTs in daily urban transport & & \\ \hline Brisbane [32] & & Pedestrian spines & & \\ \hline Adelaide [32] & & Wired communities & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: \(Cont.\)  
Figure 3: Map of smart cities across the world. The shade of color represents different number of smart cities per country. Cities are represented with colored points in the map, identified in six main regions: Africa, Asia, Europe, North America, Oceania, and South America. The bars on the right side show the countries with highest number of smart cities.  
provides healthcare services through smart gadgets (e.g., smartphones, smartwatches, wireless smart glucometer, etc.) and networks (e.g., body area and wireless local area network), offering different stakeholders (e.g., doctors, nurses, patient caretakers, family members, and patients) timely access to patient information and the ability to deploy the right procedures and solutions, which reduces medical errors and costs [44].  
Biosensors are fundamental when monitoring health, and different applications can be identified in medical diagnoses [45], and antigen detection [46], among others. Inorganic flexible electronics have witnessed relevant results, including E-skin [47], epidermal electronics (see Figure 4) [48], and eye cameras [49]. Some common materials used to create these sensors are carbon-based or conductive organic polymers, which present poor linearity [50,51]. However, more reliable, and flexible sensors have been created at lower cost, better linearity, and shorter response time, such as piezoresistive sensors integrating nano-porous polymer substrates [52].  
New sensor developments are creating relevant opportunities in the health industry. Current procedures for sensing proteins are commonly based on noisy wet-sensing methods. A more robust procedure is carried out by means of graphene sensors that avoid the drifting of electrical signals, resulting in more stable and reliable signals. These sensors also reduce detection times [53]. Nonetheless, when having these robust sensors connected to the human body, one critical challenge is their communication through the wireless sensor network, since the IEEE 802.15.4 standard can hardly be adapted to multi-user interfaces [54]. Still, some solutions have been proposed, such as replacing the ultra-wideband (UWB) with a gateway, so sensor nodes stop and switch to'sleep mode' until new information transmission is needed again. This allows lowering the energy consumption and collisions and increase the speed and number of users [55].