# Body Area Networks in Healthcare: A Brief State of the Art
## 3 Physical Layer Requirements
### Wearable Sensors in Healthcare  
Medicine is always searching non-intrusive methods for monitoring people health. The goal of a wearable sensor is to extract information from the body without implanting a sensor inside the body. This means that sensors must be attached to the epidermis in most cases. The epidermis by nature acts as a barrier. The epidermis is the first line of defense of the immune system also protecting the body from the loss of water, nutrients, and solutes. For this reason, the epidermis presents the first challenge to the wearable sensors to be able to obtain information from the body [27]. Wearable sensors allow for monitoring vital signs of a patient [31]. This amount of information helps doctors to detect diseases with more confidence and also helps professional athletes to improve their performance. The health of a person can be measured with a collection of physiological aspects. Roles of each aspect and the interaction between them help doctors to identify disorders. These indicators are grouped into (1) body motions, (2) vital signs and (3) metabolism parameters [19]. Body motion is usually measured using strain and pressure sensors. Possible applications for these kinds of sensors are rehabilitation, gesture identification, gait detection or expression identification [32; 33]. As an example, please consider a smartwatch application named ADAM (Advanced Daily Activity Monitor) [34]. ADAM is only one node of a BAN solution. This solution monitors elderly people daily activities. The BAN application contains algorithms for fall detection, gait alteration detection and activity recognition. Very recently, an IoT-based health monitoring system uses electrocardiogram sensor, body temperature sensor, pulse rate, and oxygen saturation sensor [35]. Body motion detection is fundamental in some healthcare-related areas like sports. BANs applied to sports are used to help people for improving their progress, or professional athletes for finding defects in their performance. They are also widely used in rehabilitation of injuries caused by sport [36; 37]. The measurements are typically focused on data of movements performed by the user, which are acquired by sensors such as accelerometers, gyroscopes, magnetometers, pressure gauges, etc. Data provided by these sensors are analyzed to assess, for instance, the physical ability of users, determining if a movement is properly executed, to know the speed reached, the amount of calories and fat burned, your heart rate, or the distance traveled in the exercise [4]. Moreover, BANs are also utilized in virtual and augmented reality to detect body movements or hand gestures to interact with objects within a virtual environment [38]. Another approach presents a wearable composed of three-axis absolute orientation sensor, accelerometer and gyroscope [39]. Vital signs like heart rate, skin temperature or respiration use temperature, strain, pressure and humidity devices. These sensors are applied for the detection of cardiovascular diseases, heart failures, apnea and emotional control, or wound healing and physiological status. An example of a BAN system that monitors vital signs of patients that suffer from hyperhyreosis, sleep apnea, or sudden infant death syndrome has been proposed [40]. The application uses a wearable digital respiratory sensor and a breathing pattern monitor that helps to identify an abnormal breathing. The tests have shown that the application detects abnormal respiratory effectively. However, the system has only been tested eight times with six different subjects. More tests are needed to verify the accuracy of this solution. A couple of recent papers have introduced the estimation of mental distress from photoplethysmography [41] and electrodermal activity [42]. In case of the metabolism, chemical sensors are used in sweating areas and cavities to evaluate the body status by the concentration of alcohol, pH, lactic acid and electrolytes. These sensors can be combined with multiple configurations depending on the application.