## 2 Various Types of Flexible Sensors (Temperature, Pressure, Humidity and Chemical)
### Pressure Sensors  
Pressure sensors have gained attention in flexible sensor research due to their versatility. These sensors detect the forces exerted on a surface area. Pressure sensors are used in various applications including medical diagnosis and touch screen devices. The most cost effective and efficient type of pressure sensors are the capacitive and resistive types, which convert force/pressure into electrical signals [27; 28]. Current research in pressure sensors is centred on improving the flexibility, stretchability, durability, linearity and sensitivity of the sensors [11; 12; 29].  
Currently, most touch sensors are rigid and are made from substrates such as glass or thermosets and sensing materials such as microelectrodes or indium tin oxide [9]. These materials are not recyclable and are bad for the environment. Microelectrodes require expensive disposal methods at the end of their use. A research team from the South China University of Technology developed highly sensitive touch sensors which are biodegradable. The sensors are composed of nanopaper made from cellulose nanofibrils with dimensions of 3.5 cm \(\times\) 2.5 cm \(\times\) 0.05 mm [9]. Currently, there exist three major issues in flexible sensor research, namely reliability under bending cycles, sensitivity and transparency. All three issues were addressed by the research team during the development of paper-based sensors. In addition to developing usable and reliable sensors, the team mentioned that each sensor would cost about EUR 0.06, which is cost effective. The low cost comes from the fact that cellulose, the main component of the touch sensor, is the most abundant biopolymer on earth.  
The availability of cellulose makes cellulose paper a cheap and sustainable substitute for microelectrodes. The only issue with cellulose is its low transparency, which was addressed by the research team by using cellulose nanofibrils composed of cellulose [9]. Nanomaterials tend to have better physicochemical and plasmonic effects than their bulk  
Figure 3: A picture of a patient wearing a flexible respiratory disease monitoring sensor (humidity sensor), reprinted with permission from [26].**(a)** Flexible respiratory sensor attached to a person’s nose for monitoring.**(b)** Output from the flexible respiratory sensor showing distinct peaks for inhalation and exhalation.  
counterparts. Cellulose nanofibrils have a high transparency of 80% at 400 nm, a value much higher than the bulk counterpart. These sensors can withstand 1000 bending cycles at a minimum bending radius of 5 mm without significant degradation or loss in sensing capabilities. The research team demonstrated another advantage of flexible sensors which is their ease of fabrication. A simple inkjet printing process was used to fabricate the capacitive sensors. Capacitive sensors have an advantage over resistive and inferred-sensing techniques including low power requirements, high reliability and ease of fabrication [30].  
Biocompatibility and disposal after use remains an issue in the flexible sensor research. The research by [PERSON] et al. tackles these issues, developing touch sensors made from paper which are non-toxic to the human body and can be disposed safely in the environment with a decomposition period of 3-4 weeks when placed into soil [9].  
Nanocrystal-based strain gauge sensors are used in fabricating wearable devices for monitoring heartbeat, breathing rhythms, human motion and others [11]. Strain sensors can measure the electromechanical deformation of objects. Nanomaterial-based strain sensors have advantages over the conventional metal thin film-based sensors, including higher sensing capacity and flexibility. This enables nano-based sensors to be used in advanced cases such as modelling the human body by measuring the curvature of the human arm and in voice recognition by measuring the resistance of the sensors with respect to the movement of the vocal cords, thereby distinguishing the words spoken [31].