###### Abstract
explored various types of sensors and fabrication methods for flexible sensors. The review was centred on applications of the flexible sensors including soft robots but did not present wearable sensing applications and chemical sensors in depth. This review paper presents the most recent developments for the materials and methods used in the fabrication of flexible sensors. This review paper explores the limitations, advantages and advances in current methods and materials, including the use of additive manufacturing (3D printing) in the fabrication of these sensors. Herein, various 3D printing methods being used in the fabrication of various types of flexible sensors including temperature, humidity, pressure, medical monitoring and chemical are explored. A summary of mechanisms and current methods employed in the development of self-healable flexible sensors are presented. Throughout the review, demonstrator examples of the advantages of the flexible sensors are provided. These include their ease of fabrication and increased room for innovative and smart solutions. Some smart sensors that can self-power and change shape upon exposure to a stimuli are mentioned herein. These stimuli-responsive sensors are developed via a new additive manufacturing method called 4D printing. Sensors are being incorporated in food packaging to enable the detection of cracks in the packaging material and to detect if the food has gone bad. Sensors have enabled the development of smart cars that can fully or partially self-drive. Newer cars can detect road markings and can warn the driver about obstacles such as other cars and trees, which has made driving manoeuvres such as reverse parking much safer and easier. Sensors can be divided into two main categories, namely non-flexible and flexible sensors. Non-flexible sensors are also termed rigid sensors. Although this type of sensor has pros, such as low cost of substrate material and low power losses, they are limited in flexibility. The lack in flexibility limits their use in health monitoring and other uses whereby the sensor is required to be continually bent, stretched or put under pressure. On the other hand, flexible sensors thrive under deformations. Their electrical properties are not affected by the bending/stretching. In fact, these sensors can use the bending and deformations to detect motion, such is the case of sensors used in robotics and human motion detection. Flexible sensors have several advantages over rigid sensors. They have impeccable sensing capabilities even at harsh bending stresses of 1500 \(\mu\varepsilon\)[8]. Some of these sensors can be subjected to 8000 bending cycles and still retain their sensing capabilities [8]. Flexible sensors also tend to have enhanced thermal and mechanical properties and lighter weight than rigid sensors. The use of flexible sensors is sometimes hindered by the low electrical conductivity of the flexible material. To overcome this, nanomaterials of carbon, silver, copper and others are being incorporated into the flexible component to enhance electrical properties of the device [3]. Polymers are the most used pliable material in flexible sensors due to their high flexibility yet resilience under bending stresses and their ease of fabrication. However, most polymers do not have the required electrical properties for the fabrication of electronics such as health monitoring devices and environmental monitoring devices [3] whereby high sensitivity and conductivity is required. Most efforts in the research of flexible sensors are centred on enhancing the flexibility and conductivity of the materials used in the fabrication. Areas such as robotics, prosthetics, implantable medical devices, electronic skin and smart watches require flexible and thin sensors. The flexibility of the sensors enables for a simple integration process on curved surfaces such as the neck and wrist. The flexibility property also allows for the development of complex shapes. With shape complexity comes mass, volume and cost reductions.