###### Abstract  
The use of flexible sensors has tripled over the last decade due to the increased demand in various fields including health monitoring, food packaging, electronic skins and soft robotics. Flexible sensors have the ability to be bent and stretched during use and can still maintain their electrical and mechanical properties. This gives them an advantage over rigid sensors that lose their sensitivity when subject to bending. Advancements in 3D printing have enabled the development of tailored flexible sensors. Various additive manufacturing methods are being used to develop these sensors including inkjet printing, aerosol jet printing, fused deposition modelling, direct ink writing, selective laser melting and others. Hydrogels have gained much attention in the literature due to their self-healing and shape transforming. Self-healing enables the sensor to recover from damages such as cracks and cuts incurred during use, and this enables the sensor to have a longer operating life and stability. Various polymers are used as substrates on which the sensing material is placed. Polymers including polydimethylsiloxane, Poly(N-isopropylacrylamide) and polyvinyl acetate are extensively used in flexible sensors. The most widely used nanomaterials in flexible sensors are carbon and silver due to their excellent electrical properties. This review gives an overview of various types of flexible sensors (including temperature, pressure and chemical sensors), paying particular attention to the application areas and the corresponding characteristics/properties of interest required for such. Current advances/trends in the field including 3D printing, novel nanomaterials and responsive polymers, and self-healable sensors and wearables will also be discussed in more detail. flexible sensors; additive manufacturing; 3D printing; self-healing; nanocomposites; advanced manufacturing +
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and exercise-related signals. The implementation of flexible sensors into devices allows the creation of a multi-functional device, thereby breaking the limitation of traditional watches. According to Scopus statistics, the number of articles on flexible sensors doubled to 3710 between 2013 and 2019 which shows that the research field is growing rapidly [3]. According to the review paper [3], the market share of flexible displays was 8% in 2016 and rose to 27% by 2020, corresponding with the recent rapid growth of this research area. There are other review papers on flexible sensors published within the last 2 years [3; 4; 5; 6; 7], showing rapid growth of this research topic. One report published in 2020 [4] reviewed 3D printed sensors covering force sensors, pressure sensors and others. The report did not capture other manufacturing methods or the use of nanomaterial in depth. Another report published in 2021 [5] concentrated on flexible pressure and strain sensors used in health monitoring. They covered the sensing mechanisms and the use of nanomaterials such as carbon nanotubes in depth, however they did not cover manufacturing methods such as 3D printing which has recently gained much attention for polymer processing. [PERSON] et al. [6] discussed the materials, fabrication methods and electrical performance of flexible strain sensors. The sensitivity, linearity, response time and durability properties of the sensors were captured. The materials including flexible polymers and nanomaterials were disused and a short review of 3D printing was presented, however self-healable sensors and the various types of sensors were not presented in detail with the focus only on strain sensors. [PERSON] et al. [7] presented the use of PEDOT:PSS in electrochemical sensors. The review examined sensors that can detect ions, pH levels and hydrogen peroxide. The review was limited/focused on one polymer (PEDOT:PSS) and one type of sensor (chemical sensor). [PERSON] et al.