###### Abstract  
The development of wearable sensor devices brings significant benefits to patients by offering real-time healthcare via wireless body area networks (WBANs). These wearable devices have gained significant traction due to advantageous features, including their lightweight nature, comfortable feel, stretchability, flexibility, low power consumption, and cost-effectiveness. Wearable devices play a pivotal role in healthcare, defence, sports, health monitoring, disease detection, and subject tracking. However, the irregular nature of the human body poses a significant challenge in the design of such wearable systems. This manuscript provides a comprehensive review of recent advancements in wearable and flexible smart sensor devices that can support the next generation of such sensor devices. Further, the development of direct ink writing (DIW) and direct writing (DW) methods has revolutionised new high-resolution integrated smart structures, enabling the design of next-generation soft, flexible, and stretchable wearable sensor devices. Recognising the importance of keeping academia and industry informed about cutting-edge technology and time-efficient fabrication tools, this manuscript also provides a thorough overview of the latest progress in various fabrication methods for wearable sensor devices utilised in WBAN and their evaluation using body phantoms. An overview of emerging challenges and future research directions is also discussed in the conclusion. w +
Footnote †: journal: Sensors  
available that can adapt to any surface without breaking and have the capability to elongate or deform under stress without losing functionality. They are usually made from an elastic; flexible substrate that has active sensing components bonded to it and can be employed in numerous applications. Recently, a self-powered and repairable humidity sensor constructed from silk nanofibre (SNF) film advanced the development of wearable technology by offering novel approaches to the domains of intelligent control, health monitoring, and next-generation wearable technology, including self-powered triboelectric nanogenerator (TENG) sensors utilising polyethylene oxide and copper-oxide nanofibres to monitor humidity as well as ethanol in simulated exhalation [2; 3]. A range of wearable devices has been developed incorporating different sensing applications. These devices utilise systems-on-chips (SoCs) to combine the sensing and processing of bio signals [4]. The most current developments in skin-worn, fattoo-based wearable electrochemical devices, electrolyte and metabolite sensors, biofuel cells, batteries, and nanomaterial-based wearable skin devices were investigated [5]. The application of liquid metal or its alloys as printing inks holds great potential in the realm of flexible electronics. It presents a convenient and direct approach to fabricating electronic components by simply writing them out using these liquid materials. This innovative method involves incubating the liquids to create the desired electronic structures [6]. Within the context of the DIW method, there is a comprehensive evaluation of crucial components for wearable electronic devices. These components include nanogenerators for energy generation, lithium-ion batteries for energy storage, and strain sensors for managing energy consumption. The focus of this examination revolves around assessing their performance metrics and exploring various fabrication techniques [7]. Shoe-based sensor systems can handle various vertical ground reaction forces (vGRFs), motion intent learning, plantar pressure, and fall detection, and they are useful in the calculation of the centre of mass displacement while walking, as well as pedestrian navigation and tracking [8]. A loop-based sensing antenna was designed using NinjaFlex material on a 3D hemispherical-shaped substrate [9]. The printing method was fused deposition modelling (FDM).