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Flexible sensors, actuators, and energy harvesters are critical technologies for wearable electronics   with applications in health monitoring, athlete training, prostheses, soft robots, and haptic human-machine interactions. For wearable devices, there is an added need for energy storage to power data collection and transmission.  

Most existing sensors are powered by batteries, which are bulky and need to be replaced or recharged and may pollute the environment once thrown away. To overcome these challenges, active, self-powered sensors that also harvest energy from mechanical deformation, have the potential to eliminate the use of batteries.  

Flexible energy harvesters can power sensors for real-time monitoring of the vital biophysical signals of end users. Stretchable self-powered sensors and energy harvesters have many applications including compression garments, smart socks, and patient gowns where the sensors need to undergo large deformation during normal wear.  

Piezoelectric nanogenerators 

Piezoelectric energy harvesters are promising technologies that can convert mechanical deformation to electricity based on different mechanisms. Biocompatibility and mechanical durability of self-powered sensors are also important characteristics for human-machine interactions of robotic systems and medical implantable electronics.  

Piezoelectric materials can generate electric power from mechanical deformation and vice versa. They are increasingly used in self-powered sensors that harvest energy from the ambient environment (such as human movement) and actuators that give haptic sensation for robotics and biomedical devices.  

However, the piezoelectrically driven devices that utilise inorganic materials are usually rigid and brittle andare not suitable for wearable purposes in the bulk form. To embed the piezoelectric ceramic particles into the polymer matrix either as film or fibres can make the piezoelectric materials flexible and even stretchable.  

Electrospun polyvinylidene fluoride (PVDF)/boron nitride nanosheets (BNNSs) composite nanofibres are prepared and show piezo-triboelectric response under impact and tensile loading. Without further poling, adding small amount of functionalized BNNSs to PVDF yields great enhancement in triboelectricity, stretchability, and in-plane thermal conductivity, compared to the un-modified PVDF nanofibers. The stretchable, durable piezo-triboelectric nanogenerator has potential as self-powered wearable sensors for human motion detection. 

Direct current triboelectric nanogenerators 

Direct current triboelectric nanogenerators (DC TENGs) have emerged as a paradigm shifting technology which generate DC power output without the need of a rectifier. They show advantages over conventional triboelectric nanogenerators in terms of simpler power management design, better system integration capability and more flexible application scenarios. Stretchable DC TENGs are developed for enhanced performance.