Thermal and electrical energy converters and interfaces for the internet of humans

The emerging Internet of Humans (IoH) can shape future healthcare and consumer devices. The energy supply of IoH devices and unobtrusive interfacing with the human body remain fundamental challenges yet to be resolved. To address these obstacles, this work investigates thermal energy harvesting of body heat and soft thermal and electric skin interfaces. The thermal harvesting potential of the human body is assessed over the mechanisms of heat generation and thermoregulation. Experimental verification is provided through a user study with 56 participants. The study demonstrates that 2 mW cm -2 to 40 mW cm -2 of thermal energy are available for harvesting, depending on the subject, body location, environment, and activity. A significant decrease of thermal energy with age is observed and can be correlated to an increase in thermal skin resistance. To access the available energy, three wearable thermoelectric energy converters for different IoH applications are presented. A user-centered design approach, incorporating the complete pathway from body core to application, is pursued to combine performance with wearability. Special attention is paid to the critical interaction between thermal and electric energy conversion. A miniaturized thermal harvester for IoH sensor nodes generates μW, a wristband harvester supplying a multi-sensor bracelet hundreds of μW and a forehead harvester designed for a medical wearable mW of power. With a power density of 18.75 μW cm -3 at room temperature, the forehead harvester delivers the highest power output per volume compared to existing systems and demonstrates that wearability can be drastically improved without compromising the performance. The wearability and performance of body heat harvesters is also defined by the thermal interfaces with the skin and the environment. A soft composite of silver particles and PDMS (Ag = 25 vol.%, λ = 1.42 W m -1 K -1) improves the comfort of skin interfaces without considerable power losses by adopting to the irregular body surface. A heat sink incorporating biomimetic microactuators with high temperature sensitivity (SD = 0.053K -1) extends the applicable temperature range of thermal harvesters by dynamically changing the thermal interface resistance to the ambient. Lastly, to interface the human body electrically, soft, skin-conformal, and selfadhesive biopotential electrodes are presented. The dry skin interfaces show similar or superior performance in terms of skin-contact impedance (47 kΩ cm2) and motion artifact rejection (RMS = 30 μV) as compared to clinical standard gel electrodes. At the same time, skin irritations are minimized and the long-term stability in wet environments improves. The performance is validated by high-fidelity recordings of the ECG of athletes during workout in challenging environments and of the EEG through dense hair. The interfaces and energy converters presented in this thesis are integrated into a proof-of-concept wearable EEG system that combines different energy harvesting sources with ultra-low power electronics and smart algorithms for automated patient monitoring.