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Wireless sensor nodes (WSNs) and embedded microsystems have recently gained tremendous traction from researchers due to their vast sensing and monitoring applications in various fields including healthcare, academic, finance, environment, military, agriculture, retail, and consumer electronics. An essential requirement for the sustainable operation of WSN is the presence of an uninterrupted power supply; which is currently obtained from electrochemical batteries that suffer from limited life cycles and are associated with serious environmental hazards. An alternative to replacing batteries of WSNs; either the direct replacement or to facilitate battery regular recharging, is by looking into energy harvesting for its sustainable drive. Energy harvesting is a technique by which ambient energy can be converted into useful electricity, particularly for low-power WSNs and consumer electronics. In particular, vibration-based energy harvesting has been a key focus area, due to the abundant availability of vibration-based energy sources that can be easily harvested. In vibration-based energy harvesters (VEHs), different optimization techniques and design considerations are taken in order to broaden the operation frequency range through multi-resonant states, increase multi-degree-of-freedom, provide nonlinear characteristics, and implement the hybrid conversion. This comprehensive review summarizes recent developments in VEHs with a focus on piezoelectric, electromagnetic, and hybrid piezoelectric-electromagnetic energy harvesters. Various vibration and motion-induced energy harvesting prototypes have been reviewed and discussed in detail with respect to device architecture, conversion mechanism, performance parameters, and implementation. Overall sizes of most of the reported piezoelectric energy harvesters are in the millimeter to centimeter scales, with resonant frequencies in the range of 2-13 900 Hz. Maximum energy conversion for electromagnetic energy harvesters can potentially reach up to 778.01 μW/cm 3. The power produced by the reported hybrid energy harvesters (HEHs) is in the range of 35.43-4900 μW. Due to the combined piezoelectric-electromagnetic energy conversion in HEHs, these systems are capable of producing the highest power densities.

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This paper presents the low-cost, fine-resolution printing of conductive copper patterns on silicon substrate. The colloidal solution containing copper nanoparticles is deposited through electrohydrodynamic printing technology. Conductive copper tracks of different width are printed by varying the operating conditions (applied voltage and flow rate) and controlling the jet diameter. The minimum pattern width achieved was approximately 12 μm with the average thickness of 82 nm across the width after the sintering process. The achieved pattern width is five times smaller than the capillary used for patterning. The morphology and purity of the printed copper tracks were analyzed through scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray diffraction (XRD). The current-voltage (I-V) characteristic of the printed copper tracks showed linear Ohmic behavior and exhibited resistivity ranging from 5.98 × 10 −8 m −1 to 2.42 × 10 −7 m −1 .

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In this paper, we report an alternate technique for the deposition of nanostructured TiO 2 thin films using the electrohydrodynamic atomization (EHDA) technique using polyvinylpyrrolidone (PVP) as a stabilizer. The required parameters for achieving uniform TiO 2 films using EHDA are also discussed in detail. X-ray diffraction results confirm that the TiO 2 films were oriented in the anatase phase. Scanning electron microscope studies revealed the uniform deposition of the TiO 2 . The purity of the films is characterized by using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), confirming the presence of Ti-O bonding in the films without any organic residue. The optical properties of the TiO 2 films were measured by UV-visible spectroscopy, which shows that the transparency of the films is nearly 85% in the visible region. The current-voltage (I -V ) curve of the TiO 2 thin films shows a nearly linear behavior with 45 m cm of electrical resistivity. These results suggest that TiO 2 thin films deposited via the EHDA method possess promising applications in optoelectronic devices.

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Harvesting biomechanical energy is a viable solution to sustainably powering wearable electronics for continuous health monitoring, remote sensing, and motion tracking. A hybrid insole energy harvester (HIEH), capable of harvesting energy from low-frequency walking step motion, to supply power to wearable sensors, has been reported in this paper. The multimodal and multi-degrees-of-freedom low frequency walking energy harvester has a lightweight of 33.2 g and occupies a small volume of 44.1 cm3. Experimentally, the HIEH exhibits six resonant frequencies, corresponding to the resonances of the intermediate square spiral planar spring at 9.7, 41 Hz, 50 Hz, and 55 Hz, the Polyvinylidene fluoride (PVDF) beam-I at 16.5 Hz and PVDF beam-II at 25 Hz. The upper and lower electromagnetic (EM) generators are capable of delivering peak powers of 58 µW and 51 µW under 0.6 g, by EM induction at 9.7 Hz, across optimum load resistances of 13.5 Ω and 16.5 Ω, respectively. Moreover, PVDF-I and PVDF-II generate root mean square (RMS) voltages of 3.34 V and 3.83 V across 9 MΩ load resistance, under 0.6 g base acceleration. As compared to individual harvesting units, the hybrid harvester performed much better, generated about 7 V open-circuit voltage and charged a 100 µF capacitor up to 2.9 V using a hand movement for about eight minutes, which is 30% more voltage than the standalone piezoelectric unit in the same amount of time. The designed HIEH can be a potential mobile source to sustainably power wearable electronics and wireless body sensors.