You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review 
Chapter Title:  Electrostatic Energy Harvesting 
1. Abstract of Cited Paper (BIB001):  Past research on vibration energy harvesting has focused primarily on the use of magnets or piezoelectric materials as the basis of energy transduction, with few experimental studies implementing variable-capacitance-based scavenging. In contrast, this paper presents the design and demonstration of a variable-capacitance vibration energy harvester that combines an asynchronous diode-based charge pump with an inductive energy flyback circuit to deliver 1.8 /spl mu/W to a resistive load. A cantilever beam variable capacitor with a 650-pF dc capacitance and a 348-pF zero-to-peak ac capacitance, formed by a 43.56cm/sup 2/ spring steel top plate attached to an aluminum base, drives the charge pump at its out-of-plane resonant frequency of 1.56 kHz. The entire harvester requires only one gated MOSFET for energy flyback control, greatly simplifying the clocking scheme and avoiding synchronization issues. Furthermore, the system exhibits a startup voltage requirement below 89 mV, indicating that it can potentially be turned on using just a piezoelectric film. 
2. Abstract of Cited Paper (BIB002):  A capacitive electrostatic generator with a structure suitable for a vibrational energy harvester is described. The device is a variation of the classical “doubler of electricity”. The device has only one moving part, only diodes as electronic components, and can generate any voltage, limited only by a nonlinear loading device. A simple version suitable for recharging a battery is discussed, and the results of a practical demonstration implementation are shown. 
3. Abstract of Cited Paper (BIB003):  Abstract This Letter exposes an electrostatic-based vibration energy scavenging scheme using a two-layer configuration made of a high permittivity material and a low permittivity layer of variable thickness. Because of the significant change in the permittivity allowed by this process, the capacitance ratio variation is much larger than in the case of a single layer (whatever the permittivity of the latter), therefore allowing an increased efficiency and a significant energy gain, by a typical factor of 50. 
4. Abstract of Cited Paper (BIB004):  This brief describes several ways in which a variation of the electrostatic generator known as “doubler of electricity” or “Bennet's doubler” can be used as a practical micropower generator and a battery charger. The device has a structure that can be constructed as a microelectromechanical system device powered by ambient vibration. A simple battery charger that does not require any control circuit for operation is detailed, and the inherent instability of the device is exploited to allow greater power output, with experimental results presented. 
5. Abstract of Cited Paper (BIB005):  In this paper, we report on an electrostatic energy harvester with an out-of-the-plane gap closing scheme. Using advanced MEMS technology, energy harvesting devices with a four wafer stack are batch fabricated and fully packaged at wafer scale. CYTOP polymer is used both as an electret material and an adhesive layer for low temperature wafer bonding. The overall size of the device is about 1.1×1.3 cm2. With an external load of 13.4 MΩ, a power output of 0.15 μW is achieved when vibration at an acceleration amplitude of 1 g (~9.8 m/s2) is applied at a low frequency of 96 Hz. The frequency response of the device is also measured and a broader bandwidth is observed at higher acceleration amplitude. 
6. Abstract of Cited Paper (BIB006):  Abstract In this paper, we examine the power generation performance of a rotational electret power generator theoretically and experimentally using electrostatic induction based on the properties of hydroxyapatite (HAp) ceramic electrets; the two surfaces of the electrets have opposing polarities. The HAp electrets have long-term stability in comparison with polymer electrets, which are more commonly used. The theoretical power output is proportional to the square of the total surface area of electrets per block of the stator, frequency of the rotor, and surface electric potential of the electret. Moreover, the power output is proportional to the square of the inverse of the air gap between the rotor and electrets. The behavior of power outputs obtained under different experimental conditions correlated with our theoretical prediction. The maximum output power generated by our prototype electret generator was 3.6 μW with the frequency of the rotor being 24 Hz. 
7. Abstract of Cited Paper (BIB007):  In this paper, an electrostatic vibration energy harvester is proposed and fabricated with both broad bandwidth and high normalized power density (NPD, harvested power/volume/acceleration2). The device is made up of two parts, top movable plate and bottom fixed plate, both of which are fabricated from silicon wafers using advanced micro-electro-mechanical system (MEMS) technology. A CYTOP polymer layer is spray coated for an electret material as well as an adhesive layer for low temperature wafer bonding. The air damping effect in energy harvesting has been investigated. At a low pressure of 3 Pa in a vacuum chamber, maximum power output of 4.95 μW has been harvested at low vibration amplitude of 0.09 g, resulting in a bandwidth of 12 Hz and NPD of 3 mW/cm3/g2, which outperforms most of the previous harvesters. A high harvester effectiveness of 67.9% is therefore achieved. The response on random vibrations is also tested. An average output power of 2.22 μW is harvested when random vibration is applied at a frequency range of 160 ± 12.5 Hz with RMS acceleration of 10.5 m/s2. The excellent overall performance gives promising application for energy harvesting from random sources and multi-device stack for wireless sensor networks.

Literature Review:
The working principle of the electrostatic EH is based on the variation of the capacitance, which converts mechanical energy into electrical form. The electrostatic EH is mainly achieved by using a parallel plate capacitor with one plate fixed and the other one movable. Consequently, the electrostatic EH can be more specifically named electrostatic kinetic EH, which mainly harvests the vibration energy in the machines. The vibration can provide great opportunities for EH since these are present in all machines to some degree. Generally, the electrostatic EH device consists of a resonator, a variable capacitor and a conditioning circuit, which is shown in Figure 15 BIB008 . From the references, it is apparent that triboelectric harvesters are normally designed as nanogenerators with advantages of robustness, easy integration, high conversion efficiency and low cost. However, the friction and the inevitable wear of the two triboelectric layers results in a limited lifetime, also rotating contact can reduce the material losses to some degree. The wear of materials leads to lower conversion efficiency and gradually cannot meet the energy requirement. Furthermore, heat generated by the wear may cause catastrophic accidents in some environments. The working principle of the electrostatic EH is based on the variation of the capacitance, which converts mechanical energy into electrical form. The electrostatic EH is mainly achieved by using a parallel plate capacitor with one plate fixed and the other one movable. Consequently, the electrostatic EH can be more specifically named electrostatic kinetic EH, which mainly harvests the vibration energy in the machines. The vibration can provide great opportunities for EH since these are present in all machines to some degree. Generally, the electrostatic EH device consists of a resonator, a variable capacitor and a conditioning circuit, which is shown in Figure 15 BIB008 . To activate the electrostatic generator, the charge is injected into the capacitor at maximum capacitance and pulled off at minimum capacitance. The change of the gap between the electrode of the variable capacitor can generate the electric outputs. The mass of the movable electrode, the spring and the damper form a resonator. The resonator can constrain the motion of the movable electrode of the capacitor and capture the external vibration at a certain range of the frequency band in high efficiency, usually at the natural frequencies of the resonator. The implementation of electrostatic To activate the electrostatic generator, the charge is injected into the capacitor at maximum capacitance and pulled off at minimum capacitance. The change of the gap between the electrode of the variable capacitor can generate the electric outputs. The mass of the movable electrode, the spring and the damper form a resonator. The resonator can constrain the motion of the movable electrode of the capacitor and capture the external vibration at a certain range of the frequency band in high efficiency, usually at the natural frequencies of the resonator. The implementation of electrostatic generator usually needs a DC to DC converter for extracting energy and furthermore, the operation of the converter has to be carefully synchronized at certain instants of the vibration cycle. The electrostatic generators can be widely applied in various scale machines. Numerous applications of the electrostatic EH have been developed to harvest the mechanical vibration energy. Most of the electrostatic generators are based on a single variable capacitor. The electrostatic EH working principle shows that the harvesting system requires an external power source to polarise the capacitance or an electret material that induces charges spontaneously BIB008 . A passive electrostatic device can be applied as an auxiliary to an external power supply, but it is difficult to implement a maintenance-free sensor node BIB004 . With the electrets, the drawback is that charges cannot exceed a maximum threshold as it will reduce the conversion efficiency and retain the charges for a limited time. With advanced MEMS technology, some designers BIB006 BIB007 have investigated and manufactured electret-based electrostatic generators, but only achieving microwatt power levels. Miyazaki et al. BIB001 has proposed an electrostatic generator which delivers 120 nW energy at the conversion efficiency of 21% with the excitation frequency of 45 Hz. Yen and Lang BIB002 have developed an asynchronous diode-based charge pump-based electrostatic harvester, which can generate 1.8 µW outputs. To overcome the pre-charging problem, the idea of the "doubler of electricity" or "Bennet's doubler" is adapted to eliminating the need of batteries or other devices for pre-charging  BIB005 BIB003 . Although the researchers have designed various circuit structures to exclude the external power supply requirements, the low output power means that electrostatic EH technology cannot be used alone in the process of self-powered WSN-based machine condition monitoring. Through making a survey on various techniques and prototypes for mechanical EH, it is shown clearly that structures of mechanical energy harvesters are complicated and with finite lifespan caused by the deformation and abrasion. In addition, the vibration energy is limited and most of the rotational energy is not wasted energy. Therefore, the mechanical EH harvesters are suitable only as auxiliary for thermal EH devices to supply power for WSNs.