Table 7.

Comparison of nanomechanical energy harvesting methods in terms of pros and cons, performance, techniques for efficient utilization, and challenges

Comparison/ types	Electromagnetic	Piezoelectric	Triboelectric
Pros	No requirement of contacts [106] No requirement of voltage source [106] Smaller mechanical damping [106] Higher current [107] Operation is durable and robust [107] Lower impedance [108]	Smaller mechanical damping [106] No need for a voltage source [106] Higher capacitance [107] No requirement of mechanical stoppers [106] High energy density [106] High output voltage (2–10 V) [106]	Flexibility in device structure [109] Higher power density [110] Can operate at lower frequencies [109] Easy to fabricate with nanoscale size [109] High energy conversion efficiencies [110]
Cons	Low efficiency at low frequency [107] Difficult miniaturization [111] High coil losses [112] Lower efficiency [112] Complex integration [106] Lower output voltage [106]	Low current and high impedance [108] Incompatible for CMOS process [111] Poor coupling at microscale [106] Difficult to integrate [106] Requirement of special piezoelectric materials [112] Can be self-discharged at lower frequencies [111]	Durability is not good [110] The mechanism is not fully understood [110] High voltage and low current [109] Challenging to be integrated [110] Electrostatic charge accumulation
Strategies for effective energy harvesting	Frequency up-conversion [113] Sprung eccentric rotor [114] Elimination of spring [115] Spring clockwork mechanism [116] Induce non-linearity [117]	Induce non-linearity [118] Proper circuit management [119] Frequency up-conversion [120] Use a double pendulum system [121]	Development of core–shell structure [122] Design an ultrathin and flexible structure [123] To use single-electrode mode Use liquid metal electrode [124] Use of air-cushion mechanism [124]
Optimal locations for biomechanical energy harvesting	Center of gravity of upper body [125] Wrist movements [126] Knee movements [127] Feet motion [128] Legs and arms [113]	Movements of arms and legs [121] Human feet [129] Palms and fingers [130]	Relaxation and contraction of lung and cardiac muscles [131] Human skin [132] Clothes [133] Hand tapping [132]
Range of power output on nano-scale	0.5‒32 mW [113, 128, 134]	0.0002‒45.6 mW [121, 135, 136]	0.3‒4.67 mW [133]
Challenges	Difficult miniaturizing [137] Difficulties in integration [138] Design of flexible system [139]	Toxicity of piezoelectric materials Ultralow frequencies of human motions [140] Requirements of complex human movements [141] Rigidity and brittleness of Piezoelectric materials [142]	Need of surface modifications Humidity challenges The inflexibility of the electrode [143] Biocompatibility [144] Washability [122]