In today’s environmental friendly society, it is increasingly urgent to replace chemical batteries with biomechanical energy. With the development of low power products such as the portable electric devices, GPS and MEMS, whose power is low to milliwatts, biomechanical energy harvesting from human motion presents a promising clean alternative  .
Biomechanical energy harvesters generate electricity from people as they go about their activities of daily living resulting in power generation over much longer durations  . The conversion modes of biomechanical energy from human motion are mainly electromagnetic, mechanical, thermoelectric and piezoelectric  . Among these, the piezoelectric mode has become one focus in that its simple structure, pollution-free, high power density and so on. There have been three main ways to collect human motion energy with piezoelectric effect: bending piezoelectric material to produce electricity by using the weight of human body; causing vibration of piezoelectric beam by using acceleration pulse generated when heel collision with the ground or leg swinging. Besides, gripping palm and expanding arm’s muscles can also be used to generate electricity from piezoelectric material  . The objective of this paper is thus to review the currently available piezoelectric energy harvester from different forms of human motion, then to summary the technology. The main challenges that must be overcome to reach this goal are finally discussed.
2. PEHs from Human Motions
Energy Harvesting from Walking
Based on walking, the PEHs were triggered through heel-strike, knee-joint, arm motion, center of mass. The developed PEH devices mainly included shoe, kneepad, floor, dancing blanket, and backpack.
PEHs through heel strike
Kymissis and et al.  proposed three different devices that could be built into a shoe and generated electrical power parasitically while walking, as shown in Figure 1. The devices included a ‘‘Thunder’’ actuator constructed of piezoceramic composite material located in the heel, a rotary magnetic generator also located under the heel, and a multilayer PVDF foil laminate patch located in the sole of the shoe. In order to compare the performance of the three devices, three working prototypes were constructed and their performances were measured. The peak powers were observed to approach 20 mW for the PVDF stave, 80 mW for the PZT unimorph, and the shoe mounted rotary generator averaged to about 250 mW. A follow-up energy storage circuit and a radio frequency signal transmitting device were also designed. The energy generated by walking 3 - 6 steps per second could transmit a radio frequency signal. This design replaced batteries to supply power for GPS positioners, walkthroughs and other devices.
Figure 1. PEH on shoe proposed by Kymissis and et al.  .
On this basis, further research had been done on structural optimization design and experimental verification by Fourie  and Mateu  .
Haghbin  made a PEH with an air pump, as shown in Figure 2(a). PZT films were deformed by compressed air, which generated electrical energy. The whole PEH device was sealed to protect the piezoelectric material by airbag. It was very convenient to implant soles and generate electricity by treading on heels, as shown in Figure 2(b). Experiments on a running machine showed that the average power of 1.24 mW could be generated at a walking speed of 4 miles/h.
Besides, Leinonen and et al.  designed a cymbal PEH and implanted it into the sole, as shown in Figure 3. PZT-5H disc with Ф35 mm was selected to stamp into “cymbal” structure, which produced deformation under the weight of human body. The experimental results showed that the maximum average power of about 800 uW could be generated at 1 Hz gait. The theoretical and experimental errors were only within 7%.
Li and et al.  firstly designed a PEH with a curve L-shape mass at free end of the piezoelectric cantilever beam from the point of view of improving the energy collection density and reducing the resonant frequency. The piezoelectric beam was mounted in a cavity and embedded in sole, as shown in Figure 4. The power density could reach 1.45 mW/cm3, which was about 68% higher than that of traditional piezoelectric cantilever beam. The experimental results showed
Figure 2. PEH proposed by Haghbin  . (a) Air pump type PEH; (b) Inserted in shoe
Figure 3. Piezoelectric energy harvesting shoe proposed by Leinonen  . (a) Cymbal PEH; (b) Inserted in shoe.
that the average output power was 49 uW at a walking speed of 3 miles per hour.
Yoshiyasu Takefuji et al.  developed a piezoelectric carpet, which was tested at the entrance of the subway or the places where there are more pedestrians in the corridors of shopping malls. The results showed that the 25 m2 of the developed carpet produced 1400 kW of electricity per day. Japan’s East Japan Raiway also installed the “power generation mat” for harvesting energy from pedestrian to power the “ticket automatic door”, as shown in Figure 5.
The piezoelectric power generated floor was exhibited on Shanghai Science and Technology Festival of 2011, as shown in Figure 6. The floor was developed by Shanghai Silicate Research Institute of the Chinese Academy of Sciences. When pedestrians were jumping or walking on this floor, it could instantly generate electricity to light up the LED lattice and display the harvested energy data on the large screen  .
Figure 4. Piezoelectric energy harvesting shoe proposed LI and et al.  .
Figure 5. Power generation mat in Japan  .
Figure 6. Piezoelectric power generated floor  .
Hwang et al.  designed a pedal-type piezoelectric vibration energy collection device, as shown in Figure 7. The experimental results show that when a person of 68 kg walked on the tiles at both ends of the device, four piezoelectric beams was dived by the spring connected with the tiles for vibrating, an average power of about 0.12 mW in a cycle was generated. When a 80 g steel ball fell from 1 m height to the developed ceramic tile, the average power of about 707 uW could be generated.
Because the resonance frequency band of the traditional linear piezoelectric cantilever beam is too narrow, many researchers have begun to introduce non-linearity into the field of piezoelectric vibration energy harvesting from human motion. Wang Wei et al.  researched effect of output power on different harvesters, including bi-stable energy harvester (BEH), linear energy harvester (LEH) and mono-stable energy harvester (MEH), as shown in Figure 8. They designed a bistable magnetically coupled piezoelectric cantilever beam for harvesting human motion. The potential well was obtained by acceleration through leg swing and the foot impact. The energy capture efficiency improved very much.
Figure 7. A pedal-type PEH device  .
Figure 8. Non-linear PEH device proposed by Wang et al.  . (a) Non-linear PEH device; (b) Output power via different kinds of harvesters.
PEHs through knee joint
PEH device through knee joint was developed by Donelan and et al. in 2008  . This 1.6 kg device comprised an orthopedic knee brace configured such that knee motion drove a gear train (113:1) through a unidirectional clutch, transmitting only knee extension motion to a DC brushless motor that served as the generator. The generated electrical power was dissipated by a load resistor. This method generated 2.5 W per knee at a walking speed of 1.5 m/s.
To improve working stability and comfortability, Yao et al.  subsequently proposed a harvester installed on keen, as shown in Figure 9(a). The device adopted a mechanism of cam and roller (in Figure 9(b)) to transfer leg movement to piezoelectric ceramics, which generated electrical energy in each step cycle. In the experiment, the piezoelectric harvester with Φ40 mm substrate and Φ30 mm piezoelectric ceramic wafer was selected. When 100 kΩ was applied, the maximum output voltage was 80 V and the maximum output power was 58.2 mW, and about 27.5 mJ of energy could be generated per walk.
Pozzi and et al.  designed a PEH (shown in Figure 10) by using the leg swing motion as the power source. It consisted of four parts: rotor, stator, dial and piezoelectric beam. The PEH was installed near the knee joint. When the leg swings back and forth, the piezoelectric cantilever beam was continuously
Figure 9. A PEH proposed by Yao and et al.  . (a) Setup installed on keen; (b) Mechanism of cam and roller.
Figure 10. A PEH proposed by Pozzi and et al. 
picked by the dial to generate electricity. The experimental result showed that up to 2.06 mW of power was generated while a man weared the device on leg and carried a 24 kg backpack.
PEHs through arm motion
Bomm Yang and et al.  made full use of the soft properties of PVDF materials and proposed an effective shell structure for energy harvesting from twist joint. PVDF polymer film was attached on the curved shell structure, which was used to protect twist joint, as shown in Figure 11. The experiment validated that the output voltage of the shell structure was higher than that of the flat one. When the angular velocity of elbow joint was 9 rad/s, the maximum voltage was up to 40 V.
Renaud and et al.  developed a hand-crashed piezoelectric energy collector, as shown in Figure 12. A piezoelectric cantilever beam was installed at both ends of a cavity, and a track which accommodated the reciprocating motion of the slider was designed in the vertical direction. The experimental results showed that the slider with a mass of about 4 g generated electricity about 600 uW when it collided with the piezoelectric beams on both sides of the track at 10 Hz.
Similarly, Halim proposes a hand-operated vibration energy harvester with hybrid piezoelectric and electromagnetic power generation modes  . The piezoelectric beam impacted by ball was driven to generate high frequency resonance. Meanwhile the fixed magnet on the piezoelectric beam moved relative to the coil, so that the piezoelectric and electromagnetic power generation could be realized simultaneously. Compared with the same type of electromagnetic power generation device, the power density was greatly increased.
Figure 11. Flexible PEH by Momm Yallg and et al.  .
Figure 12. Hand-crashed PEH proposed by Renaud and et al.  .
Figure 13. Piezoelectric energy harvesting backpack proposed Joel and et al.  . (a) Piezoelectric backpack; (b) the stack amplifier.
PEHs through backpack with center of mass
Granstrom and et al.  took full advantage of the flexible properties of PVDF piezoelectric materials and installed PVDF in the backstrap of shoulder bags. PVDF could be strained by the weight of the backpack to generate electricity. Experiment showed that the average power of 45.6 mW was generated. Obviously, the flexible material using PVDF was easier to excite piezoelectric effect for capturing more energy than hard ceramics such as piezoelectric stack.
To collect more energy from backpack, Joel and et al.  proposed a piezoelectric energy harvesting backpack, as shown in Figure 13(a). It was accomplished by replacing the strap buckle with a mechanically amplified piezoelectric stack actuator (as shown in Figure 13(b)). The instrument allowed the relatively low forces generated by the pack to be transformed to high forces on the piezoelectric stack. Using the instrumented backpack carrying a 220 N load at ~2.75 Hz, the mean power output of ~0.4 mW could be available from each piezoelectric device. Because piezoelectric stack was used and the material was too hard to deform, the effect of energy collection was not very good.
The technology of piezoelectric energy harvesting from walking, knee or muscle motion, and palm grasping, has been researched. The developed PEHs were applied in shoe, backpack, and wrist protector. The harvested electricity was generally at the milliwatt level. It was mainly used in low power electronic equipment for power supplying. However, there exists a long way to go for practical application. Some problems must be overcome in the future research including lightweight, flexibility, stretchable, multi-direction and wideband, to be more fit to human body structure and movement.
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