3D printing of negative Poisson's ratio on polymer film-based PENG

The rapid development of wearable systems requires a sustainable energy source that can harvest energy from the environment without frequent charging. Piezoelectric polymer films are ideal candidates for fabricating piezoelectric nanogenerators (PENGs) to harvest energy from the environment due to their flexibility, good piezoelectricity, and stable performance that is not affected by the environment due to their inherent polarization. However, their applications are mostly limited to piezoelectric mode energy harvesting based on the 3-3 direction piezoelectric effect due to molecular polarization and non-stretchability.

According to MEMS Consulting, researchers from Nanyang Technological University, Singapore, The Hebrew University of Jerusalem, Israel, and other institutions recently published a paper titled "3D Printed Auxet Structure-Assisted ezoelectric Energy Harvesting and Sensing" in the journal Vanguard Energy Materials. In this study, by 3D printing a negative Poisson's ratio (auxetic) structure on a film-based PENG, the bending deformation of the PENG can be converted into a well-controlled in-plane tensile deformation, thereby realizing the piezoelectric effect in the 3-1 direction. The synclastic effect of the negative Poisson's ratio structure was applied to a flexible energy harvesting device for the first time, making the previously undeveloped film bending deformation a valuable energy harvesting device and increasing the bending output voltage of the PENG to 8.3 times. The researchers installed the PENG assisted by the negative Poisson's ratio structure on different joints and soft fingers of the human body to demonstrate its function of sensing bending angles and monitoring movement.

Among mechanical metamaterials, negative Poisson's ratio structures are the most widely used structural designs, which can achieve negative Poisson's ratios that are rare in natural materials. All applications of negative Poisson's ratio structures in flexible electronic devices utilize their planar negative Poisson's ratio properties, while the unique properties of negative Poisson's ratio structures in out-of-plane bending, namely the synclastic effect, have not been fully utilized. The synclastic effect is produced by the formation of a dome-shaped hyperboloid when the negative Poisson's ratio material is bent (Figure 1a). On the other hand, uniform materials tend to form a single curved surface under bending, while materials with hexagonal honeycomb structures tend to form a saddle-shaped surface (Figure 1b).

Figure 1 Schematic diagram of synclasc and anticlastic effects and the structure of the auxetic-PENG device

In this study, by using the synclastic effect of negative Poisson's ratio structures and 3D printing negative Poisson's ratio structures using digital light processing (DLP), the researchers developed a thin-film PENG that can generate electricity in a 3-1 mode under bending (Figure 1c). This unprecedented approach enables the auxetic-PENG to harvest energy in a bending mode, which is not possible in typical piezoelectric polymer-based thin-film PENGs. The unique negative Poisson's ratio structure can also precisely control tensile strain without any overstretching. It can be used as a bending motion to monitor human body movements. The structure of the auxetic-PENG (Figure 1d) consists of four layers: bottom electrode, piezoelectric material, top electrode, and negative Poisson's ratio structure. The printed negative Poisson's ratio structure will guide the in-plane tensile deformation of the piezoelectric device under bending.

To experimentally investigate the bending mode energy harvesting caused by the synclastic effect of the auxetic-PENG, the researchers bent the sample in a cantilever bending manner, with one end fixed and the other end pushed by a linear motor with a displacement of 5 mm (the curvature on the sample was 17 mm), and the voltage generated was measured to be ≈ 1 V (Figure 2a, red line). The negative Poisson's ratio structure was then peeled off, and the researchers performed the same measurement on the M-BTO/P(VDF-TrFE) film without the negative Poisson's ratio structure, and obtained an output voltage of ≈ 0.12 V (Figure 2a, blue), which is 0.88 V less than the auxetic-PENG. To find the optimal working state of the device, a load ranging from 1 kΩ to 1 GΩ was connected in parallel to the PENG, and then the output voltage and density were tested at 5 mm bending displacement (17 mm curvature) and 1.5 Hz frequency, as shown in Figure 2c. The output voltage decreases with increasing load resistance, while the output current density increases with increasing load resistance. The maximum instantaneous output power density is obtained by multiplying the output voltage by the output current density, as shown in Figure 2d. The researchers also studied the sensing properties of the auxetic-PENG, and the results showed that the output voltage is proportional to the bending displacement and has a quadratic relationship with the bending curvature (Figure 2e). The predictable relationship between the output voltage and curvature suggests that it has the potential to be a sensor.

Figure 2 Experimental study of bending mode energy harvesting of auxetic-PENG

Since the in-plane tensile strain on the piezoelectric film is caused by the synclastic effect of the attached negative Poisson's ratio structure, the shape factor of the negative Poisson's ratio structure will affect the magnitude of the in-plane strain on the piezoelectric film. In order to study the effect of the size of the negative Poisson's ratio structure on the in-plane strain of the piezoelectric film, the researchers conducted and experimental studies, and the results are shown in Figure 3.

Figure 3 Simulation and experimental results

To demonstrate the possible applications of the auxetic-PENG, the negative Poisson's ratio M-BTO/P(VDF-TrFE) sample was subjected to bending (5 mm displacement, 1.5 Hz frequency) after rectification with a 1 µF (Figure 4a). The device was fully charged in 32 seconds, showing the energy harvesting function of the auxetic-PENG. The auxetic-PENG was mounted on the outer surface of a cabinet door to demonstrate the energy harvesting from the door opening/closing motion as a bendable sensor, as shown in Figures 4b-4d. The auxetic-PENG also has the potential for wind energy harvesting, as shown in Figures 4e and 4f. Taking advantage of the flexibility and sensitivity of the auxetic-PENG, it was mounted on the inside of a human joint to sense the bending motion of the human body, thus becoming a self-powered physiological monitoring sensor (Figure). Due to its output stability, bending amplification through the synclastic effect, light weight, flexibility, and self-powered characteristics, which distinguish it from triboelectric, piezoresistive, and capacitive sensors, the auxetic-PENG can also be used as a promising choice for bending sensors in soft robots, as shown in Figures 4h and 4i.

Figure 4 Application demonstration of auxetic-PENG

In summary, this study developed a negative Poisson's ratio structure-assisted piezoelectric nanogenerator (PENG) and sensor based on a surface-modified piezoelectric ceramic barium titanate nanoparticle (BTO NP) and P(VDF-TrFE) composite. Compared with the unmodified BTO-NP/P(VDF-TrFE) composite, the modified BTO NP by 3-(trimethoxysilyl)propyl methacrylate (TMSPM) improved the piezoelectricity, ferroelectricity, and dielectric constant of the composite, which was due to the more uniform distribution of the modified particles in the P(VDF-TrFE) matrix, which enhanced the force transmission to the BTO NP. For the first time, the researchers used the synclastic effect of the negative Poisson's ratio structure to realize a bending energy harvesting mode with a 3-1 direction piezoelectric effect, which is not achievable on typical non-stretched piezoelectric polymer film energy harvesters. The researchers studied the size factor of the negative Poisson's ratio structure through simulation and experiment, and the results showed that the finer the structure, the lower the output. They studied the effect of the negative Poisson's ratio structure form factor on bending energy harvesting, providing guidance for the optimization and customization of PENG. In terms of applications, this auxetic-PENG can be used as both an energy harvester and a self-powered sensor for physiological monitoring in personal health assessment and medical diagnosis.

Review editor: Liu Qing