Electrospinning Machine | A Universal Orientation-Engineering Strategy for Enhancing Mechano-Electric Conversion Performance in Semi-Crystalline Biopolymers

Triboelectric nanogenerators (TENGs), as an emerging mechano-electric conversion (MEC) technology, are receiving increasing attention for their excellent self-powering and self-powered sensing capabilities. Due to wide material sources, diverse structural designs, minimal environmental impact, and satisfactory conversion efficiency at low frequencies, TENGs have broad application prospects in wearable emergency power sources, personalized healthcare, and human-machine interaction. However, their further development and practical application are limited by their relatively low MEC performance.

To effectively improve the triboelectric output of triboelectric materials, a series of strategies such as structural optimization, charge compensation, and energy management have been proposed. But most triboelectric materials, especially commonly used wearable biopolymers, still exhibit significant deficiencies in mechanically driven charge transfer and transport capabilities. Currently, researchers attempt to address this issue by enhancing the electron-donating or electron-accepting capabilities of materials, for example, using conventional chemical modification methods such as grafting, polymerization, and doping to suppress charge dissipation in triboelectric biopolymers. However, these methods often rely on complex chemical processing techniques, which not only increase preparation costs but also risk degrading the original properties of the materials (such as biocompatibility and mechanical flexibility), making it difficult to meet the demands for low-cost, large-area production. Therefore, directly designing triboelectric materials with high charge output from the perspective of intrinsic composition or structural optimization is crucial.

Recently, the research team led by Dong Kai at the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, published their latest research findings titled "A Universal Orientation-Engineering Strategy for Enhancing Mechano-Electric Conversion Performance in Semi-Crystalline Biopolymers" in the journal Advanced Materials. The first author is He Jin, a Ph.D. candidate jointly trained by Tiangong University and the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. The team developed oriented silk fibroin nanofibers (SFNs) with phase transition polarization and enhanced carrier mobility using high-voltage high-speed collaborative electrospinning technology, and proposed an orientation-engineering strategy to improve the mechano-electric conversion performance of semi-crystalline biopolymers.

The study found that stress induction during high-voltage polarization endows SFNs with excellent triboelectric properties, increasing their output power density by three times compared to non-oriented fibers. This is mainly attributed to the stress-induced transformation of the molecular structure from disordered α-helix to ordered stacked β-sheet structures during the gradual orientation of SFNs, where intramolecular hydrogen bonds transition to intermolecular hydrogen bonds, shortening the distance between molecular chains and enhancing carrier transport between molecular chains. Additionally, the ordered arrangement of molecular chains not only increases the polarity of the polymer but also optimizes the proportional distribution of charge traps, thereby significantly improving the material's interfacial charge transfer capability and bulk charge transport efficiency. This study further validates the effectiveness of the orientation-engineering strategy in improving triboelectric performance and holds significant importance for theoretical mechanism research and practical application development of high-performance triboelectric biopolymers.

Figure 1 Multi-level structure of silk fibroin and its orientation-induced aggregation structure regulation strategy. a) Schematic diagram of the multi-level hierarchical structure of silk fibroin. b) Schematic diagram of the preparation of orientation-regulated SFNs under the synergistic action of the high-voltage electrostatic traction force of the electrospinning needle and the high-speed winding force of the receiving roller. c) Aggregation structure evolution of SFNs at different stages of electrospinning.

As shown in Figure 1a, natural silk has intricate multi-level structures, primarily including the primary polypeptide chain structure composed of amino acid residues, secondary structures characterized by β-sheets and α-helices, and tertiary structures formed by the assembly of β-phases, among others. High-speed electrospinning technology can effectively regulate the molecular chain aggregation structure of silk fibroin. As shown in Figure 1b, under the synergistic action of the high-voltage electrostatic traction force of the electrospinning needle and the high-speed winding force of the receiving roller, the orientation degree of SFNs can be controlled.

Figure 2 Stress-induced orientation design and characterization of SFNs. a) High-speed snapshots, SEM images, and 2D SAXS diffraction patterns of the electrospinning process for SFNs with different orientations. b) Distribution ratio of SFNs at different winding speeds. c) Correspondence between winding speed and orientation coefficient. d) Tensile modulus of SFNs with different orientation coefficients.

Figure 2 compares high-speed photographs and SEM images of the electrospinning process at different winding speeds. As the winding speed increases from 200 r/min to 3200 r/min, the orientation coefficient of SFNs increases from 0 to 0.36, and the fiber morphology gradually transitions from disordered and random distribution to ordered alignment along the stress field direction. The corresponding orientation coefficients of SFNs collected at different winding speeds were calculated through SASX diffraction patterns, providing support for quantitative analysis of fiber orientation.

Figure 3 Influence of orientation coefficient on the structure and performance of SFNs. a) FTIR spectra of SFNs with different orientation coefficients. b) Proportion of α-helix and β-sheet structures at different orientation coefficients. c) One-dimensional electron density correlation function K(z). d) Crystalline region thickness (dc) and long period length (L) of SFNs at different orientation coefficients. e) Proportion of bound and free hydrogen bonds at different orientation coefficients. f) Multi-scale structural evolution model of SFNs orientation.

Figure 3 compares the changes in the internal structure of SFNs under different coefficients. As the orientation coefficient increases, the molecular conformation gradually transitions from disordered α-helix structures to ordered stacked β-sheet structures; simultaneously, molecular chains slip and reorganize along the force field direction, forming more regular crystalline regions. The orientation process involves changes at three levels: hydrogen bond reorganization in microscopic molecular chains (including breakage of hydrogen bonds within α-helices and formation of hydrogen bonds between β-sheets), disorder-to-order phase transition in mesoscopic aggregate structures, and anisotropic assembly of macroscopic fiber networks.

Figure 4 Triboelectric output performance of oriented SFNs. a-c) Comparison of charge density, power density, and charging capability of SFNs with different orientation coefficients. d) Potential maps of SFNs with different orientation coefficients measured by Kelvin probe (using a size of 15×15 mm²). e) Potential distribution curves extracted along the horizontal direction. f) Simulated potential distribution of SFNs with different orientation coefficients. g) 3D AFM surface morphology and 2D surface potential map of SFNs with low orientation coefficient and h) high orientation coefficient.

Figure 4 compares the triboelectric characteristics of SFNs with different orientation coefficients. The triboelectric performance of orientation-regulated SFNs is significantly improved. The triboelectric output performance of SFNs is strongly correlated with the orientation coefficient. Charge density increases significantly with the orientation coefficient, and the peak power density increases from 1 mW/m² to 3 mW/m²; the time to charge a 0.47 μF capacitor to 1.5 V decreases from 60 s to 30 s. Surface potential analysis shows that highly oriented SFNs form obvious potential gradient distributions due to their ordered structure, while the potential distribution of disordered fibers is relatively uniform. This difference further verifies the enhancement effect of orientation on charge output.

Figure 5 Interfacial charge transfer and bulk charge transport characteristics regulated by orientation. a) Dielectric constant of SFNs with different orientation coefficients. b) Band gap changes of SFNs with different orientation coefficients. c) Trap energy level and density distribution. d) Distribution ratio of deep traps and shallow traps in SFNs with different orientation coefficients. e) Carrier mobility of SFNs with different orientation coefficients. f) Dielectric loss of SFNs with different orientation coefficients. g) Bulk leakage current and h) surface conduction current of SFNs with different orientation coefficients. i) Potential charge dissipation pathways in SFNs.

Figure 5 analyzes the electrical characteristics of SFNs with different orientation coefficients. The orientation-engineering strategy optimizes the charge transfer and transport mechanisms, improving their charge performance. In terms of interfacial charge transfer, the dielectric constant of highly oriented SFNs increases and the band gap narrows, which can enhance polarity and reduce the charge transition energy barrier; in terms of bulk charge transport, the proportion of shallow traps increases and carrier mobility improves, promoting charge transport within the SFNs polymer.

Figure 6 Theoretical analysis of the mechanism for improving triboelectric performance through orientation optimization. a) Molecular chain structure evolution model of silk fibroin with different orientation coefficients. b) Bader charge density difference of the silk fibroin molecular chain model. c) Comparison of electron coupling and reorganization energy between different orientation coefficients. d) Changes in average intramolecular and intermolecular hydrogen bond lengths with orientation coefficient. e) Evolution of intramolecular and intermolecular hydrogen bonds in silk fibroin chains. f) HOMO-LUMO energy levels at different orientation coefficients. g) Projected density of states (PDOS) at different orientation coefficients. h) Schematic diagram of charge transport pathways.

In summary, the team developed a universal orientation-engineering strategy that successfully significantly improves the triboelectric performance of semi-crystalline biopolymers by inducing phase transition polarization and enhancing carrier mobility in SFNs. During the research, the team constructed a multi-scale structural evolution model of SFNs, systematically elucidating the synergistic mechanism of multi-scale structural evolution (covering molecular conformation transition, aggregation structure ordering, and fiber alignment regularization) on mechano-electric conversion. This work provides key theoretical guidance for the molecular design and structural regulation of high-performance triboelectric biopolymers and offers important support for promoting the application of TENGs and the innovative development of wearable energy devices.

Original link: https://doi.org/10.1002/adma.202510157