Molecular engineering of piezoelectricity in collagen-mimicking peptide assemblies

Piezoelectric materials convert mechanical energy into electrical energy, and are typically made from inorganic materials or organic polymers. However, these materials often have limitations such as toxicity, complex synthesis, and poor sustainability, hindering their use in biocompatible applications like health monitoring and regenerative medicine. Natural materials like collagen exhibit piezoelectricity, but their response is generally weak. This research aims to overcome these limitations by leveraging the biocompatibility and engineerability of short peptides to create a high-performance, biocompatible piezoelectric device. The study utilizes a minimalistic approach, employing self-assembling short peptides as building blocks to mimic the hierarchical structure of collagen and enhance its piezoelectric properties. The use of computational modeling, specifically density functional theory (DFT), plays a crucial role in predicting and guiding the design of these peptides, optimizing their structure for maximal piezoelectricity. The ultimate goal is to demonstrate a proof-of-concept biocompatible piezoelectric generator that is both highly sensitive and reliable.

Previous research has demonstrated piezoelectricity in various natural materials, including bone, collagen, viruses, cellulose, and chitosan. However, the piezoelectric response of these biomaterials is typically low (0.1–10 pm V⁻¹), limiting their applications. Collagen, known for its extensibility, high tensile strength, and swelling properties, has been studied extensively for its piezoelectric characteristics, with fibrillar rat tail collagen exhibiting the highest measured shear piezoelectric coefficient (d14) of 12 pm V⁻¹ among biomaterials. Biomimetic approaches using self-assembling peptides offer a promising route to creating biocompatible functional materials due to their inherent biocompatibility and tunable properties. While short peptides often self-assemble into β-sheets, the helical structure of collagen is crucial for its piezoelectric response. The high number of hydrogen bonds in collagen's helical structure creates a macroscopic dipole, enabling coupling with electric fields and shear forces. Hydroxyproline (Hyp), a post-translationally modified amino acid in collagen, exhibits the highest piezoelectric response among collagen components. Studies have also shown that aromatic amino acids can enhance electrical conductivity in peptide-based structures. However, designing ultra-short peptides that mimic collagen's supramolecular architecture and achieve a high piezoelectric response has been challenging. Computational modeling, specifically DFT, has proven effective in predicting and rationalizing the piezoelectric response of various materials, including amino acids, peptides, and biomineral crystals. Combining DFT with experimental techniques like piezoresponse force microscopy (PFM) allows for accurate prediction and characterization of nanoscale electromechanical phenomena. Classical molecular dynamics (MD) simulations further contribute to understanding the kinetics of piezoelectric systems, particularly in liquid environments and across temperature ranges.

This study employed a multi-faceted approach combining experimental techniques and computational modeling. The researchers used two collagen-mimicking tripeptides: Pro-Phe-Phe and Hyp-Phe-Phe. The peptides were synthesized and self-assembled into fibrillar structures. The secondary structures of the peptide assemblies were characterized using Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopy. FTIR analysis revealed the presence of a predominantly helical structure in Hyp-Phe-Phe, supported by CD spectroscopy results. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to visualize the supramolecular assembly, revealing the formation of uniform high aspect ratio fibers. Single-crystal X-ray diffraction was conducted to elucidate the molecular-level interactions governing the supramolecular organization. Quantitative nanomechanical mapping AFM (QNM-AFM) was used to determine the Young's modulus of Hyp-Phe-Phe fibrils. Density functional theory (DFT) calculations were performed on the obtained single-crystal structures to predict the elastic, dielectric, and piezoelectric constants of the tripeptides. Piezoresponse force microscopy (PFM) measurements were carried out to experimentally validate the DFT predictions, using point measurements on single crystals to determine the vertical (d33) and shear (d15) piezoelectric coefficients. Molecular dynamics (MD) simulations were performed to study the effects of hydroxylation on the supramolecular packing, hydrogen bonding, and conformational freedom of the peptides. Finally, a coin-sized power generator was fabricated by sandwiching the peptide assembly film between two silver electrodes. The generator's performance was characterized by measuring the short-circuit current and open-circuit voltage under various applied forces. A control device using the β-sheet forming dipeptide Phe-Phe was also fabricated for comparison.

The study revealed that Hyp-Phe-Phe, with the addition of a hydroxyl group, exhibits a significantly enhanced piezoelectric response compared to Pro-Phe-Phe. DFT calculations predicted a maximum piezoelectric strain constant (d35) of -27.3 pm V⁻¹ for Hyp-Phe-Phe, significantly higher than the values reported for most biomaterials. Experimental PFM measurements confirmed a substantially improved piezoelectric response for Hyp-Phe-Phe compared to Pro-Phe-Phe: d33 for Hyp-Phe-Phe was 4.03 ± 1.96 pm V⁻¹, while d15 was 16.12 ± 2.3 pm V⁻¹. This shear value exceeds that of materials like LiNbO3, ZnO, and other biomaterials like collagen films. MD simulations showed that the hydroxylation of proline to hydroxyproline strengthens the hydrogen bonding network, while also increasing conformational freedom in the phenylalanine rings, contributing to the enhanced piezoelectric response. The fabricated peptide-based power generator using Hyp-Phe-Phe demonstrated a stable maximum current >50 nA and potential >1.2 V. In contrast, the β-sheet-forming Phe-Phe control showed significantly lower current and voltage outputs under similar applied force, highlighting the importance of the helical structure and specific amino acids for optimizing piezoelectricity. The mechanical rigidity of the peptide assemblies allowed sustained power generation under cyclic force, with no degradation over 1000 cycles.

The findings of this study demonstrate a successful approach to molecular engineering of piezoelectricity in peptide-based materials. The significant increase in piezoelectric response observed in Hyp-Phe-Phe compared to Pro-Phe-Phe highlights the crucial role of specific chemical modifications in enhancing the electromechanical properties of these biomaterials. The strong correlation between DFT predictions and experimental PFM measurements validates the effectiveness of computational modeling in guiding the design of high-performance piezoelectric peptides. The observation that the helical arrangement of the peptides, mimicking the structure of collagen, leads to a much higher piezoelectric response than the β-sheet structure of Phe-Phe clearly indicates the significance of secondary structure in determining the material's electromechanical properties. The results address the limitations of current piezoelectric materials by providing a biocompatible, sustainable, and readily synthesizable alternative. The successful demonstration of a functional piezoelectric generator based on these peptides opens up new possibilities for energy harvesting and sensing applications.

This research successfully demonstrated a significant enhancement in piezoelectricity using rationally designed collagen-mimicking peptides. The integration of DFT predictions and experimental validation showcases the potential of computational modeling in the design of biomaterials. The high performance of the peptide-based power generator suggests practical applications in various fields. Future research could explore further optimization of peptide sequences, the creation of aligned peptide arrays for improved energy output, and the integration of these materials into more complex devices.

While this study provides compelling evidence for the enhanced piezoelectricity of Hyp-Phe-Phe, further studies are needed to fully explore the long-term stability and performance of the peptide-based devices under diverse environmental conditions. The relatively small size of the single crystals used in PFM measurements could potentially impact the generalizability of the findings. The current device design is a prototype and optimization of device architecture for enhanced energy output remains an area for future research.