The environmental impact of petroleum-based polymers like PET and PVC necessitates the development of sustainable alternatives. Plant-derived proteins are attractive candidates due to their abundance, biodegradability, and low environmental impact. However, generating functional materials from plant proteins at scale has been hindered by the lack of efficient methods to control their micro- and nanoscale structures, which are crucial for desirable material properties. This study addresses this challenge by presenting a scalable approach to control the self-assembly of water-insoluble plant proteins into high-performance films. The use of plant-based proteins offers a significant advantage over animal-derived proteins, which can be expensive, potentially allergenic, and have a higher environmental impact. Most plant-based proteins are water-insoluble, posing a major obstacle to controlled self-assembly. Previous methods have focused on complex protein purification to extract water-soluble components, limiting scalability. This research aims to overcome this limitation by developing a new processing method that enables the controlled self-assembly of water-insoluble plant proteins into functional, robust materials, opening avenues for sustainable material production.

Prior work on protein-based films through controlled self-assembly has largely utilized synthetic peptides or animal-derived proteins like silk, β-lactoglobulin, and lysozyme. While successful, these approaches face challenges related to cost, allergenicity, and environmental impact. Research on plant-based protein films has been limited by the proteins’ poor water solubility, typically necessitating complex and costly purification to isolate water-soluble fractions, hindering scalability. This study explores a novel approach that tackles the challenge of water insolubility directly, enabling the creation of high-performance films from readily available and sustainable plant sources.

The researchers employed a multiscale self-assembly approach using soy protein isolate (SPI) as a model plant protein. SPI, known for its abundance as a byproduct of soybean oil production, was dissolved in a binary mixture of water and acetic acid. This solvent system, unlike previous approaches using non-volatile chaotropic agents, is environmentally friendly and does not leave residues in the final product. The mixture was subjected to ultrasonication and elevated temperatures (90°C) for 30 minutes to improve protein solubility and break down large aggregates into smaller, soluble units. The resulting solution could reach concentrations as high as 10 w/v%. Lowering the temperature of the solution initiated self-assembly, forming a hydrogel. Glycerol was then added as a plasticizer (30 w/w% to the total dry mass) before casting the solution onto a preheated glass Petri dish. The uniform distribution of the protein solution is enhanced by the heat of the petri dish during the casting step, followed by formation of a thin hydrogel upon cooling to room temperature. Drying at room temperature for three days yielded a free-standing film. The self-assembly process was monitored using various techniques including turbidity measurements, solubility measurements, dynamic light scattering (DLS), optical microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). X-ray diffraction (XRD) analysis was used to further investigate the protein secondary structure in the final film. The mechanical properties of the films were assessed through tensile testing. UV-vis spectroscopy was used to measure the optical transmittance. The method's scalability was demonstrated by producing a 30 x 40 cm film. For coating applications, a dip-coating process was employed, involving immersing a substrate (e.g., paperboard) into the heated SPI solution, allowing a gel coating to form, and then drying to solidify the coating. The water barrier properties of the coating were evaluated through water absorption measurements and a colorimetric test using cobalt chloride. Micro- and nanopatterning was achieved using soft lithography techniques to create various surface features such as micropillars and periodic nanogrooves. The hydrophobicity of micropatterned films was assessed by contact angle measurements, and the structural color of nanopatterned films was observed. Another commercial SPI feedstock from a different manufacturer was also tested, confirming the versatility and reproducibility of the method.

The study successfully produced meter-scale, mechanically robust plant-based films with high optical transmittance. The films exhibited high tensile strength (15.6 ± 2.07 MPa) and Young’s modulus (209 ± 39.1 MPa) comparable to engineering plastics like LDPE and PTFE, even without reinforcing agents. FTIR and XRD analysis revealed a significant increase in intermolecular β-sheet structures (from 49% in the original SPI powder to 65% in the film) – a key factor contributing to enhanced mechanical strength. The self-assembled films showed significantly higher transparency (93.7% transmittance at 550 nm) than conventional SPI films (77.4%). The developed process uses a water/acetic acid solvent system, offering a cost-effective and environmentally friendly alternative to existing methods. The scalability of the approach was validated by producing a large (30 x 40 cm) film. The films showed potential for coating applications; dip-coating produced coatings that significantly reduced water uptake in paperboard. Adding a small amount of hydrophobic corn zein further enhanced water barrier properties. Soft lithography enabled successful micro- and nanopatterning of the films, creating hydrophobic surfaces (increased contact angle from 45° to 99°) and structural color through diffraction.

The findings address the critical need for sustainable alternatives to petroleum-based polymers. The development of a scalable method for producing high-performance films from readily available and inexpensive plant-based proteins represents a significant advance in materials science. The superior mechanical properties and transparency of the self-assembled films, comparable to those of synthetic plastics, highlight the potential for replacing petroleum-based materials in various applications. The enhanced water barrier properties obtained through dip-coating, along with the ability to further tailor the hydrophobicity and functionality of the films using additional proteins demonstrate the versatility of this approach. The demonstrated micro- and nanopatterning capabilities open up further possibilities for functionalizing the films, such as creating specialized surfaces for applications in photonics and drug delivery. The research significantly contributes to the field by showcasing a sustainable and cost-effective path towards creating next-generation materials.

This study successfully demonstrated a scalable method for generating high-performance, multifunctional films from plant proteins. The method offers a sustainable and cost-effective alternative to synthetic polymers, exhibiting impressive mechanical strength, optical transparency, and facile processability. Further research could explore the application of this technology in various fields such as packaging, coatings, and biomedical devices, and investigate the use of other plant protein sources to further expand the diversity of materials produced.

While the study demonstrates the potential of this method, further investigation is needed to assess the long-term stability and durability of the films under various environmental conditions. The use of a specific plant protein (SPI) might limit the generalizability of the findings. Further research should explore other plant proteins to determine the applicability of this method across a broader range of protein types. Additionally, a comprehensive life cycle assessment would be beneficial to fully assess the environmental benefits of this approach compared to conventional methods.