Advancing sustainable plastics through supramolecular design

Main text

Petroleum-based plastics are prized for their durability and versatility, but these very qualities have made them a double-edged sword. Their resistance to degradation ensures long-term utility but also leads to persistent environmental pollution, particularly in marine ecosystems, where microplastic accumulation causes devastating ecological harm.¹ While efforts to address this issue have led to advancements in recycling and biodegradable alternatives like poly(lactic acid) (PLA), these solutions still fall short. For instance, PLA’s water insolubility hinders microbial breakdown, meaning it ultimately contributes to microplastic pollution just like conventional plastics.² Addressing this critical challenge, Aida and colleagues have developed mechanically strong but metabolizable supramolecular plastics (SPs) through innovative noncovalent synthesis (salt bridge crosslinking) and liquid-liquid phase separation (LLPS) technology. This breakthrough directly tackles two key industry pain points: the poor degradability of traditional plastics and persistent microplastic pollution, while maintaining exceptional material performance.³ SPs achieve this by being inherently dissociable into metabolizable monomers under biologically relevant conditions, like seawater.

The key innovation lies in the noncovalent synthesis of SPs through salt bridging between sodium hexametaphosphate (SHMP) and guanidinium ion (Gu)-based monomers, facilitated by LLPS.⁴ In aqueous environments, SHMP undergoes facile hydrolysis to yield phosphates, and Gu derivatives can be metabolized by cyanobacteria into amines through hydrolysis. SHMP (0.1 M) and ^alkylGu₂ (SO₄²⁻, 0.01 M) aqueous solutions were mixed at a 1:3 molar ratio. Following this, the solution underwent spontaneous LLPS, resulting in the emulsification of the mixture into a water-based dispersion of droplets, referred to as the condensed layer. The original inorganic compound undergoes selective phase partitioning during LLPS, as illustrated in Figure 1. The schematic details the critical ion expulsion mechanism: initial homogeneous mixing of SHMP and Gu monomers, spontaneous phase separation into polymer-rich coacervate and ion-rich aqueous phases, and complete exclusion of Na⁺/SO₄²⁻ counterions from the supramolecular networks. The efficiency of this desalination process was quantitatively verified through advanced spectroscopic characterization. Compared to conventional SPs, Prof. Aida’s salt-bridged SPs achieve an unprecedented combination of high mechanical strength and rapid seawater-triggered degradation through their innovative LLPS desalination strategy, overcoming the long-standing trade-off between durability and environmental persistence. This breakthrough not only demonstrates a solvent-free, energy-efficient processing route using commodity chemicals but also establishes a new design paradigm for developing sustainable structural materials that bridge industrial performance requirements with circular economy principles.

Figure 1.

^alkylSP₂: From eco-friendly synthesis to recyclable applications

(A) Preparation of SPs by phase separation desalination.

(B) Thermal breakage of H bonds in the salt bridge and thermal reshaping image.

(C) Closed-loop recycling of ^alkylSP₂ by immersing a 1-mm-thick processed ^alkylSP₂ film in a 3.5 wt % aqueous NaCl solution for 0.5 h.

(D) The potential applications of ^alkylSP₂ plastic.

Inductively coupled plasma atomic emission spectrometry and energy-dispersive X-ray spectroscopy analyses revealed that the concentrations of Na⁺ and SO₄²⁻ in the resulting SPs were negligible, thereby confirming the occurrence of desalination during the LLPS process. The condensed liquid phase was dried by evaporation and subsequently hot pressed, yielding a colorless, glass-like ^alkylSP₂ film with a high density of 1.62 g/cm³ and exceptional optical transmittance (97%). Combining rheology, NMR spectroscopy, and scattering techniques, the authors demonstrate that the condensed liquid phase formed during LLPS exhibits thermoplastic-like behavior with a distinct glass-to-rubber transition. The SPs exhibit Young’s moduli of up to 18 GPa, surpassing many thermoset polymers, while remaining thermally reshapable above their glass transition temperatures (T_g). For instance, ^amineSP₃ exhibits a T_g of 135°C and a tensile strength of 25.9 MPa, rivaling conventional plastics in performance. Moreover, SPs exhibited remarkable adhesive properties that directly correlate with the dynamic nature of their noncovalent networks. The reversible salt bridge interactions enable adaptive interfacial bonding, allowing effective adhesion to glass plates in both aqueous and air environments. When subjected to a load of 350 g, the supramolecular networks maintained adhesive integrity for 120 h in deionized water through dynamic bond reformation. In contrast, the bonded glass plates failed within 8.5 h in artificial seawater, as electrolyte competition disrupted the critical salt bridge interactions needed for interfacial adhesion. This phenomenon was ascribed to the electrolytes in seawater, which promoted the dissociation of the polymer into monomers. Upon dissolving ^alkylSP₂ in deuterated artificial seawater, NMR signals began to appear in both the hydrogen and phosphorus spectra within 10 min. In contrast, when dissolved in deuterated deionized water, ^alkylSP₂ exhibited no detectable NMR signals for 192 h.

The dielectric and rheological analyses further revealed that the supramolecular network’s stability arises from a delicate balance of salt bridges and hydrogen bonds, which reversibly break and reform. The observed behavior was consistent with that of covalent thermoplastics, and it was achieved exclusively through noncovalent interactions. This breakthrough fundamentally challenged the established view that supramolecular polymers lacked the necessary mechanical strength for structural uses. Prof. Aida et al. extended this design to polysaccharide-based SPs, such as chondroitin sulfate, further underscoring the versatility of their approach, yielding materials with tensile strength up to 94 MPa and compatibility with 3D printing.

The environmental benefits of SPs were multifaceted. First, their dissociation into monomers in marine environments prevented microplastic accumulation, addressing a critical gap in existing biodegradable polymers. More significantly, the closed-loop recyclability of SPs, evidenced by the high-yield recovery of SHMP and Gu monomers, was consistent with the principles of a circular economy.⁵ Experimental results indicated that the recovery yields of the monomers SHMP and ^alkylGu₂ were 91% and 82%, allowing for repeated reuse without degradation of material properties.

While SPs represent a significant breakthrough in sustainable materials, several key challenges must be overcome to enable their widespread practical application. The inherent water sensitivity of SPs, even when treated with hydrophobic coatings like parylene C, poses limitations for extended use in moist or aquatic environments, necessitating innovations in either advanced coating technologies or the development of inherently hydrophobic monomers that maintain the material’s unique dissociability. Scaling up production presents another hurdle, as the current synthesis process depends on precise stoichiometric ratios and LLPS conditions that require optimization for industrial-scale manufacturing. Furthermore, comprehensive life cycle assessments are needed to evaluate the energy requirements and environmental impacts of monomer recovery and recycling processes to ensure that their sustainability claims hold true. Looking ahead, research should focus on integrating SPs with other eco-friendly materials, like cellulose or lignin, to enhance their properties and applications; their excellent biocompatibility and metabolizable make them particularly promising candidates for biomedical uses, such as temporary medical implants or controlled drug delivery systems. In drug delivery, SPs stand out due to their dynamic noncovalent networks, which enable stimulus-responsive release mechanisms. Unlike conventional biodegradable polymers, which often cause inflammatory responses due to acidic degradation by-products, SPs dissociate into non-toxic monomers that can be naturally metabolized by cellular processes.

Cheng et al. have delivered a transformative approach to plastic design, merging the mechanical prowess of conventional polymers with the environmental benignity of supramolecular systems.³ This work demonstrates how noncovalent design bridges the performance-sustainability gap in plastics. Consequently, this work constitutes a significant milestone in the pursuit of sustainable materials, establishing a robust foundation for the eventual comprehensive substitution of traditional non-biodegradable plastics across multiple sectors. With the continuous optimization and widespread application of related technologies, SPs will be deeply integrated into daily life, driving a green material revolution across diverse sectors, including food packaging, medical devices, and building materials. The development of sustainable plastics will effectively reduce the ecological burden on earth and usher in a new era of harmonious coexistence between humans and nature.

Funding and acknowledgments

This work was supported by the National Natural Science Foundation of China (22278049 and U24A20559) and the Dalian High-Level Talent Innovation Program (2024RJ017).

Declaration of interests

The authors declare no competing interests.

Published Online: July 15, 2025