PHA, the Greenest Plastic So Far: Advancing Microbial Synthesis, Recovery, and Sustainable Applications for Circularity

Abstract

The global proliferation of nonbiodegradable petrochemical plastics presents severe environmental, health, and economic challenges, underscoring the urgent need for sustainable alternatives. Polyhydroxyalkanoates (PHAs), a family of biobased and biodegradable polyesters, offer a promising solution with their environmental compatibility, versatility, and ability to mimic conventional plastics. This review explores PHA production, encompassing microbial biosynthesis, metabolic pathways, and innovative extraction methods to optimize production and material properties. Genetic and metabolic engineering advances address cost and scalability barriers, and novel feedstock utilization. Innovations in copolymerization, blending, and nanocomposites have expanded PHA applications across packaging, agriculture, and biomedical fields. The evolution of smart and functionalized PHAs for high-value sectors such as regenerative medicine, drug delivery, and smart packaging further demonstrates their expanded potential beyond conventional bioplastics. Emphasis is placed on integrating PHAs into circular economy models by leveraging waste-derived feedstocks, emerging microbial strategies, including mixed microbial cultures (MMCs), halophilic systems, and aquaculture-integrated production, offer scalable and resilient routes for cost-effective, sustainable PHA biosynthesis. Despite their potential, PHAs remain hindered by challenges involving high production costs, inconsistent mechanical properties, and environmental trade-offs in downstream processing. PHAs are sustainable bioplastics that align competently with Green Chemistry principles, though improvements in energy efficiency and catalytic optimization are required for broader commercial applications. The review highlights novel recovery techniques aligned with a green bioeconomy, including green solvents, supercritical fluid extraction, enzymatic, mealworm, and other biorecovery methods. Integration with circular economy frameworks through waste valorization, biobased feedstocks, and zero-waste biorefinery models further enhances the sustainability of PHA production. Future research must optimize extraction methods, address microplastic risks, and expand into high-value markets. PHAs represent one of the most environmentally promising bioplastics to date due to their complete biodegradability, non-persistent microplastics, and lower emissions, with advancements in production further enhancing their viability. By advancing PHA technologies and fostering interdisciplinary collaboration, PHAs can accelerate the transition to a circular bioeconomy, offering a unique combination of full biodegradability, material tunability, and circular design that positions them as frontrunners in the transition away from fossil-based plastics.

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TEM image of accumulated PHA granules in the cell of Azotobacter chroococcum before recovery (Reprinted from dos Santos et al., Ing. Cienc. 2017, 13 (26), 269–298. 10.17230/ingciencia.13.26.10. Licensed under CC BY 4.0.).

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Inherent properties of PHA supporting sustainability and circularity. PHA exhibits various properties, including nontoxicity, a tensile strength of 40 MPa, full biodegradability in natural environments, water-insolubility with resistance to hydrolytic degradation, a glass transition temperature of 2–4 °C, a high melting temperature around 175 °C, biocompatibility for medical applications and tissue engineering, and partial crystallinity.

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Chemical, mechanical, and biological approaches to PHA extraction. This summarizes processes such as chemical dissolution (e.g., solvents like chloroform, dichloromethane), alkaline treatments (e.g., NaOH, KOH), and mechanical techniques like high-pressure fragmentation, grinding with beads, and supercritical CO₂ extraction. Adapted from various studies on PHA recovery methods. ,

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Process Flow of Solvent-Based PHA Extraction. Basic steps of the solvent-based PHA extraction process, illustrating the steps from biomass collection to cell disruption, PHA solubilization, recovery, purification, characterization, and application. This method highlights the efficiency of solvent-based techniques in isolating high-purity PHA for diverse applications.

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Efficiency of different PHA extraction methods. The bar chart compares recovery rates and purity of PHA extraction methods. Halogenated solvents achieve the highest recovery (99%) and purity (95%) but are environmentally hazardous. Green solvents (90% recovery, 92% purity) and alkali treatment (96.8% recovery, 88.6% purity) offer sustainable alternatives but face scalability and contamination challenges. Mechanical methods provide moderate recovery (85–88%) but require pretreatment to avoid degradation. Supercritical fluid extraction (SCFE) delivers high purity (98%) but low recovery (63%), making it suitable for niche applications.

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Applications of PHAs. Diverse applications of polyhydroxyalkanoates (PHAs) across various sectors, including biomedical uses (e.g., surgical implants, tissue engineering), environmental solutions (e.g., biodegradable bags and oil spill remediation), packaging, pharmacological and veterinary encapsulation, agricultural uses (e.g., fertilizers and pesticides), industrial recovery processes, and automotive components (e.g., biofuels and car parts).

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Worldwide distribution of selected PHA manufacturers.

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Reasons why PHAs appear to be the greenest plastic alternative so far. PHAs offer a sustainable solution to plastic pollution by combining renewable resources, eco-friendly production, complete biodegradability, and industrial versatility. With ongoing advancements, they support a circular bioeconomy and a greener future.