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Polyhydroxybutyrate
Polyhydroxybutyrate
Polyhydroxybutyrate
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Polymeric crystals of PHB observed by polarizing optical microscope.

Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that are of interest as bio-derived and biodegradable plastics.[1] The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but other polymers of this class are produced by a variety of organisms: these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers.

Biosynthesis

[edit]

PHB is produced by microorganisms (such as Cupriavidus necator, Methylobacterium rhodesianum or Bacillus megaterium) apparently in response to conditions of physiological stress;[2] mainly conditions in which nutrients are limited. The polymer is primarily a product of carbon assimilation (from glucose or starch) and is employed by microorganisms as a form of energy storage molecule to be metabolized when other common energy sources are not available. [citation needed]

Microbial biosynthesis of PHB starts with the condensation of two molecules of acetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.[3] PHAs granules are then recovered by disrupting the cells.[4]

Structure of poly-(R)-3-hydroxybutyrate (P3HB), a polyhydroxyalkanoate
Chemical structures of P3HB, PHV and their copolymer PHBV

Thermoplastic polymer

[edit]

Most commercial plastics are synthetic polymers derived from petrochemicals. They tend to resist biodegradation. PHB-derived plastics are attractive because they are compostable and derived from renewables and are bio-degradable.

ICI had developed the material to pilot plant stage in the 1980s, but interest faded when it became clear that the cost of material was too high, and its properties could not match those of polypropylene. Some bottles were made for Wella's "Sanara" range of shampoo; an example using the tradename "Biopol" is in the collection of the Science Museum, London.

In 1996, Monsanto (who sold PHB as a copolymer with PHV) bought all patents for making the polymer from ICI/Zeneca including the trademark "Biopol".[5] However, Monsanto's rights to Biopol were sold to the American company Metabolix in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. Monsanto began to focus on producing PHB from plants instead of bacteria.[6] But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB.[7]

Biopol is currently used in the medical industry for internal suture. It is nontoxic and biodegradable, so it does not have to be removed after recovery.[8]

TephaFLEX is a bacterially derived poly-4-hydroxybutyrate, manufactured using a recombinant fermentation process by Tepha Medical Devices, intended for a variety of medical applications that require biodegradable materials such as absorbable sutures.[9]

Properties

[edit]
  • Water-insoluble and relatively resistant to hydrolytic degradation. This differentiates PHB from most other currently available biodegradable plastics, which are either water-soluble or moisture-sensitive.
  • Good oxygen permeability.
  • Good ultra-violet resistance but poor resistance to acids and bases.
  • Soluble in chloroform and other chlorinated hydrocarbons.[10]
  • Biocompatible and hence is suitable for medical applications.
  • Melting point 175 °C., and glass transition temperature 2 °C.
  • Tensile strength 40 MPa, close to that of polypropylene.
  • Sinks in water (while polypropylene floats), facilitating its anaerobic biodegradation in sediments.
  • Non-toxic.
  • Less 'sticky' when melted.

History

[edit]

Polyhydroxybutyrate was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne.[11]

Biodegradation

[edit]

Firmicutes and proteobacteria can degrade PHB. Bacillus, Pseudomonas and Streptomyces species can degrade PHB. Pseudomonas lemoigne, Comamonas sp. Acidovorax faecalis, Aspergillus fumigatus and Variovorax paradoxus are soil microbes capable of degradation. Alcaligenes faecalis, Pseudomonas, and Illyobacter delafieldi, are obtained from anaerobic sludge. Comamonas testosteroni and Pseudomonas stutzeri were obtained from sea water. Few of these are capable of degrading at higher temperatures; notably excepting thermophilic Streptomyces sp. and a thermophilic strain of Aspergillus sp.[12]

References

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[edit]
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Polyhydroxybutyrate

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Polyhydroxybutyrate (PHB), also known as poly(3-hydroxybutyrate), is a naturally occurring biodegradable belonging to the polyhydroxyalkanoate (PHA) family, synthesized by various as an intracellular carbon and energy storage compound under nutrient-limited conditions. It serves as a promising biocompatible and eco-friendly alternative to petroleum-based plastics due to its properties and complete biodegradability in natural environments, breaking down into , , and biomass. First discovered in 1926 by French microbiologist Maurice Lemoigne in the bacterium Bacillus megaterium, PHB gained commercial attention in the late as a sustainable material. Chemically, PHB is a linear homopolymer composed of (R)-3-hydroxybutyric acid monomers, with the repeating unit [-O-CH(CH₃)-CH₂-C(=O)-]ₙ, forming a semicrystalline structure that includes orthorhombic α-crystals arranged in lamellar spherulites. Its physical properties include a density of 1.18–1.26 g/cm³, a melting temperature of 165–180°C, a glass transition temperature of 0–15°C, and high crystallinity (typically 50–70%), which contribute to its stiffness and brittleness but also excellent oxygen and water vapor barrier capabilities. These characteristics make PHB suitable for applications requiring durability and impermeability, though its mechanical limitations often necessitate blending with other polymers or fillers to enhance flexibility. PHB is primarily produced through microbial fermentation by bacteria such as Cupriavidus necator (formerly Ralstonia eutropha) or Bacillus species, using renewable carbon sources like glucose, agricultural wastes, or even CO₂ under controlled conditions of carbon excess and nitrogen limitation. Industrial-scale production was pioneered in the 1980s by Imperial Chemical Industries (ICI), which commercialized the PHA copolymer PHBV under the trade name Biopol for uses in packaging and biomedical devices, though early efforts faced challenges with high costs and production scalability. Recent advances focus on cost-effective methods, including genetic engineering of producer strains and utilization of low-cost substrates, to support broader adoption in sustainable manufacturing. Key applications of PHB span , , and , where its enables uses in systems, surgical implants, and scaffolds, while its biodegradability supports films and disposable items that decompose in or . Despite its potential to address , challenges like high production costs (approximately 4–6 times that of conventional plastics as of ) and variable mechanical performance continue to drive research into copolymers like poly(3-hydroxybutyrate-co-3-hydroxyvalerate) () for improved properties. As of 2025, the PHB market is projected to grow to USD 679 million by 2034, driven by for sustainable materials. Overall, PHB exemplifies the shift toward bio-based materials in a , with ongoing innovations enhancing its viability as a .

Chemical Structure

Monomer Composition

Polyhydroxybutyrate (PHB) is a composed of repeating units of the (R)-3-hydroxybutyric acid, which has the molecular formula C₄H₈O₃. This features a hydroxyl group attached to the third carbon atom of butanoic acid, resulting in the CH₃-CH(OH)-CH₂-COOH. In bacterial , the is derived from the intermediate (R)-3-hydroxybutyryl-CoA, which serves as the direct precursor for polymerization into the PHB chain. The chain of PHB consists of linkages formed between the carboxyl and hydroxyl groups of the 3-hydroxybutyrate units, yielding a repeating structural unit of –[O–CH(CH₃)–CH₂–C(=O)]–. As a homopolymer, pure PHB is exclusively composed of these identical (R)-3-hydroxybutyrate , arranged in a strictly isotactic configuration with all chiral centers having the R . In contrast, copolymers such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) incorporate additional units like 3-hydroxyvalerate alongside 3-hydroxybutyrate, altering the overall composition, though pure PHB remains the simplest and most studied form.

Polymer Characteristics

Polyhydroxybutyrate (PHB) is a linear formed by the of 3-hydroxybutyric acid monomers through ester linkages, creating a highly regular chain structure. This yields an isotactic configuration, where all chiral centers at the C3 position are predominantly in the (R)- form, ensuring stereoregularity that is essential for its material properties. Bacterial PHB typically has a weight-average molecular weight (Mw) of 1–2 million Da, with values varying from 0.1 to 3 million Da depending on production conditions, corresponding to approximately 1,000 to 35,000 units. This high molecular weight contributes to the 's behavior and processability. PHB displays a semi-crystalline morphology, with crystallinity levels often exceeding 60% and reaching up to 95% in highly ordered samples, driven by the packing of its chains into crystalline lattices. In the dominant α-crystalline form, the adopts a 3₁ helical conformation, which organizes into orthorhombic unit cells and imparts a degree of due to restricted chain mobility in the crystalline domains. Compared to other (PHAs), PHB is distinguished by its short methyl side chain (R = CH₃), which promotes higher crystallinity and rigidity relative to PHAs with longer alkyl side chains, such as poly(3-hydroxyhexanoate), that enhance flexibility and reduce brittleness.

Biosynthesis and Production

Microbial Biosynthesis Pathways

Polyhydroxybutyrate (PHB) is synthesized intracellularly by various as a carbon and compound, primarily under conditions of limitation combined with excess carbon availability. In microorganisms such as (formerly Ralstonia eutropha), PHB accumulation is triggered when essential nutrients like or are restricted, while carbon sources such as glucose are abundant. This metabolic shift redirects excess carbon flux toward PHB production, allowing cells to store up to 80% of their dry cell weight as PHB granules, serving as a reserve for survival during starvation. The biosynthetic pathway consists of three sequential enzymatic steps starting from , the central derived from carbon . First, 3-ketothiolase (encoded by phbA) catalyzes the reversible condensation of two molecules to form acetoacetyl-CoA. Next, acetoacetyl-CoA reductase (encoded by phbB) reduces acetoacetyl-CoA to (R)-3-hydroxybutyryl-CoA using NADPH as a cofactor. Finally, PHA (encoded by phbC) polymerizes (R)-3-hydroxybutyryl-CoA into PHB, releasing CoA. These enzymes assemble the (R)-3-hydroxybutyrate units into high-molecular-weight PHB chains, which are stored as insoluble granules within the . The genes encoding these enzymes are organized in the phbCAB , a polycistronic unit that ensures coordinated expression of phbC, phbA, and phbB in the order phbC-phbA-phbB. Transcription of the operon is upregulated under nutrient stress, integrating signals from environmental cues to activate PHB synthesis. Factors such as and further modulate enzyme activity and overall pathway efficiency; for instance, optimal pH around 6.8–7.0 and temperatures of 30–35°C enhance synthase performance and granule formation in C. necator. A representative yield for PHB production follows the simplified pathway: carbon substrate (e.g., glucose) → acetyl-CoA → PHB, with typical conversions achieving 0.3–0.5 g PHB per g of substrate consumed under optimized nutrient-limited conditions in C. necator.

Industrial Production Techniques

Industrial production of polyhydroxybutyrate (PHB) primarily relies on microbial processes, with Cupriavidus necator (formerly Ralstonia eutropha) as a key bacterium due to its high PHB accumulation capacity under nutrient-limited conditions. Conventional strategies employ two-stage fed-batch : an initial growth phase to build using balanced nutrients, followed by a production phase with excess carbon sources like glucose or vegetable oils to induce PHB synthesis, achieving intracellular PHB contents of 50-80% of dry cell weight. Downstream recovery typically involves solvent extraction with to dissolve PHB from , or enzymatic using proteases and lysozymes to disrupt cell walls, followed by precipitation and purification; however, these methods contribute significantly to costs, with historical production expenses ranging from $2-5 per kg due to high energy and solvent demands. Recent advancements from 2020 to 2025 have focused on cost reduction through sustainable feedstocks and . Waste substrates such as orange peel waste, shells, and agricultural residues like sugarcane have been utilized as carbon sources in fermentations with , yielding PHB accumulations up to 40-50% of dry cell weight and reducing raw material costs by 50-70% compared to pure glucose. approaches have engineered strains like harboring the phbCAB from native producers, enabling PHB production from inexpensive substrates such as or waste , with optimized pathways enhancing yield by redirecting metabolic fluxes. These innovations have lowered projected costs to below $3 per kg in scaled processes. Alternative plant-based production involves transgenic approaches to express bacterial PHB biosynthesis genes in crops. In , engineered lines have achieved PHB levels up to 4% of fresh weight in leaves, while transgenics reach 1.8% of leaf dry weight without major impacts on overall ; however, challenges in extraction from plant tissues and low accumulation rates limit commercial scalability compared to microbial methods. Global PHB production capacity reached approximately 34,000 tons in 2024, driven by key players like Danimer Scientific (acquired by Teknor Apex in July 2025), with projections estimating growth to over 119,000 tons by 2032 amid rising demand for biodegradable plastics.

Properties

Physical and Thermal Properties

Polyhydroxybutyrate (PHB) exhibits a of 1.23–1.25 g/cm³, which is notably higher than that of at approximately 0.90–0.91 g/cm³. This reflects PHB's compact molecular packing, influenced by its linear isotactic chain . The properties of PHB are characterized by a of around 175°C and a temperature typically ranging from -10°C to 5°C, enabling flexibility at low temperatures but rigidity above the . PHB demonstrates high crystallinity, often 50–70%, which arises from its regular chain conformation and results in challenging processability during melt due to a narrow temperature window between and degradation. occurs above 250°C, with onset temperatures reported around 220–270°C depending on processing conditions, limiting conventional thermal molding without additives. Optically, PHB appears translucent in its amorphous state but becomes opaque in the crystalline form due to light from spherulitic structures. Regarding solubility, PHB is insoluble in , contributing to its hydrophobicity, yet it dissolves readily in and hot solvents such as (DMSO).

Mechanical and Chemical Properties

Polyhydroxybutyrate (PHB) exhibits notable mechanical properties that position it as a stiff yet brittle , with a tensile strength of approximately 40 MPa and a of 3.5 GPa, rendering it comparable in rigidity to conventional engineering plastics but limited in . Its elongation at break typically ranges from 3% to 8%, highlighting its inherent brittleness under tensile loading, in contrast to (PET), which achieves a higher tensile strength of around 50 MPa and greater overall . This low extensibility arises from PHB's high crystallinity, which restricts chain mobility and promotes fracture propagation, though its processability is influenced by a relatively low near 175°C. In terms of , PHB shows moderate resistance to (UV) radiation, though prolonged exposure can lead to including chain scission and reduced mechanical properties. It is also hydrolytically stable in neutral aqueous environments, showing minimal chain scission or molecular over time. However, PHB exhibits sensitivity to strong bases, where alkaline conditions accelerate bond hydrolysis and reduce material durability. Additionally, its piezoelectric properties stem from the chiral, helical conformation of the chains, enabling the generation of under mechanical stress due to the asymmetric arrangement of polar groups. PHB possesses favorable barrier properties for certain gases, with oxygen permeability higher than that of (EVOH) copolymers but offering effective protection against oxidative ingress in applications. In contrast, its barrier is relatively poor, characterized by higher transmission rates that can limit utility in moisture-sensitive environments. To address PHB's , blending with poly(3-hydroxyvalerate) (PHV) has been employed as a modification strategy, which disrupts crystallinity and enhances chain flexibility, thereby increasing elongation at break to over 50% in forms like .

Applications

Biomedical Applications

Polyhydroxybutyrate (PHB) has garnered significant interest in biomedical applications due to its , biodegradability, and properties, making it suitable for and tissue-contacting devices. PHB exhibits non-toxicity and supports and proliferation without eliciting adverse immune responses, as demonstrated in various and studies. Its degradation products, primarily 3-hydroxybutyric acid, are naturally occurring metabolites in the , further enhancing its safety profile. , PHB degrades primarily through by macrophages, leading to surface erosion and gradual resorption over months to years, depending on implant size and . In surgical applications, PHB and its copolymers have been explored for resorbable sutures and implants. PHB-based sutures provide sufficient mechanical strength for myofascial , comparable to in rat models, and promote tissue regeneration without chronic inflammation. For cardiovascular applications, PHB patches have been implanted as pericardial substitutes and for closure in animal models, reducing postoperative adhesions by up to 27% and fully resorbing in 12–24 months via enzymatic and cellular activity. These patches support endothelial cell growth and vessel patency, leveraging PHB's mechanical durability for implant longevity. PHB microspheres enable controlled , particularly for hydrophilic therapeutics like insulin. Encapsulation of insulin in PHB or PHB-co-hexanoate (PHBHHx) nanoparticles achieves high efficiency, around 70%, with sustained release over weeks, minimizing burst effects and improving in diabetic models. This approach facilitates targeted delivery, reducing dosing frequency and side effects. For , PHB scaffolds promote regeneration by providing a porous, biocompatible matrix that enhances osteogenic differentiation. Recent advances include PHB-lignin/ nanofiber nanocomposites (2023), which improve mechanical properties and for defect repair . Similarly, PHB-based nanocomposites have been developed for dressings, accelerating through activity and moisture retention in preclinical models. These applications capitalize on PHB's ability to integrate with cells and degrade into non-inflammatory byproducts.

Packaging and Industrial Uses

Polyhydroxybutyrate (PHB) has emerged as a promising alternative to (PET) in packaging applications, particularly for films and bottles, due to its biodegradability and compatibility with renewable feedstocks. Blends of PHB with (PLA) exhibit enhanced mechanical properties suitable for , offering a sustainable substitute for conventional petrochemical-based materials while maintaining barrier functions against oxygen and moisture. In 2023, advancements in PHB-based blends, such as multilayer blown films incorporating (PBS) and nanocomposites, demonstrated improved oxygen barrier properties and mechanical strength, enabling their use in active to extend . Beyond packaging, PHB finds industrial applications in adhesives, where its thermoplastic nature allows formulation into biodegradable bonding agents for various substrates. It is also utilized in coatings for paper and metals, providing hydrophobic and protective layers that degrade environmentally without residue. For 3D printing, PHB filaments, often blended with cellulose or starch, enable the production of sustainable prototypes and biomedical scaffolds with good printability and biocompatibility. In agriculture, PHB-based mulching films help control weeds, retain soil moisture, and decompose naturally, reducing plastic pollution compared to traditional polyethylene films. Commercialization of PHB traces back to Biopol, a historical product developed by (ICI) in the 1980s as the first microbial PHB and resins for packaging like bottles, though production ceased in 1999 due to economic challenges. Modern equivalents include Nodax, a PHA copolymer produced by Danimer Scientific since 2007, tailored for flexible films and coatings in consumer goods. The global PHB market is projected to reach $643 million by 2032, driven by demand in and , reflecting a of 15.9% from 2024. Copolymers like poly(3-hydroxybutyrate-co-3-hydroxyvalerate) () enhance PHB's flexibility for bottle applications, allowing into durable, shatter-resistant containers with tunable mechanical properties based on hydroxyvalerate content. In 2022, PHB Industrial S.A. launched heat-resistant grades, expanding their viability for hot-fill packaging processes that require thermal stability up to 120°C.

History

Discovery and Early Research

Polyhydroxybutyrate (PHB) was first discovered in 1925 by French microbiologist Maurice Lemoigne at the , who isolated it as intracellular lipid granules from the bacterium Bacillus megaterium during studies on microbial autolysis. Lemoigne identified these granules as a polymerized form of , serving as a storage compound in the bacterial cells. In his seminal 1926 publication, he detailed the chemical nature of these "granules lipidiques," establishing their empirical formula as (C₄H₆O₂)ₙ and noting their properties upon extraction with hot . Research on PHB advanced significantly in the and , with key contributions from British microbiologists R.M. Macrae and J.F. Wilkinson, who examined its biosynthesis and degradation in washed suspensions of and B. megaterium. Their 1958 study demonstrated that PHB accumulates as discrete, plastic-like granules under nutrient-limited conditions, functioning as a carbon and energy reserve in . During this era, structural analyses, including early applications of (NMR) spectroscopy, confirmed PHB's isotactic configuration, consisting of repeating (R)-3-hydroxybutyrate units linked by bonds. These investigations also revealed PHB's insolubility in and its degradability via intracellular depolymerases, solidifying its biological role. By the 1960s, studies expanded to recognize PHB as the prototypical member of the polyhydroxyalkanoate (PHA) family, with publications identifying related polymers incorporating longer-chain hydroxyalkanoates in various bacterial species. Notable works, such as those by Lundgren et al., highlighted the diversity of PHA inclusions across genera like and , emphasizing their universal function as stress-response reserves. In the , initial explorations of PHB's practical utility emerged, particularly by (ICI) in the UK, which investigated its potential as a material through pilot-scale extraction and processing from bacterial cultures. This work laid the groundwork for commercial interest, focusing on PHB's processability without delving into large-scale production.

Commercial Development

The commercial development of polyhydroxybutyrate (PHB) gained momentum in the 1980s when (ICI) in the UK commercialized Biopol, a of PHB and polyhydroxyvalerate produced via large-scale bacterial using eutrophus (now ). Biopol was initially marketed for applications such as bottles and razor handles, marking the first industrial-scale production of a microbial polyester at capacities up to 300 tons per year. However, high production costs, primarily driven by expensive carbon sources like glucose and energy-intensive downstream processing, limited its market penetration; by the early 1990s, Biopol priced at approximately $16 per kg, far exceeding conventional plastics at under $1 per kg. The Biopol business, spun off from ICI to Zeneca, was sold to in 1996 amid these economic challenges; Monsanto discontinued its bioplastics operations in 1998. In 1996, Monsanto acquired ICI's Biopol technology and patents, aiming to integrate it into for cost-effective production. shifted focus toward engineering PHB biosynthesis into crop plants like soybeans and canola during the early , seeking to leverage for cheaper raw materials and avoid expenses. However, low yields in transgenic plants—typically under 5% dry weight—proved insufficient for commercial viability, prompting a pivot back to microbial methods. In 2001, sold the assets to Metabolix, a U.S. startup specializing in of for PHA production. Metabolix advanced pilot-scale microbial , achieving yields up to 80 g/L, but struggled with scaling and market adoption amid volatile oil prices and regulatory hurdles for genetically modified organisms. The company filed for protection in 2017, selling its PHA to for $10 million, which continues development under the name CJ Bio. As of 2025, CJ Bio has expanded with a 5,000-tonne annual capacity plant operational since 2022, launching innovative PHA products like PHACT for coatings and masterbatches, and winning the 2025 Bioplastics Innovation Award. Recent progress has revitalized PHB commercialization through cost-reduction strategies, including waste-based feedstocks like agricultural residues and by-products, which can lower raw material expenses by 40-50%. Current production costs have declined to $4-6 per kg, with projections estimating further reductions to below $3 per kg via optimized strains and integrated biorefineries. In , Biomer in has sustained small-scale commercial production since the 1990s, supplying medical-grade PHB granules, while Asian firms like TianAn Biopolymer in expanded capacities to over 2,000 tons annually by 2024. Key innovations include for efficient PHA extraction, such as US Patent 6,043,063 (2000), which uses non-halogenated solvents to recover up to 95% purity without toxic residues, facilitating sustainable scaling. These advancements position PHB for broader adoption in high-value sectors, with global market capacity exceeding 50,000 metric tons by 2024.

Biodegradation

Mechanisms of Degradation

Polyhydroxybutyrate (PHB) degradation is predominantly driven by enzymatic processes involving polyhydroxyalkanoate (PHA) depolymerases secreted by and aquatic microorganisms. These extracellular enzymes, notably from bacterial genera such as and , catalyze the of linkages in the PHB backbone through an exo-acting mechanism that preferentially initiates at the polymer chain ends, releasing (R)-3-hydroxybutyrate monomers or oligomers. The depolymerase adsorbs onto the hydrophobic PHB surface via its catalytic domain, while substrate-binding domains facilitate chain alignment for bond cleavage, ensuring efficient breakdown without intracellular uptake of the intact . This process is highly specific to PHAs, with enzyme activity optimized at neutral to slightly alkaline and moderate temperatures around 30–40°C. The kinetics of PHB degradation typically follow a surface erosion model, where microbial colonization and action progressively the material from the exterior, minimizing bulk changes until advanced stages. In environments, PHB half-lives range from 2 to 4 months under ambient conditions, though this can extend to 6 months or more in drier or cooler s; key influencing factors include (optimal at 25–30°C), levels (faster at 40–100% relative ), and microbial community density. In marine settings, degradation rates are often accelerated due to higher microbial diversity and salinity-tolerant depolymerase producers, with half-lives varying from 54 days in warm field conditions (29°C) to 116 days in lab benthic tests at 20°C; thicker samples may require several months. These rates underscore PHB's environmental responsiveness, with surface preventing fragmentation into during early degradation. Abiotic factors often precondition PHB for biotic degradation by inducing initial chain scission and increasing surface hydrophilicity. Thermo-oxidative degradation occurs under elevated temperatures (above 50°C) and oxygen exposure, generating peroxides and carbonyl groups that fragment the , while from UV (wavelengths 290–400 nm) causes Norrish-type reactions leading to radical formation and cross-linking or cleavage. These non-enzymatic processes weaken the crystalline structure of PHB, exposing more sites for microbial attachment and accelerating overall breakdown in combined abiotic-biotic scenarios. Recent advances include the development of engineered PHA depolymerases based on wild-type enzymes from species to improve efficiency in PHB hydrolysis for circular economy applications. In 2025, studies have identified new bacterial strains capable of degrading PHB and introduced surface modification strategies, such as cellulose triacetate coatings, to control biodegradation rates.

Environmental Considerations

Polyhydroxybutyrate (PHB) offers sustainability advantages over conventional plastics through its lifecycle, particularly in reducing and mitigating persistent . Lifecycle assessments indicate that PHB production from renewable feedstocks like biomethane results in a of approximately 1.6 kg CO₂ equivalents per kg, compared to 1.8 kg CO₂ equivalents per kg for (PP) under similar energy conditions. With optimized integration, PHB emissions can drop to negative values, such as -9.3 kg CO₂ equivalents per kg, highlighting its potential for during biomass growth. Furthermore, PHB's complete biodegradability in natural environments minimizes the accumulation of , addressing a key driver of and from non-degradable polymers like PP. Despite these benefits, PHB faces challenges in scalability that limit its broader environmental impact within the . Current production processes, reliant on microbial , struggle with high costs and low yields, hindering large-scale adoption to displace fossil-based plastics effectively. Additionally, the use of food-derived carbon sources such as sugars and starches for PHB synthesis raises concerns about competition with , potentially exacerbating issues in resource-limited regions. Recent advancements in 2024 and 2025 have focused on PHA strategies to enable closed-loop , transforming degraded PHB into reusable oligomers for new synthesis without loss of material quality. These approaches support a by reducing waste and enhancing recyclability, while PHB's deployment is increasingly recognized for curbing ocean through faster marine degradation compared to persistent alternatives. Regarding ecotoxicity, PHB degradation products are generally non-harmful to ecosystems, with studies showing no significant adverse effects on aquatic organisms and even positive influences on terrestrial environments. In , PHB stimulates microbial respiration and carbon turnover, thereby supporting overall without introducing toxic residues. This contrasts with conventional plastics, which contribute to long-term disruption via microplastic persistence. A 2025 review further confirms low ecotoxicity of PHA degradation products across environments.

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