Polyhydroxyalkanoates (PHA)
Polyhydroxyalkanoates (PHA) are a family of biodegradable polyesters synthesized by numerous microorganisms as intracellular carbon and energy storage compounds. Unlike synthetic plastics, PHAs are fully biocompatible, compostable, and derived from renewable resources, making them a sustainable alternative to petroleum-based polymers like polyethylene (PE) and polypropylene (PP). Over 150 different PHA monomers exist, allowing tunable properties from rigid thermoplastics to flexible elastomers.
PHAs are classified by monomer chain length: short-chain-length (scl-PHA, e.g., poly(3-hydroxybutyrate) or PHB, C3-C5), medium-chain-length (mcl-PHA, C6-C14), and long-chain-length (lcl-PHA). PHB, the most common, mimics PP in properties but degrades completely in soil, marine, or compost environments.
Polyhydroxyalkanoates (PHA) – Biosynthesized by Bacteria
PHA Life Cycle – From Waste to Biodegradable Plastic
Synthesis of PHA
PHAs are biosynthesized via microbial fermentation of carbon-rich substrates under nutrient-limited conditions (e.g., nitrogen or phosphorus limitation), inducing accumulation up to 90% of cell dry weight. Common producers include Cupriavidus necator, Pseudomonas spp., Bacillus spp., and halophiles like Haloferax mediterranei.
Industrial methods favor mixed microbial cultures (MMCs) for cost reduction, using waste streams (e.g., agro-industrial residues) in feast/famine cycles to boost yields. Downstream processing involves cell disruption (e.g., solvent extraction, enzymatic digestion) for polymer recovery.
- Bacterial Fermentation: Primary method; bacteria convert sugars, lipids, or wastes into PHA granules via acetyl-CoA pathways (e.g., PhaA, PhaB, PhaC enzymes).
- Substrate Influence: Glucose yields PHB; vegetable oils produce mcl-PHAs; wastes like rice husk or glycerol enable low-cost production (up to 70% yield).
- Emerging Approaches: Transgenic plants or recombinant E. coli for in planta synthesis; halophilic cultures reduce sterilization costs.
The biosynthetic pathway: Glucose → Acetyl-CoA → Acetoacetyl-CoA → 3-Hydroxybutyryl-CoA → PHA (n monomers). Yields: Up to 9.8 g/L PHB from pretreated rice straw.
1. Carbon Source Preparation
Waste substrates (e.g., crop residues) are hydrolyzed for fermentation.
2. Fermentation
Microbes accumulate PHA under stress: C6H12O6 + nutrients → PHA granules + biomass.
3. Extraction
Cells lysed via solvents (e.g., chloroform) or mechanical methods; purity >95%.
Properties of PHA
- Biodegradable in diverse environments (soil, sea, compost)
- Biocompatible and non-toxic
- Tunable: From brittle (PHB) to elastomeric (mcl-PHA)
- Processable via extrusion, molding, 3D printing
- Excellent barrier to moisture, oxygen, aromas
| Property | Value (PHB) | Comparison |
|---|---|---|
| Density | 1.23–1.25 g/cm³ | Similar to PP (0.90 g/cm³) |
| Glass Transition Temperature (Tg) | 0–5 °C | Lower than PET (70 °C) |
| Melting Temperature (Tm) | 175 °C | Comparable to LDPE (110 °C) |
| Tensile Strength | 40 MPa | Similar to PP (30 MPa) |
| Young’s Modulus | 3.5 GPa | Stiffer than PE (1 GPa) |
| Elongation at Break | 3–8% | Brittle vs. PP (400%) |
| Biodegradability | 100% in 6 months (compost) | Superior to PET (centuries) |
Applications of PHA
1. Packaging
- Food trays, films, bottles; compostable bags
- Single-use items: Straws, cutlery
2. Biomedical
- Sutures (e.g., P4HB), stents, scaffolds
- Drug delivery microspheres; tissue engineering
- Wound dressings, bone/cartilage implants
3. Agriculture
- Mulch films, controlled-release fertilizers
- Biocontrol agents for pest management
4. Other Industries
- Textiles (fibers), electronics casings
- Consumer goods: Cosmetics, diapers
Environmental Impact & Sustainability
PHAs significantly reduce plastic pollution: Fully biodegradable (no microplastics), carbon-neutral lifecycle (CO2 from biomass offsets emissions).
- Renewable Feedstocks: Wastes (e.g., glycerol, crop residues) minimize fossil fuel use
- Low Carbon Footprint: 0.5–1.5 kg CO₂/kg PHA vs. 3–4 kg for PE
- Versatile Degradation: 3–6 months in compost/soil; marine-safe
- Circular Economy: Recyclable via chemical/biological routes; supports waste valorization
Challenges
- Cost barriers despite regulatory support (e.g., EU plastic bans)
- Brittleness in some types; requires blending
- Scalability: Production expected to quadruple by 2025
- Feedstock competition with food crops (mitigated by wastes)
Related Topics
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