PHA

Polyhydroxyalkanoates (PHAs) are a family of naturally occurring polyesters produced by many bacteria as intracellular carbon and energy storage compounds. They accumulate as granules in the cytoplasm when microorganisms experience an excess of carbon and a limitation of other nutrients such as nitrogen, phosphate, or oxygen.

PHAs are diverse in monomer composition. The most common member is polyhydroxybutyrate (PHB), and many polymers are copolymers such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Monomer units are typically hydroxyalkanoates, and the polymer is synthesized by PHA synthase enzymes that polymerize these units. The properties of PHAs range from stiff and brittle to flexible and elastomeric, depending on the monomer composition and molecular weight.

Properties and applications: PHAs are thermoplastic, biodegradable, and generally biocompatible. They can be processed by conventional plastic methods such as extrusion and injection molding. Applications span packaging films and disposable items to medical devices, sutures, and tissue engineering scaffolds. Biodegradation occurs via microbial action in soil, water, and compost environments, with rates influenced by environment, polymer composition, and crystallinity. Some PHAs decompose under industrial composting conditions, while home compostability is variable.

Production and economics: PHAs are produced by bacterial fermentation using renewable or low-cost feedstocks, followed by extraction and purification. Costs remain higher than conventional petroleum-based plastics, though advances aim to improve yield, reduce processing energy, and enable mixed-weedstock fermentation.

Environmental and regulatory context: PHAs offer an alternative to conventional plastics with reduced persistent environmental impact when disposed of properly. Regulatory acceptance varies by polymer grade and application, with ongoing research to expand medical and food-contact approvals.