Biological upcycling of PET: Opportunities, limitations, and design principles

Poly(ethylene terephthalate) (PET) is one of the most widely produced synthetic polymers, yet its recycling remains constrained by energy demand, contamination, feedstock heterogeneity, and, in some routes, material downcycling. In recent years, biological PET upcycling has emerged as a strategy to couple PET depolymerization, whether performed by isolated enzymes or by enzymes produced in microbial hosts, with microbial assimilation of the resulting monomers to generate higher-value chemicals, materials, and biopolymers. This review examines the progression of PET bio-upcycling strategies, from hybrid chemical-biological systems to increasingly integrated biological processing schemes. The performance of these systems is determined by multiple interacting factors, including enzyme stability, PET crystallinity, microbial metabolism, intermediate toxicity, and the balanced assimilation of terephthalic acid (TPA) and ethylene glycol (EG). This review integrates mechanistic, physiological, and process-level insights with techno-economic considerations to identify the principal constraints that limit current approaches and distil a set of design principles that provide a framework for the rational design of scalable PET bio-upcycling systems.