A Circular Plastics Concept That Applies Underutilized Biomass and Cell-Plastics Technology in Japanese Industries and Regions

Bioplastics are increasingly expected to function not only as alternatives to fossil-derived plastics but also as components of circular plastic systems. However, currently bioplastics remain limited by cost, feedstock availability, achievable biomass content, and end-of-life compatibility. This review examines these limitations by organizing recent technological and policy trends in bioplastics, with particular attention to Japan’s social and industrial infrastructure. On this basis, we discuss a systems-level framework for circular plastics that integrates regionally underutilized non-edible biomass, decentralized production concepts, and the emerging possibility of cell-plastics based on unicellular green algae. We argue that the practical dissemination of biomass plastics requires not only material development but also compatibility with molding processes, recycling and biodegradation pathways, and regional collection and treatment systems. In this context, cell-plastics derived from Chlamydomonas reinhardtii are positioned as an emerging technological platform for direct biomass utilization and interfacial material design, although their large-scale implementation remains limited by current cultivation and manufacturing constraints. We propose that circular biomass-plastics systems in Japan should be developed as regionally adapted production frameworks with clearly defined end-of-life pathways, rather than as simple substitutes for petroleum-derived plastics.