Applied Sciences  
Article  
Fabrication and Biodegradability of Starch Cell-Plastics as Recyclable Resources  
[PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Fabrication and Biodegradability of Starch Cell-Plastics as Recyclable Resources.  
Accepted: 14 January 2021  
Submitted: 18 January 2021  
Recently, cell-plastics have been suggested as new raw materials [12; 14; 15]. Cell-plastics are bioplastics constructed of unicellular green algal cells, such as _Chlamydomonas reinhardtii_, as ingredients and biodegradable compounds such as glycerol, bovine serum albumin (BSA), and polybutylene succinate (PBS) as fillers. Cell-plastics have several properties as follows: (1) green algae such as _Chlamydomonas_ sp., which have a higher carbon assimilating ability than general terrestrial plants, are used as ingredients, resulting in easier preparation as a biomass resource [16]; (2) these cells are substantially rigid [17], providing cell-plastics with the requisite strength as a raw material; (3) there is no need to extract and refine the cell components as the end product because the cells can be directly used; and (4) unicellular cells can be freely placed and a filler can be used to connect each cell. Various cell-plastics have been fabricated so far, with cell-plastics composed of PBS exhibiting a tensile strength of 8.8 \(\pm\) 3.8 MPa and a Young's modulus of 240 \(\pm\) 90 MPa [14], similar to the properties of low-density polyethylene (LDPE), which has a Young's modulus of 165.3 MPa and a tensile strength of 10.3 MPa [18]. The mechanical characteristics of the PBS cell-plastics suggested its potential as a substitute to polypropylene (PP) and polyethylene (PE), which have a high demand and constitute \(\sim\)92% of plastics produced from petroleum, such as wrapping and sheets used as commodities [19]. Although these cell-plastics are fabricated using biodegradable cells and fillers, their degradation has not been studied. Therefore, the biodegradability of cell-plastics as carbon-recyclable bioplastic resources needs to be proven.  
In this study, cell-plastics were fabricated using the unicellular green alga _C. reinhardtii_ as an ingredient and starches as fillers. Raw starch is hydrophilic; therefore, its use is restricted to producing practical plastics because of its non-waterproof nature, unlike general plastics. In a previous study, PBS cell-plastics composed by adding cells into PBS showed enhanced water repellency than that displayed by PBS, presumably due to the lotus effect [14]. Thus, for the fabrication of starch cell-plastics, water repellency was expected by adding algal cells. Additionally, enhanced water repellency was achieved by using oxidized starch to control its hydrophilic property by changing the hydroxyl group to a carbonyl group in raw starch [20]. Next, cell-plastics were fabricated according to modification of contents as follows: (1) the ratio of the ingredient and the filler; and (2) the ratio of raw starch and oxidized starch. The fabricated cell-plastics were analyzed for their surface characteristics using scanning electron microscopy (SEM), mechanical characteristics using tensile tests, and biodegradability using \(\alpha\)-amylase. Although the biodegradation of starch by \(\alpha\)-amylase is well known [21], the biodegradation of starch cell-plastics has not been proven yet. This is the first report to describe starch cell-plastics from their fabrication to biodegradability and to introduce their potential as recyclable carbon resources.