Many commercially available polymers have some associated environmental problems. The increase in the production and subsequent disposal of synthetic polymers causes severe environmental pollution because they do not deteriorate in the natural environment. Accordingly, researchers have begun to focus on developing biodegradable polymers as environmentally benign materials degraded by microorganisms into carbon dioxide. Furthermore, the production of synthetic polymers contributes to the depletion of fossil resources. To reduce fossil resource usage, researchers have also begun to develop bio-based polymers derived from biomass resources.
Different type of biodegradable polymer
Biosynthetic poly((R)-3-hydroxyl butyrate) (P3HB), which many microorganisms can use as an energy source, has been developed as a potentially biodegradable polymer. Chemosynthetic polyesters, i.e., poly(lactic acid) (PLA), polycaprolactone (PCL), poly(butylene succinate) (PBS), and poly(butylene adipate-co-butylene terephthalate) (PBAT), have also been developed as biodegradable polymers since the latter part of the twentieth century. Some of them are manufactured from biomass and used commercially.
Natural Biodegradable Polymers
Natural polymers are also used as biodegradable polymers with or without modification. For instance, starch modified by glycerol or chemosynthetic polymers, i.e., PCL and cellulose partially esterified with a fatty acid, are thermoplastic and biodegradable. The low mechanical and thermal properties of commercially available biodegradable polymers limit their adoption. Therefore, these biodegradable polymers have only been used as alternatives to general-purpose polymers. PLA, a relatively high glass transition temperature (Tg), flexural strength, and flexural modulus, i.e., 60 °C, 80–100 MPa, and 3 GPa, is used in applications requiring rigid material. However, the biodegradation of PLA is limited to high-temperature compost environments, and PLA does not show biodegradability in the natural environment.
Natural Protein Biodegradable polymers
Natural protein materials, such as silk, have been used for fiber material since ancient times. They are bio-based and degrade in the natural environment. Although soybean isolate is an abundant protein resource, it is impossible to use it industrially due to poor processability. Therefore, chemical modification and polymer blending procedures were studied to endow the material with moldability and improved mechanical properties while maintaining biodegradability.
Natural structural proteins, such as elastin, resilin, mussel byssus thread, squid suckering, silks produced by various insects, and others are gaining attention due to their remarkable mechanical properties. For instance, some spider species have silk fiber with a tensile strength of 1.1 GPa and toughness of 160 MJ m−3, more robust and more challenging than any commercially available fiber. Additionally, the biodegradability of naturally occurring structural proteins has been demonstrated in some environments. For example, wool susceptibility to fungal breakdown and degradation of regenerated silkworm silk has been established. On the other hand, while natural protein material performance is high, the mammals mentioned earlier and insects’ low productivity prevent commercial use. To manufacture structural protein materials, microbial fermentation using genetic engineering has been developed.
Genetic engineering allows customization of properties, i.e., processability, mechanical properties, thermal properties, and hydrophilicity, even beyond what is observed in natural proteins. Commercial-scale production of protein materials is just starting globally.
Spiber Inc. is developing recombinant structural protein under the trade name Brewed Protein from sugar via a microbial fermentation process. The recombinant structural protein material can be spun or molded into the fiber, sheet, and bulk material and used as an alternative to conventional plastics. Its structure and amino acid sequence control the mechanical properties of protein material. Protein material is composed of natural amino acids linked together by peptide bonds.
Testing of biodegradable protein plastic
Although amino acids can be metabolized to inorganic compounds by micro-organisms, suggesting that protein material is potentially biodegradable in the natural environment, the cleavage of peptide bonds in a given bulk protein material is not entirely ensured. To employ any novel polymer as a biodegradable polymer, it is necessary to evaluate the natural environment’s degradability.
In this study, we evaluated the thermal and mechanical properties of the recombinant structural protein material BP1 using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and three-point flexural testing, and we evaluated the biodegradability of BP1. Hydrolysis by isolated bacteria was evaluated using a straightforward zone method with BP1-containing emulsified media. The enzymatic degradation was assessed using compression-molded sheet samples. The aerobic biodegradability of BP1 in an aqueous medium using a soil inoculum was determined by estimating the oxygen demand in a closed respirometer.
Bio-based and biodegradable materials are essential to establish a circular economy in pursuit of a more sustainable society. The recombinant structural protein (BP1) tested in this study is a bio-based material, as it is manufactured via a microbial fermentation process with glucose as the feedstock. The mechanical properties of currently available biodegradable materials are suitable for use as general-purpose polymers. Because the glass transition temperatures of commercially available biodegradable plastics except PLA are below room temperature, they are pliable at room temperature and thus used as polyethylene alternatives. Although the glass transition temperature of PLA is 60°C and it is relatively rigid at ambient temperatures, it cannot degrade in the natural environment—only in compost at high temperatures.
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