Rapid biodegradation of microplastics generated from bio-based thermoplastic polyurethane

Abstract

The accumulation of microplastics in various ecosystems has now been well documented and recent evidence suggests detrimental effects on various biological processes due to this pollution. Accumulation of microplastics in the natural environment is ultimately due to the chemical nature of widely used petroleum-based plastic polymers, which typically are inaccessible to biological processing. One way to mitigate this crisis is adoption of plastics that biodegrade if released into natural environments. In this work, we generated microplastic particles from a bio-based, biodegradable thermoplastic polyurethane (TPU-FC1) and demonstrated their rapid biodegradation via direct visualization and respirometry. Furthermore, we isolated multiple bacterial strains capable of using TPU-FC1 as a sole carbon source and characterized their depolymerization products. To visualize biodegradation of TPU materials as real-world products, we generated TPU-coated cotton fabric and an injection molded phone case and documented biodegradation by direct visualization and scanning electron microscopy (SEM), both of which indicated clear structural degradation of these materials and significant biofilm formation.

Figure 1

Generating microplastics particles and tracking their biodegradation in compost. Microplastic particles were generated by sanding solid sheets of plastic materials and size selection of the resulting particulates by sieving. Particles smaller than 5 mm and larger than 350 µm were selected and mixed with fresh compost in equal mass ratios. Biodegradation of microplastic particles in compost was tracked by extraction and direct particle visualization. Additionally, biodegradation and mineralization of microplastics to CO₂ in compost was monitored by aerobic respirometry performed at 45 °C. Microbial enrichments were performed to isolate strains that utilize biodegradable microplastics as a carbon source and to identify possible depolymerization products.

Figure 2

Comparison of persistant EVA and transient TPU-FC1 microplastic particles. (a) Representative images of microplastic extraction filters. Microplastic particles were stained with Nile Red and illuminated with blue light for imaging. Particles appear bright against the darker filter background. Full color images and images with quantified microplastics using ImageJ are provided in Supplemental Fig. 1. (b) Particle counts of various microplastics materials over time in compost and background compost at Day 0, 90, and 200. Values represent the average of at least 3 independent microplastic extraction procedures. Two-way ANOVA, ****p < 0.0001; **p < 0.01; ns = p > 0.05.

Figure 3

Biodegradation of materials in compost monitored by CO₂ evolution respirometry. Percent theoretical biodegradation calculated as described in Materials and Methods. Dashed lines indicate 75% theoretical biodegradation at 45 days, which the cellulose control material must reach as an experimental validation condition of the ASTM5338 standard.

Figure 4

Characterization of bacteria capable of utilizing thermoplastic polyurethane as a sole carbon source. (a) Growth curves of various strains grown with TPU-FC1 as a sole carbon source in minimal media at 22 °C. (b–d) Growth of various strains on diacids, diol, and diamine monomers derived from the TPU-FC1 formulation, each compound was provided as sole carbon sources in minimal media at a concentration of 1 g/L. Data presented are the average of three biological replicates and error bars represent standard deviations. Note the change in y-axis scale for Panel D.

Figure 5

Depolymerization of TPU-FC1 into monomers and oligomers. GC chromatograms of cell-free supernatants from a 2-day old culture of Rhodococcus sp. 2b grown with TPU-FC1 as a sole carbon source (top) and a cell-free media control (bottom). Peaks 1 and 3 appeared only in the presence of Rhodococcus after 2 days incubation. Peak 2 is the diethyl succinate internal standard. Peak 4 was present in both the sample and control and was assumed to be a component of the media. Peak 1 was identified as the TPU-FC1 diol monomer based on its fragmentation pattern and compared against an authentic standard. Peak 3 was proposed to be a TPU-FC1 oligomer based on fragmentation structure prediction.

Figure 6

Scanning electron microscopy of biodegraded TPU products compared to controls. (a,b) TPU-coated fabric after 2-week incubation in compost. (c) Control image of TPU-coated fabric that was not placed in compost. (d,e) Injection molded TPU phone case after 12-month incubation in compost. (f) Control image of injection molded TPU phone case prior to placement in compost.