Is white pollution difficult to solve? Try this "living" plastic!

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

The invention of plastic has brought great convenience to our daily lives. However, the large-scale generation of plastic waste and improper disposal methods have made plastic waste, also known as white pollution, one of the most serious environmental problems today.

The reason why the white pollution problem is difficult to solve is that petroleum-based plastics take hundreds of years to degrade in nature, causing soil and environmental pollution. To solve white pollution from the source, biodegradable plastics (such as polylactic acid PLA) should be used to replace petroleum-based plastics.

In order to speed up the degradation of degradable plastics, Chinese scientists have invented a "living" plastic. They have genetically edited microorganisms to produce spores that are tolerant to extreme environments and can secrete plastic-degrading enzymes under specific conditions. The spores are then embedded in a plastic matrix through plastic processing methods.

In daily use, the spores remain dormant and the plastic can maintain stable performance. Only under certain conditions (surface erosion, composting) will the spores in the plastic be activated and the degradation process will be started, completing the complete degradation of the plastic.

Programming biodegradable "living" plastics by engineering spores

(Image source: Reference 1)

Non-degradable plastics

Plastic is a synthetic polymer material, and its history can be traced back to the late 19th century. In the mid-20th century, with the development of the petrochemical industry, the production cost of plastics was greatly reduced, and the application scope of plastics was further expanded, becoming an indispensable part of modern society. The widespread use of plastics has also brought many environmental problems. The white pollution that can be seen everywhere has seriously threatened the survival and development of the earth and human beings.

Why is it so difficult to degrade plastic? Because plastic is a high molecular polymer that has only appeared in the past 100 years. 100 years may not be short for humans, but it is just a blink of an eye for nature. In such a short time, nature has not yet evolved microorganisms that can quickly degrade these "new things".

According to statistics, traditional petroleum-based plastics, such as polypropylene (PP), high-density polyethylene (HDPE), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET), take more than 100 years to degrade.

How long does it take for various plastics to degrade?

(Image source: Reference 2)

Degradable plastics

As white pollution becomes increasingly serious, humans have also realized the urgency of the problem and started looking for alternatives to petroleum-based plastics. my country has also introduced many policies to restrict the use of non-degradable plastics, from "plastic restrictions" to "plastic bans". In this context, degradable plastics are gaining more and more attention. Petroleum-based plastics take hundreds of years to degrade, and can basically be defined as "non-degradable plastics" for humans.

There is a class of bio-derived polymers that are called "degradable plastics" because there are microorganisms and enzymes in nature that can quickly degrade these polymers. They can be naturally degraded in less than a year, so they are called "degradable plastics", such as polylactic acid (PLA), polybutylene adipate/terephthalate (PBAT), polyhydroxyalkanoic acid (PHA), polybutylene succinate (PBS), polycarbonate (PCL), etc.

Currently, the use of degradable plastics to replace petroleum-based plastics has become a development trend at home and abroad. For example, supermarkets provide degradable plastic bags for a fee, degradable plastic straws in the catering industry, and degradable surgical sutures used in hospitals for surgery (no need to remove stitches).

Degradable plastic bags

(Photo source: veer photo gallery)

Improving the degradation rate of biodegradable plastics

In order to increase the degradation rate of degradable plastics, increasing the amount of degrading enzymes is the key. If the degrading enzymes can be put into the plastics, they will be automatically released when the plastics are discarded, and the degradation rate can be increased by using enzymes to degrade the plastics. However, how to store the degrading enzymes in the plastics so that they will not degrade when they are used normally, but will start to degrade when they are discarded? Researchers thought of a special structure of bacteria - spores.

In the billions of years of natural evolution, many microorganisms have evolved resistance to harsh environmental conditions. When extreme environments come and are no longer suitable for survival and reproduction, bacteria will transform into spores, which can give bacteria super strong resistance. Spores can withstand extreme dryness, temperature and pressure, which happen to exist in the environment of plastic processing.

Therefore, Dai Zhuojun's team from the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, proposed to modify Bacillus subtilis through synthetic biology methods, introduce the genetic circuit for controllable secretion of plastic-degrading enzymes (Holderella cepacia lipase, Lipase BC) into Bacillus subtilis, and force Bacillus subtilis to "hibernate" in an environment of divalent manganese ions to form a spore form.

The generated spores also carry the edited gene circuits and are more tolerant to high temperature, high pressure, organic solvents and drying than bacteria. A series of spore-containing plastics were prepared by directly mixing the genetically engineered spore solution with polycarbonate PCL plastic masterbatch through high-temperature melt extrusion or organic solvent methods.

In various tests of physical properties, there is no significant difference between the "living" plastic and ordinary PCL plastic in parameters such as yield strength, stress limit, maximum deformation and melting point. In daily use environment, the spores remain dormant and the plastic can maintain stable performance. Without the addition of any other exogenous agents, the "living" plastic in the soil environment can be completely degraded within 25-30 days, while traditional degradable PCL plastics take about 55 days to be degraded to the point where they are invisible to the naked eye.

In order to verify the universality of the system, the researchers continued to try other plastic systems, mixing spores with polymer materials such as PBS, PBAT, PLA, PHA and PET to prepare corresponding "living" plastics.

Degradation process of "living" plastic in soil

a. “Living” plastic can be completely degraded in soil in 30 days; b. Ordinary PCL plastic can be completely degraded in soil in 55 days

(Image source: Reference 1)

The researchers also immersed the "living" plastic in a common carbonated beverage environment for two months. Without any external influence, the "living" plastic was able to maintain a stable shape, indicating that living plastics can be used like traditional plastics, and the degradation process will only start when they are destroyed or discarded. This study provides a new perspective and method for the development of new biodegradable plastics, which is expected to help solve the current serious plastic pollution dilemma.

Conclusion

The invention of "living" plastics provides new ideas and solutions to solve the global problem of white pollution. Through bioengineering technology, scientists have successfully combined the natural evolutionary advantages of microorganisms with modern materials science to create a plastic that can self-degrade under certain conditions. This innovative achievement not only demonstrates the possibility of sustainable development in theory, but also brings practical hope for reducing the environmental impact of plastic waste in practice.

However, the promotion and application of "living" plastics still need to overcome many challenges, including production costs, technical maturity, and social acceptance of large-scale applications. Only with the dual promotion of scientific and technological progress and policy guidance can this new material truly enter the market and become a powerful tool to deal with white pollution.

In the future, we look forward to more technological innovations like this to reduce plastic pollution at the source and achieve harmonious coexistence between man and nature. Let us work together to contribute to protecting the earth's environment.

References:

1.Tang et al. Degradable living plastics programmed by engineered spores. Nature Chemical Biology. 2024, 206, 20.

2.Choi et al. Sustainable production and degradation of plastics using microbes. Nature Microbiology. 2023, 8, 2253–2276.

3.Geyer, et al. Production, use, and fate of all plastics ever made. Science Advance. 2017, 3, e1700782.