Biodegradable polymers are materials that can be broken down by living things. Most polymers are made to last a long time, but biodegradable polymers are not. These polymers can come from natural materials, petroleum-based chemicals, or a mix of both.

Polymers are the main part of most plastics, so talking about biodegradable plastics and polymers is closely connected. The terms "bioplastic" and "biodegradable polymer" are similar but not the same. Some bioplastics (plastics made partly or fully from plant or animal matter) are not biodegradable, and some biodegradable plastics are made entirely from petroleum. As more companies want to appear environmentally friendly, they are exploring and using bioplastics. However, the definition of bioplastics is still being discussed. The term is often used for a wide variety of products that may be made from natural materials, biodegradable, or both. This could mean that plastics made from oil might be called "bioplastics" even if they have no natural parts. Some people are unsure if bioplastics will solve environmental problems as expected.

History

Early work on biodegradable materials came before the time when synthetic polymers were made. These synthetic polymers need oil-based chemicals. This early work focused on natural materials or materials made from them. One of the first medical uses of a biodegradable polymer was the catgut suture, which was used as early as 100 AD. The first catgut sutures were made from sheep intestines, but modern versions are made from purified collagen taken from the small intestines of cattle, sheep, or goats.

In the 1830s, cellulose was changed into gun cotton (cellulose nitrate) and then into cellulose acetates. These are likely the first biodegradable (semi-synthetic) polymers. Early research on a biopolymer called polyhydroxyalkanoate (PHA) helped prepare the way for its use in industry. Later attempts by W.R. Grace & Co. (USA) did not succeed. In 1973, when OPEC stopped sending oil to the United States to raise oil prices, efforts to make polyhydroxybutyrate (PHB) using the bacteria Alcaligenes latus by Imperial Chemical Industries (ICI UK) also ended. The PHA produced in this case was a type called short-chain-length polyhydroxyalkanoate (scl-PHA). Work on PHA continues today. A related material is polylactic acid (PLA). Studies on how to make PLA from lactic acid and its related materials began at DuPont in the 1930s. In the 1970s, a mixture of PLA and polyglycolic acid led to the creation of Vicryl, a type of absorbable suture material.

The idea of synthetic biodegradable plastics and polymers was first introduced in the 1980s. In 1992, an international meeting was held where experts on biodegradable polymers discussed a definition, standards, and testing methods for these materials. Organizations like the American Society for Testing of Materials (ASTM) and the International Standards Organization (ISO) were also created to oversee these efforts. In the late 2010s, some clothing and grocery store companies began using biodegradable bags.

Large-scale production of biodegradable polymers started in the late 1990s.

Types of biodegradable polymers

Most biodegradable polymers are polyesters. The ester group (RC(O)OR') can break down in water or with the help of enzymes. It is also important to consider whether additives used with polymers can break down naturally.

Biologically made polymers come from natural sources like plants, animals, or microorganisms.

Polyhydroxyalkanoates (PHAs) are a type of biodegradable plastic made naturally by some microorganisms, such as Cupriavidus necator. Examples of PHAs include poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV), and polyhydroxyhexanoate (PHH). PHAs are created when microorganisms are given too much carbon and lack other nutrients like phosphorus, nitrogen, or oxygen. Once PHA is formed, it is removed by breaking open the microorganisms.

PHAs can be divided into two groups:
– scl-PHA: Made from short-chain fatty acids (3 to 5 carbon atoms) by bacteria like Cupriavidus necator and Alcaligenes latus (PHB).
– mcl-PHA: Made from medium-chain fatty acids (6 to 14 carbon atoms) by bacteria like Pseudomonas putida.

Scientists are working to improve the production of PHAs using synthetic biology.

Polylactic acid (PLA) is a type of polyester made from renewable materials like corn, cassava, sugarcane, or sugar beet pulp. In 2010, PLA was the second most widely used bioplastic globally.

PLA can break down in composting conditions but is not considered biodegradable outside of these artificial environments according to American and European standards.

Starch blends are made by mixing starch with plasticizers. Starch alone is brittle, so plasticizers are added to make it more flexible. However, not all plasticizers are biodegradable, so the biodegradability of the blend depends on the plasticizer used.

Biodegradable starch blends include combinations like starch/polylactic acid, starch/polycaprolactone, and starch/polybutylene-adipate-co-terephthalate. Blends like starch/polyolefin are not biodegradable.

Cellulose-based bioplastics include materials like cellulose acetate and nitrocellulose, as well as their derivatives, such as celluloid. When modified, cellulose can become thermoplastic.

Common non-biodegradable petroleum-based plastics include polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). These plastics are used for their strength, such as PVC in sewage pipes. Some petroleum-based chemicals are used to make biodegradable polymers.

Polyglycolic acid (PGA) is a thermoplastic made from glycolic acid. It is used in medical applications, like sutures, because it breaks down safely in the body. The chemical bonds in PGA make it unstable in water, allowing it to break down into harmless glycolic acid. Enzymes can speed up this process. Glycolic acid is then processed in the body and excreted as water and carbon dioxide. PLA and blends of PLA and PGA are also biodegradable.

Polybutylene succinate (PBS) is made from succinic acid and 1,4-butanediol. It is used in food and cosmetic packaging and as a biodegradable film in agriculture. Some bacteria, like Amycolatopsis sp. HT-6 and Penicillium sp. strain 14-3, can break down PBS. Other microbes, such as Microbispora rosea and Excellospora species, can also consume PBS.

Polycaprolactone (PCL) is made by combining caprolactone molecules. It is used in medical implants. Some bacteria, like Bacillota and Pseudomonadota, can break down PCL. Penicillium sp. strain 26-1 and Aspergillus sp. strain ST-01 can also degrade PCL, though more slowly. Clostridium species can break down PCL in environments without oxygen.

Polybutylene adipate terephthalate (PBAT) is a biodegradable plastic made from butanediol, adipic acid, and terephthalic acid.

Polyethylene terephthalate (PET) is not biodegradable, despite being a polyester. Over 80 million tons of PET are produced yearly. Some bacteria can break down PET, but the process is very slow.

Poly(vinyl alcohol) (PVA) is a biodegradable vinyl polymer that dissolves in water. Because water is a safe and inexpensive solvent, PVA is used in many areas, such as 3D printing, food packaging, textiles, paper coatings, and healthcare products.

Many biodegradable polymers, both natural and synthetic, have been developed for medical use. Examples include polyanhydrides, polyacetals, poly(ortho esters), polyurethanes, polycarbonates, and polyamides.

Biodegradation pathways and mechanisms

Most biodegradable polymers are polyesters. These materials break down through a process called hydrolysis, which uses water to split the polymer into smaller parts, such as a carboxylic acid (RCO2H) and an alcohol (ROH).

When polyesters are made from hydroxyl carboxylic acids (like PLA, PCA, and PHB), the carboxylic acid and alcohol parts come from the same single monomer. This makes the hydrolysis equation simpler.

At neutral pH, the carboxylic acid in the molecule becomes a carboxylate ion. Hydrolysis can occur through chemical methods (without enzymes) or with the help of enzymes. These enzymes are released from cells or released when cells break apart. Polymers are too large to enter cells. Chemical hydrolysis is often slow, but acids, bases, and mineral surfaces can speed up the process. Once polyesters fully break down into monomers, these smaller molecules can enter cells, where they are broken down further. Microbial degradation is sometimes described as a three-step process. In the end, biodegradation produces water (H2O) and carbon dioxide (CO2).

Biodegradability depends on the environment. Whether a plastic item breaks down depends not only on the material itself but also on conditions like temperature, the item’s size, and the presence of specific microorganisms. Synthetic polyolefins, such as polyethylene, are among the hardest plastics to break down.

Low plastic recovery rates often happen because conventional plastics mix with organic waste (like food scraps, wet paper, and liquids), leading to more waste in landfills and natural areas. Composting mixed organic materials (such as food scraps, yard trimmings, and wet, non-recyclable paper) can help reduce waste and improve recycling efforts. In 2015, food scraps and wet, non-recyclable paper made up 39.6 million and 67.9 million tons of municipal solid waste, respectively.

Applications and uses

Biodegradable polymers are important in medicine, agriculture, and packaging.

A major area of research involves using biodegradable polymers for controlled drug delivery. For a biodegradable polymer to be used in medicine, it must meet these conditions:

• It must not be harmful to the body to avoid immune reactions;
• The time it takes to break down must match the time needed for treatment;
• The materials left after breakdown must not harm cells and must leave the body easily;
• The material must be easy to shape to fit the needs of the task;
• It must be easy to clean to ensure safety;
• It must remain stable and usable for a long time before use.

Biodegradable polymers and biomaterials are also used in tissue engineering and regeneration. Tissue engineering uses artificial materials to help grow new tissue. These systems can create tissues and cells in a laboratory or use a biodegradable scaffold to build new structures and organs. A biodegradable scaffold is preferred because it reduces the risk of the body rejecting the material. While some advanced systems are not yet ready for human use, studies on animals have shown success. For example, scientists grew rat smooth muscle tissue on a scaffold made of polycaprolactone and polylactide. Future research may allow this technology to help repair, support, or improve human tissues. A major goal of tissue engineering is to create organs, like kidneys, from basic parts. A scaffold helps grow the organ, and the polymer eventually breaks down and leaves the body safely. Scientists have used polyglycolic acid and polylactic acid to create blood vessel tissue for heart repair. These scaffolds help form healthy arteries and blood vessels.

Biodegradable polymers are also used in orthopedic applications, such as replacing bones and joints. Many non-biodegradable materials, like silicone rubber, polyethylene, and acrylic resins, have been used for these purposes. These materials often act as biocompatible cement to fix prostheses or replace joints. Other biodegradable materials include polyglycolide, polylactide, polyhydroxobutyrate, chitosan, hyaluronic acid, and hydrogels. For example, poly(L-lactide) (PLA) is used to make screws and darts for repairing meniscal tissue. It is sold under the name Clearfix Meniscal Dart/Screw. PLA breaks down slowly, taking more than two years to be absorbed by the body.

Biodegradable plastics can replace traditional plastics that remain in landfills and reduce plastic pollution. A 2010 report by the United States Environmental Protection Agency (EPA) found that the U.S. produced 31 million tons of plastic waste, which was 12.4% of all household waste. Only 2.55 million tons were recovered, or 8.2% of the total. This recovery rate was much lower than the 34.1% recovery rate for all household waste.

Biodegradable plastics can help reduce waste by allowing composting to handle large amounts of nonrecoverable waste. Compostable plastics are useful because they are lightweight, strong, and inexpensive, and they fully break down in industrial composting facilities. Supporters say certified biodegradable plastics can be mixed with organic waste, making it easier to compost more waste than recycling.

Using biodegradable plastics allows large amounts of household waste to be fully recovered through composting and other methods, rather than being buried in landfills or burned.

In addition to medicine, biodegradable polymers help reduce waste in packaging. Efforts are being made to replace petroleum-based materials with biodegradable ones. One common polymer used in packaging is polylactic acid (PLA). PLA can be shaped into films, wrappings, and containers, including bottles and cups. In 2002, the FDA approved PLA for use in all food packaging. BASF sells a product called ecovio®, which is a mix of a compostable polyester and PLA. This material is used for plastic films like shopping bags and organic waste bags. It can also be used for other products, such as thermoformed or injection-molded items. This versatile polymer can even be used to coat paper or make foam-like products.

Regulations/standards

To ensure that products labeled as "biodegradable" are accurate, specific standards have been created. In the United States, the Biodegradable Products Institute (BPI) is the main group that certifies these products. ASTM International sets rules for testing biodegradable plastics, including tests done with and without oxygen, as well as in ocean environments. A group called Committee D20.96, part of ASTM International, is responsible for these rules. The current ASTM standards include two types: specifications, which determine if a product meets requirements, and test methods, which describe how to conduct tests for specific time periods and safety levels.

Under anaerobic conditions (without oxygen), a product must have at least 70% of its material broken down within 30 days (ASTM D5511-18) or during the testing period (ASTM D5526-18) to be considered biodegradable. These test methods explain how to perform tests but do not decide if the results are successful or not.

For aerobic composting (with oxygen), plastics must fully break down into carbon dioxide, with at least 90% of the material turned into carbon dioxide within 180 days (about 6 months). These specifications include clear criteria to determine if a product passes or fails.

Similar rules apply in Europe, where 90% of the material must break down into carbon dioxide within 6 months. In November 2022, the European Commission proposed new rules to replace an older law from 1994 and to clarify labels for biobased, biodegradable, and compostable products.

In October 2020, British Standards introduced new rules for biodegradable plastics. To meet these rules, plastics must break down into a wax-like material that contains no microplastics or nanoplastics within two years. This breakdown can be caused by sunlight, air, or water. Niall Dunne, chief executive of Polymateria, stated that his company developed polyethylene film that breaks down in 226 days and plastic cups that break down in 336 days.