A polymer broken down in the body or the environment by hydrolysis or the action of microorganisms. Polylactic acid and polyglycolic acid are used in absorbable sutures and bone fixation devices.
Japanese name: 生分解性高分子
Biodegradable polymer appears in 1 practice question across 1 topic, including Biomaterials.
Field: Biomaterials
The term biodegradable polymer is used in two settings. In medicine it means materials that are broken down and absorbed in the body, also called bioresorbable materials. Typical examples are poly(lactic acid) (PLA), poly(glycolic acid) (PGA), their copolymer PLGA and polycaprolactone (PCL), used for sutures, bone fixation devices and carriers that release drugs gradually. In the environmental field it means plastics broken down by microorganisms ultimately to carbon dioxide, water and the like. The difference is that degradation in the body proceeds mainly by hydrolysis by water, whereas in the environment the action of microbial enzymes plays a large part.
In hydrolysis, polymer chains are first cut at random and the molecular weight falls, then strength is lost, and finally small fragments dissolve and the mass decreases. When water penetrates quickly into the interior and the whole material degrades almost uniformly, this is called bulk erosion; when the surface reaction is faster than water penetration and the material thins gradually from the surface, it is called surface erosion. Aliphatic polyesters such as PLA and PGA mainly undergo bulk erosion, and in large parts acid accumulates inside and degradation speeds up from the center. Materials that show surface erosion, such as polyanhydrides, are suited to carriers that release a drug at a constant rate.
The rate of degradation varies greatly with chemical structure, crystallinity, molecular weight and part size. PGA is highly hydrophilic and loses its strength within a few weeks, so it suits short-term sutures. Crystalline poly(L-lactic acid) (PLLA) has a glass transition temperature of about 60 °C and a melting point of about 170–180 °C, and takes several years to be fully absorbed in the body. PCL has a low glass transition temperature of about −60 °C, is soft at room temperature and also degrades slowly, on a scale of years. Depending on the use, the material and composition are chosen so that strength is kept until the bone or tissue heals and the material is absorbed afterward.
Biodegradable describes whether a material is broken down, and biobased describes whether its raw material comes from plants or other biomass; the two are separate matters. Polyethylene made from plants does not biodegrade, while petroleum-based PBAT and PCL do. Biodegradability tests are also carried out under defined conditions. For example, PLA degrades in industrial compost at about 58 °C, but in soil or seawater, below its glass transition temperature, it degrades only very slowly. Even products labeled biodegradable are not necessarily broken down in the environment where they are discarded, so it is important to check under what conditions they are biodegradable.
Answer: C. About 54 days
From exp(−kt) = 0.20, t = ln(1/0.20)/k = ln 5/0.030 = 1.609/0.030 ≈ 53.6 days, about 54 days. About 23 days uses the common logarithm log 5 ≈ 0.699 instead of the natural logarithm. About 27 days assumes the molecular weight falls linearly in proportion to time, giving 0.80/0.030. In reality, acid accumulating inside a large part speeds up degradation there and crystallization proceeds during degradation, so the fall deviates from a simple exponential. Also, strength is lost later than the molecular weight falls, and mass loss (resorption) begins even later. For this reason the composition and molecular weight of the material are chosen so that it keeps its strength until the bone heals and is resorbed afterward.
In medicine they are often used almost interchangeably, but strictly, biodegradable means the material is broken down, while bioresorbable also includes removal of the products from the body by metabolism or excretion. A material that only breaks into small pieces that remain in the body cannot be called bioresorbable.
Lactic acid and glycolic acid are metabolized to carbon dioxide and water, so they are usually not a problem. However, if a large part degrades in a short time, acid can build up locally and cause an inflammation-like reaction, so the rate of degradation and the size of the part are adjusted.
The basic method is copolymerization to change crystallinity and hydrophilicity; PLGA, made by copolymerizing glycolic acid with PLA, has a degradation period that can be changed through its composition. Molecular weight, crystallinity, part thickness and porosity also have an effect. Raising crystallinity or molecular weight slows degradation.
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