9.2.2021
The word ‘bio’ is a prefix widely used in the present-day society and it is related to very different kinds of products. Typically, it is assumed to evoke something positive and nature-related in us, something natural and hopeful, sustainable and green. A lot of products are sold under these terms: biologically grown bio-food referring to organically produced products, biofuel and -oils, as well as biomaterials like bio-plastics and bio-fibres. For engineers, the prefix ‘bio’ might be an important indication of the origin of the raw-material, its behaviour in nature or compatibility with human body. Also, in our project we are focused on bio-based industry: biocomposites, paper and packaging and medical textiles. However, discussing in a diverse and multidisciplinary context, the prefix ‘bio’ can be highly misleading. Here, I use plastics to exemplify this issue.
Plastics consist of a base polymer together with fillers and additives and are a commodity without which our society would struggle. Plastics play an important role in packaging, medical applications, transportation and many other fields. Simultaneously, it is acknowledged that typical plastic raw-material is non-renewable and the waste management infrastructure (and human nature) does not support the collection of all plastic waste. This, together with the very durable nature of plastics, has led to the plastic pollution and different actions and attempts have been made to tackle this issue. One of these is the introduction of bio-plastics. However, this term may mean a variety of things.
First, bio-plastic can refer to plastics that are biodegradable, an aspect specifically interesting for single-use products which have a high potential to be falsely dumped in nature. The definition of biodegradability means that the material is capable of undergoing biological degradation. These organic polymers are typically consisting of ester, amide, or ether bonds, which allow decomposition e.g. through hydrolysis or biological processes. However, for a consumer the term biodegradable might be understood as a material, that is easily compostable in a natural environment. Instead, a biodegradable plastic might require an industrial biorefinery environment for a complete degradation while in nature its degradation might take years. Further, the product requirements, such as the temperature resistance of a coffee cup lid, might be at odds with the degradability: high degree of crystallinity of a biodegradable polylactic acid (PLA) coffee cup lid improves its mechanical properties at high temperatures which are introduced by the hot beverage. However, the high degree of crystallinity also slows down degradation.
Second, bio-plastic may refer to a plastic which is made from renewable raw-material. Sugars and oils extracted from plants are examples of raw-materials, which can be used as the basis of polymers. Although these bio-based plastics are not dependent on crude oil or natural gas, their structure can be identical with the traditional synthetic polymers. Therefore, the bio-based origin does not guarantee anything about the behaviour of these plastics in nature. Instead, both bio-based plastics and those derived from petrochemical processes include examples of non-biodegradable and biodegradable grades. However, regarding the circular economy of plastics, these bio-based but non-biodegradable plastics can be recycled for new plastic products which is not the case for biodegradable plastics. The variety of recyclable plastics is limited because from a mixture of plastic grades it is typically impossible to generate high-quality plastic products. Therefore, only commodity and mono-material plastics are recycled.
Third, the term bio-plastic can refer to a biomedical plastic. Tissue engineering and wound treatment applications, as well as drug delivery, are examples of biomedical applications in which plastics play an important role. An example from tissue engineering are the polymeric bioresorbable materials, which can slowly degrade in human body through hydrolysis or enzymatic pathways simultaneously acting as a substrate for new bone cells. These medical devices can be based e.g. on polyglycolic acid (PGA) or polylactic acid (PLA), the latter of which can also be found from the coffee cup lid. This renders the need for a second surgery to remove the implant unnecessary. Another interesting example is from the world of drug delivery. The solubility as a function of temperature and pH of so-called copolymers, in which two different polymer structures are combined to one, can be controlled by the selection of the structure, which enables controlled drug delivery into human body.
These examples illustrate the diversity of the ‘bio’ prefix and remind us that it is hardly a synonym for anything specific. Instead, caution is required when using or drawing conclusions based on it.
Writer:
Essi Sarlin, Associate Professor
Tampere University
