The Need for Biodegradable Agricultural Plastics
The agriculture industry has long been a huge consumer of single-use plastics due to its dependence on polyethylene (PE) bed mulch films. PE films are secured over crop beds establishing a barrier between the crop and soil which hinders microbial and insect digestion, keeps produce clean, and lengthens produce shelf life [4]. PE films are an essential component of contemporary agricultural practice as they regulate soil temperature and humidity, prevent weed growth, and reduce fertilizer loss, all of which increase crop yield. But despite their utility, the difficulties associated with their disposal and management are mounting environmental threats.
Figure 1: An example of a white PE bed mulch film used in strawberry farming.
Plastic bed mulch waste can be recycled, stockpiled for later disposal, burned, or buried directly into the soil, though the majority is dumped into landfills [9]. Agricultural PE films are notoriously difficult to recycle, as films become contaminated with dirt throughout the growing season. While the plastic pollution associated with bed mulch films is alarming, these films are still essential to feeding our growing population; therefore the development of a sustainable bed mulch film is imperative.
We intend to promote the development of a cellulose-based biodegradable thermoplastic as an alternative to contemporary bed mulch films to help combat the issue of overproduction of agricultural plastic waste.
Our inspiration was taken partially from our PI and advisor, Dr. David Bernick, whose investigation into the possibility of using cellulose from almond hulls to develop bed mulch films sparked the idea for Komaplastics. Additionally the work by the 2016 Imperial College of London iGEM team in their isolation of a strain of bacteria proven to produce bacterial cellulose (BC) in relatively large quantities, Komagataeibacter rhaeticus iGEM, enabled us to create our project’s foundation.
Cellulose as Our Material
Bacterial cellulose (BC) was of particular interest to our team because of its promise of biodegradability. However, the structure of the strands make it difficult to work with. Hydrogen bonding occuring between hydroxyl groups of neighboring BC strands form cellulose fibrils of various sizes. These dense crystalline fibrils pose an obstacle for the penetration and homogeneous integration of a plasticizer. Cellulose dissolution or decrystallization is therefore a necessary precursor to plasticizer addition.
Bacterial cellulose was of particular interest to our team because of its promise of biodegradability. But the structure of cellulose strands make it difficult to work with. Hydrogen bonding that occurs between hydroxyl groups of neighboring BC strands form dense, crystalline fibrils of various sizes, decreasing general flexibility and preventing the penetration and homogeneous integration of a plasticizer. Cellulose dissolution or decrystallization is therefore a necessary precursor to plasticizer addition.
Plasticization of cellulose may be achieved through covalent or hydrogen bonding of the plasticizer compound to the hydroxyl groups of cellulose; simple addition of plasticizers, co- polymers, or polymers; or a combination of both techniques. Current experimental data has pointed us in the direction of Glycerol as a plasticizer.
Figure 2: Diagram showing the structure of BC with a plasticizing molecule (navy) integrated into the cellulose fibrils (orange). The plasticizers introduce space between cellulose microfibrils, increasing elasticity and decreasing crystallinity.
Our cellulose-based thermoplastic will be designed to mimic the properties of polyethylene (PE) film with the added quality of being biodegradable. These qualities include high ultraviolet (UV) light resistance, tensile strength, elasticity, gas and water impermeability, low crystallinity index, and low melting temperature. Our thermoplastic will likely be useful beyond bed mulch films by potentially replacing other agricultural plastics.
Overcoming Cellulose’s Rigidity- CBMs
Besides plasticization, cellulose can be modified by Carbohydrate Binding Molecules (CBMs) to become less crystalline and rigid. CBMs in nature are subunits in enzymes responsible for cellulose degradation, and have specifically been shown to interfere with cellulose crystalline structure. As they can be biologically produced, are non-toxic unlike many plasticizers, and for their effects on cellulose, CBMs have become the backbone of our project design.
