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 . 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. 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 contribute to the development of a cellulose-based biodegradable thermoplastics as an alternative to contemporary bed mulch films to help combat the issue of overproduction of agricultural plastic waste.
Figure 1: An example of a white PE bed mulch film used in strawberry farming.
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 where they isolated a strain of bacteria proven to produce bacterial cellulose (BC) in relatively large quantities, Komagataeibacter rhaeticus iGEM, enabled us to establish the foundation for our project.
Cellulose as Our Material
Bacterial cellulose was of particular interest to our team because of its promise of biodegradability. Instead of PE films filling up our landfills, a biodegradable plastic will simply degrade into organic sugar monomers in the soil. However, 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. Inserting a plasticizer, a compound that disrupts hydrogen bonding between strands, would increase flexibility and decrease crystallinity. Because cellulose is so crystalline though, plasticizers have trouble penetrating cellulose and homogeneously integrating into it. Cellulose dissolution or decrystallization is therefore a necessary precursor to plasticizer addition.
Plasticization of cellulose may be achieved through covalent or hydrogen bonding of a compound to the hydroxyl groups of cellulose, co-polymerization, or addition of polymer blends. 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). Plasticizers introduce space between cellulose microfibrils, increasing elasticity and decreasing crystallinity.
Our cellulose-based thermoplastic will be designed to mimic the properties of PE films with the added quality of being biodegradable. PE film qualities include high UV light resistance, higher tensile strength, elasticity, gas and water impermeability, low crystallinity index, and low melting temperature. Our thermoplastic has potential beyond use as a bed mulch film, and it may be used to replace other agricultural plastics as well.
Overcoming Cellulose's Rigidity - CBMs
Besides plasticization, cellulose can be bound to Carbohydrate Binding Molecules (CBMs) to become less crystalline and rigid. CBMs are naturally occurring enzyme subunits responsible for carbohydrate binding and a they are a key part of carbohydrate degradation . They can be found in fungi, bacteria, eukaryotes, and archaea, effectively spanning all forms of life. Due to their variety and ubiquity, there are vast quantities of CBMs that bind to different carbohydrates and at different binding sites, such as cellulose binding modules, a type of CBM. On top of these traits, these versatile compounds can be biologically produced and are non-toxic unlike many plasticizers. For these reasons, CBMs have become the backbone of our project design.