Organic Regulations against Synthetic Biology
United States Department of Agriculture (USDA) organic regulation 7CFR 205.601 (source) describes approved characteristics and techniques for bed mulches used in organic agriculture. The excluded methods section of this regulation states that “organisms and feedstocks must not be derived from excluded methods. Excluded methods as defined by the organic standards include the following:
“A variety of methods used to genetically modify organisms or influence their growth and development by means that are not possible under natural conditions or processes and are not considered compatible with organic production. Such methods include cell fusion, microencapsulation and macroencapsulation and recombinant DNA technology (including gene deletion, gene doubling, introducing a foreign gene and changing the positions of genes when achieved by recombinant DNA technology.”
This policy presents a significant barrier to implementation of biodegradable films in organic farming. In speaking with growers and experts in the field of film development, we were informed that polyethylene (PE) and other petroleum-derived films are the only agricultural films offered for organic farming. These films must be disposed of after each growing season adding to the continuously mounting issue of plastic waste.
We see that this an obvious place for intervention via Bioengineering. Our design involves the use of synthetic biology (DNA technology) to introduce genes into our naturally occurring bacterial cellulose that would allow entirely bio-based products to supply these desired qualities as opposed to accepted petroleum derivatives.
The prohibition of DNA technology in the development of biodegradable agricultural films further stigmatizes this approach and places us even further from a long term solution: zero plastic waste. This policy insinuates that the USDA places priority on adhering to the negative stigma surrounding genetically modified organisms above the issues of countering global warming, plastic waste, and use of carcinogenic materials.
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 Rigidity
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.
References
