Cellulose is the world’s most abundant organic compound on Earth. It is estimated to make up 30% of the entire globe’s non-fossil organic carbon. It is used as a structural support biopolymer by plants to build up cell walls. You can find cellulose in many different forms, as humans have learned to exploit this naturally occurring material. We have learned to build houses and furniture from hardwood, make clothing out of cotton fibers, and press the material into thin sheets of paper. A team from MIT now wants to add to cellulose’s versatility by using it as source material for 3D printing.
Lead researcher Sebastian Pattinson and associate professor of mechanical engineering A. John Hart demonstrate how cellulose can be used as a renewable, inexpensive building material for the emerging technology of 3D printing. The manufactured products are strong but are biodegradable, making them environmentally friendly. What makes 3D printing so attractive is that customization is only limited by the operator. The process described by the MIT team is fast, inexpensive, and easy.
What is cellulose? It is a polysaccharide or starch that plants and some bacteria make as a result of photosynthesis. In the presence of light and water, plants can convert carbon dioxide gas into a sugar molecule and oxygen. The plant will string a line of these sugar molecules together to form cellulose. Unlike the sweet sugars mammals consume, these carbohydrates are not digested and we refer to them as fiber.
The MIT team notes that using cellulose for manufacturing is not a novel idea, but previous research was met with substantial road blocks. The sugars within the starch have strong intermolecular bonds that hold the biopolymers together. The temperature needed to melt the hydrogen bonding network exceeds its thermal decomposition limit. This H-bonding network is so strong that the viscosity of any liquid solution would be great for extrusion.
This image from a scanning electron microscope shows a cross section of an object printed using cellulose. The inset shows the surface of the object. (MIT)
The solution the MIT team uses is to eliminate the H-bonding network by converting the starch to a cellulose acetate derivative. By ‘capping off’ hydrogen donating groups, the cellulose acetate will disassociate itself in a benign organic solvent like acetone. The solution is workable and can be extruded from a nozzle. After a quick evaporation of the solvent, the cellulose acetate solidifies in place. A second treatment with lye removes the capping, restoring the H-bond network, increasing the strength of the printed construct. When compared to styrene or polylactic acid, the MIT team shows that the cellulose materials are generally stronger.
Cellulose is a biocompatible material that has found uses outside of building materials, textiles, and paper. Due to their biocompatibility, some cellulose derivatives are used in pharmaceuticals, medical devices, and food additives. Pattinson and Hart 3D printed a pair of surgical tweezers that were doped with a small amount of antimicrobial dye. The tool killed off bacteria by shining a fluorescent light on them. The MIT team envisions using these printable tools in remote locations. They would be durable, easy to keep clean, and inexpensive to make.
Adding a small amount of antimicrobial dye to cellulose acetate ink allowed researchers to 3-D-print a pair of antimicrobial surgical tweezers(Credit: MIT)
Another advantage to the cellulose acetate system is that the extrusion process is not heat dependent. Common 3D plastic printing uses heat extrusion; this limits printing because the plastic polymers can decompose if they get too hot. The cellulose acetate sets quickly at room temperature. Blowing warm air over the print would quicken the evaporation process. The team would also like to develop a method to recapture the acetone solvent for recycling, making the process more environmentally friendly. Combined with the low production cost of cellulose acetate, bulk material can also be purchased at the same cost as thermoplastics. The result is that this process is primed to deliver fast, affordable, and durable 3D prints for a wide range of applications.
Top image: Origami Paper Tree. (YouTube)