The basis for paper, the material that is the most used for printing on, is cellulose. Cellulose may soon also become a material that is used to do 3-D printing. New research done at MIT shows that this abundant material may potentially providing a renewable, biodegradable alternative to the polymers currently used for 3-D printing.
The new system is described in the journal Advanced Materials Technologies. Lead author of the paper, MIT postdoctoral student Sebastian Pattinson describes cellulose as the most abundant organic polymer in the world.
Pattinson explains that cellulose is bio renewable, biodegradable and inexpensive. It is also chemically very versatile and is the most important component in wood, giving it its mechanical properties.
Cellulose is used in a host of widely different products, including medical devices, pharmaceuticals, as food additives, and in clothing and building materials. Many of these types of products would benefit from the kind of customization that 3-D printing enables. Pattinson noted that one of 3D printing’s benefits is that it allows for individually customizing each product. He added that 3-D printing technology is growing rapidly.
It is not a new idea to use cellulose as a material for additive manufacturing (3-D printing). It has been attempted many times, but there are numerous major obstacles. When cellulose is heated, it becomes flowable only after it has decomposed thermally. This is partly due to the hydrogen bonds that are found between cellulose molecules. The intermolecular bonding also results in high concentration cellulose solutions being too viscous to extrude easily.
The MIT team therefore elected to work with cellulose acetate instead. This material is made from cellulose easily and is readily available as it is already widely produced. The number of hydrogen bonds in this material has been reduced by acetate groups. Cellulose acetate can be dissolved in acetone and then be extruded through a nozzle. The acetone evaporates quickly, allowing the cellulose acetate to solidify in place. An optional treatment afterwards replaces the acetate groups and will increase the strength of the printed parts should this be required.
Pattinson explained that the hydrogen-bonding network is restored through a sodium hydroxide treatment. The resultant strength and toughness of the parts are greater than that of many materials commonly used for 3-D printing, including polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS).
A. John Hart is an associate professor of mechanical engineering and the Mitsui Career Development Professor in Contemporary Technology and the co-author of the paper. Pattinson and Hart added an extra dimension to the innovation to demonstrate the chemical versatility of the production process. They 3-D-printed a pair of surgical tweezers with antimicrobial functionality by adding a small amount of antimicrobial dye to the cellulose acetate ink.
This demonstrated that the parts kill bacteria when a fluorescent light is shone on them. In remote medical settings where surgical tools are needed but it’s difficult to deliver new tools when they break, such custom-made tools could be useful. If the sterility of the operating room is not ideal, the antimicrobial function could be essential.
The production speed of most existing extrusion-based 3-D printers is limited by the amount of heat that can be delivered to the polymer without damaging it, as they rely on heating polymer to make it flow. The new cellulose process simply relies on evaporation of the acetone at room temperature to solidify the part. Pattinson notes that this could potentially be faster.
There are also various other methods that could be used to speed the process up even further. These include blowing hot air over it to speed evaporation, or laying down thin ribbons of material to maximize surface area. A production system would also look for ways to recover the evaporated acetone to make the process environmentally friendly and more cost effective.
The researchers note that cellulose acetate is widely available as a commodity product. When bought in bulk, the material is much less expensive than the typical filament materials used for 3-D printing. The price is in fact comparable to that of thermoplastics used for injection molding. When this factor is combined with the ability to functionalize cellulose in a variety of ways and the room temperature requirements of the process, it could make the material and process commercially viable.