Polyhydroxybutyrate-Polybutylene Succinate/Chitosan-Coated Macadamia Nutshell Composite Filaments with Antibacterial Functionality for Material Extrusion 3D Printing

In the world of medical care, the tools used to heal broken bones and support injured limbs often come with a heavy price tag, both financially and environmentally. Traditional splints and braces are frequently made from metals that are heavy and uncomfortable, or from synthetic plastics that are strong but never break down, eventually piling up as permanent waste in landfills and oceans. While scientists have developed biodegradable plastics that can replace these materials, they often lack the necessary strength or the ability to fight off infections, which is a critical risk when a device sits against the skin for days or weeks. The challenge lies in creating a material that is strong enough to hold a body part in place, light enough to be comfortable, safe enough to dissolve harmlessly after use, and capable of stopping bacteria from growing on its surface.

Researchers at Chulalongkorn University in Thailand have taken a significant step toward solving this problem by turning agricultural waste into a high-tech medical material. They developed a new type of filament, a thin thread used in 3D printers, that combines two biodegradable plastics with a powder made from macadamia nutshells. To make this mixture work, they treated the nutshells to clean them and then coated them with chitosan, a natural substance derived from shellfish shells that is known to kill bacteria. The result is a printable material that is not only strong and stable but also actively fights infection, offering a sustainable path forward for custom-made medical devices.

The journey began with a simple observation: macadamia nutshells are a massive waste product, with hundreds of thousands of tons discarded globally every year. The research team collected these shells, ground them into a fine powder, and then subjected them to a chemical bath to remove unwanted plant fibers, leaving behind a cleaner, more reactive material. They then mixed this powder into a blend of two biodegradable polymers, polyhydroxybutyrate and polybutylene succinate, which were chosen because they complement each other well; one provides stiffness while the other adds flexibility. The team tested different amounts of the nutshell powder, ranging from none to ten percent of the total weight, to see how much filler the plastic could hold before it became too brittle or difficult to print.

They found that adding too much powder made the material hard to push through the 3D printer and caused it to lose its structural integrity. However, a mixture containing five percent of the nutshell powder struck the perfect balance, maintaining the strength of the plastic while adding the benefits of the natural filler. But the real breakthrough came when they introduced the chitosan coating. By dipping the nutshell powder into a chitosan solution before mixing it with the plastic, they created a bridge between the natural filler and the synthetic polymer. This coating did more than just help the ingredients stick together; it transformed the entire filament into an antibacterial shield.

When the researchers printed test pieces and subjected them to rigorous testing, the results were clear. The uncoated nutshell powder actually weakened the material slightly and offered no protection against bacteria. In contrast, the chitosan-coated version retained the mechanical strength needed for medical use while demonstrating a powerful ability to kill two common types of bacteria: one that causes skin infections and another often found in the gut. The printed objects held their shape remarkably well, with very little shrinking or warping during the printing process, a common problem with biodegradable materials. The coating also helped the material flow smoothly through the printer nozzle, ensuring that the final product was consistent and reliable.

The study suggests that this approach offers a viable solution for creating custom orthopedic devices, such as splints for broken limbs, that are tailored to a patient's specific needs. Because the material is biodegradable, it can be worn until the bone heals and then safely discarded without contributing to the growing crisis of medical waste. The researchers demonstrated that by using agricultural byproducts and natural antibacterial agents, it is possible to create a material that is not only functional and safe but also environmentally responsible. While the material still has some limitations regarding how much it can stretch before breaking, the combination of strength, printability, and infection control marks a promising advance in the field of sustainable medical manufacturing.