Understanding Morphology, Bulk Properties, and Binding of Silk Particles for 3D Printing

Abstract

Silk fibroin has emerged as a leading biomaterial for biomedical applications. 3D printing has been successfully used for printing with silk fibroin, albeit in the form of a bioink, in direct-write 3D printers. However, in the form of bioinks, stability and mechanical attributes of silk are lost. An innovative alternative to producing 3D printed solid silk constructs is silk milled into powder for printing in a binder jetting printer. In this work, we focus on characteristics of silk powder to determine suitability for use in 3D printing. Two different silk powders are compared with hydroxyapatite powder, a known biomaterial for biomedical constructs. We have investigated powder size and shape by Camsizer X2 and Scanning Electron Microscope and bulk behaviour, dynamic flow behaviour, and shear behaviour by FT4 powder rheometer. Preliminary printing tests were conducted in an in-house custom-built printer designed for silk powder. It was found that silk powder has low flowability and stability. Therefore, to print solely out of silk powder, a 3D printer design will need sophisticated techniques to produce flow to ensure even distribution and consistent thickness of powder layers during the printing process. It was also found that high concentrations of formic acid (>75 to 99 wt.%) can fuse particles and therefore be used as a binder ink for 3D printing. The printer design challenges for silk powder are discussed.