3D Printing of a Thermoset, Citrate Base Polymer

Kirk, Gerald
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 17, 2017
Committee Members:
  • Jian Yang, Thesis Advisor
  • 3D Printing
  • Additive Manufacturing
  • Citrate Base Polymers
  • Thermoset
  • Skin Scaffold
  • Tissue Engineering
Significant effort in the field of Bioengineering has been devoted to the fabrication and application of scaffold materials as one third of the tissue engineering trifecta: cells, materials, and growth factors. While traditional fabrication techniques, notably porogen leaching and gas foaming, have resulted in widely applied and functional scaffolds, such constructs exhibit a large degree of variability and are difficult to replicate more complex, physiological geometries. These limitations have increased interest in additive manufacturing as an alternative, generating significant research and development focus on both scaffold materials and scaffold fabrication techniques. Currently, additive manufacturing is split into two major categories: bio (cell) printing, and scaffold printing. While the former relies on gel-based materials, including alginate, collagen, gelatin, Polyethylene Glycol PEG, and mixtures of the above (due to their water solubility, relatively mild printing conditions, and ability to facilitate nutrient transfer to encapsulated cells from the surrounding tissue), the latter has traditionally used thermoplastic, biocompatible polymers such as Poly Capralactone (PCL), Poylactic Acid (PLA), Poly Lactic-co-glycolic Acid (PLGA), as well as ceramics, and metals. The latter are capable of post-print cell seeding or of direct cell recruitment in vivo. The material choice for use in additive manufacturing is determined primarily by the ability to form solid constructs rapidly upon printing through temperature- or pH-regulated gelation, free radical crosslinking, shear thickening, or solidification from a melted state, among other methods. In this study, we explore the use of citrate based, thermoset polymers as a new class of materials for additive manufacturing. This study includes the modification of an off-the-shelf 3D printer commonly used for filament extrusion of PLA thermoplastics, with a micro extrusion system consisting of a pressurized pneumatic system, syringe reservoir, and needle extruder. Further, we formulate novel composite bio-inks to facilitate the printing of thermoset, citrate based pre-polymers prior to irreversible crosslinking. This was done to increase the manufacturability of citrate based polymers, allowing these class of materials to expand its suitable tissue scaffold applications. 3D printed constructs were explored as a skin substitute, using additive manufacture allowed thin scaffolds to be manufacture and tested for this application. Three materials are studied to evaluate printability using the modified printer: PCL, a known printable thermoplastic, Poly-(Octamythlene Citrate) (POC), a well-studied thermoset, and POC-Ca, a thermoset modified with the addition of calcium ions. Composite inks are formed with hydroxyapatite and salt with vary concentrations to determine the optimal concentration for composite inks. The printability study is conducted via evaluations of a printed scaffold by three methods: visual observation of extrudability and filament formation, quantifying the circularity of a single printed layer (demonstrating shape fidelity), and comparison between the theoretical areas of printed holes and their experimentally determined values. Scaffolds were designed for the printability evaluations were 12 mm x 12 mm with 9, 2mm holes, with a scaffold thickness of 0.4 mm. Overall, results demonstrate that citrate based composites can be designed as functional bio-inks printable to 2 mm resolution into consistent, thin films. Thus, this work demonstrates the ability to adapt additive manufacturing to generate thermoset tissue engineering scaffolds that could not previously be created with traditional manufacturing techniques.