3D Printing Silver- and Copper-Doped Hydroxyapatite Scaffolds for Use as Bone Substitute
Date
2022-08
Authors
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Publisher
New York State College of Ceramics at Alfred University. Inamori School of Engineering.
Abstract
With the objective of 3D printing antibacterial bone scaffolds in mind, silver- and copper-doped hydroxyapatite was synthesized through precipitation. The formulation was created with the intention of ten percent dopant substitution in calcium sites. The resulting solid was then milled into to a powder. Phase identification through X-ray diffraction verified the structural composition was hydroxyapatite. Elemental analysis through X-ray fluorescence (XRF) showed the silver-doped powders had 0.3 atomic percent dopant and the copper-doped powders had 1.1 atomic percent dopant, both below the expected goal of 2.7 atomic percent. Secondary electron imaging through scanning electron microscopy provided visualization of porous particles with a wide particle size distribution. Energy dispersive spectroscopy (EDS) showed little evidence of any detectable dopant concentration in the powders, which was consistent with the low dopant content detected through XRF. Particle size analysis through dynamicc light scattering revealed mean particle sizes of 2.2 μm and 1.2 μm for the silver-doped and copper-doped hydroxyapatite samples, respectively. Surface area analysis through the Brunauer-Emmett-Teller method measurements reported 80.0 ± 0.2 m²/g for the silver-doped sample and 342.0 ± 0.6 m²/g for the copper-doped sample.
The powders were sent to Lithoz America for slurry preparation using their proprietary recipes. The porosity and agglomeration of the powders proved to be an obstacle for achieving a slurry with an appropriate solids loading for Lithoz's lithography-based ceramic manufacturing system. After a series of failed attempts, it was discovered that the powders' agglomerates were absorbing the resin. As a result, the effective solids loading of the slurries widely strayed from the measured solids loading. With this added knowledge, both powders were successfully prepared into printable slurries.
Both slurries were printed into nine scaffolds, all 10 mm in length, 10 mm in width, and 5 mm in height. Both compositions underwent curing complications during the run. Scaffolds were then cleaned, with difficulty, using the Lithoz proprietary solvent, LithoSol 20. They underwent three firing cycles: preconditioning, debinding, and sintering. The preconditioning cycle was held at a maximum temperature of 120°C for 72 hours, the debinding cycle was held above 600°C for 9 hours with a maximum instantaneous temperature of 1000°C, and the sintering cycle was held at a maximum temperature of 1100°C for 2 hours.
After sintering, the scaffolds were considerably smaller in size. The shrinkage of the silver-doped scaffolds was ~35% in the XY directions and ~30% in the Z direction. The shrinkage of the copper-doped scaffolds was ~40% in the XY direction and ~35% in the Z direction. The Archimedes density was calculated to be 2.01 g/m³ for the silver-doped scaffolds and 2.20 g/m³ for the copper-doped scaffolds, both densities higher than that of human bone. EDS data indicated a lack of homogeneity within the silver-doped scaffolds and phase separation in the copper-doped scaffolds. The silver-doped scaffolds consisted of regions that ranged in concentration from undetectable amounts of silver to 6.25 atomic percent silver. Conversely, the copper-doped scaffolds had copper-rich deposits with 56.26 atomic percent copper and a region with an undetectable copper concentration.
Scaffolds were tested for antibacterial behavior through exposure to Staphylococcus aureus, a gram positive species of bacteria, and Escherichia coli, a gram negative species of bacteria. These tests showed little evidence of antibacterial activity. Each inhibition zone for the twelve tested scaffolds was 0mm. It was concluded that this was a result of multiple factors: non-homogeneity, agar levels in the petri dish, and low dopant concentrations.
In order to achieve a viable antibacterial bone substitute customizable to a patient's needs, a lower density, a higher degree of homogeneity, and a higher dopant concentration must be achieved in 3D-printed parts. Additionally, achieving a controllable and adjustable density is necessary for meeting the needs of a variety of patients.
Description
Thesis completed in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis
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Keywords
Biomedical materials, Tissue scaffolds