Injection Molding of 3-3 Hydroxyapatite Composites

Abstract

The manufacturing of ideal implants requires fabrication processes enabling an adjustment of the shape, porosity and pore sizes to the patient-specific defect. To meet these criteria novel porous hydroxyapatite (HAp) implants were manufactured by combining ceramic injection molding (CIM) with sacrificial templating. Varied amounts (Φ = 0-40 Vol%) of spherical pore formers with a size of 20 µm were added to a HAp-feedstock to generate well-defined porosities of 11.2-45.2 Vol% after thermal debinding and sintering. At pore former contents Φ ≥ 30 Vol% interconnected pore networks were formed. The investigated Young's modulus and flexural strength decreased with increasing pore former content from 97.3 to 29.1 GPa and 69.0 to 13.0 MPa, agreeing well with a fitted power-law approach. Additionally, interpenetrating HAp/polymer composites were manufactured by infiltrating and afterwards curing of an urethane dimethacrylate-based (UDMA) monomer solution into the porous HAp ceramic preforms. The obtained stiffness (32-46 GPa) and Vickers hardness (1.2-2.1 GPa) of the HAp/UDMA composites were comparable to natural dentin, enamel and other polymer infiltrated ceramic network (PICN) materials. The combination of CIM and sacrificial templating facilitates a near-net shape manufacturing of complex shaped bone and dental implants, whose properties can be directly tailored by the amount, shape and size of the pore formers.

Keywords: bone grafts; ceramic injection molding; dental implants; interpenetrating composites; porous hydroxyapatite; sacrificial templating.

Figure 1

Microstructure (A,B) and particle size distributions (C) of the type A (d₅₀ = 20 µm) and type B (d₅₀ = 200 µm) phenolic resin spheres utilized as spherical pore formers.

Figure 2

Porosity fractions (total, open and closed) of the porous hydroxyapatite (Hap) samples in dependence of the pore former content containing 20 µm spherical pore formers (A): The linear fit of (A) shows the linear relation (continuous line) between the total porosity and the pore former content, the dashed lines are only a guidance for the eye. (B) shows the dependence of the real surface-to-surface inter pore distance (data points represented by square symbols), determined by image analysis from SEM-micrographs, on the pore former content for the 20 µm spherical pore formers. The experimental data was compared to the model of Equation (1) inserting the measured particle size distribution of the 20 µm spheres of Figure 1C (see material and methods, here highlighted in the blue section) and for different pore sizes including 5, 100 and 200 µm (dashed lines).

Figure 3

SEM-micrographs of fracture surfaces showing the microstructure of the porous HAp samples containing 10 (A,D), 20 (B,E) and 40 (C,F) Vol% of 20 µm spherical pore formers. The lower 250× magnification images of (A–C) display the homogeneous distribution of the pores, while the higher 1000× magnification images of (D–F) show the change of connectivity from isolated to interconnected pores.

Figure 4

Mechanical properties of the porous HAp ceramics and HAp/urethane dimethacrylate (UDMA) composites: Young’s modulus and flexural strength of the porous HAp ceramics in dependence of the total porosity (A) and Young’s modulus and Vickers hardness of the HAp/UDMA composites in dependence of the UDMA-polymer content (B).

Figure 5

Microstructure of the fabricated HAp/UDMA composites with 40 Vol% of pore formers (A,B): SEM-micrographs show the complete polymer infiltration of all HAp pores (A) and polymerization shrinkage at the HAp/UDMA interface, highlighted by the orange arrows (B). Visualization of the interconnected UDMA-based polymer network (C,D): SEM-micrograph after etching-out the HAp ceramic matrix (C) and reconstructed, skeletonized µCT-polymer network indicating the pore sizes by a color heat-map with blue for pores >1 µm and red color >10 µm (D). Fracture behavior of the HAp/UDMA composites (E,F): The two fracture surfaces of one sample from the flexural testing, showing the fracture at the polymer/ceramic interface associated with 100% pullout effects of all polymeric spheres, highlighted as exemplary by three red arrows (E,F).

Figure 6

Future perspectives for porous HAp ceramics and HAp/UDMA composites fabricated by porous injection molding: Microstructure of a porous HAp sample with a bimodal pore size distribution containing 5 Vol% of 200 µm and 15 Vol% of 20 µm spherical pore formers (A). Polished SEM-micrograph of a HAp sample with a graded porosity, showing the defect-free interface between two layers containing 20 Vol% and 40 Vol% of 20 µm spherical pore formers (B). Potential non-load bearing cranial (1) and maxillofacial (2) implants for the porous HAp ceramics (C). Fabricated porous HAp bending bars with 0, 10 and 30 Vol% of pore formers (1) and potential two-part dental implants for the interpenetrating HAp/UDMA composites (2), showing a ceramic crown and three types of abutments with different anchoring geometries (D).