Porous Gelcast Ceramics for Bone Repair Implants

Lightweight, porous ceramic structures are being developed for a variety of biologically-related applications. In bioengineering, there is interest in replacing metal-based implants with more functional implants consisting of porous bioceramics that are bioresorbable. In the design of bioinspired ceramic structures, there is a desire to understand the coupling between geometric complexity and porosity in natural structures such as bone. However, it has been difficult to process ceramics with controlled porosity and geometric complexity. In this research program, the ceramic processing technology known as “gelcasting” has been employed to study the fabrication of model functional alumina implants that can possess physical characteristics similar to real bone. This technology, often used to make turbine blades and reactor parts, has been used here with added carbon impurity particles that are burned out during sintering to obtain porous microstructures. Gelcast alumina was used to fill a mold generated with a new mold design algorithm and machined using a new path planning algorithm that operated on a 3-D image of a real bone structure, permitting the creation of artificial bone structures. For this effort, the fabrication of porous microstructures was studied using two different types of carbon: activated and graphitic. The mechanical performance of the artificial bone was characterized to gain insight into the structure/property/performance relationship of these geometrically complex, lightweight structures. It was determined that the activated carbon can be used to create more closed porosity at porosity levels of up to 35%. Bone structures with 35% porosity formed from 10% activated carbon exhibited about 1/7 the strength, 1⁄4 the stiffness, and about 30% less ductility than structures with 10% porosity formed with no pore-forming agents. Introduction In bioengineering, there is a desire to create lightweight, porous ceramic structures for a variety of applications. For bone repair, these structures can provide more functional implants than conventional metal-based implants. There is also interest in creating bioinspired ceramic structures that take advantage of the coupling between geometric complexity and porosity that is present in natural structures such as bone. The challenge has been to fabricate geometrically complex porous ceramic structures with conventional manufacturing technologies. Recently, a ceramic processing technology known as “gelcasting” has been used to create parts with complex geometries. In gelcasting, ceramic powders are mixed with a monomer solution and polymerized to create a mixture with gel-like characteristics. Upon heating, the ceramic particles are immobilized by the polymer, which allows for complex shapes to be produced without any secondary processes. The gel can then be poured or injected into a mold of any shape, dried, and sintered [1]. If a secondary process is necessary, the parts can also be machined in the green state, allowing for more tool versatility. In addition to geometrically complex shapes, gelcasting can be used to tailor mechanical properties for specific applications. Since ceramics are usually strong but brittle, a metallic powder can be added to the slurry to create a composite with increased ductility. In some applications, specifically biomedical ones, porosity is desired to allow for tissue in-growth and fluid transport [2]. Porosity can be obtained using several different techniques, such as polymer sponge burnout, use of large ceramic particles, and polymers and impurity particles used as pore-forming agents. The first of these methods includes cutting a polymer sponge to the desired shape and impregnating the sponge with the slurry [3], [4]. The use of powders comprised of large ceramic particles, can produce the desired porosity provided the particle diameters are around 20.0 μm [5]. Another method involves the use of the polymer polyvinyl butyral (PVB) as the pore forming agent [6]. Maca et al. used cold isostatic pressing, another ceramic forming process, with carbon as the impurity particle to obtain highly controlled porous structures [7]. In this research effort, gelcast ceramics with pore-forming agents are used to fill molds to create model artificial bone structures. The molds are generated using a new mold design algorithm [8] and machined using a new path planning algorithm that operated on a 3-D image of a real bone structure. The gelcast ceramics incorporate two different types of carbon particles: activated and graphitic. The model artificial bones were then mechanically tested to gain insight into the structure/property/performance relationship of these geometrically complex, lightweight structures. Method and materials The specific recipe that is being used in the current research for the gelcasting slurry is illustrated in Table 1. Alumina (Al2O3) is the main powder constituent. Ultimately, a ceramic phase found in bone, hydroxyapatite (HAp) will be used, but alumina serves as a good alternative for model structures because it has similar characteristics with the advantage of being easier to acquire and more affordable. Two different carbon powders are added as pore-forming agents: activated carbon and graphite, in varying weight percentages up to 10% of the total mass of added powder, which are burned out when the sample is sintered. Table 1 Gelcasting Recipe Component Name Weight Percent (%) Volume Percent (%) Monomer 15% HMAM solution 19.43 45.31 Dispersant Darvan C as received 2.34 5.93 Polymerizing Agent 10% AZIP solution 1.73 3.93