Orienting structure to serve medical functions
This thesis explores how the change in material and the structure can better serve medicalfunctions. A 3D printed physical phantom and a hydrogel coated orthopedic implant were developed. The purpose of physical phantom work was to characterize and improve the ability of fused filament fabrication to create anthropomorphic physical phantoms for CT research. Specifically, we sought to develop the ability to create multiple levels of x-ray attenuation with a single material. CT images of 3D printed cylinders with different infill angles and printing patterns were assessed by comparing their 2D noise power spectra to determine the conditions that produced a minimal and uniform noise. A backfilling approach in which additional polymer was extruded into an existing 3D printed background layer was developed to create multiple levels of image contrast. A print with nine infill angles and a rectilinear infill pattern was found to have the best uniformity, but the printed objects were not as uniform as a commercial phantom. An HU dynamic range of 600 was achieved by changing the infill percentage from 40% to 100%. The backfilling technique enabled control of up to 8 levels of contrast within one object across a range of 200 HU, similar to the range of soft tissue. A contrast detail phantom with 6 levels of contrast and an anthropomorphic liver phantom with 4 levels of contrast were printed with a single material. In conclusion, this work improves the uniformity and levels of contrast that can be achieved with fused filament fabrication, thereby enabling researchers to easily create more detailed physical phantoms including realistic, anthropomorphic textures. The goal of the orthopedic implant work is to replace the damaged cartilage with a synthetic hydrogel. This requires a method for securing the hydrogel in a defect site with the same shear strength as the cartilage-bone interface. Bonding hydrogel to a titanium base that can in turn bond to bone could enable long-term fixation of the hydrogel, but current methods of forming bonds to hydrogels do not have the shear strength of the cartilage-bone interface. This thesis reports the first method for attaching a hydrogel to metal with the same shear strength as the cartilage-bone interface. The average shear strength of the junction between 1.2-mm-thick hydrogel and metal made in this manner exceeded the shear strength of porcine8 cartilage-bone interface. The shear strength of attachment increased with the number of bacterial cellulose layers and with the addition of cement between the bacterial cellulose layers. This new method of attachment will be useful to the creation of hydrogel-coated orthopedic implants for treatment of osteochondral defects. After creating the bonding between hydrogel and metal base, the thesis then introduces the work of a synthetic hydrogel. The goal of the work is to increase the mechanical strength of the hydrogel, which further increases the shear strength between the hydrogel and metal base. This work shows that reinforcement of annealed PVA with BC leads to a 3.2-fold improvement in the tensile strength (from 15.6 to 50.5 MPa) and a 1.7-fold increase in the compressive strength (from 56.7 to 95.4 MPa). The highly crystallized BC-PVA hydrogel that results from annealing is the first hydrogel with a tensile and compressive strength that exceeds that of cartilage. When tested against cartilage, annealed BC-PVA wore an opposing cartilage surface to the same extent as cartilage and was three times more resistant to wear than cartilage. The improved tensile strength of annealed BC-PVA enabled it to attach to a metal base with a shear strength 68% greater than the shear strength of cartilage on bone. The high strength, high wear resistance, and low COF of annealed BC-PVA make it an excellent material for replacing damaged cartilage. The wear performance of the BC-PVA hydrogel was further improved by doping nanoclay into original hydrogel network. By using a two-step infiltration, a tensile strength of 37.98 MPa was achieved. The wear of opposing cartilage against the hydrogel was much better compared to cartilage itself. Thus, the nanoclay doped hydrogel also has the potential to be used in actual cartilage repair.
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