Rizhi Wang

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
Fracture mechanisms and structural fragility of human femoral cortical bone (2018)

Hip fracture has serious repercussions at both the societal and personal levels. For better fracture prevention, it is essential to understand the material changes of femoral cortical bone that contribute to hip fragility, and the deformation and fracture process during hip fractures. Therefore, the aim of this dissertation was to study the mechanisms of hip fracture from both structural and mechanical perspectives. Using quantitative backscattered electron (qBSE) imaging and polarized Raman microspectroscopy, periosteal hypermineralization in aged human proximal femur was found with significantly higher mineral content/mineral-to-matrix ratio than lamellar bone. Accompanying the increased mineralization was the “brittle” cracking behavior upon microindentation in the hypermineralized tissue. Small- and wide-angle X-ray scattering (SAXS/WAXS) measurement showed substantially thinner, shorter and more irregularly distributed mineral platelets in the hypermineralized region, indicating the material changes at the ultrastructural level. Combined second harmonic generation (SHG) and two photon excitation fluorescence (TPEF) techniques were used to study shear microcracking and its association with the organization of collagen fibrils in the femoral cortical bone. Unique arc-shaped shear microcracks, differing from either tensile or compressive microcracks, were identified at the peripheral zone of the osteons. These microcracks were further located within the “bright” lamellae where collagen fibrils are primarily oriented at the circumferential direction to the osteons’ long axes. Microcracking analysis on clinically retrieved femoral neck components identified shear, compressive and tensile microcracks associated with major fractures. The results pointed to the central role of the superior cortex in resisting a hip fracture, whereby higher density of microcracks and buckling failure were found in the superior cortical bone. BSE imaging at the fracture sites found the direct involvement of hypermineralization, which lacked crack deviation and had fewer microcracks than the tough lamellar bone. This dissertation answered fundamental questions regarding the role of femoral cortical bone in clinical hip fractures, and elucidated the underlying failure mechanisms due to microstructural changes and the complex stress states under external loading. The findings thus provided new insights into better identifying at-risk population of hip fracture.

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Local delivery of antimicrobial peptides from titanium surface for the prevention of implant-associated infections (2013)

Titanium (Ti) is a key biomedical material extensively used in orthopaedic implants. Prevention of implant-associated infections has been one of the main challenges in orthopaedic surgery. This challenge is further complicated by the concern over the development of antibiotic resistance as a result of using traditional antibiotics for infection prophylaxis. One of the promising alternatives is the family of antimicrobial peptides (AMPs). The present dissertation develops progressive approaches that enable the loading and local delivery of a unique group of cationic antimicrobial peptides through titanium implant surfaces. In the first technique, a thin layer of micro-porous calcium phosphate (CaP) coating was processed by electrolytic deposition onto the surface of titanium as the drug carrier. The AMP-loaded CaP coating was not cytotoxic for MG-63 osteoblast-like cells, and the implants showed high antimicrobial activity against both Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria with 10⁶-fold reductions of both bacterial strains within 30 min and ∼92% and ∼77% inhibition of luminescence at 4 h and 24 h, respectively. Second study investigated the in vitro AMP release, antimicrobial performance, and cytotoxicity of a modified Tet213 (HHC36), as well as the in vivo bone growth of AMP loaded into calcium phosphate coated Ti implants in a rabbit model. Burst release during the first few hours followed by a slow and steady release for 7 days was observed. In vivo bone growth study showed that loading of AMP did not impair bone growth onto the implants. In the last study multilayer thin films of titania nanotubes (NT) and CaP coatings were formulated with AMP and were topped with a thin phospholipid film similar to cell membrane. The films were shown to be non-cytotoxic, hydrophilic, with the potential of tuning loading and release kinetics of AMP. The best model describing the AMP release was first-order model.The first two approaches demonstrated a promising method for an early stage peri-implant infection treatment. The last study proposed a technique to improve the kinetics of AMP release and total loaded AMP quantity, and to increase the Ti interfacial strength while maintain the osteconductivity by applying CaP coating.

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Experimental study of deformation and microcracking in human cortical bone (2011)

Human bone is a complex biological material with up to seven levels of hierarchical structure. Due to this complexity, it is still not fully understood how the various structures contribute to the macroscopic mechanical response. Such understanding is important to assess the mechanical contributions of the bone material to whole bone fractures. It is well known that microcracking is associated with bone’s inelastic deformation and contributes to its resistance to fracture. Multiple microcracks suggest control over their development. Yet, the structure – microcracking interactions in cortical bone, particularly at the lamellar and Haversian systems levels, are still unclear. Following a qualitative, structure – mechanical function relations approach, the present dissertation provides further insight into how bone resists fracture by distributed microcracking. This was achieved through a detailed study of bone’s deformation and fracture processes using mechanical testing on human cadaver bones and a combination of microscopy techniques, including laser scanning confocal microscopy, to characterize the structure – microcracking relations. Particular interest was given to compression and bending, two loading modes involved in falls resulting in hip fractures. Haversian bone derived part of its fracture resistance through microcracking largely controlled by the concentric lamellae and underlying fibrillar organisation surrounding each Haversian canal. Multiple microcracks developed stably within the osteonal wall due to different fibrillar orientation in each lamella. Such process happened to most osteons resulting in well-distributed damage, hence providing inelastic deformation to the tissue. Haversian bone’s resistance to fracture would thus depend on its intact lamellar structure. Changes in number and organisation of the lamellae would likely alter bone’s ability to control microcracks and may lead to bone fragility. Based on a tibia study, long bones’ fracture resistance in bending was found to be linked to Haversian bone’s behavior. As a result of post-yield strain redistribution associated with tensile and compressive microcracking, bone’s compressive behavior was also found to play an important role in the bending response. Directly applying fundamental research to the clinical field, a preliminary analysis of the superior cortex of fractured femoral necks retrieved from patients revealed compressive microcracking. Such evidence emphasizes the importance of bone’s hierarchical structure in hip fracture.

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Master's Student Supervision (2010-2017)
Bioprocessing of nacreous coatings on orthopedic implant materials (2012)

Nacre from mollusc shells has a complex hierarchical structure composed of an organic-inorganic composite, and exhibits remarkable mechanical properties. In addition, nacre is biocompatible and bioactive making it an excellent candidate for biological coatings for orthopaedic applications. The bioprocessing of nacreous coatings on conventional orthopedic materials via biomineralization of abalone shells was examined in this thesis. The animal reaction to the materials was evaluated by the coating surface morphology, thickness and coating-implant interface, which were characterized using SEM, EDS, XRD and Raman spectroscopy.In the first test, poly(methyl methacrylate) (PMMA), high density polyethylene (HDPE), and titanium (Ti) substrates were implanted separately on the growth surface of abalone shells to examine the effect of different materials on mineral growth. The abalones were under restricted diet. PMMA and HDPE implants resulted in thicker coatings and were able to achieve the desired nacre structure (thickness of 38.1 ± 28.8 μm and 38.7 ± 22.2 μm, respectively). The titanium implants showed thin and sparse coating and were not able to achieve nacre (thickness of 5.3 ± 3.4μm).In the second test, the effect of Ti surface modification (micro-porous, nano-porous and smooth surface) was examined. The substrates were implant together on one location of the shell and were under normal feeding conditions. Thick nacreous coatings, 50 to 280 µm, were formed on the Ti surfaces. There was no apparent trend between the type of Ti surface and the coating formed; however, it appeared that coatings on the implants were similar within the same animal. Thus, this indicates that feeding conditions and location of implantation may play a role in coating mineralization. In addition, two new unique features were found in the implants that have not been reported in literature before: vaterite and alternating bands of nacre towers and aragonite grains across the coating surface. The findings in this thesis therefore suggest that nacreous coatings can be processed on both polymeric and metallic implant materials as long as proper abalone culturing conditions are maintained. The biofabrication techniques developed in this project can be applied to the development of new classes of surface coatings for biomedical implants.

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Local delivery of antimicrobial peptides using self-organized TiO₂ nanotubes for implant-related infections (2011)

Among the various complications that lead to the failure of orthopaedic implants, prosthetic-related infections have been reported as one of the major causes. Local delivery of antimicrobial agents through the implants surface is an ideal solution to the peri-implant infection problem. Due to the increasing resistance of pathogens to the current therapy utilizing antibiotics, developing novel antimicrobial agents has received much attention recently. Among the potential alternatives are the antimicrobial peptides (AMPs). Because of their broad-spectrum bactericidal ability, low toxicity and immunogenicity, as well as complex killing mechanisms, AMPs have much lower possibility of developing resistance than traditional antibiotics.In the past decade, fabrication of TiO₂ nanotubular structures by anodization method has attracted great interests because of its controllable, reproducible results as well as the simple process. In light of their high surface-to-volume ratio, controllable dimensions, excellent biocompatibility, adjustable wettability and other promising properties, TiO₂ nanotubes are considered as an ideal carrier for drugs.In the current study, self-organized, vertically-oriented TiO₂ nanotubes were successfully prepared by anodization method in both water based electrolytes (phosphoric acid based and ammonium sulphate based electrolytes) and organic based electrolytes (Glycerol based and Ethylene glycol based electrolytes). The nanotube coatings prepared in ethylene glycol based electrolytes, with ~80nm diameter and ~7 μm thickness, were selected for the drug delivery purpose. HHC-36, one of the most potent broad-spectrum AMPs with the sequence of (KRWWKWWRR) was loaded onto the titanium dioxide nanotubes via a simple vacuum assisted physical adsorption method. Antimicrobial activity test against Gram-positive bacteria (Staphylococcus aureus) demonstrated that this novel AMP-loaded nanotube surface significantly inhibited bacteria proliferation and effectively reduced bacterial adhesion on the surface. It was also found that the antimicrobial activities of the samples were highly dependent on the drug loading conditions. By changing the loading conditions, the bacteria killing rate after 4 hour incubation increased dramatically from 90% to 99.9%. In vitro study showed that the AMP-loaded nanotube samples are not cytotoxic for MG-63 osteoblast-like cells.

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