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Title: Shape memory polymer composites for bone repair
Authors: Zhang, Yuanchi
Degree: Ph.D.
Issue Date: 2019
Abstract: Bone defect has become one of the most concerning issues in public health area due to its high cause of disability and morbidity. In the United States alone, there are more than 1.5 million cases of osteoporotic vertebral fracture which costs US$ 12-18 billion every year. The treatment for large bone defects has been studied with the "gold standard" using the autograft while the chronic inflammation, disease transmission and graft rejection hinder its development. In addition, metal-based materials are the most used implants while issues such as stress shield, infection and inflammation lead to researchers highly expect new materials to replace them. The terrible processability limits their employments in minimally invasive surgery that is desired because extensile surgical exposure could negatively affect the biological environment of the damaged site. As one of the smart materials, shape memory polymers (SMPs) like shape memory polyurethane (SMPU) could achieve the minimally invasive function due to its shape memory effect (SME). Nevertheless, the insufficient mechanical performances are still the major limitation of SMPUs in bone repair. To improving the mechanical properties of SMPUs, nanofillers e.g. hydroxyapatite (HA), graphene oxide (GO), reduced GO (rGO) are utilized to modify the composites. However, issues still exist in the SMPU composites: (1) in many published reports, claimed mechanical properties of SMP composite were still not satisfied; (2) in some cases, the nanofillers could disturb strength of SMPU composites; (3) several SMPU composites were reported to achieve high modulus with sacrifice of SME. Besides, as bone repairing material, biological studies on the composites are unsatisfactory. For example, screw has been such a common treatment choice to fix fragments of the fracture bone or support defect while the memory screw with in vitro and/or in vivo investigations have rarely been published.
In order to narrow the research gaps, this study is to design, preparation, and investigate the biocompatible SMP nanocomposites and implants. Specifically, this work is to (a) design the adaptable shape memory polymer composites according to the biological environment and the characteristics of bone repair; (b) study the blending approach and structure of the shape memory polymer composite; (c) investigate the mechanical and memory properties of the shape polymer composites; (d) study the influence of shape memory composites on the cell behavior, tissue growth and biomechanical performance. Based on these objectives, we selected SMPU as the polymer matrix; HA, GO, rGO as the reinforcing nanofillers, RGD peptide as the tool to improve biocompatibility. We prepared physical blending SMP composites with single-modification (SMPU/HA, SMPU/GO, SMPU/rGO) and multi-modification (SMPU/HA/rGO/RGD). Then, chemical bonding SMP composites with a single-modification (GO-SMPU composites, omG-SMC) and with a multi-modification (SMPU/HA/RGD) were prepared. The memory screw, as the implants for bone repair was designed and fricated using the SMPU/HA/RGD composite. The SMPU nanocomposites were characterized in terms of morphology and structure, thermal and mechanical properties as well as shape memory behaviours. Cell experiment and animal model were conducted to investigate the biocompatibility of nanocomposites and the memory screw, respectively. It was demonstrated that SMPU/HA, SMPU/GO and SMPU/rGO nanocomposites possessed significantly enhanced mechanical properties (e.g. >100% in Young's modulus and ~100% in tensile stress). In addition, the shape memory properties of our nanocomposites stay a high level (e.g. >90% in shape memory fixity ratios and ~80% in shape memory recovery ratios). For the SMPU/HA/rGO/RGD nanocomposite, the results showed an approximately 200% enhancement in Young's modulus and more than 300% reinforcement in tensile strength of SMPU/HA/rGO/RGD nanocomposite compared with that of pristine SMPU. Besides, RGD highly promotes the adhesion, viability and proliferation of the cells resulting in the improved biocompatibility of the nanocomposites. Based on the omG-SMC, this study provides a novel "trampoline" nanocomposite with coalesced-excellent mechanical (~456.72MPa of Young's modulus) and memory properties (~100% of Rr) for high load-bearing and minimally invasive bone repair. As for the SMPU/HA/RGD, the remarkably enhanced mechanical and memory properties e.g. ~250 MPa of Young's modulus, Rr of ~96% is a litter lower than that of omG-SMC, but the introducing of HA and RGD offers nanocomposite with the osteoconduction and osteoinduction as well as excellent biocompatibility. As results, the memory screw made by the SMP/HA/RGD has could boost the growth of the new bone tissues and enhance biomechanical properties of the defected bones. We have full confidences these nanocomposites could pave a new way to design and prepare the biomedical materials. Besides, these diverse composites systems could meet various requirements in bone repair and the memory screw has a huge practically potential for high load-bearing locations and minimally invasive therapy.
Subjects: Hong Kong Polytechnic University -- Dissertations
Bone substitutes
Shape memory polymers
Biomedical materials
Nanocomposites (Materials)
Pages: 239 pages : color illustrations
Appears in Collections:Thesis

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