Hong, Daeho
(2016)
Fundamental Study of the Design and Development of Magnesium-Zinc Based Alloys for Biodegradable Implant Devices.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Inert but biocompatible metals and biodegradable polymers are currently used as orthopedic fixations devices to achieve bone healing. However, the high stiffness of permanent metals such as stainless steel (SS), cobalt-chromium (Co-Cr), and titanium (Ti) alloys can cause stress shielding and loosening of the surrounding bone. Long term use of these devices can also result in wear related debris and associated risks of metallosis requiring secondary removal surgeries causing further complications. Inferior strength and acidic degradation products of biodegradable polymers on the other hand, also limit their use as orthopedic devices. Recently, biodegradable magnesium (Mg) and Mg-based alloys have received wide attention and have become the target of intense research due to the benefit of degradability, suitable mechanical properties matching natural bone and the desired biocompatibility. The degradation mechanism of biodegradable magnesium is primarily corrosion which tends to be highly rapid for pure Mg and most Mg based alloys resulting in rapid evolution of unwanted hydrogen gas pockets. Hence, controlling the corrosion of magnesium has been the key challenge limiting the technological development, thwarting the accelerated progress of implementation of Mg based alloys for various biomedical applications. Rapid corrosion of magnesium in the body and the associated hydrogen gas evolution results in device failure, and consequent risks of pain and potential infection. Alteration of alloy composition and the associated microstructure has been typically employed to improve the corrosion resistance and achieve the desired mechanical properties while maintaining the essential biocompatibility suitable for a desired biomedical application.
In this work, Mg-Zn based alloys were developed for biodegradable orthopedic fixation device applications. Magnesium-zinc alloys have the advantage of using zinc as a major alloying element that is considered biocompatible up to a daily dose of 11 mg. Mg-Zn-Zr was thus selected as the alloy system that was accordingly processed systematically varying the elemental composition, and characterizing the resultant system for phase, structure, microstructure, in-vitro corrosion and cytocompatibility properties yielding promising preliminary results. Results of characterization showed that the processing related Mg-Zn intermetallic precipitates formed at the grain boundaries acting as initiation centers for corrosion leading to reduction in the corrosion resistance. Thus, strontium and cerium were selected and added as micro alloying elements to improve the corrosion resistance due to alteration of the alloy phase dynamics and thermodynamic stability of the precipitates. In addition, the osteogenic attributes of Mg, Sr, and Ce ions were studied using human mesenchymal stem cells.
Furthermore, to demonstrate the biosafety of Mg-Zn alloys, Mg-Zn-Sr-Zr pins were implanted in a rat femoral fracture model. The model was selectively designed to result in extreme stress on the machined Mg-Zn pin hardware and also test their biocompatibility under load-bearing conditions. As intended, higher degradation rates and device failures were observed in the micro-computed tomography (micro-CT) images. Despite the pre-designed extreme stress assisted corrosion observed, normal fracture healing response was still exhibited in the micro-CT images and bone histology demonstrating the efficacy of the alloy system. Moreover, no systemic or local tissue toxicity were detected from blood, liver and kidney assessments further validating the beneficial aspects of this alloys system for orthopedic applications.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
5 February 2016 |
| Date Type: |
Publication |
| Defense Date: |
20 November 2015 |
| Approval Date: |
5 February 2016 |
| Submission Date: |
17 November 2015 |
| Access Restriction: |
5 year -- Restrict access to University of Pittsburgh for a period of 5 years. |
| Number of Pages: |
182 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Bioengineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Biomaterials,Magnesium alloys, Biodegradable Metal, Orthopedic aplication, bone fixation, bone regeneration |
| Date Deposited: |
05 Feb 2017 06:00 |
| Last Modified: |
05 Feb 2021 06:15 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/26355 |
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