2012-2013 Research Highlights

ERC-RMB continues to make strong progress in advancing technologies related to bioresorbable metals and testing their use in clinically relevant device prototypes. Some key accomplishments are highlighted below, a more complete listing and greater detail will be included in Volume II of the Annual Report to NSF.

• TMJ Screw: Second round in vivo testing has begun with a novel ERC alloy, WXQK, which has been implanted as TMJ screws in the rabbit mandible.
• ACL Screw: we have designed and manufactured the 2nd generation Mg-based interference screws, and successfully performed in vitro evaluation and an in vivo application. Mg-based interference screws were found to perform as well as titanium interference screws at time zero and after 12 weeks of implantation, healing results are positive.
• ACL-Ring: The design and surgical procedure for the Mg-based ring to assist ACL healing, and the new ring has been tested and shown to restore joint stability in vitro. The results are a significant improvement over suture repair alone.
• Bone Fracture Plate/Screw: Histological staining and microCT analysis demonstrated that Mg device degradation (implanted in Year 4) does not inhibit fracture healing or bone growth. Fractures stabilized with Mg devices showed full thickness healing similar to commercial devices. Conducting a second rabbit ulna fixation study in Year 5 with novel fixation device designs, uniquely based on Mg’s degradation properties.
• AV-Fistula Stent: A new porcine model and new stent delivery method were tested to support the AV-Fistula Stent Project. Gen II stents were tested in a porcine model. The preliminary animal experiment showed no systemic toxicity from the stent, and the fistulae were patent.
• Nerve Repair: Repeated in vivo experiments to repair short gaps in rat nerves with a device containing Mg. Refined techniques. Conducted first in vivo long nerve gap experiment in rats and developed simplified behavioral techniques and novel micro CT analysis methods to analyze results.
• In vivo testing capabilities were extended to the NCAT campus via partnership between the ERC RMB PIs, NCAT’s Laboratory Animal Sciences group, and ERC RMB industrial partners nanoMAG, Biomet and Preclinical Surgical Services. Biodegradation and safety of a nanoMAG alloy are being studied for orthopedic applications in a rabbit model. Through close collaboration, this team is helping nanoMAG advance its technology to an investment inflection point, and in doing so, demonstrating the value that collaboration with the ERC RMB can bring to its partners.

• A carefully-planned study of the corrosion of ERC RMB’s new magnesium alloys using nanotomography revealed deeper insights into their in vitro and in vivo behavior in terms of types and mechanisms of corrosion product formation. This investigation for the first time helps bridge the gap between static and dynamic environments. The knowledge gained is allowing the team to establish protocols for the corrosion testing of biodegradable metals, with an emphasis on experimental design. Iterative application of these protocols are informing the judicious selection of alloying elements that lead to a better Mg system with smaller and more uniform grain boundaries and smaller cathodic areas.
• ERC researchers developed a two-piece implant design for low voltage electronic control of corrosion of implants on demand. A coiled stent using AZ61 material was successfully dissolved electrochemically using corrosion control in eight days in vitro. In vivo experiments were conducted using mice to measure the “live impedance” of an Mg disk electrode, and for corrosion control of an Mg disk. Based on the initial results, improvements and miniaturization of the design is underway and further testing will be performed.
• The chemical byproducts of corrosion are being characterized by a corrosion characterization systems used at the three universities. Stripping voltammetry sensors to detect different ions from alloys in Mg materials and in vivo needle sensors are being developed and will be introduced into the sensors toolbox. Also a fiber optic sensor is being developed.
• Electrochemical sensors for Zn²⁺ (based on CNT thread from Dr. Shanov’s group) and Eu³⁺ to be used for monitoring corrosion of Mg alloys containing these elements during immersion tests were developed. A dynamic in vitro corrosion control system was transferred to CS to Dr. Sfeir’s group at Pitt.

• The materials team developed three series of Mg-based novel ERC alloys by permanent mold casting routes exhibiting controlled corrosion under physiological condition, moderate room temperature mechanical properties, and cytocompatibility comparable to commercial Pure Mg (US Magnesium Inc., 99.97% pure) and commercial AZ31 alloys. Post-processing of hot extrusion and rolling of as-cast/solution treated feedstock showing promising bio-corrosion and mechanical properties were selected for further screening in vivo
• The materials team also developed rare-earth containing Mg alloys with improved corrosion and mechanical properties. Using rare-earth elements and appropriate heat treatment procedures a new alloy was produced that is significantly improved over the best Mg-Zn-Ca alloys developed in previous years.
• Extrusion capabilities for RMB ERC alloys are being developed at NCAT. A runoff test and several extrusions will be conducted in April-May, 2013.
• Three Fe-based amorphous alloy powder and amorphous thin films have been successfully generated by HEMA and PLD, respectively. All coated films exhibited good cell viability. Preliminary bio- corrosion test of Fe based film and pure iron shows slightly increased corrosion rate of the new Fe based alloys compared to pure iron.
• 3D inkjet printing has been used successfully for generating porous biodegradable constructs of Fe-Mn alloys demonstrating the ability of the novel approach for generating customized systems using CT scans. Printed scaffolds exhibited tensile properties very similar to natural bone as well as desired accelerated corrosion compared to pure Fe.
• An optical method for evaluating thin film degradation rates in aqueous media at both ambient and physiological temperatures was established and tested on Mg, Ag, Cu, Fe and Zn metallic films and Mg/MgO, Mg/alumina multilayered coatings.
• The use of metal/metal-oxide nanolaminates for fine tuning of delayed corrosion of implants was proposed and studied. Nanoporosity, thickness, degradation rate and number of layers can be effectively used for the development of degradable coatings with targeted resorption times. Degradation rates in different media, hardness, elasticity, wear properties and biocompatibility of Mg/MgO and Mg/Al₂O₃ nanolaminates were evaluated. Currently we are testing these nanolaminate coatings on new metallic alloys developed by ERC RMB.
• The concept of metal-oxide nanolayers as a dielectric barrier against galvanically accelerated corrosion or loosening of metallic coating was proposed and tested for Mg and Cu coatings. We found that 50 nm layer of MgO is a prospective biocompatible degradable galvanic separator for metallic coatings on metallic substrates.
• Synthesized novel functional biodegradable polymer (poly sulfobetaine ester urethane (PSBEU)) to improve drug release functionality from non-thrombogenic biodegradable coating. A patent application was filed for Biodegradable, Non-thrombogenic Elastomeric Polyurethanes used to coat stents and other blood contacting medical devices.
• A method for evaluating corrosion in a subcutaneous mouse model was developed and used to evaluate ERC RMB alloys.  Novel materials include alloys of Mg/Zn/Ca with variations of Zn (1, 2, or 5%) and a coating from Dr. Yun’s group (NCAT), Mg/Zn/Ca alloys developed by Dr. Xu’s group (NCAT), the alloys of ZK, WXK, and WXQK developed by Dr. Kumta’s group (Pitt), and pellets of Mg single crystal from Dr. Shanov’s group (UC) were studied or are under investigation. Data show that: 1) none of the novel alloys generated in the ERC RMB caused significant toxicity in mice; and 2) corrosion rates of novel alloys varied significantly, suggesting that ERC RMB generated materials are potentially suitable for different therapeutic purposes.

• Examination of gene expression following stimulation of Bone Marrow Stromal Cells with Mg²⁺ in media for 3wks revealed that several bone matrix genes were up-regulated. Western blotting revealed that Mg²⁺ increased the production of collagens, revealing possible explanations for the increase in in vitro proliferation and mineralization, as well as in vivo osteogenesis observed as a result of Mg²⁺ exposure.
• Incorporation of divalent ions into calcium phosphate coatings was found to significantly enhance the osteogenic differentiation of human mesenchymal stem cells cultured on coated substrates.

• AV- Fistula Stent: 3^rd Generation stents made from optimized ERC RMB alloys being prepared for in vivo testing. An improved stent design will be used. Study endpoints will include time to degradation, quality of formed fistula, time to fistula maturation, local toxicity and systemic toxicity.
• Degradable Plates and Screws for Fracture Fixation: Plates and screws made from promising ERC RMB alloys are being prepared. Device design has been refined computationally. Testing of new alloys in a rabbit model will begin late 2Q / early 3Q 2013. Study endpoints will include fracture healing, nature of device degradation (time, uniformity, etc.), local toxicity and systemic toxicity.
• Magnesium Based Scaffolds for Bone Grafts: Methods for preparation of porous metal scaffolds are being pursued to support this approach.
• Mg based Ring for ACL Healing: Improved fixation has been demonstrated ex vivo. Prototype devices are being made from commercially available alloys for a proof of concept study designed to demonstrate improved soft tissue healing and to collect information for preparation of an improved device (materials and design).
• Mg Based Soft Tissue Fixation Device: Next generation ACL-screws are being prepared from novel ERC RMB alloys. Large animal testing will begin in 2Q / 3/Q 2013 to evaluate performance of the device through clinically relevant timeframes for both healing and device resorption. A provisional patent application has been filed.
• Nerve Repair Guide: Proof of principle studies are ongoing for long gap nerve repair. Current efforts are focused on optimizing choice of Mg alloy and corrosion control method for preparation of Mg wire suitable for use in the nerve guides. A provisional patent application has been filed.
• New Projects: Initial feasibility is being explored for new projects: a Mg coated dental implant for improved osteointegration, a Mg containing injectable bone putty that has the potential to be replaced by bone, and the use of restorable metal stents for placement of extracellular matrix materials in trachea, esophagus, etc. Commercial viability and technical feasibility are being assessed prior to elevation to full project status.
   

• On November 28, 2012, ASTM and FDA co-sponsored a workshop to provide a forum for industry, academia and FDA to discuss test methods for establishing correlations between in vitro and in vivo degradation of absorbable implant devices, and the interaction of mechanical loading and mechanical performance with degradation, with a particular focus on cardiovascular devices.   This workshop (ASTM-FDA Workshop on Absorbable Medical Devices: Lessons Learned from Correlations of Bench Testing and Clinical Performance) which was held at the FDA’s White Oak, MD, campus, was unique in that it brought researchers working in the absorbable polymers and metals fields together to share their experiences and challenges. ERC-RMB was invited to represent the state of the art in the absorbable magnesium alloy space. ERC members Lisa Ferrara (IAB Chair), Yeoheung Yun (Corrosion Group Leader), and Frank Witte (Global Coordinator) were presenters at this meeting, which was attended by other ERC members as well. These presentations, along with contacts made or strengthened at the meeting (ASTM, ISO, FDA, etc.) helped to establish ERC-RMB as one of the trailblazers in establishing future protocols and standards for degradable medical device development.
• The 4th Symposium on Biodegradable Metals was held in Maratea, Italy from August 28 to September 1, 2012. The symposium is characterized by its academic-conference-style with an openly discursive format, rather than a lecture and question–answer format with panel discussions at the end of each session. Suggestions for standard materials screening tests, device performance tests and reference standard alloys were a key topic of discussion, leading to the 1st Minimum Consensus on Standardization in Biodegradable Metals. Results of this discussion were shared by Dr. Witte at the ASTM-FDA Workshop. Several of the ERC RMB leadership and key investigators attend and were central to discussions relating to development of standards.