Amos Doepke

Revolutionizing biodegradable metals

Development of biodegradable metal implants is a complex problem because it combines engineering and medical requirements for a material. This article discusses the development of sensing and corrosion control techniques that can help in the design of biodegradable metallic implants. Biodegradable metallic implants dissolve as new tissue is formed. One of the most important factors in the design of biodegradable implants is to study the active interface, which should be monitored and controlled to address the medical concern of biocompatibility. Thus miniaturized and nanotechnology-based sensors that measure the activities of the degradation process and the formation of tissue are discussed for use with in vitro and in vivo experiments. These sensors can monitor chemical components and also cell activity and can provide new knowledge about biodegradable interfaces and how to actively control the interface to provide the best bioactivity to regenerate new tissue in a short time. Development of new alloys, nano-materials, miniature sensors, corrosion control coatings, and auxiliary applications such as biodegradable drug delivery capsules is expected to open up a new era in the engineering of materials for medicine.

Carbohydrate-based label-free detection of Escherichia coli ORN 178 using electrochemical impedance spectroscopy.

A label-free biosensor for Escherichia coli (E. coli) ORN 178 based on faradaic electrochemical impedance spectroscopy (EIS) was developed. α-Mannoside or β-galactoside was immobilized on a gold disk electrode using a self-assembled monolayer (SAM) via a spacer terminated in a thiol functionality. Impedance measurements (Nyquist plot) showed shifts due to the binding of E. coli ORN 178, which is specific for α-mannoside. No significant change in impedance was observed for E. coli ORN 208, which does not bind to α-mannoside. With increasing concentrations of E. coli ORN 178, electron-transfer resistance (R(et)) increases before the sensor is saturated. After the Nyquist plot of E. coli/mixed SAM/gold electrode was modeled, a linear relationship between normalized R(et) and the logarithmic value of E. coli concentrations was found in a range of bacterial concentration from 10(2) to 10(3) CFU/mL. The combination of robust carbohydrate ligands with EIS provides a label-free, sensitive, specific, user-friendly, robust, and portable biosensing system that could potentially be used in a point-of-care or continuous environmental monitoring setting.

A system for characterizing Mg corrosion in aqueous solutions using electrochemical sensors and impedance spectroscopy

Understanding Mg corrosion is important to the development of biomedical implants made from Mg alloys. Mg corrodes readily in aqueous environments, producing H2, OH- and Mg2+. The rate of formation of these corrosion products is especially important in biomedical applications where they can affect cells and tissue near the implant. We have developed a corrosion characterization system (CCS) that allows realtime monitoring of the solution soluble corrosion products OH-, Mg2+, and H2 during immersion tests commonly used to study the corrosion of Mg materials. Instrumentation was developed to allow the system to also record electrochemical impedance spectra simultaneously in the same solution to monitor changes in the Mg samples. We demonstrated application of the CCS by observing the corrosion of Mg (99.9%) in three different corrosion solutions: NaCl, HEPES buffer, and HEPES buffer with NaCl at 37°C for 48 h. The solution concentrations of the corrosion products measured by sensors correlated with the results using standard weight loss measurements to obtain corrosion rates. This novel approach gives a better understanding of the dynamics of the corrosion process in realtime during immersion tests, rather than just providing a corrosion rate at the end of the test, and goes well beyond the immersion tests that are commonly used to study the corrosion of Mg materials. The system has the potential to be useful in systematically testing and comparing the corrosion behavior of different Mg alloys, as well as protective coatings.

The morphology and evolution of bipyramidal gold nanoparticles.

We examine the growth and evolution with time of bipyramidal gold nanoparticles grown by a seed-mediated process. The nanoparticles are characterized both by their physical dimensions determined by transmission electron microscopy and by the wavelength position of their localized surface plasmon resonance. Each growths physical dimensions correspond to particular initial conditions, and we observe two distinct modes of temporal evolution during growth. The effects of varying silver nitrate concentration and growth time are also explored. We observe a linear relationship between the tip radius of curvature and the wavelength of the longitudinal localized surface plasmon resonance (LSPR) peak. Critical parameters for synthesizing bipyramidal nanoparticles with sharp tips and correct length to width ratio are determined.

Biodegradable Mg for Bone Implants: Corrosion and Osteoblast Culture Studies

Corrosion and cell culture experiments were performed to evaluate magnesium (Mg) as a possible biodegradable implant material. The corrosion current and potential of a Mg disk were measured in different physiological solutions. The corrosion currents in cell culture media were found to be higher than in deionized water, which verifies that corrosion of Mg occurs faster in chloride solution. Weight loss, open-circuit potential, and electrochemical impedance spectroscopy measurements were also performed. The Mg specimens were also characterized using an environmental scanning electron microscope and energy-dispersive x-ray analysis (EDAX). The x-ray analysis showed that in the cell culture media a passive interfacial layer containing oxygen, chloride, phosphate, and potassium formed on the samples. U2OS cells were then co-cultured with a Mg specimen for up to one week. Based on visual observation, cell growth and function were not significantly altered by the presence of the corroding Mg sample. These initial results indicate that Mg may be suitable as a biodegradable implant material. Future work will develop small sensors to investigate interfacial biocompatibility of Mg implants.Copyright

Responsive Biosensors for Biodegradable Magnesium Implants

A biosensor is an electronic device that measures biologically important parameters. An example is a sensor that measures the chemicals and materials released during corrosion of a biodegradable magnesium implant that impact surrounding cells, tissues and organs. A responsive biosensor is a biosensor that responds to its own measurements. An example is a sensor that measures the corrosion of an implant and automatically adjusts (slows down or speeds up) the corrosion rate. The University of Cincinnati, the University of Pittsburgh, North Carolina A&T State University, and the Hannover Medical Institute are collaborators in an NSF Engineering Research Center (ERC) for Revolutionizing Metallic Biomaterials (RBM). The center will use responsive sensors in experimental test beds to develop biodegradable magnesium implants. Our goal is to develop biodegradable implants that combine novel bioengineered materials based on magnesium alloys, miniature sensor devices that monitor and control the corrosion, and coatings that slow corrosion and release biological factors and drugs that will promote healing in surrounding tissues. Responsive biosensors will monitor what is happening at the interface between the implant and tissue to ensure that the implant is effective, biosafe, and provides appropriate strength while degrading. Corrosion behavior is a critical factor in the design of the implant. The corrosion behavior of implants will be studied using biosensors and through mathematical modeling. Design guidelines will be developed to predict the degradation rate of implants, and to predict and further study toxicity arising from corrosion products (i.e., Mg ion concentrations, pH levels, and hydrogen gas evolution). Knowing the corrosion rate will allow estimations to be made of implant strength and toxicity risk throughout the degradation process.Copyright

Carbon nanotube responsive materials and applications

Carbon nanotubes (CNTs) have attracted a lot of interest in the past 20 years. Superior mechanical, …