Biodegradable metals are breaking the current paradigm in biomaterial science to develop only corrosion resistant metals. In particular, metals which consist of trace elements existing in the human body are promising candidates for temporary implant materials. These implants would be temporarily needed to provide mechanical support during the healing process of the injured or pathological tissue. Magnesium and its alloys have been investigated recently by many authors as a suitable biodegradable biomaterial. In this investigative review we would like to summarize the latest achievements and comment on the selection and use, test methods and the approaches to develop and produce magnesium alloys that are intended to perform clinically with an appropriate host response.

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Degradable biomaterials based on magnesium corrosion

The development of binary Mg–Ca alloys for use as biodegradable materials within bone

http://lbmd.coe.pku.edu.cn/PDF/2009biomaterials01.pdf

In vitro corrosion and biocompatibility of binary magnesium alloys

The knowns, known unknowns and unknowns of sequestration of soil organic carbon

Soils are the most heterogeneous parts of the biosphere, with an extremely high differentiation of properties and processes within nano- to macroscales. The spatial and temporal heterogeneity of input of labile organics by plants creates microbial hotspots over short periods of time – the hot moments. We define microbial hotspots as small soil volumes with much faster process rates and much more intensive interactions compared to the average soil conditions. Such hotspots are found in the rhizosphere, detritusphere, biopores (including drilosphere) and on aggregate surfaces, but hotspots are frequently of mixed origin. Hot moments are short-term events or sequences of events inducing accelerated process rates as compared to the average rates. Thus, hotspots and hot moments are defined by dynamic characteristics, i.e. by process rates. For this hotspot concept we extensively reviewed and examined the localization and size of hotspots, spatial distribution and visualization approaches, transport of labile C to and from hotspots, lifetime and process intensities, with a special focus on process rates and microbial activities. The fraction of active microorganisms in hotspots is 2–20 times higher than in the bulk soil, and their specific activities (i.e. respiration, microbial growth, mineralization potential, enzyme activities, RNA/DNA ratio) may also be much higher. The duration of hot moments in the rhizosphere is limited and is controlled by the length of the input of labile organics. It can last a few hours up to a few days. In the detritusphere, however, the duration of hot moments is regulated by the output – by decomposition rates of litter – and lasts for weeks and months. Hot moments induce succession in microbial communities and intense intra- and interspecific competition affecting C use efficiency, microbial growth and turnover. The faster turnover and lower C use efficiency in hotspots counterbalances the high C inputs, leading to the absence of strong increases in C stocks. Consequently, the intensification of fluxes is much stronger than the increase of pools. Maintenance of stoichiometric ratios by accelerated microbial growth in hotspots requires additional nutrients (e.g. N and P), causing their microbial mining from soil organic matter, i.e. priming effects. Consequently, priming effects are localized in microbial hotspots and are consequences of hot moments. We estimated the contribution of the hotspots to the whole soil profile and suggested that, irrespective of their volume, the hotspots are mainly responsible for the ecologically relevant processes in soil. By this review, we raised the importance of concepts and ecological theory of distribution and functioning of microorganisms in soil.

Microbial hotspots and hot moments in soil: Concept & review

In vitro and in vivo corrosion measurements of magnesium alloys.

Biodegradable magnesium–hydroxyapatite metal matrix composites

In vivo corrosion and corrosion protection of magnesium alloy LAE442

Pore network geometries of intra-aggregate pore spaces are of great importance for water and ion flux rates controlling C sequestration and bioremediation. Advances in non-invasive three-dimensional imaging techniques such as synchrotron-radiation-based x-ray microtomography (SR-pCT), offer excellent opportunities to study the interrelationships between pore network geometry and physical processes at spatial resolutions of a few micrometers. In this paper we present quantitative three-dimensional pore-space geometry analyses of small scale (∼5 mm across) soil aggregates from different soil management systems (conventionally tilled vs. grassland). Reconstructed three-dimensional microtomography images at approximate isotropic voxel resolutions between 3.2 and 5.4 pm were analyzed for pore-space morphologies using a suite of image processing algorithms associated with the software published by Lindquist et al. Among the features quantified were pore-size distributions (PSDs), throat-area distributions, effective throat/pore radii ratios as well as frequency distributions of pore channel lengths, widths, and flow path tortuosities. We observed differences in storage and transport relevant pore-space morphological features between the two aggregates. Nodal pore volumes and throat surface areas were significantly smaller for the conventionally tilled (Conv. T) aggregate (mode ≈ 7.9 x 10 -7 mm 3 /≈ 63 μm 2 ) than for the grassland aggregate (mode ≈ 5.0 x 10 -6 mm 3 /≈ 400 μm 2 ), respectively. Path lengths were shorter for the Conv. T. aggregate (maximum lengths 600 μm). In summary, the soil aggregate from the Conv. Tsite showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system.

Three-Dimensional Quantification of Intra-Aggregate Pore-Space Features using Synchrotron-Radiation-Based Microtomography

In autumn 2005 the GKSS-Research Center Geesthacht in cooperation with Deutsches Elektronen-Synchrotron
DESY, Hamburg, started operation of the new synchrotron radiation beamline HARWI-2. The beamline is
specialized for materials science experiments using hard X-rays. Recently the fixed-exit monochromator for
imaging application was installed. Using different sets of crystals in combination with an adapted setup of the
beamline optics allow for using an intense and large monochromatic X-ray beam in the energy range of 15 to
200 keV. Investigations performing microtomography in the different energy regions are presented. Furthermore
the user experiment for microtomography operated by the GKSS at beamline BW2 was enhanced to perform
continuous tomographic investigations.

J. Fischer

Papers

In vitro and in vivo corrosion measurements of magnesium alloys.

Biodegradable magnesium–hydroxyapatite metal matrix composites

In vivo corrosion and corrosion protection of magnesium alloy LAE442

Three-Dimensional Quantification of Intra-Aggregate Pore-Space Features using Synchrotron-Radiation-Based Microtomography

New developments for synchrotron-radiation-based microtomography at DESY