No abstract

We have put forth the hypothesis that collagen is mineralized during bone formation by means of a polymer-induced liquid-precursor (PILP) process, in which a liquid-phase mineral precursor could be drawn into the gaps and grooves of the collagen fibrils by capillary action, and upon solidification, leave the collagenous matrix embedded with nanoscopic crystallites of hydroxyapatite. This hypothesis is based upon our observations of capillarity seen for liquid-phase mineral precursors generated with calcium carbonate. Here, we demonstrate proof-of-concept of this mechanism by mineralizing Cellagen sponges (type I reconstituted bovine collagen) in the presence of a liquid-precursor phase to calcium carbonate. Scanning electron microscopy (SEM) was used to examine the mineralized collagen, which in combination with selective etching studies, revealed the extent to which the mineral phase infiltrated the collagenous matrix. A roughly periodic array of disk-like crystals was found to be embedded within the collagen fibers, demonstrating that the mineral phase spans across the diameter of the fibers. Some of the morphological features of the mineralized fibers in our in vitro model system are similar to those seen in natural bone (albeit of a different mineral phase), lending support to our hypothesis that these non-equilibrium morphologies might be generated by a PILP process. SEM provides a different perspective on the morphology of bone, and has been useful here for examining the extent of mineralization in composite structures generated via the PILP process. However, further investigation is needed to examine the nanostructural arrangement of the crystallites embedded within the collagenous matrix.

The synthesis of calcite fibers has been confined to nature and is observed most prominently in sea urchin teeth and bacterial deposits. By generating a liquid-phase mineral precursor, induced by the addition of acidic macromolecules to a crystallizing solution, calcite fibers with diameters ranging from 100 to 800 nm have been deposited on existing calcite substrate crystals. A solution-precursor-solid (SPS) mechanism, which has features similar to both the vapor-liquid-solid (VLS) and solution-liquid-solid (SLS) mechanisms, is proposed as the growth mechanism. Because this aqueous-based SPS mechanism occurs under physiological conditions (and down to temperatures as low as 4 °C), it is feasible that it may be used by organisms to form their fibrous biomineral structures. This discovery suggests an interesting link between two seemingly unrelated processes, high-temperature semiconductor fiber formation and biological mineralization.

The efficient capture of radionuclides with long half-lives such as technetium-99 ((99)Tc), uranium-238 ((238)U), and iodine-129 ((129)I) is pivotal to prevent their transport into groundwater and/or release into the atmosphere. While different sorbents have been considered for capturing each of them, in the current work, nanostructured chalcogen-based aerogels called chalcogels are shown to be very effective at capturing ionic forms of (99)Tc and (238)U, as well as nonradioactive gaseous iodine (i.e., a surrogate for (129)I2), irrespective of the sorbent polarity. The chalcogel chemistries studied were Co0.7Bi0.3MoS4, Co0.7Cr0.3MoS4, Co0.5Ni0.5MoS4, PtGe2S5, and Sn2S3. The PtGe2S5 sorbent performed the best overall with capture efficiencies of 98.0% and 99.4% for (99)Tc and (238)U, respectively, and >99.0% for I2(g) over the duration of the experiment. The capture efficiencies for (99)Tc and (238)U varied between the different sorbents, ranging from 57.3-98.0% and 68.1-99.4%, respectively. All chalcogels showed >99.0% capture efficiency for iodine over the test duration. This versatile nature of chalcogels can provide an attractive option for the environmental remediation of the radionuclides associated with legacy wastes from nuclear weapons production as well as wastes generated during nuclear power production or nuclear fuel reprocessing.

Biologically mineralized tissues are well recognized for their unusual crystal morphologies and hierarchically organized composite structures. The soluble acidic macromolecules associated with biominerals are thought to play an important role in modulating the mineral morphology. Our in vitro studies, which use acidic polypeptide additives to modify crystal growth of calcium-based minerals, have demonstrated a crystallization mechanism that proceeds via a liquid-phase mineral precursor. Various features of the crystals produced via this mechanism, such as "extruded" mineral fibers and mineralized collagen composites, have led us to propose the hypothesis that an amorphous, liquid-phase precursor could play a fundamental role in the morphogenesis of calcium-based biominerals. Although in vivo evidence of this process remains to be determined, we demonstrate crystallization features that mimic bone and dental enamel and suggest that this process could be relevant to biomineralization in both vertebrates and invertebrates.