▪ Abstract  The term bone refers to a family of materials, all of which are built up of mineralized collagen fibrils. They have highly complex structures, described in terms of up to 7 hierarchical levels of organization. These materials have evolved to fulfill a variety of mechanical functions, for which the structures are presumably fine-tuned. Matching structure to function is a challenge. Here we review the structure-mechanical relations at each of the hierarchical levels of organization, highlighting wherever possible both underlying strategies and gaps in our knowledge. The insights gained from the study of these fascinating materials are not only important biologically, but may well provide novel ideas that can be applied to the design of synthetic materials.

Amorphous calcium carbonate (ACC) in its pure form is highly unstable, yet some organisms produce stable ACC, and cases are known in which ACC functions as a transient precursor of more stable crystalline aragonite or calcite. Studies of biogenic ACC show that there are significant structural differences, including the observation that the stable forms are hydrated whereas the transient forms are not. The many different ways in which ACC can be formed in vitro shed light on the possible mechanisms involved in stabilization, destabilization, and transformation of ACC into crystalline forms of calcium carbonate. We show here that ACC is a fascinating form of calcium carbonate that may well be of much interest to materials science and biomineralization.

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Acidic matrix macromolecules are intimately involved in biological crystal growth. In vitro experiments, in which crystals of calcium dicarboxylate salts were grown in the presence of aspartic acid-rich proteins, revealed a stereochemical property common to all the interacting faces. Calcite crystals are nucleated on stereochemically analogous faces when proteins are adsorbed onto a rigid substrate. The importance of this property in biomineralization is discussed. studies of the effect of "tailor-made" low molecular weight additives on the growth of organic crystals (13,14). These studies showed that stereoselective adsorption of an additive onto a specific crystal face results in a drastic decrease in its growth rate relative to that of unaffected faces. Since the crystal morphology is determined by the relative growth rate of the slowest-growing faces, the interaction of the additive with the crystal affects the overall morphology (Scheme 1).Biologically formed crystals are an integral part of many organisms. Their presence in the skeletons of invertebrates and vertebrates is particularly widespread. The crystals often are all of uniform size, have oriented crystallographic axes, and adopt sizes and shapes ("crystal habits") quite different from those found in their nonbiological counterparts. These properties indicate that the crystals form under wellcontrolled conditions. A common mode of crystal growth in these tissues is through the initial formation of a structural framework (the "organic matrix") in which the crystals subsequently grow (1). The regulation of crystal growth is partly accomplished by an array of matrix macromolecules, many of which are synthesized by specific cells for this purpose (2-4). A subset of the matrix macromolecules are closely associated with the mineral phase and hence are thought to regulate crystal growth by some as yet unknown mechanisms (2). These macromolecules are characteristically acidic in nature (2-5).One widely used approach for studying the functions of these acidic macromolecules is to examine in various ways their effect on crystal growth in vitro. The kinetics of crystal growth have been measured in the presence of different matrix macromolecules under a variety of conditions (6-9).Combinations of matrix components have been used to detect a collaborative effect (10), and the ability of demineralized matrix to induce crystal nucleation has been examined (11, 12). In this study, we used a different in vitro approach.We examined the manner in which acidic matrix macromolecules interact with different structured surfaces of various crystals. Our objective was to understand the principles that govern these interactions and to gain insight into the mechanisms by which these matrix constituents regulate crystal growth in vivo. We report here a particular stereochemical property of the crystal surfaces we examined that appears to be an essential requirement for interaction to occur with one of the major groups of matrix constituents, the acidic proteins. These ...

Sea urchin larvae form an endoskeleton composed of a pair of spicules. For more than a century it has been stated that each spicule comprises a single crystal of the CaCO $ mineral, calcite. We show that an additional mineral phase, amorphous calcium carbonate, is present in the sea urchin larval spicule, and that this inherently unstable mineral transforms into calcite with time. This observation significantly changes our concepts of mineral formation in this well-studied organism.

The control of crystal formation has been developed to a remarkable degree by many organisms. Oriented nucleation, control over crystal morphology, formation of unique composites of proteins and single crystals, and the production of ordered multicrystal arrays, are all well within the realm of biological capability. Understanding the control and design principles in biomineralization is a fascinating subject that may well contribute to the improved fabrication of synthetic materials on the one hand, and to the solution of many serious pathological problems involving mineralization, on the other.

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Communications CED MATERIALS mono-thiophene (N-[(6-(thien-3-yl)hexanoyloxy]-pyrroli-dine-2,5-dione; see Scheme 1) instead of the bithiophene derivative, we have been able to prepare analogous glucose oxidase-modified polymer films. The functionalized polythiophene film has been obtained using a similar multisweep regime, however, with potential scans up to a vertex potential of 1.7 V vs. SCE, reflecting the higher potential of the radical cations formation. The second step, the covalent immobilization of the enzyme is of course equivalent and independent from the specific needs for the formation of the polymer film. The obtained enzyme electrodes show a slightly lower response as those obtained with the functionalized poly(bithiophene) films, despite there are double active ester groups available to react with the enzyme. Probably, due to the size of an individual enzyme molecule the increased number of binding sites at the electrode surface does not concomitantly increase the immobilized enzyme activity.To conclude, we have shown that, despite the high oxidation potential of thiophene and its derivatives, polythiophene films can be advantageously used for the entrapment or covalent immobilization of enzymes. This was realized by entrapment of glucose oxidase using dimeric or trimeric thiophenes as parent compounds for the polymerization process in CH3CN/water mixtures or, according to a two-step procedure, based on functionalized polythiophene or poly(bithi0phene) films and covalently bound enzyme. In principle, polythiophene as immobilization matrix in amperometric enzyme electrodes has at least two advantages over other conducting polymer films: First, derivatization of thiophene in the 3-position and the synthesis of oligomers is much easier than for pyrrole. Second, due to the higher redox potential of the polymer film itself, deterioration e.g. by enzymatically generated H202 is less probable. This is even more important for a next sensor generation, in which the conducting polymer is expected to act as molecular wire between the active site of the enzyme and the electrode surface. Possibly, the improved stability of polythiophene against oxidation in ambient environments will allow the construction of biosensors with improved sensor characteristics and life-time.Some organisms form stable minerals that would never precipitate under ambient conditions in an inorganic environment, or, if formed, are unstable precursors of more stable compounds.['321 One of the most fascinating examples is biogenic amorphous calcium carbonate, because its transformation into crystalline polymorphs is not only thermodynamically favored, but also kinetically fast.I3] We report here an example of a single skeletal element, spicules from the calcareous sponge Clathrina, composed of crystalline calcite in one layer and stable amorphous CaC03 in another. Differential dissolution of the amorphous phase of these spicules released macromolecules with proteins rich in glutamic acid (and/or glutamine), serine, glycine and polysaccharide...