Directed nucleation and growth by balancing local supersaturation and substrate/nucleus lattice mismatch.

Controlling nucleation and growth is crucial in biological and artificial mineralization and self-assembly processes. The nucleation barrier is determined by the chemistry of the interfaces at which crystallization occurs and local supersaturation. Although chemically tailored substrates and lattice mismatches are routinely used to modify energy landscape at the substrate/nucleus interface and thereby steer heterogeneous nucleation, strategies to combine this with control over local supersaturations have remained virtually unexplored. Here we demonstrate simultaneous control over both parameters to direct the positioning and growth direction of mineralizing compounds on preselected polymorphic substrates. We exploit the polymorphic nature of calcium carbonate (CaCO3) to locally manipulate the carbonate concentration and lattice mismatch between the nucleus and substrate, such that barium carbonate (BaCO3) and strontium carbonate (SrCO3) nucleate only on specific CaCO3 polymorphs. Based on this approach we position different materials and shapes on predetermined CaCO3 polymorphs in sequential steps, and guide the growth direction using locally created supersaturations. These results shed light on nature's remarkable mineralization capabilities and outline fabrication strategies for advanced materials, such as ceramics, photonic structures, and semiconductors.

Elastocapillary coalescence of plates and pillars.

When a fluid-immersed array of supported plates or pillars is dried, evaporation leads to the formation of menisci on the tips of the plates or pillars that bring them together to form complex patterns. Building on prior experimental observations, we use a combination of theory and computation to understand the nature of this instability and its evolution in both the two- and three-dimensional setting of the problem. For the case of plates, we explicitly derive the interaction torques based on the relevant physical parameters associated with pillar deformation, contact-line pinning/depinning and fluid volume changes. A Bloch-wave analysis for our periodic mechanical system captures the window of volumes where the two-plate eigenvalue characterizes the onset of the coalescence instability. We then study the evolution of these binary clusters and their eventual elastic arrest using numerical simulations that account for evaporative dynamics coupled to capillary coalescence. This explains both the formation of hierarchical clusters and the sensitive dependence of the final structures on initial perturbations, as seen in our experiments. We then generalize our analysis to treat the problem of pillar collapse in three dimensions, where the fluid domain is completely connected and the interface is a minimal surface with the uniform mean curvature. Our theory and simulations capture the salient features of experimental observations in a range of different situations and may thus be useful in controlling the ensuing patterns.

Control of bacterial biofilm growth on surfaces by nanostructural mechanics and geometry.

Surface-associated communities of bacteria, called biofilms, pervade natural and anthropogenic environments. Mature biofilms are resistant to a wide range of antimicrobial treatments and therefore pose persistent pathogenic threats. The use of surface chemistry to inhibit biofilm growth has been found to only transiently affect initial attachment. In this work, we investigate the tunable effects of physical surface properties, including high-aspect-ratio (HAR) surface nanostructure arrays recently reported to induce long-range spontaneous spatial patterning of bacteria on the surface. The functional parameters and length scale regimes that control such artificial patterning for the rod-shaped pathogenic species Pseudomonas aeruginosa are elucidated through a combinatorial approach. We further report a crossover regime of biofilm growth on a HAR nanostructured surface versus the nanostructure effective stiffness. When the 'softness' of the hair-like nanoarray is increased beyond a threshold value, biofilm growth is inhibited as compared to a flat control surface. This result is consistent with the mechanoselective adhesion of bacteria to surfaces. Therefore by combining nanoarray-induced bacterial patterning and modulating the effective stiffness of the nanoarray--thus mimicking an extremely compliant flat surface--bacterial mechanoselective adhesion can be exploited to control and inhibit biofilm growth.

Coexistence of amorphous and crystalline calcium carbonate in skeletal tissues.

We describe a new type of composite skeletal tissues in which calcite and stabilized amorphous calcium carbonate (ACC) coexist in well-defined domains. The organisms that form such structures are widely separated in the animal kingdom phylogenetic tree: calcareous sponges and ascidians. This paper compares the microstructures of their composite skeletal elements: The triradiate spicules from the sponge Clathrina are composed of a core of calcite embedded in a thick layer of ACC and covered by a thin calcitic envelope; the tunic spicules from the ascidian Pyura pachydermatina are composed of a core of ACC enveloped by an insoluble organic sheath and covered by a thick calcitic layer. We compare and contrast the macromolecules associated with different amorphous and crystalline phases and their ability to induce the formation of stabilized ACC in vitro.

Calcitic microlenses as part of the photoreceptor system in brittlestars.

Photosensitivity in most echinoderms has been attributed to 'diffuse' dermal receptors. Here we report that certain single calcite crystals used by brittlestars for skeletal construction are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma showed that in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design. These structures are absent in light-indifferent species. Photolithographic experiments in which a photoresist film was illuminated through the lens array showed selective exposure of the photoresist under the lens centres. These results provide experimental evidence that the microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4-7 micrometer below the lenses) coincides with the location of nerve bundles-the presumed primary photoreceptors. The lens array is designed to minimize spherical aberration and birefringence and to detect light from a particular direction. The optical performance is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical functions.

Patterned colloidal deposition controlled by electrostatic and capillary forces.

We use substrates chemically micropatterned with anionic and cationic regions to govern the deposition of charged colloidal particles. The direct observation of the colloidal assembly suggests that this process includes two steps: an initial patterned attachment of colloids to the substrate and an additional ordering of the structure upon drying. The driving forces of the process, i.e. , screened electrostatic and lateral capillary interactions, are discussed. This approach makes it possible to fabricate complex, high-resolution two-dimensional arrays of colloidal particles.

Imaging profiles of light intensity in the near field: applications to phase-shift photolithography.

We describe a method of imaging the intensity profiles of light in near-field lithographic experiments directly by using a sensitive photoresist. This technique was applied to a detailed study of the irradiance distribution in the optical near field with contact-mode photolithography carried out by use of elastomeric phase masks. The experimental patterns in the photoresist determined by scanning electron microscopy and atomic force microscopy were compared with the corresponding theoretical profiles of intensity calculated by use of a simple scalar analysis; the two correlate well. This comparison makes it possible to improve the theoretical models of irradiance distribution in the near field. Analysis of the images highlights issues in the experimental design, provides a means for the optimization of this technique, and extends its application to the successful fabrication of test structures with linewidths of ~50 nm.

Intracrystalline macromolecules are involved in the morphogenesis of calcitic sponge spicules.

Control over the shapes of biologically formed crystals is generally not well understood. We have studied the morphogenesis of the different-shaped calcareous sponge spicules using high-resolution synchrotron X-ray diffraction. We show that a remarkable correlation exists between the distribution of defects within spicule crystals at the nanometer level and their macroscopic morphologies at the millimeter level. These defects are produced by controlled intercalation of specialized macromolecules into the crystals. We also show that such intracrystalline macromolecules are involved in the regulation of the shapes of synthetic crystals grown de novo from solution, and epitaxially overgrown on the spicule surfaces. We conclude that intracrystalline macromolecules play an important role in modulation of the morphologies of the forming biogenic crystals. Possible mechanisms that may account for the observed growth patterns are supported by fluorescence labeling experiments in vivo.

Morphogenesis of calcitic sponge spicules: a role for specialized proteins interacting with growing crystals.

Crystals formed in biological tissues often adopt remarkable morphologies that are thought to be determined mainly by the shapes of the confined spaces in which they grow. Another possible way of controlling crystal shape, demonstrated only in vitro, is by means of specialized proteins preferentially interacting with certain crystal faces. In so doing, they reduce the rate of growth in these directions and consequently change the overall crystal shape. In an X-ray diffraction study of the distribution of defects within the lattice of calcite crystals produced by certain sponges, we show that a remarkable correlation exists between the defect patterns or crystal texture and the macroscopic morphology of the spicules. This was observed in two cases in which proteins are present within the spicule crystal, but not in a third case where such intracrystalline proteins are absent. Furthermore, one of the spicules exhibited marked differences in texture even within families of structurally identical crystal planes, demonstrating that the organisms exert exquisite control over the microenvironment in which crystals grow. We conclude that highly controlled intercalation of specialized proteins inside the crystals is an additional means by which organisms control spicule growth.