Advanced Materials

The cover image shows an SEM image of a squid tentacle, revealing the individual sucker rings. These toothed ringlike structures within the suckers provide additional gripping power during prey capture and handling. The background image depicts the nanoscale network of parallel tubular elements from which the rings are constructed. The structural, mechanical and biochemical properties of this wholly organic material is discussed in detail by David Kisailus, Henrik Birkedal, and co-workers on p. 401.

The shell of a turtle is required to be a shield that is stiff at high loads, but must provide sufficient flexibility for respiration and locomotion at smaller loads. On p. 407, Peter Fratzl and co-workers show that these seemingly contradictory requirements are met by a self-locking material, whereby stiff bony elements are connected by a much softer suture with a complex three-dimensional shape.

The exoskeleton of the arthropod Homarus americanus is a hierarchically organized nanocomposite material consisting of a chitin-fiber matrix associated with proteins and various amounts of nanoscopic biominerals. We show that the combination of two design principles in this material - twisted plywood and honeycomb - results in remarkable macroscopic mechanical properties, which are highly variable depending on factors like hydration and mineral content.

The individual toothed sucker rings of squid tentacles (highlighted in blue) provide additional gripping power during prey capture and handling. These rings comprise a nanoscale network of parallel tubular elements, as shown in the background image. The structural, mechanical, and biochemical properties of this wholly organic material are discussed in detail.

The shell of turtles is a shield which needs to be stiff at high loads but should provide sufficient flexibility for respiration and locomotion at smaller loads. We show that this seemingly contradictory requirement is met by a self-locking material, whereby stiff bony elements are connected by a much softer suture with a complex three-dimensional shape.

Using electronic speckle pattern interferometry minipig molars were tested under load inside the bone socket and when embedded in a stiff polymer. It is demonstrated that the molar bends in the direction of the load in both configurations even at low loads. This shows that the intrinsic reaction of the tooth crown to load is complemented by the structures supporting the tooth.

The cellular uptake of nanoparticles by living cells is predicted to be strongly size-dependent, according the thermodynamic analysis, and an optimal particle radius of ∼ 25–30 nm at which the cellular uptake reaches a maximum of several thousand is shown to exist. The theoretical prediction provides valuable guidance for the rational design of nanoparticle-based drug-delivery systems.

An artificial, in vitro biomineralization process that utilizes a nanofiber gel to as a substrate for biomimetic hydroxyapatite mineralization in three dimensions is presented. The system employs the natural enzyme alkaline phosphatase and a phosphorylated, anionic nanofiber gel matrix to template hydroxyapatite nanocrystals with size, shape, and crystallographic orientation resembling natural bone mineral.

A new surface bioconjugation strategy is presented. A polydopamine surface coating provides chemical activation on material surfaces, is resistant to hydrolysis, and offers selectivity in coupling of biomolecules via nucleophilic groups through simple pH control. Control of orientation of immobilized biomolecules may be possible using terminally modified DNA or His-containing proteins.

To date, the knowledge of the multiple roles of additives in calcium carbonate crystallization results from empirical control of crystal growth. We present a crystallization assay generating controlled supersaturation, which allows the categorization of different modes of additive action. This facilitates a novel understanding of the action of crystallization additives.

Mussel byssal threads are functionally graded biological fibers adapted for tethering the soft tissue of the organism (∼1 MPa) to hard surfaces !(25 GPa) in the rocky seashore. A stiffness gradient mitigates contact deformation and is defined by an incrementally regulated distribution of two protein block copolymer variants with stiff and pliable domains, respectively.

Patterned paper wet with multivalent cations can template the production of films of ionotropic hydrogels in topologically complex shapes such as Möbius strips and interlocking rings (see figure). The films have lateral dimensions as low as 2mm, and range in thickness from 0.2 to 1.3mm. The films are magnetically responsive when cross-linked by Ho³⁺ or Gd³⁺.

High doses of sodium fluoride in bones lead to severe softening, by weakening interfacial properties between the inorganic minerals and the organic components, while leaving mineralization unchanged. This leads to reduction of microdamage and associated stress-whitening pointing to a change in failure mode. Accordingly, elastic modulus, failure stress, and indentation-distance increase are decreased, whereas failure strain is increased.

Intracellular distribution of iron oxide nanoparticles incorporated within a ferritin mutant that displays genetically introduced cell-targeted peptides (RGD-4C) on its exterior surface are investigated using scanning transmission electron microscopy with a high-angle annular dark-field detector. The particles (indicated by arrows) internalized into macrophages much more effectively than those with noncell-targeted ferritin.

Bio-inspired, multifunctional, high-aspect-ratio nanostructured surfaces are fabricated in a variety of materials with controlled geometry and stiffness. A soft-lithography method that allows the one-to-one replication of nanostructures and renders it possible to produce arbitrary nanostructures with cross-sectional shapes, orientations, and 2D lattices that are different from the original master is presented. The actuation of the posts is demonstrated.

Calcite crystals grown in agarose hydrogels incorporate the gel network while retaining their single-crystal nature. The amount of gel network that is incorporated is determined by two competing factors: crystallization pressure promotes the exclusion of the gel network, while faster growth rates favor its incorporation.

Porous 3D metal-nanoparticle assemblies with selectable morphologies are synthesized. Biosilica templates (diatom frustules) are first converted into silicon replicas via magnesiothermic reduction. Electroless metal deposition, followed by silicon dissolution, then yields metal-nanoparticle assemblies with the 3D template morphology (figure reveals a silver assembly; inset images reveal internal features after partial ion milling and EDX analysis).

The role of hierarchy in gecko-like adhesive surfaces is presented. Experiments on two-level structured surfaces with micropillars of different dimensions demonstrate that hierarchy does not seem to positively influence adhesion of structured surfaces to planar substrates, but may be relevant when stiff materials or rough substrates come into play, where adaptability of the adhesive structure is required for successful contact.

Hexagonal micropattern mimicking the attachment pads of bush crickets is reported to control dry and wet elastomer friction. This micropattern eliminates completely the stick–slip disturbances when in dry conditions, and prevents hydroplaning when in wet conditions, rendering the surface behavior stable and predictable.

Two factors are critical for understanding silks: the nanoscale semicrystalline folding structure, which gives high strength and toughness, and the degree of hydration of the disordered fraction, which modifies these material properties. The combination of these two factors allows for a massive range of mechanical properties. Understanding and controlling these two factors are the key to the functionality of protein elastomers.