Natural structural materials are built at ambient temperature from a fairly limited selection of components. They usually comprise hard and soft phases arranged in complex hierarchical architectures, with characteristic dimensions spanning from the nanoscale to the macroscale. The resulting materials are lightweight and often display unique combinations of strength and toughness, but have proven difficult to mimic synthetically. Here, we review the common design motifs of a range of natural structural materials, and discuss the difficulties associated with the design and fabrication of synthetic structures that mimic the structural and mechanical characteristics of their natural counterparts.

Bioinspired structural materials

In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.

The cucurbit[n]uril family

This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400042527

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

http://fcbt.mse.gatech.edu/PDF/Bersra2007.pdf

A review on fundamentals and applications of electrophoretic deposition (EPD)

Materials that are strong, ultralightweight, and tough are in demand for a range of applications, requiring architectures and components carefully designed from the micrometer down to the nanometer scale. Nacre, a structure found in many molluscan shells, and bone are frequently used as examples for how nature achieves this through hybrid organic-inorganic composites. Unfortunately, it has proven extremely difficult to transcribe nacre-like clever designs into synthetic materials, partly because their intricate structures need to be replicated at several length scales. We demonstrate how the physics of ice formation can be used to develop sophisticated porous and layered-hybrid materials, including artificial bone, ceramic-metal composites, and porous scaffolds for osseous tissue regeneration with strengths up to four times higher than those of materials currently used for implantation.

http://science.sciencemag.org/content/sci/311/5760/515.full.pdf

Freezing as a path to build complex composites.

THE major problem with the use of ceramics as structural materials is their brittleness. One way of overcoming this problem is to introduce weak interfaces which deflect a growing crack1. Polymer composites of this sort can be easily prepared by surrounding fibres with liquid plastic. To make similar structures with ceramic matrices and fibres is difficult and expensive, however, because traditional ceramic processing techniques of powder compaction and sintering prevent densification and cause cracking2–4. Here we describe a simple, inexpensive way of preparing a ceramic material that contains such weak interfaces. Silicon carbide powder is made into thin sheets which are coated with graphite to give weak interfaces and then pressed together and sintered without pressure. Relative to the monolithic material, the apparent fracture toughness for cracks propagating normal to the weak interfaces is increased more than fourfold, and the work required to break the samples increases by substantially more than a hundredfold. The technique should be readily applicable to other ceramics.

A simple way to make tough ceramics

Rather than using fibres or other dense inclusions as a means of introducing weak interfaces to deflect cracks, a much simpler route has been developed where composite elements, in this case sheets, are made from silicon carbide powder. After coating, these sheets are stacked, compacted together and sintered without any pressure. It is shown that the graphite layers will deflect cracks preventing catastrophic failure whilst raising the apparent fracture toughness from 3.6 MPa✓m to 17.7 MPa✓m and the work required to break the sample from 28 J m −2 . Further crack growth through the sample occurs when the unbroken part reaches is failure strees as determined from an unnothched beam. This allows the apparent fracture toughness to be simply related to the failure stress and this has been confirmed by experiments where the strength of the beam has been controlled by adding artificial flaws. Crack growth is not much affected by the thickness of the interface except at low thicknesses where small gaps in the interface are more likely to occur, allowing the delamination crack to kink out of the interface and cross the next lamina. The thermal stability of the laminate up to 1500°C in air is limited only by the oxidation resistance of the graphite. The potential for the process is also discussed.

The fabrication and failure of laminar ceramic composites

Microcompression has attracted considerable interest in the study of size effects, mainly in soft metals. Little data is available in the literature on experiments on materials with a higher bulk flow stress, although it has been shown that the technique can be successfully employed to suppress cracking due to the small specimen dimensions. Here, microcompressions on MgO were carried out to demonstrate the possibility of individually activating different slip systems. The yield stresses obtained in conjunction with transmission electron microscopy show that both the hard and soft slip system in MgO can be characterised individually. Microcompression is, therefore, a potential alternative to macroscopic testing of brittle materials under confining pressure or at high temperatures. To determine the influence of size on such measurements, results on the two slip systems in MgO and from the literature are compared. It is found that the bulk yield stress of a material might be used to estimate the effect of si...

Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO

Micropillar compression of ceramics at elevated temperatures

A simple and general method for making ceramic laminates with porous crack-deflecting interlayers is demonstrated. Both the strong laminae and the porous interlayers are made by tape casting suitable slurries using the same ceramic powders. Porosity is introduced into the interlayer by adding starch particles to the slurry. The effects of the starch on the burn-out and sintering behavior of the laminates has been fully described. The influence of resulting porosity on the ability of an interlayer to deflect a growing crack and to remain stable on prolonged heating has also been investigated. For the pore morphologies studied here, it is the volume fraction of pores that controls whether crack deflection takes place. Using existing mechanics solutions, a simple theoretical criterion for this volume fraction of porosity is given and is consistent with all the experimental observations.

William Clegg

Papers

A simple way to make tough ceramics

The fabrication and failure of laminar ceramic composites

Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO

Micropillar compression of ceramics at elevated temperatures

Fabrication and Crack Deflection in Ceramic Laminates with Porous Interlayers