Smart materials

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The concept of 'smart' materials has been around for a little while, although it's never been entirely clear exactly what the term embraces. The Wikipedia definition is '…designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields'. However, the concept is sometimes taken to incorporate the presence of a built-in sensing and/or actuation capability and it is also quite common to consider material with a capacity for 'self-healing' to fall within the 'smart' family, although this tends to refer to local changes, rather than an alteration in overall properties. Perhaps a more appropriate focus for the concept is that of a material capable of some sort of active response to a change in environment, which could arise spontaneously in service or could be deliberately imposed. Of course, many devices (ranging from strain gauges to piezoelectric actuators) are based on coupling between fields of stress, strain, temperature, electromagnetism, etc. The unifying idea in the 'smart' concept, however, is that the 'intelligence' is embodied in the material, which is macroscopically homogeneous and thus should not be considered to be a 'structure' or a 'device'.
This special issue does not represent an attempt to provide a comprehensive overview of all of the types of material or effect that could be considered 'smart'. Rather it is a snapshot of the current state of play, covering a range of types of material and effect, and giving a flavour of how interactions with an environment can be manipulated so as to create some sort of improvement in performance. There is relatively little in the way of cost-benefit analyses or practical examples of commercial viability. This reflects the state of maturity of the field, which could be summarised as no longer embryonic, but still some way from fully fledged and extensive industrial usage.
In fact, the most highly developed manifestation of the 'smart' concept is that of the shape memory effect (SME), which is the subject of the first five papers. This has the longest history of successful application in the smart materials field, with the first research papers on the topic appearing around 1970 and practical exploitation now ranging from medical devices to greenhouse windows to satellite solar panels. In most cases, these depend on alterations in macroscopic shape arising from (reversible) martensitic phase transformations stimulated by changes in temperature and/or stress. While the basic effects are well understood, there are many subtleties that require further work to deliver commercially applicable smart materials, both concerning fundamentals of the phase transformations (see the contributions from Wang et al. 1 and Karaca et al. 2 ) and in terms of exploiting the SME via a range of stimuli, such as magnetic field application (see the papers of Faran and Shilo 3 and Heczko, 4 which focus particularly on the mobility of twin boundaries). The article from Zink and Mayr 5 addresses the application of a magnetically stimulated SME to promote cell growth in biomedical systems.
In the context of commercial exploitation of the SME, it is notable that extremely strong, precipitation strengthened, shape memory alloys for high temperature applications, for example the NiTiHf based alloys described by Karaca et al., 2 have recently been developed to achieve properties that meet aerospace and automotive specifications. This may herald the long awaited breakthrough in shape memory alloy technology, parallelling the widespread success of superelastic NiTi in medical devices.
The article by Markaki and Justin 6 also concerns promotion of (bone) cell growth by magnetically stimulated shape changes (in an assembly of ferromagnetic fibres). However, it does not involve a SME, being based on the tendency for elongated ferromagnetic bodies to become magnetised along their length by an external magnetic field and then to align with the field. As with many such effects, the key issue is often the magnitude of the induced strain and the energy available to drive it against opposing forces. Assessing and optimising these parameters commonly requires a detailed understanding of the phenomena involved.
The diversity of the range of 'smart' applications is highlighted by the following three papers. Ryder and Tan 7 describe recent advances in the development of metal-organic framework materials, illustrating their tremendous versatility through emergent applications in the fields of energy, sustainability, water purification and medicine. Energy harvesting via conversion of vibrations in polymer-based piezoelectric generators is described in the paper by Crossley et al. 8 Piezoelectric polymers offer a robust technology for self-powered devices, but require performance enhancement for commercial development. The review of multiferroic oxide composites presented by Sreenivasulu et al. 9 concerns exploitation of mechanical strain-mediated coupling between magnetic and ferroelectric components, potential applications and further requirements for fabrication of high quality composite materials.
The final pair of papers concern fibre-reinforced polymer composite materials with a capability for monitoring and/or repairing damage occurring during service. The first, by van der Zwaag et al., 10 gives an overview of strategies for 'self-healing', based on designing polymer composite microstructures with a capability to respond in a useful way to loss of mechanical strength or integrity (to heal damage suffered in service). That of Lau, 11 on the other hand, is focused on how embedded sensors can provide information that (automatically) leads to corrective measures.
All the papers were solicited and the editors are grateful to the authors for responding in a positive and timely manner. The reviewers have also done an excellent job. The editors would also like to thank the MST editorial staff, particularly Rose Worrell and Mark Hull, for their invaluable assistance in completing this operation smoothly within the planned timescale.