Evolution of multifunctional systems and levels of complexity
Current challenges of this beginning of the 21rst century are related to energy, space and time optimization. Indeed, as the human population keeps growing at a faster pace, the consumption has been booming to target the demand for the increasing need of goods, electricity, data, health treatment… As an example, the production of aluminium alone has increased by 386% between 1950 and 2004 (Chryssolouris et al. 2008). In addition to this dramatic augmentation in population and consumption, environmental issues and diminution of natural resources call for novel manufacturing concepts and goods production.
Multifunctional materials and devices are multi-component systems that appear as a promising approach to yield multi-performance, reduced space and materials amount. Following the description given by Peter Matic, three types of multifunctionalities can be listed (Matic 2003). The first type of multifunctionality consists in added-subsystems, where systems performing one specific function are connected together to form a multifunctional larger system. The second type is composed of co-located components. A single composite system or device is formed by multiple small elements embedded inside. Finally, the third type is integrated materials, where a composite system contains carefully selected materials that perform multifunctions. Such a sectioning of multifunctional systems follows their chronological development. Indeed, typical examples of the preliminary multifunctional devices of type two developed during the 20th century are home appliances featuring multiple programs such as washing machines for wool, delicate clothing and high temperature washing, or cooking robots for soups, drinks or cut meats. Such devices had huge impacts on daily life. Nowadays, research efforts are focusing on the development of devices with multifunctionality three such as injectable drug-delivery platforms for combined diagnostic and therapeutic actions.
Numerous excellent reviews describe novel multifunctional composite materials and structures (Gibson 2010; Christodoulou & Venables 2003; Salonitis et al. 2010). To cite only a few, recent years have witnessed the development of wearable devices of local and instantaneous autonomous monitoring, stretchable electronics for wireless command, flexible solar cells for optimized energy generation, implantable drug delivering scaffolds for faster bone regrowth. However, despite this plethora of excellent devices, it is still hard to find concrete examples of their implementation into industrial and large scale applications. Indeed, most of these multifunctional platforms rely on complex designs and manufacturing processes where interfaces are of upmost importance. The interplay of each function is still only partially understood requesting the development of much simpler systems.
To understand the evolution of multifunctional systems and establish what could be their future, we will first describe the current strategies adopted for the development of devices combining structural and functional tasks and highlight their ultra-complexity, drastically limiting their industrial development. In a second step, we will see how natural systems are the ultimate multifunctional systems and how they provide solutions for the fabrication of multifunctional platforms with an apparent simplicity. Finally, we will discuss how these bio-inspired or biomimetic structures move the complexity of multifunctional systems from the macro and micro scale towards the nanoscale, opening the path for a large range of new multifunctional composite materials.
Technological advances and request for multifunctionality has lead to the development of ultra-complex layered devices
More than a century ago, combining properties and functionalities in a system was achieved by combining single-task devices. Temperature was measured by a thermometer, humidity by a hygrometer, pressure by a pressuremeter etc. Multiple devices, placed parallel or in series, provide the advantage of a multitude of measures with the optimum system selected for each task. In addition, the order of inputs and outputs desired is flexible since the connections can be arbitrarily determined. There is not necessary correlation and coupling between each function. Therefore, an impaired function can be easily repaired by only replacing one device, a new function can be added by only connecting a new device to the system. However, the size of a multi-device machine is consequent and therefore drastically limits its applicability. Since then, multifunctionality is achieved by the juxtaposition of miniaturized systems within one composite material.
As a typical example, a patient suffering from cancer is submitted to a multitude of tests repeated along his/her illness. These are needed to fulfill sensing functions – e.g. where is the cancer located? what type of cancer? –, actuation functions – e.g. injections of drugs for chemotherapy – real time monitoring – e.g. is the cancer spreading?- and so on. Each of these functions is performed by a separate team of people, a specialized machine, requiring the motion in space and in time of the patient. This system corresponds to a multifunctionality of type one. Using a device that connects both the sensing and the monitoring functions as in multifunctionality of type two would already save time and space, therefore increasing the performance and impact of the therapy. Creating an implantable material that can fulfill several tasks would push one step further the efficiency of the treatment by combining real time monitoring, real time sensing and real time actuation. This last approach has attracted the attention of researchers and has led to the development of the field of nanotheranostics where nanoparticles are used as platform fusioning diagnostics and therapeutic agents (Rajeeva et al. 2014). Reducing the volume and weight of the devices simplify the system by eliminating structural parts as found in multiple device systems, therefore reducing costs and volume of raw materials (Bennett 2010). Such systems also present the advantage of being much more adaptable for various applications.
Similar approach has been successfully applied for the design of prosthetic electronic skin that contains skin’s mechanical flexibility, durability, biodegradability and sensing properties (Chortos et al. 2016). This is achieved by the design of a multilayered system resulting from high tech processing methods and materials. Each layer of the system contains elements that have been optimized separately. Nevertheless the complexity of the multilayered device needs a highly sophisticated power interface and actuation systems in order to coordinate the functions in each layer. As a result, multilayered platforms appear more as ultra-complex platforms where each layer is produced at high cost. The lack of interaction between each layer, and therefore each function, complicates the external powering and control of the platform, limiting the implementation of autonomous capabilities within the structure.
Bio-inspiration provides hierarchical design principles for apparent simplicity while maintaining the multifunctionality
To optimize the fabrication of current multifunctional systems, inspiration can be taken from natural systems (Salonitis et al. 2010). Bones, for examples, are naturally performing multiple tasks in an autonomous manner: they display load-bearing capabilities, are strong and tough, growing and adapting to the specific solicitations, have a particular shape and participate to the transport of blood and nutrients throughout the body. The striking specificity of natural systems is that all these functions are achieved by a relatively limited variety of chemicals and compounds, usually restricted to a few minerals, proteins. Some of nature’s properties indeed rise directly from a hierarchical assembly of the single components into a complex architecture. Bones are assemblies of aligned collagen fibers surrounding cylindrical channels, the osteons, while varying their orientation and mineral content (Fratzl & Weinkamer 2007). Each component of this hierarchical structure has some intrinsic properties, but it is mainly its assembly and interaction with other components that makes the composite material performing as desired. Remarkably, most natural systems present such assemblies and exhibit properties surpassing the single properties of their building blocks (Naleway et al. 2015; Barthelat et al. 2016).
Biomimetics therefore provides tools for the fabrication of multifunctional materials based on the microstructural arrangement of building blocks. Applying the design principles as found in nature into synthetic composites, materials displaying multiple functions and properties could be manufactured at a lesser cost. Indeed, additive manufacturing techniques such as 3D printing are now readily available and are efficient tools for building nature-like architectures (Studart 2016). This brings closer the potential industrialization of those new multifunctional materials. Finally, natural materials are intrinsically autonomous. There is therefore a high opportunity to prepare self-shaping, self-repairing or self-sustaining composite multifunctional devices by mimicking biological strategies.
In the coming years, more natural tricks will be discovered. In particular there is still a lack of understanding of the underlying mechanisms of these hierarchical structures. Since the role of interfaces and coupling between each building block is crucial for the performance of the whole, it is of upmost importance to characterize and analyze the multiple interactions at and between each hierarchical level. Despite the simpler design of the materials, relying on architecture rather than on assembly of separate optimized layers, the complexity of the material is shifted towards the micro- and nano-scopic scale, where the interactions take place. The apparent simplicity of bio-inspired design therefore reveals a hidden complexity that remains to be explained.
Using the bio-inspired designs to bring multifunctionality at the nano or atomic scales.
Mimicking bio-inspired architectures in artificial composites is a simple approach to bring multifunctionality into a material. Contrary to nature, there is an infinite variety of chemical elements and building blocks that are available to the scientists. What about assembling highly multifunctional elements into bio-inspired structures? Building blocks such as 2D layered graphene or phospherene sheets, gold core-shell nanoparticles of even newly designed and fabricated multiferroics intrinsically present outstanding performance (Service 2015, Fiebig et al. 2016). Bio-inspired architectures using synthetic building blocks such as hard alumina and copper have already provided examples of composite materials exhibiting excellent mechanical and functional properties (Le Ferrand et al. 2015). The potential of creating multifunctional materials by utilizing intrinsically multifunctional building blocks pushes the variety of devices and applications beyond imagination. The level of complexity of those new multifunctional composites will therefore mostly rely on the complexity of the design of the building blocks, which could be studied separately, in addition of the complexity of their interactions at each hierarchical levels.
The ultimate design for a truly multifunctional material would be to combine microstructures based on newly synthesized building blocks with the incorporation of living cells. Indeed, cells are multifunctional highly optimized nanomachines strongly interacting with their environment. Using cells as building blocks in bio-inspired structures would consist in building a multifunctional device with multifunctional nano-devices providing inherent and autonomous sensing and actuation. Recent studies have explored the possibility to use additive manufacturing techniques to build on-demand cellular tissues and organs (Murphy 2014) as well as plants (Wicaksono 2015). Building bio-inspired microstructures based on multifunctional synthetic building blocks and natural machineries as found in cells, truly innovative multifunctional composites can now be envisioned, such as organs able to sense and talk with monitoring devices or customized hydrophobic conductive wood. Thanks to the increased interest in applying bio-inspired designs to composite materials as well as the development of new building blocks and the understanding of the highly complex mechanisms of living cells with their environment, major breakthroughs in the fields of multifunctional materials can be expected in the next decades. Luckily, scalable production of new materials, like graphene for example (Paton 2014) and the industrial implementation of additive manufacturing techniques are also likely to take one this new generation of multifunctional materials for larger production. The increasing number of 3D printers available for particulars will also widens the range of possible device and in particular customized on-site and real-time fabrication of multifunctional elements.
Multifunctional devices are currently achieved by the layering of optimized composites, facilitating the integration and combination of functions but limiting their autonomous capabilities and requiring highly complex monitoring and manufacturing tools. Bio-inspiration on the other hand provides design principles to create multifunctional composites based on hierarchical designs. In this respect, bio-inspired composites are apparently simpler but the complexity relies in the interaction at the molecular levels between the components. Nowadays, new materials have been synthesized that perform multiple functions and that can be used as building blocks in bio-inspired architectures. The properties of the final structure then result from the individual properties of the building blocks as well as from their interactions within the hierarchical architecture. While transposition the macroscopic complexity found into multilayered systems to a complexity in the design of the building blocks and their interactions, at the nano and micro scales, the possible combination of functions gets well extended.
The large scale production of multifunctional devices based on bio-inspired designs and newly synthetized building blocks will nevertheless need tremendous efforts (Momoda 2004). Alongside with the implementation of these new technologies and synthesis, the current manufacturing processes will need to be adapted. The performance and the functions of the final product will need to be integrated from the start, which is not the traditional approach (Gardiner 2015). The systems departs from linearity with cause and effect to non-linearity, which adds another challenge (Miles 2009; Morin & Kelly 1992; Morin 2006). Alongside with multifunctionality, there is an urgent need for multi disciplinarity and multi adaptability to people and society (Chryssolouris et al. 2008; Chryssolouris et al. 2016).
Finally, the creation of novel materials using living cells and for potential implantation in human bodies should be considered carefully. Ethical issues are expected to rise and a global coordination and measures will be necessary to avoid. There is an urgent need for international ethical commissions to ensure the well founded applications of the recent technological advancements, more specifically in the fields of robots and human enhancement.
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