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The ability to produce distributed sensors by tailoring materials readily available on the market is becoming an emerging strategy for Internet of Things applications. Embedding sensors into functional substrates allows one to reduce costs and improve integration and gives unique functionalities inaccessible to silicon or other conventional materials used in microelectronics. In this paper, we demonstrate the functionalization of a commercial polyurethane (PU) foam with the conductive polymer PEDOT:PSS: the resulting material is a modified all-polymeric foam where the internal network of pores is uniformly coated with a continuous layer of PEDOT:PSS acting as a mechanical transducer. When an external force causes a modification of the foam microstructure, the conductivity of the device varies accordingly, enabling the conversion of a mechanical pressure into an electric signal. The sensor provides a nearly linear response when stimulated by an external pressure in the range between 0.1 and 20 kPa. Frequency-dependent measurements show a useful frequency range up to 20 Hz. A simple micromechanical model has been proposed to predict the device performance based on the characteristics of the system, including geometrical constrains, the microstructure of the polymeric foam, and its elastic modulus. By taking advantage of the simulation output, a flexible shoe in sole prototype has been developed by embedding eight pressure sensors into a commercial PU foam. The proposed device may provide critical information to medical teams, such as the real-time bodyweight distribution and a detailed representation of the walking dynamic.

Commercial carbon fibers have been coated by a brush-like layer of ZnO nanowires using a seeded chemical bath deposition. Both single carbon fibers or large tows can be functionalized in such a way. Exploiting the electrical conductivity of carbon fibers and the functional properties of ZnO nanostructures, we demonstrated that the intersection at the crossing of two functionalized carbon fibers can act not only as a piezoelectric strain sensor, but also as a chemoresistive gas sensor. The small volume of added nanostructures prevents delamination issues. The “crossing” configuration, which is typical of carbon fiber fabrics used for composites, paves the way to transform a typical texture with simple mechanical and structural properties into a “smart” material with noninvasive integrated arrays of micron-scale sensors.

Zinc oxide (ZnO) nanostructures can be grown in different morphologies by means of a wide range of techniques. Although the strong evidence of their gas sensing capabilities has been reported in several papers, not all of them are suitable for a large-scale or industrial scale production of gas sensor devices. Among the several ZnO nanostructures that have been grown so far at IMEM-CNR Institute, we focused our attention on tetrapods (TPs), because a vapour phase growth process that can produce grams of these nanostructures have been designed and optimized in our laboratories. This quantity of nanostructures, produced in a small lab-scale reactor, is enough to prepare several thousands of gas sensing devices. Moreover, the produced ZnO-TPs are free-standing and not constrained to a growth substrate, and can be easily suspended in a liquid media. The highly-porous entangled network of ZnO-TPs, which can be obtained by direct deposition on sensor substrate, demonstrated to efficiently work as chemoresistor, while its sensing properties (sensitivity, selectivity, ...) can be tuned or modified through the functionalization of TPs surface with other materials (noble metals, inorganic semiconductor nanoparticles, organic semiconductor layers). At the same time, ZnO nanostructures and their multifunctional properties can be used to functionalize the surface of other materials and, eventually, add to them sensing capabilities. As a meaningful example, we can illustrate the adding of ZnO nanorods (ZnO-NRs) to the surface of carbon fibers with a wet chemical process that is easily scalable to a larger scale. It has been recently demonstrated that two crossing ZnO-functionalized carbon fibers can be used to sense and transduce a piezoelectric signal or UV light, as well as a chemoresistive information.

A piezoelectric investigation of a nano-engineered transducer consisting of zinc oxide nanorods grown in situ on carbon fibers has been performed by means of dynamic hysteresis and capacitance measurements. The device has been stimulated using both static and dynamic stress: the occurrence of characteristic current vs voltage polarization lobes of a ferroelectric material (stressed piezoelectric) and the corresponding saturation polarization of 290 mC cm-2 (at 2.4 V mm-1 electric field) have been recorded under static stress application. Under dynamic stress conditions a 400% capacitance increase has been measured with respect to the unstressed device. It is noteworthy that these results have been achieved using the carbon fiber itself as conductive element, without the need for external wiring, providing a true integration of the piezoelectric transducer into carbon fiber based materials. The presented nano- engineered device acts as stress sensor and can actively react to the applied stress to hinder the deformation, enabling the design of smart carbon fiber based composites.

The physical and operating principle of a stress sensor, based on two crossing carbon fibers functionalized with ZnO nanorod-shaped nanostructures, was recently demonstrated. The functionalization process has been here extended to tows made of one thousand fibers, like those commonly used in industrial processing, to prove the idea that the same working principle can be exploited in the creation of smart sensing carbon fiber composites. A stress-sensing device made of two functionalized tows, fixed with epoxy resin and crossing like in a typical carbon fiber texture, was successfully tested. Piezoelectric properties of single nanorods, as well as those of the test device, were measured and discussed.

The functionalization of industrial materials for innovative applications is crucial in the competitive development of industrial products.

The various functional properties of ZnO are very well known and for this reason ZnO is studied for applications in different fields, such as optoelectronics, transparent electronics, piezoelectric devices, sensing and catalysis. In particular, when ZnO is obtained in form of nanostructures, most of these properties become strongly surface and/or size dependent. Authors here present both their results in the optimized growth of ZnO nanostructures with selected morphologies and an overview on different kinds of surface functionalizations that has been used to add further and new functional properties to them. Inorganic and organic semiconductor, superparamagnetic iron oxide nanoparticles and noble metals have been coupled with ZnO nanostructures by means of different physical or chemical techniques that have been developed in order to optimize the coupling between the different materials (Fig. 1). Clear enhancements or modifications in their surface reactivity, light absorption, magnetic or other peculiar properties are reported and a few examples for gas sensing application, photocatalytic degradation of water pollutants and some new ideas for complex geometries in hybrid photovoltaic applications are presented as case studies. Further perspectives for other feasible applications of these multifunctional composite nanostructures are finally discussed.