The transition to sustainable or green(er) cities requires the development and implementation of many innovative technologies. It is vital to ensure that these technologies are themselves as sustainable and green as possible. In this context, smart materials offer excellent prospects for application. They are capable of performing a number of tasks (e.g., repair, opening/closing, temperature measurement, storage and release of thermal energy) without embedded electronics or power supplies. In this short review paper, we present some of the most promising smart material-based technologies for sustainable or green(er) cities. We will briefly present the state-of-the-art in smart concrete for the structural health monitoring and self-healing of civil engineering structures, phase-change materials (PCM) for passive air-conditioning, shape-memory materials (SMA) for various green applications, and meta-surfaces for green acoustics. To better illustrate the potential of some of the solutions discussed in the paper, we present, where appropriate, our most recent experimental results (e.g., embedded SAW sensors for the Structural Health Monitoring of concrete structures). The main aim of this paper is to promote green solutions based on smart materials to engineers and scientists involved in R&D projects for green(er) cities.

In this paper, we present a fully tunable system able to generate mode localization between a 170 000 Q -factor quartz crystal microbalance at 1 MHz and a digital device (field programmable gate array) simulating in real time the presence of an identical and weakly-coupled second resonator. Indeed, this method allows to precisely select each parameter value and thus to reach the optimal configuration with the maximum sensitivity to perturbations. In addition, this design gives a perfect adaptability to the geometry of the piezoelectric resonator, that allows to work with much higher frequencies and Q -factors than conventional cantilevers or tuning-forks usually selected for the design of mode-localized sensors. The experimental sensitivities reached in this work are at least two orders of magnitude higher than the ones found in the literature, which is promising for the design of a new generation of ultrasensitive sensors based on Anderson localization.

Nowadays, 3D printing is becoming an increasingly common option for the manufacturing of sensors, primarily due to its capacity to produce intricate geometric shapes. However, a significant challenge persists in integrating multiple materials during printing, for various reasons. In this study, we propose a straightforward approach that combines 3D printing with metal coating to create an array of resistive force sensors from a single material. The core concept involves printing a sensing element using a conductive material and subsequently separating it into distinct parts using metal-coated lines connected to the electrical ground. This post-printing separation process involves manual intervention utilizing a stencil and metallic spray. The primary obstacle lies in establishing a sufficient contact surface between the sprayed metal and the structure, to ensure effective isolation among different zones. To address this challenge, we suggest employing a lattice structure to augment the contact surface area. Through experimental validation, we demonstrate the feasibility of fabricating two sensing elements from a single-material 3D-printed structure, with a maximum electrical isolation ratio between the sensors of above 30. These findings hold promise for the development of a new generation of low-tech 3D-printed force/displacement sensor arrays.

Topology optimization and generative design are methods used to lighten structures under specific loads. The resulting lightweight components are usually softer in the directions perpendicular to the applied efforts, such as in the case of cranes. This results in the possible excitation of structural modes at very low frequencies, which can lead to premature aging of lightweight structures. In this paper, we present a new design of acoustic black hole (ABH), able to mitigate vibration modes in a test 3D-printed lightweight structure, from 20 Hz upwards. The novel ABH weights less than 10 grams and can be readily screwed on the structure. Its design is based on that of a classic ABH, with additional winglets and/or rods, to increase the vibrating mass without significantly changing the stiffness of the ABH. This results in a strongly reduced ABH cut-on frequency. The winglets and/or rods also result in a greatly increased contact area between the ABH and the viscoelastic dissipative material, with positive effects on the ABH damping efficiency. The new ABHs could successfully be used, first to damp the fundamental vibration mode of aluminum cylinders, and then to reduce the amplitude of the first three structural modes of the test lightweight structure. The new ABH manages to damp modes at frequencies several times lower than the ones found in the literature, which are promising results.

Moulded-pulp packaging stands poised to replace traditional polystyrene packaging due to its eco-friendly nature and recyclability. However, optimizing the protective pins within moulded-pulp packaging has been a challenge, hindering its widespread adoption. This paper presents an innovative approach to swiftly and cost-effectively optimize pin shapes, which is crucial for enhancing impact resistance. We hereby propose a simple try and error pin optimization methodology by utilizing an hand-moulded pulp recipe and PLA 3D-printed moulds to rapidly craft tailored cardboard pins. The validation of the material fabrication was carried out with static characterization and the classification of the homemade moulded-pulp pins with impact testing analysis. This exploratory project demonstrates the feasibility of this approach, laying the foundation for future advancements in moulded-pulp packaging design.