# Smart Materials for Green(er) Cities, a Short Review
## 1 Introduction  
Over and above the environmental problems that the use of fossil fuels poses or may pose, the probable depletion of these same resources in the following decades is prompting us to rethink the infrastructure of our cities so as to drastically reduce their need for fossil fuels. The classic way to achieve this is to replace fossil fuels with renewable energy. However, it will be very difficult to completely replace fossil fuels with renewable energies in the short or even medium term [1]. It is therefore important to find solutions to reduce the overall energy consumption of our cities.  
The most popular route to smarter, energy-efficient cities is the Smart City. Here, the latest Internet-of-Things technologies are used on a massive scale to acquire as much Data as possible and then use it to optimize, among other parameters, energy consumption [2; 3; 4]. However, this approach has a number of disadvantages, not least the need to manufacture and integrate millions of tiny electronic circuits into everyday objects and surrounding structures, then interrogate these communicating objects and store and process the data. These operations consume a lot of energy. The presence of electronics in everyday objects also poses obvious recycling problems. The disadvantages of using IoT-type technologies can therefore quickly outweigh the benefits when it comes to making cities greener [5].  
To make cities both smarter and more energy efficient, we believe we also need to explore other possible families of solutions. One of these is based on the use of intelligent materials, capable of reacting on their own to external stimuli or performing useful tasks without the need for electronics or on-board energy. \"Green Tech\" solutions based onthese materials are, in the vast majority of cases, simpler to manufacture, implement and recycle than their conventional counterparts based on embedded electronics and/or mechatronic automation.  
In the following short review paper, we will focus on a number of smart materials whose applications we believe could make a significant contribution to reducing the energy consumption of cities, while at the same time making them smarter. Our review is by no means exhaustive. Rather, our aim is to highlight, through some examples, the potential of smart materials (as a whole) for greener cities.  
Several classes of smart materials can be used to make cities smarter and more energy-efficient. These include phase-change materials (PCM), shape-memory alloys (SMA) and self-healing materials. Here, we use the intrinsic physicochemical properties of materials and their ability to react to external stimuli (e.g., increased temperature, increased sunlight, appearance of a crack) to ultimately reduce the environmental impact of the structures of which they are an integral part. In this paper, we will also look at how metamaterials, and more specifically meta-surfaces, can be used to make buildings greener. We will also study one example of a classical material made \"smarter\" and greener by the integration of sensors, namely \"smart concrete\". In our case, the integrated sensors are themselves fully passive and wireless, with no embedded electronics of battery. This gives this solution a number of advantages over existing solutions. We will notably discuss possible applications of the aforementioned smart materials for passive air-conditioning applications, green acoustic insulation, and product life-span extension. Where appropriate, we will provide new experimental results obtained as part of our own research projects.