# Coaxial Fibres Incorporated with Phase Change Materials for Thermoregulation Applications
## 1 Introduction  
Civil engineering materials such as cement and asphalt concretes absorb a large amount of solar thermal energy in the form of heat, thus contributing to heat retention far more than the natural elements in rural landscapes [1; 2]. As a result, city areas often record higher air temperatures than their rural counterparts, leading to significant consequences for the environment, economy, and society [3]. Called the urban heat island (UHI), this phenomenon becomes more pronounced with urbanization through the construction of asphalt pavements and built structures [4; 5]. It is important to emphasize that the world's population is expected to increase at least 60% in the second half of this century, and thisexcessive growth will aggravate urbanization [1; 6]. Thus, the thermoregulation of these structures is essential. As an alternative, researchers have been studying the application of latent heat thermal energy storage (LHTS) materials or, more simply, phase change materials (PCMs) within civil engineering materials in order to mitigate and prevent the effects of the UHI. According to this approach, the integration of PCMs is a promising solution because they have dual benefits: the ability to store and release thermal energy latently, coupled with an isothermal nature during storage, thus reducing temperature fluctuations [7; 8]. However, their direct application can affect the mechanical properties of the composite material. PCMs have gained significance in thermoregulating environments since their introduction in the 1980s [9]. With their high latent heat capacity, PCMs are excellent for storing energy from cooling systems or lower night-time temperatures to be released during peak heat periods, improving thermal comfort [10]. [PERSON] et al. [11] adopted an approach to study PCMs and observed that they differ significantly from high-albedo materials. While high-albedo materials focus on reflecting solar radiation to reduce heat gain and cooling loads [11], PCMs adopt a more dynamic strategy. They absorb excess heat during periods of high temperature (such as during the day) and release it during cooler periods (such as at night) [12]. This phase change allows PCMs to store and release thermal energy efficiently. So, unlike high-albedo materials that passively prevent heating by reflecting solar radiation, PCMs actively balance thermal energy, absorbing and releasing heat as needed [13]. This ability to actively adjust surface temperatures makes PCMs a more versatile solution for climate control in different applications, adapting to thermal variations in the environment throughout the day. PCMs are versatile in their applications within the civil engineering [7], encompassing areas such as air conditioning systems [14], solar energy storage [15], and more broad integration in various construction materials [16]. Their primary purpose in these applications is to prevent damage caused by abrupt temperature changes, thus lowering the energy per area needed for heating/cooling these materials. PCMs can be incorporated directly [17; 18] or encapsulated [19; 20]. [PERSON] et al. [21] carried out various studies with PCMs applied directly or encapsulated and, in response, the direct application method caused the PCMs to leak, thus inhibiting water migration and interfering with the hydration process, thereby negatively affecting the development of the materials' strength [22]. Therefore, in order to achieve stable PCM incorporation so that their phase change occurs without leakage, different methods can be used to protect and encapsulate them, such as microencapsulation by sol-gel, electrospinning [23], and wet spinning [24]. [PERSON] et al.