The Nanjing University team has developed PLA nanofiber membranes that can efficiently passive refrigerate
2024-08-23
As global warming increases, people are at increased risk of exposure to hot environments and heat-related diseases. Personal heat/humidity management (PTMM) offers a promising solution for maintaining human thermal comfort with the advantages of local temperature regulation, zero energy consumption, and unlimited working scenes. PTMM can be used with a wearable cooling device or a body-hugging material. Refrigerant cycling and thermoelectric coolers are reported to be effective technologies for local cooling. However, their application always requires an external power source to control the temperature regulation mechanism. Textiles based on passive cooling provide effective PTMM without external energy and are therefore suitable for different environments and activity scenarios. However, there is still a lack of smart textiles that provide continuous cooling and drying for the human body, especially in complex and dynamic environments. Polylactic acid (PLA) is a biocompatible, degradable polymer that has been explored for use in daytime passive radiative refrigeration, but its inherent infrared emissivity is too low and achieving consistently efficient passive refrigeration in diverse environments remains a challenge.
Recently, Professor Tang Shaochun's team at Nanjing University and Professor Xia Zhengcai's team at Huazhong University of Science and Technology have reported an advanced PLA Janus fiber membrane (PLA-JFM) that combines highly nanoparticle enhanced radiative cooling and directed sweat transfer evaporative cooling with excellent PTMM capability. Janus membrane consists of PLA as the base layer and modified PLA (CaSO3/PLA) as the upper layer. The addition of CaSO3 achieved high infrared emissivity (96.1%) and high solar reflectance (96.6%), while the in-situ growth of CaSO3 nanoparticles changed the PLA fiber membrane from hydrophobic to hydrophilic. By effectively combining radiation and evaporative cooling, PLA-JFM exhibits superior passive cooling efficiency even in high temperature and humidity environments. The directional water transfer capability provides a continuous drying path for perspiration based evaporative cooling and creates a dry microenvironment around the human body. The PLA-JFM achieves synchronous temperature and humidity management, greatly expanding its applicability to various scenarios of human activities. In addition, its commendable safety, biodegradability and stability make it a promising PTMM material.
Recently, Professor Tang Shaochun's team at Nanjing University and Professor Xia Zhengcai's team at Huazhong University of Science and Technology have reported an advanced PLA Janus fiber membrane (PLA-JFM) that combines highly nanoparticle enhanced radiative cooling and directed sweat transfer evaporative cooling with excellent PTMM capability. Janus membrane consists of PLA as the base layer and modified PLA (CaSO3/PLA) as the upper layer. The addition of CaSO3 achieved high infrared emissivity (96.1%) and high solar reflectance (96.6%), while the in-situ growth of CaSO3 nanoparticles changed the PLA fiber membrane from hydrophobic to hydrophilic. By effectively combining radiation and evaporative cooling, PLA-JFM exhibits superior passive cooling efficiency even in high temperature and humidity environments. The directional water transfer capability provides a continuous drying path for perspiration based evaporative cooling and creates a dry microenvironment around the human body. The PLA-JFM achieves synchronous temperature and humidity management, greatly expanding its applicability to various scenarios of human activities. In addition, its commendable safety, biodegradability and stability make it a promising PTMM material.
The solar reflectance and infrared emissivity of PLA fiber membrane on one side were improved by utilizing the inherent strong infrared radiation and minimum ultraviolet absorption characteristics of CaSO3 nanoparticles (up to 96.1%, 15% higher than that of PLA). Moreover, the in situ growth of CaSO3 nanoparticles caused the PLA membrane to change from hydrophobic to hydrophilic. The unidirectional transmission index of asymmetric wettability Janus film is as high as 945%. The directional water transport accelerates the evaporation of sweat, creating a dry microenvironment around the human body and achieving a cooling effect.
The structural design of Janus fiber membrane (PLA-JFM) for PTMM is shown in Figure 1. The hydrophilic layer of PLA-JFM faces outward, and the hydrophobic layer is in contact with the skin. This gradient structure formed by hydrophilic and hydrophobic helps the sweat transfer from the skin to the outer surface layer quickly, thus accelerating the evaporation and heat dissipation of sweat. In addition, the three-dimensional porous structure and nanoparticles formed by the nanofibers in the PLA-JFM membrane improve the reflectivity of sunlight and reduce the absorption of solar energy. At the same time, CaSO3's high red emission rate can radiate heat into cold outer space. Traditional cotton fabrics can absorb sweat, but will stick to the skin surface for a long time, while the PLA-JFM can one-way transfer sweat to the hydrophilic layer away from the skin, speeding up the cooling of heat. The infrared radiation capability of PLA-JFM is also stronger in dry environments (Figure 1d).
(a) The manner in which the human body dissipates heat; (b) Passive cooling schematic diagram of PLA-JFM combined with radiation cooling and evaporative cooling; (c) Comparison of thermal management functions between conventional cotton cloth and PLA-JMF; (d) Infrared thermograms of different samples covered on dry skin
PLA membrane has a three-dimensional network structure composed of smooth nanofibers interleaving each other. After CaSO3 is grown on one side of PLA membrane, a large number of uniformly dispersed particles are distributed on the surface, as shown in Figure 2a-b. Figure 2c shows the statistical distribution of fiber diameter and particle size. The peak values of fiber size distribution and particle size distribution are ~ 0.34μm and ~ 2.76μm respectively, which are equivalent to the wavelength of solar radiation and can lead to the strong scattering of all-band sunlight. As can be seen from Figure 2e, the in-situ growth of CaSO3 particles on PLA fiber changes the surface properties of the film from superhydrophobic to superhydrophilic. The cotton cloth reaches water absorption saturation (about 285%) within about 10 s, while the PLA-JFM reaches saturation within 20 s, with water absorption as high as 708%.
2. SEM images of (a) PLA and (b) CaSO3/PLA membrane; (c) PLA fiber diameter and CaSO3 particle size distribution; (d) Surface water contact Angle of PLA film before and after CaSO3 growth; (e) Water absorption comparison
As shown in Figure 3, the solar reflectivity of pure PLA film is 91.3%; The solar reflectance of CaSO3/PLA reaches 96.6% due to the enhanced resonance scattering after CaSO3 particles are grown in situ. In addition, the infrared emissivity of CaSO3/PLA reached 96.1% in the range of 8~13 μm, which was much higher than that of pure PLA (only 83.3%). According to the thermal balance calculation, the theoretical net cooling power of PLA-JFM during the day is as high as 79.4W /m2.
In the dry state, the microstructure of randomly interleaving ultra-slender nanofibers results in a strong scattering of visible light. In contrast, when a liquid with a matched refractive index wets the surface of the fiber film, scattering at the air-polymer interface is significantly reduced. Thus, sunlight passing through the hydrophilic surface layer of PLA-JFM is re-scattered in the hydrophobic layer below. The PLA-JFM double layer structure has obvious advantages in wet cooling after water absorption, and can maintain high solar reflectivity and low transmittance even under wet conditions.
Figure 3. (a) Spectral reflectance of the upper and lower layers of PLA-JFM; (b) infrared absorption spectrum and bond vibration infrared emission pattern; (c) Theoretical net cooling power of PLA-JFM film during the day and (d) at night; (e) The scattering path diagram of light in PLA-JFM films under dry (left) and wet (right) conditions; (f) reflectance and (g) transmittance of PLA-JFM and CaSO3/PLA films tested in wet state
The PLA-JFM with Janus wettability enables directional water transfer. When water droplets contact the hydrophobic PLA layer, they will gradually penetrate into the lower layer and continue to diffuse until completely transferred. However, when water comes into contact with the hydrophilic CaSO3/PLA layer, it only diffuses through the hydrophilic layer and does not penetrate the lower layer. Therefore, in practical applications, the hydrophobic layer acts as the bottom layer in contact with the skin, which can absorb excessive sweat and promote its rapid evaporation. When water is transported from the hydrophobic layer to the hydrophilic layer, the one-way transfer index R reaches 945%, and when it is transported from the opposite direction, the R is -927%, indicating that PLA-JFM has excellent directional water transport performance. In addition, the water evaporation of PLA-JFM significantly exceeds that of traditional cotton cloth and CaSO3/PLA, which can dry the skin quickly in practical applications.
FIG. 4 (a) Schematic diagram of directional water transport; Water transport process of water droplets on (b) hydrophobic side and (c) hydrophilic side; When water droplets are in (d) hydrophobic layer and (e) hydrophilic layer, the relative water content on the top and bottom surfaces of PLA-JFM; (f) Comparison of water evaporation rates of three film samples; (g) Infrared thermogram of the sample covered with wet skin
PLA-JFM consistently shows an effective cooling effect under dry and wet conditions, as shown in Figure 5. In both sunny and cloudy weather, the PLA-JFM achieves daytime radiation cooling lower than the ambient temperature, and has strong adaptability to different weather conditions. Under dry and wet conditions, the temperature of PLA-JFM is reduced by 7.5°C and 2.1°C, respectively, compared with traditional cotton cloth, showing excellent passive cooling effect.
(a) Schematic diagram of experimental equipment for outdoor thermal measurement; (b) Real-time solar irradiation intensity and sample base temperature for outdoor thermal measurements; Sample temperatures in (c) sunny and (d) cloudy environments; (e) Real-time bottom temperature of the wet sample without direct sunlight.
Figure 6 shows the actual outdoor test results. In the outdoor environment, PLA-JFM radiates more energy to the outside, and the wet PLA-JFM dries quickly in a short time after the moisture transfer, maintaining a comfortable environment for the skin. PLA-JFM has good moisture permeability, 1.33 times that of traditional cotton. Its UV reflectivity at 320~400 nm is more than twice that of cotton cloth, which can more effectively block UV radiation. Therefore, the Janus double mask material developed in this work has great application potential in the effective management of human temperature and humidity.
Figure 6. (a) Infrared thermal imaging of the sample during evaporation of arm sweat in outdoor environment; (b) water vapor transmittance of the sample; (c) ultraviolet reflectance of the sample; (d) Mechanical strength of PLA-JFM
In summary, this work is based on electrospinning PLA thin film and single-sided inorganic nano modification method to prepare a thin film material that effectively combines radiation refrigeration and directional sweat evaporation refrigeration. In practical outdoor applications, the dry PLA-JFM is 4.5°C lower than the ambient temperature and 5.6°C lower than traditional cotton cloth at a light intensity of 60 mW/cm2. Thanks to the synergistic effect of unidirectional water transport, radiant refrigeration and evaporative refrigeration, PLA-JFM dries 46% faster than traditional cotton in humid environments at 2°C lower than traditional cotton. This work provides an effective way for the design and development of intelligent thermal management fiber membrane materials, which has important theoretical significance and practical application value.
Recently, Professor Tang Shaochun's team at Nanjing University and Professor Xia Zhengcai's team at Huazhong University of Science and Technology have reported an advanced PLA Janus fiber membrane (PLA-JFM) that combines highly nanoparticle enhanced radiative cooling and directed sweat transfer evaporative cooling with excellent PTMM capability. Janus membrane consists of PLA as the base layer and modified PLA (CaSO3/PLA) as the upper layer. The addition of CaSO3 achieved high infrared emissivity (96.1%) and high solar reflectance (96.6%), while the in-situ growth of CaSO3 nanoparticles changed the PLA fiber membrane from hydrophobic to hydrophilic. By effectively combining radiation and evaporative cooling, PLA-JFM exhibits superior passive cooling efficiency even in high temperature and humidity environments. The directional water transfer capability provides a continuous drying path for perspiration based evaporative cooling and creates a dry microenvironment around the human body. The PLA-JFM achieves synchronous temperature and humidity management, greatly expanding its applicability to various scenarios of human activities. In addition, its commendable safety, biodegradability and stability make it a promising PTMM material.
Recently, Professor Tang Shaochun's team at Nanjing University and Professor Xia Zhengcai's team at Huazhong University of Science and Technology have reported an advanced PLA Janus fiber membrane (PLA-JFM) that combines highly nanoparticle enhanced radiative cooling and directed sweat transfer evaporative cooling with excellent PTMM capability. Janus membrane consists of PLA as the base layer and modified PLA (CaSO3/PLA) as the upper layer. The addition of CaSO3 achieved high infrared emissivity (96.1%) and high solar reflectance (96.6%), while the in-situ growth of CaSO3 nanoparticles changed the PLA fiber membrane from hydrophobic to hydrophilic. By effectively combining radiation and evaporative cooling, PLA-JFM exhibits superior passive cooling efficiency even in high temperature and humidity environments. The directional water transfer capability provides a continuous drying path for perspiration based evaporative cooling and creates a dry microenvironment around the human body. The PLA-JFM achieves synchronous temperature and humidity management, greatly expanding its applicability to various scenarios of human activities. In addition, its commendable safety, biodegradability and stability make it a promising PTMM material.
The solar reflectance and infrared emissivity of PLA fiber membrane on one side were improved by utilizing the inherent strong infrared radiation and minimum ultraviolet absorption characteristics of CaSO3 nanoparticles (up to 96.1%, 15% higher than that of PLA). Moreover, the in situ growth of CaSO3 nanoparticles caused the PLA membrane to change from hydrophobic to hydrophilic. The unidirectional transmission index of asymmetric wettability Janus film is as high as 945%. The directional water transport accelerates the evaporation of sweat, creating a dry microenvironment around the human body and achieving a cooling effect.
The structural design of Janus fiber membrane (PLA-JFM) for PTMM is shown in Figure 1. The hydrophilic layer of PLA-JFM faces outward, and the hydrophobic layer is in contact with the skin. This gradient structure formed by hydrophilic and hydrophobic helps the sweat transfer from the skin to the outer surface layer quickly, thus accelerating the evaporation and heat dissipation of sweat. In addition, the three-dimensional porous structure and nanoparticles formed by the nanofibers in the PLA-JFM membrane improve the reflectivity of sunlight and reduce the absorption of solar energy. At the same time, CaSO3's high red emission rate can radiate heat into cold outer space. Traditional cotton fabrics can absorb sweat, but will stick to the skin surface for a long time, while the PLA-JFM can one-way transfer sweat to the hydrophilic layer away from the skin, speeding up the cooling of heat. The infrared radiation capability of PLA-JFM is also stronger in dry environments (Figure 1d).
(a) The manner in which the human body dissipates heat; (b) Passive cooling schematic diagram of PLA-JFM combined with radiation cooling and evaporative cooling; (c) Comparison of thermal management functions between conventional cotton cloth and PLA-JMF; (d) Infrared thermograms of different samples covered on dry skin
PLA membrane has a three-dimensional network structure composed of smooth nanofibers interleaving each other. After CaSO3 is grown on one side of PLA membrane, a large number of uniformly dispersed particles are distributed on the surface, as shown in Figure 2a-b. Figure 2c shows the statistical distribution of fiber diameter and particle size. The peak values of fiber size distribution and particle size distribution are ~ 0.34μm and ~ 2.76μm respectively, which are equivalent to the wavelength of solar radiation and can lead to the strong scattering of all-band sunlight. As can be seen from Figure 2e, the in-situ growth of CaSO3 particles on PLA fiber changes the surface properties of the film from superhydrophobic to superhydrophilic. The cotton cloth reaches water absorption saturation (about 285%) within about 10 s, while the PLA-JFM reaches saturation within 20 s, with water absorption as high as 708%.
2. SEM images of (a) PLA and (b) CaSO3/PLA membrane; (c) PLA fiber diameter and CaSO3 particle size distribution; (d) Surface water contact Angle of PLA film before and after CaSO3 growth; (e) Water absorption comparison
As shown in Figure 3, the solar reflectivity of pure PLA film is 91.3%; The solar reflectance of CaSO3/PLA reaches 96.6% due to the enhanced resonance scattering after CaSO3 particles are grown in situ. In addition, the infrared emissivity of CaSO3/PLA reached 96.1% in the range of 8~13 μm, which was much higher than that of pure PLA (only 83.3%). According to the thermal balance calculation, the theoretical net cooling power of PLA-JFM during the day is as high as 79.4W /m2.
In the dry state, the microstructure of randomly interleaving ultra-slender nanofibers results in a strong scattering of visible light. In contrast, when a liquid with a matched refractive index wets the surface of the fiber film, scattering at the air-polymer interface is significantly reduced. Thus, sunlight passing through the hydrophilic surface layer of PLA-JFM is re-scattered in the hydrophobic layer below. The PLA-JFM double layer structure has obvious advantages in wet cooling after water absorption, and can maintain high solar reflectivity and low transmittance even under wet conditions.
Figure 3. (a) Spectral reflectance of the upper and lower layers of PLA-JFM; (b) infrared absorption spectrum and bond vibration infrared emission pattern; (c) Theoretical net cooling power of PLA-JFM film during the day and (d) at night; (e) The scattering path diagram of light in PLA-JFM films under dry (left) and wet (right) conditions; (f) reflectance and (g) transmittance of PLA-JFM and CaSO3/PLA films tested in wet state
The PLA-JFM with Janus wettability enables directional water transfer. When water droplets contact the hydrophobic PLA layer, they will gradually penetrate into the lower layer and continue to diffuse until completely transferred. However, when water comes into contact with the hydrophilic CaSO3/PLA layer, it only diffuses through the hydrophilic layer and does not penetrate the lower layer. Therefore, in practical applications, the hydrophobic layer acts as the bottom layer in contact with the skin, which can absorb excessive sweat and promote its rapid evaporation. When water is transported from the hydrophobic layer to the hydrophilic layer, the one-way transfer index R reaches 945%, and when it is transported from the opposite direction, the R is -927%, indicating that PLA-JFM has excellent directional water transport performance. In addition, the water evaporation of PLA-JFM significantly exceeds that of traditional cotton cloth and CaSO3/PLA, which can dry the skin quickly in practical applications.
FIG. 4 (a) Schematic diagram of directional water transport; Water transport process of water droplets on (b) hydrophobic side and (c) hydrophilic side; When water droplets are in (d) hydrophobic layer and (e) hydrophilic layer, the relative water content on the top and bottom surfaces of PLA-JFM; (f) Comparison of water evaporation rates of three film samples; (g) Infrared thermogram of the sample covered with wet skin
PLA-JFM consistently shows an effective cooling effect under dry and wet conditions, as shown in Figure 5. In both sunny and cloudy weather, the PLA-JFM achieves daytime radiation cooling lower than the ambient temperature, and has strong adaptability to different weather conditions. Under dry and wet conditions, the temperature of PLA-JFM is reduced by 7.5°C and 2.1°C, respectively, compared with traditional cotton cloth, showing excellent passive cooling effect.
(a) Schematic diagram of experimental equipment for outdoor thermal measurement; (b) Real-time solar irradiation intensity and sample base temperature for outdoor thermal measurements; Sample temperatures in (c) sunny and (d) cloudy environments; (e) Real-time bottom temperature of the wet sample without direct sunlight.
Figure 6 shows the actual outdoor test results. In the outdoor environment, PLA-JFM radiates more energy to the outside, and the wet PLA-JFM dries quickly in a short time after the moisture transfer, maintaining a comfortable environment for the skin. PLA-JFM has good moisture permeability, 1.33 times that of traditional cotton. Its UV reflectivity at 320~400 nm is more than twice that of cotton cloth, which can more effectively block UV radiation. Therefore, the Janus double mask material developed in this work has great application potential in the effective management of human temperature and humidity.
Figure 6. (a) Infrared thermal imaging of the sample during evaporation of arm sweat in outdoor environment; (b) water vapor transmittance of the sample; (c) ultraviolet reflectance of the sample; (d) Mechanical strength of PLA-JFM
In summary, this work is based on electrospinning PLA thin film and single-sided inorganic nano modification method to prepare a thin film material that effectively combines radiation refrigeration and directional sweat evaporation refrigeration. In practical outdoor applications, the dry PLA-JFM is 4.5°C lower than the ambient temperature and 5.6°C lower than traditional cotton cloth at a light intensity of 60 mW/cm2. Thanks to the synergistic effect of unidirectional water transport, radiant refrigeration and evaporative refrigeration, PLA-JFM dries 46% faster than traditional cotton in humid environments at 2°C lower than traditional cotton. This work provides an effective way for the design and development of intelligent thermal management fiber membrane materials, which has important theoretical significance and practical application value.
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