Stretchable Anti-fogging Tapes for Diverse Transparent Materials 
 
Shaoting Lin1*, Yueying Yang1†, Jiahua Ni1†, Xinyue Liu1, Yoichiro Tsurimaki1,  
Baoyang Lu1, Yaodong Tu1, Jiawei Zhou1,2, Xuanhe Zhao1,3*, Gang Chen1*  
 
1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 
02139, USA 
2Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 
3Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 
Cambridge, MA 02139, USA 
 
† These authors contributed equally to this work. 
* Email: shaoting@mit.edu, zhaox@mit.edu, gchen2@mit.edu 
 
Keywords 
 
This is the author manuscript accepted for publication and has undergone full peer review but has not 
been through the copyediting, typesetting, pagination and proofreading process, which may lead to 
differences between this version and the Version of Record. Please cite this article as doi: 
10.1002/adfm.202103551. 
 
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Anti-fogging, transparent materials, hydrogel adhesion, personal protection equipment, solar water 
purification  
 
One Sentence Summary 
We report a stretchable anti-fogging tape that can be adhered to versatile transparent materials 
with various curvatures for persistent fogging prevention, enabling new applications including fog-
free eyeglasses, protective goggles and efficient solar-powered freshwater production. 
 
One Figure Summary 
 
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ABSTRACT 
 
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Surface wetting prevents surface fogging on transparent materials by facilitating filmwise 
condensation with specific chemistry, but suffers from material and geometry selectivity. Extreme 
environments associated with high humidity and mechanical loading further limit their anti-fogging 
persistence. Here, we report a stretchable anti-fogging tape (SAT) that can be applied to diverse 
transparent materials with varied curvatures for persistent fogging prevention. The SAT consists of 
three synergistically-combined transparent layers: i) a stretchable and tough layer with large elastic 
recovery, ii) an endurant anti-fogging layer insensitive to ambient humidity, and iii) a robustly and 
reversibly adhesive layer. The SAT maintains high total transmittance (>90%) and low diffuse 
transmittance (<5%) in high-humidity environments, under various modes of mechanical 
deformations, and over a prolonged lifetime (193 days tested so far). Two applications are 
demonstrated, including the SAT-adhered eyeglasses and goggles for clear fog-free vision, and the 
SAT-adhered condensation cover for efficient solar-powered freshwater production. 
 
 
 
  
 
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1. Introduction 
Transparent materials allowing light to pass them through without appreciable light scattering are 
indispensable for a wide range of applications[1, 2] ranging from protective personnel equipment, 
optical instruments to solar-powered devices. When exposed to high-humidity environments or 
sudden change of temperature, surface fogging on transparent materials causes unintended light 
scattering[3], followed by decreased visibility of optical devices[4], lowered energy-conversion 
efficiency of solar stills[5] and solar cells[6], and reduced crop yield in greenhouses[7]. Fogging on 
glasses also induces annoyance in daily life, and especially triggers health risks in fighting the Covid-
19 pandemic as it increases the frequency of hand-face touching[8].  
Surface fogging is caused by droplet condensation on the surface which scatters light, and it 
can be avoided by facilitating filmwise condensation through surface wetting [2, 9-11]. One way is to 
spray hydrophilic agents onto the surface[11], but it suffers from a short lifespan (less than a few 
days) due to the weak interaction between the hydrophilic agents and the surface (Figure S1a, 
Supporting Information). Micro- and nano-fabrication are also used to engineer surface topology for 
modifying surface hydrophobicity[12-14], but their manufacturing processes are technically 
complicated and time-consuming, thereby limited to laboratory research and impractical for large-
scale massive production[15]. Whereas strong anchorage of crosslinked hydrogels to transparent 
materials has been developed to prevent surface fogging[10], it usually leads to the reduced 
transparency in high-humidity environments due to the swelling-induced mechanical instabilities of 
hydrogels (Figure S1b, Supporting Information). Additionally, the abovementioned anti-fogging 
methods are usually selective to a certain specific transparent material. For example, the method of 
 
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covalently anchoring hydrophilic polysaccharides to glass[16] cannot be applied to polyethylene films 
due to the different interfacial chemistry. The challenges associated with existing anti-fogging 
methods are further amplified when transparent materials are curved (as in eyeglasses) and/or 
experience large deformations (as in stretch wraps). Thus, a universal strategy to maintain anti-
fogging performance of transparent materials in humid and dynamic environments for a prolonged 
period of time remains a prevailing challenge. 
Here, we propose a universal anti-fogging strategy based on a stretchable anti-fogging tape 
(SAT) that can be adhered to a wide range of transparent materials with various curvatures for 
persistent fogging prevention. The SAT synergistically integrates three distinct layers to address the 
above-mentioned limitations (Figure 1a): i) a stretchable intermediate layer with high elastic 
recovery made of low-density polyethylene (LDPE) to keep the SAT tightly bound to transparent 
materials, ii) an anti-fogging top layer made of uncrosslinked polyacrylamide (PAAm) to impart 
hydrophilicity to transparent materials, and iii) an adhesive bottom layer made of loosely crosslinked 
polydimethylsiloxane (PDMS) to form robust yet reversible adhesion between transparent materials 
and the SAT. Given these superior mechanical and physical properties, the SAT can be elastically pre-
stretched and conformably adhered to targeted transparent substrates with seamless contact and 
no surface wrinkles, thereby effectively preserving the high transparency of the pristine transparent 
materials adhered with SAT (Figure 1b). We show that the SATs can be broadly applied to diverse 
transparent materials including glass, polyethylene (PE), polyethylene terephthalate (PET), 
polystyrene (PS), poly(methyl methacrylate) (PMMA), and PDMS. Meanwhile, the SATs can be 
conformably adhered to curved transparent surfaces with varied radii of curvature. The SAT can 
effectively maintain high total transmittance (>90%) and low diffuse transmittance (<5%) in high 
 
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humidity environments. Such anti-fogging performances further persist during the 193-day testing 
period, even under various modes of mechanical loadings such as uniaxial tension, punching, and 
cyclic scratching. We further demonstrate that the SAT can be readily integrated with eyeglasses and 
protective goggles for clear fog-free vision and used as the condensation cover in a solar still to 
improve its efficiency of freshwater production. 
2. Results and Discussion 
2.1 Fabrication of stretchable anti-fogging tapes. 
To fabricate the stretchable anti-fogging tape, we biaxially prestretch an LDPE film and tightly attach 
and fix the pre-stretched LDPE film to a flat surface (e.g., acrylic substrate). The prestretch of the 
LDPE film can effectively eliminate its surface wrinkles, which otherwise reduces its optical 
transparency. On the upper surface of the prestretched LDPE film, we spin coat a layer of loosely 
crosslinked PDMS, followed by thermal curing at a mild temperature (i.e., 50 °C) for 12 hours. We 
select a mild temperature for PDMS curing to prevent surface wrinkles induced by the thermal-
induced deformation of the LDPE film. Once the PDMS layer is cured, we adopt the benzophenone-
induced grafting photopolymerization[17, 18] to covalently graft long-chain polymers of hydrophilic 
PAAm to the branched polymers of LDPE (Figure 1a). The surface with grafted PAAm chains of the 
resultant SAT film is thoroughly rinsed with deionized water to remove the unreacted reagents (i.e., 
acrylamide monomers) (Figure S2, Supporting Information). We further perform the fourier-
transform infrared spectroscopy (FTIR) to characterize the chemical bonds of the SAT film after 
thorough rinsing with deionized water. As shown in Figure 2g, the resultant SAT film shows 
pronounced peaks associated with amide groups, indicating that the interaction between PAAm and 
 
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LDPE should be strong covalent anchorage rather than weak deposition or absorption of PAAm 
chains. The details on fabrication and chemical synthesis of the SAT are provided in Methods and 
Figure S3, Supporting Information.  
2.2 Surface and mechanical characterizations of stretchable anti-fogging tapes. 
We first perform atomic force microscopy to characterize the surface topology of the PAAm 
layer in the dry state. As shown in Figure 2a, b, the thickness of the PAAm layer is about 55 nm in the 
dry state and its root mean square roughness is 19.10 nm, slightly higher than the roughness of the 
pristine LDPE film (i.e., 12.54 nm). We further use the confocal microscopy imaging to visualize the 
PAAm layer in the hydrated state. The PDMS layer and the PAAm layer are stained with Nile red and 
fluorescein, respectively. As shown in Figure 2c, the thickness of the PAAm layer is much greater 
than its thickness in the dry state due to the superior swelling of the grafted PAAm polymers. 
We further conduct contact angle measurements to characterize the surface wetting property of 
the anti-fogging tape. As shown in Figure 2d and Video S1, Supporting Information, a total volume of 
10 µL deionized water is deposited on the dry surface of the PAAm layer. As the volume of the 
deposited water increases, the advancing contact angle gradually decreases and reaches a steady-
state value of 49°. As the volume of the deposited water decreases, the measured receding contact 
angle decreases drastically from 49° to 0°, suggesting the super hydrophilicity of the PAAm layer in 
the hydrated state. We further deposit deionized water on the hydrated surface of the PAAm layer 
(Figure 2e and Video S2, Supporting Information). The water droplet on the nozzle spreads out in 0.1 
s once it contacts the PAAm layer, further demonstrating its super hydrophilicity.  
 
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In addition to surface characterizations, we also characterize mechanical properties of the 
SAT film. As shown in Figure 2f and Video S3, Supporting Information, the SAT can be stretched up to 
3.8 times its original length without interfacial delamination. In addition, the SAT exhibits a 
remarkable elastic recovery with elastic recoverable strain of 0.72 (Figure 2f), which keeps it tightly 
bound to targeted transparent materials. The elastic recoverable strain of the SAT exceeds that of 
the common transparent materials (e.g., <5% for PET, PS, PMMA)[19]. The SAT is also tough with a 
high fracture energy of 2,126 J m-2 (Figure S4 and Video S4, Supporting Information), which renders 
its mechanical robustness when elastically pre-stretched and conformably adhered to targeted 
transparent materials. In addition, we characterize the adhesiveness between the SAT and the 
backing layer (i.e., PET), which demonstrates a robust and reversible adhesive strength of 4 kPa for 
1000 cycles of attachment and detachment (Figure S5, Supporting Information).  
2.3 Optical characterizations of stretchable anti-fogging tapes 
We first characterize water condensation on both the pristine LDPE film and the LDPE-PAAm 
film. As shown in Figure S6a, Supporting Information, a water container is heated by a hot plate at a 
controlled temperature for 20 minutes to reach the steady state. A thermocouple is used to monitor 
the temperature of the water inside the container. Thereafter, the sample is covered on top of the 
water container with heated water vapor condensing on its bottom surface. We use optical 
microscopy to capture the real-time morphology of the condensed water. As shown in Figure S6b, c 
Supporting Information, the pristine LDPE film facilitates dropwise condensation due to its 
hydrophobicity, while the LDPE-PAAm film promotes filmwise condensation because of its super 
hydrophilicity in the hydrated state.  
 
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To systematically characterize the effect of the condensed water on optical properties of the 
film, we further perform UV-vis-NIR spectroscopy to quantify the total transmittance and diffuse 
transmittance throughout samples before and after condensation (Video S5, Supporting 
Information). The measurements follow the standard of ASTM D1003. The total transmittance 
characterizes the total percentage of the incident light directly and diffusely transmitting through 
the sample, while the diffuse transmittance measures the percentage of the incident light diffusely 
transmitting through the sample. As illustrated in Figure 3a, b, the total transmittance Tt is 
calculated by the ratio of the light transmitted through the specimen T2 versus the incident light T1, 
that is, Tt = T2/ T1 (Figure 3a). The diffuse transmittance Td is calculated by Td = [T4 - T3(T2/T1)]/T1, 
where T3 is the light scattered by the instrument and T4 is the light scattered by both the instrument 
and the sample (Figure 3b). When exposed to heated water vapor at 50°C (the temperature refers to 
the measured temperature of the liquid water in the container), the total transmittance of the 
pristine LDPE decreases significantly from 92% to 75% in the visible spectrum and decreases from 
92% to 60% in the near-infrared spectrum due to the backscattering of light by water droplets 
(Figure 3c, d). In contrast, the LDPE film with covalently grafted uncrosslinked PAAm (i.e., LDPE-
PAAm) shows negligible loss of transmittance over the entire spectrum in spite of a slight decrease 
of total transmittance between 1700 and 1800 nm due to the light absorption by water (Figure 3f, 
g). We also measure the diffuse transmittance of both the pristine LDPE and LDPE-PAAm to 
characterize their haze when exposed to heated water vapor at 50°C. The diffuse transmittance of 
the pristine LDPE film increases from 8% to 48% in the visible spectrum due to its dropwise 
condensation (Figure. 3e). In contrast, the filmwise condensation maintains a low diffuse 
transmittance of the LDPE-PAAm below 5% across the spectrum, manifesting the low haze of a 
 
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uniform water film (Figure 3h). The total transmittance and diffuse transmittance of the LDPE-PAAm 
after condensation in this work outperform those of the commercially available anti-fogging 
products including the anti-fog spray (Optix55), anti-fog wipes (KarisVisual), anti-fog mirror window 
film (Kerkoor), anti-fog greenhouse film (Agfabric) as summarized in Figure S7, Supporting 
Information. These commercially available anti-fogging products mostly rely on spraying hydrophilic 
agents or addition of surfactants. 
Water vapor temperature is a well-known factor affecting the optical transparency of the 
film after condensation due to the altered size of the condensed water droplets[20]. As shown in 
Figure S8a, Supporting Information, the increase of the water vapor temperature drastically 
decreases the total transmittance of the pristine LDPE film because of the enlarged water droplet on 
the film. In contrast, the water vapor temperature has little effect on the LDPE-PAAm film (Figure 
S8b, Supporting Information), which indicates the water vapor temperature does not change the 
thickness of the condensed water film. We also check the effect of the coated PDMS layer on the 
optical properties of the SAT. As shown in Figure S9, Supporting Information, the PDMS layer slightly 
decreases the total transmittance from 92% to 88% in the visible and near-infrared spectrum and 
slightly increases the diffuse transmittance from 2% to 5% across the entire spectrum. In particular, 
we find the presence of the PDMS layer dramatically decreases the total transmittance in the 
ultraviolet spectrum because of the high absorption of ultraviolet light of PDMS. 
To support the crucial role of filmwise water condensation in the high total transmittance of 
the LDPE-PAAm film, we then perform wave optics simulation based on the transfer matrix method 
for a two-layer structure made of a water film on top of a 20 µm-thick LDPE film. The predicted total 
transmittance for the 16-µm thick water film agrees well with the experimental measurement as 
 
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shown in Figure 3g. We further carry out ray optics simulation in a pristine LDPE film with water 
droplets. By assuming the uniform droplet radius of 40 µm, contact angle of 110° and droplets area 
density of 0.55 as measured experimentally (Figure S6d, Supporting Information), the model predicts 
a drastic reduction of total transmittance, matching with the experimental data shown in Figure 3d. 
The details of the wave and ray optics modeling can be found in Methods. 
2.4 Persistence and universality of anti-fogging properties. 
Persistent anti-fogging properties is a central challenge faced by existing anti-fogging 
approaches but highly desirable for practical deployment [21, 22]. Here, we show our approach can 
potentially address this challenge. As one example shown in Figure S1a, Supporting Information, the 
LDPE film sprayed with hydrophilic agents exhibits decreased optical transparency when the film is 
exposed to heated water vapor at 60 °C for 14 minutes (Figure 4a, c). The hydrophilic agents are 
washed away by condensed water on the surfaces continuously due to the weak interaction 
between hydrophilic agents and transparent materials[2]. As another example shown in Figure S1b, 
Supporting Information, the LDPE film covalently coated with a layer of crosslinked hydrogel loses its 
optical transparency when the film is exposed to heated water vapor at 60 °C for 20 minutes, 
because the crosslinked hydrogel tends to develop swelling-induced crease instability on the surface 
[23] (Figure 4b, c and Figure S1b, Supporting Information). In contrast, we show that both the 
covalent anchorage and uncrosslinked hydrophilic polymers in our approach are essential for long-
term robust and humidity-insensitive optical properties (Figure 4a, b). The total transmittance of our 
LDPE-PAAm film can consistently maintain high total transmittance over 90% and low diffuse 
transmittance below 10% across the visible and near-infrared spectrum for 193 days (Figure 4c, d 
and Figure S10a,b, Supporting Information). The durable anti-fogging properties of our approach 
 
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further justify that the interaction between PAAm and LDPE is strong covalent anchorage rather than 
weak deposition or absorption of PAAm polymers. 
We further testify the anti-fogging properties of the LDPE-PAAm film under various modes of 
mechanical loadings, including biaxial tension and dynamic scratching. We first apply punching 
loading on a circle-shaped film with a diameter of 65 mm to simulate the biaxial tensile loading. As 
shown in Figure 4e, the LDPE-PAAm film can maintain high total transmittance over 90% and low 
diffusive transmittance below 5% even when the punching displacement increases up to 15 mm. 
When the applied punching displacement reaches 20 mm, the diffuse transmittance increases due to 
the excessive plastic deformation induced in the film. Dynamic scratching is another common 
mechanical load that might damage the anchored PAAm layer. To simulate the load of dynamic 
scratching, we use a probe to compress the LDPE-PAAm film and apply a reciprocating motion of the 
probe. The compressive forces are monitored from 0 to 4.8 N, corresponding to the normal pressure 
of 0 to 20 kPa as the contact area between the probe and the LDPE-PAAm film is around 200 mm2. 
Both the total transmittance and diffuse transmittance are measured after 100 cycles of dynamic 
scratching under various compressive forces. As shown in Figure 4f and Figure S10c, d, Supporting 
Information, the LDPE-PAAm maintains a high total transmittance above 90% and low diffuse 
transmittance below 10%, indicating its remarkable mechanical robustness under various loads of 
dynamic scratching.  
To demonstrate the anti-fogging tape can be applicable to universal substrates, we 
adhere the SAT to substrates with diverse materials (e.g., PET, PDMS, PMMA, PS, glass) 
and geometries (e.g., cylindrical tubes with varied curvatures). Given the high stretchability, 
large elastic recovery, high toughness, and reversible adhesion of SAT, the SAT can be pre-
 
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stretched and then adhered to substrates with seamless contact and no surface wrinkles. When 
further exposed to heated water vapor, the taped transparent materials show superior anti-
fogging properties, giving clear fog-free vision (Figure 5a, Videos S6-S9, Supporting 
Information). The total transmittance of the visor materials with SATs can reach as high as 
90%, and the diffuse transmittance are as low as about 5% in the visible spectrum (Figure 5b 
and Figure. S11, Supporting Information). We further conformably attached the SAT to the 
inner surfaces of PMMA tubes with altered radii of curvature ranging from 3.2 mm to 19.1 
mm. As shown in Figure 5c, the SAT can effectively impart the curved surfaces with anti-
fogging properties (high total transmittances and low diffuse transmittances), giving clear 
fog-free vision when exposed to high humidity and high temperature (Figure 5d and Videos 
S10, Supporting Information). Given the high stretchability of the anti-fogging tape, we 
further demonstrate new functions for eliminating fog on soft, stretchable, and foldable 
transparent materials. As shown in Figure 5e, we demonstrate the stretchable transparent 
materials (i.e., PDMS) with anti-fogging tape can combat fogging issue at highly deformed 
state when exposed to heated water vapor (60 °C). As another example shown in Figure 5f, 
we further show a foldable transparent material can maintain its transparency for 30 minutes 
when exposed to heated water vapor (60 °C). 
2.5 Applications of stretchable anti-fogging tapes. 
After validation of the superior anti-fogging performance and remarkable mechanical 
robustness of the SAT, we further explore the applications of SATs on optical and solar 
devices that are susceptible to fogging, including eyeglasses, goggles, and solar stills.  
 
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First, we apply our SAT to solve the fogging issue of eyeglasses and safety goggles (Figure 
6a). As shown in Figure 6b, the lens with SAT attached at the inner surface can maintain clear vision 
when exposed to humid environment and a sudden change of temperature. To test the performance 
in real environments, we wear the glasses with a SAT adhered on the left lens. As shown in Figure 6c, 
d, our SAT can effectively maintain the clear vision in the indoor environment with a room 
temperature of 22 °C and at the outdoor environment with a cold temperature of -5 °C. In 
comparison, the fogging on the pristine lens severely block the vision.  
Next, we demonstrate the use of LDPE-PAAm film as a condensation cover in a sunlight-
powered water purification system to illustrate its potential for efficient solar energy harvesting and 
water desalination (Figure 7a). Normally, the evaporated water vapor from brine can condense as 
droplets on the collection cover of the solar still, resulting in a reduction of optical transparency by ~ 
35%[5]. We build a prototype of the water purification system under natural sunlight, which consists 
of a black super-absorbent foam as the absorber, the LDPE-PAAm film as the condensation cover, a 
beaker as the purified water collector, and a bubble wrap as the thermal insulation layer (Figure S12, 
Supporting Information). A metallic ball is placed on top of the condensation cover, directing the 
condensed water to drip into the beaker by gravity. As a control comparison, we also set up a 
reference system with the pristine LDPE film as the condensation cover. 
We conduct outdoor water purification tests at Massachusetts Institute of Technology 
campus from 8:00 to 20:00 on June 23th, 2020. Figure S13a, Supporting Information plots the 
recorded solar flux with a total solar energy of 7.17 kW·h m-2 during the testing period. Figure S13 
b,c , Supporting Information shows the measured ambient temperature and relative humidity on the 
 
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15 
 
 
 
 
same day with an average value of 35 °C and 40%, respectively. Purified water is collected around 
every hour. The LDPE-PAAm film as the condensation cover maintains high transparency with 
negligible fogging for the entire sunny day, while the pristine LDPE film shows significant surface 
fogging from 9:00 (Figure 7b). Due to the superior anti-fogging performance, the LDPE-PAAm film 
enables the solar water purification system to reach a water collection rate of 3.8 L/m2 per day, 2.4 
times that of the reference system (pristine LDPE film as the condensation cover, 1.6 L/m2 per day) 
(Figure 7c). The reduction of the light backscattering by the LDPE-PAAm condensation cover is 
further validated by the increased absorber temperature, vapor temperature, and bulk water 
temperature shown in Figure S14, Supporting Information. To test the durability of the film after 
long-term storage, we perform the other outdoor water purification test from 8:00 to 20:00 on Sep 
8th, 2020. Our water purification system with the HPE film as the condensation cover still shows a 
190% boost for freshwater production as shown in Figure 7d. The weather conditions on Sep 8th, 
2020 are summarized in Figure S13 d-e, Supporting Information. To further demonstrate the 
capability of solar-powered water purification, we measure the concentrations of four primary ions 
(Na+, Mg2+, K+, and Ca2+) before and after desalination as shown in Figure 7e. These ions in the 
collected water are significantly reduced and below the values for drinkable water according to US 
Environmental Protection Agency (EPA)[24]. 
We further evaluate the collection efficiency of both of the water purification systems, using 
the following definition[25] 
mwaterhwater mwaterCp Twater Tambient 
water     (1) 
Aevap  qsolardt
 
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where mwater  is the collected purified water per day, hwater  2400 kJ/kg is the latent heat for 
evaporation of water at 50 °C, C  4186 J/(°C kg) is the specific heat of water, Tp water  is the 
temperature of water at the surface with an average value of 50 °C for the HPE film and 45 °C for the 
pristine PE film , Tambient  is the ambient temperature with an average value of 35 °C, A  is the evap
evaporation area, and qsolar  is the time-dependent solar flux. Following Eq. (1), the water 
purification system using the LDPE-PAAm film as the condensation cover reaches a water collection 
efficiency as high as 36.2%, 2.4 times of the same system using the pristine LDPE film as the 
condensation cover (i.e., 15.1%). Note that the water collection efficiency for the state-of-the-art 
floating solar still is 24%[5]. Our LDPE-PAAm film, as the simplest design of condensation structure, 
can be readily integrated with existing evaporation structures with no significant investment in cost 
while increasing the water collection efficiency.  
 
3. Conclusions 
Motivated by the ubiquitous fogging and their threatening effects on environment and human 
health, we report a universal anti-fogging approach by designing a stretchable anti-fogging tape (SAT) 
that can be generally applicable to diverse transparent materials and various curved surfaces of 
optical devices and solar products. We demonstrate the superior anti-fogging properties of the SAT 
with high total transmittance above 90% and low diffuse transmittance below 5% in high-humidity 
environments, under various modes of mechanical deformations (uniaxial tension, punching, and 
cyclic mopping), and over a prolonged lifetime (over 193 days tested so far). We show potential 
 
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17 
 
 
 
 
applications of the SAT for efficient solar stills and personal protective equipment. This work also 
suggests new applications of hydrogels towards functional flexible transparent materials by 
harnessing tailored properties of polyethylene films[6, 26, 27] and hydrogels[28, 29]. 
 
4. Experimental Section 
Materials. Chemicals including acrylamide, benzophenone, Irgacure-2959, and Sylgard 184 
were purchased from Sigma-Aldrich. Saran Premium Wrap as low-density polyethylene was 
purchased from Johnson. NHS-Fluorescein was purchased from Thermo Fisher Scientific. Visor 
materials including PET, Glass, PMMA were purchased from McMaster-Carr. All chemicals were used 
without further purification. Deionized water (from a Milli-Qsystem) was used throughout the 
experiments. 
Fabrication of LDPE-PAAm. Figure S3, Supporting Information schematically illustrates the 
procedure and benzophenone-induced grafting photopolymerization[17] for fabricating the LDPE-
PAAm. The pristine low-density polyethylene film (Saran™ Premium Wrap) was first thoroughly 
cleaned with ethanol and deionized water, and completely dried with nitrogen gas. Thereafter, the 
benzophenone solution (10 wt.% in ethanol) was applied onto the polyethylene film to evenly cover 
the entire surface for 10 min at room temperature. The benzophenone-treated polyethylene film 
was washed with ethanol three times and completely dried with nitrogen gas. Precursor solution of 
hydrogel polymers was prepared by mixing aqueous solutions of hydrophilic monomer (i.e., 
acrylamide) and hydrophilic initiator (i.e., Irgacure-2959, 1 wt.%). The typical concentration of the 
hydrophilic monomer ranges from 5 wt.% to 40 wt.%, which determines the chain density anchored 
 
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18 
 
 
 
 
on the surfaces of polyethylene film. The mixture of the precursor solution was poured onto the 
benzophenone-treated polyethylene film in an acrylic mold and then covered by a glass plate with a 
hydrophobic coating. Both the precursor solution and the benzophenone-treated polyethylene film 
were subjected to ultraviolet irradiation in an ultraviolet chamber (365 nm ultraviolet; UVP CL-1000) 
for an hour. Under UV radiation, the hydrophilic monomer forms PAAm polymers via free radical 
polymerization. Meanwhile, surface absorbed benzophenone mediates the grafting of PAAm 
polymers onto the reactive sites on the polyethylene chains. After UV radiation, the treated 
polyethylene film was thoroughly rinsed with deionized water to remove the unreacted reagents on 
the surface of the LDPE film. 
Fabrication of SAT. A pristine LDPE film was first pre-stretched and tightly attached to an 
acrylate plate with no surface wrinkles. Thereafter, the pre-stretched LDPE film was uniformly 
coated with a PDMS solution, using the spin coater (Specialty coating systems, 6800 Spin Coater 
Series) with a rotation speed of 250 rpm for 3 minutes. The thickness of the PDMS layer can be 
tuned by adjusting the rotational speed. The mixing ratio of PDMS is set as 1/30 for the reversible 
adhesion. The entire sample was further cured at a mild temperature (i.e., 50 °C) for 12 hours to 
ensure no surface wrinkles formed by the thermal-induced deformation of the LDPE film. Once the 
PDMS layer is cured, we followed the same protocol of synthesizing LDPE-PAAm to covalently graft 
uncrosslinked PAAm on the other surface of the LDPE film. The resultant SAT consists of a laminated 
structure with the LDPE film as the intermediate layer, the PDMS as the adhesive layer, and the 
PAAm as the anti-fogging layer. 
AFM imaging. AFM topology images were acquired with an atomic force microscope (MFP-
3D, Asylum Research). Dry freestanding pristine LDPE and SAT films were directly attached onto the 
 
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19 
 
 
 
 
sample stage with a double-sided carbon tape. Surface topology was evaluated for areas covering 10 
×10 µm2. 
Confocal microscopy imaging. Due to the optical transparency of the SAT film, different dyes 
were utilized to facilitate imaging and characterization of the SAT film. A hydrophobic Nile red dye 
(λemission ≈ 600 nm) was added to Sylgard 184 mixture to allow the visualization of the PDMS layer 
while the anti-fogging tape was immersed in an aqueous NHS-fluorescein solution (λemission ≈ 518 nm) 
to enable the visualization of the PAAm layer. 
Contact angle measurement. A total volume of 10 µL deionized water was deposited on both 
dry and hydrated surfaces of the pristine LDPE film and the PAAm layer of the anti-fogging tape. 
Videos were recorded to measure the advancing and receding contact angles. 
UV-vis-NIR measurement. The UV-vis-NIR measurement was conducted by the Cary 5000 
UV-vis-NIR spectrophotometer (Agilent Technologies), following the standard ASTM D1003. The first 
test was run with no specimen in position but with a standard high-reflectance reference material in 
position to measure the intensity of incident light T1. The second test was run with both specimen 
and the reference in position to measure the total light transmitted through the specimen T2. The 
total transmittance can be calculated by the ratio of T2 over T1 (e.g., Tt = T2/ T1). The third test was 
run with no specimen in position but with a light trap in position to measure the light scattered by 
the instrument T3. The last test was run with both specimen and light trap in position to measure the 
light scattered by the instrument and the specimen T4. The diffuse transmittance can be calculated 
by Td = [T4 - T3(T2/ T1)]/ T1.  
 
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20 
 
 
 
 
Simulation of the total transmittance of the films after condensation. The wave optics 
simulation of a LDPE-PAAm is performed by the transfer matrix method. The structure is shown in 
Figure S6, Supporting Information where it is a two-layer structure made of the LDPE film on top of a 
water film. The entire structure is in the air. The incident light is normally incident on the structure 
from the LDPE film side and is assumed unpolarized because our primary interest is in solar radiation 
as well as ambient light. The optical properties of water are taken from the work [30]. The optical 
properties of LDPE reported in the literature [31] are used for wavelengths from 1.1 to 2.5 μm and the 
complex refractive index at the shortest wavelength reported in the literature [31], i.e.,         , is 
extrapolated to the shorter wavelengths. To take into account the exponential increase of the 
absorption coefficient of LDPE in the UV range, their optical properties at the UV range (  
         reported in the literature [32] is approximated as                              
and is smoothly connected to those at the visible range. As a result, the optical properties of LDPE 
used in the simulation are shown in Figure S16a, Supporting Information. The thickness of the LDPE 
film is determined as 20 μm by fitting the predicted total transmittance to the experimentally 
measured transmittance without water condensation as shown in Figure S16b, Supporting 
Information. To suppress the oscillation of transmittance due to the nature of perfectly coherent 
wave optics, we calculate the total transmittan zce with 0.1 nm interval and take the average over 
adjacent points over ± 25 nm. The water thickness of 16 μm is selected so that the predicted total 
transmittance best agrees with experimental measurements. We also conduct the ray optics 
simulation, giving the same results as the wave optics simulation (Figure S16b, Supporting 
Information). 
 
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21 
 
 
 
 
    To model the total transmittance through untreated LDPE film with water droplets, the 
ray optics simulation is done by using the commercial software COMSOL Multiphysics ®. The 
structure is shown in Figure S16a, Supporting Information. The same LDPE film thickness as that of 
treated LDPE film, and uniform radius of water droplets are assumed. The other geometrical 
parameters of the system are the diameter, contact angle, and areal density of water droplets. The 
diameter of the water droplets is set to be 80 μm, which approximately is the peak position of the 
radial distribution, and the contact angle of 110° is assumed. Those values are experimentally 
obtained as shown in Figure S6, Supporting Information.  The areal density is treated as a fitting 
parameter and the best fit to experimentally measured total transmittance at 50 °C is produced 
when 0.55 is assumed.  
Antifogging on stretchable and foldable transparent materials. To demonstrate the 
capability of eliminating fog on a stretchable transparent material, we first thermally cured a Sylgard 
184 with a mixture ratio of 10:1 to fabricate a stretchable transparent material, and then 
prestretched our anti-fogging tape and gently attached the tape on the surface of the PDMS 
substrate. We exposed the surface of the PDMS substrate with our anti-fogging tape to heated 
water vapor (60 °C) to test its anti-fogging property. To demonstrate the capability of eliminating fog 
on a foldable transparent material, we first cut a PET film to a 2D shape that can be folded into a 3D 
cube, and thereafter prestretched our anti-fogging tape and gently attached the tape on the surface 
of the PET film. The 2D PET film was folded into a 3D cube with the anti-fogging tape as its inner 
surface. We exposed the inner surface of the 3D cube to heated water vapor (60 °C) to test its anti-
fogging property. 
 
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22 
 
 
 
 
Outdoor water purification tests under natural sunlight. The solar water purification system 
was constructed from the super-absorbent foam (McMaster Carr, 8884k41) painted in black (Rust-
Oleum Specialty), the pristine LDPE film or LDPE-PAAm film as the condenser, the glass beaker as the 
water collection tube, the bubble wrap as the thermal insulation layer, and the glass container as the 
basin filled with water of around 600 ml. A metallic ball was placed at the center of the condenser, 
driving the flow of the condensed water by its gravitational force. The outdoor water purification 
tests were conducted at Massachusetts Institute of Technology campus on June 23th in 2020. The 
solar flux, the ambient temperature, and the ambient relative humidity were recorded by the 
rooftop weather station at MIT Center for Advanced Urbanism. The temperatures of the absorber 
Tabsorber, vapor (1 mm above the absorber) Tvapor, and the bulk water Twater were measured using 
thermocouples every hour. Collected purified water was measured using a balance with 0.001 g 
resolution. The ion concentrations of the collected water were measured using Inductively Coupled 
Plasma Mass Spectrometry with calibrated results shown in Figure S15, Supporting Information. 
 
Supporting Information 
Supporting information is available from the Wiley Online Library or from the author. 
 
Acknowledgements 
 
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23 
 
 
 
 
The authors acknowledge the discussion with Dr. Xiaoyu Chen at MIT on the FTIR result, the 
discussion with Prof. Christoph Reinhart at MIT Center for Advanced Urbanism on the measurement 
of solar flux. The authors also acknowledge the Center for Environmental Health Sciences at MIT for 
supporting the usage of Inductively Coupled Plasma Mass Spectrometry, the support of which was 
provided by a core center grant P30-ES002109 from the National Institute of Environmental Health 
Sciences, National Institutes of Health. The authors thank A. F. Schwartzman in the NanoMechanical 
Technology Laboratory at MIT for help with AFM imaging. This work was supported by MIT. 
 
Conflict of Interest 
S.L., Y.Y., Y. T., X. Z., and G. C. have a provisional patent application on the design of a transparent 
and anti-fog polyethylene film. 
 
Data Availability Statement 
The data that support the findings of this study are available from the corresponding author upon 
request. 
 
 
Author Contributions 
 
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24 
 
 
 
 
S. Lin and G. Chen conceived the idea. S. Lin designed the study. Y. Yang and J. Ni contributed equally 
to this work. S. Lin, Y. Yang, J. Ni, X. Liu, B. Lu prepared the sample, conducted the UV-vis-NIR 
measurements, mechanical characterizations, FTIR measurements, HPLC measurements, outdoor 
solar tests, and anti-fogging eyeglass tests. Y. Tsurimaki performed the simulations. S. Lin, Y. Yang, J. 
Ni, X. Liu, Y. Tsurimaki, Y. Tu, J. Zhou, X. Zhao, and G. Chen analyzed and interpreted the results. S. 
Lin and G. Chen wrote the manuscript with inputs from all authors. 
 
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Figure legends 
 
 
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27 
 
 
 
 
Figure 1 | Schematic illustration of stretchable anti-fogging tape (SAT) applicable to diverse 
transparent materials with various curvatures. a) The SAT consists of three synergistically-combined 
transparent layers: i) a stretchable middle layer with high elastic recovery made of low-density 
polyethylene (LDPE) to keep transparent materials tightly bound, ii) an anti-fogging top layer made 
of uncrosslinked polyacrylamide (PAAm) to impart hydrophilicity to transparent materials, and iii) an 
adhesive bottom layer made of loosely crosslinked polydimethylsiloxane (PDMS) to form robust yet 
reversible adhesion between transparent materials and SATs. The SAT is originally adhered to a 
backing layer (e.g. PET) and can be readily peeled from the backing layer for usage. b)  Schematic 
illustration of the working principle for applying the SAT. Once peeled from the backing layer, the 
SAT is first pre-stretched to remove wrinkles from its surface, and thereafter adhered to versatile 
transparent materials with various curvatures with seamless contact. When exposed to high-
humidity environments, the transparent materials with SATs facilitate filmwise condensation, 
allowing the incident light to transmit them through with negligible light scattering.  
 
 
Figure 2 | Surface and mechanical characterizations of the stretchable anti-fogging tape. a) AFM 
surface topology of the pristine LDPE film, measuring its root mean square of roughness. b) AFM 
 
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28 
 
 
 
 
surface topology of the LDPE-PAAm film, measuring the thickness of the LDPE-PAAm layer and its 
root mean square of roughness. c) Confocal microscopy images of the anti-fogging tape to illustrate 
the uniform thickness of the PAAm layer in the hydrated state. PDMS and PAAm are stained with 
Nile red and fluorescein, respectively. d) Contact angle measurements of the anti-fogging tape in the 
dry state. e) Contact angle measurement of the anti-fogging tape in the hydrated state. f) Nominal 
stress versus stretch curves of SAT under monotonic uniaxial tensile loading and cyclic uniaxial 
tensile loading. The red dot indicates the failure point. Inset images showing the SAT at undeformed 
state (i.e.,  1 ) and deformed state (i.e.,   3.4 ). g) Transmission FTIR spectrum of the pristine 
LDPE film and the LDPE-PAAm film. The NH2 at 3200 cm
-1 and 3349 cm-1, the C=O stretching at 1665 
cm-1, and N-H bending at 1613 cm-1 indicate the anchored polyacrylamide chains. The scale bars in 
c), d), e), and f) are 500 μm, 1 mm, 1 mm, and 10 mm, respectively. 
 
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29 
 
 
 
 
 
 
Figure 3 | UV-vis-NIR characterization. a) Schematic illustration of the measurement of total 
transmittance Tt  T2 / T1 , where T1  is the intensity of incident light and T2  is the intensity of 
transmitted light in total. b) Schematic illustration of the measurement of diffuse transmittance 
Td  T4 T3 T2 / T1  / T1 , where T3  is the intensity of the scattered light by the instrument 
measured with no sample in position but with the light trap in position, T4  is the intensity of the 
diffusively transmitted light with both the sample and the light trap in position. c) Schematically 
illustration of the diffusively transmitted and reflected light in the pristine LDPE film. d) The 
 
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30 
 
 
 
 
measured total transmittance of the pristine LDPE film before condensation and after condensation 
when exposed to hot water vapor. The bold line is the corresponding simulation result of the total 
transmittance of the pristine LDPE film after condensation when exposed to hot water vapor. e) The 
measured diffuse transmittance of the pristine LDPE film before condensation and after 
condensation when exposed to hot water vapor. f) Schematically illustration of the directly 
transmitted light in the LDPE-PAAm film. d) The measured total transmittance of the LDPE-PAAm 
film before condensation and after condensation when exposed to hot water vapor. The bold line is 
the corresponding simulation result of the total transmittance of the LDPE-PAAm film after 
condensation when exposed to hot water vapor. e) The measured diffuse transmittance of the LDPE-
PAAm film before condensation and after condensation when exposed to hot water vapor. 
  
 
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31 
 
 
 
 
 
Figure 4 | Characterizations of anti-fogging properties of the SAT in extreme environments. a) The 
existing method of spraying hydrophilic agents suffers short lifetime due to the weak interface 
between the hydrophilic agents and transparent materials. In contrast, our method of covalently 
grafting uncrosslinked hydrophilic polymers maintains the mechanical robustness by resisting 
condensed water flow. b) The existing method of coating crosslinked hydrogels suffers low optical 
transparency when exposed to high-humidity environments because the swelling of crosslinked 
hydrogels results in crease instability. In contrast, swelling of uncrosslinked hydrophilic polymers in 
our method does not affect the transparency of the film, giving high transparency in high-humidity 
 
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32 
 
 
 
 
environments. c) Comparison of the measured diffuse transmittance among existing anti-fogging 
methods and our method as a function of water vapor exposure time. d) Measured total 
transmittance and diffuse transmittance of the LDPE-PAAm film through the test period over 193 
days. e) Measured total transmittance and diffuse transmittance of the LDPE-PAAm film at various 
punching displacements of 0, 5, 10, 15, and 20 mm. f) Measured total transmittance and diffuse 
transmittance of the LDPE-PAAm film at various loads of dynamic scratching. 
 
 
 
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33 
 
 
 
 
Figure 5 | Anti-fogging properties for diverse transparent materials. a) Dropwise condensations on 
the pristine transparent films (i.e., PE, PET, PDMS, PMMA, PS, Glass) block the visions when exposed 
to heated water vapor due to their fogging surfaces. Filmwise condensations on the same films with 
anti-fogging tape maintain clear visions when exposed to heated water vapor due to their anti-
fogging surfaces. b) Measured diffuse transmittance of the pristine transparent films (i.e., PE, PET, 
PDMS, PMMA, PS, Glass) and the same films with anti-fogging tape. c) Dropwise condensations on 
the pristine curved PMMA tubes with various radii of curvature (i.e., 3.2, 6.4, 9.5, 14.3, 19.1 mm) 
block the visions when exposed to heated water vapor due to their fogging surfaces. Filmwise 
condensations on the same tubes with anti-fogging tape maintain clear visions when exposed to 
heated water vapor due to their anti-fogging surfaces. d) Measured diffuse transmittance of the 
pristine curved PMMA tubes with various radii of curvature (i.e., 3.2, 6.4, 9.5, 14.3, 19.1 mm) and the 
same tubes with anti-fogging tape. e) Demonstration of fogging on the pristine stretchable 
transparent material and anti-fogging on the stretchable transparent material with anti-fogging tape. 
f) Demonstration of anti-fogging on the foldalbe transparent material with anti-fogging tape. The 
scale bars in a), c), e), f) are 10 mm, 5 mm, 5mm, and 10 mm. 
 
 
  
 
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34 
 
 
 
 
 
Figure 6 | Anti-fogging eyeglasses and protective goggles. a) Schematic illustration of the working 
principle for applying the stretchable anti-fogging tape to eyeglasses and safety goggles. b) The 
resultant eyeglass with anti-fogging tape adhered onto the inner surface of the right-side glass. c) 
The resultant goggle with anti-fogging tape adhered onto the inner surface of the left-side goggle. d) 
Outdoor fogging test of eyeglass at the temperature of -5 °C. e) Indoor fogging test of protective 
goggle at room temperature of 20 °C. f) Outdoor fogging test of protective goggle at the 
temperature of -5 °C. The scale bars in (b-f) are 20 mm. 
 
  
 
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35 
 
 
 
 
 
Figure 7 | Outdoor water purification tests under natural sunlight. a) Schematic illustration of the 
design of solar still using LDPE-PAAm as the condensation cover to enhance solar-powered 
freshwater production. b) Images of two solar stills with the pristine LDPE film and the LDPE-PAAm 
film as the condensation cover from 09:00 to 20:00 on June 23th, 2020. c)  The collected purified 
water over time for the two water purification systems with the pristine PE film and LDPE-PAAm film 
as the condensation cover from 8:00 to 20:00 on June 23th. d)  The collected purified water over 
time for the two water purification systems with the pristine LDPE film and LDPE-PAAm film as the 
condensation cover from 8:00 to 20:00 on Sep 8th after prolonged storage of the film for 77 days. e) 
Measured concentration of the primary ions Na+, Mg2+, K+, and Ca2+ before and after solar-powered 
desalination. The scale bars in b) are 2.5 cm. 
 
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36