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\begin{document}

\title{Reinforced nanowrinkle electrospun photothermal membranes via solvent
induced recrystallization}



\author[1]{Jinlin Chang}%
\author[2]{Weiling Wang}%
\author[1]{Zhaoxin Li}%
\author[1]{Yujiao Wang}%
\author[1]{Yacong Hou}%
\author[2]{Zhiyuan Cao}%
\author[1]{Zhenwei Liang}%
\author[1]{Yuan Ma}%
\author[1]{Ding Weng}%
\author[2]{Jun Song}%
\author[1]{Yadong Yu}%
\author[1]{Lei Chen}%
\author[1]{Jiadao Wang}%
\affil[1]{Tsinghua University}%
\affil[2]{Shanghai University}%


\vspace{-1em}



  
  \date{June 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Wearable photothermal materials can capture light energy in nature and
convert it into heat energy, which is critical for flexible outdoor
sports. However, the conventional flexible photothermal membranes with
low specific surface area restrict the maximum photothermal capability,
and loose structure of electrospun membrane limits durability of
wearable materials. Here, an ultrathin nanostructure candle soot/
multi-walled carbon nanotubes / poly (l-lactic acid) (CS/MWCNTs/PLLA)
photothermal membrane is first prepared via solvent-induced
recrystallization. The white blood cell membrane-like nanowrinkles with
high specific surface area are achieved for the first time and exhibit
optimal light absorption. The solvent-induced recrystallization also
enables the membrane to realize large strength and durability.
Meanwhile, the membranes also show two-sided heterochromatic features
and transparency in thick and thin situations, respectively, suggesting
outstanding fashionability. Nanowrinkled photothermal membranes by novel
solvent-induced recrystallization show high flexibility, fashionability,
strength, and photothermal characteristics, which have huge potential
for outdoor warmth.%
\end{abstract}%



\sloppy


\textbf{1. Introduction}

With the advancement of net-zero carbon emission targets, the effective
utilization of renewable resources has attracted widespread attention
{[}1, 2{]}. Photothermal materials can capture near-infrared light
dissipated in the atmosphere and convert it into thermal energy to be
utilized, which is highly promising for wearable outdoor devices,
disease-assisted therapy, seawater evaporation, catalysis and so on
{[}3-6{]}. Flexible electrospun-based photothermal membranes (such as
Poly (L-lactic acid) (PLLA) and nylon) can offer high comfort, large
specific surface area, and wide applicability, so have great potential
in the field of wearable photothermal applications {[}7{]}; however,
durability, excellent photothermal conversion and simplicity of
fabrication of flexible photothermal membranes are still key challenges.

The photothermal performance of membranes is directly related to surface
micromorphology. Functional micro/nanostructures can exchange or promote
the engineering characteristics of materials {[}8{]}. Organisms in
nature often evolve special microstructures to better adapt to their
environment {[}9{]}. The wrinkles of the cerebral cortex enhance
intelligence {[}10{]}; the wrinkles of the guts enhance nutrient
absorption {[}11{]}; and the wrinkled membranes of leukocytes ensure a
higher specific surface area and enhance deformability {[}12, 13{]}.
Inspired by this, scientists have tried to explore thin membranes with
wrinkled microstructures through various methods to enhance the specific
surface area and absorption capacity, however, the current imitation of
wrinkled sizes still remains at the micrometer scale {[}9{]}. The usual
methods for obtaining wrinkle microstructures, such as mechanical force
{[}9{]}, photopolymerization {[}14{]}, infrared induction {[}15{]}, and
post-polymerization {[}16{]}, have been able to achieve the desired
wrinkle morphology. 3D printing allows for the generation of
controllable nanowrinkles {[}17{]}, but its high preparation cost limits
the preparation of large-area nanowrinkles. Mechanical force and
photopolymerization, etc. enable fast batch preparation, but it is
difficult to form nanoscale wrinkles. Post-polymerization enables
low-cost preparation of nanowrinkles, but the long cross-linking
reaction time and instability limit the enhancement. The realization of
low-cost, large-area and durable nanowrinkle membranes remains a
challenge. Also, ultra-thin electrospun membranes are often easily
damaged {[}18{]}. Therefore, an effective treatment procedure based on
electrospun membranes is urgently needed to develop the durability of
the material while realizing high specific surface area photothermal
films.

Here, a solvent-induced recrystallization approach is developed firstly
to obtain high-strength CS/MWCNTs/PLLA photothermal membranes with
nanowrinkled structures. The adhesive polydimethylsiloxane (PDMS)
dissolved in the solvent will residue on the fiber membrane after the
solvent evaporates, which not only tightly connects the photo-thermal
particles, but also forms an adhesive network to facilitate the
intensive cross-linking between the fibrils, so significantly enhancing
the membrane strength. The membranes also show two-sided heterochromatic
features or transparency because of the gradient photothermal particle
concentration, which considerably extends the fashionability and
customizability. Remarkably, the membrane also has excellent response
sensitivity and can heat up immediately after exposure to light. The
ultrathin nanowrinkled photothermal membrane by solvent-induced
recrystallization enables rapid batch preparation of enhanced functional
composite membranes, showing broad prospects in snow clothing, Winter
Olympics garments, and so on.

\textbf{2. Methods}

\textbf{2.1. Materials}

Poly (L-lactic acid) (PLLA, PL 65) was obtained from PURAC Biomaterials.
Tetrahydrofuran (99.5\%) was bought from Beijing Bailingwei Science and
Technology CO. LTD. Acetone (99.5\%) was supplied by Modern Oriental
(Beijing) Technology Development CO. LTD. Sylgard\textsuperscript{TM}184
silicone elastomer kit (PDMS) was purchased from Dow. MWCNTs were
acquired from Shenzhen Suiheng Science and Technology CO. LTD. Dimethyl
formamide (DMF, 99.80\%) was bought from Fisher Scientific Ltd.
Dichloromethane (DCM, 99.99\%) was purchased from Sigma-Aldrich. Human
Skin Fibroblasts (HSF) cells and HSF specialized culture medium were
obtained from Shanghai Saibekang Biotechnology. Phosphate buffer saline
(PBS) was bought from Procell. Pancreatic enzyme was supplied by
Biosharp. Cell Counting Kit-8 (CCK8) were purchased from Invigentech.
All chemicals were of analytical grade without any further purification.

\textbf{2.2. Preparation of PLLA nanofibers and CS}

The PLLA was dissolved in DCM and DMF with a mass ratio of 1.8: 93.29:
4.91, then was electrospun by electrospun equipment. Electrospun
equipment kept fixed parameters: 30 ml syringe; 19 G needle; 30 cm
distance between the needle and the collector; 17 kV voltage; 5 ml/h
feeding speed; 2 h collection {[}18{]}.

CS was obtained by the following method. First, a candle was lit and a
glass plate was placed above the candle flame. Then, the black CS
gradually adhered to the glass plate and lasted for 3 minutes. Finally,
the glass plate was removed and cooled, and the CS was gained by
scraping off the coating on the glass plate.

\textbf{2.3. Preparation of photothermal membranes by solvent-induced
recrystallization}

The first step is the recrystallization of PDMS and photothermal
particles dispersion. CS and MWCNTs were mixed at a mass ratio of 1:1,
dispersed in tetrahydrofuran and stirred, followed by the addition of
PDMS and ultrasonication. The content ratio of substances was CS:
MWCNTs: PDMS: tetrahydrofuran=0.1g: 0.1g: 3g: 100ml, and the mass ratio
of PDMS to curing agent was 3:1. The PLLA fiber membrane was fixed on a
metal frame, immersed into the above mixing solution with continuous
ultrasonication for 20 seconds.

The second step is the recrystallization of PLLA. The membrane treated
by tetrahydrofuran are quickly taken out and immersed into acetone for 5
minutes, and finally removed and dried at room temperature to obtain the
double-sided heterochromatic photothermal membrane. The volume of
tetrahydrofuran was changed to 200 ml and 400 ml and the experiment was
repeated. For easy differentiation, in the Results and Discussion
section, we named the PDMS-containing solvent-treated PLLA as APLLA.

Other solvent-induced recrystallization were also explored. Method 1:
Acetone treatment, drying and then treated with tetrahydrofuran
containing PDMS and CS/MWCNTs. Method 2: Treated by the solvent of
tetrahydrofuran containing PDMS and CS/MWCNTs. Method 3: Treatment with
tetrahydrofuran containing PDMS and CS/MWCNTs, drying and then acetone
treatment. Method 4: Acetone treatment and then treatment with
tetrahydrofuran containing PDMS and CS/MWCNTs. Method 5: Treated by the
solvent of acetone containing PDMS and CS/MWCNTs (Little amounts of
tetrahydrofuran were used to dissolve PDMS in acetone).

\textbf{2.4. Characterizations}

The morphologies of the photothermal films were acquired with a Scanning
Electron Microscope (SEM, SU8200, HITACHI, Japan), and the elemental
maps and percentage were obtained by the corresponding Energy Dispersive
Spectrophotometer (EDS). The Transmission Electron Microscopy (TEM) of
samples is gained by JEM2010. X-ray diffraction patterns (XRD) of the
samples were characterized by D8 Advance, Bruker. Strain-Stress curves
were collected by Electronic Universal Material Testing Machine
(WDW-3020). Autosorb iQ\textsubscript{2} was used to achieve
Brunauer-Emmett-Teller (BET). The water contact angle of the solar
heating membranes was measured by Dataphysics OCA25, Germany. The
Ultraviolet-Visible- near infrared (UV-vis-NIR) absorption spectroscopy
was achieved by Perkin Elmer, Lambda 1050+. The Discovery TGA was
utilized for thermogravimetric analysis (TGA). The differential scanning
calorimetry (DSC) was collected by DSC Q2000.

The photothermal data of the materials were acquired by FLIR T620 and
analyzed using the matching software FLIR Tools. Sunlight in the
laboratory is obtained through a simulated daylight xenon light source.
Solar irradiation was obtained by a TES 132 Datalogging solar power
meter. Temperature information was gained from windy.com. Location
coordinates were obtained from Google map.

CCK-8 was used to detect the changes of cell viability in the co-culture
of HSF cells and samples for 12h and 24h. HSF cells in logarithmic
growth stage were taken, inoculated into the 24-well plate according to
4\selectlanguage{ngerman}×104/well, and the culture medium was incubated at 5\% CO2 and 37\selectlanguage{english} for
12h and 24h. After that, the medium was removed, and each well was
washed with PBS for three times, and incubated for 2 hours at 37\selectlanguage{english} in a
constant temperature incubator by adding medium containing 10\% CCK-8,
5\% CO2 at 1mL/well. The absorbance at 450nm was detected by zymography
and the relative viability was calculated.

\textbf{3. Results and discussion}

\textbf{3.1. Nano-wrinkled photothermal membranes by solvent induced
recrystallization}

The electrospun fibrous membranes with solvent-induced recrystallization
have microscopic wrinkles (Fig. 1a and b). The wrinkle morphology is
similar to that of human white blood cells (Fig. 1c), and this surface
wrinkled structure greatly enhances the specific surface area and thus
the light absorption ability of the film. Unlike white cell membranes,
the size of the wrinkles in our photothermal membranes is much smaller
and reaches the nanometer scale. Nano-wrinkles have superior surface
properties but are more difficult to achieve, which highlights the
advantages of solvent-induced recrystallization for the preparation of
nano-wrinkles. The process of solvent-induced recrystallization is as
follows: The ultrathin photothermal membrane firstly is prepared by the
electrospinning technique, afterwards, the photothermal particles
CS/MWCNTs and the adhesive PDMS are sufficiently dispersed in the porous
membrane by a short ultrasonication (20 s), and then the optimized
photothermal membrane is obtained by acetone treatment for 5 min. Later,
after the evaporation of the organic solvent, the photothermal membranes
are gained, as shown in Fig. 1d.

The whole solvent-induced recrystallization process can be divided into
two steps, the first step is the recrystallization of PDMS with the
solubility of PDMS in tetrahydrofuran, which aims to enhance the
strength of the fiber membrane and make the photo-thermal particles
tightly attached; the second part is the recrystallization of PLLA with
the fact that PLLA is slightly soluble in acetone, which forms a
hierachical morphology on the surface of the fibers after
volatilization. fiber membrane before solvent-induced recrystallization
is shown in Fig. e-g and Fig. S1, and there is no wrinkle structure.
After solvent treatment, the photothermal particles form a wrapping
relationship with PLLA fibers and acquire the photothermal capability
(Fig. 1h). The photothermal performance and the schematic diagram of the
principle are shown in Fig. 1i, j and Fig. S2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1. The nano-wrinkled structure enhanced flexible photothermal
membranes by solvent induced recrystallization. (a, b) The
micromorphology of nano wrinkled photothermal membranes. (c) White blood
cell with wrinkled membrane morphology {[}12{]}. (d) The solvent induced
recrystallization process. (e) The original PLLA membrane. (f, g) The
TEM of CS and MWCNTs. (h) The structure of photothermal fibrils. (i) The
comparison of reflectance and absorptance by UV-vis-NIR spectroscopy for
photothermal membranes. UV: wavelength \textless{} 400 nm; vis: 400 nm
\textless{}= wavelength \textless{}= 780 nm; NIR: wavelength
\textgreater{} 780 nm. (j) The schematic diagram of photothermal
conversion of membranes.

\textbf{3.2. The physical properties of photothermal membranes}

The light absorption characteristics of the photothermal membranes can
be obtained by UV-vis-NIR spectroscopy (Fig. 2a-c). APLLA,
CS/MWCNTs/APLLA light-colored surfaces and dark-colored surfaces all
exhibit lower reflection and higher absorption at shorter wavelengths of
UV. However, as the UV wavelength increases, the reflections of APLLA
and light-colored surfaces increase steeply, while the dark surface
remains constant or even decreases slowly. In the near-UV band,
dark-colored surface shows excellent absorption (86.71\%), while the
light-colored surface is slightly inferior to that of the dark-colored
surface, but still presents a high absorption (80.07\%). In the vis-NIR
band, the absorption of both light and dark surfaces decreases slowly,
but the absorption of the dark surface is always larger than that of the
light surface, and the absorption of APLLA rises slowly and is much
smaller than that of CS/MWCNTs/APLLA. In the vis-NIR band, the
reflections of the dark surfaces are always maintained at very low
values. The light side and APLLA decrease, and the reflection of the
light side is much lower than that of APLLA. In conclusion, the dark
side of CS/MWCNTs/APLLA shows excellent absorption in the full band, and
the light side is a little inferior to the dark side, but the absorption
is excellent and much higher than that of APLLA, which demonstrates the
excellent photothermal performance. The reflectance, transmittance and
absorptance satisfy the following equation {[}19{]}:

\(R+T+A=1\) (1)

The XRD patterns of the dark surface show more peaks than those of the
APLLA and light surfaces, appearing at 2 Theta of 108 and 118 (Fig. 2d).
This is because the dark surface has a higher concentration of CS and
MWCNTs. This also indicates that the light surfaces have a very low
content of CS and MWCNTs, which is almost close to pure APLLA. The
CS/MWCNTs/APLLA TGA curve is almost identical compared to APLLA, and the
presence of CS and MWCNTs causes the material to decompose more rapidly
during thermal degradation (Fig. 2e). The DSC data (Fig. 2f) exhibits
the variation of the heat flow with temperature versus the sapphire (see
Fig. S3 for those with baseline). The specific heats of APLLA and
CS/MWCNTs/APLLA can be obtained by Temperature - [?]Heat Flow curves and
comparative calculation (Fig. 2g). c\textsubscript{p} is the specific
heat, m is mass, and p is heat flow. The specific heat of sapphire has a
standard, so the specific heat of APLLA and CS/MWCNTs/APLLA can be
calculated, as shown in Table S1. The loose structure of the electrospun
membrane also leads to weak connections between fibrils and poor
strength {[}20{]}. The strain-stress curves (Fig. 2h and Supplementary
Note S1) show that the strength of the treated photothermal film
increases dramatically compared to the pristine PLLA, with Young's
modulus exceeding four times that of PLLA. The water contact angle of
APLLA was 127.65\selectlanguage{ngerman}°, showing high hydrophobicity, and the contact angle
slightly increased to about 137°. The contact angles are almost the same
for the darker and lighter surfaces after adding photo-thermal particles
(Fig. S4). The material exhibits superior hydrophobicity, which enhances
the application of wearable membranes in variable weather {[}21, 22{]}.
These physical characteristics further demonstrate that the double-sided
heterochromatic photothermal film has a gradient concentration
distribution, which explains the formation of a double-sided
heterochromatic structure and shows the absorption features of
UV-vis-NIR spectra.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2. The physical properties of nano wrinkled photothermal
membranes. (a) The reflectance of UV-vis-NIR spectroscopy for
photothermal membranes. (b) The transmittance of UV-vis-NIR spectroscopy
for photothermal membranes. (c) The absorptance of UV-vis-NIR
spectroscopy for photothermal membranes. (d) The XRD of photothermal
membranes. (e) The TGA of the samples. (f) The DSC of photothermal
membranes. (g) The specific heat of photothermal membranes. (h) The
strain-stress curves of photothermal membranes.

\textbf{3.3. Analysis of the effect of nanowrinkled structures on
photothermal performance}

A novel solvent-induced recrystallization procedure is proposed for the
first time in this study, which enables the high specific surface area
and functional modification of electrospun membranes in a very short
period of time (less than 6 min). It simplifies the previously
cumbersome four-step process: morphology modification, mechanical
reinforcement, adhesive generation and adherence and allows obtaining
high-performance photothermal membranes. Moreover, this method can help
photothermal membranes realize special nano-wrinkled structure. As shown
in Fig. 3a, compared with the porous structure, the nano wrinkled
membrane will provide more attachment sites for the photothermal
particles, which is beneficial to the photothermal performance.

In this study, acetone exist as a modifier for hierachical porous
morphology, and PDMS can achieve good dispersion and rapid
volatilization only by tetrahydrofuran {[}23, 24{]}. Thus, we studied
various combinations of the two solvents. Tetrahydrofuran does not have
a solubilizing effect on PLLA, so only tetrahydrofuran as a solvent
cannot modify the fiber morphology and causes dense accumulation between
the fiber layers, resulting in a significant agglomeration of
photothermal particles on the surface (Method 2, Fig. 3b and S5a). Since
PDMS is insoluble in acetone, when treated only with acetone, PDMS
adheres to PLLA before the solvent evaporates, resulting in the
inability to form wrinkled structures (Method 5, Fig. 3c and S5b). For
acetone treatment, drying and then treating with tetrahydrofuran
containing CS/ MWCNTs, PLLA was insoluble in tetrahydrofuran, making it
difficult to form a distinct wrinkled structure (Method 1, Fig. 3d and
S6a). For treatment with tetrahydrofuran containing CS/MWCNTs, drying
and then acetone treatment, PDMS was deposited before acetone
evaporation and could not form wrinkles (Method 3, Fig. 3e and S6b). For
acetone treatment and then treatment with tetrahydrofuran containing
CS/MWCNTs, PLLA was insoluble in tetrahydrofuran and it was difficult to
form a distinct wrinkled structure (Method 4, Fig. 3f and S6c). The
tetrahydrofuran treatment and subsequent treatment with acetone
harvested more desirable morphology and photothermal characteristics, as
shown in Fig. 3e and S7 (Method in this study). We also evaluate the
photothermal performance of the membranes under different morphology and
specific surface area. Heat distribution plots of the membranes before
(Fig. S8) and after (insert images in Fig. 3b-g) 5 min of 1 sunlight
irradiation and the maximum temperature are plotted in Fig. 3h. Methods
1, 2, and 4 have poor photothermal performance, while methods 3 and 5
both have superior photothermal properties, but their vanishing or very
small wrinkled structures limit the potential upper limit of the
photothermal film and need to be further optimized. The method in this
study shows best photothermal performance. BET data show that the film
containing the photothermal particles has only a slight decrease in
specific surface area compared to APLLA, they all exhibit an
exceptionally high specific surface area (Fig. 3i and Table S2). The
nano-wrinkled surface greatly increased the specific surface area about
164 times, from 0.3057 m\textsuperscript{2}/g (macroscopic) to 50.00
m\textsuperscript{2}/g (macroscopic).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 3. Analysis of the influence of nano wrinkled structure on the
photothermal performance. (a) Comparison of original structure, porous
structure and nano-wrinkled structure. (b-g) Comparison of
micromorphology of nano-wrinkled and non-nanowrinkled structures (The
inset images are comparison of photothermal performance of nano-wrinkled
and non-nanowrinkled structures after 5min 1 sunlight irradiation.). (b)
The photothermal membrane treated by solvent of tetrahydrofuran. (c) The
photothermal membrane treated by solvent of acetone. (d) Acetone
treatment, drying and then treated with tetrahydrofuran containing
CS/MWCNTs. (e) Treatment with tetrahydrofuran containing CS/MWCNTs,
drying and then acetone treatment. (f) Acetone treatment and then
treatment with tetrahydrofuran containing CS/MWCNTs. (g) Nano-wrinkled
structure in this study. (h) The photothermal comparison of membranes
before and after 5min sunlight irradiation by the 5 solvent-induced
recrystallization in b-g. (i) The BET of photothermal membranes.

\textbf{3.4. Double-sided heterochromatic characteristics and light
transmittance of photothermal membranes}

Membranes of different thicknesses will achieve two-sided
heterochromatic features and translucency through solvent-induced
recrystallization. The loose and porous structure of the electrospun
membrane provides a deep penetration network for the photothermal
particles, creating a microscopic 3D photothermal absorption space (Fig.
4a and b). The gradient photothermal network creates a two-sided
heterochromatic appearance (Fig. 4c, thick membranes
(\textasciitilde{}20 \selectlanguage{greek}µ\selectlanguage{english}m, Fig. 4d and Fig. S9)) and allows for excellent
photothermal performance on the light side, which greatly improves the
fashionability of photothermal materials {[}25{]}. Then, with the rapid
evaporation of the solvent and the action of PDMS, the fiber film
rapidly densifies and tightly crosslinks, binding the photothermal
particles inside the membrane, which greatly enhances the internal
photothermal factor embedding and the integrated photothermal
performance. Also, it creates a gradient distribution between dark and
light faces (Fig. 4e).

When the fiber membrane is very thin, the photothermal particles can
easily pass through the membrane, so the two-sided heterochromatic
feature disappears and is replaced by transparency (Fig. 4f and Fig.
S10, thin thickness (\textasciitilde{}6.5 \selectlanguage{greek}µ\selectlanguage{english}m, Fig. 4g, h and Fig. S11
and S12)). The EDS distribution of the transparent cross section is
shown in Fig. S13 and S14. The main substance compositions of the
photothermal particles CS and MWCNTs are shown in Fig. S15 and S16. Both
the double-sided heterochromatic feature and the transparency can
improve the shortcoming of the conventional photothermal membrane, which
can only be dark in color, and enhance the fashionability of wearable
textiles. For the SEM images of APLLA and photothermal membranes (Fig.
S17), the EDS distribution (Fig. 4i-k and Fig. S18) show that the
adhesive PDMS is uniformly distributed throughout the membrane,
providing sufficient adhesive sites. The CS/MWCNTs/APLLA light and dark
surfaces show the obvious existence of elemental C, indicating the large
distribution of CS particles and MWCNTs particles. Obviously,
agglomerated C is also observed in its holes, suggesting that the
photothermal particles can penetrate into the interior of the film. The
elemental spectrum of the light-colored photothermal film surface is
shown in Fig. S19. C and O are the main components, and Si also has big
components, displaying the arrangement of the adhesive and the
photothermal components.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Figure 4. Double-sided heterochromatic characteristics and light
transmittance of photothermal membranes. (a) The membrane without
solvent induced recrystallization (photothermal particles can only be
attached to the surface). (b) The membrane with solvent induced
recrystallization (photothermal particles form networks inside the
membrane). (c) The pictures of double-sided heterochromatic photothermal
membranes (20 \selectlanguage{greek}µ\selectlanguage{english}m). (d) The cross-section of double side heterochromatic
photothermal membranes. (e) The gradient distribution of photothermal
particles during solvent induced recrystallization. (f) Transparency of
the membrane (on the fabric, the membrane covers the letter `A' in
`TSINGHUA'). (g) Cross-section of transparent membranes. (h)
Amplification of (g). (i) The EDS maps of the treated membrane without
CS and MWCNTs. (j) The EDS maps of the light side of photothermal
membranes. (k) The EDS maps of the dark side of photothermal membranes.
The red represents C, the blue represents Si, and the green represents
O.

\textbf{3.5. Solar heating performance and outdoor tests}

The photothermal properties of transparent or double-sided
heterochromatic nano wrinkled fiber films were evaluated by thermal
imaging. PLLA and APLLA without CS/MWCNTs showed almost no temperature
rise (Fig. 5a-d). As shown Fig. 5e, at the initial CS/MWCNTs
concentration, the temperature of the dark side of the film continued to
increase with the increase of the sunlight radiation time, and the
higher the concentration, the higher the temperature. However, when the
concentration of CS/MWCNTs was reduced to 1/4 of the original
concentration, the photothermal response of the film was elevated, and
it could be almost instantaneously boosted to high temperatures, but
with large temperature fluctuations. Comparing the dark and light sides
of CS/MWCNTs/APLLA (Fig. 5f), it can be found that the light side has a
more rapid photothermal response and no significant enhancement with
time, but is unstable; the dark side has a slow increase in temperature
with time and better photothermal stability. Comparing the photothermal
performance of CS/MWCNTs/APLLA dark-colored surface under different
sunlight radiations (Fig. 5g), it can be found that the temperature of
the photothermal film is higher with the same time as the radiation
enhancement, and the higher the radiation intensity, the more rapid the
response. However, the temperature distinction is not obvious for
sunlight densities of 1000 kWm\textsuperscript{-2} and 1250
kWm\textsuperscript{-2}.

We also evaluate the photothermal performance of the membranes on
low-temperature substrates, as shown in Fig. 4h. CS/MWCNTs/APLLA light
side (top left), dark side (top right), dark side 1/2 concentration
(bottom left), and dark side 1/4 concentration (bottom right) were
placed on cryogenic glass (-20 @\selectlanguage{english}C), and the temperature changes were
recorded under 1 sunlight radiation (1000 kWm\textsuperscript{-2}). The
Max represents the maximum temperature of the 4 photothermal films, and
Sp1 represents the collimation temperature. It can be seen that the
photothermal membrane on the cryogenic substrate can also heat up
rapidly and the maximum temperature reaches nearly 55 @\selectlanguage{english}C within 4
minutes, with the darker side of the membrane with a high concentration
of CS/MWCNTs/APLLA showing the best low-temperature photothermal effect.
We also evaluate the photothermal uniformity of the films. As shown in
the Fig. 4i-k, comparing the thermal distribution of the material before
and after 5 minutes of 1 sunlight irradiation, it can be seen that the
thermal distribution of CS/MWCNTs/APLLA photothermal film is very
homogeneous and shows high in the middle and low at both ends {[}26{]}.

The evaluation of wetting the photothermal film is also important since
wearable materials are often faced with situations such as sweat-soaking
{[}27{]}. As shown in Fig. S20 and Supplementary Video 1, the heat
distribution of the photothermal film before and after the drop of pure
water is recorded. Even under wetting conditions, the film can rise to
37 @\selectlanguage{english}C within 4 minutes and promote liquid evaporation for self-drying
recovery. Fig. S21 demonstrates the variation of the maximum temperature
(Max) of the area and the collimation temperature (Sp1) under the
wetting condition, and the photothermal film exhibits a fast temperature
recovery. The thermal management capability of the film is also an
important criterion. The membrane was heated in an oven at 60 @\selectlanguage{english}C for 5h,
after which it was taken out and the photothermal distribution was
recorded as shown in the Fig. S22 and 23. The photothermal film has
excellent thermal dispersion and can be cooled from 60 @\selectlanguage{english}C to room
temperature in 10 s. The distribution of the temperature along the
collinear horizontal line indicate that the film has excellent
uniformity {[}28{]}. Furthermore, we test the thermal performance of the
film in different orientations (parallel, 40 degrees and perpendicular
to the light). The orientation perpendicular to the light gives the best
performance (Fig. S24a-c). The film also shows good thermal behavior
when folded (Fig. S24d).

We conduct outdoor evaluations of CS/MWCNTs/APLLA membranes. The outdoor
test equipment was shown in Fig. S25 and the detailed outdoor test
parameters were shown in Table S3 and the test locations are shown in
Fig. 6a. At night without sunlight irradiation, the temperature of the
photo-thermal film decreases with the air temperature and is slightly
higher than the air temperature by 2 @\selectlanguage{english}C (Fig. 6b). At 7-8 am, the
photothermal membrane can reach nearly 50 @\selectlanguage{english}C with less than 0.5 sunlight
irradiation. The sharp fluctuation of the membrane temperature in two
places between 7:0-7:10 am is caused by obstacles blocking the membrane,
while the slow decrease and increase of the temperature between
7:10-7:20 am is caused by the decrease of sunlight radiation due to the
blocking of sunlight by buildings (Fig. 6c). At midday, when the
sunlight was 800 kWm\textsuperscript{-2}, the temperature of the solar
thermal film was even close to 75 @\selectlanguage{english}C and remained consistently high. One
of the short temperature drops was affected by clouds covering the sun
(Fig. 6d). At sundown, both air temperature and solar irradiation
gradually decrease, the temperature of the film first rises and then
falls, and the temperature of the film can only be maintained in the
range of 38-40 @\selectlanguage{english}C when the solar irradiation falls to 100
kWm\textsuperscript{-2} and below (Fig. 6e). As the weather changes from
sunny to cloudy, solar radiation drops dramatically and the
effectiveness of the solar thermal membranes decreases. In rainy
weather, the surface of the membrane gets wet with rainwater, thus
affecting the photothermal performance (Fig. 5f-h). By comparing the
photothermal performance of the membrane under different weather
conditions, it shows the universality of the double-sided
heterochromatic photothermal membrane under varies environments
{[}29{]}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 5. The photothermal performance of double-sided heterochromatic
nano wrinkled photothermal membranes. (a) The original PLLA membranes
before sunlight irradiation. (b) The original PLLA membranes after
sunlight irradiation. (c) The treated PLLA membranes without CS and
MWCNTs before sunlight irradiation. (d) The treated PLLA membranes
without CS and MWCNTs after sunlight irradiation. (e) The photothermal
comparisons of membranes in different CS/MWCNTs concentrations. (f) The
photothermal comparisons of dark side and light side. (g) The
photothermal comparisons of membranes in different sunlight irradiation.
(h) The photothermal performance of membranes on low-temperature glass
substrates (-20 @\selectlanguage{english}C) (Top left, original concentration light side; top
right, original concentration dark side; bottom left, C/2 concentration
dark side; bottom right, C/4 concentration dark side). (i, j) The
photothermal comparisons of membranes before (i) and after (j) sunlight
irradiation, the blue triangle represents the location of the lowest
temperature on the line, the red triangle represents the location of the
highest temperature on the line. (k) The temperature distribution on the
line of (i and j).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Figure 6. The outdoor solar heating performance of double-sided
heterochromatic nano wrinkled photothermal membranes. (a) The test
location. (b-e) The outdoor solar heating test on a sunny day at 0-1am
(b), 7-8am (c), 12-13pm (d) and 17-18pm (e). (f-h) The outdoor solar
heating test at 14-15pm on a sunny day (f), a cloudy day (g) and a rainy
day (h).

\textbf{3.6. Wearability}

The performance of the photothermal membranes is evaluated on human skin
as shown in Fig. 7a, b, and Supplementary Video 2. The top is the
initial CS/MWCNTs concentration, the bottom left is 1/2 of the initial
concentration, the bottom right is 1/4 of the initial concentration, and
in the middle of the three is PLLA (which is difficult to distinguish
from the figure due to its very poor photothermal performance). It can
be seen that the high concentration of CS/MWCNTs/APLLA always maintains
the highest temperature, which slowly increases with time up to 46 @\selectlanguage{english}C
without discomfort to human skin. The photothermal membrane was pasted
on clothes and evaluated for its photothermal performance (Fig, 7c and
d). It can rise 7.6 @\selectlanguage{english}C in two minutes outdoors (865
Wm\textsuperscript{-2}), which is significantly better than PLLA,
demonstrating its applicability to wearable photothermal devices. By
co-culturing with HSF for 12 and 24 hours, the photothermal membrane
also demonstrated excellent biocompatibility (Fig. 7e and f).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Figure 7. The wearability of photothermal membranes. (a) The
photothermal performance of membranes on human skin. (b) The
photothermal maps of membranes on human skin. (c) Indoor performance of
wearable photothermal membranes (0.3Wm\textsuperscript{-2} sunlight, 21
@\selectlanguage{english}C). (Sp1 is the temperature of PLLA; Sp2 is the temperature of the
photothermal membrane; Sp3 is the temperature of human skin;The
membranes (20 \selectlanguage{greek}µ\selectlanguage{english}m) are glued to cotton fabrics and contact directly with
the human skin.) (d) Outdoor performance of wearable photothermal
membranes (865Wm\textsuperscript{-2}, 21 @\selectlanguage{english}C). (e) Investigation of cell
viability (e: 12h; f: 24h), showing the good biocompatibility of
photothermal membranes.

\textbf{4. Conclusion}

In summary, reinforced CS@MWCNTs/PLLA photothermal membranes obtained by
solvent-induced recrystallization are reported in this work. The
photothermal membrane exhibits a nanowrinkled structure with a high
specific surface area, which provides more light absorption and enhances
the photothermal effectiveness of the membrane. The photothermal film
can heat up instantaneously and maintain between 60\textasciitilde{}75@\selectlanguage{english}C
at 800Wm\textsuperscript{-2} sun irradiation, which provides excellent
photothermal performance. The innovative solvent-induced
recrystallization procedure can greatly improve the mechanical
properties and micro-morphology of the electrospun membrane without
sacrificing the wearing comfort of the flexible membrane and is
potentially applicable to other electrospun membranes.

\textbf{Author contributions}

This manuscript was written with the contributions of all authors. All
authors have approved the final version of the manuscript.\textbf{Jinlin
Chang:} Methodology, Conceptualization, Investigation, Formal analysis,
Data curation, Writing -- review \& editing.\textbf{Weiling Wang} :
Conceptualization, Data curation. \textbf{Zhaoxin Li, Yujiao Wang,
Yacong Hou, Zhiyuan Cao, Zhenwei Liang} : Data curation.\textbf{Ding
Weng, Yuan Ma} : Methodology. \textbf{Yadong Yu, Jun Song} :
Methodology, Conceptualization, Investigation. \textbf{Lei Chen, Jiadao
Wang} : Project administration, Supervision, Methodology,
Conceptualization, Formal analysis, Investigation, Writing -- review \&
editing.

\textsuperscript{\textbf{#}} : \textbf{Jinlin Chang} and \textbf{Yadong
Yu} contributed equally to this work.

\textbf{Acknowledgments}

We acknowledge Dr. Jiashen Li at The University of Manchester and Zixuan
Li at Imperial College London, UK. This work is financially supported by
Shanghai Pu jiang Program (22PJD026).

\textbf{References}

{[}1{]} S. Garc\selectlanguage{ngerman}ía-Freites, C. Gough, M. Röder, The greenhouse gas
removal potential of bioenergy with carbon capture and storage (BECCS)
to support the UK's net-zero emission target, Biomass and Bioenergy 151
(2021) 106164.

{[}2{]} M. Shahbaz, M.A. Nasir, E. Hille, M.K. Mahalik, UK's net-zero
carbon emissions target: Investigating the potential role of economic
growth, financial development, and R\&D expenditures based on historical
data (1870--2017), Technological Forecasting and Social Change 161
(2020) 120255.

{[}3{]} C. Murugan, V. Sharma, R.K. Murugan, G. Malaimegu, A.
Sundaramurthy, Two-dimensional cancer theranostic nanomaterials:
Synthesis, surface functionalization and applications in photothermal
therapy, Journal of Controlled Release 299 (2019) 1-20.

{[}4{]} C. Chen, Y. Li, J. Song, Z. Yang, Y. Kuang, E. Hitz, C. Jia, A.
Gong, F. Jiang, J. Zhu, Highly flexible and efficient solar steam
generation device, Advanced materials 29(30) (2017) 1701756.

{[}5{]} L. Zhu, M. Gao, C.K.N. Peh, G.W. Ho, Recent progress in
solar-driven interfacial water evaporation: Advanced designs and
applications, Nano Energy 57 (2019) 507-518.

{[}6{]} W. Chen, H. Miao, G. Meng, K. Huang, L. Kong, Z. Lin, X. Wang,
X. Li, J. Li, X.Y. Liu, Polydopamine\selectlanguage{english}-induced multilevel engineering of
regenerated silk fibroin fiber for photothermal conversion, Small 18(11)
(2022) 2107196.

{[}7{]} T.M. Makinen, J. Hassi, Health problems in cold work, Industrial
health 47(3) (2009) 207-220.

{[}8{]} W. Wang, J. Chang, L. Chen, D. Weng, Y. Yu, Y. Hou, G. Yu, J.
Wang, X. Wang, A laser-processed micro/nanostructures surface and its
photothermal de-icing and self-cleaning performance, Journal of Colloid
and Interface Science (2023).

{[}9{]} Y. Tan, B. Hu, J. Song, Z. Chu, W. Wu, Bioinspired multiscale
wrinkling patterns on curved substrates: An overview, Nano-Micro Letters
12 (2020) 1-42.

{[}10{]} V. Fernandez, C. Llinares\selectlanguage{english}-Benadero, V. Borrell, Cerebral cortex
expansion and folding: what have we learned?, The EMBO journal 35(10)
(2016) 1021-1044.

{[}11{]} A.E. Shyer, T. Tallinen, N.L. Nerurkar, Z. Wei, E.S. Gil, D.L.
Kaplan, C.J. Tabin, L. Mahadevan, Villification: how the gut gets its
villi, Science 342(6155) (2013) 212-218.

{[}12{]} M.B. Hallett, C.J. von Ruhland, S. Dewitt, Chemotaxis and the
cell surface-area problem, Nature reviews Molecular cell biology 9(8)
(2008) 662-662.

{[}13{]} L. Wang, C.E. Castro, M.C. Boyce, Growth strain-induced
wrinkled membrane morphology of white blood cells, Soft Matter 7(24)
(2011) 11319-11324.

{[}14{]} X. Gao, J. Li, T. Li, Z. Su, X. Ma, J. Yin, X. Jiang,
Photo\selectlanguage{english}-polymerization induced hierarchical pattern via self\selectlanguage{english}-wrinkling,
Advanced Functional Materials 31(49) (2021) 2106754.

{[}15{]} W. Xu, S. Chen, M. Yao, X. Jiang, Q. Lu, A
near-infrared-triggered dynamic wrinkling biointerface for noninvasive
harvesting of practical cell sheets, ACS Applied Materials \& Interfaces
13(28) (2021) 32790-32798.

{[}16{]} W. Guo, C.M. Reese, L. Xiong, P.K. Logan, B.J. Thompson, C.M.
Stafford, A.V. Ievlev, B.S. Lokitz, O.S. Ovchinnikova, D.L. Patton,
Buckling instabilities in polymer brush surfaces via postpolymerization
modification, Macromolecules 50(21) (2017) 8670-8677.

{[}17{]} X. Fan, C. Deng, H. Gao, B. Jiao, Y. Liu, F. Chen, L. Deng, W.
Xiong, 3D printing of nanowrinkled architectures via laser direct
assembly, Science Advances 8(31) (2022) eabn9942.

{[}18{]} J. Chang, C. Meng, B. Shi, W. Wei, R. Li, J. Meng, H. Wen, X.
Wang, J. Song, Z. Hu, Flexible, breathable, and reinforced ultra-thin
Cu/PLLA porous-fibrous membranes for thermal management and
electromagnetic interference shielding, Journal of Materials Science \&
Technology (2023).

{[}19{]} B. Tang, W.L. Li, Y. Chang, B. Yuan, Y. Wu, M.T. Zhang, J.F.
Xu, J. Li, X. Zhang, A Supramolecular Radical Dimer: High\selectlanguage{english}-Efficiency
NIR\selectlanguage{english}-II Photothermal Conversion and Therapy, Angewandte Chemie
International Edition 58(43) (2019) 15526-15531.

{[}20{]} Z. Zeng, F. Jiang, Y. Yue, D. Han, L. Lin, S. Zhao, Y.B. Zhao,
Z. Pan, C. Li, G. Nystrom, Flexible and ultrathin waterproof cellular
membranes based on high\selectlanguage{english}-conjunction metal\selectlanguage{english}-wrapped polymer nanofibers for
electromagnetic interference shielding, Advanced Materials 32(19) (2020)
1908496.

{[}21{]} C. Keiser, C. Becker, R.M. Rossi, Moisture transport and
absorption in multilayer protective clothing fabrics, Textile Research
Journal 78(7) (2008) 604-613.

{[}22{]} J. Chang, L. Shi, M. Zhang, R. Li, Y. Shi, X. Yu, K. Pang, L.
Qu, P. Wang, J. Yuan, Tailor\selectlanguage{english}-Made White Photothermal Fabrics: A Bridge
between Pragmatism and Aesthetic, Advanced Materials (2023) 2209215.

{[}23{]} Z. Lu, B. Zhang, H. Gong, J. Li, Fabrication of hierarchical
porous poly (L-lactide)(PLLA) fibrous membrane by electrospinning,
Polymer 226 (2021) 123797.

{[}24{]} Y. Yu, L. Chen, D. Weng, Y. Hou, Z. Pang, Y. Zeng, J. Wang,
Solvent volatilization-induced cross-linking of PDMS coatings for
large-scale deicing applications, ACS Applied Polymer Materials 5(1)
(2022) 57-66.

{[}25{]} H. Jia, J. Guo, J. Zhu, Comparison of the photo-thermal energy
conversion behavior of polar bear hair and wool of sheep, Journal of
Bionic Engineering 14(4) (2017) 616-621.

{[}26{]} W.-H. Han, Q.-Y. Wang, Y.-Y. Kang, X. Zhou, C.-C. Hao,
Electrospun polymer nanocomposites for thermal management: a review,
Nanoscale 15(5) (2023) 2003-2017.

{[}27{]} M. Chung, W.H. Skinner, C. Robert, C.J. Campbell, R.M. Rossi,
V. Koutsos, N. Radacsi, Fabrication of a wearable flexible sweat pH
sensor based on SERS-active Au/TPU electrospun nanofibers, ACS applied
materials \& interfaces 13(43) (2021) 51504-51518.

{[}28{]} Y. Ahn, D.-H. Hu, J.H. Hong, S.H. Lee, H.J. Kim, H. Kim, Effect
of co-solvent on the spinnability and properties of electrospun
cellulose nanofiber, Carbohydrate polymers 89(2) (2012) 340-345.

{[}29{]} C. Shi, D. Yuan, L. Ma, Y. Li, Y. Lu, L. Gao, X. San, S. Wang,
G. Fu, Outdoor sunlight-driven scalable water-gas shift reaction through
novel photothermal device-supported CuO x/ZnO/Al 2 O 3 nanosheets with a
hydrogen generation rate of 192 mmol g\selectlanguage{english}- 1 h\selectlanguage{english}- 1, Journal of Materials
Chemistry A 8(37) (2020) 19467-19472.

\textbf{Hosted file}

\verb`000.graphical abstract.pptx`  available at \url{https://authorea.com/users/754736/articles/724582-reinforced-nanowrinkle-electrospun-photothermal-membranes-via-solvent-induced-recrystallization}

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