Cotton-Inspired Hollow Fibers for Portable Atmospheric Water Harvesting
- Amin Mojiri
- 14 hours ago
- 7 min read
Original Authors: Yaxuan Wang, Jiarui Zhang, Ting Xu, Xuan Wang, Meng Zhang, Junjie Qi, Han Zhang, Kun Liu, Liyu Zhu, Lin Dai, Chuanling Si
Original paper is accessible at: https://doi.org/10.1002/adfm.74928
Why portability matters in atmospheric water harvesting
Atmospheric water harvesting (AWH) is often discussed through the lens of sorbent chemistry, adsorption capacity, or solar-driven regeneration. However, real-world deployment also depends on another practical factor: form factor.
Many high-performance AWH materials, including MOFs, aerogels, hydrogels, and salt-loaded porous composites, can capture large amounts of water. But many of them are rigid, bulky, fragile, or difficult to integrate into portable systems.
This paper addresses that limitation by asking a practical materials-design question:
Can atmospheric water harvesting materials be made flexible, textile-like, and portable while still working across a wide humidity range?
The authors answer this by developing a bioinspired hollow fiber that mimics the moisture-transport function of mature cotton fibers. The result is a flexible, wearable, solar-driven AWH textile capable of harvesting water under both dry and humid conditions.
Cotton as the biological inspiration
The design is inspired by mature cotton fibers.
After cotton fibers mature, their internal protoplasts dry and disappear, leaving behind a hollow cavity running through the center of the fiber. This internal hollow structure acts as a capillary channel, helping cotton absorb, retain, and transport moisture efficiently.
The authors translate this biological architecture into an engineered AWH fiber.
Instead of a dense solid filament, they fabricate a hollow fiber with:
an internal transport channel
hydrophilic cellulose-based networks
LiCl water-capture sites
CNT-based photothermal functionality
flexibility suitable for textile formation
This gives the material both high water uptake and fast adsorption–desorption kinetics, while also allowing it to be woven into flexible devices.
Material design: s-CNF/CNT/SA-LiCl hollow fibers
The hybrid fiber is composed of:
sulfonated cellulose nanofiber (s-CNF)
sodium alginate (SA)
carbon nanotubes (CNTs)
lithium chloride (LiCl)
Each component has a specific function.
s-CNF and SA form the hydrophilic structural network. Their abundant hydroxyl, carboxylate, and sulfonate groups help bind water molecules and support LiCl loading.
CNTs provide photothermal conversion, allowing the fiber to heat under sunlight and release captured water.
LiCl serves as the hygroscopic salt, enabling strong water adsorption even under low relative humidity.
The fiber is fabricated using coaxial wet-spinning, where a shell solution and core solution are extruded together into a coagulation bath. Calcium ions induce cross-linking, forming a stable hollow structure through a dual-diffusion process.
Why the hollow structure matters
The hollow architecture is the central innovation of the paper.
Compared with a solid fiber, the hollow fiber provides two key advantages:
During adsorption
The hollow cavity provides additional binding sites, greater surface exposure, and faster water vapor transport pathways.
During desorption
The cavity increases the effective evaporation area and helps confine photothermal heat generated by CNTs, improving water release efficiency.
The optimized hollow fibers made with a 17G–22G coaxial needle had:
outer diameter: 350–400 µm
inner diameter: 300–350 µm
hollow cavity volume: approximately 73% of total fiber volume
By contrast, the solid comparison fibers had a diameter of about 180–230 µm.
This geometry is not only structurally important; it directly improves LiCl loading and water transport.
Optimizing LiCl loading
LiCl was introduced into the hybrid fibers to enhance water adsorption. The authors tested fibers impregnated with 5, 10, 15, 20, and 25 wt.% LiCl solutions, producing CSC5, CSC10, CSC15, CSC20, and CSC25 samples.
The optimized sample was CSC15.
The actual LiCl impregnation amounts were:
CSC5: 10 wt.%
CSC10: 34 wt.%
CSC15: 50 wt.%
CSC20: 44 wt.%
CSC25: 48 wt.%
Interestingly, higher LiCl solution concentration did not always produce higher loading or better performance. At excessive concentrations, LiCl agglomeration and pore blockage reduced adsorption kinetics. Therefore, CSC15 was selected for subsequent experiments.
Wide-humidity water adsorption performance
The optimized CSC15 hollow fibers demonstrated strong water uptake across a wide humidity range.
At:
11% RH: 0.41 g/g
33% RH: 1.48 g/g
57% RH: 2.22 g/g
75% RH: 3.36 g/g
95% RH: 5.00 g/g
The adsorption rate also increased from 0.10 to 1.92 kg/(kg·h) within the first hour across the tested humidity range.
These values are important because they show that the fiber can function in both dry and humid environments. At low RH, adsorption is driven mainly by chemisorption from LiCl and sulfonated groups. At high RH, capillary condensation becomes more important, supported by the porous and hollow structure.
Hollow fibers outperform solid fibers
The hollow architecture produced a clear performance advantage over solid fibers.
When solid fibers were impregnated with the same 15 wt.% LiCl solution, their LiCl loading reached only 24 wt.%, approximately half that of the hollow fibers.
Their final water uptake values were also much lower:
0.13 g/g at 11% RH
1.00 g/g at 57% RH
2.69 g/g at 95% RH
Compared with hollow fibers:
0.41 g/g at 11% RH
2.22 g/g at 57% RH
5.00 g/g at 95% RH
This difference was attributed to higher available loading sites and faster penetration kinetics in the hollow structure.
This is one of the strongest design lessons of the study:
The fiber architecture is not passive; it actively controls salt loading, vapor transport, and overall AWH performance.
Fast photothermal desorption
CNTs enabled efficient solar-driven desorption.
The s-CNF/CNT/SA-LiCl fibers showed more than 94% light absorption in the 300–780 nm spectral range. Under 1-sun irradiation, the fiber surface temperature increased rapidly:
46.7°C within 1 min
55.3°C after 5 min
By comparison, fibers without CNTs only reached 36.7°C after 5 min, confirming the importance of CNT-based photothermal conversion.
During desorption at 55°C and 0% RH, the fibers released most of their absorbed water within the first 180 minutes. The released fractions were:
89% after adsorption at 11% RH
86% after adsorption at 33% RH
92% after adsorption at 57% RH
92% after adsorption at 75% RH
96% after adsorption at 95% RH
The hollow fibers also showed a high desorption rate of 4.52 kg/(kg·h) in the first 60 minutes.
The abstract reports an even higher desorption-rate metric of 5.88 kg/(kg·h), approximately twice that of solid fibers.
To avoid overstatement, the key point is:
The hollow design substantially accelerates desorption compared with solid fibers, because both the inner and outer surfaces participate in water release.
Stable adsorption–desorption cycling
Cycling stability is essential for practical AWH systems.
The fiber was tested over ten adsorption–desorption cycles using a lab-built system. After ten cycles, the change in adsorption efficiency was negligible, indicating that LiCl leakage was minimal.
This matters because salt leakage is a common weakness of salt-based AWH materials. By embedding LiCl within a hydrophilic fiber network and using hollow transport channels, the material improves both uptake and stability.
From fiber to wearable water-harvesting textile
A major practical advantage of this work is that the fibers can be woven into textiles.
The authors wove the hybrid fibers onto a nylon mesh to form a flexible fiber cloth. The textile could be:
rolled
folded
worn on the body
suspended outdoors for water harvesting
This makes the material more portable than many bulk sorbents, aerogels, or rigid AWH devices.
The outdoor fiber cloth measured:
60 cm × 48 cm
and contained:
13.4 g of s-CNF/CNT/SA-LiCl hybrid fiber
63.4 g total fiber cloth mass before adsorption
Five outdoor adsorption–desorption experiments were conducted. Water uptake values were:
4.03
4.21
3.81
3.89
4.12 g/gfiber
The resulting water production rates were:
2.82
3.03
2.62
2.96
2.80 g/gfiber/day
Two-cycle daily water harvesting
To improve daily productivity, the authors performed two adsorption–desorption cycles within 24 hours.
During the first nighttime adsorption cycle, conducted under average nighttime conditions of:
27.9°C
70–90% RH
the fiber cloth absorbed:
55.0 g water
corresponding to 4.10 g/gfiber
Solar desorption released:
43.5 g water
and successfully collected:
39.1 g liquid water
This corresponds to:
79% evaporation efficiency
71% collection efficiency
2.92 g/gfiber water production rate
During the second cycle, the fiber absorbed:
30.82 g water
corresponding to 2.30 g/gfiber
and collected:
21.9 g water
corresponding to 1.63 g/gfiber
Across the two cycles, the total water collection capacity reached:
4.55 g/g/day
This result demonstrates that the textile format can support repeated daily operation rather than a single adsorption–desorption cycle.
Collected water quality
The authors measured ion concentrations in collected water after three adsorption–desorption cycles.
The reported ion concentrations were:
Li⁺: 0.51 mg/L
Ca²⁺: 5.4 mg/L
Mg²⁺: 4.2 mg/L
Cl⁻: 3.1 mg/L
NO₃⁻: 5.5 mg/L
SO₄²⁻: 6.1 mg/L
The study reports that these values comply with World Health Organization drinking water standards.
This is especially relevant because LiCl-based AWH materials can raise concerns about salt leakage. Here, the measured Li⁺ concentration was relatively low compared with several previously reported LiCl-based systems discussed by the authors.
Why this work is important
This paper contributes to AWH in three connected ways.
First, it shows that bioinspired architecture can improve water transport. The hollow cotton-like structure reduces diffusion resistance and increases adsorption/desorption efficiency.
Second, it demonstrates that AWH materials do not have to be rigid or bulky. These fibers can be woven into flexible textiles, suggesting opportunities for portable and wearable water-harvesting systems.
Third, the material works across a wide humidity range, from 11% RH to 95% RH, making it relevant for both dry and humid environments.
The work therefore shifts AWH design from only “how much water can a material hold?” toward a broader question:
Can the material be shaped into flexible, scalable, and deployable forms?
Key insights
Cotton-inspired hollow fibers provide efficient water vapor transport channels.
The fiber combines s-CNF, CNT, SA, and LiCl for moisture capture, photothermal heating, and structural flexibility.
The optimized CSC15 fiber achieved 0.41 g/g at 11% RH and 5.00 g/g at 95% RH.
The hollow fiber strongly outperformed solid fibers because of higher LiCl loading and shorter diffusion pathways.
Under 1-sun irradiation, the fiber reached 46.7°C in 1 min and 55.3°C after 5 min.
The fiber released up to 96% of absorbed water within 180 min, depending on the adsorption humidity.
The abstract reports a desorption rate of 5.88 kg/(kg·h), while the detailed text reports 4.52 kg/(kg·h) in the first 60 min under tested conditions.
The fiber retained stable performance over 10 adsorption–desorption cycles with negligible LiCl leakage.
The material can be woven into textile form and used in flexible, wearable AWH devices.
A two-cycle outdoor test achieved 4.55 g/g/day water production.
Takeaway
This study shows that atmospheric water harvesting can move beyond powders, gels, and rigid porous blocks.
By taking inspiration from the hollow structure of cotton fibers, the authors developed a flexible fiber-based AWH material that combines:
wide-humidity water uptake
fast vapor transport
solar-driven desorption
textile processability
wearable device potential
The most important contribution is not only the high water uptake, but the translation of AWH into a portable fiber-textile platform.
In short, this work suggests a future where atmospheric water harvesting materials may be woven, worn, folded, and deployed as flexible water-generating textiles.
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