A stomata-inspired superhydrophobic portable filter system

Stomata, specialized functional openings distributed on the leaf surface, are used for plant respiration by allowing gas exchange, i.e., taking in carbon dioxide and releasing oxygen, and for water content regulation. Their function is vital to plant survival. Leaves with different wettability exhibit different stomata densities. In this study, we find that stomata on Pistia stratiotes L. leaves are protected by superhydrophobic setae, which prevent direct contact between the stomata and water in humid environments by suspending water droplets on the top of the setae. Thus, oxygen and carbon dioxide are freely exchanged through the stomata. This structure inspired us to design and develop a mask for filtering solid particles and noxious gas from the atmosphere. The incoming gas is in convective contact with water, achieving a filtering efficiency. The solid particles and potential harmful gas in air are wetted and captured by water, leaving fresh air for healthy breathing. This novel design has potential applications in the treatment of respiratory diseases.

A stoma is a plant gas exchange channel that is used for carbon dioxide intake and oxygen release, playing a crucial role in plant survival in nature. [1][2][3] The distribution and density of stomata on the leaf surface may vary with surface wettability. On the hydrophilic leaves of a locust tree, the stomata density on the lower surface is higher than that on the upper surface. This unique design is benecial for respiration and photosynthesis, even in rainy conditions. Some leaves with superhydrophobic surfaces exhibit a higher stomata density on the upper surface than that on the lower surface, because of their robust waterrepellence. [4][5][6][7] Rough micro-/nano-topography may allow significantly more air ow between solid-liquid interfaces with water droplets' suspension on the peak region of the rough solid surface. [8][9][10][11] Thus, the superhydrophobic topography suppresses direct contact between the stomata and water, preventing contamination and yielding healthy plants.
Many plants in nature possess superhydrophobic leaves, e.g., lotus, taro, and Pistia stratiotes L. [12][13][14] Different from other superhydrophobic plants with micro/nano-topography, Pistia stratiotes L. plants have millimetre-level superhydrophobic setae. Their high topographical features are especially benecial for long-lasting and steady breathing in the water environment. Inspired by this natural design, we developed a novel air ltration system. It consists of stomata arrays integrated with superhydrophobic micro-papillae. The micro-hole arrays lter micro-sized solid particles from the air and provide fresh air for breathing.
Pistia stratiotes L. is a self-cleaning oating herbaceous plant that lives on the water surface. The plant's surface is covered with superhydrophobic millimeter-sized setae (mm-setae) arrays for keeping a stable respiration shown in Fig. 1a and b, enabling the plant survival in high-humidity environments. Water droplets with a large surface tension are easily suspended on the surface of setae arrays ( Fig. 1c and Movie Online Resource 1 †). Thus, they rarely contact the stomata under the setae arrays, maintaining a layer of owing air at the solidliquid interface, further enhancing the leaf survivability. Even when the leaf surface is 1 m below the water level, the leaf can survive for more than 24 h due to the amount of air retained between the mm-setae. The ability of the Pistia stratiotes L. leaf to survive underwater is better than that of the lotus leaf. 5,15 In addition, solid dust may be easily captured by a water droplet, and once it is wetted, it cannot get out of the water droplet due to the large surface tension and excellent wettability of water. [16][17][18][19][20] Fig. S1 and Movie Online Resource 2 † show a captured solid particle that oats in the water droplet. In this process, water lters the air. 21 The surface of the Pistia stratiotes L. leaf is composed of mmsetae and stomata. The mm-setae and stomata play two vital roles for plant survival, i.e., they provide the airow and serve as a gateway for gas exchange, respectively. Fig. 1d-f shows the SEM micrographs of the leaf surface for better insight into topographical details. The mm-setae, with a height of $1 mm, are covered by nano-petals and wax material for achieving the superhydrophobic function, and the stomata are located in the root region of the mm-setae (Fig. 1f). Large setae effectively hinder direct contact between stomata and water. Droplet suspends on the top region of rough surface and shows Cassie's state. 22,23 The mm-setae are distributed not only on the upper surface but also on the lower surface of the leaf (Fig. S2 †), both playing a signicant role in maintaining water-repellence and respiration, further enhancing the survival ability underwater. 24 The mm-setae on the leaf surface exhibit excellent mechanical properties required to resist the external force exerted by water and avoid wetting. We used a micromechanical balance to test the mechanical properties of mm-setae on the upper and lower surfaces (Fig. 2). A stress of 3.2 Â 10-2 N is generated when the mm-setae are bent at a deformation of 1 mm. The relationship between the deformation and the stored elastic energy (G) is shown in eqn (1) where K is the elastic modulus of the mm-setae, h 0 and Dh are the initial height and the deformation of mm-setae, respectively. The bending of the mm-setae becomes challenging as the deformation increase. 25 The average pressure is larger than 104 Pa, so the water-repellence function is stable even when the structure is soaked in water at a depth of 1 m, making the structure substantially more durable than that of the lotus leaf.
As the wetting of stomata is suppressed, the exchange of oxygen and carbon dioxide can take place freely through the stomata even in rainy environment (Fig. 3), resulting in normal respiration. This design improves the vitality and avoids the leaf's degradation. Inspired by the functionality of the combination of stomata and superhydrophobic mm-setae, we designed a superhydrophobic membrane with upper surface composed of superhydrophobic micro-papillae and micro-stomata arrays (Fig. 4). The micro-papillae, with a height of 500 mm and a base diameter of 500 mm, could provide more air ow and induce a liquid droplet suspension on their tips (Fig. 4b). The contact angle of the 10 mL-droplet is larger than 152 , and the droplet quickly sheds off the surface with a tilting angle of 2 , providing a passage for air (inset in Fig. 4b). The micro-stomata are located between the micro-papillae and extend through the membrane. The long and short diameters of elliptical stomata are 1 mm and 500 mm, respectively. The space between two neighboring stomata, with a width of 500 mm, provides the superhydrophobic function and a lot of air between the solid surface and water. The stomatal channels with a length of 4 mm   pass through the whole membrane (Fig. S3 †). When the surface is covered by water, the micro-papillae generate small bubbles and air layer at liquid/solid interface (Fig. 4c). The three-phase contact line is very unstable, as it shrinks and detaches from the solid surface when the surface is tilted at an angle of 2 (Fig. 4d). The excellent superhydrophobic performance indues liquid droplet suspension on the tip of micro-papillae, preventing liquid from contacting with the stomata (Fig. 4e and S4 †). In the magnied view of the surface shown in Fig. 5, the surface is covered by micro sphere and ZnO nano-rod, which enhances the roughness and further water-repellence function.
The designed bio-fabricated respiratory system is constructed by this membrane equipped with a polymethyl methacrylate (PMMA) shell, and it is used to capture micro-sized solid particles and gas pollutants from the atmosphere (Fig. 6). The membrane is locked in the PMMA shell as a gate to prevent the inltration of water as shown in the Movie 3. † A lter mask is obtained aer mounting the respiratory system on a mask. Fig. 7a shows its working schematic. During the breathing process, the air moves through the valley of the superhydrophobic micro-papillae aer entering the respiratory system, and it is ltered by water before being transported into the breathing system (Fig. 7b). Most of solid micro particles and odorous gas once contact the water, they are wetted and absorbed by the water. As the stale gas passes through the sample, it turns into cleaner air. Practical application process     . 7 The bio-fabricated respiratory system. (a) Schematic drawing of the system. The bio-fabricated surface is embedded in a groove. The groove is filled with water. The air is filtered by water, so the clean air moves through the stomata. (b and c) Practical application process by medical staff. The mask system not only filter the solid dust in atmosphere, but also harmful gases from the air. by medical staff illustrate that the mask system lters not only solid dusts but also harmful gases in the air (Fig. 7c). Most of the solid particles from the air are absorbed due to their excellent water wettability (Fig. S5 †). This design effectively improves respiratory system.
The water in the PMMA shell could be replaced by a volatile drug to treat diseases. Drugs are transported through the stomata and enter the body by the respiratory tract. The size of stomata controls the drug owing speed. Moreover, this system can be recycled. When the water/drug solution in the PMMA shell is dry, the shell could be reused aer washing with water. The number of reuses can exceed 120 times.
In conclusion, inspired by the topography of the natural leaf surface, we developed a novel kind of air lter. The biofabricated surface is equipped with superhydrophobic microsetae and perforating structural micro-stomata. Water suspends on the superhydrophobic micro-setae, providing signicantly more owing air at the valley of the micro-setae. The air is ltered when it goes through the valley and the stomata, providing fresh, comfortable, and moderately humid air for the human respiratory system. This study provides an effective fabrication method for respiratory ltration systems, having potential applications for masks, breather valves, and medical treatment.

Conflicts of interest
There are no conicts to declare.