Fast and Efficient Water Absorption Material Inspired by Cactus Root

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Letter Cite This: ACS Macro Lett. 2018, 7, 387−394

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Fast and Efficient Water Absorption Material Inspired by Cactus Root Hyejeong Kim, Junho Kim, and Sang Joon Lee* Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea S Supporting Information *

ABSTRACT: Analogous to the morphological and functional features of cactus root, a novel cactus root-inspired material (CRIM) was fabricated by integrating cellulose fibers, microparticles, and agarose-based cryogels. Without undergoing sophisticated chemical synthesis or surface modification, the CRIM exhibited efficient water absorption and retention ability with high structural stability. 82% of the total water absorption capacity was recovered within 1 min, with a swelling rate nearly 930fold faster than the evaporation rate, while only about 17% of the length extension occurred. Given that efficient water absorption and storage without physical change is crucial to the design and fabrication of water management devices, the CRIM is a promising material for various applications, including cosmetics or healthcare products, functional fabrics, and drug delivery devices. istics. Meanwhile, cellulose fibers absorb or adsorb water hygroscopically with physical changes, particularly variations in volume, boiling point, and viscosity.9,16,17 The fibers are often blended with various polymeric materials or subjected to various treatments, providing tough mechanical properties, stretchability, antimicrobial activity, and structural uniformity.18−23 Currently, developing highly durable functional materials that can withstand harsh environments, such as fast draining and limited water supply, through simple methods while retaining their physical properties remains challenging.10,15 In this work, we propose a novel cactus root-inspired system that can efficiently absorb and retain water. The morphological structures and water absorptive abilities of the cactus roots were first investigated through advanced bioimaging techniques. Figure 1a shows the optical image of a representative Opuntia microdasys, revealing the structural characteristics of the cactus roots. The cactus roots were covered with rhizosheath composed of root hair, soil, and mucilage, as shown in Figure 1a(i,ii).8 The average thickness of root hair was about 5.2 ± 1.2 μm, and they were complicatedly clotted with soil particles and mucilage [Figure 1a(iii,iv)]. The water absorption ratio of a cactus root is defined as Ra = (rf − ri)/ri, where ri is the initial mass of the root immediately after taking out from dry soil, and

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eserts experience limited rainfall. On intermittently rainy days, only a little rainwater is absorbed hygroscopically by superhydrophilic sand, and most rain drops are quickly drained out.1 Most desert plants have adaptive strategies to maximize water uptake from wet soil and minimize water loss to dry soil.2−5 As a representative desert plant, cacti can manage water effectively under harsh water shortage conditions.1,6 Cactus roots can absorb available water quickly, resist water loss when soil becomes dry, and resume water uptake upon the cessation of drought.2,7,8 Owing to the morphological and functional features, cactus roots can be considered good water absorption and retention systems from a biomimetic perspective. However, the unique water management role of cactus roots has not been fully examined, and mimicking its advantageous features has not been attempted yet. The ability of materials to absorb and store liquid in a welldefined structure is highly important not only to high-tech fields in material science, nanotechnology, and bioengineering but also to various commonly used applications, such as liquid filtration, medical treatment, hygiene, apparel, food packing, and horticulture.9 Thus, polymeric materials, such as hydrogels or fiber-based materials, are widely studied as candidate materials for liquid absorption.10−13 As superabsorbent polymers, hydrogels absorb water volume that is 10−100× its own volume by holding water molecules through hydrogen bonding.10,14,15 However, handling hydrogels is difficult because they are mostly brittle and requires continuous hydration because of their volatile superabsorbent character© XXXX American Chemical Society

Received: January 5, 2018 Accepted: March 7, 2018

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DOI: 10.1021/acsmacrolett.8b00014 ACS Macro Lett. 2018, 7, 387−394

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Figure 1. Structural and functional features of cactus root. (a) Optical images of (i) whole body, (ii) root, and (iii) cross section of a typical O. microdasys. (b) SEM image of O. microdasys. (c) Comparison of water absorption ratios in the roots of cactus and general plant that belongs to a noncactus species. Insets show a typical cactus, M. backebergiana (left), and one of noncactus plants, C. macrocarpa (right); Scale bar: 1 cm. (d) Comparison of water absorption ratio of M. backebergiana root with that of the rhizosheath-removed root. (e) X-ray images showing morphological structure of cactus root. (i) 3D CT image and (ii) cross-sectional image of a dried cactus root. (iii) 3D CT image and (iv) cross-sectional image of a wetted cactus root.

rf is the final mass of the root after dipping in water for 1 min. The average Ra value of six different species of cacti is approximately 1.13 ± 0.43. This is 2.4× higher than that of general plants, 0.47 ± 0.24 (Figure 1c, Supporting Information). The surfaces of the roots were extremely rough because of the presence of many micron-scale soil particles, root hairs, and layered epidermal cell wall [Figure 1e(i,ii)]. The complicated structure enabled the cactus roots to absorb large amounts of water rapidly and prevent the cortex cells from losing water by evaporation [Figure 1d,e]. Based on the morphological structures and functional features of cactus roots, a cactus root-inspired material (CRIM) capable of fast water absorption and long water retention was fabricated. Figure 2 shows a schematic of virtual artificial root which consists of a water reservoir inspired by cortex cell and water-absorbing CRIM. The proposed CRIM system is inspired by the rhizosheath and the layered epidermal cell wall of cactus root. To imitate the dominant structural components of the rhizosheath, we adopted cellulose fibers, microparticles, and agarose cryogel to represent the root hairs, soil particles, and mucilage, respectively. A presolution that contains agarose solution, cellulose fibers, and microparticles

was coated on the cylindrical gel and cryogelated. Owing to directional freezing, the fabricated CRIM model exhibited layered concentric circles around the cylindrical gel. This concentric circular shape is similar to the layered structure of the epidermis of a cactus root [Figures 1e(ii), 2a(i)]. Through the selective combination among 23.4 g/L of cellulose fibers, 45.5 g/L of microparticles, and 2 wt % of agarose solution, three different presolutions for the CRIM models, namely, CC (cryogel + cellulose fibers), CM (cryogel + microparticles), CCM (cryogel + cellulose fibers + microparticles), were prepared. By cryogelating the presolutions, the three CRIM models were fabricated [Figure 2a(ii−v)]. In the CCM model, microparticles with an average diameter of 38.2 μm were entangled with 10.5 μm thick cellulose fibers, and cryogel covered two components [Figures S1, S2, and Figure 2a(ii,iii)]. In the CC model, the cellulose fibers were clotted with cryogel, and a layered structure was formed during the cryogelation procedure [Figures 2a(iv) and S3]. The combinational composites obtained by the unidirectional freeze casting of the presolution and cellulose fibers were observed to possess highly aligned cellulose fibers (Figures S3 and S4).24 When only microparticles and cryogel were combined, the 388

DOI: 10.1021/acsmacrolett.8b00014 ACS Macro Lett. 2018, 7, 387−394

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Figure 2. Fabrication of the CRIM models and their water management abilities. (a) (i) Schematic diagram, (ii) optical image, and (iii) SEM image of the CCM (cryogel + cellulose fibers + microparticles) model. Optical images of (iv) the CC (cryogel + cellulose fibers) and (v) CM (cryogel + microparticles) models. (b) Comparison of swelling ratios of five CRIM models at 1 and 5 min, and full swelling states. (c) Temporal variations in recovery ratios of the five CRIM models. Comparisons of (d) water evaporation rates and (e) SE ratios of the CC, CM, and CCM models. Scale bar: 100 μm.

respectively. After 1 min of swelling, the average SR1 value of the CCM model presented the highest value of 4.75, followed by those of CM and CC models with average values of 3.30 and 1.78, respectively. The recovery ratios of the CRIM models were defined as SRt/SRf (Figure 2c). Within 1 min of swelling, the CCM model almost recovers 82% of total water content, whereas the CM, CC, and cryogel models recover 52%, 45%, and 28% of their full water absorption capacities at 1 min of swelling, respectively. The cellulose fibers model was nearly fully swollen at 1 min but showed a low full swelling ratio. After 5 min of swelling, the CCM model almost recovers 92% of its water absorption capacity, whereas CM, CC, and cryogel models recover were 76%, 69%, and 49% of their full water absorption capacities, respectively. As the water retention performance, the CC model exhibited the highest evaporation ratio, followed by the CM and CCM models (Figure 2d). To compare the water

microparticles were scattered in cryogel and no layered structure was observed [Figure 2a(v)]. The water absorptive abilities of the three CRIM models and the role of each component in water management were elucidated by comparing the water absorptive abilities of cellulose fibers and cryogel as control models (Figure 2b). The swelling ratio obtained after t min of swelling, SRt, and the swelling ratio after each CRIM was fully swollen, SRf, were measured. The results show that cellulose fibers swelled rapidly but exhibited a relatively low swelling ratio of about 1.73 even when it was at a fully swollen state. The control model cryogel demonstrated the highest SRf (7.01) among all the test samples but had a small SR1 value (1.95). When the cryogel, cellulose fibers, and microparticles were combined together, the water absorption features of the CRIM models improved. When the three CRIM models were fully swollen, the average SRf values of the CC, CM, and CCM models were 3.84, 6.18, and 6.03, 389

DOI: 10.1021/acsmacrolett.8b00014 ACS Macro Lett. 2018, 7, 387−394

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Figure 3. Water absorption and retention mechanisms of the CRIM models. (a) (i) 3D and (ii) cross-sectional images of the dried CC model. (iii) 3D and (iv) cross-sectional image of the wet CC model (orange-colored region, cellulose fibers; blue-colored region, water). (b) (i) 3D and (ii) cross-sectional images of the dried CCM model, (iii) 3D and (iv) cross-sectional images of the wet CCM model. Orange-, blue-, and green-colored regions indicate cellulose fibers, water, and microparticles, respectively. Scale bar in (a, b): 100 μm. (c) Schematic diagrams illustrating the water absorption mechanism of the CCM CRIM model. (d) Optical images of (i) CC, (ii) CM, and (iii) CCM models at the initial dry state, (′) 1 min swelling, and (″) full swelling states. Comparison of (e) average length strains and (f) volume increase ratios of the three CRIM models. Scale bar: 1 cm.

cellulose fibers were loosely interconnected and blended with microparticles, forming many airspaces [Figure 3b(i)]. Due to high image contrast of Ag-coated hollow particles, the microparticles are clearly distinguished from other components of CRIM. When the cellulose fibers absorbed water, most of the airspaces were filled with water, excluding only the regions containing microparticles. Through direct observation, we analogized a simple mechanism for the efficient water absorption of the CCM model. The fabricated CRIM models subjected to unidirectional freezing sustained cracks at the micron-scale, and the polymeric materials resulted in typical alignments along the frozen ice. In these cracks and alignments, a large amount of capillary water was quickly captured. The

absorption levels and retention features of the models, we defined and evaluated the determinant constant swelling/ evaporation (SE ratio) = SR1/ER1 (Figure 2e), where ER1 is the evaporation rate during 1 min of evaporation. The CCM model demonstrated the highest value (933), implying that the amount of water that can be absorbed by the model is 933× the amount of water lost by evaporation. The corresponding values of the CM and CC models were 326 and 189, respectively. In the CC model, the cellulose fibers were densely packed by clotting with cryogel and forming a layered structure [Figure 3a(i)]. As they absorbed water, cellulose fibers became loose and water permeated through them [Figure 3a(ii)]. In the CCM where microparticles were added in the CC model, the 390

DOI: 10.1021/acsmacrolett.8b00014 ACS Macro Lett. 2018, 7, 387−394

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Figure 4. Effect of microparticles concentration on water management abilities of the CRIM model. Variations in (a) swelling ratio, (b) recovery ratio, (c) evaporation rate and swelling/evaporation ratio, and (d) length strain and volume increase ratio of the CRIM model according to microparticles concentration.

micron-scale cellulose fibers favorably absorbed water within their complex fibrous structure [Figure 3c(i)]. The hygroscopic water was tightly held on the surface of the hydrophilic silvercoated microparticles. The superporous structure of cryogel also favorably absorbed water. When both components of cellulose fibers and microparticles were mixed, the complex composition was clotted with cryogel. Thus, capillary water accumulated and the water absorption effect was enhanced [Figure 3c(ii)]. Loosely dispersed micron-scale airspaces were formed when only cellulose fibers or cryogel was used. However, when they were combined with microparticles, the interconnection among cryogel, cellulose fibers, and microparticles worked as a strong backbone that supported the porous structure and maintained the stability. For a while, the absorbed capillary water gradually swelled the cryogel by passing through the nanoscale polymeric structure [Figure 3c(iii)]. As the CRIM models absorbed water hygroscopically, their volumes increased (Figure 3d). To quantitatively compare the geometrical changes, we measured the width strain (εW), height strain (εH), and total volume increase ratio of the CRIM models (Figure 3e,f). In the CC model, the average values of εW and εH were 0.7 and 0.23, respectively, indicating that the volumetric increase of the CC model was mainly induced by height variation. The average values of εW and εH in the CM

model were 0.36 and 0.33, and those of the CCM model exhibited the lowest values of 0.19 and 0.16, respectively. For the cases of CM and CCM models, the general trend of volumetric changes was similar to the directional variations of both width and height strains. The overall volume increase ratios of the CC, CM, and CCM models were 2.58, 2.35, and 1.59, respectively. This result indicates that the proposed CRIM demonstrated the lowest volumetric changes when all three components of cellulose fibers, cryogel, and microparticles were combined together. The water absorptive and retention abilities of the CRIM models were investigated with varying microparticle concentrations (Figure 4). Detailed compositions of the test samples with various microparticle concentrations are summarized in Table 1. When 1M of microparticles were added in the CC model, where M is equal to 1.4 wt %, the water swelling ratios of the test samples largely increased (Figure 4a). On the other hand, as microparticles were additionally added, the swelling ratio of the CCM model decreased. For all cases, the recovery ratios of the CCM model were higher than 0.8 after 1 min of swelling. After a little while, they were higher than 0.9 at 5 min of swelling (Figure 4b). As microparticles were added to the CRIM models, their water evaporation rates decreased. The SE value decreased as more microparticles were added to the CRIM models. When 1M of microparticles were added, the 391

DOI: 10.1021/acsmacrolett.8b00014 ACS Macro Lett. 2018, 7, 387−394

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The water absorptive and retention abilities of the CRIM models according to cellulose fibers concentration were also investigated (Figure 5). Detailed compositions of the test samples with various concentrations of cellulose fibers are presented in Table 1. As cellulose fibers were additionally added, the swelling ratios of the CCM model increased (Figure 5a). For all cellulose fibers concentrations, the recovery ratios of the CRIM models were higher than 0.8 after 1 min of swelling and become higher than 0.9 at 5 min of swelling (Figure 5b). As cellulose fibers were added to the CRIM models, their water evaporation rates decreased (Figure 5c). On the other hand, the SE values increased as more cellulose fibers were added to the models. When 1C and 5C of cellulose fibers were added, where C corresponds to 0.9 wt %, the CCM model absorbed 468× and 614× more water than the water lost by evaporation, respectively. The values did not vary significantly when the amount of cellulose fibers addition was more than 3C, indicating that the cellulose fibers addition of about 3C gives rise to saturated values at which the water absorption and storage ability of the CRIM models were maximized. However, the addition of more cellulose fibers induced less geometrical changes, while the models were fully swollen from the dried state (Figure 5d). As 5× more cellulose fibers were added, the average values of εW and εH increased from 0.12 and 0.11 to 0.16 and 0.25, respectively. Both values were maintained in a

Table 1. Composition of 12 Different Presolutions microparticle concentration effect M = 1.4 wt %

0M

1M

2M

3M

4M

5M

base presolution

2 wt % of agarose powder + 64 mL of DI water + 23.4 g/L of cellulose fibers microparticles (g) 0 1 2 3 4 5 cellulose fiber concentration effect C = 0.9 wt % base presolution cellulose fibers (g)

0C

1C

2C

3C

4C

5C

2 wt % of agarose powder + 64 mL of DI water + 45.5 g/L of microparticles 0 0.65 1.3 1.95 2.6 3.25

CRIM could absorb 1200 times more water compared to the water lost by evaporation. As 5M of microparticles were added, the CRIM model could absorb 700 times more water than the water loss. On the other hand, the addition of more microparticles induced less geometrical changes (Figure 4d). As the amount of microparticles increased 5 times, the average values of εW and εH decreased from 0.23 and 0.39 to 0.14 and 0.12, respectively. Accordingly, the volume increase ratio of the model with addition of 1 M microparticles was 2.09, whereas the corresponding value for 5M microparticles addition was 1.34. This result indicates that height variation dominantly induced the volumetric changes of the CRIM models.

Figure 5. Effect of cellulose fibers concentration on water management abilities of the CRIM model. Variations in (a) swelling ratio, (b) recovery ratio, (c) evaporation rate and SE ratio, and (d) length strain and volume increase ratio of the CRIM model according to cellulose fibers concentration. 392

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similar range until 3C of cellulose fibers were added to the CRIM models. In addition, the volume increase ratios of the CCM model added with 1C and 5C cellulose fibers were 1.39 and 1.69, respectively. These results indicate that the height variation dominantly induced the volumetric changes of the tested models. In summary, a novel biologically inspired water management material was developed on the basis of the survival strategy of cactus roots living in arid deserts. The experimental results of water absorption ability of cactus indicate that the effect of rhizosheath is crucial for water absorption of cactus root. The advantageous aspects of cactus root inspired us to explore its hydraulic strategy for efficient water management, especially for water recurement. We used three criteria for estimating efficient water management, namely, (1) large amount of water absorption, (2) rapid water absorption, and (3) minimal water loss by evaporation. To ensure that the fabricated CRIM models met the criteria, their swelling ratio, recovery ratio, and evaporation rate were evaluated. The CRIM model with high value of SE can be considered as a suitable model for efficient water management. The volumetric change caused by water absorption should be small to guarantee better material stability. Each of the constitutional elements cellulose fibers, cryogel, and microparticles can be freely changed for multiple functionalities. For instance, hydrophobic microparticles can be added to the CRIM system used for oil-based engineering applications, such as oil separation and selective filtration. Instead of cryogel, other cryogelable hydrogels can be used to apply the material in different areas of biotechnology.25 In addition, as efficient water management without physical damage of material is crucial in the design of practical water handling devices, the CRIM may have wide engineering applications satisfying the mechanical and hydrodynamical requirements. Therefore, it can be effectively applied to various liquid management systems, especially those used in cosmetics or healthcare products, functional fabrics, drug delivery devices, and agriculture applications.

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Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00014. Measuring water absorption ability of cactus root, X-ray microimaging, normal distributions of particle diameter and fiber thickness of the CRIM model, SEM images of the CC model, and schematic of composites obtained by the directional freeze casting possessed a highly aligned cellulose fibers along freezing direction (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Fax: +82-54-279-3199. Phone: +82-279-2169. E-mail: sjlee@ postech.ac.kr. ORCID

Sang Joon Lee: 0000-0003-3286-5941 Author Contributions

H.K. and S.J.L. proposed the study. H.K. and J.K. performed the experiment and they processed images. H.K. analyzed the experimental data. All authors discussed the results. All authors participated in completing the manuscript. Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; No. 2017R1A2B3005415). Notes

The authors declare no competing financial interest.

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REFERENCES

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EXPERIMENTAL SECTION

Fabrication of CRIM Models. A typical cellulose paper (Double A Premium 80, Double A Public Co.) was ground into fibers and dipped into deionized water for the preparation of a cellulose solution. A total of 2 wt % of agarose powder (Sigma Chem. Co.) was well mixed into the cellulose solution and then heated for complete dissolution. Silvercoated hollow particles (Potters Industries, Conduct-O-Fil SH230S33) with an average diameter of 38.2 μm were mixed into the presolution containing cellulose fibers and agarose gel. As the hollow hydrophilic microparticles have a low density of 0.5 g/cc, they are well dispersed in the aqueous presolution without sedimentation problem in fabrication of the CRIM. The presolution was frozen at −20 °C for 12 h for cryogelation and then melted at room temperature. The detailed compositions of the presolutions used for fabricating CRIM models are summarized in Table 1. Characterization of the Fabricated CRIM Models. The swelling ratio based on body weight is defined as SR = (ms − md)/ md, where ms and md denote the swollen and dried masses of a test sample, respectively. The evaporation ratio was evaluated using the formula ER = (mf − mi)/(mi·t), where mf is the final mass of the sample after t min evaporation and mi is the initial mass of the sample before evaporation starts. The length strain is defined as (Lf − Li)/Li, where Lf and Li represent the final and initial lengths of the test sample, respectively. The error bars in the all graphs represent standard deviation. 393

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