Anisotropic, Mesoporous Microfluidic Framework with Scalable

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Anisotropic, Mesoporous Microfluidic Framework with Scalable, Aligned Cellulose Nanofibers Chao Jia, Feng Jiang, Piao Hu, Yudi Kuang, Shuaiming He, Tian Li, Chaoji Chen, Alan Murphy, Chunpeng Yang, Yonggang Yao, Jiaqi Dai, Christopher B. Raub, Xiaolong Luo, and Liangbing Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17764 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Anisotropic, Mesoporous Microfluidic Framework with Scalable, Aligned Cellulose Nanofibers Chao Jia,1, a Feng Jiang,1, a Piao Hu,2 Yudi Kuang,1 Shuaiming He,1 Tian Li,1 Chaoji Chen,1 Alan Murphy,1 Chunpeng Yang,1 Yonggang Yao,1 Jiaqi Dai,1 Christopher B. Raub,3 Xiaolong Luo,2 Liangbing Hu1, * 1. Department of Materials Science and Engineering, University of Maryland College Park, College Park, Maryland 20742, USA 2. Department of Mechanical Engineering, Catholic University of America, Washington, DC, 20064, USA 3. Department of Biomedical Engineering, Catholic University of America, Washington, DC, 20064, USA a

Equally contributed.

*

Email: [email protected]

Keywords: Anisotropic, Mesoporous, Cellulose nanofibers, Mass transport, Microfluidics

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Abstract Cellulose paper has been extensively used in microfluidic analytical devices due to its hydrophilic nature. However, cellulose is randomly packed in paper without any particular orientation or channels within the bulk of the material, necessitating a complicated design of hydrophilic microchannels to guide liquid flow. Herein we develop an anisotropic mesoporous microfluidic framework (named as white wood) with aligned cellulose nanofibers and inherent microchannels via a facile one-step delignification process from natural wood. The hydrophilic nature of the innate microchannels in white wood makes it ideal for application as a pump-free microfluidic chip, exhibiting fast and anisotropic liquid and large solid particle (as demonstrated with carbon nanotubes) mass transport, with a high transport speed along the channel direction approximately five times faster than that perpendicular to the channel direction. The anisotropic mass transport is further exemplified in the fabrication of chitosan film with aligned microstructure and birefringence, formed by virtue of the unidirectional capillary forces exerted by the microchannels. We envision that our anisotropic mesoporous framework can have great application to pump-free microfluidics, and the simple preparation process will pave a new way for the development of microfluidic devices based on chemically-modified wood.

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Introduction Using paper as an inexpensive platform for analytical devices can be traced back to the 1940s, when it was applied in partition chromatography as a micro-analytical tool to separate pigments,1 sugars,2 amino acids,3 and phosphoric esters.4 However, it was not until six decades later that the renaissance of paper-based microanalysis technique found its inception, as led by the Whitesides group in successful fabrication of microfluidic paper-based analytical device (µPAD).5, 6 Owing to the many advantages of µPAD including affordability, sensitivity, portability, disposability, power-free functioning and ease of use in multiplex assays, substantial efforts have been devoted to the development of analytical devices based on paper substrates.7-9 The application of µPADs has been expanded in various fields, such as environmental analysis,10 health diagnosis,11 and food quality monitoring.12 Despite the aforementioned advantages, paper primarily consists of randomly packed cellulose fibers, lacking defined hydrophilic microchannels within the substrate. Hence, extra efforts are necessary to define these channels for directing fluids, including physical methods of wax printing,13 screen printing,14 laser cutting and etching,15 flexographic printing,16 as well as chemical methods of photolithography,17 wet etching,18 plasma treatment,19 and inkjet printing.20 All of these channel-defining processes will inevitably complicate the fabrication process, as well as increase the production cost. In addition, the papermaking process is very complex, involving a large number of energy and chemical intensive steps.21 As a renewable and environmentally-friendly biomaterial, wood has been extensively used in the fields of green electronics, biological devices, energy storage, and composite materials.22-27 In nature, wood develops into hierarchical structures of micrometer-scale elongated conducting cells including vessel elements and fiber tracheids, which are responsible for efficient conducting of water, ions, and other nutrients from soil to support the tree’s metabolism.28 These inherent microchannels are oriented along the tree growth 3



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direction, resulting in anisotropic microstructures in wood. The unique hierarchical structure of trees for upward mass transport inspires us to develop a new material for pump-free µPADs. Herein we developed an anisotropic mesoporous microfluidic framework (named as white wood) with inherent microchannels by removing the colored and hydrophobic lignin from natural wood. The preparation process of the white wood is quite simple and timesaving, including just one delignification procedure. More importantly, the aligned cellulose nanofibers are retained during the process. The anisotropic mass transport property was demonstrated by transporting water and carbon nanotube (CNT) ink. The mass transport in the white wood can be ascribed to the capillary action produced in the micron-sized and nano-sized channels in wood. The unique anisotropic mass transport was applied to the fabrication of chitosan film with aligned microstructure and birefringence signal that was formed by virtue of the unidirectional capillary forces exerted by the microchannels. We anticipate great applications of our anisotropic mesoporous framework in the field of pump-free microfluidics, and its development will pave the way for the direct use of natural wood in microfluidic devices.

Results and discussion Natural basswood was used as the raw material to prepare the mesoporous framework. Basswood, as a typical type of hardwood, is primarily made of three kinds of cells, i.e. fiber tracheids, vessels, and parenchyma.28 Vessels, owing to the large lumen diameter and open-ended wall structures with simple perforation plates, have been regarded as the primary conduits for mass transport, whereas the thinner fiber tracheids with closed tapered ends are mostly responsible for mechanical support.28 In addition, basswood possesses an average vessel element length of 300-600 µm, and an average fiber tracheid length of 800-1500 µm.29

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The anisotropic mass transport of our mesoporous framework was vividly depicted in Figures 1a and 1b. In this paper, both fiber tracheids and vessels are collectively called microchannels, which can be clearly visualized under SEM to show the hierarchical mesoporous structure (Figures 1c, 1d and S1). These straight microchannels along the tree growth direction have a low tortuosity, which are conducive to the fast lignin removal and mass transport. Some small pits with an average diameter of ~2.5 µm are present on the internal surface of the microchannels (Figures S2 and S3), which enable slow transverse mass transport between adjacent channels.30, 31 It should be noted that all kinds of wood possess similar anisotropic microstructures even though they may exhibit differences in terms of species, age, location in trees, so our one-step delignification process can be generally used to prepare white wood with anisotropic mass transport properties.

Figure 1. Schematic and images of wood hierarchical structure. (a-b) Schematic to show the mesoporous structures in wood where the channels are aligned vertically. The straight channels in wood have a low tortuosity, which is beneficial to the fast lignin removal and mass transport. Apart from the 5



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microchannels, lots of nanochannels and cellulose nanofibers are exposed among cellulose microfibers after lignin removal, which also play an important role in fluid transport. (c) SEM image of wood hierarchical structure. Vertically aligned fiber tracheids and vessels can be clearly seen. (d) High magnification SEM image of the aligned cellulose nanofibers.

In wood, lignin can be found in the entire wood cell walls, including middle lamella, primary and secondary cell walls (S1, S2 and S3), while the cellulose microfibers are mainly distributed in primary and secondary cell walls.32 After lignin removal, the hydrophilic cellulose microfibers can be exposed with inter-fiber spacing previously occupied by lignin emptied as nanochannels, which can also play an important role in fluid transport by increased hydrophilicity and surface areas (Figure 1b). All these inherent channels (microchannels of vessels and fiber tracheids as well as nanochannels between cellulose nanofibers after lignin extraction) enable the application of wood in microfluidics. Applying wood in microfluidics has the following advantages: 1) fast mass (both liquid and solid) transport within the naturally defined microchannels; 2) pump-free mass transport via capillary forces; 3) anisotropic mass transport owing to the alignment of microchannels. A piece of basswood, cut along the longitudinal direction, was selected as the starting material for white wood preparation (Figure 2a). The basswood with about 22% lignin exhibits a yellowish color due to the strong light-capturing capability of lignin. In addition, lignin is also a complicated amorphous phenolic-type polymer, and wholly distributed in the wood cell walls.28 To fabricate white wood, a simple delignification process employing acidic NaClO2 solution was selected to remove the lignin in bulk wood. After the delignification process, the original yellowish wood slice became white, indicating successful removal of the light-absorbing lignin, which was further confirmed by the chemical composition analysis of original wood and white wood. As shown in Figure 2b, the lignin in white wood is almost completely removed during the delignification process. Most critically, with around 30 % of weight reduction due to the removal of lignin and other wood components, the white wood 6


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does not show obvious dimensional shrinkage after this treatment, suggesting the original microchannels can be well preserved. As lignin embeds in the cellulose and hemicellulose matrix and coheres the wood tissue as a whole,32 it is also reasonable to deduce the generation of numerous hydrophilic nanochannels among the cellulose microfibers. These hydrophilic nanochannels were generated at spaces previously occupied by the hydrophobic lignin, which can enhance the water transport.

Figure 2. Preparation of white wood and morphology characterization. (a) Digital images of the original flavescent basswood (left) and white wood after lignin removal (right). (b) Relative content of cellulose, hemicellulose and lignin before and after delignification obtained by chemical composition analysis. (c) SEM image of the white wood microchannels. (d) High magnification SEM image of the individual microchannel to show the presence of pits (average diameter of ~2.5 µm) responsible for 7



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transverse mass transport. (e) High magnification SEM image of the microchannel to present the aligned cellulose nanofibers and the nanochannels among these nanofibers. (f) Cross-sectional SEM image of the white wood. (g) Diameter distribution of fiber tracheids and vessels in white wood. The inset shows the diameter distribution of the enlarged section for vessel lumens.

Vertically aligned microchannels along the tree growth direction can be clearly visualized from SEM image (Figure 2c), showing low tortuosity and open channel structures. These vertically aligned microchannels are the primary paths for transporting water, ions, and other necessary nutrients in living trees to support the photosynthesis, which would be used as conduits for mass transport in microfluidics. The microstructures of wood contain numerous boarded pits with pit membrane on the secondary cell wall, connecting adjacent vessels/fiber tracheids to allow water transport while blocking the transport of embolism and vascular pathogens.30,

31

Pit membranes can be broken during the delignification process

(Figure 2d) because the membranes contain significant amount of lignin.31,

33

Therefore,

removing lignin from wood perforated the microchannels, making it possible to transport liquid and solid mass between adjacent channels as well as along the otherwise closed fiber tracheids. Furthermore, lignin removal exposed the cellulose microfibers on the cell wall surfaces, containing nanochannels between the aligned hydrophilic cellulose microfibers (Figure 2e). Brunauer-Emmett-Teller (BET) pore size distribution determination indicates that the white wood possesses a BET surface area of 22.3 m2 g-1, and most pores are distributed in the 2-30 nm range, falling into the category of mesopores (2-50 nm), demonstrating the white wood has a mesoporous structure (Figure S4). The cross-sectional SEM image of white wood in Figure 2f shows the distribution of heterogeneous pores, including narrower fiber tracheids and much bigger vessels. The fiber tracheids and vessels in white wood show two distinct lumen diameters averaging at 12 and 51 µm, respectively, similar to those of original wood (Figures 2g, S5 and Table S1). Although the number of vessel lumens is much less than that of fiber tracheid lumens, it 8


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accounts for about 41% of the total channel volume, which is beneficial to the transport of large particles, considering its larger diameter and open-ended structure. Unidirectional mass transport is critical in applications where functional materials need to be transported to target locations (such as drug delivery and biological diagnosis). The microchannels and nanochannels along the tree growth direction are ideal for unidirectionally transporting water and large particles in white wood. To demonstrate this property, we examined the transport efficiency of white wood for water and carbon nanotube (CNT) aqueous suspension. Figure 3a shows the schematic of filter paper strip, and CNT ink transport status in filter paper at the beginning and after 150 s of ink transport. It is known that filter paper is a thin sheet of material that is produced by pressing randomly dispersed cellulose fibers (Figure S6). From the figure we can clearly see that CNT cannot be effectively transported in filter paper due to the blockage of fibers, while the transport of water can be seen. Besides, the CNT transport is isotropic in filter paper due to the random stack of cellulose fibers (Figure 3e). Obvious anisotropic transport behavior can be observed for CNT transport in white wood. Figure 3b presents the schematic of white wood strip with vertical channels, and the transport status of CNT ink in white wood at the beginning and after 150 s of ink transport. As we stated earlier, the white wood has intrinsic straight microchannels to guarantee sufficient space to accommodate big particles. CNT carried by water can transport upward along the channels under capillary forces, and the absorbed CNT can be clearly seen in the channels (Figures 3f-h). The longest transport distance of 17.5 mm at 150 s and 4.8 mm transport distance in the first second were achieved for CNT transport in this direction (Figures 3d and 3e). On the contrary, the upward CNT ink transport in the white wood strip with horizontal channels is very slow, showing only 3 and 0.4 mm transport distance within 150 s and in the first second, respectively (Figures 3c-e). The CNT transport behavior in this 9



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direction can be attributed to the existence of small pits on the internal surface of microchannels. The effectiveness of lignin removal on CNT transport was further corroborated by the CNT transport in the original wood strip, showing as low as 3 mm upward transport distance within 150 s (as compared to 17.5 mm in white wood) with vertical channel direction and no transport (as compared to 3 mm in white wood) with horizontal channel alignment (Figure S7), which is possibly due to the presence of hydrophobic lignin, closed fiber tracheids, and pit membranes. These obtained results once again confirm the SEM imaging and chemical composition analysis that lignin has been removed and the microchannels have been perforated in white wood during the delignification process. In summary, white wood can transport CNT by virtue of the inherent microchannels, while filter paper does not have this ability.

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Figure 3. CNT ink transport in filter paper and white wood. The schematics and CNT ink transport status at the beginning and after 150 s of ink transport for (a) filter paper, (b) white wood with vertical channels, and (c) white wood with horizontal channels. (d) Transport distance of CNT ink over time for white wood and filter paper. (e) The initial transport speed of CNT ink during the first second for white wood and filter paper. (f) SEM image of wood microchannels after CNT transport. (g) SEM image of the absorbed CNT in white wood. (h) High magnification SEM image of absorbed CNT in white wood.

The transport of CNT aqueous suspension includes both CNT and water, where liquid transport could be interfered by the CNT movement and blockage. To investigate the capability and effectiveness of pure liquid transport for our anisotropic mesoporous framework, we investigated and compared the water transport through both filter paper and 11



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different directions of white wood. A small quantity of green dye was added in water to enhance the visual contrast. Figure 4a shows the schematic of filter paper strip, and water transport status in filter paper at different times. Filter paper made of pure cellulose with abundant hydroxyl groups is highly hydrophilic, and water can be absorbed in filter paper due to capillary action. In addition, water transport in filter paper is isotropic because of the homogeneous structure (Figure 4e). As expected, anisotropic water transport in white wood can also be observed. Figure 4b presents the schematic of white wood strip with vertical channels, and water transport status in white wood at different time points. The white wood exhibits fast water transport due to the synergistic effect of micron- and nano-sized channels, resulting in a 6 mm water transport distance in the first second and a 32.5 mm transport distance within 150 s (Figures 4b, d and e), almost double that of CNT transport (Figure 3). Similar to CNT transport, the water transport in white wood strips with horizontal channels is very slow, and water only transports 0.4 mm in the first second and 5 mm in 150 s, as shown in Figures 4c-e, which is slightly higher than that of CNT transport. Compared with CNT transport, water transport shows a longer distance in the same period of time, indicating less resistance for water transport. From Figures 4a and 4b we can also observe that the top line of transported water in the filter paper is flat, while that in the white wood is jagged. The jaggedness of the line in the white wood results from its uneven pore size. Similarly, original wood also demonstrated low efficiency in water transport, as evidenced from the 10 mm water transport distance within 150 s with vertical channels, and negligible water transport for horizontal channels (Figure S8). The dramatic transport phenomena between white wood and original wood suggest that delignification is effective in perforating fiber tracheids and pit membranes, which, together

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with the removal of hydrophobic lignin, is beneficial for the efficient liquid transport required by microfluidics.

Figure 4. Water transport in filter paper and white wood. The schematics and water transport status at different time points for (a) filter paper, (b) white wood with vertical channels, and (c) white wood with horizontal channels. (d) Water transport distance as a function of time for white wood and filter paper. (e) The initial water transport speed during the first second for white wood and filter paper. (f) Unit mass water absorption capacity of white wood and filter paper.

The unit mass water absorption capacity is presented in Figure 4f, which shows 3.18 ± 0.03 g g-1 water absorption capacity for white wood with vertical channels. Slightly lower water absorption capacity can be observed for the white wood with horizontal channels, possibly due to less water filling in this transverse direction through the inter-channel pits system. Compared with white wood, significantly less amount of water (1.85 ± 0.06 g g-1) 13



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can be absorbed by the random filter paper strip, owing to the smaller pore volume fraction. In addition, the absorbed water in white wood and filter paper is higher than their theoretical water absorption capacity calculated based on the pore volume fraction (Table S1) (2.54 g g-1 for white wood and 1.32 g g-1 for filter paper), which can be attributed to the presence of some water on the sample surface. As comparison, the water absorption capacity of original wood with vertical channels can only reach to 1.50 ± 0.05 g g-1, significantly less than the 3.18 ± 0.03 g g-1 for white wood. The significantly higher water absorption capacity for white wood can be ascribed to its large pore volume fraction (79.2% for white wood vs. 70.3% for original wood) and improved hydrophilicity. It should be noted that the 1.50 ± 0.05 g g-1 water absorption capacity of original wood is slightly less than its theoretical water absorption capacity of 1.58 g g-1, which can be attributed to the presence of hydrophobic lignin and close-ended fiber tracheids that are less accessible to water absorption. 3D mass transport in the white wood block was also demonstrated in Figure 5. Figure 5a is the schematic to show the transport of a colored water droplet in the white wood block, in which microchannels are perpendicular to the horizontal plane. When a dyed water droplet (100 µL) is placed on the top surface of the white wood, it spreads into a uniform circle due to the transverse diffusion of water through the inter-channel pits (Figure 5b). The circular diffusion pattern also indicates that the transport speed of the colored water is same along all directions due to the isotropic property of the top surface. In this 3D wood block system, the primary water transport is along the channel direction, showing that the dyed water can penetrate through the entire wood block to the backside of the cross-sections. Besides, the dyed circular pattern is uniform throughout of the channel direction, suggesting that the liquid transport is confined within the channels without excessive transverse diffusion. The water transport in the white wood block with the channel direction parallel to the horizontal plane is presented in Figure 5c. When a dyed water droplet (100 µL) was dropped 14


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on the top surface of the white wood with the channels parallel to the horizontal direction, it spreads along the channels into an elliptical shape (Figure 5d), indicating an anisotropic water transport with much faster transport speed along the channel directions. In addition, the depth of the water transport along the thickness of the white wood is relatively small, showing a circular-arc pattern, which indicates minimal water transport between channels through the pits. From the results we can conclude that the white wood possesses the anisotropic mass transport property, with the maximum transport speed and distance being achieved along the channels, whereas the transverse transport between channels is small considering both the scarcity of pits and smaller pit diameter.

Figure 5. 3D mass transport demonstration. (a) Schematic to show the transport of a colored water droplet in the white wood block. The channel direction is perpendicular to the horizontal plane. (b) Digital images of top view and cross section of white wood after colored water transport. A drop of colored water with a volume of 100 µL was dripped on the top surface of the white wood, and the water transported 15



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along the channels from top to bottom. (c) Schematic to show the transport of a colored water droplet in the white wood block. The channel direction is parallel to the horizontal plane. (d) Digital images of top view and cross section of white wood after colored water transport. A drop of colored water with a volume of 100 µL was dripped on the top surface of the white wood, and the water transported along the channels in the horizontal direction.

The anisotropic mesoporous framework can be applied in pump-free microfluidics, which was exemplified by the assembly of chitosan film with aligned microstructure and birefringence. Chitosan is extracted from the second most abundant polysaccharide chitin from the exoskeletons of crustaceans. Chitosan can be dissolved in acidic conditions below its pKa value of about 6.3, and form a film or hydrogel structure in pH conditions above 6.3 (Figure 6a). With its unique reactive functional groups, gel-forming capability and biocompatibility, chitosan has been widely used in drug delivery and tissue engineering,34, 35 and in biofabrication as a spatiotemporally programmable interface between bio-entities and microdevices.36, 37 Birefringence, quantified as optical retardance, is a direct indication of the molecular alignment of chitosan.38 The molecular alignment of chitosan film conveniently prepared with the anisotropic mass transport in white wood is expected to further expand the applications of chitosan in tissue engineering and microfluidics, where anisotropic fiber alignment of extracellular matrix is highly desirable but difficult to achieve. The white wood and filter paper samples were first pretreated using 1 M NaOH solution to create an alkaline environment, and then the tips of the samples were immersed in a chitosan solution (Figure 6b). Chitosan molecules carried by water can transport upward under capillary forces, and solidify when they are contact with the alkaline samples. At the same time, hydroxyl ions can diffuse from the NaOH-treated samples into chitosan solution, forming a front of gelation that expands into the solution over time. As time went on, solid chitosan films were formed at the tips of the samples (Figures 6c-e). The transmitted light images show that the obtained chitosan films from different samples possess distinctly 16


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different width (distance from the edge of the sample to the edge of chitosan film). As shown in Figure 6f, the maximum width of the chitosan films formed at the tips of the white wood with vertical channels, the white wood with horizontal channels and filter paper are 1069, 97, and 194 µm, respectively. This phenomenon can be ascribed to the different capillary force, hydroxyl ion diffusion, and mass transport speed in different samples. The anisotropic flow in white wood samples results in the alignment of chitosan molecule during the chitosan film formation process, which can be confirmed by the birefringence of chitosan film. Optical retardance Γ can be used to quantify the birefringence,38 and the data is shown in Figure 6g. Higher optical retardance of up to 29.9 nm likely indicates more aligned molecules in the chitosan film obtained from the white wood with vertical channels. In contrast, the optical retardance in the chitosan films obtained from white wood with horizontal channels and filter paper is about 10-12 nm, which is similar to that of pure chitosan film (11 nm) prepared by simple evaporation, rinsing and rewetting. The chitosan film from filter paper is loose and small, and demonstrates the smallest optical retardance, which can be attributed to the isotropic capillary force in filter paper. The formed chitosan film on the white wood with horizontal channels is very small and uniform, and the optical retardance is slightly higher than that of chitosan film on filter paper, which can be explained by the anisotropic mass transport from the small boarded pits visible in the fiber tracheids and vessels by electron microscopy. These obtained results are in line with the previous study38, which demonstrated that the birefringence and microalignment of chitosan film is parallel to the direction of chitosan flow. It is noted that the optical retardance of the chitosan film on the white wood with vertical channels decreases from 29.9 nm to 13.4 nm within the profile range, owing to the decreased capillary force and/or the lower pH in the chitosan film, further from the samples. In summary, chitosan film with aligned microstructure was conveniently formed on the white wood with vertical channels by virtue of the unidirectional capillary forces. The 17



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anisotropic mesoporous framework has great potential to rearrange and align other polymers, suggesting extensive applications in pump-free microfluidics.

Figure 6. Fabrication of chitosan film with aligned microstructures. (a) Gelation reaction of chitosan molecule with hydroxyl ions. (b) Digital image of the experimental setup for chitosan film formation. Schematics (left), transmitted light images of chitosan films at the tip of the strips (middle), and optical retardance of the polarized chitosan films from MATLAB process (right) for (c) filter paper, (d) white wood with vertical channels, and (e) white wood with horizontal channels. (f) The maximum width of the chitosan films formed at the tips of the samples. (g) The optical retardance of the profile plots of chitosan films versus distance from the sample edges to 75 µm away.

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Conclusion In conclusion, an anisotropic mesoporous microfluidic framework (white wood) was developed from natural wood by a facile one-step delignification process. The white wood inherits the vertically aligned cellulose nanofibers and microchannels of the original wood, and can be employed as a pump-free microfluidic chip. The fast and anisotropic liquid and solid mass transport was demonstrated by using DI water and CNT ink as the simulants. The experimental results show that the white wood can efficiently transport large solid particles without the assistance of an external pump, while the counterpart commercial filter paper does not have this capability. On the other hand, the water transport speed in the white wood is much faster than that in the filter paper. In addition, anisotropic mass transport property in white wood block was also demonstrated. The mass transport speed along the channel direction is about 5 times faster than that of the direction perpendicular to the channel. The chitosan film with aligned microstructure and birefringence was prepared by virtue of the unidirectional capillary forces exerted by the microchannels. Therefore, our anisotropic mesoporous framework demonstrates great potential in pump-free microfluidics, and its development will dramatically expand the application range of natural wood.

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Experimental Materials and chemicals Basswood (American basswood) was purchased from Walnut Hollow Company. Filter paper (VWR® Grade 413, Qualitative) was purchased from VWR International. Sodium chlorite (NaClO2, puriss. p.a., 80 %), acetic acid, sodium hydroxide, and chitosan (85% deacetylated, medium molecular weight) were purchased from Sigma-Aldrich, and all these chemicals were used as they are without further treatment. Ethanol and deionized (DI) water were employed to wash the obtained white wood. P3-SWCNT with 1.0-3.0 atomic% carboxylic acid (Carbon Solutions, Inc.) was used for mass transport. The P3-SWCNT possesses a length of 500-1500 nm and a diameter of 4-5 nm. The green dye used was green ink purchased from LAMY. Lignin removal from basswood The NaClO2 solution was prepared by dissolving NaClO2 powder in DI water. The pH value was adjusted by adding acetic acid in the lignin removal solution. The pH value was set as about 4.6 in this study. The wood samples were immersed in the NaClO2 solution and kept boiling without stirring until they became white completely. Afterwards, the obtained white wood was rinsed in ethanol water solution three times to remove the remaining chemicals. Finally, the white wood samples were freeze dried at -51°C. Mass transport experiments Two kinds of mass was used in this study, which are deionized (DI) water and CNT ink. A small quantity of green dye was added in DI water to increase the visual contrast. The CNT ink with a concentration of 0.1 wt% was prepared by dispersing P3-SWCNT in DI water and sonicating using an ultrasonic cleaner (FS110D, Fisher Scientific). Continuous videos were recorded to determine the location of the mass. Each determination was repeated three times to evaluate the variability of the results. 20


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Fabrication of chitosan film with aligned microstructures The white wood and filter paper samples were immersed in 1M NaOH solution for 30 s to create an alkaline environment. Chitosan was dissolved in DI water to obtain the chitosan solution with a concentration of 0.5 % (w/v) and pH of 5.8. The tips of the pretreated samples were put in the chitosan solution and kept for 150 s to obtain the chitosan films. Three chitosan films were obtained for each sample and the averages of the maximum width were presented. Characterizations The morphologies of the original wood and the white wood were characterized using a field emission scanning electron microscopy (FESEM, HITACHI SU-70). The pore diameter distribution and pore area percentage were obtained by analyzing the SEM images of wood top view using Image-Pro Plus software, and the volume fraction of the vessels was estimated from their pore area percentage. BET pore size distribution and surface area were determined by a surface area analyser (Tristar II 3020). The pore volume fraction of the samples was calculated by the following equation: Pore volume fraction (%) = (1 -

( ) (

)

) × 100%, ρ

(sample) denotes sample density (kg m-3), ρ (cell wall) is the cell wall density, which is about 1500 kg m-3.39 To measure water absorption capacity, the front tips of the samples (5 mm × 8 mm) were put in contact with water to initiate capillary force driven water absorption. Water absorption was terminated until water reached the top of the samples (note that different absorption times are expected for white wood with different channel directions). The water absorption capacity was calculated by the following equation: Water absorption capacity (g g-1) =

( ) ( ) ( )

, m (before) is the mass of sample before absorbing water (g), m

(after) is the mass of sample after absorbing water (g). Theoretical water absorption capacity (g g-1) =

!"#$ × (%" ) ( )

, ρ (sample) is sample density (kg m-3), ρ (water) is

water density (1000 kg m-3). The bright field images were obtained with a Nikon Ts100 21



Page 22 of 27

inverted microscope, and the birefringence signals were determined using a transmitted quantitative polarized light microscope (qPLM, MT9900 series, Meiji Techno America, Inc.). The birefringence images were analyzed with MATLAB using a digital image processing adaptation of the de Sénarmont technique,38 and ImageJ was used to extract and plot the quantitative data. The chemical composition analysis of original wood and white wood was performed by the Analytical Chemistry and Microscopy Lab at USDA Forest Products Lab. Supporting Information SEM images of original basswood and filter paper. BET pore size distribution of white wood. Diameter distribution of fiber tracheids and vessels in original wood. Comparison of physical properties for white wood, filter paper, and original wood. CNT and water transport in original wood. Acknowledgements Chao Jia would like to thank the China Scholarship Council (CSC) for its financial support. We acknowledge the help from Dr. J.Y. Zhu at USDA Forest Products Lab with the chemical composition analysis of original wood and white wood. We also acknowledge Xiulei (David) Ji and Zhifei Li at Oregon State University for BET determination. Author contributions L. Hu, C. Jia and F. Jiang designed the experiments and analyzed data. C. Jia, S. He and A. Murphy performed the fabrication processes of white wood. C. Jia, F. Jiang and T. Li carried out the mass transport experiments. P. Hu, X. Luo, and C. Raub performed the preparation and characterization of chitosan films. C. Chen, Y. Yao, and C. Yang did the SEM characterization. Y. Kuang and J. Dai drew the schematics. All authors contributed to the manuscript writing. References (1) Muller, R. H.; Clegg, D. L. Automatic Paper Chromatography. Anal. Chem. 1949, 21, 22


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Anisotropic, Mesoporous Microfluidic Framework with Scalable

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