Microfluidic Fabrication of Bioinspired Cavity-Microfibers for 3D

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Microfluidic Fabrication of Bioinspired Cavity-Microfibers for 3D Scaffolds Ye Tian, Jian-Chun Wang, and Liqiu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09212 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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ACS Applied Materials & Interfaces

Microfluidic Fabrication of Bioinspired Cavity-Microfibers for 3D Scaffolds Ye Tian, † ‡ Jianchun Wang, §and Liqiu Wang*† ‡ †

Department of Mechanical Engineering, the University of Hong Kong, Hong Kong

‡

HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), Hangzhou, Zhejiang, 311300,

China §

Center for Transport Phenomena, Energy Research Institute, Qilu University of Technology

(Shandong Academy of Sciences), Jinan, 250014, China KEYWORDS: microfluidics, microfiber, cavity-microfiber, gas-in-water, cell culture, dehumidifying

ABSTRACT: We present a gas-in-water microfluidic method to precisely fabricate wellcontrolled versatile microfibers with cavity knots (named cavity-microfiber), like tiny-cavitymicrofiber, hybrid-cavity-microfiber, cavity-microfiber and chained microfiber. The cavitymicrofibers are endowed with tunable morphologies, unique surface properties, high specific surface area, assembling ability, flexibility, cytocompatibility and hydroscopicity. We assemble cavity-microfibers as 3D scaffolds for culturing the human umbilical vein endothelial cells (HUVECs) and dehumidifying. The HUVECs on the scaffolds demonstrate good cell viability and 3D HUVECs frameworks, confirming the unique cytocompatibility of cavity-microfiber. And the cavity-microfibers and their scaffolds also demonstrate excellent dehumidifying ability

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and large-scale dehumidifying, respectively. Our cavity-microfiber can offer a broad range of applications in sensor, wearable electronics, dehumidifying, water collection engineering, drug delivery, biomaterials and tissue engineering.


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Fiber-based materials have attracted more and more attentions due to their broad range of applications in diverse fields, including sensors,

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controlled drug delivery,

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wearable

electronics, 4, 5 and tissue engineering.6-9 Bioinspired microfibers, inspired by spider silk, as an emerging functional fiber-based materials have been widely used for water collection engineering10-16 due to perfectly resembling the periodic spindle-knots and joints of water-wetted spider silk. Although many efforts on bioinspired microfiber for water collection have been made, almost only the functions and the water collection application of single bioinspired microfiber have been investigated. The functions and applications of scaffolds assembled by bioinspired microfibers have not been sufficiently studied. Bioinspired microfibers can be generally fabricated by Rayleigh instability methods, 10, 14-17 electrodynamics technology18-22 and microfluidic technology23-26. In Rayleigh instability methods, the polymer film attached to the nylon fiber immediately breaks up into many polymer droplets hanging on the nylon fiber due to the Rayleigh instability of the polymer solution, resulting in formation of the periodic spindle-knots of the fiber after drying the droplets. However, this method is difficult to have control over the morphology of microfiber and highly susceptible to outside influences. With electrodynamics technology, the formation of the beaded fibers can be treated as the capillary breakup of the electrospinning jets by surface tension, altered by the presence of electrical forces from high voltage. Similarly, this method also has limited control over the fiber structure, and it is energy-consuming and extremely dangerous resulting from high-voltage. Microfluidic technology can achieve precise control of small droplets and micro-jets, enabling precisely-tuned bioinspired microfibers. However, the fibers fabricated by traditional oil-in-water microfluidic method only have uniform material composition and surface roughness almost without difference.11 In addition, bioinspired


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microfibers are generally used for water collection, the more functions and applications of bioinspired microfibers, like cell culture and dehumidifying, have not been sufficiently investigated. Therefore, it is urgent demand to develop a new technique to fabricate wellcontrolled bioinspired microfibers with unique surface properties and investigate the functions and applications of microfiber-scaffolds. Here we developed a gas-in-water microfluidic method to fabrication versatile microfibers with cavity knots (named cavity-microfiber). We successfully fabricated tiny-cavity-microfiber, hybrid-cavity-microfiber, cavity-microfiber and chained microfiber with the unique cavity structures in a well-controlled manner. The bioinspired cavity-microfibers with the unique surface and morphology demonstrate precisely-tuned morphologies, unique surface roughness, high specific surface area, assembling ability, flexibility, cytocompatibility and hydroscopicity. Further, we assembled these bioinspired cavity-microfibers fabricated by gas-in-water microfluidic method into different 3D scaffolds. These 3D scaffolds were used for 3D human umbilical vein endothelial cells (HUVECs) culture and dehumidifying. The results demonstrated they possessed unique performance for 3D cell culture and excellent dehumidifying/large-scale dehumidifying ability. Our facial and low-cost technique can offer versatile unique bioinspired microfibers and assembled fiber-scaffolds. Our bioinspired-microfiber-fabrication technique is immediately available to practical applications and offers promising applications in fiber-based sensors, oil-water separation, fluid control, smart functional materials, drug delivery, cell culture, biomaterials and tissue engineering. In our experiments, a capillary-based microfluidic system (Figure 1a) was employed to form gas-in-water templates, consisting of alginate-based composite solution (ABC solution) as the outer phase and nitrogen or air as inner phase. The nitrogen or air (Figure 1a1) was used to


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generate bubbles for cavity formation due to the breakup of gas under the shear of ABC solution. Figure 1a2 shows the partial enlarged detail of internal structure of the microfluidic device, enabling well-controlled generation of gas bubbles (Figure 1a3 and Figure 1a4). Then the ABC solution wrapping

gas bubbles was extruded from the orifice into CaCl2 solution for the

fabrication of microfiber with cavity knots (named hydrated cavity-microfiber) because ABC solution can be crosslinked quickly into gelatinized alginate calcium upon touching with the Ca2+ ions (Figure 1b and Supporting Information Movie 1). Finally, the hydrated cavity-microfiber was dehydrated as ambient temperature: 23-26.5℃ and relative humidity: 57%-60% for more than 1 hour to obtain the water-wetted-spider-silk-like dehydrated cavity-microfiber. During the fabrication process of cavity-microfiber, with the slow increase of gas pressure under a fixed flow rate of outer phase, six different phases (Figure 1c) will appear in the microchannel of microfluidic device which directly determines the morphology of cavitymicrofiber. Firstly, there is no any bubble (Figure 1d) from the beginning of Pr = 0 (Regime 0 in Figure 1c) because the gas pressure is too small. Then, with the increase of gas pressure, tiny bubbles with around 10μm diameter (Figure 1e) can be generated slowly one by one (Regime 1 in Figure 1c). After that, the system reaches an unstable transition stage and one tiny bubble with several big bubbles (Figure 1f) can be generated (Regime 2 in Figure 1c). Next, uniform big bubbles with fixed distance between two bubbles (Figure 1g) are generated one by one (Regime 3 in Figure 1c). With the continuous increase of gas pressure, there would be no distance between two bubbles. At this time, coherent bubbles (Figure 1h) can be continuously produced (Regime 4 in Figure 1c). At last, due to too large gas pressure, a gas jet (Figure 1i) appears (Regime 5 in Figure 1c). Therefore, through tuning the gas pressure, we can obtain versatile microfibers.


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Especially, in Regime 3, the morphology of cavity-microfiber is highly affected by the gas pressure of inner phase with a fixed flow rate of outer phase. With the increase of gas pressure, the volume of gas bubble, knot volume of hydrated cavity-microfiber and knot volume of dehydrated cavity-microfiber are increasing linearly, respectively (Figure S1 and Figure S2). Generally, the knot volume of hydrated cavity-microfiber is larger than the volume of gas bubble and the knot volume of dehydrated cavity-microfiber. When gas pressure is smaller, the knot volume of dehydrated cavity-microfiber is larger than the volume of gas bubble but less than the knot volume of hydrated cavity-microfiber. However, with the increase of gas pressure, the volume of gas bubble will become larger than the knot volume of dehydrated cavity-microfiber but less than the knot volume of hydrated cavity-microfiber. Moreover, the gas pressure of inner phase also has an important effect on the distances between two bubbles, two knots of hydrated cavity-microfiber and two knots of dehydrated cavity-microfiber (Figure S3), respectively. With the increase of gas pressure, the distances between two bubbles, two knots of hydrated cavitymicrofiber and two knots of dehydrated cavity-microfiber gradually decrease (Figure S4 and Figure S5). Generally, the distance between two bubbles is less than the distance between two knots of hydrated cavity-microfiber. And the distance between two knots of hydrated cavitymicrofiber is less than the distance between two knots of dehydrated cavity-microfiber. In addition, the drying time of cavity-microfiber also plays an important role in the morphology of cavity-microfiber. Firstly, we investigated the effect of drying time on the diameter of joint part of cavity-microfiber (Figure S6). At the beginning of drying, the diameter of joint part of hydrated cavity-microfiber decreased rapidly and almost linearly (from 408.3μm to 158.1μm) due to the rapid loss of moisture in the radial direction of cavity-microfiber. After that, the cavity-microfiber was dried completely and the diameter of joint part of cavity-


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microfiber can be fixed. Meanwhile, the major axis and minor axis of knot of cavity-microfiber were changing during the drying process as shown in Figure S7 and Figure S8, respectively. Drying process can make the cavity-microfiber elongated in the axial direction and shrunken in the radial direction. Therefore, the major axis of knot of cavity-microfiber can be elongated a lot (from 327.3μm to 540.0μm) during the drying process (Figure S7) and the minor axis of knot of cavity-microfiber also has slightly decrease (from 417.8μm to 316.6μm) in the radial direction with the loss of moisture (Figure S8). The resultant cavity-microfibers we fabricated are shown in Figure 2. Optical microscope images show the normal hydrated cylindrical microfiber for Regime 0 (Figure 2a), the hydrated tiny-cavity microfiber for Regime 1 (Figure 2b), the hydrated hybrid-cavity microfiber for Regime 2 (Figure 2c), the hydrated cavity-microfiber for Regime 3 (Figure 2d) and versatile hydrated chained microfibers for Regime 4 (Figure 2e-h) (named them depending on the type of cavity in the microfibers). Due to uncontrollable gas in Regime 5, there is no any fiber fabricated. The resultant dehydrated cavity-microfibers are shown in Figure 2 i-l, which have periodic spindle-knots and joints, resembling the structure of water-wetted spider silk. The SEM images (Figure 2m-n) show cavity-microfibers on the micro-scale. Figure 2m and Figure 2n exhibit the fine overall structures of two cavity-microfibers with periodic spindle knots and joints. Interestingly, the joint of cavity-microfiber exhibits the sparse transverse cracking surface (Figure 2n1), while the knot of cavity-microfiber displays high-density alligator cracking surface (Figure 2n2 and Figure 2n3), which results in that the surface roughness of knot is much larger than that of joint. This intriguing surface feature results in that the knot part is more hydrophilic than the joint part, facilitating the water droplet towards spindle knot. Figure 2o and Figure 2p demonstrate the cavity structure of knot of cavity-microfiber and the solid cross section of joint


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part of cavity-microfiber, respectively. These cavity-microfibers can be fabricated by the gas-inwater microfluidic method in large quantities and collected for storage and further use (Figure 2q). The cavity-microfibers have excellent performance to assemble into different 3D scaffolds due to their assembling ability and flexibility. To confirm this, we assembled cavity-microfibers into wooden-raft-like scaffold (Figure 3a), cylindrical scaffold (Figure 3b left), conical scaffold (Figure 3b middle), hexagonal-prisms-like scaffold (Figure 3b right), spherical scaffold (Figure 3c), grid-like scaffold (Figure 3d) and spider-web-like scaffold (Figure 3e). These versatile 3D scaffolds demonstrate the assembling ability and flexibility of cavity-microfibers and enable the broad range of applications in different fields. In cell culture and tissue engineering fields, 3D scaffolds are widely used. To investigate the cytocompatibility of cavity-microfibers and their scaffolds, at an alternative, wooden-raft-like scaffold was used to culture the human umbilical vein endothelial cells (HUVECs). We seeded uniformly the HUVECs on the sterilized and Poly-L-Lysine modified wooden-raft-like scaffold. After cell culture continuously, the HUVECs attached and grew on the cavity-microfibers well. Optical image shows the HUVECs on the cavity-microfiber scaffold (Figure 3f). Meanwhile, the fluorescence microscopic images of HUVECs stained by Calcein-AM on the cavity-microfiber scaffold were also investigated to demonstrate the good cell viability (Figure 3g-h). Live/dead staining of HUVECs on the cavity-microfiber scaffold was also explored to show the high cell viability (Figure 3i-k). Moreover, HUVECs stained by Alexa FluorTM 488 Phalloidin and DAPI on the cavity-microfiber scaffold were also investigated to show the 3D HUVECs frameworks and good cell status (Figure 3l-n). Fluorescence microscopic magnified images confirmed the good cell status further (Figure 3o-q). In addition, the 3D reconstruction images from confocal


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microscope show the semicircular 3D HUVECs framework (Figure 3r), the single-hollow 3D HUVECs framework (Figure 3s) and triple-hollow 3D HUVECs framework (Figure 3t), respectively, confirming the good performance of 3D cell culture on the cavity-microfiber scaffold. These results indicate the cavity-microfiber scaffold provided a real 3D space for HUVECs growth. Meanwhile, the cavity-knot structure of cavity-microfiber offers not only the precise positioning but also the storage of cell growth medium for cell culture. These results enable the cavity-microfibers and their scaffolds to have wide applications in 3D cell culture, biomaterials, tissue engineering and biomedical engineering. Finally, we also investigated the dehumidifying ability of cavity-microfibers and their scaffolds. The cavity-microfibers and spider-network-like scaffold were stuck on home-made frames in an airtight container with desired relative humidity (RH) for dehumidifying, respectively. First, we selected the dehydrated cavity-microfiber with the length of major axis, 428.513μm, the length of minor axis, 261.011μm, and the distance between two neighboring knots, 966.916μm (Figure 4a) for dehumidifying test. The dehumidified water volume was controlled by the length of cavity-microfiber. We varied the length of cavity-microfiber from 4cm to 12cm within 1min under RH=99%, the volume of dehumidified water increased from ~0.214μL to ~0.602μL (Figure 4b). Similarly, the dehumidified water volume was also dependent on the dehumidifying time. Varying the dehumidifying time from 30s to 150s with the length of cavity-microfiber, 8cm, and RH=87%, the volume of dehumidified water increased from ~ 0.154μL to ~0.715μL (Figure 4c). Meanwhile, we also investigated the relationship between the dehumidified water volume and the length of cavity-microfiber and dehumidifying time under the same RH=92%, respectively. The results demonstrated the dehumidified water volume was increased from ~ 0.174μL to ~0.558μL when we varied the length of cavity-


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microfiber from 4cm to 12cm within 1min (Figure 4d) and ~ 0.174μL to ~0.742μL when we varied the dehumidifying time from 30s to 150s with the length of cavity-microfiber, 8cm (Figure 4e), showing the similar trend to Figure 4b and Figure 4c, respectively. Further, we also investigated the saturation dehumidifying ability of the cavity-microfiber, ~983±50μL g-1 (that means each gram of cavity-microfiber can dehumidify ~983±50μL). We also compared the dehumidifying performance of microfibers with different structures (Figure 4f). Because the knot-joint structure and the unique surface properties of cavity-microfibers resulted in the high specific surface area and strong water mist adhesion, the cavity-microfiber demonstrated better dehumidifying performance than that of cylindrical microfiber, tiny-cavity microfiber and hybrid-cavity microfiber. Due to the robustness of cavity-microfiber, the cavity-microfiber can keep the knot structure and dehumidify for cycles. Figure 4g shows that the similar volume of dehumidified water in different dehumidifying cycles and the cavity-microfiber still maintains the similar structure after 6 dehumidifying cycles, enabling the repeated dehumidifying process over time. To achieve the large-scale dehumidifying, at an alternative, we assembled a spiderweb-like 3D scaffold of ~105cm-cumulative-length with four radius cavity-microfibers (Figure S9). The results demonstrated that the volume of dehumidified water of the spider-web-like 3D scaffold increased from ~2.134μL to ~9.645μL with the increase of dehumidifying time from 30s to 150s under the RH=92% (Figure 4h). The above results indicate that the cavity-microfibers have unique dehumidifying ability and the cavity-microfiber scaffolds can be capable of the large-scale dehumidifying. In conclusion, we fabricated versatile cavity-microfibers with the unique cavity structures via gas-in-water microfluidic method, including tiny-cavity-microfiber, hybrid-cavity-microfiber, cavity-microfiber and chained microfiber with a well-controlled manner. The cavity-microfibers


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are endowed with precisely-tuned morphologies, unique surface roughness, high specific surface area, assembling ability, flexibility, cytocompatibility and hydroscopicity. Moreover, we assembled cavity-microfibers we fabricated into different 3D scaffolds. These 3D scaffolds were used for 3D human umbilical vein endothelial cells (HUVECs) culture and dehumidifying. The results indicate the HUVECs on the scaffolds have good cell viability and form the real 3D HUVECs frameworks, confirming the unique cytocompatibility of cavity-microfiber. The cavitymicrofibers have excellent dehumidifying ability and cavity-microfiber scaffolds can be capable of large-scale dehumidifying. Our facial and low-cost technique can offer versatile unique bioinspired microfibers and assembled fiber-scaffolds. Our bioinspired cavity-microfiber fabrication technique is immediately available to practical applications and offers promising applications in fiber-based sensors, wearable electronics, oil-water separation, fluid control, smart functional materials, drug delivery, cell culture, biomaterials and tissue engineering. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami……. The following files are available free of charge. Methods: Materials, fabrication of gas-in-water microfluidic device, fabrication of cavitymicrofibers via gas-in-water microfluidic method, characterization of cavity-microfiber morphology, cell culture on cavity-microfiber scaffolds and dehumidifying using cavitymicrofiber scaffolds (PDF).


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Figures: Effects of gas pressure on bubbles and knots under flow rate of outer phase 1 mL h-1, effects of gas pressure on bubbles and knots under flow rate of outer phase 1.4 mL h-1, diagram of three distance parameters, effects of gas pressure on distances of bubbles and knots under flow rate of outer phase 1 mL h-1, effects of gas pressure on distances of bubbles and knots under flow rate of outer phase 1.4 mL h-1, the effect of drying time on the diameter of joint part of cavitymicrofiber, the effect of drying time on the major axis of knot of cavity-microfiber, the effect of drying time on the minor axis of knot of cavity-microfiber, spider-web-like 3D scaffold for large-scale dehumidifying and relationship between the shear rate and viscosity of ABC solution (PDF). Notes: The difference between water collection and dehumidifying of bioinspired microfiber, the reason for the formation of two different surfaces of joint and knot (PDF). Movies: The fabrication of cavity-microfiber via gas-in-water microfluidic method (AVI). AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions Y.T. and L.W. designed the project. Y.T. performed the experiments and analyzed the experimental data. Y.T. and J.W performed bio-experiments. Y.T. and L.W. wrote the manuscript. L.W. supervised the study. All authors commented on the paper. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The financial support from the Research Grants Council of Hong Kong (GRF 17237316, 17211115, 17207914 and 717613E), the University of Hong Kong (URC 201511159108, 201411159074 and 201311159187), the pilot project scheme and basic research grant (2015.42017.4) of Shandong Academy of Sciences, Shandong Province Natural Science Foundation (No.ZR2016BB15) and the Youth Science Fund of Shandong Academy of Sciences (No.2016QN006) is gratefully acknowledged. This work was also supported in part by the Zhejiang Provincial, Hangzhou Municipal and Lin’an County Governments.

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Figure 1. Fabrication of cavity-microfibers via gas-in-water microfluidic method. (a) Schematic diagram of gas-in-water microfluidic system: black: gas; gray: cylindrical glass capillary; brown: ABC solution; red: square glass capillary. (a1) Cross-section view of A-A cross section; (a2) partial enlarged detail of internal structure of the microfluidic device; (a3) schematic diagram of gas bubbles in the microfluidic device; (a4) Cross-section view of B-B cross section. (b) The optical image showing the fabrication of cavity-microfiber. (c) Phase diagram showing the different regimes during fabrication of cavity-microfiber: Pr = the actual pressure : a standard atmosphere; Cao is capillary number of outer phase in microfluidic system. (d) No bubble corresponding to Regime 0. (e) Tiny bubbles corresponding to Regime 1. (f) Hybrid bubbles corresponding to Regime 2. (g) Uniform bubbles corresponding to Regime 3. (h) Coherent bubbles corresponding to Regime 4. (i) Gas jet corresponding to Regime 5. Scale bars: 100μm.


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Figure 2. Morphology of cavity-microfibers. Optical microscope images showing (a) normal hydrated cylindrical microfiber for Regime 0; (b) hydrated tiny-cavity microfiber for Regime 1; (c) hydrated hybrid-cavity microfiber for Regime 2; (d) hydrated cavity-microfiber for Regime 3; (e-h) versatile types of hydrated chained microfibers for Regime 4; (i-l) different dehydrated cavity-microfibers. SEM images showing (m-n) two dehydrated cavity-microfibers; (n1) partial enlarged detail of joint part of cavity-microfiber corresponding to (n); (n2) partial enlarged detail of middle knot part of cavity-microfiber corresponding to (n); (n3) partial enlarged detail of right knot part of cavity-microfiber corresponding to (n); (o) the hollow cavity of the knot; (p) the solid cross section of the joint part of cavity-microfiber. (q) The collected dehydrated cavitymicrofibers in large quantities. Scale bars: 100μm in (a-l), 250μm in (m), 350μm in (n), 30μm in (n1-n3), 50μm in (o-p) and 10mm in (q).


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Figure 3. 3D scaffolds and cell culture. (a-e) Optical images showing different 3D scaffolds assembled using cavity-microfibers. (f) Optical images showing the HUVECs attached on the cavity-microfiber scaffold. (g) Fluorescence microscopic images of HUVECs attached on the cavity-microfiber scaffold, Green: Calcein-AM staining to show cell viability corresponding to (f). (h) The superimposed image of (f) and (g). Fluorescence microscopic images showing live/dead staining for HUVECs (Green: Calcein-AM, Red: PI): (i) Calcein-AM staining, (j) PI staining and (k) the superimposed image of (i) and (j). Fluorescence microscopic images showing (l) ALEXA 488 Phalloidin (green), (m) DAPI (blue) staining and (n) the superimposed image to show the 3D HUVECs frameworks and cell status cultured on cavity-microfiber scaffolds. Fluorescence microscopic magnified images showing (o) ALEXA 488 Phalloidin (green), (p) DAPI (blue) staining and (q) the superimposed image to confirm the good cell status. (r) The superimposed image of ALEXA 488 Phalloidin (green) and DAPI (blue) staining to show the semicircular 3D HUVECs framework. (s-t) Calcein-AM staining to show the single-hollow and triple-hollow 3D HUVECs framework, respectively. Scale bars: 10mm in (a), 6mm in (b), 5mm in (c-e), 100μm in (f-k, r-t), 200μm in (l-n) and 50μm in (o-q).


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Figure 4. Dehumidifying of cavity-microfibers. (a) The cavity-microfiber used for dehumidifying. (b) A plot of the volume of dehumidified water against the length of cavitymicrofiber (time: 1min, RH: 99%, R=0.98275). (c) A plot of the volume of dehumidified water against the dehumidifying time (length of cavity-microfiber: 8cm, RH: 87%, R=0.99362). (d) A plot of the volume of dehumidified water against the length of cavity-microfiber (time: 1min, RH: 92%, R=0.98691). (e) A plot of the volume of dehumidified water against the dehumidifying time (length of cavity-microfiber: 8cm, RH: 92%, R=0.99056). (f) The performance of dehumidifying of different microfibers under the same conditions (length of cavity-microfiber: 4cm, RH: 92%, time: 1min). (g) A plot of the volume of dehumidified water against the dehumidifying cycle (length of cavity-microfiber: 5cm, time: 1min, RH: 90%), insets showing that the cavity-microfibers maintain their shapes after dehumidifying cycles. (h) A plot of the volume of dehumidified water against the dehumidifying time for cavity-microfiber-network (total length of cavity-microfiber: ~105cm, RH: 92%, R=0.99892). R means the coefficient of determination. All error bars indicate the standard deviations over three independent measurements. Scale bars: 300μm in (a, g).


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Microfluidic Fabrication of Bioinspired Cavity-Microfibers for 3D

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