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Flexible topological liquid diode catheter
Journal Pre-proof Flexible topological liquid diode catheter Jiaqian Li, Huanxi Zheng, Xiaofeng Zhou, Chao Zhang, Minjie Liu, Zuankai Wang PII:
S2542-5293(19)30150-6
DOI:
https://doi.org/10.1016/j.mtphys.2019.100170
Reference:
MTPHYS 100170
To appear in:
Materials Today Physics
Received Date: 27 September 2019 Revised Date:
25 November 2019
Accepted Date: 26 November 2019
Please cite this article as: J. Li, H. Zheng, X. Zhou, C. Zhang, M. Liu, Z. Wang, Flexible topological liquid diode catheter, Materials Today Physics, https://doi.org/10.1016/j.mtphys.2019.100170. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Flexible topological liquid diode catheter Jiaqian Li1, Huanxi Zheng1, Xiaofeng Zhou2, Chao Zhang1, Minjie Liu1, Zuankai Wang1,3* 1
Department of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, China.
2
Shanghai Key Laboratory of Multidimensional Information Processing, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China.
3
Shenzhen Research Institute of City University of Hong Kong, Shenzhen 518057, China.
Corresponding author: [email protected]
Abstract: Spontaneous and directional transport of various liquids in open and closed spaces are of great importance in various energy, water and medical applications. Despite extensive progress in engineering diverse open surfaces to rectify liquid transport, the implementation of the diode-like liquid transport in a closed system is still rare. In this study, we present the design of flexible liquid diode catheters internally decorated with asymmetric reentrant structure that allows for the efficient and unidirectional transport of diverse liquids. We also elucidate that the hydrodynamic transport behaviors in the closed diode catheters are primarily dominated by the interplay between the surface effect and the bulk effect, which are distinctly different from those on open liquid diodes. The flexible liquid diode catheters can preserve their directional transport behaviors even under mechanical bending or twisting, and under various liquids from low-surface-tension liquids, medical solutions to lubricant oils. The combination of extraordinary flexibility, durability and stability promises the development of liquid transport devices that can work in various configurations and applications.
Keywords: flexible catheter, liquid diode, liquid transport, soft replication, surface effect
1
Introduction Spontaneous and directional liquid transport in closed catheters, tubes or conduits is a ubiquitous phenomenon in nature as exemplified by blood vessels and tree xylem, and also plays an important role in a hybrid of industrial applications ranging from microfluidics, advanced printing, wearable electronics to biomedical devices[1-4]. Although various flexible catheters have been rapidly developed for liquid transportation, directional liquid transport in these flexible catheters generally requires auxiliary control devices, such as pumps and valves. To eliminate the requirement of additional pumps or valves, surfaces with responsive properties have been engineered to leverage the external stimuli to trigger liquid motion. For example, one feasible approach has been explored by the design of photo-responsive multilayer catheters to control the motion of liquid slugs through the formation of curvature gradient induced by photo-induced asymmetric deformation of the tube[5,6]. However, these approaches are still limited by the need of external energy or the proper selection of stimuliresponsive materials, which in turn restrict their wide applications. Therefore, the development of novel flexible catheters that enable the passive, efficient and directional liquid transport remains a long-standing challenge. Over the past several years, engineering open surfaces to achieve directional liquid transport has flourished[7-16], especially inspired by numerous biological examples[17-27]. These open surfaces, termed as liquid diodes, mainly take advantage of wettability gradient, structural curvature gradient, asymmetric capillary or their combinations[28-45] to promote liquid flow in a single direction and meanwhile preventing its backflow. In spite of exciting progress, it is difficult to directly translate these features responsible for directional liquid transport in open systems such as structures, curvatures, shapes, and roughness into the closed systems owing to the spatial confinement. For example, even if the emerging 3D printing 2
technology have extended our fabrication capability of flexible materials[46-48], it is still challenging to pattern asymmetric nano- and micro-structures such as mushroom[49,50] and sharp edges[51,52] that are vital to the pinning of the backflow, inside the closed catheter. Previously, we have successfully developed a topological liquid diode that enables the spontaneous, fast, directional transport of various liquids[30]. In spite of many advantages, such a directional liquid transport is limited to the open surface as well as hard silicon material. So far, it is difficult to reproduce the complicated and fragile reentrant structures on the flexible materials. Such disadvantages greatly restrict the wide applications of liquid diodes in various settings, especially for flexible medical catheter application. Herein we report the design and fabrication of flexible topological catheters decorated with reentrant structure that allows for the continuous and directional liquid transport in a closed space. Moreover, the flexible liquid diode catheter can preserve its transport behavior even under mechanical bending or twisting, as well as under various liquids from low-surface-tension liquids, medical solutions to lubricant oils.
Material and methods Sample fabrication. We have developed the topological liquid diode on the silicon wafer via a standard Microelectromechanical System (MEMS) manufacturing technique as reported in our previous work[30]. To fabricate soft diode membrane, we utilized a simple two-step molding-casting method to transfer the asymmetric microstructures from the silicon liquid diode
to
soft
material.
The
silicon
liquid
diode
was
firstly
silanized
by
trichloro(1H,1H,2H,2H-perfluorooctyl)silane in a 1mM n-hexane solution, followed by heat treatment at ∼120 ºC in air for one hour to render it hydrophobic. The polydimethylsiloxane (PDMS, DC SYLGARD™ 184) and curing agent was firstly mixed at a mass ratio of 10:1, then stirred for 10 minutes, and finally vacuumed for 1 hour to remove internal gas before 3
casting onto the silicon liquid diode. In order to peel negative PMDS mold out from the cavity with reentrant structure, we cured the PDMS with heating treatment at 60 ºC for ~30 minutes, resulting in soft PDMS mold in contrast to high-temperature curing. The negative PDMS mold was then completely cured by heating at 80 ºC for 2 hours in an oven. Subsequently, the negative PDMS mold was silanized with vacuum treatment for 12 hours at room temperature. The soft UV resin (viscosity 3500-5000 cps at 25 ºC) was directly casted on the negative PDMS mold in the photolithography room before vacuuming gas for 10 minutes at 40 ºC in a vacuum oven. The sample was then completely cured by a UV light (50 W) for 4 minutes. Then, we obtained soft liquid diode membrane after peeling the cured resin out from the PDMS mold. Finally, we bent soft UV resin diode membrane into a medical PVC catheter to fabricate the new flexible liquid diode catheter. Surface characterization. All soft diode membranes and catheters were characterized by Scanning Electron Microscope (SEM, QuantaTM 450 FEG) after coating a thin gold layer (about 20 nm) on the samples with the Quorum Q150TS Sputter coater. Surface treatment and contact angle measurement. To create the hydrophilic wetting property, we treated the diode membrane and catheter with Plasma Cleaner PDC-32G (Harrick Plasma limited.) at high RF level for about 2 minutes. To obtain the hydrophobic wetting property, the diode membrane and catheter were modified via the vacuum deposition of trichloro(1H,1H,2H,2H-perfluorooctyl)silane for 5 hours inside a fume hood. After hydrophilic and hydrophobic modification, the apparent contact angles on the original, hydrophilic and hydrophobic membranes were measured by ramé-hart M200 Standard Contact Angle Goniometer at room temperature with 50% relative humidity. We tested five sets of the contact angles and calculated their average results and error margins. Liquid transport experiments. We conducted the experiments of liquid transport on the diode membranes and within the diode catheters at room temperature. To improve the visual clarity and the resolution of liquid transport, we stained the deionized water with a blue dye. 4
In our experiments, the infusion rate of the liquid was precisely controlled by a syringe pump (KDScientific Model 200 Series). The transport behaviors of various liquids were observed and recorded by the Nikon D5200 digital camera connected with a Nikon micro-lens.
Results and discussion In this work, we designed the flexible liquid diode catheter based on two polymer materials, with the polyvinyl chloride (PVC) as the supporting outer layer and the ultraviolet (UV) resin diode membrane as the inner layer. To construct the soft and flexible diode catheter with hydrophilic property, we first implemented a three-step process to fabricate the UV inner layer. First, we designed and fabricated a silicon-based liquid diode template, which consists of tiny capillary channels and reentrant structures[30]. Then, we transferred these tiny structures from the silicon template into the PDMS by using a modified soft-curing process (Fig. S1). By curing the PDMS at a relatively lower temperature than the conventional curing process, the PDMS mold is soft enough to being peeled out, allowing for the successful replication of even tiny structures. Subsequently, UV resin-based flexible liquid diode membrane was duplicated from the PDMS mold. The as-fabricated diode membrane can be bent or twisted into various configurations, exhibiting an extraordinary flexibility (Fig. S2). Eventually, the liquid diode catheter was engineered by bending the liquid diode membrane into the PVC catheter as schematized in Fig. S3. Figure 1a shows the cross-section SEM image of the as-fabricated two-layer liquid diode catheter. To validate whether the reentrant structure in the silicon template is successfully translated into the UV material, we compared the specific microstructures in both materials. As shown in Figure 1b and 1c, both the reentrant structures and capillary channels are exquisitely replicated from the silicon template (Fig. S4), demonstrating the efficacy of our soft-curing process. Remarkably, the liquid diode catheter yields superior robustness 5
under mechanical bending or twisting, thus making it possible to construct different configurations as depicted in Figure 1d. We first compare the liquid transport in the catheters with and without the texture under a continuous liquid pumping. To clearly visualize the transport behavior of water, we stained the water with a blue dye. As shown in Figure 2a, when the stained water is infused into the diode catheter at a volumetric rate Q ≈ 5 µl/s, it first spreads along the inner wall of the catheter, and then fills the catheter soon at the time tc ≈ 10 s. After this initial stage, liquid diode catheter delivers the water toward the direction entering the cavity, defined as spreading direction, and stably blocking the flow in the reverse direction at the same time (Movie S1). By contrast, the original smooth PVC catheter (Fig. S5) prefers to transport the continuous water bi-directionally in two directions without the rectification effect as shown in Figure 2b and Movie S2. Thus, we can conclude that the closed catheter with the asymmetric topography can achieve the directional liquid transport. To elucidate how the asymmetric topography regulates the transport behavior, we further analyze the retention force within the smooth catheter and liquid diode catheter[53,54], respectively. For the smooth cylindrical catheter, the retention force (F0) impeding the motion of the fluid acts equally in both directions (Figure 3a), resulting in a typical bidirectional liquid transport (Fig. 2b). In contrast, the retention forces in the case of the structured diode catheter become direction-dependent owing to the presence of asymmetric reentrant structures as schematized in Figures 3b and 3c. In the spreading direction, the liquid front locates on the top surface (Fig. 3b, ω = 0) or the sidewall (Fig. 3b, ω = π/2) of U-shaped pillars. Thus, the retention force in the spreading direction can be expressed as Fs = max { Fω =0 , Fω =π 2 } , where 1 1 Fω = 2π Dγε sin (θ R 0 + θ A0 ) sin ω + ∆θ 0 2 2
6
is the retention force derived by Extrand’s geometric model[53] in the range of 0 ≤ ω ≤ π/2, D is the inner diameter of the catheter, γ is the surface tension of the liquid, ε is the surface roughness, ω is the rising angle of the surface, and ∆θ0 = θ A0 − θ R 0 is the contact angle hysteresis. Distinct from the spreading direction, the liquid front can contact the bottom surface of the reentrant structures in the pinning direction, exhibiting three possibilities as schematically shown in Fig. 3c. As a result, the retention force in the pinning direction can be expressed as Fp = max {Fω =0 , Fω =π 2 , Fω =π } , where Fω=0 and Fω=π/2 can be calculated by the above Extrand’s model[53,54], and
Fω =π = π Dεγ ( cos θ R 0 + 1) . Based on the above analytical analysis, for the diode catheter as shown in Fig. 2a, Fp is 52% larger than Fs, which explains why there is a directional liquid spreading in the hydrophilic liquid diode catheter. To quantify the capability of directional liquid transport in the liquid diode catheter, we define the ratio of the spreading length Ls in the spread direction relative to that in the pinning direction Lp (inset image in Figure 4a) as the rectification coefficient, k. Figure 4a presents the variation of k in the diode catheters with different wetting properties (Fig. S6). It is clearly observed that the hydrophilic liquid diode catheter has a larger k compared with the hydrophobic diode catheter. For the hydrophobic surface, the retention force[55] in the pinning direction (Fig. S7) can be expressed as Fp = π Dγε ( cos θ R 0 + 1) .
With the increase in the intrinsic contact angle, the retention force in the pinning direction on the hydrophobic surface (θ ≈138º) is approximately twice smaller than that on the hydrophilic surface (θ ≈56º). As a result, the backflow of liquid in the hydrophilic diode catheter can be fully suppressed as shown in Fig. 2a and Fig. S8. In contrast, there is a relatively strong 7
backflow in the reverse direction within the hydrophobic diode catheter (Fig. S9), leading to a small k as shown in Fig. 4a. Such a phenomenon in the closed system is also consistent with the spreading behaviors of the water in the open system, in which the hydrophilic membrane can stably arrest the backflow (Fig. S10) whereas the hydrophobic membrane slowly delivers the water flow along the pinning direction (Fig. S11). Distinct from capillary-induced liquid transport on the diode membrane, we also find that the liquid transport velocity is almost independent on the wettability of the inner surface of the diode catheter (Fig. 4b), for example, the average velocities are about 0.6 mm/s when Q ≈ 5.0 µl/s regardless of the wetting properties of the diode catheters. Distinct from the open-space surface on which the directional liquid transport originates from the combination of the surface topography and wettability[8-11,13,15,16], the preferential confinement of a local thin liquid layer in the inner surface of catheter is also dependent on the global liquid flow. To verify it, we fabricated diode catheters with various inner diameters ranging from 1 to 3 mm, and carried out liquid transport experiments at the same flow rate. All these diode catheters are capable of delivering the liquid in single direction (Fig. S12). With the decrease of catheter size, the surface effect gradually dominates the bulk effect and dictates the liquid transporting in the liquid diode catheter, rendering a better directional liquid transport as evidenced by the rectification coefficient and delivery speed as shown in Figs. 4c and 4d. Specifically, compared with the larger catheter, smaller catheter yields a longer transport distance in the spreading direction and a shorter distance in the reverse direction, resulting in a lager k as shown in Fig. 4c. Similarly, the increase of h/D also gives rise to the increase of the delivery speed of liquid in the diode catheters (Fig. 4d). The transport velocity increases from 0.42 mm/s to 1.34 mm/s as h/D increases when Q ≈ 5.0 µl/s. As a consequence, smaller liquid diode catheter exhibits the superior directional transport of the water at a higher efficiency and fidelity by leveraging of the surface effect. 8
Figure 4e shows the time-resolved evolution of spreading distance Ls in the diode catheter and smooth catheter. The infused liquid initially spreads along the wall of the catheter and then fills the catheter (Figs. 2a and 2b). After the flow in the catheter fully develops, the transport distance in the spreading direction varies almost linearly over time in the liquid diode catheter (Fig. 4e, orange line), which is the same to the linear spreading behavior (Fig. 4e, green line) in the smooth catheter. Through comparing the slopes of these lines, it is found that the delivery speed of the liquid in the diode catheter (vdiode ≈ 0.85 mm/s) is much larger than that in the smooth catheter (vsmooth ≈ 0.25 mm/s), evidencing that surface effect facilitates the directional liquid delivery in the diode catheter in contrast to the smooth catheter. The manifestation of superior and directional transport dictated by the surface effect in the diode catheter requires the delicate control of the water infusion. Figure 4f shows the variation of transport velocity in different catheters under a function of the volumetric rate Q. Obviously, v varies linearly with Q, but consists of two different regions separated by a critical volumetric rate Qc ≈ 15 µl/s. The varying slope between v and Q in the region of Q < Qc (Fig. 4f, red dash line) is slightly larger than that in the range of Q > Qc (Fig. 4f, black dash line). Such a small decrease originates from the breakdown of surface or pinning effect in the reverse direction, a case where the bulk effect overcomes the hysteresis force induced by reentrant structure. In addition, the interplay between surface effect and bulk effect also regulates the transport state of the liquid in the diode catheter as displayed in Figure 5a. When the water is slowly injected into the diode catheter Q < Qc, the surface effect outweighs the bulk effect to enable the typical trait of unidirectional liquid transport (Fig. 5a, blue area). Beyond this range, the liquid transport behavior in the diode catheter transitions into the bidirectional regime as a result of the dominant role of the bulk effect over the surface effect. In this state, the infused water starts to spread in the reverse direction (inset image in Fig. 5a) with a smaller velocity than that in the spreading direction. 9
We continue to perform the liquid transport experiments under bending and twisting conditions to examine the durability and stability of our liquid diode catheter. As shown in Figures 5b and 5c, in each bending or twisting cycle, the flexible liquid diode catheter was first bent or twisted, and then recovered to the straight shape. As shown in Figure 5b, after one hundred bending cycles, there is no notable degradation in the magnitude of k, suggesting the remarkable durability and stability of the directional liquid transport. Similarly, the magnitude of k also remains stable in the twisting test as depicted in Figure 5c. In contrast to the hard liquid diode, such as silicon, soft liquid diode enables the more stable characteristic of directional liquid transport in the long-term operations, making it more applicable in various applications and configurations. The diode-like liquid transport in closed catheters is also applicable to various liquids. As shown in Figure 5d, liquid diode catheter is capable of transporting the low-surfacetension liquids such as hexane and ethanol although with the lower rectification coefficients compared with the deionized water. Second, liquid diode catheter can also transport medical solutions including 5% glucose solution and 0.9% saline solution in single direction at the higher fidelity in contrast to other tested liquids. These results indicate the promising applications of liquid diode catheter in the medicine area, such as preventing the infusion reflux of medical solutions and body fluids. Furthermore, liquid diode catheter also shows the directional transport of the lubricant oils including mineral oil, Krytox 103 and hydroxy terminated Polydimethylsiloxanes (hydroxy PDMS with viscosity ~ 65cSt and ~ 25cSt). Such a unique characteristic imparts superior functions including anti-fouling and anti-friction.
Conclusion We have reported a simple soft molding-casting method to prepare flexible topological catheter that allows for the spontaneous, rapid and directional liquid transport. Through the 10
analysis of the retention forces in two directions, we developed a theoretical model to interpret the observed behavior of unidirectional liquid transport within the liquid diode catheter. We demonstrated that the transport of the liquid within the liquid diode catheter is primarily regulated by the interplay between the surface effect and the bulk effect, distinct from capillary-induced liquid transport as manifested on the diode membrane. We also elucidated the durability and stability of the liquid diode catheter in the long-term bending and twisting operations, as well as in the transport of diverse liquids including low-surface-tension liquids, medical solutions and lubricant oils. This work advances our fundamental understanding in the hydrodynamics of liquid transport in closed systems, and provides important routes for the development of liquid transport devices that can work in various configurations and applications.
Acknowledgements Z.W. is grateful for financial support from Research Grants Council of Hong Kong (Grants No. 11275216 and 11213915), the Research Grants Council of Hong Kong under Collaborative Research Fund (No. C1018-17G), and City University of Hong Kong (No. 9360140, No.9667139). Z. X. acknowledges the financial support from National Natural Science Foundation of China (No. 51975215). The authors declare no competing interests.
Appendix A. Supplementary data Supplementary data to this article can be found online at ……
Data availability
11
The data that support the plots within this paper and other findings of this study are available in the main text and in the Supplementary Information. Additional information is available from the authors upon reasonable request.
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Figure 1. Flexible topological liquid diode catheter. (a) Scanning electron microscope (SEM) image showing the cross section of liquid diode catheter. The diode catheter is made of two layers, the inner is the UV diode membrane and the outer is the supporting PVC catheter. (b) SEM image of liquid diode membrane consisting of U-shaped island arrays distributed in the parallel fences. (c) Magnified SEM image of U-shaped island and reentrant structures. Here h is the height of these microstructures. (d) Flexibility and softness characterization of liquid diode catheter. The liquid diode catheter can be easily bent or twisted into various configurations.
16
Figure 2. Liquid transport within the diode catheter and smooth catheter. (a) Selected snapshots of directional liquid transport within the liquid diode catheter. The stained water prefers to flow in a single direction entering the cavity and keeps pinned in the reverse direction. The intrinsic contact angle of the diode catheter is 56.3 ± 3.2°. (b) Selected snapshots
of
bidirectional
liquid
transport
17
within
the
smooth
catheter.
Figure 3. Triple-phase contact line dynamics. (a) Schematic of a liquid slug and its advancing and receding edges in the smooth catheter. Here θA0 and θR0 represent the advancing and the receding contact angles in the smooth catheter. (b, c) Triple-phase contact line in the spreading (b) and pinning (c) direction. In the spreading direction, the contact line can contact either the top surface (ω = 0) or the sidewall (ω = π/2) of the pillars. In the pinning direction, the contact line can contact either the top surface (ω = 0), the sidewall (ω = π/2) or the bottom surface (ω = π) of reentrant structure. ω is the rising angle of the surface. The macroscopic contact angles θA and θR between the advancing and receding menisci and the substrate are dependent on the intrinsic contact angles θA0 and θR0 of the material, and the contact face of the pillars. The white and black arrows indicate the directions of the flow and the retention force. Note that the substrate constrains θA and θR to be between 0 and 180°.
18
Figure 4. Characterization of transport properties of liquid diode catheter. (a) The rectification coefficients of liquid diode catheters with different wettability. The inset image shows a snapshot of liquid spreading within the hydrophobic liquid diode catheter. Ls and Lp represent the transport distance in the spreading direction and reverse direction, respectively. (b) Variation of liquid transport velocity in the liquid diode catheters for different CA and Q. (c) Comparison of k for liquid diode catheters with different h/D and Q. These results are calculated based on the injection of 100µl water. (d) Liquid transport velocities in the liquid diode catheters for different h/D and Q. (e) Time-dependent variation of Ls in the liquid diode catheter and the smooth catheter. (f) Variation of liquid transport velocity within the liquid diode catheters under different CA and Q. The red and black dash lines are the fitting lines in different regions. Here Qc is the critical volumetric rate where the water flow overcomes the pinning
effect
and
propagates
slowly
19
in
the
reverse
direction.
Figure 5. Durability and stability of liquid diode catheter. (a) Variation of k under different transport modes (unidirectional and bidirectional) and Q. The inset image represents the transport behavior in the diode catheter when Q ≈ 55 µl/s, in which bidirectional flow has developed. (b) Bending of the catheter has no notable effect on the directional transport of water. The inset shows the optical image of a diode catheter before and after the bending. (c) Twisting of diode catheter has no significant effect on the directed transport of water. The inset shows the optical image of a diode catheter before and after the twisting. (d) Directional transport of various liquids including low-surface-tension liquids (hexane and ethanol), medical solutions (glucose and saline solutions) and lubricate oil (hydroxy PDMS-25, hydroxy PDMS-65, Krytox 103 and mineral oil) in the diode catheters.
20
Highlights
Developing a flexible liquid diode catheter for unidirectional liquid transport Liquid behavior in the diode catheter is dominated by surface effect and bulk effect The diode catheter can preserve its function under mechanical bending or twisting The diode catheter can transport various liquids including medical solutions and oils
Author Contributions Zuankai Wang supervised the research. Jiaqian Li and Zuankai Wang designed the experiments. Jiaqian Li, Huanxi Zheng and Xiaofeng Zhou fabricated the samples. Jiaqian Li, Minjie Liu and Chao Zhang analyzed the data. Jiaqian Li, Xiaofeng Zhou and Zuankai Wang wrote the paper. All the authors commented on the paper.
Conflict of Interest Statement
The authors declare no competing interests.
S2542-5293(19)30150-6
DOI:
https://doi.org/10.1016/j.mtphys.2019.100170
Reference:
MTPHYS 100170
To appear in:
Materials Today Physics
Received Date: 27 September 2019 Revised Date:
25 November 2019
Accepted Date: 26 November 2019
Please cite this article as: J. Li, H. Zheng, X. Zhou, C. Zhang, M. Liu, Z. Wang, Flexible topological liquid diode catheter, Materials Today Physics, https://doi.org/10.1016/j.mtphys.2019.100170. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Flexible topological liquid diode catheter Jiaqian Li1, Huanxi Zheng1, Xiaofeng Zhou2, Chao Zhang1, Minjie Liu1, Zuankai Wang1,3* 1
Department of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, China.
2
Shanghai Key Laboratory of Multidimensional Information Processing, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China.
3
Shenzhen Research Institute of City University of Hong Kong, Shenzhen 518057, China.
Corresponding author: [email protected]
Abstract: Spontaneous and directional transport of various liquids in open and closed spaces are of great importance in various energy, water and medical applications. Despite extensive progress in engineering diverse open surfaces to rectify liquid transport, the implementation of the diode-like liquid transport in a closed system is still rare. In this study, we present the design of flexible liquid diode catheters internally decorated with asymmetric reentrant structure that allows for the efficient and unidirectional transport of diverse liquids. We also elucidate that the hydrodynamic transport behaviors in the closed diode catheters are primarily dominated by the interplay between the surface effect and the bulk effect, which are distinctly different from those on open liquid diodes. The flexible liquid diode catheters can preserve their directional transport behaviors even under mechanical bending or twisting, and under various liquids from low-surface-tension liquids, medical solutions to lubricant oils. The combination of extraordinary flexibility, durability and stability promises the development of liquid transport devices that can work in various configurations and applications.
Keywords: flexible catheter, liquid diode, liquid transport, soft replication, surface effect
1
Introduction Spontaneous and directional liquid transport in closed catheters, tubes or conduits is a ubiquitous phenomenon in nature as exemplified by blood vessels and tree xylem, and also plays an important role in a hybrid of industrial applications ranging from microfluidics, advanced printing, wearable electronics to biomedical devices[1-4]. Although various flexible catheters have been rapidly developed for liquid transportation, directional liquid transport in these flexible catheters generally requires auxiliary control devices, such as pumps and valves. To eliminate the requirement of additional pumps or valves, surfaces with responsive properties have been engineered to leverage the external stimuli to trigger liquid motion. For example, one feasible approach has been explored by the design of photo-responsive multilayer catheters to control the motion of liquid slugs through the formation of curvature gradient induced by photo-induced asymmetric deformation of the tube[5,6]. However, these approaches are still limited by the need of external energy or the proper selection of stimuliresponsive materials, which in turn restrict their wide applications. Therefore, the development of novel flexible catheters that enable the passive, efficient and directional liquid transport remains a long-standing challenge. Over the past several years, engineering open surfaces to achieve directional liquid transport has flourished[7-16], especially inspired by numerous biological examples[17-27]. These open surfaces, termed as liquid diodes, mainly take advantage of wettability gradient, structural curvature gradient, asymmetric capillary or their combinations[28-45] to promote liquid flow in a single direction and meanwhile preventing its backflow. In spite of exciting progress, it is difficult to directly translate these features responsible for directional liquid transport in open systems such as structures, curvatures, shapes, and roughness into the closed systems owing to the spatial confinement. For example, even if the emerging 3D printing 2
technology have extended our fabrication capability of flexible materials[46-48], it is still challenging to pattern asymmetric nano- and micro-structures such as mushroom[49,50] and sharp edges[51,52] that are vital to the pinning of the backflow, inside the closed catheter. Previously, we have successfully developed a topological liquid diode that enables the spontaneous, fast, directional transport of various liquids[30]. In spite of many advantages, such a directional liquid transport is limited to the open surface as well as hard silicon material. So far, it is difficult to reproduce the complicated and fragile reentrant structures on the flexible materials. Such disadvantages greatly restrict the wide applications of liquid diodes in various settings, especially for flexible medical catheter application. Herein we report the design and fabrication of flexible topological catheters decorated with reentrant structure that allows for the continuous and directional liquid transport in a closed space. Moreover, the flexible liquid diode catheter can preserve its transport behavior even under mechanical bending or twisting, as well as under various liquids from low-surface-tension liquids, medical solutions to lubricant oils.
Material and methods Sample fabrication. We have developed the topological liquid diode on the silicon wafer via a standard Microelectromechanical System (MEMS) manufacturing technique as reported in our previous work[30]. To fabricate soft diode membrane, we utilized a simple two-step molding-casting method to transfer the asymmetric microstructures from the silicon liquid diode
to
soft
material.
The
silicon
liquid
diode
was
firstly
silanized
by
trichloro(1H,1H,2H,2H-perfluorooctyl)silane in a 1mM n-hexane solution, followed by heat treatment at ∼120 ºC in air for one hour to render it hydrophobic. The polydimethylsiloxane (PDMS, DC SYLGARD™ 184) and curing agent was firstly mixed at a mass ratio of 10:1, then stirred for 10 minutes, and finally vacuumed for 1 hour to remove internal gas before 3
casting onto the silicon liquid diode. In order to peel negative PMDS mold out from the cavity with reentrant structure, we cured the PDMS with heating treatment at 60 ºC for ~30 minutes, resulting in soft PDMS mold in contrast to high-temperature curing. The negative PDMS mold was then completely cured by heating at 80 ºC for 2 hours in an oven. Subsequently, the negative PDMS mold was silanized with vacuum treatment for 12 hours at room temperature. The soft UV resin (viscosity 3500-5000 cps at 25 ºC) was directly casted on the negative PDMS mold in the photolithography room before vacuuming gas for 10 minutes at 40 ºC in a vacuum oven. The sample was then completely cured by a UV light (50 W) for 4 minutes. Then, we obtained soft liquid diode membrane after peeling the cured resin out from the PDMS mold. Finally, we bent soft UV resin diode membrane into a medical PVC catheter to fabricate the new flexible liquid diode catheter. Surface characterization. All soft diode membranes and catheters were characterized by Scanning Electron Microscope (SEM, QuantaTM 450 FEG) after coating a thin gold layer (about 20 nm) on the samples with the Quorum Q150TS Sputter coater. Surface treatment and contact angle measurement. To create the hydrophilic wetting property, we treated the diode membrane and catheter with Plasma Cleaner PDC-32G (Harrick Plasma limited.) at high RF level for about 2 minutes. To obtain the hydrophobic wetting property, the diode membrane and catheter were modified via the vacuum deposition of trichloro(1H,1H,2H,2H-perfluorooctyl)silane for 5 hours inside a fume hood. After hydrophilic and hydrophobic modification, the apparent contact angles on the original, hydrophilic and hydrophobic membranes were measured by ramé-hart M200 Standard Contact Angle Goniometer at room temperature with 50% relative humidity. We tested five sets of the contact angles and calculated their average results and error margins. Liquid transport experiments. We conducted the experiments of liquid transport on the diode membranes and within the diode catheters at room temperature. To improve the visual clarity and the resolution of liquid transport, we stained the deionized water with a blue dye. 4
In our experiments, the infusion rate of the liquid was precisely controlled by a syringe pump (KDScientific Model 200 Series). The transport behaviors of various liquids were observed and recorded by the Nikon D5200 digital camera connected with a Nikon micro-lens.
Results and discussion In this work, we designed the flexible liquid diode catheter based on two polymer materials, with the polyvinyl chloride (PVC) as the supporting outer layer and the ultraviolet (UV) resin diode membrane as the inner layer. To construct the soft and flexible diode catheter with hydrophilic property, we first implemented a three-step process to fabricate the UV inner layer. First, we designed and fabricated a silicon-based liquid diode template, which consists of tiny capillary channels and reentrant structures[30]. Then, we transferred these tiny structures from the silicon template into the PDMS by using a modified soft-curing process (Fig. S1). By curing the PDMS at a relatively lower temperature than the conventional curing process, the PDMS mold is soft enough to being peeled out, allowing for the successful replication of even tiny structures. Subsequently, UV resin-based flexible liquid diode membrane was duplicated from the PDMS mold. The as-fabricated diode membrane can be bent or twisted into various configurations, exhibiting an extraordinary flexibility (Fig. S2). Eventually, the liquid diode catheter was engineered by bending the liquid diode membrane into the PVC catheter as schematized in Fig. S3. Figure 1a shows the cross-section SEM image of the as-fabricated two-layer liquid diode catheter. To validate whether the reentrant structure in the silicon template is successfully translated into the UV material, we compared the specific microstructures in both materials. As shown in Figure 1b and 1c, both the reentrant structures and capillary channels are exquisitely replicated from the silicon template (Fig. S4), demonstrating the efficacy of our soft-curing process. Remarkably, the liquid diode catheter yields superior robustness 5
under mechanical bending or twisting, thus making it possible to construct different configurations as depicted in Figure 1d. We first compare the liquid transport in the catheters with and without the texture under a continuous liquid pumping. To clearly visualize the transport behavior of water, we stained the water with a blue dye. As shown in Figure 2a, when the stained water is infused into the diode catheter at a volumetric rate Q ≈ 5 µl/s, it first spreads along the inner wall of the catheter, and then fills the catheter soon at the time tc ≈ 10 s. After this initial stage, liquid diode catheter delivers the water toward the direction entering the cavity, defined as spreading direction, and stably blocking the flow in the reverse direction at the same time (Movie S1). By contrast, the original smooth PVC catheter (Fig. S5) prefers to transport the continuous water bi-directionally in two directions without the rectification effect as shown in Figure 2b and Movie S2. Thus, we can conclude that the closed catheter with the asymmetric topography can achieve the directional liquid transport. To elucidate how the asymmetric topography regulates the transport behavior, we further analyze the retention force within the smooth catheter and liquid diode catheter[53,54], respectively. For the smooth cylindrical catheter, the retention force (F0) impeding the motion of the fluid acts equally in both directions (Figure 3a), resulting in a typical bidirectional liquid transport (Fig. 2b). In contrast, the retention forces in the case of the structured diode catheter become direction-dependent owing to the presence of asymmetric reentrant structures as schematized in Figures 3b and 3c. In the spreading direction, the liquid front locates on the top surface (Fig. 3b, ω = 0) or the sidewall (Fig. 3b, ω = π/2) of U-shaped pillars. Thus, the retention force in the spreading direction can be expressed as Fs = max { Fω =0 , Fω =π 2 } , where 1 1 Fω = 2π Dγε sin (θ R 0 + θ A0 ) sin ω + ∆θ 0 2 2
6
is the retention force derived by Extrand’s geometric model[53] in the range of 0 ≤ ω ≤ π/2, D is the inner diameter of the catheter, γ is the surface tension of the liquid, ε is the surface roughness, ω is the rising angle of the surface, and ∆θ0 = θ A0 − θ R 0 is the contact angle hysteresis. Distinct from the spreading direction, the liquid front can contact the bottom surface of the reentrant structures in the pinning direction, exhibiting three possibilities as schematically shown in Fig. 3c. As a result, the retention force in the pinning direction can be expressed as Fp = max {Fω =0 , Fω =π 2 , Fω =π } , where Fω=0 and Fω=π/2 can be calculated by the above Extrand’s model[53,54], and
Fω =π = π Dεγ ( cos θ R 0 + 1) . Based on the above analytical analysis, for the diode catheter as shown in Fig. 2a, Fp is 52% larger than Fs, which explains why there is a directional liquid spreading in the hydrophilic liquid diode catheter. To quantify the capability of directional liquid transport in the liquid diode catheter, we define the ratio of the spreading length Ls in the spread direction relative to that in the pinning direction Lp (inset image in Figure 4a) as the rectification coefficient, k. Figure 4a presents the variation of k in the diode catheters with different wetting properties (Fig. S6). It is clearly observed that the hydrophilic liquid diode catheter has a larger k compared with the hydrophobic diode catheter. For the hydrophobic surface, the retention force[55] in the pinning direction (Fig. S7) can be expressed as Fp = π Dγε ( cos θ R 0 + 1) .
With the increase in the intrinsic contact angle, the retention force in the pinning direction on the hydrophobic surface (θ ≈138º) is approximately twice smaller than that on the hydrophilic surface (θ ≈56º). As a result, the backflow of liquid in the hydrophilic diode catheter can be fully suppressed as shown in Fig. 2a and Fig. S8. In contrast, there is a relatively strong 7
backflow in the reverse direction within the hydrophobic diode catheter (Fig. S9), leading to a small k as shown in Fig. 4a. Such a phenomenon in the closed system is also consistent with the spreading behaviors of the water in the open system, in which the hydrophilic membrane can stably arrest the backflow (Fig. S10) whereas the hydrophobic membrane slowly delivers the water flow along the pinning direction (Fig. S11). Distinct from capillary-induced liquid transport on the diode membrane, we also find that the liquid transport velocity is almost independent on the wettability of the inner surface of the diode catheter (Fig. 4b), for example, the average velocities are about 0.6 mm/s when Q ≈ 5.0 µl/s regardless of the wetting properties of the diode catheters. Distinct from the open-space surface on which the directional liquid transport originates from the combination of the surface topography and wettability[8-11,13,15,16], the preferential confinement of a local thin liquid layer in the inner surface of catheter is also dependent on the global liquid flow. To verify it, we fabricated diode catheters with various inner diameters ranging from 1 to 3 mm, and carried out liquid transport experiments at the same flow rate. All these diode catheters are capable of delivering the liquid in single direction (Fig. S12). With the decrease of catheter size, the surface effect gradually dominates the bulk effect and dictates the liquid transporting in the liquid diode catheter, rendering a better directional liquid transport as evidenced by the rectification coefficient and delivery speed as shown in Figs. 4c and 4d. Specifically, compared with the larger catheter, smaller catheter yields a longer transport distance in the spreading direction and a shorter distance in the reverse direction, resulting in a lager k as shown in Fig. 4c. Similarly, the increase of h/D also gives rise to the increase of the delivery speed of liquid in the diode catheters (Fig. 4d). The transport velocity increases from 0.42 mm/s to 1.34 mm/s as h/D increases when Q ≈ 5.0 µl/s. As a consequence, smaller liquid diode catheter exhibits the superior directional transport of the water at a higher efficiency and fidelity by leveraging of the surface effect. 8
Figure 4e shows the time-resolved evolution of spreading distance Ls in the diode catheter and smooth catheter. The infused liquid initially spreads along the wall of the catheter and then fills the catheter (Figs. 2a and 2b). After the flow in the catheter fully develops, the transport distance in the spreading direction varies almost linearly over time in the liquid diode catheter (Fig. 4e, orange line), which is the same to the linear spreading behavior (Fig. 4e, green line) in the smooth catheter. Through comparing the slopes of these lines, it is found that the delivery speed of the liquid in the diode catheter (vdiode ≈ 0.85 mm/s) is much larger than that in the smooth catheter (vsmooth ≈ 0.25 mm/s), evidencing that surface effect facilitates the directional liquid delivery in the diode catheter in contrast to the smooth catheter. The manifestation of superior and directional transport dictated by the surface effect in the diode catheter requires the delicate control of the water infusion. Figure 4f shows the variation of transport velocity in different catheters under a function of the volumetric rate Q. Obviously, v varies linearly with Q, but consists of two different regions separated by a critical volumetric rate Qc ≈ 15 µl/s. The varying slope between v and Q in the region of Q < Qc (Fig. 4f, red dash line) is slightly larger than that in the range of Q > Qc (Fig. 4f, black dash line). Such a small decrease originates from the breakdown of surface or pinning effect in the reverse direction, a case where the bulk effect overcomes the hysteresis force induced by reentrant structure. In addition, the interplay between surface effect and bulk effect also regulates the transport state of the liquid in the diode catheter as displayed in Figure 5a. When the water is slowly injected into the diode catheter Q < Qc, the surface effect outweighs the bulk effect to enable the typical trait of unidirectional liquid transport (Fig. 5a, blue area). Beyond this range, the liquid transport behavior in the diode catheter transitions into the bidirectional regime as a result of the dominant role of the bulk effect over the surface effect. In this state, the infused water starts to spread in the reverse direction (inset image in Fig. 5a) with a smaller velocity than that in the spreading direction. 9
We continue to perform the liquid transport experiments under bending and twisting conditions to examine the durability and stability of our liquid diode catheter. As shown in Figures 5b and 5c, in each bending or twisting cycle, the flexible liquid diode catheter was first bent or twisted, and then recovered to the straight shape. As shown in Figure 5b, after one hundred bending cycles, there is no notable degradation in the magnitude of k, suggesting the remarkable durability and stability of the directional liquid transport. Similarly, the magnitude of k also remains stable in the twisting test as depicted in Figure 5c. In contrast to the hard liquid diode, such as silicon, soft liquid diode enables the more stable characteristic of directional liquid transport in the long-term operations, making it more applicable in various applications and configurations. The diode-like liquid transport in closed catheters is also applicable to various liquids. As shown in Figure 5d, liquid diode catheter is capable of transporting the low-surfacetension liquids such as hexane and ethanol although with the lower rectification coefficients compared with the deionized water. Second, liquid diode catheter can also transport medical solutions including 5% glucose solution and 0.9% saline solution in single direction at the higher fidelity in contrast to other tested liquids. These results indicate the promising applications of liquid diode catheter in the medicine area, such as preventing the infusion reflux of medical solutions and body fluids. Furthermore, liquid diode catheter also shows the directional transport of the lubricant oils including mineral oil, Krytox 103 and hydroxy terminated Polydimethylsiloxanes (hydroxy PDMS with viscosity ~ 65cSt and ~ 25cSt). Such a unique characteristic imparts superior functions including anti-fouling and anti-friction.
Conclusion We have reported a simple soft molding-casting method to prepare flexible topological catheter that allows for the spontaneous, rapid and directional liquid transport. Through the 10
analysis of the retention forces in two directions, we developed a theoretical model to interpret the observed behavior of unidirectional liquid transport within the liquid diode catheter. We demonstrated that the transport of the liquid within the liquid diode catheter is primarily regulated by the interplay between the surface effect and the bulk effect, distinct from capillary-induced liquid transport as manifested on the diode membrane. We also elucidated the durability and stability of the liquid diode catheter in the long-term bending and twisting operations, as well as in the transport of diverse liquids including low-surface-tension liquids, medical solutions and lubricant oils. This work advances our fundamental understanding in the hydrodynamics of liquid transport in closed systems, and provides important routes for the development of liquid transport devices that can work in various configurations and applications.
Acknowledgements Z.W. is grateful for financial support from Research Grants Council of Hong Kong (Grants No. 11275216 and 11213915), the Research Grants Council of Hong Kong under Collaborative Research Fund (No. C1018-17G), and City University of Hong Kong (No. 9360140, No.9667139). Z. X. acknowledges the financial support from National Natural Science Foundation of China (No. 51975215). The authors declare no competing interests.
Appendix A. Supplementary data Supplementary data to this article can be found online at ……
Data availability
11
The data that support the plots within this paper and other findings of this study are available in the main text and in the Supplementary Information. Additional information is available from the authors upon reasonable request.
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Figure 1. Flexible topological liquid diode catheter. (a) Scanning electron microscope (SEM) image showing the cross section of liquid diode catheter. The diode catheter is made of two layers, the inner is the UV diode membrane and the outer is the supporting PVC catheter. (b) SEM image of liquid diode membrane consisting of U-shaped island arrays distributed in the parallel fences. (c) Magnified SEM image of U-shaped island and reentrant structures. Here h is the height of these microstructures. (d) Flexibility and softness characterization of liquid diode catheter. The liquid diode catheter can be easily bent or twisted into various configurations.
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Figure 2. Liquid transport within the diode catheter and smooth catheter. (a) Selected snapshots of directional liquid transport within the liquid diode catheter. The stained water prefers to flow in a single direction entering the cavity and keeps pinned in the reverse direction. The intrinsic contact angle of the diode catheter is 56.3 ± 3.2°. (b) Selected snapshots
of
bidirectional
liquid
transport
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within
the
smooth
catheter.
Figure 3. Triple-phase contact line dynamics. (a) Schematic of a liquid slug and its advancing and receding edges in the smooth catheter. Here θA0 and θR0 represent the advancing and the receding contact angles in the smooth catheter. (b, c) Triple-phase contact line in the spreading (b) and pinning (c) direction. In the spreading direction, the contact line can contact either the top surface (ω = 0) or the sidewall (ω = π/2) of the pillars. In the pinning direction, the contact line can contact either the top surface (ω = 0), the sidewall (ω = π/2) or the bottom surface (ω = π) of reentrant structure. ω is the rising angle of the surface. The macroscopic contact angles θA and θR between the advancing and receding menisci and the substrate are dependent on the intrinsic contact angles θA0 and θR0 of the material, and the contact face of the pillars. The white and black arrows indicate the directions of the flow and the retention force. Note that the substrate constrains θA and θR to be between 0 and 180°.
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Figure 4. Characterization of transport properties of liquid diode catheter. (a) The rectification coefficients of liquid diode catheters with different wettability. The inset image shows a snapshot of liquid spreading within the hydrophobic liquid diode catheter. Ls and Lp represent the transport distance in the spreading direction and reverse direction, respectively. (b) Variation of liquid transport velocity in the liquid diode catheters for different CA and Q. (c) Comparison of k for liquid diode catheters with different h/D and Q. These results are calculated based on the injection of 100µl water. (d) Liquid transport velocities in the liquid diode catheters for different h/D and Q. (e) Time-dependent variation of Ls in the liquid diode catheter and the smooth catheter. (f) Variation of liquid transport velocity within the liquid diode catheters under different CA and Q. The red and black dash lines are the fitting lines in different regions. Here Qc is the critical volumetric rate where the water flow overcomes the pinning
effect
and
propagates
slowly
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in
the
reverse
direction.
Figure 5. Durability and stability of liquid diode catheter. (a) Variation of k under different transport modes (unidirectional and bidirectional) and Q. The inset image represents the transport behavior in the diode catheter when Q ≈ 55 µl/s, in which bidirectional flow has developed. (b) Bending of the catheter has no notable effect on the directional transport of water. The inset shows the optical image of a diode catheter before and after the bending. (c) Twisting of diode catheter has no significant effect on the directed transport of water. The inset shows the optical image of a diode catheter before and after the twisting. (d) Directional transport of various liquids including low-surface-tension liquids (hexane and ethanol), medical solutions (glucose and saline solutions) and lubricate oil (hydroxy PDMS-25, hydroxy PDMS-65, Krytox 103 and mineral oil) in the diode catheters.
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Highlights
Developing a flexible liquid diode catheter for unidirectional liquid transport Liquid behavior in the diode catheter is dominated by surface effect and bulk effect The diode catheter can preserve its function under mechanical bending or twisting The diode catheter can transport various liquids including medical solutions and oils
Author Contributions Zuankai Wang supervised the research. Jiaqian Li and Zuankai Wang designed the experiments. Jiaqian Li, Huanxi Zheng and Xiaofeng Zhou fabricated the samples. Jiaqian Li, Minjie Liu and Chao Zhang analyzed the data. Jiaqian Li, Xiaofeng Zhou and Zuankai Wang wrote the paper. All the authors commented on the paper.
Conflict of Interest Statement
The authors declare no competing interests.