Morphology and Surface Properties of Roach Water Transport Arrays

Article Cite This: ACS Appl. Bio Mater. 2019, 2, 2650−2660

www.acsabm.org

Morphology and Surface Properties of Roach Water Transport Arrays Sunghan Kim,*,†,§ Volodymyr F. Korolovych,† Marc J. Weissburg,‡ and Vladimir V. Tsukruk*,† †

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States § School of Mechanical Engineering, Chung-Ang University, Seoul 06974, South Korea ‡

Downloaded via NOTTINGHAM TRENT UNIV on August 13, 2019 at 12:47:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: We report on morphological studies of wharf roaches, Ligia exotica, which can passively absorb and transport water through the microscopic protrusions on their legs. We systematically investigated the geometrical variables of the protrusions on each podite of legs to reveal a particularized structural complexity. For the morphological analysis, each podite was split into nine different zones by grouping the protrusions with similar shapes and organization. The protrusions are shown to possess three different types of shapes located on each specific zone of the podite. In addition, the nanoscale surface morphologies of the protrusions on the wharf roach legs were probed by using atomic force microscopy, and the surface properties of the hairy arrays were determined for identifying the localized hydrophobicity distribution. The protrusion surface possessed a nanoscale periodic patterned texture, and both the valley and ridges of a periodic pattern on the protrusion surface exhibited an identical low surface energy. We suggest that the structural morphologies and distinct hydrophobicity of the protrusions can be critical in determining the directional wettability of an entire leg and important for designing a sturdy water transport and passive water-absorbing system without external energy consumption. KEYWORDS: wharf roach, Ligia exotica, water transport, scanning electron microscopy (SEM), atomic force microscopy (AFM), surface properties

■

INTRODUCTION Biological surfaces have variety admirable properties that are studied to understand how they achieve their superior functionalities and enable biomimetic engineering applications. The microscopic and nanostructural morphologies of biological surfaces play key roles in achieving various advanced characteristics including low-frequency signal transmission, superior mechanical properties, and biological infrared (IR) receptors.1−5 Many recent scientific and engineering advances such as advanced optical imaging and appearance,6−8 surface wettability manipulation,9−11 and adhesion and friction control12−14 are based on the biological examples encountered in nature. The unique surface characteristics of insects and animals depend heavily on the specific surface morphology and not just on the chemical composition of the topmost skin layer.15−17 For instance, hairy structures on wandering spiders’ legs act as vibration sensors that help them hunt prey.18−21 Snake skin combines an ultramicropore system with ordered microfibrillar arrays to provide functional tribological properties with reduced adhesive forces and directional friction.17 The arrays of microhair cells on the skin of the Blind Cave fish detect water flow for navigation in the underwater environment.22 The © 2019 American Chemical Society

microstructured riblets of shark skin control vortices on the skin resulting in a lower drag force.23 Finally, the microhair array on butterfly glasswings enhances the hydrophobicity of the butterflies’ wings for better dewetting and color appearance.16 Wharf roaches, Ligia exotica, which are isopod crustaceans, have attracted attention because of the unique morphological structures of the protrusions on their legs.16,18−21,24,25 The shape and dimensions of these protrusions are crucial factors that provide unique surface properties related to the transport of water within open structural arrays on their legs.24,25 The different types of microscale protrusions on wharf roach legs support water transport to the gills when the sixth and seventh legs are placed held together.24 In previous research, the arrays of protrusions on wharf roaches were analyzed in order to understand the crucial water transport mechanism for creating other systems that can efficiently collect and transport water.24,25 It has been suggested that the physical topography and chemical composition of these arrays control critical surface properties for Received: April 14, 2019 Accepted: May 14, 2019 Published: May 14, 2019 2650

DOI: 10.1021/acsabm.9b00318 ACS Appl. Bio Mater. 2019, 2, 2650−2660

Article

ACS Applied Bio Materials

Figure 1. (a) Photograph of a wharf roach in lab conditions. (b) Schematic diagram showing wharf roach legs: sixth and seventh legs on both left and right sides. SEM images of the (c) sixth leg on the right side, with each podite numbered, (d) sixth leg on the left side, (e) seventh leg on the right side, and (f) seventh leg on the left side. Scale bars for all images are 5 mm.

microscopy (AFM).29,30 With SEM, we collected information on the shape and dimensions of protrusions at a large scale critical for water transport using a size range of roaches to identify potential scaling relationships. We found that three different types of protrusions are distributed on a single podite and can be divided in nine different zones, and the shape of protrusions are determined by various geometric variables. The direct probing of biological surfaces using AFM allows to identify their nanoscale crucial morphological features and prospective wetting properties, which were not probed to date.2,31−33 We observed that a single protrusion possesses a highly textured nanofibrillar morphology and surprisingly low hydrophobicity, which might be critical for confinement of the capillary flow. Understanding structural details of protrusions is important to understand the wharf roach’s water transport mechanism in future studies.

their survival such as directional wettability, local adhesion, friction and surface energy to create efficient capillary transport. The features on different podites include hair-like protrusions (HLPs) located predominantly on the edges of the open capillary region and paddle-like protrusions (PLPs) located at the center of the open capillary region.24,25 It has been suggested that the open capillaries on each podite of wharf roach legs present microfluidic systems for water collection and transport.24,25 However, detailed information on the nano/microscale characteristics of the protrusions on wharf roach’s is absent. Moreover, previous attempts to design a biomimetic passive transport system have not considered the detailed microscopic geometrical features and surface wetting properties. Rather, simple pillar-like structures including microblade-type arrays with scales of different widths,26 arrow-like micropillar arrays,27 and nanospike array structures28 were considered to mimic the protrusions of wharf roach legs, and no information is available on fine morphology and shapes of hair structures, which might be critical for further exploration of these biological features. In this study, we report the detailed observations on different geometric features of the protrusions on the podites of wharf roach’s sixth and seventh legs using a unique combination of scanning electron microscopy (SEM) and atomic force

■

EXPERIMENTAL METHODS

Animal Collection and Maintenance. Wharf roaches were collected by a hand from boat docks on Skidaway Island, Georgia, USA, and transported to our lab. They were kept in 20 gal aquaria lined with a pebble substrate moistened with artificial seawater with 25 ppt salinity (Figure 1a). This routine allowed the animals to access water 2651


Article


Figure 2. SEM images and geometric variables of representative protrusions. (a) Top-view SEM image of the left-side fourth podite. (b) Tilted-view SEM image of the left-side fourth podite with highlighting representative protrusion features (c) type 1, (d) type 2, (e) type 3. Zoomed-in SEM images of protrusion features (c) type 1, vertical protrusion; (d) type 2, tilted protrusion; (e) type 3, tilted protrusion with a radius of curvature. while not being submerged. Animals were fed with commercial flaked fish food, and the water was changed daily. Preparation of Samples of Wharf Roach Legs with a Fixation Process. As known, open capillaries that transport water to the gills are located on podites of the sixth and seventh wharf roach legs.24,25 Thus, we selected both sixth and seventh legs of the left and right sides of the wharf roach’s body (Figure 1b). Individual legs were amputated by gently squeezing the coxa-basis joint with fine forceps in animals that had been chilled to quiescence for roughly 10 min at 4 °C. The amputated legs were prepared using standard experimental protocols for invertebrate tissue.24 Briefly, the legs were immediately prefixed in 3% glutaraldehyde in 0.2 M Sorenson’s phosphate buffer (pH = 7.4) for 3 h. Following this treatment, the post fixation process was performed for 2 h in 1% osmium tetraoxide in 0.2 M Sorenson’s buffer. The postfixed legs were briefly washed in Tween 80 buffer (2 drops per 100 mL of buffer) to remove surface particulates, followed by a brief buffer wash. The rinsed legs were then dehydrated in a graded ethanol series (50%, 70% 80%, 90%, and 100%) for 30 min each. Finally, the legs were post dehydrated in hexamethyldisilazane (HMDS) for 1 h, and stored in a desiccator. SEM and AFM Imaging. The podites of the legs at moderate resolution were investigated using SEM (Hitachi-3400SN) with a 10 kV accelerating voltage. For SEM observations, all fixed legs were coated with a gold coating (thickness ∼2.5 nm) before measurement (DESK-IV Cold Sputter; Denton Vacuum). The nanoscale surface morphology and nanoscale surface properties were investigated by using an ICON AFM (Bruker) in ambient conditions according to the usual procedure.34 The soft tapping mode was used to obtain AFM images using silicon AFM tips (MicroMasch, HQ, XSC11/Al BS; tip radius, 8 nm; tip height, 18 ± 2 μm; cantilever length, 500 ± 5 μm; cantilever width, 30 ± 3 μm). The AFM images of the samples were taken with scan sizes from 10 μm × 10 to 1 μm × 1 μm at 512 × 512 pixel resolution. The scanning rate was fixed at 0.3 Hz. The microroughness was determined by NanoScope Analysis software within 500 nm × 500 nm surface areas on multiple locations.

AFM Pull-off Force Tests. To determine the surface energy of the wharf roach’s protrusion comparatively, pull-off force tests were conducted on three different surfaces: piranha-cleaned silicon substrates (highly hydrophilic surface) and trichlorosilane-treated silicon wafers (highly hydrophobic surface) in direct comparison with protrusions on fresh raw wharf roach legs. Surface adhesive forces were estimated by measuring the pull-off forces in the contact mode and analyzing the force−distance curves (FDC) obtained under various humidity conditions: in fluid (100% RH), at 5% RH and 45% RH humidity.35,36 Wharf roach legs were allowed to equilibrate for 10 min at each given environmental condition (RH) prior to FDC tests. The AFM cantilevers’ sensitivities were evaluated by collecting the FDC data on a sapphire surface.37,38 The spring constant of the sharp-type AFM cantilever (0.19 ± 0.06 N/m) was determined by the thermal tuning method.39

■

RESULTS AND DISCUSSION SEM Images of Wharf Roach Legs. The SEM images of the entire sixth and seventh legs of a wharf roach, on both the right and left sides, show six different podites that are connected to one another (Figure 1b−f). Protrusions occur on the second to fifth podites on the sixth leg on both sides and on the sixth podite of the seventh leg on both sides (Figure 1c−f, Figure S1).24 As reported, the sixth legs’ third to fifth podites have representative structural details of the microstructural open capillaries, which play a crucial role for collecting and transporting water with specific functional geometric conditions.24 Characteristics of Protrusions on Wharf Roach Legs. To determine the shapes of the protrusions on the wharf roach legs systematically, three representative characteristic shapes of protrusions were identified and analyzed from SEM images taken from each podite (Figure 2). Vertical protrusions (type 1) 2652


Article


Figure 3. SEM images of (a) interprotrusion distance in longitudinal (ln) and transversal (tn) directions on the fourth podite and (b) density of protrusions on the third podite in the unit area of 50 μm × 50 μm. Scale bars for all images are 50 μm.

Figure 4. (a) SEM image of the fourth podite with the determined two-dimensional (2D) X−Y coordinate system. (b) Magnified SEM images of two different protrusions with opposite orientation angles. (c) Examples of structural models of protrusions built with determined orientation angles. The scale bar for the image is 500 μm.

were mostly located at the third podite and at the edges of the fourth and fifth podites (Figure 2c). The fourth and fifth podites had two other types of protrusions: tilted protrusions (type 2) and tilted protrusions with a radius of curvature (type 3) (Figure 2d,e). Seven different geometrical variables were determined by analyzing the SEM images of the protrusions on each podite. The shape of the type 1 protrusion was characterized by height 1 (H1), thickness (T), and width (W). Tilting angle (θz) was added to further characterize the shape of the type 2 protrusion. When the tilting angle of type 2 has an angle of zero, it corresponds to the type 1 protrusion feature (type 1 = type 2 having tilting angle of zero). The shape of the type 3 protrusion was characterized by the radius of curvature (R), height 2 (H2), and length (L) in addition to the other four geometric variables. Both type 1 and type 2 protrusions were found throughout the third, fourth, and fifth podites of the wharf roach legs, while type 3 protrusions were observed on the fourth and fifth podites alone

(Figure S2). We measured interprotrusion distances to characterize further the arrays present on each podite. Next, the interprotrusion distances, thus, in both the longitudinal (ln) and transverse (tn) directions were measured (Figure 3a), and the density of the protrusions were also identified to specify the denseness of the protrusions in the defined area (Figure 3b). The orientation angle (θXY) of the protrusions systematically varied in different podite regions that affect the configuration of the channels for water transport (Figure 4). The protrusion orientation angle was quantified using a coordinate system where the y-axis was fixed parallel to the longitudinal direction of the fourth podite (Figure 4a). Protrusions were oriented oppositely on different sides of the podite (Figure 4b). For example, one protrusion was located at an orientation angle of approximately 190°, based on the determined coordinate system, and another protrusion had an orientation angle of approximately 320°. By using this approach, 2653


Article


Figure 5. Structure of protrusion features established by using the information on the geometrical variables. (a) Schematic images of the combining process of two determined orientation angles θXY, A°; θZ, B°. (b) Representative 3D models of protrusions using determined geometry variables. (c) Illustrative protrusion array generated by interprotrusion distances in both the longitudinal (ln) and transverse (tn) directions.

Figure 6. Nine different zones of protrusions on the third podite. (a) SEM image of the third podite of a wharf roach leg divided into nine different zones: three zones (top, α; middle, β; bottom, γ) in the longitudinal direction and three zones (outside, A; center, B; inside, C) in the transverse direction. (b) Mean width in each of the 9 zones on the third podite. (c) Mean height in each of the 9 zones on the third podite. (d) Mean direction (tn) in each of the 9 zones on the third podite. (e) Mean direction (ln) in each of the 9 zones on the third podite. The scale bar for SEM image is 400 μm. Each bar is represented as mean ± STD, n = 10.

The orientation of a protrusion can be defined using the θz and θXY parameters with shapes given by the essential geometric

the orientation angle of each protrusion feature was determined in the specified two-dimensional plane (Figure 4c). 2654


Article


Figure 7. Different sizes of the fourth podites on the sixth leg of different sized animals. (a) Fourth podite SEM image of a small animal. (b) Fourth podite SEM image of a medium animal. (c) Fourth podite SEM image of a large animal. (d) Variation in size of the fourth podite in longitudinal and transversal directions for different sizes of animals. The body length of small-, medium-, and large-sized animals was 10.9, 17.5, and 27.7 mm, respectively. The scale bars for images are 500 μm. Each bar is represented as mean ± STD, n = 5.

determine the values of all geometric variables in the nine different zones, separately. Variations in the protrusion’s geometric characteristics in different zones of the same podite were compared for the left and right legs of the wharf roach (Figure S5). The density, tilting angle, and orientation angle of the protrusions also were different in different zones of the same podite and showed identical values for left and right legs, except for the tilting angle. This difference in the tilting angle was probably affected by the fixation process and drying on supporting substrates. Both the fourth and fifth podites of the wharf roach leg had different types of protrusions (Figure S6). As in the case of the third podite, geometric variables of the protrusions are different for different zones on the fourth and fifth podites. Overall, zone B had greater geometric variables of the protrusions than zone A or zone C, as observed in the third podite. Type 3 geometry mainly was observed in zone A (outside) of the fourth and fifth podites. Specifically, all of zone A of the fourth podite possessed type 3 protrusions, but zone α-A had only a type 3 protrusion at the fifth podite (Figure S7). The radius of curvature of all type 3 protrusions was almost identical, at 5.1 ± 0.2 μm, for both the fourth and fifth podites, and other geometric variables (length and height 2) of the type 3 protrusion were different in the fourth and fifth podites. As determined in previous studies, the distribution of protrusions on the podite, in terms of width of protrusions, is one of the critical geometric parameters that define the shape of the protrusions.24,25 Whereas the third podite contained only protrusions having a relatively small width, the fourth and fifth podites contained protrusions with large and small widths (7− 16 μm and 17−25 μm, respectively). In the fourth and fifth podites, the small-width protrusions were predominantly located in zone A and zone C (HLPs), while the large-width protrusions were present in zone B (PLPs), as defined previously.24,25 The orientation angle of the protrusions showed sets of uniform directional values throughout each zone on the third, fourth, and fifth podites. In the third podite, most of the protrusions were oriented similarly to the direction of joints that connect the third and second podite, with similar values of orientation angle ranging between 250 and 300°. The orientation angles of the protrusions in the fourth and fifth

variables to build a typical structural protrusion (Figure 5a,b). The aggregate system properties can be analyzed by using defined both longitudinal and transverse interprotrusion distances to produce arrays of the appropriate shape and spacing (Figure 5c). In addition, we found that the shape, density, and orientation of the protrusions differed by region, even in a single podite. Each entire podite displays a number of different zones based on the structural variables used to define the protrusions. To characterize this variation, the podite was divided into nine different zones that represented homogeneous regions defined by the shapes and size differences of the protrusions. All protrusions of the podites were divided into three zones (top, α; middle, β; bottom, γ) in the longitudinal direction and three zones (outside, A; center, B; inside, C) in the transverse direction. Longitudinal direction zones were grouped by the similar width size of the protrusions, and transverse direction zones were partitioned to highlight the orientation angle difference for further analysis. The size (width and length) of each podite and the size of each zone were almost identical on the left and right sides of the wharf roach legs (Figures S3 and S4). The values of geometric variables in these nine different zones on the same podite are summarized in Figure 6. As mentioned, the third podite chiefly possessed type 1 and type 2 shapes of protrusions (Figure 6b,c). The specific values of each geometric variable were different in the different zones, except thickness, which was similar for all protrusions (around 1 μm), and the total body length of the wharf roach was 27.7 mm in this case. As a representative case, the height of protrusions in zone α-B, zone β-B, and zone γ-B were 39.1 ± 1.1 μm, 37.7 ± 2.1 μm, and 29.7 ± 3.2 μm, respectively (Figure 6). The width of protrusions in zone α-B, zone β-B, and zone γ-B was 14.5 ± 1.3 μm, 12.4 ± 1.7 μm, and 11.7 ± 2.0 μm, respectively. The thickness of the protrusions is almost identical over all zones, at 1.1 ± 0.3 μm. Notably, both the width and height of the protrusions located in zone B throughout the three different regions (α, β, γ) in the longitudinal direction of the podite were larger than that of any other protrusion located in the same podite. The lengths of the directions tn and ln and the height of the protrusions varied in the different zones of one podite as well. Thus, it was important to 2655


Article


Figure 8. (a) AFM scans on three different surfaces of protrusion. (b) RMS microroughness of three different surfaces of protrusions. Each bar is represented as mean ± STD, n = 10. Representative topographical AFM images (Z scale = 150 nm) of three different locations on the protrusion: (c) top surface, (d) outside surface, and (e) inside surface. Scale bars for all images are 400 nm. 3D AFM topographical images of three different locations on the protrusion: (f) top surface, (g) outside surface, and (h) inside surface.

while the density of protrusion decreased with an increase in the size of the podite (Figure S9). It is possible to highlight that the width, thickness, and orientation angle are similar, regardless of the podite size and animal size. On the other hand, the height, interprotrusion distance, and density depend upon the size of the podite. AFM Analysis of the Protrusion Surface. The fine morphologies of the top and side (in/out) surfaces of the protrusions were investigated with a nanoscale resolution using AFM (Figure 8a). To avoid an unexpected disruption of the protrusion surface structure in the process of plucking for protrusions, the protrusion surface analysis was conducted using naturally formed protrusions on the legs. The top surface of the protrusions was flat and smooth, whereas both the inside and outside surfaces possess unique nanoscale morphologies (Figure 8c−h, Figures S10 and S11). In particular, the top surface of the protrusions shows a low root-mean-square (RMS) microroughness of 3.1 ± 0.6 nm. The outside and inside surfaces of the protrusions possess a higher RMS microroughness of 15.8 ± 2.8 nm and 22.1 ± 3.1 nm, respectively. We suggest that the absence of the clear texture on the top surface might be related to the excessive erosion of the top surface of the protrusion as a result of repeated contact with stiff surfaces.40 Indeed, traces of a patterned texture were observed at the edge of protrusion (Figure S12). Notably, the side surfaces of protrusion possess a welldeveloped nanoscale periodic patterned texture oriented parallel

podites were not identical throughout the podites but differed in different zones. Almost all protrusions in the fourth podite possessed an orientation angle of approximately 353 ± 3°, which indicated protrusions oriented toward the inside of the wharf roach leg, excluding those in zone γ-B and zone γ-C, which exhibited an orientation angle of 270 ± 3°. In the fifth podite, the orientation angle could be grouped into three different zones in the longitudinal direction: zone-α, 325 ± 3°; zone-β, 354 ± 3°; zone-γ, 312 ± 2°. Additionally, the protrusions located around the joints that connect different podites were irregularly distributed. Specifically, highly dense groups of protrusions with irregular orientation angles were located at the joints, which may function as a bridge for water transport from one podite to another (Figure S8). Finally, animal size had clear consequences for podite size and protrusion geometry. The podite size increased gradually with the animal size (Figure 7), which also affected some geometric parameters of protrusions. It is noteworthy that the three representative characteristic shapes of protrusions were consistently observed through all different sizes of wharf roach legs. The width, thickness, and orientation angle of the protrusions were almost identical for different sizes of the fourth podite in different zones; however, the other geometric variables such as the interprotrusion distance, height, and density, differed according to the size of the fourth podite. The height of the protrusions and the interprotrusion distance increased with an increase in the size of the podite in all zones, 2656


Article


Figure 9. (a) Pull-off forces between the AFM tip and target surface under various humidity conditions. (b) Surface energy of wharf roach protrusion, trichlorosilane-treated silicon, and piranha-cleaned silicon in-fluid conditions. Pull-off force histograms of (c) wharf roach protrusions, (d) trichlorosilane-treated hydrophobic silicon substrates, and (e) piranha-cleaned hydrophilic silicon substrates in various humidity conditions % RH. Each bar is represented mean ± STD, n = 10.

surface with a contact angle below 5°) and trichlorosilanetreated silicon substrates (a purely hydrophobic surface with a contact angle around 120°) under different humidity conditions.46,47 The pull-off forces for the protrusions on fresh wharf roach legs exhibited the lowest values among the three surfaces in each humidity condition: 1.52 ± 0.18 nN for the in-fluid condition, 2.22 ± 0.33 nN for 5% RH condition, and 3.58 ± 0.43 nN for 45% RH condition (Figure 9a,c−e). A lower pull-off force is observed on a surface having a low surface energy such as the hydrophobic surfaces.35 To properly estimate the surface energies of each surface, the effect of the capillary interactions between the AFM tip and each target surface was eliminated using the pull-off force data collected in the fluid (Figure 9b). The pull-off forces measured on both valley and ridges of textured surfaces were similar (Figure S14). The surface energies of the piranha-cleaned silicon surfaces, trichlorosilane-treated silicon surfaces, and wharf roach protrusions were determined by this approach as 44.93 ± 0.64 mJ/m2, 22.49 ± 0.26 mJ/m2, and 15.48 ± 0.18 mJ/m2, respectively. The values for comparative silicon surfaces follow the trend known from literature: the surface energies of silicon wafer substrates showed relatively higher values, in the range from 62 to 73 mJ/m2,48,49 and the surface energies of the hydrophobic silane-functionalized surfaces exhibited lower values ranging from 15 to 22 mJ/ m2.50,51 The values of surface energy of protrusions determined here are generally lower than that of common natural biopolymers such as keratin = 25.5 mJ/m2 and chitosan = 30.1 mJ/m2 and

to the longitudinal direction of protrusion (Figure 8d,e). The peak-to-peak distance (periodicity) of the nanostructural patterns of 140 nm was determined from the one-dimensional fast Fourier transform (1D-FFT) analysis of AFM images (Figure S13).41,42 Estimation of the Surface Energy of the Protrusions. By measuring the pull-off force between two different surfaces, it is possible to estimate the relative surface energy.35,43 The Johnson−Kendall−Roberts (JKR) contact model was employed to identify the relation between the work of adhesion and the pull-off force:35 Wadh =

Fpull‐off (3/2)πR

(1)

where Wadh is the work of adhesion, Fpull‑off is the pull-off force between two surfaces, and R is the radius of the tip. The surface energies were calculated using the relationship W1/2 = γ1/vapor(or liquid) + γ2/vapor(or liquid) − γ1/2,44 where W is the work of adhesion, γ is the surface energy, and the index numbers 1 and 2 represent different surfaces in physical contact.45 The surface energy of the AFM silicon tip can be examined through the work of adhesion for similar surfaces between a piranhacleaned silicon substrate and clean AFM silicon tip with the known relation for identical surfaces: Wadh = 2γ.35,45 The prospective wettability of the wharf roach’s protrusion was identified in comparison with other representative hydrophobic and hydrophilic surfaces. Specifically, pull-off force tests of protrusions on fresh wharf roach legs were compared to tests using piranha-cleaned silicon substrates (a purely hydrophilic 2657


Article

ACS Applied Bio Materials natural hydrophobic wax materials: carnauba wax = 24 mJ/m2 and beeswax = 28 mJ/m2.52,53 However, the surface energy of the protrusions is comparable to that of some natural biological surfaces and synthetic polymers, such as beetle elytra (18.5 mJ/ m2), strawberry skin (18.8 mJ/m2), tomato skin (15.04−17.4 mJ/m2), limb bud tissue cells (20.1 mJ/m2), eucalyptus leaves (17.4 mJ/m2), Fluorinert FC-70 lubricant (17.1 mJ/m2), trifluoromethyl (−CF3) end groups containing polymers (9.3 mJ/m2), and paraffin wax (19.2 mJ/m2).54−62 Some biological surfaces show much lower values of the surface energy, generating superhydrophobicity, and lower adhesion due to surface topography. For example, the epicuticular wax covered slippery surface of a carnivorous plant, Nepenthes, has a surface energy of 2.5−5.4 mJ/m2 and neural retina tissue of chick embryo organs = 1.6 mJ/m2.63,64 The surface energy is a function of a surface’s chemical composition, and thus, the surface energy of protrusions evaluated here can be partially related to the chemical composition of the protrusions’ surface.61,65 Specifically, the chemical composition of a natural waxy-like surface coating layer such as lipids can possibly cover the surface of the protrusions on fresh wharf roach legs. Thus, the surface energy of the surface of the protrusions on fresh wharf roach legs can be mediated by the layer of lipids, which are natural hydrophobic wax materials possessing a lower surface energy, but it remains to be elucidated in future studies. The overall calculated values of surface energies indicated that the wharf roach protrusions exhibited hydrophobicity on their surfaces throughout both valley and ridges of the nanostructural periodic pattern. Additionally, the determined value of a low surface energy of protrusions was measured on both smooth valley and ridges of the nanopatterned surface of protrusions and exhibited identical values, which indicates homogeneity of surface chemistry and physical properties over the entire protrusion surface.61 Moreover, the nanofibril surface texture in combination with a lower surface energy might affect the wettability of the entire protrusion surface, which can be critical for the water transport mechanism of wharf roach legs.66,67 As suggested in the previous studies, the wharf roach legs are wetted easily by water due to the open capillaries on wharf roach legs.24 As known, the macroscopic wetting phenomenon is mainly affected by structural topography groove and wettingrelated phenomena on the surfaces. Water can be transported through open capillaries comprised by protrusions when the sixth and seventh legs are apposed together. In accordance with the Wenzel wetting theory, microstructural shape details of open capillaries make the protrusions regions more wettable so that protrusions are able to transport water in a defined direction.68 In addition, the periodic surface texture might assist in the transportation of water from the wetted surface to the wharf roach’s body. Although the wharf roach legs exhibit a hydrophilic wetting behavior on a macro level, the surface of the protrusion itself is hydrophobic and shows a lower surface energy. Hydrophobicity of protrusions can enhance the water flow, while water is transported through open capillaries by confining the water wetting behavior and preventing uniform spreading in all directions.69 We suggest that the lower surface energy of protrusions can prevent water from stagnating on the surface of the wharf roach’s leg during the water transport through the open capillaries.70 It must be emphasized in this context that the structural characteristics of the open capillaries on the wharf roach legs and the patterned structure on protrusions guide the absorption of external water, and the original surface hydrophobicity of protrusions helps to transport

water without excessive local retention. Moreover, by reducing adhesion and friction forces between the two legs, the lower surface energy of protrusions may diminish the risk of abrading or tearing protrusions when the sixth and seventh legs are placed in proximity to each other to transport water. This may help to extend the wharf roaches’ water collecting ability, so it remains useful over the animal’s entire life.

■

CONCLUSIONS In conclusion, the geometric variables of protrusions were determined through statistical analyses of measurements obtained from SEM images, and the nanoscale details of the surface structures of the protrusions were identified for the first time by using AFM and surface force studies. Overall, the geometric variables were found to be identical and well matched on both left and right sides of the legs. The width, thickness, and orientation angle of protrusions on a given podite were almost identical regardless of the podite size, while the height, interprotrusion distance, and density were size dependent. We observed the nanostructural periodic patterns on the surfaces of the protrusions, which exhibited a highly textured nanoscale morphology with a periodicity of 140 nm and such a texture might influence the surface wettability of protrusions. Among various types of water transport mechanisms inspired by nature, the open-capillary-based passive water collecting mechanism of wharf roach legs enable systematical design water transport channels using structurally organized information on geometric variables of the protrusion features. The ydrophobic surface property of protrusion is critically important to fast water transport and reducing the stagnant water layer on each single protrusion. On the other hand, we can suggest that the hydrophobicity of protrusions may facilitate closed channeling within structural arrays for directional collecting and efficiently transporting water from an outside water-wet area to the gills promptly and rapidly. Overall, this study emphasizes the functionality of wharf roach protrusion features by offering critical geometric information and fundamental surface characteristics of protrusion that have not been addressed before. The current results demonstrate that in order to put a passive water transport mechanism on surfaces, the decisive geometrical shapes of the protrusion features observed on a wharf roach leg may need be accurately modeled and functionally distributed on the surfaces. The proposed types of protrusion features including information on geometric variables at the multilength scale revealed here can be used for building a passive water collecting and transporting system as critical design factors. These results also indicate that appropriately utilizing the hydrophilic function of open capillaries of the microstructural protrusions and the hydrophobicity of surface of a single protrusion can be a crucial framework to establish an effective water collecting mechanism. These findings will be useful for suggesting critical design factors to model and build biomimetic passive water transport systems. Furthermore, comprehensive consideration of the geometric property-based and surface property-based passive water transport mechanism will be significant to design for drinking water saving systems, rapid liquid drainage systems, selective liquid collecting surfaces, and other microfluidic systems with taking advantages of such liquid transporting mechanism. On the basis of the findings of this research, exploring not only the mechanism of capillary effect on water transport but also the effect of surface lipid layers would be helpful for expending the scope of future studies of the efficient bioinspired water 2658


Article


retaining nanofur for drag reduction. ACS Appl. Mater. Interfaces 2015, 7, 10651. (12) Watson, G. S.; Green, D. W.; Schwarzkopf, L.; Li, X.; Cribb, B. W.; Myhra, S.; Watson, J. A. A gecko skin micro/nano structure−A low adhesion, superhydrophobic, anti-wetting, self-cleaning, biocompatible, antibacterial surface. Acta Biomater. 2015, 21, 109. (13) Wang, Y.; Hu, H.; Shao, J.; Ding, Y. Fabrication of well-defined mushroom-shaped structures for biomimetic dry adhesive by conventional photolithography and molding. ACS Appl. Mater. Interfaces 2014, 6, 2213. (14) Kamei, J.; Abe, H.; Yabu, H. Biomimetic bubble-repellent tubes: Microdimple arrays enhance repellency of bubbles inside of tubes. Langmuir 2017, 33, 585. (15) McConney, M. E.; Schaber, C. F.; Julian, M. D.; Eberhardt, W. C.; Humphrey, J. A.; Barth, F. G.; Tsukruk, V. V. Surface force spectroscopic point load measurements and viscoelastic modelling of the micromechanical properties of air flow sensitive hairs of a spider (Cupiennius salei). J. R. Soc., Interface 2009, 6, 681. (16) Siddique, R. H.; Gomard, G.; Hölscher, H. The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly. Nat. Commun. 2015, 6, 6909. (17) Hazel, J.; Stone, M.; Grace, M.; Tsukruk, V. Nanoscale design of snake skin for reptation locomotions via friction anisotropy. J. Biomech. 1999, 32, 477. (18) McConney, M. E.; Anderson, K. D.; Brott, L. L.; Naik, R. R.; Tsukruk, V. V. Bioinspired material approaches to sensing. Adv. Funct. Mater. 2009, 19, 2527. (19) Young, S. L.; Chyasnavichyus, M.; Barth, F. G.; Zlotnikov, I.; Politi, Y.; Tsukruk, V. V. Micromechanical properties of strain-sensitive lyriform organs of a wandering spider (Cupiennius salei). Acta Biomater. 2016, 41, 40. (20) McConney, M. E.; Schaber, C. F.; Julian, M. D.; Barth, F. G.; Tsukruk, V. V. Viscoelastic nanoscale properties of cuticle contribute to the high-pass properties of spider vibration receptor (Cupiennius salei Keys). J. R. Soc., Interface 2007, 4, 1135. (21) Young, S. L.; Chyasnavichyus, M.; Erko, M.; Barth, F. G.; Fratzl, P.; Zlotnikov, I.; Politi, Y.; Tsukruk, V. V. A spider’s biological vibration filter: micromechanical characteristics of a biomaterial surface. Acta Biomater. 2014, 10, 4832. (22) McConney, M. E.; Chen, N.; Lu, D.; Hu, H. A.; Coombs, S.; Liu, C.; Tsukruk, V. V. Biologically inspired design of hydrogel-capped hair sensors for enhanced underwater flow detection. Soft Matter 2009, 5, 292. (23) Bixler, G. D.; Bhushan, B. Fluid drag reduction with shark skin riblet inspired microstructured surfaces. Adv. Funct. Mater. 2013, 23, 4507. (24) Ishii, D.; Horiguchi, H.; Hirai, Y.; Yabu, H.; Matsuo, Y.; Ijiro, K.; Tsujii, K.; Shimozawa, T.; Hariyama, T.; Shimomura, M. Water transport mechanism through open capillaries analyzed by direct surface modifications on biological surfaces. Sci. Rep. 2013, 3, 3024. (25) Tani, M.; Ishii, D.; Ito, S.; Hariyama, T.; Shimomura, M.; Okumura, K. Capillary rise on legs of a small animal and on artificially textured surfaces mimicking them. PLoS One 2014, 9, No. e96813. (26) Ito, S.; Ishii, D. Overwritable liquid selective open channel. Surf. Interface Anal. 2016, 48, 1199. (27) Muto, K.; Ishii, D. Effects of anisotropic liquid spreading on liquid transport in arrow-like micropillar arrays. Colloids Surf., A 2018, 544, 86. (28) Hirai, Y.; Mayama, H.; Matsuo, Y.; Shimomura, M. Uphill Water Transport on a Wettability-Patterned Surface: Experimental and Theoretical Results. ACS Appl. Mater. Interfaces 2017, 9, 15814. (29) Zhang, S.; Geryak, R.; Geldmeier, J.; Kim, S.; Tsukruk, V. V. Synthesis, assembly, and applications of hybrid nanostructures for biosensing. Chem. Rev. 2017, 117, 12942. (30) Geryak, R.; Quigley, E.; Kim, S.; Korolovych, V. F.; Calabrese, R.; Kaplan, D. L.; Tsukruk, V. V. Tunable Interfacial Properties in Silk Ionomer Microcapsules with Tailored Multilayer Interactions. Macromol. Biosci. 2019, 19, 1800176.

transport systems. Thus, identifying capillary-driven condensation of the protrusion arrays using a colloidal type AFM tip will be beneficial in order to further elucidate the role of open capillaries and presence of lipid layers on the macroscopic wetting behavior of wharf roach legs as the future work.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsabm.9b00318. Details of SEM, optical microscope, and AFM characterization data of wharf roach legs and 1D-FFT analysis for the peak-to-peak distance (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 82-2-820-5265. *E-mail: [email protected]. Tel: 1-404-894-6081. ORCID

Sunghan Kim: 0000-0003-0597-5512 Vladimir V. Tsukruk: 0000-0001-5489-0967 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Victor Breedveld for hydrophobic surfaces and valuable discussions and Jessica Pruett for collecting roaches. The authors are thankful for financial support from the Kimberly Clark Corporation and the Air Force Office for Scientific Research FA9550-17-1-0297 Award.

■

REFERENCES

(1) Young, S. L.; Gupta, M.; Hanske, C.; Fery, A.; Scheibel, T.; Tsukruk, V. V. Utilizing conformational changes for patterning thin films of recombinant spider silk proteins. Biomacromolecules 2012, 13, 3189. (2) Hazel, J.; Fuchigami, N.; Gorbunov, V.; Schmitz, H.; Stone, M.; Tsukruk, V. Ultramicrostructure and microthermomechanics of biological IR detectors: materials properties from a biomimetic perspective. Biomacromolecules 2001, 2, 304. (3) McConney, M. E.; Tsukruk, V. V. Synthetic Materials for BioInspired Flow-Responsive Structures. Frontiers in Sensing 2012, 341. (4) Bhushan, B. Biomimetics: lessons from nature−an overview. Philos. Trans. R. Soc., A 2009, 367, 1445. (5) Watson, G. S.; Watson, J. A.; Cribb, B. W. Diversity of cuticular micro-and nanostructures on insects: Properties, functions, and potential applications. Annu. Rev. Entomol. 2017, 62, 185. (6) Chung, T.; Lee, Y.; Yang, S. P.; Kim, K.; Kang, B. H.; Jeong, K. H. Mining the Smartness of Insect Ultrastructures for Advanced Imaging and Illumination. Adv. Funct. Mater. 2018, 28, 1705912. (7) Yu, K.; Fan, T.; Lou, S.; Zhang, D. Biomimetic optical materials: Integration of nature’s design for manipulation of light. Prog. Mater. Sci. 2013, 58, 825. (8) He, Y.-D.; Zhang, Z.-L.; Xue, J.; Wang, X.-H.; Song, F.; Wang, X.L.; Zhu, L.-L.; Wang, Y.-Z. Biomimetic Optical Cellulose Nanocrystal Films with Controllable Iridescent Color and Environmental StimuliResponsive Chromism. ACS Appl. Mater. Interfaces 2018, 10, 5805. (9) Wisdom, K. M.; Watson, J. A.; Qu, X.; Liu, F.; Watson, G. S.; Chen, C.-H. Self-cleaning of superhydrophobic surfaces by self-propelled jumping condensate. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 7992. (10) Blossey, R. Self-cleaning surfacesvirtual realities. Nat. Mater. 2003, 2, 301. (11) Kavalenka, M. N.; Vüllers, F.; Lischker, S.; Zeiger, C.; Hopf, A.; Röhrig, M.; Rapp, B. E.; Worgull, M.; Hölscher, H. Bioinspired air2659


Article

ACS Applied Bio Materials (31) Fuchigami, N.; Hazel, J.; Gorbunov, V.; Stone, M.; Grace, M.; Tsukruk, V. Biological thermal detection in infrared imaging snakes. Biomacromolecules 2001, 2, 757. (32) Gorbunov, V.; Fuchigami, N.; Stone, M.; Grace, M.; Tsukruk, V. Biological thermal detection: micromechanical and microthermal properties of biological infrared receptors. Biomacromolecules 2002, 3, 106. (33) Bhushan, B.; Jung, Y. C. Micro-and nanoscale characterization of hydrophobic and hydrophilic leaf surfaces. Nanotechnology 2006, 17, 2758. (34) McConney, M. E.; Singamaneni, S.; Tsukruk, V. V. Probing soft matter with the atomic force microscopies: imaging and force spectroscopy. Polym. Rev. 2010, 50, 235. (35) Tsukruk, V. V.; Bliznyuk, V. N. Adhesive and friction forces between chemically modified silicon and silicon nitride surfaces. Langmuir 1998, 14, 446. (36) Kim, S.; Polycarpou, A. A.; Liang, H. Active control of surface forces via nanopore structures. APL Mater. 2013, 1, 032118. (37) Tsukruk, V. V.; Singamaneni, S. Scanning Probe Microscopy of Soft Matter: Fundamentals and Practices; John Wiley & Sons, 2012. (38) Kim, S.; Xiong, R.; Tsukruk, V. V. Probing Flexural Properties of Cellulose Nanocrystal−Graphene Nanomembranes with Force Spectroscopy and Bulging Test. Langmuir 2016, 32, 5383. (39) Cook, S.; Schäffer, T.; Chynoweth, K.; Wigton, M.; Simmonds, R. W.; Lang, K. Practical implementation of dynamic methods for measuring atomic force microscope cantilever spring constants. Nanotechnology 2006, 17, 2135. (40) Vincent, J. F.; Wegst, U. G. Design and mechanical properties of insect cuticle. Arthropod Struct. Dev. 2004, 33, 187. (41) Brun, M.; Demadrille, R.; Rannou, P.; Pron, A.; Travers, J. P.; Grévin, B. Multiscale Scanning Tunneling Microscopy Study of Self Assembly Phenomena in Two Dimensional Polycrystals of π Conjugated Polymers: The Case of Regioregular Poly (dioctylbithiophene alt fluorenone). Adv. Mater. 2004, 16, 2087. (42) Erickson, B.; Fang, M.; Wallace, J. M.; Orr, B. G.; Les, C. M.; Banaszak Holl, M. M. Nanoscale structure of type I collagen fibrils: quantitative measurement of D spacing. Biotechnol. J. 2013, 8, 117. (43) Kim, S.; Geryak, R. D.; Zhang, S.; Ma, R.; Calabrese, R.; Kaplan, D. L.; Tsukruk, V. V. Interfacial Shear Strength and Adhesive Behavior of Silk Ionomer Surfaces. Biomacromolecules 2017, 18, 2876. (44) Schrader, M. E. Langmuir 1995, 11, 3585. (45) Adams, A.; Gast, A. John Wiley and Sons: New York, 1997. (46) Tsukruk, V. V.; Reneker, D. H. Scanning Probe Microscopy of Polymeric and Organic Molecular Films: From Self-Assembled Monolayers to Composite Multilayers. Polymer 1995, 36, 1791. (47) Tsukruk, V. V.; Bliznyuk, V. N. Adhesive and friction forces between chemically modified silicon and silicon nitride surfaces. Langmuir 1998, 14, 446. (48) Janssen, D.; De Palma, R.; Verlaak, S.; Heremans, P.; Dehaen, W. Static solvent contact angle measurements, surface free energy and wettability determination of various self-assembled monolayers on silicon dioxide. Thin Solid Films 2006, 515, 1433. (49) Fiorilli, S.; Rivolo, P.; Descrovi, E.; Ricciardi, C.; Pasquardini, L.; Lunelli, L.; Vanzetti, L.; Pederzolli, C.; Onida, B.; Garrone, E. Vaporphase self-assembled monolayers of aminosilane on plasma-activated silicon substrates. J. Colloid Interface Sci. 2008, 321, 235. (50) Gorb, E.; Gorb, S. Effects of surface topography and chemistry of Rumex obtusifolius leaves on the attachment of the beetle Gastrophysa viridula. Entomol. Exp. Appl. 2009, 130, 222. (51) Vafaei, S.; Tuck, C.; Ashcroft, I.; Wildman, R. Surface microstructuring to modify wettability for 3D printing of nano-filled inks. Chem. Eng. Res. Des. 2016, 109, 414. (52) Sionkowska, A.; Skopinska-Wisniewska, J.; Planecka, A.; Kozlowska, J. The influence of UV irradiation on the properties of chitosan films containing keratin. Polym. Degrad. Stab. 2010, 95, 2486. (53) Wang, W.; Lockwood, K.; Boyd, L. M.; Davidson, M. D.; Movafaghi, S.; Vahabi, H.; Khetani, S. R.; Kota, A. K. Superhydrophobic coatings with edible materials. ACS Appl. Mater. Interfaces 2016, 8, 18664.

(54) Voigt, D.; Tsipenyuk, A.; Varenberg, M. How tight are beetle hugs? Attachment in mating leaf beetles. R. Soc. Open Sci. 2017, 4, 171108. (55) Skurtys, O.; Velásquez, P.; Henriquez, O.; Matiacevich, S.; Enrione, J.; Osorio, F. Wetting behavior of chitosan solutions on blueberry epicarp with or without epicuticular waxes. LWT-Food Sci. Technol. 2011, 44, 1449. (56) Sánchez-Ortega, I.; García-Almendárez, B. E.; Santos-López, E. M.; Reyes-González, L. R.; Regalado, C. Characterization and antimicrobial effect of starch-based edible coating suspensions. Food Hydrocolloids 2016, 52, 906. (57) Casariego, A.; Souza, B.; Vicente, A.; Teixeira, J.; Cruz, L.; Díaz, R. Chitosan coating surface properties as affected by plasticizer, surfactant and polymer concentrations in relation to the surface properties of tomato and carrot. Food Hydrocolloids 2008, 22, 1452. (58) Forgacs, G.; Foty, R. A.; Shafrir, Y.; Steinberg, M. S. Viscoelastic properties of living embryonic tissues: a quantitative study. Biophys. J. 1998, 74, 2227. (59) Khayet, M.; Fernández, V. Estimation of the solubility parameters of model plant surfaces and agrochemicals: a valuable tool for understanding plant surface interactions. Theor. Biol. Med. Modell. 2012, 9, 45. (60) You, I.; Lee, T. G.; Nam, Y. S.; Lee, H. Fabrication of a microomnifluidic device by omniphilic/omniphobic patterning on nanostructured surfaces. ACS Nano 2014, 8, 9016. (61) Gupta, M.; Gleason, K. K. Initiated chemical vapor deposition of poly (1H, 1H, 2H, 2H-perfluorodecyl acrylate) thin films. Langmuir 2006, 22, 10047. (62) Chibowski, E. On some relations between advancing, receding and Young’s contact angles. Adv. Colloid Interface Sci. 2007, 133, 51. (63) Wang, L.; Zhou, Q. Surface hydrophobicity of slippery zones in the pitchers of two Nepenthes species and a hybrid. Sci. Rep. 2016, 6, 19907. (64) Winklbauer, R. Cell adhesion strength from cortical tension−an integration of concepts. J. Cell Sci. 2015, 128, 3687. (65) Goodwyn, P. P.; De Souza, E.; Fujisaki, K.; Gorb, S. Moulding technique demonstrates the contribution of surface geometry to the super-hydrophobic properties of the surface of a water strider. Acta Biomater. 2008, 4, 766. (66) Xue, Y.; Wang, H.; Yu, D.; Feng, L.; Dai, L.; Wang, X.; Lin, T. Superhydrophobic electrospun POSS-PMMA copolymer fibres with highly ordered nanofibrillar and surface structures. Chem. Commun. 2009, 6418. (67) Reshmi, C.; Sundaran, S. P.; Juraij, A.; Athiyanathil, S. Fabrication of superhydrophobic polycaprolactone/beeswax electrospun membranes for high-efficiency oil/water separation. RSC Adv. 2017, 7, 2092. (68) McHale, G.; Shirtcliffe, N.; Newton, M. Analyst 2004, 129, 284. (69) Dong, Y.; Kong, J.; Phua, S. L.; Zhao, C.; Thomas, N. L.; Lu, X. Tailoring surface hydrophilicity of porous electrospun nanofibers to enhance capillary and push−pull effects for moisture wicking. ACS Appl. Mater. Interfaces 2014, 6, 14087. (70) Ueda, E.; Levkin, P. A. Emerging applications of superhydrophilic superhydrophobic micropatterns. Adv. Mater. 2013, 25, 1234.

2660


Morphology and Surface Properties of Roach Water Transport Arrays

Recommend Documents