Friction-Active Surfaces Based on Free-Standing Anchored Cellulose

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Surfaces, Interfaces, and Applications

Friction-Active Surfaces Based on FreeStanding Anchored Cellulose Nanofibrils Clemens Schaber, Agnieszka Kreitschitz, and Stanislav N. Gorb ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05972 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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ACS Applied Materials & Interfaces

Friction-Active

Surfaces

Based

on

Free-Standing

Anchored

Cellulose Nanofibrils

Clemens F. Schaber1*, Agnieszka Kreitschitz1,2, Stanislav N. Gorb1

1

Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 9, 24118 Kiel, Germany

2

Department of Plant Developmental Biology, Institute of Experimental Biology, University of Wrocław, ul. Kanonia, 50-328 Wroclaw, Poland

*C. F. Schaber e-mail: [email protected] Tel. +49 431 880-4509 Fax +49 431 880-1389

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Abstract: A specific feature of fibrous surfaces is the dependence of their mechanical properties on the alignment of the fibers. Vertically aligned fibers enhance friction and adhesion, whereas horizontal fibers are known to act as lubricant reducing friction. Many plants form a specific fibrous mucilage cover around their seeds upon hydration. This mucilage consists of cellulose, hemicelluloses, and strongly hydrophilic pectins. We show that controlled critical point drying of hydrated seed mucilage of three exemplary seed mucilage-rich plant species results in exposure of free-standing cellulose nanofibers with very high aspect ratio and anchored to the seed surface. The structural dimensions of the cellulose nanofibers (CNFs) are similar to vertically aligned carbon nanotubes and the contact elements in the adhesion system of the gecko, which show outstanding high dry friction and adhesion. Tribological experiments demonstrate very high average friction coefficients when sliding a smooth and stiff probe over the surface of such arrays of dry free-standing cellulose nanofibrils in the range from 1.4 to 1.8. The high friction values most likely arise from bending of the single cellulose fibers and their alignment with the counterpart surface in close contact. We suggest the potential of free-standing cellulose nanofibrils of plant seed mucilage as a natural and ecologically friendly material where high contact forces to surfaces in dry environments are desired.

Keywords: nanofibers, seed mucilage, friction, cellulose, reversible adhesives


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Introduction Friction is very high on surfaces coated by vertically aligned nanofibers, such as carbon nanotube arrays covalently bonded to the substrate1,2. This effect can partly be explained by adhesive forces between the minute nanometers-sized fibers and a sliding probe. Here we report on a novel approach of using free-standing cellulose nanofibers of the seed mucilage of plants for generating of similar physical effects, i.e. strong contact forces based on van der Waals interactions between fibers and the counterpart surface. Cellulose represents a natural, renewable polymer characteristic for the plants, and plants are the main source of industrially used cellulose fibers3,4. Cellulose can be physically, chemically, or biochemically modified in order to adjust its properties for the specific purpose. Most commercially harvested cellulose is used in the paper industry. Nano-cellulose is important for polymer composites preparation e.g. in production of biodegradable polymers. It has also found many applications in biomedicine, cosmetics, and food industry3-5. However, in all these cases, cellulose fibers are not free-standing and anchored to the surface. The cellulose nanofibers are generally strong in tension and weak in compression. This is the reason why they are part of many biological nanocomposites6 such as the vast majority of plant cell walls, providing high breaking stress in wood and tensile strength in other plant tissues. Additionally, cellulose nanofibers of different orientation are involved in plant actuators, which are activated by changing air humidity or moisture content in the material7–13. In arthropods, molecularly similar polysaccharide chitin nanofibers reinforce the cuticular exoskeleton, which represents another remarkably stable natural nanocomposite14. Various other nanofibers are part of adhesive systems, such as keratin-based fibrous setae of geckos15 or fiber-like fillers of smooth adhesive pads of insects16,17. Nanofibers in the clingfish adhesive system enhance the sealing of suction cups and their adaptability to the wide range of substrate roughness in combination with 3 ACS Paragon Plus Environment


mucus18. In adhesive systems, nanofibers provide strong adaptability to the substrate by their strong deformability under compression, but withstand high tensile forces when they adhere in contact19. Cellulose represents a linear homopolymer built of 1,4-β-D-glucan units linked by 1,4-β glycoside bonds. Linked together in parallel, the cellulose polymer chains form fibers with a diameter of 3.5 nm typical for the plant cell wall3,20,21. In most cases, these elementary fibers bundle together to nanofibrils and are frequently called ʻmicrofibrilsʼ in literature. For the sake of clarity, in agreement with the common language in the nanosciences, and in order to specify the structures by their real size, we will stick to the terminus ʻnanofibrilsʼ for all the fibers up to approximately 100 nm in diameter. Mucilage containing cellulose as a major structural element is typical for seeds of many diverse plant families, e.g. Brassicaceae, Asteraceae, Lamiaceae, and Plantaginaceae22–24. The mucilage is produced by secreting cells, which are often part of the seed or fruit coat25,26. It consists of a mixture of different polysaccharides i.e. pectins, hemicelluloses, and cellulose. The main components of the mucilage are pectins, which absorb large amounts of water upon hydration, leading to the formation of a gel-like capsule around the seed within a few minutes24,27. Very often the cellulose in the mucilage is organized in long fibers stretching radially from the seed into the mass of the hydrogel formed by pectins25,27. The cellulose fibrils are a kind of skeleton for the mucilage and prevent its loss from the seed surface25,28,29. Different amounts of available water result in specific adhesive and frictional properties of the mucilage envelope with an impact on e.g. the dispersal mode of the seeds30,31. Also the presence and the nature of mucilage reinforcement by aligned cellulose fibrils can tune the adhesive and frictional properties of hydrated seeds.


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Recently, a method was reported for the preparation of hierarchically ordered dry seed mucilage using critical point drying (CPD) of hydrated seeds25. The method resulted in the formation of a unique network of polysaccharide nanofibrils on the surface, with the cellulose fibrils being the structurally most important backbone material. The present paper aims at mechanical testing for possible applications of the material based on the free-standing and anchored cellulose nanofibrils as a natural, environmentally friendly, and ecologically sustainable high-friction material. We performed friction tests on the CPD-prepared dry seed mucilage of three exemplary seed mucilage-rich plant species, the cress Lepidium sativum L. (L. sativum), the white wormwood Artemisia leucodes SCHRENCK (A. leucodes), and the desert plant Neopallasia pectinata L. (N. pectinata). The seeds of the selected plant species typically form cellulose-rich mucilage envelopes upon hydration with slightly different dimensions of the cellulose nanofibers. With regard to the possible impact on the remarkable frictional properties found, the specific structural arrangements of the cellulose nanofibril networks were examined using light and scanning electron microscopy.

Results

Hydrated mucilage morphology and composition. The surface of the untreated dry seeds was covered with the delicate, thin, and transparent seed coat consisting of the mucilage secreting cells, which produce the mucilage envelope when hydrated (Figures 1a–c). Upon hydration the mucilage material swelled by water uptake, and after a few minutes a gel-like envelope appeared around the seed coat (Figure 1d–f). Among the three examined species, the biggest mucilage envelope was formed by N. pectinata, which more than doubles the width of the seed when compared with the dry state (Figure 1f). 5 ACS Paragon Plus Environment


Figure 1. Mucilage formation upon hydration of the seed. Dry seeds of (a) Lepidium sativum, (b) Artemisia leucodes, and (c) Neopallasia pectinata, and the same seeds hydrated (d–f) and surrounded by the transparent gel-like mucilage envelope (me). (f) In N. pectinata the whitish color of the envelope clearly indicates the presence of cellulose fibrils.

Staining reactions allowed the determination of the main components of the mucilage. Ruthenium red specifically showed the presence of pectins, which due to their swelling constitute the big mass of the envelope (Figures 2a–c). Stained with Crystal violet, Methylene blue or Safranin, the cellulose fibrils appeared stretching out from the seed surface in the mass of pectins. In L. sativum, the cellulose fibrils were visible as uncoiled, straight, and delicate fibers spreading radially from the surface of the seed. Their length corresponded with the envelope size (Figure 2d). In case of A. leucodes and N. pectinata, the cellulose was visible in form of thicker and coiled threads (Figure 2e,f). In N. pectinata their length exceeded two to five times the size of the mucilage envelope and limited uncoiling of the fibrils during preparation (Figure 2a,f). Staining 6 ACS Paragon Plus Environment

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of cellulose using the specific dye Direct red 23 proved that it were the cellulose fibrils that extended radially from the surface of the seed core in the hydrated mucilage of all three tested plant species (Figure 2g–i).

Figure 2. Identification of mucilage components in hydrated seeds. (a) Ruthenium red staining of seeds of L. sativum, (b) A. leucodes, and (c) N. pectinata demonstrates the presence of pectins (rose to red color) forming the main mass of the mucilage envelopes (me) around the seed (se). The dark “shells” in (a) and (b) indicate increased mucilage material density due to coiling of fibrils resulting in dye accumulation, whereas in (c) N. pectinata a copious mucilage envelope of long, outreaching, and at low magnification apparently uncoiled fibrils is visible. (d) Delicate cellulose fibrils (cf) stretch radially from the seed surface in the mucilage of L. sativum (stained using Methylene blue). (e) Slightly coiled cellulose fibrils (cf) in A. leucodes stained using Crystal violet. (f) In N. pectinata the cellulose fibrils (cf) formed thick and heavily coiled fibers visible at



higher magnification (Safranin staining). (g–i) CLSM optical sections show the cellulose fibrils (cf), identified by their fluorescence after staining with Direct Red 23, embedded in the mucilage and stretching from the seed surface (se).

Mucilage structure after dehydration and critical point drying. After the water was removed from the swollen mucilage envelope by CPD, the structure and organization of the remaining components could be examined using scanning electron microscopy (SEM) (Figure 3). The dry mucilage components could be identified as fibrils of diverse sizes and lengths. At low magnification, the mucilage envelope of L. sativum appeared rather compact (Figure 3a) compared to the more fibrillar and loose structure of A. leucodes and N. pectinata (Figure 3b,c). In L. sativum, at higher magnification the fibrils were organized in a dense tangled weblike structure (Figure 3), whereas those of A. leucodes and N. pectinata mucilage appeared more ordered with a tendency to aggregate and form strands with varying widths (Figure 3e,f). In detail, the mucilage components at most formed a net-like structure made up of nanofibrils of different size and aggregation (Figures 3g–i). The long and unbranched cellulose fibrils were decorated by the narrow, short, and branched chains of other polysaccharides, such as pectins and hemicelluloses. The diameters of the fibrous single components of L. sativum mucilage were similar, with the long stretched cellulose fibrils building the backbone of the materials, and shorter, partially branched ones spread in between (Figure 3g). The cellulose fibrils of A. leucodes and N. pectinata formed thicker, longitudinal strands interconnected by thinner, shorter, branched and unbranched fibrils (Figures 3h,i). In the latter two species, the mucilage envelope was organized in a way so that the thicker cellulose fibrils extended radially from the seed surface and constituted a kind of framework for the other mucilage components located in between. 8 ACS Paragon Plus Environment

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Figure 3. SEM of different organization levels of seed mucilage envelopes. (a), (d), (g) Micrographs of the seed mucilage of L. sativum, (b), (e), (h) of A. leucodes, and (c), (f), (i) of N. pectinata after critical point drying. Arrows in (g), (h), and (i) point to cellulose fibrils, and arrowheads to cross-linking, partially branched, and shorter chains of other fibrillar polysaccharides most likely representing pectins and hemicelluloses.

The thickness of the single cellulose fibrils varied statistically significantly between the three species studied (Figure 4). The narrowest single cellulose fibrils were those of L. sativum with a mean thickness (± standard deviation) of 18.8 (± 2.5) nm, followed by those of A. leucodes with a mean thickness of 22.4 (± 4.0) nm, and thickest ones were found in N. pectinata with a mean of 30.7 (± 3.7) nm. The number of single fibrils measured for each species was 30 (n=30). 9 ACS Paragon Plus Environment


45 c

40 Thickness (nm)

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35 30

b

a

25 20 15 10 Lepidium sativum

Artemisia Neopallasia leucodes pectinata

Figure 4. Thickness of cellulose nanofibrils in dry seed mucilage of the three plant species tested. The black dots are original data from 30 measurements on each species. The boxes show the 25th and the 75th, and the whiskers the 10th and 90th percentiles of the data. Median values are indicated by the horizontal line inside the boxes. a, b, and c indicate the significant statistical difference (P

Friction-Active Surfaces Based on Free-Standing Anchored Cellulose

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