1 s2.0 S223878542300039X Main
1 s2.0 S223878542300039X Main
1 s2.0 S223878542300039X Main
Original Article
Article history: Surface texturing has been a powerful means to improve the service performance of
Received 25 November 2022 various engineering materials. The 3 mol% yttria-stabilized tetragonal zirconia polycrystal
Accepted 6 January 2023 (3Y-TZP) represents the most-used ceramic oxide in restorative dentistry due to its
Available online 17 January 2023 excellent esthetic effects, good chemical stability and superior biocompatibility. However,
such materials show the limited ability of tribological and antibacterial performance for
Keywords: dental applications. In the present work, a bio-inspired design of textured surfaces was
3Y-TZP ceramics conducted based on the microstructural analysis of butterfly wings, peacock tail feathers,
Biomimetic textures and dolphin skins to improve the service performances of 3Y-TZP ceramics for dental
Wettability behavior applications. Three types of microtextures, including micro-grids, micro-feathers, and
Tribological performance micro-grooves, were fabricated onto the 3Y-TZP ceramics using the laser ablation tech-
Antibacterial properties nique. The effects of different microtextures on the wettability, tribological and antibac-
terial behaviors of 3Y-TZP ceramics were studied. The results indicate that all the
biomimetic microtextures can effectively improve the service performance of 3Y-TZP.
Wettability acts as a decisive factor for the tribological and antibacterial performances of
textured ceramic surfaces. The bio-inspired microtextured surfaces all show hydrophobic
behavior, thus yielding an effective improvement of antibacterial properties for 3Y-TZP.
Moreover, the micro-grids inspired by the butterfly wings basically perform the best in
tribological and antibacterial tests compared with the other counterparts.
© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
* Corresponding author.
E-mail address: xujinyang@sjtu.edu.cn (J. Xu).
https://doi.org/10.1016/j.jmrt.2023.01.039
2238-7854/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
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Fig. 1 e The SEM images of microstructures of wing surfaces for (a) Stibochiona nicea (Gray) (1 mm) [35]; (b) Vanessa indica
Herbst (10 mm) [36]; (c) Pieris rapae (10 mm) [36].
To develop anti-wear and antibacterial surfaces for 3Y-TZP and other pollutants are mixed, the surfaces of butterfly wings
ceramics, the bionic design method was adopted in the cur- can always keep clean and dry [20]. Previous studies have
rent work. The paper attempts to prepare functional textures found that the unique ultrastructure of the butterfly wing
using laser ablation method to enhance the service perfor- scale surface is the key factor for its hydrophobic and self-
mances of 3Y-TZP for dental use. The topographical features cleaning performance [21]. Fig. 1 shows the scanning elec-
of three microtextures, including micro-grids, micro-feathers, tron microscope (SEM) images of microstructures of wing
and micro-grooves, were inspired by the microstructural surfaces for Stibochiona nicea (Gray) [35], Vanessa indica
analysis of butterfly wings, peacock tail feathers, and dolphin Herbst [36] and Pieris rapae [36]. These butterfly scales have
skins. Considering the high machining efficiency and grid-like micro-textures on the surface and are regularly ar-
noncontact characteristics of the laser ablation method [34], ranged, with ridges and ribs parallel to each other distributed
the picosecond laser processing system was used to prepare between the scales. Inspired by the closely-arranged grid
the designed microtextures onto the 3Y-TZP samples. The structure on the surface of butterfly scales, the biomimetic
contact angle measurements and tribological tests were car- model of micro-grids was established. The bionic surface was
ried out to compare the wettability and anti-wear behaviors of closely arranged with regular grid-like groove structures to
laser-textured 3Y-TZP samples. Staphylococcus aureus was also improve the hydrophobic performance. Additionally, previous
cultured on the nontextured and laser-textured samples to studies indicate that when the laser processing pitch
evaluate the antibacterial properties of different micro- increased to 100 mm, the surface structure was gradually
textures. The paper clarifies the mechanisms of different revealed, and the grid structure was not yet apparent. When
microtextures affecting the service performance of zirconia the laser pitch was increased to 150 mm and 200 mm, the sur-
ceramics for dental applications. face structure could be fully revealed [23]. Based on the sur-
face structure of butterfly wings and the characteristics of
laser processing technology, the texture model was designed,
2. Experimental design, implementation and as shown in Fig. 2.
measurement Peacock tail feathers are proved to possess exceptional
wetting properties. As depicted in Fig. 3(a) [37], the droplets
2.1. Bionic design of microtextures exist in an approximately spherical shape and can roll freely
at a certain inclination angle on the surface of the peacock
Butterfly wings have strong hydrophobicity and self-cleaning feather. This is due to the unique structure formed by the
properties. In the natural environment, where rain, dust, growth and arrangement of the feathers. Fig. 3(b) is a peacock
Fig. 2 e The schematic diagram of the geometrical features of the designed micro-grid textures (Unit: mm).
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feather structure magnified 10 times under an optical micro- reduction. While for water striders that can stand and walk on
scope. As shown in Fig. 3(c), when increasing the observation the water surface, their legs must have super-hydrophobic
magnification, the branches of peacock feathers are structures to hinder water infiltration. It can be seen from
composed of closely arranged scale-like structures, and there Fig. 5 (c) and (d) that their leg setae also have irregular groove-
are tiny gaps between each column of feathers. In contrast, like structures [40]. These grooves are not in the same direc-
there is a larger gap between rachides. The width of each quill tion but staggered from each other at an angle. Therefore, the
is about 40 mm, and the width from the rachis to the edge of surfaces of the above three organisms all have or can be
the quill is about 300 mm [37]. Considering the characteristics simplified into parallel grooves, and a biomimetic model can
of laser ablation methods, the bionic model shown in Fig. 4 be derived as shown in Fig. 6. The grooves in the model are not
was established, in which the microstructure characteristics rectangular grooves but have evenly distributed arc-shaped
of the peacock feathers were preserved, and some geometrical depressions with a radius of 300 mm, which is to simulate
parameters were adjusted appropriately. the uneven state of the grooves on the biological surface. It is
The dolphin surface is very smooth, containing an average speculated that this surface may be hydrophobic and friction-
roughness of only 5.3 mm. As can be seen from Fig. 5 (a), the reducing.
dolphin skin is not a perfectly smooth surface but has folds all
over the body [38]. These folds can be simplified as grooved 2.2. Fabrication of microtextures
microtextures with equal spacing and depth. The shark sur-
face is made up of numerous overlapping scales called The 3Y-TZP powders were purchased from a commercial
“dermal denticles”. It can be seen from Fig. 5 (b) that these company to fabricate the zirconia workpiece samples. At the
protrusions are a segment of ridges along the shark's initial stage, the powdery 3Y-TZP was firstly dry pressed with
advancing direction. They are oriented in the same direction a pressure of 90 MPa and then hot pressed at 300 C with a
but not strictly along a straight line [39]. The groove-type pressure of 240 MPa to form the green-compact billets [41].
microtexture can be obtained by simplifying the protrusions Then, the ceramic blocks were sintered inside a sintering
that are not along the same line as the protrusions are con- furnace at the sintering temperatures of 1500 C under the
nected in sequence. The approximate groove-type micro- atmospheric environment. The bulk density and Vickers
textures of dolphins and sharks are mainly used for drag hardness of the fully-sintered 3Y-TZP samples are 6.04 g/cm3
Fig. 4 e The schematic diagram of the geometrical features of the designed micro-feather textures (Unit: mm).
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Fig. 5 e The organisms with grooved microtextures: (a) dolphins and their folds across the skin surface [38]; (b) sharks and
their surface ridges [39]; (c) SEM image of water strider leg bristles [40]; (d) SEM image of grooves on the surface of water
strider leg bristles [40].
and 1295 HV, respectively [41]. The fully-sintered 3Y-TZP evaporate the solid material for chip separation. The method
sample is composed of 98.75% tetragonal phase and 1.25% is more flexible and can realize the mapping of 2D data to the
monoclinic phase, and its grain size is within 522.60 nm surface of the 3D model, generate 3D data, and realize the
through our previous XRD analysis [41,42]. Before the laser marking of the physical surface. The laser processing tech-
treatment, all the ceramic samples were carefully polished to nology can be seamlessly connected with the AutoCAD soft-
have the identical size of 10 mm (length) 10 mm ware without complex conversion and has a powerful
(width) 5 mm (thickness). To create designed microtextures secondary development interface, which is suitable for a
onto the 3Y-TZP samples, the laser processing method was broad range of applications. In the present study, the ultra-
adopted. It is a very promising and powerful manufacturing violet picosecond processing system was used for the surface
method being extensively used in various engineering fields. texturing operation. The relevant geometrical parameters of
Its principle lies in projecting the laser beam onto the surface the three types of microtextures are listed in Table 1, in which
of the material to obtain highly-localized energy to melt or the texture density signifies the ratio of the area cut by the
Fig. 6 e The schematic diagram of the geometrical features of the designed micro-groove textures (Unit: mm).
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Fig. 7 e The photographs of the original surface and the three kinds of the microtextured surfaces.
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Fig. 8 e The SEM morphologies of the original surface and microtextured surfaces.
original hydrophilic zirconia surface into a hydrophobic sur- remains on the surface, and the grooves are completely filled
face, in which the micro-grid texture changes the most, and with air, as illustrated in the Cassie model of Fig. 13.
the contact angle reaches over 150 . According to the formula of the Wenzel model, the theo-
On an ideal smooth and flat surface, the intrinsic contact retical contact angle of the microtextures can be calculated, as
angle of the zirconia ceramic surface can be obtained ac- listed in Table 2. By comparing the test values, it is found that
cording to the Young's equation, which is about 75 . However, the contact angle error of the micro-groove texture is very
the zirconia ceramic samples used in the experiments cannot small, while the micro-grid and micro-feather textures have
reach the ideal state, and the surface has a certain roughness.
Therefore, from the test results in Table 2, it can be seen that
there is a specific error between the surface contact angle and
the theoretical intrinsic contact angle for the textured 3Y-TZP
samples.
When analyzing the wettability of a workpiece sample,
surface roughness needs to be considered. For non-ideal sur-
faces, the contact angle of the material surface can be cor-
rected according to the Wenzel and Cassie models shown in
Fig. 13. The Cassie model assumes that the liquid fills all the
groove structures on the solid surface, and in this case, the
surface roughness amplifies the wetting behavior of the ma-
terial itself. When the roughness is large, the air will form an
air cushion in the groove and cause the failure of the Wenzel
model. In this case, it will create a situation where the liquid
Fig. 9 e The schematic diagram of the frictional tests.
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Fig. 11 e The schematic diagram showing the friction direction during the tribological tests.
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Fig. 18 e The photographs showing the bacterial colony distribution in the petri dish.
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Fig. 20 e The schematic diagram of bacterial adhesion for (a) the hydrophilic surface and (b) the hydrophobic surface.
Fig. 21 e The fluorescence staining characteristics of bacterial adhesion for different 3Y-TZP surfaces.
adsorption on the textured surfaces with different wettability, friction and anti-wear performances. The micro-feather
where hydrophobic surfaces cause bacteria to concentrate on shows a better effect on reducing friction than the
the top edge of the textures. As seen in Fig. 22, when bacteria groove-like texture, but due to the influence of structural
tend to adhere onto the textured surface, the suspended part strength and debris, its wear is more severe than that of the
will rupture and eventually collapse due to the effects of micro-groove texture.
multi-level textured structures [44]. The bacterial cell wall The adhesion and reproduction of bacteria are disturbed
attaching to the surface often undergoes a large stretch and due to the changes in surface wettability and the presence
deformation, resulting in an instant extreme increase in the of roughness. All the biomimetic surfaces designed in the
surface area. Then the irreversible mechanical rupture of cell study have an obvious inhibitory effect on Staphylococcus
walls occurs, leading to the decline of the bacterial population aureus compared with the original reference surface.
[45]. The phenomena make the bacteria more directional in Among them, the micro-grid texture exhibits the best
distribution to ensure their survival. antibacterial properties, followed by the micro-feathers
In sum, the designed biomimetic microtextures can and micro-grooves, and its antibacterial rate reaches
change the wettability of zirconia ceramics and consequently about 65.14%. It is confirmed to be the most preferred
affect bacterial adhesion. The Staphylococcus aureus in the texture type amongst the examined textures for clinical
experiment prefers to adhere to the hydrophilic surface, so applications to achieve the best antibacterial behavior of
the use of the air film generated by the super-hydrophobic dentures.
surface can inhibit and reduce the bacterial adhesion [46]. The hydrophobic surfaces designed in this work all
Meanwhile, the existence of surface texture also influences exhibit excellent wear resistance and antibacterial prop-
the adhesion, extension, and reproduction of bacteria and erties due to their influence on friction debris movement
thus exerts an antibacterial effect. This implies that the use of and bacterial adhesion. The micro-grid performs better
hydrophobic microtextures can possess the advantage of than the other two microtextures in terms of hydropho-
improving the antibacterial behaviors of dentures for the bicity, friction and wear properties, and antibacterial
clinical applications. Finally, due to the super-hydrophobic behaviors.
properties, the micro-grids yield the most outstanding anti- Finally, the current research work is limited to the perfor-
bacterial ability, followed by the micro-feathers and micro- mance evaluation of microtextures prepared on planar
grooves. surfaces of fully-sintered 3Y-TZP ceramics, not the actual
denture surfaces. To better approach realistic clinical ap-
plications, more endeavors should focus on preparing
4. Conclusions biomimetic microtextures on realistic curved tooth sur-
faces. Additionally, future research on optimizing the
The present work proposes the bio-inspired design of textured texture topographies should be carried out through inter-
surfaces based on the microstructural analysis of butterfly disciplinary research. Finally, more clinical trials are
wings, peacock tail feathers, and dolphin skins to improve the required to comprehensively evaluate the service perfor-
service performances of 3Y-TZP ceramics for dental applica- mance of textured 3Y-TZP denture surfaces.
tions. Three types of biomimetic microtextures, involving
micro-grids, micro-feathers, and micro-grooves, were pre-
pared onto the 3Y-TZP ceramics to improve their service
performances. The paper addresses the impacts of different Author statement
microtextures on the wettability, tribological and antibacterial
behaviors of textured 3Y-TZP ceramics. Based on the results Jinyang Xu: Conceptualization, Methodology, Investigation,
acquired, some key conclusions can be drawn. Funding Acquisition, Writing - Original Draft; Xiaoming
Zhang: Investigation, Data Curation, Formal analysis; Jingyu
The original reference surface basically yields a 33.1 Dai: Investigation, Data Curation, Formal analysis; Dedong
contact angle, which suggests the intrinsic hydrophilic Yu: Writing - Review & Editing; Min Ji: Writing - Review &
behavior of the nontextured 3Y-TZP ceramics. The intro- Editing; Ming Chen: Writing - Review & Editing.
duction of biomimetic microtextures has been confirmed
to have different degrees of influence on the wettability of
the 3Y-TZP surface. The micro-grid structure yields the Declaration of competing interest
largest contact angle of 151.2 and, thus, the best hydro-
phobicity. While the micro-feather is affected by factors The authors declare that they have no known competing
such as surface roughness, and its actual contact angle is financial interests or personal relationships that could have
103.2 , being smaller than that of the micro-groove appeared to influence the work reported in this paper.
texture. Therefore, its hydrophobicity is relatively the
lowest.
The original reference surface achieves the highest average Acknowledgments
friction coefficient of 0.342, indicating a very poor anti-
friction behavior of the nontextured 3Y-TZP ceramics. The work was supported by the National Natural Science
Additionally, the micro-grid texture yields the best anti- Foundation of China (Grant Nos. 52175422 and 52175425).
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