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ReviewCite this article: Xu Q, Zhang W, Dong C,

Sreeprasad TS, Xia Z. 2016 Biomimetic self-

cleaning surfaces: synthesis, mechanism and

applications. J. R. Soc. Interface 13: 20160300.

http://dx.doi.org/10.1098/rsif.2016.0300

Received: 18 April 2016

Accepted: 18 August 2016

Subject Category:Reviews

Subject Areas:biomimetics, nanotechnology

Keywords:self-cleaning, biomimetic, nanostructure,

gecko, lotus effect, adhesion

Authors for correspondence:Quan Xu

e-mail: xuquan@cup.edu.cn

Zhenhai Xia

e-mail: zhenhai.xia@unt.edu

& 2016 The Author(s) Published by the Royal Society. All rights reserved.

Biomimetic self-cleaning surfaces:synthesis, mechanism and applications

Quan Xu1, Wenwen Zhang2, Chenbo Dong3, TheruvakkattilSreenivasan Sreeprasad3 and Zhenhai Xia4

1State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249,People’s Republic of China2College of Textile, North Carolina State University, Raleigh, NC 27607, USA3Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005, USA4Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA

QX, 0000-0003-2195-2513; ZX, 0000-0002-0881-2906

With millions of years of natural evolution, organisms have achieved

sophisticated structures, patterns or textures with complex, spontaneous

multifunctionality. Among all the fascinating characteristics observed in

biosystems, self-cleaning ability is regarded as one of the most interesting

topics in biomimicry because of its potential applications in various fields

such as aerospace, energy conversion and biomedical and environmental

protection. Recently, in-depth studies have been carried out on various

compelling biostructures including lotus leaves, shark skins, butterfly

wings and gecko feet. To understand and mimic their self-cleaning mechan-

isms in artificial structures, in this article, recent progress in self-cleaning

techniques is discussed and summarized. Based on the underlying self-

cleaning mechanisms, the methods are classified into two categories:

self-cleaning with water and without water. The review gives a succinct

account of the detailed mechanisms and biomimetic processes applied to

create artificial self-cleaning materials and surfaces, and provides some

examples of cutting-edge applications such as anti-reflection, water repellence,

self-healing, anti-fogging and micro-manipulators. The prospectives and

directions of future development are also briefly proposed.

1. IntroductionRegular cleaning involving sanitizing materials and solutions is necessary to

maintain freshness on the routine surfaces that we encounter in our day-to-day

life. In addition to the economic burden, extensive cleaning potentially introduces

hazardous substances to the environment and ecosystem. By contrast, various sur-

faces in Nature exhibit a high intrinsic ability to clean themselves without any

external aid. This phenomenon, due to its unique mechanism and high adapta-

bility, has attracted tremendous research curiosity in past decades [1–10]. The

concept of self-cleaning was initially unveiled based on the superhydrophobic

nature of certain plant leaves. Among them, the most well-known example is

the lotus leaf, which could make water droplets roll off the leaf surface quickly

to achieve surface cleaning. Lotus leaves exhibit a contact angle . 1508 and a

small sliding angle , 28. The high surface tension of water will assemble the dro-

plets into spheres that drive the droplets to roll off the surface together with

embedded dirt from the surface. On the contrary, on superhydrophilic surfaces

such as Tillandsia usneoides and sphagnum moss [11], the dirt components on

surfaces can be detached using only water. Here, the extremely small or even

zero contact angle at the interface between the surface and the water droplets

makes the dirt components movable along with the water flow on the surface.

Hence, both hydrophobic and hydrophilic surfaces can be efficiently cleaned

with the aid of water [12–23]. In general, removing surface-coated/adhered

contaminants is difficult because it is hard to dissolve them without the help of

high surface energy, solubility and mobility of water. However, intelligent,

intricate architectures developed in Nature present remarkable examples of

superhydrophobic surface superhydrophilic surfacehorizontal line

10 mm

20 mm

(b)(a)

(c) (d )

(e)

Figure 1. Natural plants and their surface morphology. (a) Lotus leaf. (Reproduced from [35] with permission of The Royal Society of Chemistry.) (b) Scanningelectron microscopy (SEM) picture of lotus leaves. (Reproduced from [35] with permission of The Royal Society of Chemistry.) (c) Anubias leaves. (d ) SEM picture ofAnubias leaves. (Reproduced from [2] with permission of The Royal Society.) (e) Demonstration of self-cleaning on a superhydrophobic surface and a superhydrophilicsurface. For the superhydrophobic case, the particles on the surface can be removed by the rolling off of water droplets due to the high contact angle (usuallygreater than 1508) between the surface and the droplet. For the superhydrophilic surfaces, the particles on the surface can be easily separated by the small contactangle between the droplet and the surface and then taken away due to the good contact between the surface and the droplet.

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self-cleaning even without the aid of water. For example,

geckos can keep their feet clean through the unique, complex

structure of their foot skin. Pitcher plants have highly wetting

surfaces, which can cause water droplets to spread rapidly

across the surfaces. Wetting can enhance the slipperiness and

increase the capture rate of small insects as they slip off from

its rims [24,25]. Such self-evolved structures give inspiration

to researchers around the world to develop artificial materials

with self-cleaning capabilities by mimicking natural systems.

Although several reviews exist about self-cleaning [15,26–28],

rapid progress in this direction has led to the invention of

novel structures with enhanced properties, which closely

match nano/microstructures with their natural counterparts.

Hence, this mini-review focuses on recent developments

regarding different approaches for fabrication of biomimetic

self-cleaning surfaces with a particular emphasis on the

underlying mechanisms. A brief overview of some important

practical applications is also presented.

2. Self-cleaning with waterWater, the most abundant and freely available liquid system

with optimal density as well as polarity, is being used as an

essential medium to remove different types of contami-

nations on surfaces. In Nature, many ingenious designs

exist which combine surface properties and water energetics

to scour material surfaces. Lotus leaves, one of the earliest

discovered and investigated self-cleaning biosurfaces

[29,30], uses a specific water–surface interaction to clean

the surface of leaves. Young and co-workers [31–34]

proposed wetting models which can be used to explain the

self-cleaning mechanism of lotus leaves. Young’s equation

for the contact angle is given as

cos u0 ¼gSA � gSL

gLA

, ð2:1Þ

where u0 represents the contact angle of the liquid on the sur-

faces, and gSA and gSL are the surface energies of the solid

against air and liquid, respectively, and gLA is the surface

energy of liquid against air.

Young’s equation can successfully predict the contact

angle of a water droplet on a flat surface with a homogeneous

interface. If the surface is rough and the actual surface area is

larger than its flat projected area, the contact angle can be

given under Wenzel’s equation

cos u ¼ Rf cos u0, ð2:2Þ

gas

liquid

Young’s model Wenzel’s model

Wenzel interface

liquid liquid liquidair

solid solid solidair pockets

airliquid

air

Cassie–Baxter interface Cassie interface

Cassie–Baxter’s model

solid

q0 qW qCB

(e)

(b)

(a)

(c) (d )

Figure 2. Modelling of surface wetting. (a) The progress of wetting models for a droplet on a flat surface from Young’s model (flat surface) to the Cassie – Baxter model(with surface roughness). (b) Surface evolver of droplets, which is a calculation of the profile of the droplet with the minimal surface area, namely the minimal surfaceenergy. (Reproduced from [36] with permission of The Royal Society of Chemistry.) (c) Droplet profile on cones, which is a calculation of the droplet profile. (Reprintedwith permission from Michielsen S, Zhang J, Du J, Lee HJ. 2011 Gibbs free energy of liquid drops on conical fibres. Langmuir 27, 11 867 – 11 872. Copyright & 2011American Chemical Society.) (d ) Dynamic thermal simulation of droplets on a flat surface based on the chemistry interactions (this approach has requirement on dropletsize, which is limited by the computational capability). (Reproduced from [38] with permission of Nature Publishing Group.) (e) Schematics of configurations described bythe Wenzel equation (equation (2.2) for the homogeneous interface, Cassie equation equation (2.3) for the homogeneous interface and the Cassie – Baxter interface forthe composite interface with air pockets equation (2.4)). (Reprinted with permission from Bhushan B, Jung YC. 2011 Natural and biomimetic artificial surfaces forsuperhydrophobicity, selfcleaning, low adhesion, and drag reduction. Prog. Mater. Sci. 56, 1 – 108. Copyright & 2011 American Chemical Society.)

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where Rf represents the ratio of the actual surface area to its

flat projected area. As for a rough surface Rf . 1, which

means a hydrophobic surface becomes more hydrophobic

for rough surfaces and a hydrophilic surface becomes more

hydrophilic for rough surfaces. Similarly, for the hetero-

geneous surfaces composed of two fractions, the Cassie

equation can be derived

cos u ¼ f1 cos u1 þ f2 cos u2, ð2:3Þ

where f1 and f2 are the fractional area with contact angle u1

and u2, respectively.

For a composite interface consisting of a solid–liquid frac-

tion ( f1 ¼ fSL and u1 ¼ u0) and liquid–air fraction ( f2 ¼ fLA

and cos u21 ¼ 21), the Cassie–Baxter equation can be derived

cos u ¼ Rf cos u0 � fLAðRf cos u0 þ 1Þ: ð2:4Þ

Based on these proposed models, it is possible for a surface

to achieve self-cleaning ability by controlling surface

100 mm

20 mm

microwater droplet

10 mm

1 mmPTFE-coatedcarbon nanotube forest

(a) (b)

(c) (d )

Figure 3. Biomimetic self-cleaning surfaces. (a) Fabricated through soft-lithographic imprinting. (Reprinted with permission from [51]. Copyright & 2006 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.) (b) Fabricated through an electro-brush plating and blackening process. (Reprinted with permission from [52]. Copyright& 2015, Royal Society of Chemistry.) (c) Fabricated through aluminium oxide nanoparticles. (Reprinted with permission from Alexander S, Eastoe J, Lord AM,Guittard F, Barron AR. 2015 Branched hydrocarbon low surface energy materials for superhydrophobic nanoparticle derived surfaces. ACS. Appl. Mater. Interfaces8, 660 – 666. Copyright & 2016 American Chemical Society.) (d ) Fabricated through carbon nanotubes. (Reprinted with permission from Lau KK, Bico J, TeoKB, Chhowalla M, Amaratunga GA, Milne WI, McKinley GH, Gleason KK. 2003 Superhydrophobic carbon nanotube forests. Nano Lett. 3, 1701 – 1705. Copyright& 2003 American Chemical Society.) All these approaches increased the contact angles of droplets on them through an increase in the surface roughnessbased on the Cassie – Baxter model shown in figure 1a. (Online version in colour.)

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microstructures to promote free, spontaneous movement of a

liquid droplet on the surface, allowing it to extract contami-

nants from the surfaces. Therefore, for a surface to achieve

self-cleaning, the primary goal will be to ensure that droplets

can flow or roll off smoothly from the attached surface with-

out any resistance. Natural or biosurfaces principally

maximize or minimize the contact angles of the droplets to

facilitate free liquid droplet movement. This free movement

is achieved by modulating the surface energy of the three

phases including liquid, air and solid (on the surface).

When the contact angle of the droplets is approaching

1808, it is very easy for the droplets to roll off from the surface

(e.g. lotus leaves). Zhang et al. [35] studied this effect and

found that at this maximized contact angle droplets freely

move across the leaf’s surface and remove foreign bodies

(dirt components) by dissolving them in the liquid along

the direction of motion of the droplets (figure 1a,b). However,

several other natural systems apply the opposite approach,

e.g. pitcher plants have highly wettable surfaces, which can

minimize contact angles to form a water film. The wetting

surface is so slippery that prey slide off it [25]. When the con-

tact angle approaches zero (e.g. the smooth surface structure

of Anubias barteri in figure 1c or Heliamphora nutans), water

will flow more freely on it. In the presence of water, the

good solubility and high surface energy of water favourably

wash the contaminated area, taking out the dissolved con-

taminants. Moreover, the minuscule contact angle of water

droplets will serve as a sharp knife, which can scrape the

contaminants off the surface, separating the dirt that is

already stabilized via intramolecular forces, resulting in the

simple removal of the contaminants. The scanning electron

microscopy (SEM) pictures of the lotus leaf surface mentioned

above and A. barteri leaf surface studied by Koch & Barthlott

[2] are shown in figure 1b and 1d, respectively. The higher sur-

face roughness of lotus leaves compared with A. barteri leaves

is evident from the SEM images due to the protuberances on

lotus leaves being much smaller and denser.

In short, surfaces that apply the two opposite strategies

for maximizing and minimizing the contact angle of

droplets to promote free droplet fall-off are termed super-

hydrophobic and superhydrophilic surfaces, respectively.

Figure 1e illustrates the applied mechanism for each surface’s

self-cleaning process. Although the strategy typically requires

a large amount of water to clean the surface, water-assisted

self-cleaning is still attracting a high volume of attention

from scholars, as successful adaptation and development of

the strategies could lead to a considerable reduction in

efforts/costs in cleanliness and maintenance.

Inspired by the self-cleaning surfaces found in Nature,

many technological studies employing a variety of materials

and surface modification technologies have been conducted

to fabricate artificial superhydrophobic and superhydrophilic

surfaces with self-cleaning ability. Although significant pro-

gress has been achieved, the level of efficacy attained in

Nature is yet to be fully replicated. Figure 2 illustrates the

different models proposed to understand the surface-wetting

(e)

(b)

adhesivelamellae

arrays of setae

75 mm 20 mm 1 mm

seta spatulae

(a)

(c) (d )

Figure 4. Structure hierarchy of the gecko adhesive system. (a) Macrostructure: ventral view of a tokay gecko climbing a vertical glass wall. (b) Mesostructure: ventralview of the foot. The visible overlapping pads are the adhesion lamellae (scansors). (c) Microstructure: a proximal portion of a single lamella, with a visible array ofindividual setae. (d and e) Nanostructure: the branched structure of a single seta at upper right, terminating in hundreds of spatula tips. (Reprinted with permissionfrom [69]. Copyright & 2005 National Academy of Sciences.)

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process of diverse surfaces. Figure 2a demonstrates a very

traditional wetting model reported by Young, Wenzel and

Cassie–Baxter. This model can be used to estimate the con-

tact angle of rough surfaces where air pockets exist,

providing a theoretical approach for the design of functional

surfaces with special wettability. However, the effect of

gravity is not considered in the model, which leads to an

imprecise estimation of contact angles of droplets on

surfaces compared with the ones simulated with gravity

considered, resulting in high variability [40]. Figure 2bshows a droplet profile study by Muller et al. [36] based on

surface evolvers. This surface evolver is a simulation tool

that can determine the profile of the droplet with minimal

surface area, namely minimal surface energy, which indicates

that attaining low surface energy is a fundamental property

of self-cleaning surfaces [36]. Michielsen et al. [37] considered

surfaces to be composed of cones, a very common shape in

Nature, and computed the wetting behaviour of such surfaces

(figure 2c). This study implied that barrel-shaped droplets

could move spontaneously to any location along the cone

axis where the defined systems, including the liquid, solid

and gas phases, achieve their lowest Gibbs free energy. For

a constant cone angle, as the contact angle between the

liquid and the cone increases, the drop will move towards

the apex of the cone. Likewise, for a constant contact angle,

as the cone angle increases, the drop moves toward the

apex. According to this theory, it is possible to make a droplet

move towards the apex and then roll away from the surface to

clean the surface by controlling the geometry and surface

energy of the cone. Shih et al. [38] simulated the wetting be-

haviour of various surfaces with small droplets, such as the

tiny droplets in fog that have a complicated intramolecular

interaction (figure 2d ). This unique approach is used to

understand the formation of contact angles by calculating

the molecular interaction at the liquid–solid interface. This

study indicated that the quantum effect or atomic-level inter-

actions between liquid and solid phases could be

quantitatively revealed. However, this study is limited only

to small droplets due to the limited capability of the compu-

ter processing regarding the volume of the droplets. In all the

above modelling approaches, parameters including the Gibbs

free energy, surface evolver, and dynamic thermal modelling

are widely used. All these models are useful to understand

the intricate parameters and factors that control the self-

cleaning behaviour of the surfaces (figure 2e).

Experimentally, surface modification [41], which only

requires limited material consumption, proves to be the most

effective and widely used method to create a surface with

unique wetting properties. Diverse approaches, including

the creation of the three-dimensional gradient porous

10 mm

1 mm 20 mmPP fibres PDMS fibres

10 mm

(a) (b)

(c) (d )

Figure 5. SEM images of a polypropylene (PP) fibrillary adhesive and conventional pressure-sensitive adhesives. (a) Fibrillar adhesive contaminated by gold microspheres.(Reprinted with permission from Lee J, Fearing RS. 2008 Contact self-cleaning of synthetic gecko adhesive from polymer microfibres. Langmuir 24, 10 587 – 10 591.Copyright & 2008 American Chemical Society.) (b) Fibrillar adhesive after 30 contacts (simulated steps) on a clean glass substrate. (Reprinted with permission from LeeJ, Fearing RS. 2008 Contact self-cleaning of synthetic gecko adhesive from polymer microfibres. Langmuir 24, 10 587 – 10 591. Copyright & 2008 American ChemicalSociety.) (c) PP fibres fouled with 3210 mm glass spheres, where some can be seen deeply embedded (EM) between fibres. (Reprinted with permission from Gillies AG,Puthoff J, Cohen MJ, Autumn K, Fearing RS. 2013 Dry self-cleaning properties of hard and soft fibrillar structures. ACS Appl. Mater. Interfaces 5, 6081 – 6088. Copyright& 2008 American Chemical Society.) (d ) Polydimethylsiloxane (PDMS) fibres contaminated with 40250 mm glass spheres after 40 recovery steps. Particles canbe readily seen embedded between the fibres (EM). (Reprinted with permission from Gillies AG, Puthoff J, Cohen MJ, Autumn K, Fearing RS. 2013 Dry self-cleaningproperties of hard and soft fibrillar structures. ACS Appl. Mater. Interfaces 5, 6081 – 6088. Copyright & 2008 American Chemical Society.)

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interconnected network, hydrogel-coated mesh, fluid-infused

porous films, reversible capillary-stabilized liquid-filled pores,

assembled colloidal photonic crystals, micro/nano-hierarchical

structures and proximally immobilized ions, have been

employed to build surfaces with self-cleaning properties

[41–50]. Figure 3 summarizes some of the compelling strategies

reported recently. Liu et al. [51] used the advanced soft-litho-

graphic imprinting technique to fabricate biomimetic and self-

cleaning surfaces (figure 3a). The fabricated surface with

imprinted protuberances closely mimicked the structure of

lotus leaves. The presence of the surface protuberances drasti-

cally increases the contact angle of water droplets sitting on

this surface due to the trapped air between the droplet and

the rough surface, which shares the same scenario as the lotus

leaves. Figure 3b illustrates a similar substrate constructed by

Wei et al. [52] through electro-brush plating and blackening pro-

cesses. Figure 3c,d provides an example of a surface coated with

aluminium oxide nanoparticles fabricated by Alexander et al.[53] and a self-cleaning surface covered by carbon nanotubes

(CNTs) studied by Lau et al. [54]. All the samples in figure 3

are hydrophobic surface-based self-cleaning materials. Com-

pared with the hydrophobic surface self-cleaning materials,

the hydrophilic surface does not have much variety in fabrica-

tion approach due to the high surface tension requirements.

Materials including TiO2 [15,55,56], SiO2 [57], ZnO [58], VO2

[59], Ag [60] and polydopamine-encapsulated octadecylamine

[61] have been explored as a novel approach to make

superhydrophilic self-cleaning surfaces. Generally, hydrophilic

surfaces are more challenging to fabricate because the easily

modified surface roughness always increases the hydrophobi-

city instead of hydrophilicity based on the most fundamental

Young’s equation on contact angles.

While water-assisted self-cleaning is an efficient pathway

for surface cleaning, under some special conditions where

water is not readily accessible, such strategies can fail. For

example, in outer space and in cold areas where the tempera-

ture is below 08C, surfaces cannot access abundant amounts

of liquid water or the free movement of water will be

hindered. Such conditions may limit the usages of water-

assisted self-cleaning surfaces. Thus, there is an urgent need

to develop advanced technology for surface decontamination

in such unique extreme environments.

3. Self-cleaning without waterGeckos have the world’s most efficient and reversible adhesion

system [62–68]. Although their feet are very sticky, geckos

have an extraordinary ability to prevent them from fouling

while running on dusty ceilings and in corners. Autumn

et al. [70] explained the underlying principle behind the

gecko’s strong adhesion. A gecko’s toe pads have a complex

hierarchical structure composed of millions of small hairs

called ‘setae’. Each seta further branches into hundreds of

wall

substrate/wall

DH

case I: drop off case II: pick up

gecko toe pad

setae

separation front

distal proximal

contacting setaecrowded 10 mm

(i) (ii) (iii) (iv)

particlespatulae

seta

NRp

Wpw =–ApwRp

6Dpw

NWps = N–ApsRpRs

6Dps(Rp – Rs)

(b)(a)

(c) (d )

Figure 6. (a) Model of interactions between N gecko spatulae of radius Rs, a spherical dirt particle of radius Rp and a planar wall. (Reprinted with permission from[69]. Copyright & 2005 National Academy of Sciences.) (b) One cycle of simulated steps, with contact with an initially clean glass slide: (i) applying normalcompressive force, (ii) shear load added to the compressive load by a hanging weight, (iii) removing the compressive load ( pure shear loading) and (iv) detachingthe sample. (Reprinted with permission from Lee J, Fearing RS. 2008 Contact self-cleaning of synthetic gecko adhesive from polymer microfibres. Langmuir 24, 10 587 –10 591. Copyright & 2008 American Chemical Society.) (c) Schematics of gecko toe peeling induced by digital hyperextension. (Reproduced from [71] with permission ofThe Royal Society.) (d ) Optical image of a gecko-like log constructed using SiO2 microparticles (size d ¼ 1 – 25 mm) via precisely assembling them on a glass slide witha biomimetic micromanipulator and using an atomic force microscope. (Reproduced from [88] with permission of Nature Publishing Group.)

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even smaller hairs, each of which ends in a flattened spatula

(figure 4a–e). The van der Waals (vdW) interaction between

the millions of spatulae and the surface after coming into inti-

mate contact is sufficient for the gecko to adhere to the surface

[70]. This highly evolved mechanism allows the gecko to

adhere to surfaces irrespective of whether they are hydro-

phobic or hydrophilic, dry or wet, rough or smooth. In 2005,

Hansen & Autumn [69] discovered that, in addition to this

high adhesion ability, the gecko toe pads have a unique self-

cleaning capability. Unlike the classic ‘lotus effect’, the

gecko’s self-cleaning ability does not demand the assistance

of water. This ability is regarded as a ‘dry self-cleaning’ prop-

erty because geckos can successfully dislodge most attached

contaminants simply by contacting with external surfaces

during their movement from one place to another [71,72].

Another interesting example is tree frogs. Although their

strong adhesion is aided by the secretion of mucus, the shear

movements and ‘flushing’ play an important role in shedding

particles/contaminants and keeping their feet from fouling

[73]. Mimicking these hierarchical fibrillary or thin soft film

adhesive structures, one can create the next generation of

sticky, yet self-cleaning tapes, climbing robots, smart surfaces,

etc. that can work efficiently under various temperature,

humidity and pressure conditions [74].

Various strategies have been formulated to synthesize

gecko feet-like surfaces from different materials including

silicones [75], CNTs [76–79], poly(methyl methacrylate)

[80], polyurethane acrylate [81], polydimethylsiloxane

(PDMS) [82] and more [83–86]. The successful fabrication

of gecko-inspired functional materials requires the combi-

nation of the physical, chemical and biological principles.

The development of such novel functional systems has been

extensively discussed in recent review articles [27,39].

Although biomimetic dry self-cleaning surfaces have been

well studied, the fabrication of artificial dry self-cleaning sur-

faces is still at a very preliminary stage. Lee & Fearing [87]

reported the first gecko-inspired, dry self-cleaning adhesive

using a high-aspect-ratio fibrillar polypropylene (PP) array.

The fabricated fibrillar adhesive showed a recovery of

around 33% of the shear adhesion on clean samples after 30

simulated steps on a dry, clean and rigid surface

(figure 5a–b). Subsequently, Gillies et al. [72] fabricated a

dry self-cleaning fibrillar structure made of hard and soft

polypropylene. They discovered that hard fibres could recover

96–115% of shear adhesion after fouling with small and large

particles (some of them performed better than before). How-

ever, soft fibres showed only 55% recovery upon fouling

with large particles, as particles prefer to embed readily

within the soft polymers (figure 5c–d) and prevent the further

self-cleaning of soft polymers. Further, Menguc et al. [9]

designed vertically aligned elastomer microfibres and tested

the self-cleaning properties of the biomimetic material after

icing

400 nm

20 mm 20 mm

water

hexadecan

anti-icing

(b)

(a)

(c)

(d )

1000

500

0

(nm)

20(nm)

10

0

(nm)

0500

1000

Figure 7. Multifunctionality of self-cleaning surfaces and their applications. (a) Anti-icing surfaces obtained through increasing both the contact angle of water due to airpockets trapped in the rough surfaces and the heterogeneous nucleation energy barrier by reducing the size of particles on the surface. (Reprinted with permission fromCao L, Jones AK, Sikka VK, Wu J, Gao D. 2009 Anti-icing superhydrophobic coatings. Langmuir 25, 12 444 – 12 448. Copyright & 2009 American Chemical Society.)(b) Anti-fogging surfaces obtained through creating a superhydrophobic surface which can prevent the condensation of water. (Reprinted with permission from [93].Copyright & 2014, Royal Society of Chemistry.) (c) Water/oil separation based on the difference between the contact angles of water and oil on the same surfaces.(Reproduced from [94] with permission of The Royal Society.) (d ) Antimicrobial surface inspired by shark skin which uses a certain micro-sized pattern to resist bacteriaadhesion to the surface. (Reprinted with permission from [95]. Copyright & 2014 Springer International Publishing Switzerland.) (Online version in colour.)

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contamination. Followed by a load–drag–unload dry contact-

ing self-cleaning process, they discovered large particles rolling

during the drag process. If the particles are smaller than the

adjacent fibre distances, they may be embedded in between

those fibres. If the particles are far below the size of the

fibres, they may stay on the surfaces of those fibres and thus

be hard to remove.

The fundamental mechanism behind the self-cleaning

ability of gecko feet-like structures is also proposed. First,

Hansen & Autumn suggested that contact self-cleaning occurs

when a particle–substrate adhesive force (Fw2p) overcomes

the seta–particle adhesive forces (Fs2p), or simply Fw2p .

Fs2p (figure 6a) [69]. However, this model failed to give a com-

plete explanation of why certain particles demonstrate a

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preference to bind more strongly to the toe pads than to the sub-

strate surfaces. To account for this contradiction, Lee & Fearing

[87] proposed that the shaking of the particles during the shear-

ing process was the main reason that seta arrays shed most of the

particles (figure 6c). Menguc et al. [9] believed that the particles

rolling under the fibre could enhance the self-cleaning property

of a fibrillar adhesion system. Moreover, they suggested that the

dragging rate and average load are two critical parameters that

affect the performance of the self-cleaning process. Hu et al. [71]

reported that geckos clean their feet via a unique digital hyper-

extension (DH) process (figure 6c). With DH, the gecko can

clean 80% of the dust in only four steps of walking while only

40% of the adhesion force can be recovered without DH.

During the DH process, the detachment between the gecko

seta and substrate becomes a kinetic process. The pull-off vel-

ocity at high speed may affect the self-cleaning of gecko seta

arrays. Inspired by this, Xu et al. [88] designed experiments to

measure a single gecko seta and spatula Fw2p and Fs2p versus

different shearing and pull-off velocities. The results revealed

that the particle–wall adhesion is velocity dependent, whereas

spatula–particle adhesion is velocity independent. This differ-

ence leads to the robust self-cleaning capability of gecko feet.

During animal locomotion, DH generates high normal pull-off

and shear speed before each step. Thus, the gecko can effectively

and efficiently dislodge dust from toe pads. Furthermore, this

thorough understanding of the self-cleaning mechanism at a

single gecko seta level has led to the use of a single gecko seta

as a novel powerful micromanipulator tool for various appli-

cations. Both single gecko seta and artificially designed seta

can quickly pick up, transport and drop off microparticles, help-

ing in precisely assembling complex micro-patterns/structures

(figure 6d). The gecko-inspired manipulators [4,89–91] can

potentially open up a new window for micromanipulation

of particles which could be used in microelectromechanical

systems, biomedical devices, etc.

4. Multifunctionality of self-cleaning surfacesA self-cleaning surface usually possesses other functions due to

its unique structures and chemistry. The self-cleaning surfaces

with multifunctionality may provide additional benefits and

properties (figure 7), including anti-icing, anti-fogging, oil–

water separation and antimicrobial activity, which brings

promising applications in materials engineering, resource

reuse, healthcare, as well as safety. For example, Cao et al.[92] fabricated a superhydrophobic surface with anti-icing

capability and revealed that the size of the particles exposed

on the surface is necessary for the anti-icing property, which

gives self-cleaning surfaces new functionality (figure 7a). As

the surface is a nanoparticle composite, water on this composite

is primarily in contact with air pockets trapped in the rough

surface, which leads to superhydrophobicity or self-cleaning.

On the other hand, the nanoparticles on the surface directly

contact water, which significantly increases the nucleation

energy barrier in heterogeneous nucleation, and thus suppress

the formation of ice on the surface (i.e. anti-icing property).

Park et al. [93] prepared photoanodes with anti-fogging and

anti-reflection properties by coating with hydrophilic SiO2

nanoparticles. The SEM and atomic force microscopy figures

of the coated surfaces can be seen in figure 7b [93]. Yang

et al. [94] fabricated a porous surface made of a PDDA-PFO/

SiO2 coating which allows the passage of water but makes oil

stand on the top of the surface, making it an anti-oil surface

(figure 7c). Self-cleaning surfaces also have significant appli-

cation for anti-fouling and antimicrobial surfaces, which were

initially reported by Ball [96]. Comparison of the antimicrobial

activity of a normal surface and a micro-patterned surface,

resembling the pattern on shark skin, is given in figure 7d[95]. Meanwhile, a hierarchical structure ranging from the

microscale to the nanoscale with a gradient in the surface

energy may produce a superhydrophilic slippery surface with

continuous, directional water transport on it [97]. Recently, a

new mechanism for a slippery surface was discussed by

Chen et al. [98] by analysing the peristome surface of Nepenthesalata, which may have practical applications in developing arti-

ficial fluid-transport systems. Nowadays, more and more

studies focused on slippery surface directions are expected

after the discovery of the directional motion of fluid on

N. alata surfaces. All those functions make the self-cleaning sur-

face very favourable for different industries, including energy

harvesting, civil engineering, medical devices, aerospace and

environmental protection.

5. Conclusion and outlookRobust self-cleaning surfaces are highly desired material sys-

tems for various applications in society, including solar

energy [99], anti-fogging [100], self-healing [101], water–oil

separation [102,103], water purification surfaces [104] and

smart devices [105–107]. Deriving inspiration from Nature,

scientists are working hard to fabricate multifunctional

self-cleaning surfaces. In this article, two pathways of self-

cleaning, namely the water-assisted and water-free (dry)

pathways, are reviewed. A concise overview of the state of

the art and an in-depth analysis of the fundamental mechan-

ism of the self-cleaning capability of this fascinating class of

materials is presented with a particular focus on recent devel-

opments. The latter part of the review also demonstrates the

application possibilities of these captivating structures.

The future trend of self-cleaning surfaces is expected to see

various other cutting-edge functionalities being incorporated

into self-cleaning surfaces including anti-reflection, water

repellence, self-healing, anti-fogging, micromanipulation,

anti-stickiness, etc. From the engineering aspect, future devel-

opments in the fabrication process will make these smart

surfaces more cost-effective, flexible, sustainable, durable and

reliable. Going forward, the control of these smart surfaces

will be attained via the application of various kinds of external

stimuli, such as mechanical, thermal, electrical and magnetic

fields. At the same time, research will find ways to incorporate

additional novel and intriguing properties into self-cleaning

structures to increase their utility. Moreover, Nature has

evolved many more multifunctional systems which are wait-

ing to be discovered [108,109]. A more fundamental

understanding of the mechanisms in natural systems is necess-

ary to mimic and replicate the properties intimately in

next-generation, multifunctional self-cleaning surfaces. Hence,

a continuous, sustained, rigorous study of different biosurfaces

present in Nature and construction of futuristic multifunctional

surfaces with unprecedented properties are expected.

Competing interests. We declare we have no competing interests.

Funding. We thank the National Nature Science Foundation of China(no. 51505501) and the National Key Research and DevelopmentPlan (no. 2016YFC0303700) for support.

10

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