Directional transport of centimeter-scale object on anisotropic microcilia surface under water
Directional transport of centimeter-scale object on anisotropic microcilia surface under water
Yuefeng Wang 0 5
Xiaodong Chen 1 5
Kang Sun 2
Ke Li 3 5
Feilong Zhang 4 5
Bing Dai 0 5
Jun Shen 3 5
Guoqing Hu 1 5
Shutao Wang 0 5
0 CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences , Beijing 100190 , China
1 The State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences , Beijing 100190 , China
2 School of Engineering, Guangdong Provincial Key Laboratory of Sensor Technology and Biomedical Instruments, Sun Yat-sen University , Guangzhou 510006 , China
3 Technical Institute of Physics and Chemistry, Chinese Academy of Sciences , Beijing 100190 , China
4 Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences. Beijing , 100190 , China
5 University of Chinese Academy of Sciences , Beijing 100049 , China
Natural organisms such as cactus spines or trachea cilia have unique directional transport ability, owing to their anisotropic surface structures or asymmetric motion. However, most artificial interfacial materials are incapable of transporting macroscale object underwater. Herein, we report that anisotropic microcilia arrays, composed of cobalt fine powder and PDMS, can successfully transport the centimeterscale hydrogel underwater by periodically asymmetric stroke under alternative magnetic field. Reciprocal collective stroke of anisotropic microcilia can generate directional flow, propelling the centimeter-scale hydrogel slice forward. Accompanying computational simulation results are consistent with the directional transport behaviors observed in our experiments. This study provides a clue to design artificial anisotropic interfacial materials with capability of transporting macroscale object at low Reynolds number.
macro-object transport; anisotropic surface; artificial micro-cilia arrays
Anisotropic structures in nature exhibit many amazing
interfacial properties, ranging from reversible adhesion of
gecko feet , wavelength-selective reflection of butterfly
wings , to water transport on cactus spines  and
peristome surface of Nepenthes alata . Biomimicry of
these anisotropic surfaces has been emerging as an active
and challenging field. Many exquisite artificial anisotropic
surfaces were designed to transport milliliter droplet, for
example, linear polymer grooves , micro ratchet glass
surfaces , unidirectional inclined silicon nanowires 
and nanopillars , tilted polydimethylsiloxane (PDMS)
micropillar array  and nanofilm with tilted parylene
nanorods . Asymmetric motion of interfacial
structures in organisms also exhibits distinguished transport
capabilities. Taking natural cilia for instance, asymmetric
stroke of their tiny and dense actuator units can provide
mechanical work for directional transport of food in
paramecium [11,12], mucus gel in trachea [13,14], or
neuroblasts in adult brain . Many responsive
materials (i.e. electroactive nanowire forest  or membrane
, magnetic actuated nanorods , micropost [19, 20]
or sheet [21,22], ion or temperature responsive hydrogel
microplate [23?26] and photo-driven liquid crystal cilia
) were exploited to bend, twist or rotate in artificial
micro actuators. However, these efforts fall into dilemma
facing to directional transport of macroscale object. In air,
2 cm-length hydrogel rod can be moved at the solid-solid
interface by mechanical vibration of anisotropic surface
 and 1 cm2 floating object can be transported at the
solid-liquid interface by asymmetric bending of
lightdriven polymer . When under the water, only
molecules, ions and micro particles can be transported
directionally [18,20,21,30,31]. Macroscale directional transport
in the liquid circumstance remains a great challenge by
using an artificial interfacial material.
Here, for the first time, we demonstrate that anisotropic
microcilia PDMS arrays mixed by cobalt nanoparticles
can perform magnetically actuated transport of
centimeter-scale hydrogel slice in the liquid circumstance.
Transport capability of microcilia arrays can be
manipulated by tuning the tilt angle, the row spacing of
microcilia and the actuation frequency of magnetic field.
Furthermore, experimental results and numerical
analyses confirm that microcilia stroke driven by reciprocal
magnetic field can effectively generate directional flow,
and finally lead to transport of macroscale hydrogel slice.
Our approach opens an avenue for effective transport of
macroscale object at low Reynolds number by
engineering anisotropic and stimuli-responsive interfacial
Fabrication of magnetic-driven artificial microcilia arrays
with a tilt angle
The artificial microcilia arrays were fabricated through
replica molding method [32,33], which is described in
Fig. 1a. Firstly, polyethylene sheet (PE, low density, Alfa)
was fixed on the bevel edge of right-triangle holder and
then ordered conical cavities were punched by using a
programmable X-Y-Z controlling platform .
Therefore, by tuning the holder?s bevel angle, PE molds with
ordered tilted conical cavities were obtained.
Secondly, cobalt nanoparticles (Aladdin) (Fig. S1) were
added in PDMS prepolymer (Dow-Sylgard 184). The
weight ratio of components is 10:1:5 (prepolymer:curing
agent:cobalt nanoparticles). After degassing process, the
mixture was squeeze coated onto the as-prepared PE
mold. The samples were degassed again for 1 h and then
treated by ultrasonic for 2 h to facilitate cobalt
nanoparticles penetration. In case of the heat generated by
ultrasonic process, ice was added into the water tank.
Finally the homogeneous coating was cured at 80?C for
2 h. After peeling off the coating carefully, we obtained
the magnetic-driven, uniform and ordered microcilia
arrays with different tilt angle and row spacing (Fig. S2).
Macroscale hydrogel transport test for artificial microcilia
The artificial microcilia arrays, actuated by a moving
magnet on the robotic arm, were capable of transporting
an agar hydrogel slice at low Reynolds number. The
device was exhibited in Fig. S3. The fluid used in the
experiment was sodium hyaluronate (Xi?an Realin
Biotechnology Co.)/phosphate buffered saline (PBS, 1?,
HycloneTM) solution (150 mg sodium hyaluronate
resolved in 100 mL PBS). The viscosity of the fluid is
13.3 mPa s. This dilute biological fluid permits the
potential use of artificial microcilia arrays in biomedical
The macroscale object of hydrogel slices were produced
as follows. Firstly, 4 wt% of agar powder (OXOID
LP0011) solution was prepared at 80?C. For better
observation of the hydrogel slice, 5 wt% of titanium oxide
(100 nm, rutile, Aladdin) was blended. After intensive
mixing in 0.5 h, the hot solution was poured into plastic
petri dishes, which is treated by plasma washer, in order
to achieve a membrane with a thickness of 150?200 ?m.
The membrane turned hard when immersed in water.
Ultimately, by tailoring the membrane with a punching
bear, the slice with a diameter of 1.1 cm and weight of
66 mg was obtained.
Testing process: First, the artificial microcilia arrays
sample was adhered onto the bottom of the observation
tank by double-side tape. The above-prepared fluid was
then poured into the tank and the hydrogel slice was put
onto the microcilia surface. As the robotic arm moved a
magnet reciprocally in a particular frequency, the
hydrogel slice was transported along with the periodical
stroke of microcilia arrays. A digital camera (Cannon,
60D, Japan) or a high-speed camera (i-SPEED 3,
OLYMPUS, Japan) was applied to record the movement
of microcilia arrays and the displacement of hydrogel
slice. Finally, the average transport velocity of hydrogel
slice for each microcilia sample was counted and the
experiment for each sample was repeated three times.
Numerical simulations of magnetic flux density vectors
on the microcilium in the moving magnetic field and
numerical simulation of flow field generated by stroking
microcilia arrays were conducted by using COMSOL
software. The details for the methods and analyses are
discussed in Supplementary information.
RESULTS AND DISCCUTION
The magnetically actuated anisotropic microcilia arrays,
composed of PDMS and nano cobalt powder, were
fabricated by replica molding approach as described in Fig.
1a. The PE mold was fixed slanted and punched by a
commercial needle using X-Y-Z controlling platform.
Through tuning the tilt angle and position of pores on the
mold, we obtained the desired microcilia arrays with
different tilt angles and row spacings (Fig. S2). The
diameter of microcilia ranged from 9 to 700 ?m, and the
2. . . . . . . . . . . . . . . . . . . . . . . . . .?. .Sc.ie.nc.e.C.hi.na. P.r.es.s a.n.d.Sp.r.in.ge.r-.V.er.lag. G.m.b.H. G.e.rm.a.n.y .20.18. . . . . . . . . . . . . . . . . . . . . . . . . . .
SCIENCE CHINA Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ARTICLES
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?.. .S.ci.e.n.ce. .C.h.in.a. .P.re.s.s.a.n.d. S.p.r.i n.g.e.r-. V.e.r.la.g. G..m. b. H..G. e.r.m. a.n.y. 126.96.36.199. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
ARTICLES . . . . . . . . . . . . . . . . . . . . . . . . . SCIENCE CHINA Materials
length was 2,500 ?m (the cone angle was 8.5?). In
addition, as shown in Fig. 1d?g, the cobalt element in three
cross section of a microcilium was marked by a scanning
electron microscope (SEM) equipped with an
energydispersive X-ray spectroscope (EDS). As the section
getting close to the microcilium tip, the cobalt content
became lower (Table S1). This may be attributed to lower
locomotion capability of cobalt nanoparticles in viscous
procured PDMS. By applying an external magnetic field,
the cilia array can be operated to stroke collectively (Fig.
We observed the dynamic transport process of agar
hydrogel slice on the anisotropic artificial microcilia
arrays with the tilt angle of 45? in the sodium hyaluronate/
PBS solution (viscosity: 13.3 mPa s), monitored by digital
camera. The back and forth movement of magnet fixed in
the robotic arm (Fig. S3) can bring periodical variation of
magnetic field to actuate the anisotropic microcilia arrays.
Thus, the hydrogel slice with diameter of about 1.1 cm
was transported continuously along the tilted direction of
anisotropic microcilia arrays (Fig. 2a and Movie S1).
Further, we recorded the stroking details of microcilia
arrays by using a high-speed camera. Periodically
changing magnetic field allows reciprocal stroke and recovery
of microcilia arrays, leading to continuously step-by-step
moving of hydrogel slice (Fig. S4 and Movie S2).
In order to explore the transport capability of the
microcilia arrays, we explored the influence of the tilt angle,
the row spacing of microcilia arrays and the actuation
frequency of magnetic field on the transport velocity of
hydrogel slice. Firstly, we observed the correlation of tilt
angle of microcilia arrays to the transport velocity of
hydrogel slice when their row spacing and the actuation
frequency of magnetic field were fixed at 1.4 mm and
2.5 Hz, respectively. As the tilt angle was increased from
30? to 90? (Fig. 2b), the transport velocity of hydrogel
slice gradually increased from 0.33?0.02 cm min?1 of 30?
to 1.63?0.06 cm min?1 of 45?, a maximum transport
velocity, and then decreased to 0?0.02 cm min?1 of 90?.
Therefore, we further chose microcilia arrays with the tilt
angle of 45? to test the influence of the row spacing of
microcilia arrays on the transport velocity of hydrogel
4. . . . . . . . . . . . . . . . . . . . . . . . . .?. .Sc.ie.nc.e.C.hi.na. P.r.es.s a.n.d.Sp.r.in.ge.r-.V.er.lag. G.m.b.H. G.e.rm.a.n.y .20.18. . . . . . . . . . . . . . . . . . . . . . . . . . .
slice, when the actuation frequency of magnetic field was
fixed at 2.5 Hz. As shown in Fig. 2c, when the row
spacing increased from 0.7 to 3.5 mm, the transport velocity
of hydrogel slice decreased from 1.90 to 0.35 cm min?1.
The result indicates that larger row spacing decreased the
density of microcilia arrays, causing a reduction in the
transport velocity of hydrogel slice. In Fig. 2d, when the
tilt angle and the row spacing of microcilia arrays were
fixed at 45? and 1.4 mm, the transport velocity of
hydrogel slice increases from 0.59 to 2.00 cm min?1, with
increasing actuation frequency of magnetic field from
0.83 to 2.92 Hz. Higher actuation frequency of magnetic
field causes more stroke of microcilia in a certain time, so
the transport capability will increase accordingly.
Therefore, we can manipulate the transport velocity of hydrogel
slice within 0?2 cm min?1 by tuning the tilt angle, the row
spacing of microcilia arrays and the actuation frequency
of magnetic field.
In order to explore the mechanism of macroscale object
transport by the microcilia arrays, we observed the
transport behaviors of the 45? and the vertical microcilia
arrays by monitoring a blue-dyed droplet in the stroke
region. Note that the blue-dyed droplet was prepared by
adding solid dye powder into the solution with same
components of the experimental fluid. For the 45?
microcilia arrays, the blue-dyed droplet moved upwards
along the microcilia from their bottom while they were
stroking. The continuous stroking of microcilia propelled
blue-dyed droplet continuously moving toward the stroke
direction with slight diffusion, indicating that an oriented
fluid flow was generated in the horizontal direction (Fig.
3a and Movie S3). In contrast, for the vertical microcilia
arrays, the blue-dyed droplet moved perpendicularly
upwards along with the vertical microcilia. When
propelled to the top surface of the fluid by the stroking
microcilia, the dyed droplet began to separate and move
symmetrically in the horizontal direction. The
phenomenon indicates that the stroking by the vertical microcilia
can hardly generate directional flow in the horizontal
direction (Fig. 3b and Movie S4).
Furthermore, we probed into the reason by numerically
simulating the stroke behavior of a magnetic microcilium
in the moving magnetic field. In the simulation, the
magnetic flux density vectors perpendicular to the
microcilium were adopted since the normal magnetic force
made major contribution to its stroke and recovery. For
the 45? microcilium (Fig. 3c), as the magnet was moving
from the right side beneath the microcilium, the magnetic
flux density on the microcilium increased gradually in the
left stroke direction (negative direction in the figures).
Then, the magnet crossed under the microcilium, leading
to a peak value of magnetic flux density in the right stroke
direction (positive direction in the figures). Finally, with
the magnet moving far away from the microcilium, the
magnetic flux density decreased to zero. Based on the
reciprocal motion of experimental magnet, the simulated
magnetic flux density curve exhibited a time-dependent
periodical trend. In addition, Fig. 3d also showed a
combination of simulated magnetic flux density and
experimental extent of microcilium bending. Although the
magnetic flux density first increased in the negative
direction, its relatively low value made the leftward stroke
of microcilium negligible. As the positive magnetic
density increased, the microcilium began to bend and its tilt
angle decreased; when the magnetic flux density
decreased, the microcilium recovered to its original shape
due to the rubber elasticity. The asymmetric feature of
magnetic flux density lead to asymmetric stroke of
microcilia arrays, thus generating the directional horizontal
flow of the blue-dyed liquid (Fig. 3a). For the vertical
microcilium, although there is a similar trend of magnetic
flux density vector, the variations of magnetic flux density
in negative and positive direction were symmetric (Fig.
3e). Owing to the symmetry of magnetic field, the
bending and relaxation of the vertical microcilium would
be in the opposite directions but in the same amplitude
(Fig. 3f), causing no directional flow of blue-dyed droplet
in horizontal direction (Fig. 3b).
Besides the directional flow generated by the stroking
microcilia arrays, the anisotropic friction might also have
influence on the transport process to the hydrogel slice
[35,36]. We measured the density of the hydrogel slice as
1.15 g cm?3, which is a little higher than the density of
experimental solution (1.07 g cm?3). There is friction
force when the magnetic microcilia arrays contact with
the hydrogel slice, which has a positive effect for the
directional transport. However, as the hydrogel slice is
lubricious in the solution, their friction coefficient would be
very low. Therefore, we consider the main cause of
directional transport of hydrogel slice is the fluid flow
generated by the stroke of anisotropic microcilia arrays.
We also established a numerical model using the
COMSOL software for better verifying the
above-mentioned mechanism. In Fig. 4a, the numerical simulations
show the velocity vectors/fields of the fluid flow generated
by 45? microcilia stroking. Clearly, when the microcilia
stroke in the tilt direction, the horizontal flow generates
on the tips of microcilia, which can form a directional
propulsion force to the object on the microcilia; rather, in
the recovery process, only vortex flows occurs in the gaps
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?.. .S.ci.e.n.ce. .C.h.in.a. .P.re.s.s.a.n.d. S.p.r.i n.g.e.r-. V.e.r.la.g. G..m. b. H..G. e.r.m. a.n.y. 188.8.131.52. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
ARTICLES . . . . . . . . . . . . . . . . . . . . . . . . . SCIENCE CHINA Materials
of microcilia, which can obstruct the motion of the object.
In contrast, when the microcilia are set vertical, the
microcilia stroke symmetrically, and rightward and leftward
velocity with symmetric distribution of magnitude were
offset (Fig. 4b). Therefore, the object on the vertical
microcilia can only vibrate in situ during their stroke.
Ad6. . . . . . . . . . . . . . . . . . . . . . . . . .?. .Sc.ie.nc.e .C.hi.na. P.r.es.s a.n.d .Sp.r.in.ge.r-.V.er.lag. G. m.b.H. G.e.r m.a.n.y .20.18. . . . . . . . . . . . . . . . . . . . . . . . . . .
ditionally, we simulated the fluid flow generated by 45?
microcilia arrays with five different row spacings as in
above experiments (Fig. S7a). The mean flow velocity at a
horizontal plate above the numerical microcilia arrays
(Fig. S7b) shows a gradual decrease with the row spacing,
with the same trend of the experimental measurements of
the transport velocity of hydrogel slice. The simulation
results further reinforce that the major contribution to
macroscale object transport is a result of the generated
flow by asymmetrical stroke of microcilia arrays.
In summary, we have developed anisotropic
magneticresponsive microcilia arrays that can transport
centimeter-scale object in the liquid circumstance.
Integration of magnetic nanoparticles to the anisotropic
microcilia arrays endows them with asymmetric stroke in
the periodical magnetic field at low Reynolds number,
causing coordinated directional fluid flow and thus
propelling the hydrogel forward. The magnetic-actuated
microcilia arrays will show promising applications in
macroscale object transport. For example, the most
serious deficiency in current artificial trachea is the lack of
capability of mucus gel transport . Our study provides
a clue to design next generation of bio-inspired interfacial
materials to overcome this deficiency. Besides of magnetic
actuation, other stimuli such as electricity, temperature,
pH or light, can also be utilized to transport the
macroscale object. Responding smart materials such as
electroactive polymer, pH or temperature responsive
hydrogels and light driven liquid crystal will be the
candidates to design microcilia arrays. This topic of
macroscale object transport is still in the infant stage and will
bring a good future.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?.. .S.ci.e.n.ce. .C.h.in.a. .P.re.s.s.a.n.d. S.p.r.i n.g.e.r-. V.e.r.la.g. G..m. b. H..G. e.r.m. a.n.y. 184.108.40.206. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author contributions Wang Y and Wang S conceived the design of
the research project. Wang Y performed the experiments and
characterizations. Sun K and Dai B helped develop the fabrication of
microcilia arrays. Li K and Shen J performed the numerical simulations of
magnetic flux density vectors on the microcilium in the moving
magnetic field. Chen X and Hu G developed the numerical model of stroking
microcilia arrays. All authors participated in discussions and contributed
to the writing of the paper. Wang S conceived the project and supervised
the experiments. Wang Y, Chen X and Sun K contributed equally in this
Conflict of interest
The authors declare that they have no conflict of
Supplementary information Numerical simulation methods and
supporting data are available in the online version of the paper.
8. . . . . . . . . . . . . . . . . . . . . . . . . .?. .Sc.ie.nc.e .C.hi.na. P.r.es.s a.n.d .Sp.r.in.ge.r-.V.er.lag. G. m.b.H. G.e.r m.a.n.y .20.18. . . . . . . . . . . . . . . . . . . . . . . . . . .
Yuefeng Wang is currently a doctoral student in the Technical Institute of Chemistry and Physics, Chinese Academy of
Sciences (CAS), under the supervision of Prof. Shutao Wang. His main research focuses on bio-inspired materials.
Xiaodong Chen is currently an associate professor in the Institute of Mechanics, CAS. He received his PhD degree from
the School of Astronautics, Beijing University of Aeronautics and Astronautics. He worked in the Department of
aeronautics and astronautics, Georgia institute of technology as a postdoctorcal research assosiate for two years. His
research interests are in the areas of micro-nano fluid mechanics and aerospace propulsion theory and engineering.
Kang Sun is currently an associate research professor at Sun Yat-sen University. He received his PhD degree from the
National Center for Nanoscience and Technology, followed by a postdoctoral research in the Institute of Chemistry, CAS.
His research interests include bioinspired smart interface, organs on a chip and clinical test.
Shutao Wang is currently a full professor in the Technical Institute of Chemistry and Physics, CAS. He received his PhD
degree from the Institute of Chemistry, CAS. His research interests focus on the design and fabrication of bioinspired
interface materials with controlled surface adhesion and nanobiointerface for theranostics.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?.. .S.ci.e.n.ce. .C.h.i n.a. .P.re.s.s.a.n.d. S.p. r.i n.g.e.r-. V. e.r.la.g. G..m. b. H..G. e.r.m. a.n.y. 220.127.116.11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Autumn K , Peattie AM . Mechanisms of adhesion in geckos . Integrative Comp Biol , 2002 , 42 : 1081 - 1090
Yoshioka S , Kinoshita S . Wavelength-selective and anisotropic light-diffusing scale on the wing of the Morpho butterfly . Proc R Soc B-Biol Sci , 2004 , 271 : 581 - 587
Ju J , Bai H , Zheng Y , et al. A multi-structural and multi-functional integrated fog collection system in cactus . Nat Commun , 2012 , 3 : 1247
Chen H , Zhang P , Zhang L , et al. Continuous directional water transport on the peristome surface of Nepenthes alata . Nature , 2016 , 532 : 85 - 89
Xia D , He X , Jiang YB , et al. Tailoring anisotropic wetting properties on submicrometer-scale periodic grooved surfaces . Langmuir , 2010 , 26 : 2700 - 2706
Sandre O , Gorre-Talini L , Ajdari A , et al. Moving droplets on asymmetrically structured surfaces . Phys Rev E , 1999 , 60 : 2964 - 2972
Liu C , Ju J , Ma J , et al. Directional drop transport achieved on high-temperature anisotropic wetting surfaces . Adv Mater , 2014 , 26 : 6086 - 6091
Chu KH , Xiao R , Wang EN . Uni-directional liquid spreading on asymmetric nanostructured surfaces . Nat Mater , 2010 , 9 : 413 -417 Cao M , Jin X , Peng Y , et al. Unidirectional wetting properties on multi-bioinspired magnetocontrollable slippery microcilia . Adv Mater , 2017 , 29 : 1606869
Malvadkar NA , Hancock MJ , Sekeroglu K , et al. An engineered anisotropic nanofilm with unidirectional wetting properties . Nat Mater , 2010 , 9 : 1023 - 1028
Mast SO . The food-vacuole in paramecium . Biol Bull , 1947 , 92 : 31 - 72
Dauer DM , Ewing RM . Functional-morphology and feeding-behavior of malacoceros-indicus (polychaeta, spionidae) . Bull Mar Sci , 1991 , 48 : 395 - 400
Fahy JV , Dickey BF . Airway mucus function and dysfunction . N Engl J Med , 2010 , 363 : 2233 - 2247
Button B , Cai LH , Ehre C , et al. A periciliary brush promotes the lung health by separating the mucus layer from airway epithelia . Science , 2012 , 337 : 937 - 941
Sawamoto K , Wichterle H , Gonzalez-Perez O , et al. New neurons follow the flow of cerebrospinal fluid in the adult brain . Science , 2006 , 311 : 629 - 632
Cheng C , Weissm?ller J , Ngan AHW . Fast and reversible actuation of metallic muscles composed of nickel nanowire-forest . Adv Mater , 2016 , 28 : 5315 - 5321
Kim O , Shin TJ , Park MJ . Fast low-voltage electroactive actuators using nanostructured polymer electrolytes . Nat Commun , 2013 , 4 : 2208
Shields AR , Fiser BL , Evans BA , et al. Biomimetic cilia arrays generate simultaneous pumping and mixing regimes . Proc Natl Acad Sci USA , 2010 , 107 : 15670 - 15675
Zhou B , Xu W , Syed AA , et al. Design and fabrication of magnetically functionalized flexible micropillar arrays for rapid and controllable microfluidic mixing . Lab Chip , 2015 , 15 : 2125 -2132 Wang Y , Gao Y , Wyss HM , et al. Artificial cilia fabricated using magnetic fiber drawing generate substantial fluid flow . Microfluid Nanofluid , 2015 , 18 : 167 - 174
Belardi J , Schorr N , Prucker O , et al. Artificial cilia: generation of magnetic actuators in microfluidic systems . Adv Funct Mater , 2011 , 21 : 3314 - 3320
Hu W , Lum GZ , Mastrangeli M , et al. Small-scale soft-bodied robot with multimodal locomotion . Nature , 2018 , 554 : 81 -85 Sidorenko A , Krupenkin T , Taylor A , et al. Reversible switching of hydrogel-actuated nanostructures into complex micropatterns . Science , 2007 , 315 : 487 - 490
Zarzar LD , Kim P , Aizenberg J . Bio-inspired design of submerged hydrogel-actuated polymer microstructures operating in response to pH . Adv Mater , 2011 , 23 : 1442 - 1446
He X , Aizenberg M , Kuksenok O , et al. Synthetic homeostatic materials with chemo-mechano-chemical self-regulation . Nature , 2012 , 487 : 214 - 218
Shastri A , McGregor LM , Liu Y , et al. An aptamer-functionalized chemomechanically modulated biomolecule catch-and-release system . Nat Chem , 2015 , 7 : 447 - 454
van Oosten CL , Bastiaansen CWM , Broer DJ . Printed artificial cilia from liquid-crystal network actuators modularly driven by light . Nat Mater , 2009 , 8 : 677 - 682
Mahadevan L , Daniel S , Chaudhury MK . Biomimetic ratcheting motion of a soft, slender, sessile gel . Proc Natl Acad Sci USA , 2004 , 101 : 23 - 26
Gelebart AH , Mc Bride M , Schenning APHJ , et al. Photoresponsive 30 31 32 33 34 35 36 fiber array: toward mimicking the collective motion of cilia for transport applications . Adv Funct Mater , 2016 , 26 : 5322 -5327 Wang Y , Gao Y , Wyss H , et al. Out of the cleanroom, self-assembled magnetic artificial cilia . Lab Chip , 2013 , 13 : 3360 Vilfan M, Potocnik A , Kavcic B , et al. Self-assembled artificial cilia . Proc Natl Acad Sci USA , 2010 , 107 : 1844 -1847 Drotlef DM , Bl?mler P , del Campo A . Magnetically actuated patterns for bioinspired reversible adhesion (dry and wet) . Adv Mater , 2014 , 26 : 775 - 779
Drotlef DM , Bl?mler P , Papadopoulos P , et al. Magnetically actuated micropatterns for switchable wettability . ACS Appl Mater Interfaces , 2014 , 6 : 8702 - 8707
Li K , Ju J , Xue Z , et al. Structured cone arrays for continuous and effective collection of micron-sized oil droplets from water . Nat Commun , 2013 , 4 : 2276
Xue L , Iturri J , Kappl M , et al. Bioinspired orientation-dependent friction . Langmuir , 2014 , 30 : 11175 - 11182
Jin K , Cremaldi JC , Erickson JS , et al. Biomimetic bidirectional switchable adhesive inspired by the gecko . Adv Funct Mater , 2014 , 24 : 574 - 579
Acknowledgements This work is supported by the National Natural Science Foundation of China ( 21425314 , 21434009 , 21421061 , 11402274 and 11772343), the Program for Changjiang Scholars and the TopNotch Young Talents Program of China.