Controllable Microfluidic Fabrication of Magnetic Hybrid

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Materials and Interfaces

Controllable Microfluidic Fabrication of Magnetic Hybrid Microswimmers with Hollow Helical Structures Meng-Jiao Tang, Wei Wang, Zhi-Lu Li, Zi-Ming Liu, Zhi-Yu Guo, Hua-Yu Tian, Zhuang Liu, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01755 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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Industrial & Engineering Chemistry Research

Controllable Microfluidic Fabrication of Magnetic Hybrid Microswimmers with Hollow Helical Structures

Meng-Jiao Tang,† Wei Wang,*,†,‡ Zhi-Lu Li,† Zi-Ming Liu,† Zhi-Yu Guo,† Hua-Yu Tian,† Zhuang Liu,†,‡ Xiao-Jie Ju,†,‡ Rui Xie,†,‡ and Liang-Yin Chu†,‡ †

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R.

China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China

ACS Paragon Plus Environment

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ABSTRACT Controllable magnetic hybrid microswimmers with hollow helical structures are fabricated, by a facile strategy based on microfluidic template synthesis and biosilicification, to achieve enhanced rotation-based locomotion for cargo transport.

The magnetic hybrid

microswimmers are fabricated by first synthesizing Fe3O4-nanoparticles-containing helical Ca-alginate microfibers from microfluidics, followed with biosilicification and controllable dicing to engineer their rigid hollow helical structures.

The microswimmers show hollow

helical structures consisting of a rigid, biocompatible alginate/protamine/silica shell embedded with Fe3O4 nanoparticles.

Their helical structures can be engineered into open

tubular structures or closed compartmental structures by using microfibers or diced microfibers as templates for biosilicification.

Powered by a simple rotating magnet, the

microswimmers can achieve enhanced rotation-based locomotion, and provide good mechanical strength for supporting cargo for transportation.

This work provides a simple

and efficient strategy for fabricating controllable magnetic hybrid microswimmers with hollow helical structures to achieve enhanced rotation-based locomotion for cargo transport, encapsulation and delivery.


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INTRODUCTION Microswimmers that can achieve locomotion propelled by chemical reactions,1,2 bacteria,3-7 magnetic field,8-14 electrical field,15,16 light,17,18 and ultrasound,19,21 are promising for applications such as drug delivery,11,22-24 fluid manipulation,25-30 biosensing31,32 and environmental remediation.33-37 can

achieve

Among these microswimmers, helical microswimmers that

remotely-controllable

locomotion

based

on

simple

magnetic-guided

rotation,8,14,38,39 show advanced functions for biomedical applications due to their unique rotation behavior.

For example, with rotation-based locomotion, intravascular helical

microswimmers can achieve enhanced drilling for medical application.40

Moreover, when

helical microswimmers are used for drug delivery, their rotation can achieve enhanced mixing for improving drug diffusion.

Since the functions of helical microswimmers mainly

rely on their rotation-based locomotion,14 improvement of the rotation-based locomotion allows efficient enhancement of their performances for applications.

As compared with

these rigid microswimmers with solid helical structures, helical microswimmers with rigid hollow structures can provide internal compartment with increased capacity for encapsulation,11 and reduced weight as well as reduced hydraulic friction for enhanced rotation-based locomotion.

Meanwhile, their rigid structures ensure efficient maintenance

of their helical structures during the locomotion process for applications.

However,

although many magnetic helical microswimmers have been developed,9,10,13,14,25,30,32,41-44 most of them are not with controllable hollow helical structures. Generally, magnetic helical microswimmers can be produced by methods such as rolling-up of strained multilayer metals,10 and deposition of metals on helical templates made



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by glancing-angle deposition,45,46 3D direct laser writing,39,44 and on those from nature such as spiral plant vessels,9 and Spirulina.31

For example, deposition of multilayer metals with

internal stress can lead to their rolling-up into helical shape upon releasing from the sacrificed substrate.10

Glancing-angle deposition of SiO2 on a rotated Si wafer followed with Cobalt

coating can produce magnetic helical Co/SiO2 microswimmers.46

3D direct laser writing of

photoresist SU-8 into helical structures followed with Ni/Ti bilayer deposition can produce magnetic helical SU-8 microswimmers.13

Deposition of Ti/Ni bilayers on diced spiral

vessels of plants can also produce magnetic helical microswimmers.38

These methods

create rigid microswimmers with excellent performances for biomedical applications, but it is usually difficult for them to fabricate hollow helical structures.

Meanwhile, these methods

usually require expensive apparatus for the accurate deposition of metal materials.

By

simply coating magnetic nanoparticles on sacrificed helical Spirulina in solution, helical microswimmers with hollow shell consisting of Fe3O4 nanoparticles can be constructed.11 However, precise control of their helical structures remains difficult due to the limited shape control of Spirulina.

Based on the excellent manipulation of microflows,47-52 microfluidic

techniques allow generation of coiled flow templates for fabrication of helical microswimmers

with

more

controllable

structures.

Recently,

by

periodical

UV-polymerization of coiled flow templates, controllable hydrogel microswimmers with solid helical structures containing magnetic nanoparticles have been produced.53

However,

it is still difficult for this method to engineer hollow structures, especially those with a rigid shell.

Thus, development of a facile strategy for creation of magnetic rigid microswimmers


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with controllable hollow helical structures for enhanced rotation-based locomotion is still highly required. Here we report on a simple strategy based on microfluidic template synthesis and biosilicification to controllably fabricate magnetic hybrid microswimmers with hollow helical structures for enhanced rotation-based locomotion and cargo transport.

The magnetic

hybrid microswimmers are fabricated by first synthesizing Fe3O4-nanoparticles-containing helical Ca-alginate microfibers from microfluidic coiled flows, followed with biosilicification and controllable dicing to engineer their rigid hollow helical structures.

The magnetic

hybrid microswimmers exhibit hollow helical structures with a rigid, biocompatible alginate/protamine/silica shell embedded with Fe3O4 nanoparticles.

Their helical structures

can be engineered into open tubular structures or closed compartmental structures by switching the sequences of biosilicification and dicing.

Powered by a simple rotating

magnet, the microswimmers can achieve enhanced rotation-based locomotion in water and aqueous media with human-blood-like viscosity, and provide good mechanical strength for supporting cargo for transportation.

These magnetic hybrid microswimmers are promising

for applications such as cargo transport, encapsulation and delivery.

EXPERIMENT SECTION Materials.

Sodium alginate (Na-Alg) (Mw = 6000) and calcium chloride (CaCl2) were

purchased from Kermel Chemical Reagent Co., Ltd.

Protamine sulfate (P4380, Grade II,

from salmon), acetic acid (HAc), sodium silicate (Na2SiO3) and polyethylene glycol (Mw=20,000) were purchased from Chengdu Kelong Chemicals.


Superparamagnetic Fe3O4


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nanoparticles (MNPs) with diameter of 20 nm and ethoxylated trimethylolpropane triacrylate (ETPTA) were purchased from Aladdin Industrial Corporation. AR grade and used without further purification.

The chemicals were all of

Deionized water used in the experiments

was obtained from a Milli-Q water purification system (Millipore). Microfluidic

Fabrication

of

Magnetic

Helical

Microfiber

Templates.

A

glass-capillary microfluidic device, constructed by assembling of cylinderical and square glass capillaries,42 was used to fabricate the magnetic helical microfiber templates.

A

cylinderical capillary with outer and inner diameters of 960 µm and 550 µm, was used as the injection tube.

The injection tube was tailored into a designed orifice by micropuller

(PN-30, Narishige) and microforge (MF-830, Narishige). was ~80 µm.

The diameter of the orifice tip

The tapered injection tube was coaxially inserted into the square capillary,

used as collection tube, with inner dimension of 1 mm.

These assembled capillaries were

fixed on a glass slide, and sealed with transparent epoxy resin to construct the microfluidic device. To fabricate the magnetic helical microfiber, first, a coiled flow was generated in the microfluidic device as the template.

Briefly, an aqueous solution containing 2 wt% Na-Alg

and certain concentration of Fe3O4 nanoparticles was used as the inner fluid, while an aqueous solution containing 1 wt% CaCl2 was used as the outer fluid.

Both the inner and

outer fluids were injected into the vertically-placed microfluidic device at appropriate flow rates by the syringe pumps to produce the coiled flow template.

Due to the diffusion of Ca2+

from the outer fluid to the inner fluid, Na-Alg in the coiled flow template was crosslinked by Ca2+ to produce magnetic helical Ca-alginate (Ca-Alg) microfibers.


The fabricated

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magnetic helical microfibers were collected in an aqueous solution containing 5 wt% CaCl2 for further crosslinking, and then washed with water for further use. Fabrication of Magnetic Hybrid Microswimmers with Hollow Helical Structures. With the magnetic helical microfibers as templates, magnetic hybrid microswimmers with hollow helical shape consisting of open tubular structures were fabricated by two-step biosilicification followed with a controllable dicing process.

For the biosilicification, the

magnetic helical microfibers with negatively-charged Ca-Alg matrix were washed in deionized water for 20 min to remove the redundant Ca2+.

Then the magnetic helical

microfibers were soaked in an aqueous solution containing positively-charged protamine (0.05 g L-1) for 20 min for their surface modification, followed with water washing to remove uncoated protamine.

Next, the magnetic helical microfibers coated with protamine were

soaked in aqueous solution containing 30 mmol L-1 Na2SiO3 (pH=7.0, adjusted by HAc) for 20 min, to further coat the microfibers with a silica layer.

The high concentration of Na+

resulted in decomposition of the crosslinked Ca-Alg networks, thus hollow helical microfibers with a hybrid alginate/protamine/silica shell were produced. At last, the hollow helical microfibers were converted into magnetic hybrid microswimmers by a controllable dicing process.

Alternatively, magnetic hybrid microswimmers with hollow helical shape

consisting of closed compartmental structures were fabricated by switching the sequence of biosilicification and dicing processes.

This was achieved by first controllably dicing the

magnetic helical microfibers into segments for the following biosilicification treatment. Morphological and Structural Characterization of Magnetic Helical Microfibers and Microswimmers.

The fabrication process of the magnetic helical microfibers was



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monitored by using an inverted optical microscope (IX 71, Olympus) coupled with a high-speed digital camera (Miro3, Phantom, Vision Research).

The morphologies and

structures of magnetic helical microfibers were characterized by using a digital microscope (GE-5, Aigo) and a digital camera.

The structures and chemical compositions of magnetic

hybrid microswimmers were characterized by using an optical microscope (BX61, Olympus) and a scanning electron microscope (SEM, TM3030, Hitachi) equipped with energy dispersion X-rays (EDX). Investigation on the Rotation-based Locomotion of Magnetic Hybrid Microswimmers. The rotation-based locomotion of magnetic hybrid microswimmers was studied by using a simple rotating permanent magnet in a magnetic stirrer for powering the rotation-based locomotion.

The magnetic hybrid microswimmer was placed inside a glass capillary with

inner diameter of 550 µm, and powered by a rotating magnet of a magnetic stirrer for rotation-based locomotion.

The inverted optical microscope coupled with high-speed digital

camera was used to monitor their rotation-based locomotion behaviors.

The effects of

rotating frequency of the magnet (f), content of Fe3O4 nanoparticles (CMNPs), values of helical pitch, and media viscosity (μ) on the rotation-based locomotion behaviors were studied. Different concentrations of polyethylene glycol (Mw=20,000) were added in aqueous solution for viscosity adjustment. The ability of magnetic hybrid microswimmers for cargo transport was studied by the inverted optical microscope coupled with high-speed digital camera.

Uniform polyETPTA

microspheres, dyed with Sudan Red Ⅲ, fabricated from microfluidics were utilized as the cargo.

Both the magnetic hybrid microswimmer and polyETPTA microspheres with


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controlled numbers were placed inside the glass capillary with inner diameter of 550 µm for cargo transport.

RESULTS AND DISCUSSION Controllable Microfluidic Fabrication of Magnetic Helical Microfibers.

The

magnetic hybrid microswimmers with hollow helical structures are fabricated by first synthesis of magnetic helical microfibers from coiled flow templates (Figure 1a-d), followed with two-step biosilicification and controlled dicing processes (Figure 1e-h).

Based on

liquid rope coiling effect,54,55 the coiled flow templates are generated from a glass-capillary microfluidic device by matching the viscosities and flow rates of inner and outer fluids.

In

our study, by using outer fluid with fixed viscosity of 1.03 mPa·s, coiled flow can be generated by using inner fluid with viscosity ranging from 291.5 mPa·s to 2930 mPa·s. Typically, an aqueous solution containing 2 wt% Na-Alg and 5 wt% magnetic Fe3O4 nanoparticles with viscosity of 919 mPa·s is used as the inner fluid, while an aqueous solution containing 1 wt% CaCl2 with viscosity of 1.03 mPa·s is used as the outer fluid. Separate injection of the inner and outer fluids at appropriate flow rates into the cylindrical and square capillaries of a vertically-placed microfluidic device can stably generate coiled flow templates.

Upon contact of the inner and outer fluids during the generation process of

coiled flow templates, the Ca2+ in the outer fluid diffuses into the inner fluid to crosslink the alginate.

This allows in situ capture of the coiled structures of flow templates for

continuous synthesis of helical Ca-Alg microfibers embedded with magnetic Fe3O4 nanoparticles inside the microfluidic device.

The synthesized magnetic helical microfibers



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are then collected in a Ca2+-containing solution by using a roller at rotating rate of 8 rpm for complete crosslinking.

Such a microfluidic fabrication process enables continuous

production of magnetic helical microfibers with length of ~70 m within 1 h.

Since the

structure of coiled flow templates can be adjusted by changing flow rates, this allows precise structural manipulation of the resultant magnetic helical microfibers.

Two parameters,

helical pitch and amplitude, as marked in Figure 2a, are used to describe their helical structures.

The helical pitch is defined as the distance between every two adjacent peaks of

the magnetic helical microfibers or the distance between every two adjacent valleys, while the amplitude is defined as the distance between the peak and the valley.

As a typical

example, the helical pitches of magnetic helical microfibers can be well-controlled by tuning the flow rate of outer fluid (Qo) while fixing the flow rate of inner fluid (Qi) at 200 µL min-1 (Figure 2).

When Qo increases from 200 µL min-1 to 650 µL min-1, the helical pitches of

magnetic helical microfibers increase from 139 µm to 436 µm (Figure 2a).

Meanwhile, the

amplitude of the magnetic helical microfibers remains nearly unchanged (Figure 2b), because the amplitude mainly depends on the inner dimension of the collection tube.

All the results

show the good controllability of our strategy on the helical structures of microfibers. Fabrication of Magnetic Hybrid Microswimmers with Hollow Helical Structures. Magnetic hybrid microswimmers with hollow helical shapes consisting of closed compartmental structure and open tubular structure, can be respectively created by switching the sequences of two-step biosilicification,56-58 and controlled dicing for treating the magnetic helical microfibers.

Typically, for fabricating of the hollow microswimmers with closed

compartmental structure, the magnetic helical microfibers are first diced into controllable


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segments and then immersed in an aqueous solution containing protamine for first-stage surface modification (Figure S1a-c).

Since the crosslinked Ca-Alg matrix are negatively

charged, the microfiber segments can be coated with a layer of positively-charged protamine. Then, the protamine-coated microfiber segments are immersed in an aqueous solution containing high concentration of Na2SiO3 for second-stage surface modification.

This

soaking process leads to precipitation of a rigid silica layer on the surface of the protamine-coated microfiber segments and decomposition of the crosslinked Ca-Alg networks to produce magnetic helical microswimmers with closed compartmental structure consisting of a rigid, biocompatible hybrid alginate/protamine/silica shell (Figure S1d). Figure 3a and b show the morphologies of magnetic helical Ca-Alg microfibers with three different helical pitches fabricated under different conditions of flow rates (Figure 3a), and their diced segments with well-controlled length and similar helical shape as that of the microfibers (Figure 3b).

All the microfibers before and after dicing exhibit uniform

structures due to the excellent controllability of microfluidics on the structure of coiled flow templates.

With the diced helical Ca-Alg microfiber segments as templates for

biosilicification, magnetic hybrid microswimmers with hollow helical shape consisting of closed compartmental structure can be fabricated (Figure 3c). Alternatively, with helical Ca-Alg microfibers as templates for biosilicification and then dicing treatment (Figure 1e-f), magnetic hybrid microswimmers with hollow helical shape consisting of open tubular structure can be fabricated.

The SEM images (Figures 3d and

S2a) of the resultant magnetic hybrid microswimmer, and the magnified SEM image (Figure S2b) showing the open tubular structure at one end of the magnetic hybrid microswimmer,



confirm their hollow helical shapes consisting of open tubular structures.

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The distribution

of Fe element in the hybrid shell (Figure 3e) confirms the presence of magnetic Fe3O4 nanoparticles in the shell.

Meanwhile, the EDX result in Figure 3f shows the contents of C,

O, Si, Ca, Fe elements in the microswimmer sample.

Although the carbon tape used for

sample preparation may contribute to part of the C content, the presence of Si, Ca, and Fe still confirms the successful construction of the magnetic hybrid alginate/protamine/silica shell. Moreover, by further breaking the magnetic hybrid microswimmer for SEM characterization, a hollow helical structure with shell thickness of ~8 µm, can be clearly observed from the SEM image (Figure 3g), especially from the magnified image (Figure 3h) of its cross-section structure.

The shell thickness can be adjusted in the range from hundreds of nanometers to

several micrometers,58,59 by changing the concentration of Na2SiO3 solution and the soaking time.

Such a hybrid shell possesses porous structures for mass transfer as confirmed in our

previous work.58

As compared with soft helical microswimmers, the rigid helical

microswimmers can better maintain their helical structures to ensure efficient rotation-based locomotion for applications. Magnetic Hybrid Microswimmers for Rotation-based Locomotion.

For helical

microswimmers containing superparamagnetic nanoparticles, they can exhibit soft magnetic properties and shape-induced magnetic anisotropy.[11,60-63] By using a rotating magnetic field, a magnetic torque can be applied to the helical microswimmers, with a component of magnetization in the radial direction of the helical structure for powering their rotational locomotion.[11,53,60-63]

Similarly, the magnetic hybrid microswimmers with hollow helical

structures can achieve rotation-based locomotion under manipulation of a simple rotating


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magnet in a magnetic stirrer (Figure S3).

The helical microswimmers with closed

compartment structures are used for studying the rotating-based locomotion behaviors. Deionized water with viscosity of 1.03 mPa·s is used as the surrounding media.

The

rotation-based locomotion behaviors can be controlled by manipulating the rotation frequency of the magnet (f), the contents of magnetic Fe3O4 nanoparticles (CMNPs) and the helical pitches of the magnetic hybrid microswimmers.

As a typical example, for the

magnetic hybrid microswimmers (pitch = 720 µm) fabricated from coiled flow templates with CMNPs= 5 wt%, by increasing f from 200 rpm to 1200 rpm, their locomotion velocity increases from 2.60 mm s-1 to 3.94 mm s-1 (Figure 4a), while their rotation frequency increases from 150 rpm to 384 rpm (Figure S4).

However, for the magnetic Ca-Alg

microswimmers (pitch = 720 µm, CMNPs= 5 wt%) with solid helical structures, their locomotion velocity only increases from 0.96 mm s-1 to 1.40 mm s-1, with increasing f from 200 rpm to 1200 rpm. Thus, with same value of f, the magnetic hybrid microswimmers with hollow helical structures show a locomotion velocity ~2.6 times fast as that of the magnetic Ca-Alg microswimmers with solid structures.

The results indicate that, although the

magnetic Ca-Alg microswimmers with solid structures can provide more Fe3O4 nanoparticles for magnetic manipulation, the magnetic hybrid microswimmers can still achieve faster velocity due to their hollow helical structures.

This confirms the effect of hollow helical

structure for enhanced rotation-based locomotion.

Moreover, by simple control of the

“on-off” and rotation direction of the magnet, the “on-off” and moving direction of the rotation-based locomotion can be easily controlled (Movie S1).

Especially, when stopping

the rotation of magnetic field, the magnetic hybrid microswimmers cannot move in the



magnetic field (Movie S1).

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This indicates that, the movement of magnetic hybrid

microswimmers is mainly caused by the rotation of the magnetic field, not by the magnetophoretic effect. Manipulation of the Fe3O4 contents and pitch structures of the magnetic hybrid microswimmers also allows control of the locomotion velocity.

For example, with f= 800

rpm, the locomotion velocity of magnetic hybrid microswimmers (pitch = 720 µm) increases from 2.53 mm s-1 to 3.65 mm s-1 with increasing CMNPs from 2 wt% to 5 wt% (Figure 4b). With f= 800 rpm, the locomotion velocity of magnetic hybrid microswimmers (CMNPs= 5 wt%) increases from 1.09 mm s-1 to 4.37 mm s-1 with increasing the pitch from 407 µm to 1047 µm (Figure 5).

Besides, the viscosity of surrounding media also influences the locomotion

velocity of magnetic hybrid microswimmers.

For example, with f= 800 rpm, the locomotion

velocity of magnetic hybrid microswimmers (pitch= 720 µm, CMNPs= 5 wt%) decreases from 3.65 mm s-1 to 0.122 mm s-1 with increasing the viscosity from 1.03 mPa·s to 36.36 mPa·s (Figure 6), due to the larger drag caused by increased viscosity.

Especially, in aqueous

media with viscosity of 4.42 mPa·s, similar to the viscosity of human blood of adults, the locomotion velocity of magnetic hybrid microswimmers can still reach 1.81 mm s-1 (Figure 6a,d), showing good locomotion performance.

All the results show the enhanced and

well-controlled rotation-based locomotion of the magnetic hybrid microswimmers with hollow helical structures. Magnetic Hybrid Microswimmers for Cargo Transport.

The alginate/protamine/silica

shell of the magnetic hybrid swimmers provides a rigid structure to support cargo for transportation.

This is demonstrated by using polyETPTA microspheres synthesized from


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microfluidics, each with weight of ~0.045 mg, as the cargo.

Magnetic hybrid

microswimmers (pitch= 720 µm, CMNPs= 5 wt%) powered by the rotating magnet with f= 800 rpm, are used to transport the polyETPTA microspheres in an aqueous solution with viscosity of 1.03 mPa·s.

As shown in Figure 7a-c and Movie S2, the magnetic hybrid microswimmer

can support the microspheres on its head for cargo transport inside the capillary.

With

increasing the microsphere number from 0 to 3, the velocity decreases from 3.65 mm s-1 to 0.211 mm s-1 (Figure 7d).

During the cargo transport process, no deformation of the helical

structure is observed, indicating the good mechanical strength of the rigid hollow helical structures for supporting the microspheres.

CONCLUSIONS In summary, controllable magnetic hybrid microswimmers with hollow helical structures are developed by a facile strategy based on microfluidic template synthesis and biosilicification for cargo transport.

Each of the microswimmers shows a hollow helical structure consisting

of a rigid, biocompatible alginate/protamine/silica shell embedded with superparamagnetic Fe3O4 nanoparticles.

Their helical structures can be engineered into open tubular structures

or closed compartmental structures by switching the sequences of the biosilicification and dicing processes.

The microswimmers enable enhanced rotation-based locomotion in water

and aqueous media with human-blood-like viscosity, when powered a simple rotating magnet. Meanwhile, the rigid hollow helical structure provides good mechanical strength for supporting cargo for transportation.

Moreover, based on the excellent control of

microfluidics on microflows, the helical pitch and amplitude, and the diameter of microfibers



can be well-tailored by adjusting the flow rates and the dimension of the injection tube and collection tube for further engineering the hollow helical structures of magnetic hybrid microswimmers.

Thus, this work provides a simple and efficient strategy for controllable

fabrication of magnetic hybrid microswimmers with hollow helical structures to achieve enhanced rotation-based locomotion for cargo transport, encapsulation and delivery.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Figures S1~S4 (PDF) Movie S1, Rotation-based Locomotion of Magnetic Hybrid Microswimmer in a Capillary (AVI) Movie S2, Magnetic Hybrid Microswimmer for Cargo Transport (AVI)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (W.W.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48), the State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01), and the Specialized Research Fund for the Doctoral Program of Higher Education (20130181120063) by the Ministry of Education of China.

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Figures

Figure 1. Schematic illustration of microfluidic fabrication of magnetic hybrid microswimmers with hollow helical structures.

(a-d) Microfluidic generation of coiled flow

templates (a) for synthesizing magnetic helical Ca-Alg microfibers via interfacial crosslinking reactions (b,c) between alginate and Ca2+ (d).

(e-g) Biosilicification of

magnetic helical Ca-Alg microfibers (e) via two sequential soaking steps for protamine coating (f), and then silica coating and Ca-Alg decomposition (g).

(h) Controlled dicing of

the helical hybrid microfibers to create magnetic hybrid microswimmers with hollow helical structures.



Figure 2. Flow-rate-dependent structure changes of the magnetic helical Ca-Alg microfibers. (a) Optical micrographs of the magnetic helical Ca-Alg microfibers produced with Qi= 200 µL min-1 and different Qo.

(b) Effects of Qo on the helical pitch and amplitude of the

magnetic helical Ca-Alg microfibers.

The scale bar is 300 µm.


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Figure 3. Morphologies and structures of the magnetic helical Ca-Alg microfibers and magnetic hybrid microswimmers.

(a,b) Optical micrograph of the magnetic helical Ca-Alg

microfibers before (a) and after (b) dicing.

(c) SEM image of a magnetic hybrid

microswimmer with hollow helical shape containing closed compartmental structure.

(d-f)

SEM image of a magnetic hybrid microswimmer with hollow helical shape containing open tubular structure (d), with EDX analysis showing the surface distribution of Fe element (e), and element contents of the hybrid shell (f).

(g,h) SEM image of a broken magnetic hybrid

microswimmer (g), with a magnified cross-sectional image showing the hollow helical structure (h).

Scale bars are 500 µm in (a, c-e, g), 4 mm in (b), and 50 µm in (h).



Figure 4. Effects of rotating frequency of the magnet (f) (a) and content of magnetic nanoparticles (CMNPs) (b) on the velocity (u) of rotating-based locomotion of the magnetic hybrid microswimmers in deionized water.

For the locomotion, pitch= 720 µm, µ= 1.03

mPa·s, CMNPs= 5 wt% in (a), and f= 800 rpm in (b).


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Figure 5. Effect of helical pitches on the rotating-based locomotion of the magnetic hybrid microswimmers.

(a-c) Optical snapshots showing the rotation-based locomotion of

magnetic hybrid microswimmers with pitches of 614 µm (a), 720 µm (b), and 1047 µm (c). (d) Pitch-dependent velocity changes of the magnetic hybrid microswimmers. locomotion, μ= 1.03 mPa·s, CMNPs= 5 wt%, and f= 800 rpm.


For the


Figure 6. Effects of viscosity of aqueous media on the rotating-based locomotion of the magnetic hybrid microswimmers.

(a-c) Optical snapshots showing the rotation-based

locomotion (f= 800 rpm) of magnetic hybrid microswimmers (pitch= 720 µm, CMNPs=5 wt%) in aqueous media with viscosities of 4.42 mPa·s (a), 10.41 mPa·s (b), and 36.36 mPa·s (c). (d) Viscosity-dependent velocity changes of the magnetic hybrid microswimmers.


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Figure 7. Magnetic hybrid microswimmers for cargo transport.

(a-c) Optical snapshots

showing the magnetic hybrid microswimmers (pitch= 720 µm, CMNPs=5 wt%) for transportation (f=800 rpm) of one (a), two (b), and three (c) polyETPTA microspheres in deionized water (µ=1.03 mPa·s).

(d) Effect of microsphere number on the locomotion

velocity of the magnetic hybrid microswimmers.



Graphic for TOC


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Controllable Microfluidic Fabrication of Magnetic Hybrid

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