nihms588726.pdf

Fully-Loaded Micromotors for Combinatorial Delivery and

Autonomous Release of Cargoes

Dr. Sirilak Sattayasamitsathit

†

Huanhuan Kou

†

Wei Gao

Walter Thavarajah

Kevin

Kaufmann

Prof. Liangfang Zhang

, and

Prof. Joseph Wang

Department of Nanoengineering, University of California, San Diego, La Jolla, California 92093

USA

Liangfang Zhang: zhang@ucsd.edu; Joseph Wang: josephwang@ucsd.edu

Keywords

Self-destruction micromachines; Combinatorial drug delivery; Cargo release; Zinc; Drug delivery

Nano/micromotors are tiny synthetic devices that can effectively convert energy into

movement and forces.

[

]

Operating on locally supplied fuels, such as hydrogen peroxide,

acid, and even water,

[

]

these artificial motors can perform considerably complex

tasks.

[

]

For instance, hydrogen-peroxide powered micromotors have shown a remarkable

ability to isolate circulating tumor cells and bacteria from raw biological fluids.

[

]

Previous studies have also demonstrated that by attaching drug-loaded nanoparticles onto

the outer surface of micromotors, the motors can deliver their therapeutic payloads to a

target destination through pre-defined paths with a speed over three orders of magnitude

higher than regular Brownian motion, reflecting the large propelling and towing forces of

the motors.

[

]

Along with remarkable control over the movement directionality, these man-

made microscale devices are currently a subject of intense fundamental and practical

research activities.

[

]

While still in an early stage, attempts to explore biomedical

applications of micromotors are extremely active and encouraging.

[

]

Delivering cargoes in a defined and fast manner represents a major application of synthetic

micromotors.

[

]

The key challenge, however, is how to design, fabricate, and optimize

new motors with appropriate functions for effective delivery and efficient release of their

payloads. A reliable delivery vehicle is expected to have the capability for carrying a large

amount of cargoes for enhanced effectiveness (e.g., therapeutic efficacy in drug delivery),

delivering simultaneously different types of cargoes for multitasking (e.g., theranostics or

combination therapy to overcome drug resistance), releasing payloads in a responsive

manner (e.g., controlled drug release), and destroying itself when no longer needed.

[

]

To meet these critical multifunctionality requirements, we successfully constructed in the

Correspondence to: Liangfang Zhang,

zhang@ucsd.edu

; Joseph Wang,

josephwang@ucsd.edu

†

These authors contributed equally to this work.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

HHS Public Access

Author manuscript

Small

. Author manuscript; available in PMC 2015 July 23.

Published in final edited form as:

Small

. 2014 July 23; 10(14): 2830–2743. doi:10.1002/smll.201303646.

Author Manuscript

present study a novel chemically-powered zinc-based micromotor that displays impressive

cargo loading, delivery, and release capabilities.

The fully-loaded double-conical zinc micromotors are fabricated by coupling template-

electrodeposition with particle-infiltration techniques using silica and gold nanoparticles as

model cargo analogues. As illustrated in Figure 1, the motor fabrication route involves the

electrodeposition of zinc within the conical micropores of a polycarbonate membrane which

are tightly packed with the cargo particles. Dissolution of the membrane template results in

the release of the freestanding double-conical shaped micromotors, characterized with a

remarkably high cargo packing fraction of up to 74% of the entire motor body (for mono-

sized spherical cargoes according to Kepler conjecture).

[

]

This loading fraction can be

further increased by combining different sized spherical particles or using particles with

different shapes. When the micromotors are placed in acidic fuel media, hydrogen bubbles

are spontaneously ejected from one end, leading to a bubble thrust and a fast movement

(Figure 1e and Movie S1). Such directional locomotion is enabled by the formation of a

galvanic cell between the zinc and the sputtered gold contact.

[

]

The dissolution of the zinc

body leads to an autonomous release of the encapsulated cargoes and eventual splitting apart

of the motors (Figure 1f), while the motors are moving till they are almost fully dissolved.

The scanning electron microscopy (SEM) images demonstrate the morphology of the

template-prepared Zn micromotors loaded with monodisperse SiO

nanoparticles (diameter

~500 nm) (Figure 2a,b). Infiltrating the cargo spheres within the micropores of the

membrane template allows for convenient and efficient cargo encapsulation within the

electrodeposited zinc without any chemical linking steps. The resulting cargo-loaded

micromotors are 20 μm long and have a double-cone structure (with 2 μm diameter at both

ends and 1 μm diameter in their center), reflecting the shape of the micropores of the

polycarbonate membrane template. These SEM images illustrate that the SiO

particles are

tightly packed (with minimal gaps) and fully-loaded within the body of the zinc

micromotors. As expected, only a small portion of the particles is visible on the outside of

the fully-loaded Zn body (Figure 2b). The corresponding energy-dispersive X-ray

spectroscopy (EDX) data indicates that the SiO

particles are dispersed uniformly and

densely throughout the entire micromotor body, with zinc deposited in the voids between

these cargo particles (Figure 2c,d). Control experiments were carried out using zinc

micromotors without the SiO

particles. SEM images (Figure 2e,f) and EDX examination

(Figure 2g,h) confirm that only elemental zinc is present in these control micromotors.

Next, we demonstrated the capabilities of the zinc micromotors for combinational cargo

delivery and multifunctional operation. For this purpose, the micromotors were successfully

loaded with a binary cargo mixture of different sizes and types of nanoparticles. As

illustrated in Figure 3a, such simultaneous encapsulation of different cargo populations was

tested using SiO

nanoparticles of different sizes (500 nm and 250 nm in diameter), as well

as co-encapsulation of Au nanoparticles (diameter ~20 nm) and SiO

nanoparticles

(diameter ~500 nm). As shown in Figure 3b and c, the top and side-view SEM images of the

micromotors clearly indicate a homogeneous full loading of 500 nm and 250 nm SiO

particles within the motors. The SEM image (Figure 3d) and EDX mapping (Figure 3e-g)

further confirm that the zinc micromotors can be fully loaded with 20 nm Au nanoparticles

Sattayasamitsathit et al.

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together with 500 nm SiO

particles. Furthermore, the EDX mapping analysis shows the

uniform distribution of elemental Au and Si over the entire body of the micromotor. Note

that the Au nanoparticles cannot be observed in the SEM image owing to their small size.

The SEM image (Figure 3h) and EDX mapping (Figure 3i-k) of the control zinc

micromotors (without particle loading) confirm the absence of Au and Si within the zinc

body of the micromotors. Collectively, these results demonstrate that the micromotor

preparation route allows convenient packing of multicomponent cargoes at predetermined

sizes, compositions and proportions.

Since the maximum packing capacity for mono-sized spheres is 74% of the total container

volume according to Kepler conjecture,

the maximum packing capacity of 250 and 20 nm

diameter cargo spheres within the 36.7 μm

volume of the double-cone micromotor is

estimated to be slightly over 3,000 and 6,000,000 particles per micromotor, respectively.

However, as the particle size increases, the container wall has a greater influence on the

packing arrangement of the particles and increases the void volume within the micromotor.

By packing the micromotors with two populations of nanoparticles of greatly different sizes

(e.g., 20 and 500 nm, as the case in Figure 3d), it is likely to have a packing fraction higher

than 74%, because smaller cargoes can fill the voids between larger cargoes. Moreover,

changing to non-spherical nanoparticle shapes, such as hexagons, may further increase the

packing capacity.

Such high particle loading capacity and new functionalities do not compromise the

locomotion behavior of the motors. For example, the time-lapse images in Figure 4a

(corresponding to Movie S2) show the effective movement of several zinc micromotors

fully-loaded with SiO

nanoparticles (500 nm in diameter) in a strongly acidic environment

and the concomitant cargo release from the motors. The cargo-loaded micromotors are self-

propelled with an average initial speed of 110 μm s

-1

in a 0.7 M HCl solution while the

speed of Zn micromotors without cargoes is around 180 μm s

-1

under the same conditions. A

hydrogen bubble tail generated from one side of the micromotor is clearly observed (Figure

4a, 0 s). As their zinc body is oxidized and dissolved by the acid fuel, the cargoes are

released autonomously. Such gradual zinc dissolution leads to the breaking apart of the

motors at their narrow center region, and eventually to a complete release of the

encapsulated cargoes (Figure 4a, 6-9 s). Within less than 15 sec, the micromotors stop their

motion and nearly disappear (Figure 4a, 12 s). To better illustrate the cargo release process,

we intentionally stopped the motion of the motors by reducing the fuel (acid) concentration

and then focused on a single stationary micromotor with higher magnification. It was shown

that the motor started to break apart 3 s after adding the acid, leading to the release of a large

amount of cargo particles, and to complete dissolution of the motors (Figure 4b and Movie

S3).

The speed and life time of the acid-powered micromotors are strongly dependent on the

concentration of the surrounding acid and upon their payload loading fraction. Increasing

speed of fully-loaded micromotors from 110 to 140 μm s

-1

can be achieved by increasing

HCl concentration from 0.7 M to 1 M, however resulting in a shorter lifetime from 15s to 5s.

The zinc micromotors without SiO

particle loading are self-propelled in the 0.7 M HCl

solution at a speed of 180 μm s

-1

, which corresponds to a relative speed of 8 body lengths

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-1

. In contrast, the SiO

-loaded micromotors are moving at a slower speed of 110 μm s

-1

(5.5 body lengths s

-1

). SiO

-loaded micromotors here is slower than Zn-polyaniline reported

previously (100 body lengths s

-1

) owing to their solid structure (no opening) and being fully

packed with SiO

nanoparticles. We also demonstrated that lifetime of cargo-loaded

micromotors was affected by the amount of payloads in the micromachines. The cargo-

loaded motors are fully destroyed in this solution within 15 s, as compared to the 20 s

lifetime of the control motors which have higher zinc content.

In summary, an attractive self-propelled microscale motor has been developed that

concurrently possesses multiple functions for potential biomedical applications, including a

remarkably high loading capacity, combinatorial delivery of different cargoes and

autonomous ‘on-the-fly’ release of payloads. Moreover, unlike most existing micromotors

that are designed to withstand deterioration, these new micromotors destroy themselves

upon completing their delivery mission. While the concept was illustrated through the

loading of model SiO

and Au nanoparticle cargoes, it could be readily expanded to

simultaneous encapsulation of a wide variety of payloads possessing different biomedical

functions such as therapy, diagnostics, and imaging. This development is thus expected to

advance significantly the emerging field of cargo- towing nano/micromotors and to further

expand the opportunities for biomedical applications of nano/microvehicles.

Experimental Section

Fully-Loaded Micromotors

Detailed motor fabrication and characterization protocols used in this study can be found in

Supplementary Data 1. Briefly, cargo-loaded Zn micromotors were fabricated using a

membrane-template directed electrodeposition method in the microparticle-infiltrated

polycarbonate membrane. The Cyclopore polycarbonate membranes (Whatman 7060-2511)

containing 2 μm diameter pores with a 75 nm gold sputtered film were used for cargo

packing. Diluted 60 and 15 fold SiO

particles were filled into the membrane pore by

vacuum infiltration. The size of the micromotors was controlled by the pore size of the

membrane template and the deposition charge. Cargo-loaded micromotors were synthesized

by mechanically packing SiO

particles (diameter of 500 nm) in the membrane

templates,

[

]

followed by the zinc electrodeposition process using a potential of -1.2 V

for a total charge of 8 C. Mixture of Au nanoparticles (20 nm) with SiO

microparticles (500

nm) or 2 different sizes of SiO

particles (500 and 250 nm) were used for simulating multi-

cargo loaded micromotors. The micromotors were released from the template by mechanical

polishing and membrane dissolution in methylene chloride. The samples were washed with

methylene chloride, ethanol and water two times of each and were collected by

centrifugation.

Micromotors characterization

The morphology of the double-conical micromotors and the distribution of embedded cargos

were examined by Scanning electron microscopy (SEM) imaging and Energy-dispersive X-

ray (EDX) analysis. For long term storage of micromotors, they should be preserved in

ethanol to prevent degradation. The propulsion and controlled payload release of cargo-

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loaded micromotors were examined in acidic solutions containing 0.3-1.0 M HCl (along

with 1.7% Triton X-100). Real-time videos of propulsion of the cargo-loaded micromotors,

autonomous release of payloads and self-destruction were recorded using an inverted optical

microscope (Nikon Instrument Inc. Ti-S/L100), coupled with a 40x objective lens, and a

Hamamatsu digital camera C11440 using the NIS-Elements AR 3.2 software.

Supplementary Material

Refer to Web version on PubMed Central for supplementary material.

Acknowledgments

This project received support from the Defense Threat Reduction Agency-Joint Science and Technology Office for

Chemical and Biological Defense (Grant no. HDTRA1-13-1-0002). L. Z. acknowledges the funding support from

the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health under

Award Number R01DK095168. H. K. acknowledges the China Scholarship Council (CSC) for the financial

support. W.G. is a HHMI International Student Research fellow.

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Figure 1.

Dual-template fabrication of the fully-loaded zinc-based micromotors. (a) sputtering

membrane template with a gold conducting layer; (b) packing nanoparticle cargoes into the

membrane pores; (c) electrodeposition of zinc into the biconical pores; (d) dissolution of the

membrane template to release individual micromotors; (e-f) hydrogen-bubble propulsion in

acid and autonomous release of the cargoes while the motors are dissolved and destroyed;

(g) microscopic image of a bubble-propelled cargo-loaded micromotor in HCl (Movie S1).

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Figure 2.

SEM images (a,b,e,f) and EDX analysis (c,d,g,h) of Zn micromotors. (a-d), Zn micromotors

encapsulated with 500 nm SiO

particles: top view (a), side view (b), and EDX analysis (c,

d). (e-h), control Zn micromotors without the SiO

particles. Top view (e), side view, (f) and

EDX analysis (g, h). Scale bar, 0.5 μm (b,f), 1 μm (a,e), and 2 μm (c,d,g,h).

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Figure 3.

Multi-cargo loaded micromotors. (a) Model depicts the encapsulation of different types of

cargos, (b-c), SEM images display the top view (b) and side view (c) of the micromotors

fully loaded with two differently sized SiO

particles (500 and 250 nm in diameter). (d-g),

SEM image (d) and EDX analysis (e-g) of the micromotors fully loaded with both SiO

nanoparticles (500 nm) and Au nanoparticles (25 nm). (h-k), SEM image (h) and EDX

analysis (i-k) of a control zinc micromotor without any particle loading. All scale bars, 1

μm.

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Figure 4.

Time-lapse images of the SiO

–loaded micromotors. (a) Self-propulsion of multiple

micromotors and autonomous cargo release in a 0.7 M HCl fuel solution. (b) Close-up

images of the cargo release from a fully-loaded stationary micromotor in a 0.3 M HCl

solution. Images (a) and (b) were taken at 3 s intervals from Movies S2 and S3, respectively.

Scale bars, 20 μm.

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