Engineering, 2013, 5, 146-148
http://dx.doi.org/10.4236/eng.2013.510B031 Published Online October 2013 (http://www.scirp.org/journal/eng)
Multi-Layer Microbubbles by Microfluidics
Hongbo Zhang*, Haosu Meng, Qian Sun, Jianpu Liu, W. J. Zhang
Complex and Intelligent Research Centre, East China of Science and Technology, Shanghai, China
Received April 2013
ABSTRACT
Multi-layer microbubble has great potential in enabling the corporation of medical imaging with tumor therapy such as
drug and gene delivery of therapeutics or other functional materials in medical applications. Microfluidic technique has
advanced over the last decade and showed great promise in replacing traditional microbubble generating method. In this
paper, a multi-layer microbubble structure was produced with the aspect as potentially used for drug loaded microbub-
ble contrast agents.
Keywords: Microfluidic; Multi-Layer; Microbubbles; Medical Imaging
1. Introduction
Microbubble as contrast agents in medical imaging is
first invented in 1970s and it has greatly improved the
quality of medical imaging over the decades. Recently, it
has been found that microbubbles can be used to kill ma-
lignant cells by releasing preloaded drugs and also by
enhancing the drug transfer efficiency in ultrasound filed.
It can also assist the drug targeting to the cells by the
micro-jetting emitted by the microbubble when it col-
lapses subjected to an intense ultrasound filed. This con-
cept has encouraged researches to gener ate microbubbles
with unique structures that meet the demand, but so far,
there are still numerous obstacles remained.
Microfluidic technique has been a promising tool to
generate microbubbles owing to its number of merits,
such as high monodispersity and controllable size distri-
bution [1,2]. New attempts have been made to increase
the production, for example, multi-array microchips [3]
and sudden deepened configuration in the micro-channel
[4]. The mechanism of microbubble generation by micro -
fluidic is still not fully understood, but the most accepted
theory is that in flow focusing (or co-flow) configuration,
the microbubbles are generated by the instability of gas-
liquid jets. Therefore, it can produce much smaller mi-
crobubbles compared with the T-junction geometry [5].
The mechanism of the microbubble formation has been
studied by the ultra high speed camera [6]. Their obser-
vation suggests that the final moment of microbubble
pinch-off in a cross focusing system is purely liquid iner-
tia driven; however, surface tension is still important.
The drug loaded microbubbles are mainly produced by
attaching functional groups to the microbubble surface,
which suffers the major defect that only selects group of
drugs can be loaded [7]. Multilayer microbubbles might
hold the key to succeeding drug loading function with
high efficiency. The drug or gene can be encapsulated in
the middle layer with a protection ou ter layer in the shell.
Also due to the i mmer si on of the two layers, the outer
layer can be liposome while the drug or gene aqueous
liquid is in the middle. In this paper, a multi-channel mi-
crofluidic device was constructed to generate multi-layer
microbubbles.
2. Material
The middle layer of the microbubble is between 80
(Amresco, Ltd.) and ionized water mixture (2% w/v), the
outer layer is composed of liquid paraffin (Shanghai
Lingfen g L t d.) and air is chosen as the gas phase.
3. Microbubble Generation
The microfluidic configuration is shown in Figure 1. The
depth of the channel is 25 μm, the width of the channel is
100 μm, the width of the joint is 20 μm and the length of
the channel for each input is 7 mm.
4. Experiments
The experimental setup is shown in Figure 2. Com-
pressed air was regulated by a valve (Swagelok B-SS4-
VI, USA) and a pressure sensor was used to monitor the
pressure. Liquid phases were supplied by Syringe pumps
(Stoelting 53100, USA). High speed camera (Microview
MVC610DAM-GE110, USA) and microscopy (Sharp-
scope SF-1, USA) was used to evaluate the microbubble
generation process and s ize dist ribution of microbubbles.
*Corresponding a uthor.

H. B. ZHANG ET AL.
147
Figure 1. Microfluidic configuration.
Figure 2. Experimental setup.
5. Results and Analysis
The air pressure was fixed at 23 psi, while the flow rate
of the middle layer (tween 80 and water mixture) was set
at 20 μl/min, the flow rate of the outer layer of liquid
paraffin varied between 5 - 15 μl/min, multi-layer mi-
crobubbles were generated. Figure 3 shows the micro-
bubble generating process. The microbubbles with much
smaller size compared with the width of the channels
were produced. Due to the secondary encapsulation, the
final multi-layer microbubbles (as shown in Figure 4)
have a relative larger size compared with monolayer mi-
crobubbles. The inserted photo clearly showed the mul-
ti-layer structure. The scale bar is 100 μm. The mul-
ti-layer microbubbles have a relative uniform size. The
core of the microbubbles tends to move to the edge of the
mul ti-layer stru cture, it is more likely because of the van
der Waals force.
The flow ratio of two liquid phases plays an important
role in determine the microbubble size and distribution
when the gas phase pressure is fixed. At lower flow ratio
of 0.25 and 0.4, the microbubble production is low, at the
ratio of 0.5 between 80 and liquid paraffin, the micro-
bubbles size is becoming smaller as shown in Figure 5,
with the increase of the both liquid phase flow rate, the
microbubble becomes larger, but the dispersity of the
microbubbles reduces (as shown in Figure 6). Due to
higher flow rates, the process is becoming more stable.
As the increase of the outer layer flow rate, the outer
layer in the microbubble is becoming thicker as shown in
Figure 4 compared with Figure 5. The scale bar in Fig-
ure 5 is 50 μm. It is possible to modify the membr an e
thickness of the microbubbles, therefore, it is likely to
Figure 3. Microbubble generation.
Figure 4. Multi-layer microbubbl es (flow ratio of 0.75).
Figure 5. Microbubbles at liquid phase flow ratio of 0.5.
Figure 6. Microbubble dispersity.
adjust the amount of drugs and genes being carried by
the microbubbles.
The dispersity index was calculated by the formula
(1):
0%
10%
20%
30%
40%
50%
0.25
0.4
0.5
0.6
0.75
Dispersity
Flow rate rationof Two liquid Phases
Tween 80/Paraffin

H. B. ZHANG ET AL.
148
dm
CV D
σ = (1)
where
d
σ
is the standard derivation, and
m
D
is the
mean diameter of microbubbles.
Due to the higher surface tension of liquid paraffin to
water of 72 dyn/cm, the multi-layer microbubbles cannot
survive for more than 10 minute. The properties of the
immiscible two liquid could play an important role in
prolonging th e life time of microbubbles .
6. Conclusion
Multi-layer microbubble structures have been produced
by microfluidics. By adjusting the operating parameters,
such as flow rate ratio, the microbubble size can be va-
ried, so as the thickness of the microbubble membrane
and it could offer a potential mean for gen eration of drug
and gene loaded microbubble contrast agents to accom-
plish the dual function as imaging and therapy of can-
cers.
7. Acknowledgements
This work is supported by the “One Thousand Talented
Scheme” and the Fundamental Research Funds for the
Central Universities in Chin a.
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