2
1. INTRODUCTION
Subsequently, Harrison et al. implemented a capillary electrophoresis injection analysis
experiment using a micro-scale chip in 1993, and set the main conguration of the microuidic
system to a chip with a thickness of generally no more than 5 mm and an area of several square mil-
limeters to several square centimeters [6]. Subsequently, microuidic technologies have developed
rapidly. Woolley and colleagues rst realized the DNA sequencing using microuidic technology
in 1995 [7], and realized the on-line detection of PCR products using microuidic technology
during 1996–1998 [8] and chemiluminescence detection experiments [9]. In 1998, Bums reported
on the multi-function microuidic chip for DNA analysis, which integrates injection, mixing, re-
action, electrophoretic separation, and optical detection [10], which opens up the versatility road
of microuidic chip. In 2002, Quake et al. published an article in Science which introduced micro-
uidic large-scale integrated design methods and applications [1]. In the following year, Weigl et
al. reviewed the microuidic operation methods such as microuidic driving and cell manipulation
[11]. In the same year, Erickson et al. conducted numerical simulations and experimental studies
on microuidic motion. e transmission properties of microuidics are summarized and provided
a theoretical basis for microuidic chip design [12]. In 2006, Whiteside et al. published an article
in Nature to review the development of microuidic chips and point out the direction and trend
of microuidic development in the future [13]. In 2014, Bhatia et al. published the work of organ
chips in Nature Biotechnology [14] which was used to simulate the microenvironment and function
of human organs, and further promoted the application development of microuidic technologies.
Although the microuidic technology emerged less than 30 years ago, it has evolved from a
simple capillary electrophoresis miniaturization method to biological, chemical, mechanical, elec-
tronic, material, and medical applications. Researchers have combined microuidic technologies
with optical, mechanical, electrical, and acoustic technologies to conduct interdisciplinary research
in multiple application elds, such as drug screening, food detection, environmental monitoring,
and aerospace science.
1.1.2 MICROFLUIDIC CHIP MATERIAL AND PROCESSING METHOD
Microuidic chip material
e composition of the microuidic chip system mainly includes microchannels, microactuators,
microcontrollers, micro-sensors components, and connectors [15]. Material properties play key
roles in microuidic systems, such as the hydrophobicity, light transmission, conductivity, and bio-
3
compatibility of the microchannel surface. At present, the materials commonly used in microuidic
chips are mainly silicon materials, glass materials, and high molecular polymer materials.
(a) Silicon and glass materials
Silicon materials such as silicon wafers were the rst materials to be used in microuidic chips
[16]. However, silicon materials have limitations in their application. First, the light transmittance
is poor, which limits the observation and detection of the sample, and secondly, the electrical in-
sulating properties are poor, and the cost of the silicon material is high. Later, researchers began to
use glass materials in microuidic chips to replace pure silicon materials [17, 18].
Compared to silicon materials, glass materials have advantages of high light transmittance
and high biocompatibility. Common glass types include chrome glass, boride glass, indium tin oxide
(ITO), quartz glass, etc., which can be selected according to application requirements. For example,
ITO glass with the characteristics of transparent electrode, high electrical conductivity, and high
light transmission has been widely used to fabricate microelectrodes in microuidic chips [19].
Quartz glass has a weak absorption of ultraviolet light, and is suitable for applications requiring
UV absorption detection [20].
(b) High molecular polymer materials
In addition to silicon materials and glass materials, polymer materials are widely used in microu-
idic chips due to their high light transmittance, electrical insulation, chemical inertness, and low
cost [21]. e polymer materials commonly used in microuidic chips are thermoplastic polymers
and curable polymers.
ermoplastic polymers mainly include polyamide (PI) [22], polycarbonate (PC) [23, 24],
and polymethyl methacrylate (PMMA) [25, 26]. Such materials can be thermoformed by using a
mold, which has the characteristics of high light transmittance, low cost, and long service life, but
the surface hardness is poor, and scratches are easily aected to observe the experiment.
Curable polymers mainly include polydimethylsiloxane (PDMS), epoxy resin, and poly-
urethane. Due to the high light transmittance, high biocompatibility, and convenient processing,
PDMS has become one of the most widely used high polymer materials in the eld of microuidic
chips [27‒29].
(c) Other materials
In addition to the above two types of materials, in recent years paper materials [30‒32] and printed
circuit boards (PCB) [33, 34] have gradually been applied in microuidics in recent years. e paper
materials mainly use the capillary eect to realize micro-liquid driving, and the surface hydrophilic/
hydrophobic modication can realize the function of the microchannel on the paper, and are mostly
used in real-time detection [35]. PCB materials with the advantages of PCB microelectrode pro-
cessing are mostly used as electrodes for electrical control and detection of the sample [36].
1.1 OVERVIEW OF MICFROFLUIDS
4
1. INTRODUCTION
In short, the materials of the microuidic chip are the premise and basis of the microu-
idic technology. In practical applications, the chip materials need to be selected according to the
application requirements. e factors to be considered are: (a) the diculty of processing the ma-
terial, the processing accuracy and cost; (b) physical properties of the material, such as insulation,
transparency, and thermal conductivity; and (c) chemical properties of the material, such as surface
hydrophobicity and biocompatibility.
Processing technology of microuidic chips
MEMS technologies are the most common methods of microuidic chip processing. e process-
ing technologies for fabricating a 2D planar structure includes photolithography [37], oxidation
[38], chemical vapor deposition (CVD) growth [39], coating [40], etc. e processing technology
for fabricating the 3D structure mainly includes photolithography, chemical etching, plasma
etching [41, 42], X-ray lithography (Lithography, Electroplating, and Molding, LIGA) [43, 44],
and so on.
(a) Processing technologies of silicon and glass materials
Silicon materials are mainly used for processing liquid ow drive and control devices such as mi-
cropumps and microvalves, or as a positive mold for processing high molecular polymer materials
in hot pressing and molding. It is often processed by photolithography and etching techniques. It
usually consists of a lm deposition, photolithography, and etching process. In the processing of
quartz and glass, the surface is often modied by chemical methods, and then microchannel pro-
cessing is performed using photolithography and etching techniques.
(b) High polymer materials processing technology
e processing methods of high molecular polymer materials mainly include: molding methods
[45], hot pressing methods [46], LIGA technologies [47], and soft lithographies [48, 49]. e
molding methods rst form a convex mold of the channel by photolithography and etching, and
then pour polymer material on the mold, and nally cure the high molecular polymer material, and
peel o to obtain a chip having microchannels. e hot pressing methods bond the high molecu-
lar polymer material and the mold together in the hot pressing device. When the high molecular
polymer is softened by heating, the corresponding microstructure can be printed on the mold, and
demolding is performed to obtain a chip having microchannels after cooling.
LIGA technologies are suitable for fabricating high aspect ratio polymer chip structures,
including X-ray deep lithographies, electroforming, and injection molding. X-ray deep lithogra-
phies can obtain a high aspect ratio microchannel structure in the photoresist, electroforming mold
deposits metal in the gap of the developed photoresist image, and lift-o the photoresist to obtain
the high aspect ratio structure. Injection molding is the formation of the microchannel structure on
the polymer material by replica molding on a mold.
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