Microfluidic technology (Microfluidics) refers to the science and technology involved in the use of microtubes (tens to hundreds of micrometers) to process or manipulate tiny fluids (slightly up to nanup). It is an emerging interdisciplinary subject involving chemistry, fluid physics, microelectronics, new materials, biology and biomedical engineering. The early concept of microfluidic devices can be traced back to the 1870s with lithography, and then developed into microfluidic capillary electrophoresis machines and microreactors.

Microfluidic devices are often called microfluidic chips, also known as microchip laboratories (Lab on a Chip) and micrototal analysis systems (micro-Total Analytical System). Microfluidic chip using similar semiconductor microcomputer electrical processing technology on the chip build microflow circuit system, the experiment and analysis process to the path of each other and liquid phase chamber of chip structure, loading biological samples and reaction fluid, using micromechanical pump, electrotonic method drive the buffer flow in the chip, form microflow path, on the chip or a variety of continuous reaction. The samples were analyzed rapidly, accurately, and at high-throughput by fluorescence, electrochemical, and mass spectrometry.

1. Classification of the microfluidic techniques

 In microfluidic technology, microfluidic drive and control technology is the premise and foundation of realizing microfluidic control. There are many kinds of control modes, and the principles and forms are not the same. According to the different microfluidic driving modes, microfluidic technologies are mainly divided into two categories: active microfluidic and self-prototactic microfluidic.

 Active microfluidic control is a microfluidic control method using exogenous driving force (including pressure, centrifugal force, magnetic force, electrowetting, etc.). Pressure microfluidic is the use of pressure or hydraulic or gas-hydraulic mixing to control the movement of liquid in the chip. Centrifugal microfluidic is generally a symmetric disk configuration and uses the centrifugal force generated by rotation to drive the movement of the liquid in the chip. Magnetic microfluidic control uses the magnetic field to control the magnetic matter in the fluid to drive the motion of the fluid. Digital microfluidic control is generally based on the basic principle of electrowetting, manipulating droplets in various ways to build electrode arrays and achieve complex biochemical analysis.

Self-driven microfluidic usually refers to the use of surface hydrophilic and hydrophobic characteristics or capillary force to carry out fluid transport and treatment. It is characterized by self-drive, no additional pump source and energy.

2. Characteristics of the microfluidic chip technology

Microfluidic chip technology developed based on microelectromechanical systems (MEMS) is known as one of the seven technologies to change the future. Compared with traditional methods, it has the following advantages:

(1) Integrated miniaturization and automation

Microfluidic technology can focus multiple steps of sample detection on a small chip, integrate these operation steps through the combination of the size and curvature of the flow channel, microvalves, and cavity design, and finally miniaturize and automate the entire detection integration.

(2) High throughput

Because microfluidic control can be designed as a multi-channel, the samples to be detected can be diverted to multiple reaction units simultaneously through the microchannel network, and the reaction units can be isolated from each other, so that each reaction cannot interfere with each other, so multiple items can be detected in parallel for the same sample as needed. Compared with routine item-by-item testing, it greatly reduces the detection time and improves the detection efficiency, which is characterized by high throughput.

(3) Less consumption of detection reagents

Due to the miniaturization of integrated detection, the cavity of the reaction unit on the microfluidic chip is very small. Although the concentration of the reagent formula may be increased in a certain proportion, the amount of reagent used is much lower than that of conventional reagents, which greatly reduces the consumption of reagents.

(4) Less sample size and less demand

Since the test is only done on a few centimeters of chip, the sample size to be tested is very small, often only a slight liter or even an upgrade. At the same time, due to its high-throughput characteristics, multiple tests can be realized for samples collected at one time, so it is more advantageous for testing samples that are not easy to obtain.

(5) Less pollution

Due to the integration function of the microfluidic chip, all the operations that need to be completed manually in the laboratory are automatically integrated into the chip to complete automatically, so as to minimize the pollution of the samples to the environment during the manual operation. For example, in the molecular nucleic acid detection, the diffusion of aerosols makes the subsequent sample testing prone to false positives, which is well solved by the use of microfluidic technology.

At the same time, the microfluidic chip technology still has the following shortcomings:

(1) Lack of norms and standards for core technologies

In the industrialization of microfluidic control, because the technology is not mature and the products lack corresponding standardization and standardization, it is not possible to realize the generalization of components. In this way, there cannot be a cooperative development mode of upstream and downstream companies.

(2) High production costs

Microfluidic products themselves are new products in many fields such as microcomputer electrical processing, life science, chemical synthesis, optical engineering and electronic engineering, with high technical requirements and long development cycle, resulting in higher production costs.

(3) Technical problems

 For example, for the microfluidic immunoassay chip system, the fixation of the antibody and the sealing of the surface of the microchannel significantly affect the sensitivity of the immunoassay, which is the key problem to be solved in this kind of chip.

In addition, the integration of microfluidic chips and peripheral devices, such as automatic analysis and display devices, is also a key problem that needs to be solved.

3. Difference between microfluidic chips and biochips

 Biochip (biochip or bioarray) is the integration of biochemical analysis processes on the chip surface based on the principle of specific interactions between biomolecules, so as to achieve high-throughput rapid detection of DNA, RNA, peptides, proteins and other biological components. The narrow sense of biochip concept refers to the biomolecules formed by fixing biomolecules on solid phase transmitters such as silicon wafer, glass sheet and gel through different methods. Therefore, biochip technology is also known as microarray (microarray) technology.

Microfluidic chip takes the precise control of microfluidic fluid as the core technology, while biochip takes the static affinity reaction pairing as the core technology. In terms of principle, application and development goals, they are all chip laboratories, but they each have their own characteristics, and they belong to different discipline systems and technical fields. In practical research and application, the technologies involved in each concept are often crossed with each other. For example, biochips can be used alone or used as a detection technology of microfluidic chips.

4. Application of microfluidic chips in the field of in vitro diagnosis

Currently, in vitro diagnosis is the largest application scenario for microfluidic technology. In vitro diagnostic technology at home and abroad mainly show the development trend of automation, rapid, ultra-high sensitivity, high-throughput detection and non-invasive and minimally invasive.

(1) Application of biochemical and immunodiagnostic testing

 The principle of in vitro biochemical diagnostic detection is mainly based on enzyme kinetic detection, relying on enzyme catalytic substrate to produce signal. The characteristics of high throughput and miniaturization of microfluidic chips can solve the problems of many biochemical detection items, large sample consumption and high reagent cost. At present, commercial microfluidic biochemical analysis chips are mainly centrifugal microfluidic chips. The basic operation steps of whole blood samples, such as sampling, separation, quantification, dilution, reaction and detection, are integrated on the microchip, and each reaction chamber is connected with the microchannel network, and the precise control of fluid is realized through centrifugal force, capillary force and siphon valve.

Immunodetection is mainly based on the specific biometrance mechanism of antigen antibodies, which itself has a high specificity. Studies have found that in the microcirculation, when the hydrodynamic strength in 0.1 ~ 10 pN can divide the non-specific binding of antigen antibody, and at 6 ~ 250 pN can still retain specific binding of antigen antibody, at the same time microfluidic technology platform for microfluidic analysis is small, greatly reduce the consumption of expensive immune reagents such as antibodies. In addition, the manipulation and integration of fluid at micro-nano scale not only improves the speed of antigen and antibody reaction, effectively shortens the reaction time, and greatly simplifies the operation process of immunoassay. Therefore, microfluidic immunoassay technology has great potential applications in improving the detection performance including specificity, sensitivity and accuracy of in vitro diagnostic tests.

(2) The application of molecular diagnostic testing

At present, molecular diagnosis accounts for an increasing proportion in precision laboratory medicine. The recurrence of tumor metastasis, the screening of targeted drugs, and the prenatal diagnosis of the fetus all depend on molecular diagnosis. In terms of microfluidic chip, nucleic acid amplification technology is also the most mature development, based on different types of nucleic acid amplification methods have a lot of reports, including real-time quantitative PCR chip, reverse PCR chip, droplet PCR chip, digital PCR chip, etc., and based on constant temperature amplification technology ring mediated isothermal amplification, rolling ring amplification, recombinant enzyme polymerase amplification of microfluidic chip. A small amount of template DNA and reagents in a digital microfluidic chip are encapsulated in droplets or microwells, allowing the analysis of precious nucleic acid samples in a shorter time period relative to conventional DNA amplification protocols (e. g., PCR). At present, microfluidic chips have realized most of the technical methods in the field of molecular diagnosis, including genotyping, gene mutation, single nucleotide polymorphism locus detection, disease-related tiny RNA detection, DNA sequencing, etc.

5. Registration and review of related products

 At present, the products registered and reviewed mainly include reagents based on different chip detection methods and the testing equipment used by supporting chips. Chip reagents are mainly registered in China. At present, more than 40 three types of reagents have been approved in China, and more than 10 items of equipment related to chip testing (domestic and imported) are approved by the National Bureau. More than 100 domestic second-class products.

 The early chip reagent products were all biochips, mainly used for the detection of nucleic acid and proteins. Rereagents were packaged in advance in the form of microarray, such as nucleic acid hybridization chip or protein chip for the detection of multiple genes or antibodies. In 2013, our center issued the Guidelines for the Registration and Review of Biochip Testing Reagents to guide the registration of such products.

 With the continuous development of microfluidic technology, more and more reagents of different microfluidic forms have been applied for registration and approved for market. There are microarray chip, hybridization chip, PCR amplification chip, dish chip, capillary electrophoresis, electrochemical gene chip, etc. The applied reagent testing mainly covers pathogen testing, genotyping, gene polymorphism, genetic genes, and tumor markers; the supporting testing equipment is mainly focused on constant temperature amplification chip analyzer, digital PCR analyzer, biochip reader, etc.

At present, the degree of automation and microfluidic of domestic products is low, and the main product categories are still concentrated in the biochips with low degree of microfluidic, as well as the PCR chips with simple centrifugal method for control, and the requirements for supporting equipment are not high.

6. review thinking and prompt

 For biochips not involving the liquid route system, the overall analytical performance was evaluated with reference to other reagents. For microfluidic chips, additional speed and centrifugal force should be considered for disc chips. For other liquid circuit-controlled chips, factors such as liquid pipeline diameter, flow rate and pressure are considered. There are also some chips that are not covered with reagents, at this time the chip is not managed as a reagent, the reagent application data should be used in the matching chip selection research.

For active chips, that is, the microfluidic process is automatically realized by the chip, and the supporting equipment refers to the requirements of conventional equipment products. For passive chips, the microfluidic process is realized by applying external power, and the corresponding mechanical positioning, pressure, magnetic force and other related factors should be evaluated according to the implementation principle of external power.