# A Review on Microfluidic Platforms Applied to Nerve Regeneration
## 2 Design of Microfluidic Devices
### Structures
#### 2.2.1 2D Culture Structure  
There are many kinds of 2D culture structures. Figure 2A shows a modified structure of microfluidic device, which is added a pair of reservoirs on both sides of each compartment to store sufficient volume of medium. This structure is the first generation of microfluidic-based compartmentalized neuron culture platform designed by [PERSON] [13] and [PERSON] [41]. The structure can make a small volume difference between the two compartments, achieving fluid isolation of the axon in the microenvironment. The role of microchannels is to directly isolate the axons. Their elongated structures are designed based on the property that axons grow faster and longer than dendrites so that the axon, rather than the dendrite, can extend to the opposite isolation compartment. By varying the length of the microchannel, the axon tip can access proximal or distal sites. There are many microfluidic platform-based neurobiological researches that have adopted this structure [42; 43; 44]. Subsequent improvements to microfluidic devices include increasing the number of compartments and optimizing the alignment of compartments, micro-channels and reservoirs on the chip. As shown in Figure 2B, [PERSON] [45] designed a microfluidic device with three compartments (soma, axonal, axonal) to conduct comparison experiments. [PERSON], [PERSON] et al. [21] presented a microfluidic device with four compartments (glial, glial, axonal, soma) to control the injection of solutions in micro-channels and soma/axonal compartments, which is shown as Figure 2C. [PERSON] [46] presented a five-compart microfluidic device to screen drugs. As shown in Figure 2D, fluidic isolation between compartments allows for investigation of effects of neural networks after application of chemical injury to one of the compartment. [PERSON] [47] designed a high-throughput microfluidic device with 24 axon compartments and one somaal compartment. As shown in Figure 2E, every six axon compartments share a common reservoir for culture media. This design facilitates the use of batch culture and the conduct of controlled trials. [PERSON] [48] introduced a circular structure microfluidic device with axon guiding microgrooves for quantified axon growth and regeneration, which is shown as Figure 2F. These kinds of improvements are made to meet the requirements of experiments such as drug/NGF screening, spatio-temporal control of cell and axonal axotomy. [PERSON] [49]. presented a device constructed of eight chambers in the shape of an eight-spoked asterisk. The device is characterized by the ability to limit the interface between the cell-filled hydrogel and the chemical solution of interest to improve the time difference between the chemical solution reaching the cells. The advantages and disadvantages of different cell culture structures are listed in Table 2.  
Figure 1: A basic structure of microfluidic device for neuron regeneration. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Methods** & **Advantages** & **Disadvantages** \\ \hline Structure of & 1. & East fabrication of the single layer in PDMS microfluidic device can be realized easily. \\ Structure & 2. & Applicable to non-adherent cells. \\ in Figure 1 & 3. & It can be combined with basic flow control. \\ \hline Structure & 1. & East fabrication of the single layer in PDMS microfluidic device can be realized easily. \\ in Figure 1 & 2. & Applicable to non-adherent cells. \\  & 3. & It can be combined with basic flow control. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advantages and disadvantages of different cell culture structures. Figure 2: 2D culture microfluidic devices. (**A**) Two-compartment structure designed. (**B**) Three-compartment structure (soma, axonal, axonal). (**C**) Fourcompartment structure (glial, glial, axonal, somaal) for controlling the injection of solutions. (**D**) Five-compartments structure for drug screen. (**E**) The structure of six axon compartments sharing common reservoir for high-throughput batch culture.