# A Review on Microfluidic Platforms Applied to Nerve Regeneration
## 1 Introduction
Microfluidic platforms enable safe, high-throughput and accurate experiments at the microscopic scale. Some experiments that require many volunteers with risk of injury, such as electrical stimulation experiments [17; 18], can be performed on microfluidic platforms [19]. In recent years, commercial microfluidic platforms have been widely used in many cytological pathology studies. [PERSON] [20] used microfluidic device SND 450 (Xona Microfluidics, Inc., Durham, NC, USA) to test the hypothesis that the mTOR pathway regulates retinal ganglion cells (RGC) and promotes axonal regeneration after injury. [PERSON], [PERSON] et al. [21] investigated the regenerative factors of chemically damaged neurons through a commercial microfluidic platform. [PERSON] [22] adopted a modified rabies virus encoding a fluorescent protein to retrogradely label neurons through isolated axons. Then he created an isolated microenvironment within a compartment, and performed axotomy and immunocytochemical analysis on a pre-assembled XC450 XonaChip (Xona Microfluidics, Inc.). [PERSON] [23] proposed a method for assessing axonal degeneration using microfluidic device. [PERSON] [24] conducted an experiment on the effects of external magnetic field gradients to generate drag forces on axon growth on R150 chip (Xona Microfluidics, Inc.) to investigate strategies for axon regrowth. [PERSON] [25] used microfluidic device to investigate the mechanisms of axon-axon interaction and developed axons to guide the growth of young axons during embryonic development. The current commercial microfluidic platforms can perform several basic operations for in vitro regenerative culture of neuronal cells, including neuron loading, axon isolation, axon injury and drug/nerve growth factor (NGF) screening. Although commercial microfluidic platforms can be used to perform high-quality experiments in neural regeneration study, they still cannot fulfil the specific requirements in some experiments. For example, experiments that involve precise damage to the axon require a special coating on the top of the micro-channels. Experiments with solution gradients for axon guidance require a modification of micro-channels, by adding extra inlets and micro-wells. Therefore, many papers have proposed self-developed microfluidic platforms to satisfy the specified research requirements such as experimental protocols, study models, and specific methods. Those self-developed microfluidic platforms are introduced in following sections. In Section 2, we discuss the properties of commonly used microfluidic fabrication materials and introduce different structures for microfluidic devices, including 2D and 3D culture structures. Section 3 introduces microfluidic devices for related research according to two subcategories of nerve regeneration research: nerve repair and nerve regrowth. The studies of nerve repair often involve precise damage to neural axon. Therefore, a subsection is listed to introduce different microfluidic platforms to generate axonal injury. Axonal guidance is a significant technical point in the research of nerve regeneration; we therefore describe in detail the different platforms that guide the growth of axon in Section 4. Finally, in Section 5, we summarize this manuscript, whereafter we discuss the existing challenges and difficulties, and future perspective.