# A Review on Microfluidic Platforms Applied to Nerve Regeneration
## 2 Design of Microfluidic Devices  
### Materials  
The traditional method for the manufacturing process of cell culture devices uses glass, plastic and silicone as common raw materials. Silicone has advantages, such as resistance to organic solvents, ease of metal deposition, high thermal conductivity and stable electroosmotic mobility. However, its properties such as of impermeability, opacity and high cost make it an undesirable candidate for manufacturing microfluidic devices. Besides this, its high stiffness makes it difficult to manufacture microfluidic components [26]. Glass is an optically transparent, electrically insulating and impermeable material. Glass has relatively low non-specific adsorption and is compatible with biological samples. However, glass has similar disadvantages to silicone, wherein its high hardness and high manufacturing costs lead to many limitations for its application in microfluidics [27]. Low temperature co-fired ceramic (LTCC) technology is a commonly used fabrication method for ceramic microfluidic device. Ceramic material has high themostability, high surface stability and are lower in price than glass and silicone. However, ceramic does not have gas permeability or optical transparency either. Some manufacturers have attempted to use thermoplastic materials such as polystyrene (PS), polycarbonate (PC), poly-methyl methacrylate (PMMA) or poly-ethylene glycol (PEG) diacrylate to manufacture microfluidic device. Thermoplastics are fabricated by thermoolding that allows the manufacture of a large amount of duplicates at high speed and low cost which is profitable for commercial production, but not economical for production of prototypes. Thermoplastic material has many merits such as water stability, optical clarity, rigid mechanical property and compatibility to electrophoresis, which make it one of the common materials for microfluidic devices [28]. However, different thermoplastic materials have different characteristics. PS has the advantages of being optically transparent, biocompatible, chemically inert and having easily functionalized surfaces. Its hydrophobic surface can be hydrophilized by physicochemical means. However, PC is expensive to manufacture and process and is suitable for large-scale manufacturing [29]. PC material is made from bisphenol and phosgene polymerization. Its advantages of high transparency, high glass transition temperature, high impact resistance and low moisture absorption make it suitable for DNA thermal cycling applications. However, PC has poor resistance to some organic solvents and poor absorption of UV light [30]. The advantages of PMMA include strong mechanical properties, high optical transparency and electrolysis compatibility. Therefore, it is often used in the fabrication of disposable microfluidic chips. However, the hydrophobicity of PMMA is the worst among all thermoplastic materials [31; 32]. As PDMS, PEG diacrylate shares similar advantages, such as water stability, optical clarity and low background fluorescence. However, PEG diacrylate shows less nonspecific adsorption and has greater resistance to permeation of small hydrophobic molecules than PDMS material [33]. Over past two decades, PDMS has gradually been used for fabrication and prototyping of microfluidic chips, because of its convenient fabrication, low production cost, good optical clarity, high gas permeability and relative biocompatibility. PDMS provides a chemically inert surface with low interfacial free energy, and its surface properties tend to modify with surface coating such as plasma, covalent and dynamic modification [26]. In addition, due to the superior biocompatibility and nontoxicity, PDMS materials can be made into chips that will not be immunologically rejected by human body and have the potential to be applied in the in vivo environment for long-term coexistence with the body. There are some disadvantages of PDMS, such as permeability to water vapor, which makes it difficult to control evaporation in PDMS devices.