Gas foaming uses high pressure carbon dioxide gas for the fabrication of highly porous scaffolds. A polymer solution is saturated with carbon dioxide at a high pressure (800 psi) [47]. When the dissolved carbon dioxide becomes unstable under this condition, it will separates from the polymer, and its molecules become cluster to minimize the free energy to create pore nucleation (foaming) [197]. After completing the foaming process, a 3D porous polymeric structure of scaffold is formed with expanded polymeric volume and decreased polymer density [198]. In the recent, Gravel et al. [199] described the first attempt in using natural coralline biomaterials as reinforcing gas-forming agents for the development of 3D macroporous hybrid biomatrices of chitosan and natural coralline material. Yun et al. [200] used a gas forming method to obtain micro-sized pores in the electrospun fiber bundles for designing a dual-porosity structure of Montmorillonite reinforced PLLA nanocomposite scaffold. And the robust dual-porosity scaffold structure was used for a facile transport of metabolic nutrients and wastes through the nano-sized pores, as well as, for the cell implantation and blood vessel invasion through the micro-sized pores. Therefore, the gas foaming can be utilized as a step in the manufacture of highly porous reinforced scaffolds, in combination with other fabrication techniques.

The great advantage of gas foaming is that the technique doesn’t require the use of organic solvents compared to many manufacturing techniques requiring high temperature and organic solvents, where the residue will damage cells and nearby tissues. Such techniques widely used in industry are rarely used in the manufacture of scaffolds. Because it is hard to control the size and interconnection of pores, and large closed pores can be created inside the polymer structure during the foaming process, which is unsuitable for cells [201]. Thus, the gas foaming is mainly used to form bone scaffolds [201-203]. Recently, many modifications of the gas foaming method have been explored, such as the use of polymer-salt mixtures [204], or ammonium bicarbonate salt instead of high pressure carbon dioxide with [205].

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