Table 2.

Porous Scaffold Manufacturing Techniques

Technique	Applications
Freeze casting	Ceramic slurries are most commonly freeze cast, where water from the slurry is sublimated and results in pores with the morphological characteristics of ice crystals.
Freeze drying/lyophilization	A relatively simple technique that can be used with natural materials such as collagen, gelatin, or silk fibroin, the porosity can be modified based on changes in freezing temperature and material concentration.
Solvent casting and particulate leaching	For three-dimensional scaffolds, molds must be created for casting the polymer solution. Although leaching requires additional processing time, the use of organic solvents facilitates addition of drugs or growth factors to scaffolds.
Gas foaming	Carbon dioxide at high pressure is used to expand the polymer instead of using temperature or other solvents. Varying pressure can also produce scaffolds with a gradient porosity.
Phase separation	Thermally induced phase separation can be used to separate polymers into their solvent and solid polymer, resulting in homogenous and interconnected porosity throughout the scaffold that can be tunable based on cooling rates during processing.
Electrospinning	Electrospun fibers can vary from nanoscale to microscale, with alignment and chemical composition based on processing parameters. Previously restricted to polymers, recent advances have also allowed for electrospinning of titanium for bone tissue engineering.
Sol–gel	Traditionally used colloidal metal oxides, the sol–gel method results in a scaffold with tunable porosity and chemistry. Biphasic chitosan scaffolds with an affinity peptide have shown the ability to recruit stem cells for cartilage regeneration.
Additive manufacturing	Extrusion methods are mostly polymer based. Solid freeform through sintering can be applied to both polymers and metals, while laser melting is restricted to metals.