Table 1.

Methods of porous HGs preparation

Method	Mechanism	Advantages	Limitations
Porogen templating	The usage of porogens during the gelation process, that are removed after HGs formation.	Simple process; good pore size control	Difficulties with porogen removal
Gas foaming	High-speed stirring of gel, which generates bubbles, or addition of substance, that produces gas particles through a chemical reaction.	Simple process; inexpensive method	Insufficient possibility of pore size control
Bicontinuous emulsion templating	Preparation of emulsion, where the aqueous phase is a mixture of monomers for polymerization.	Simple process	Various pores size; water soluble polymers
Cryogelation	Polymerization at very low temperatures (formation of crystals). During controlled heating, crystals are melt and pores are formed.	More interconnective porogen structure	Use of sub-zero temperatures
3D printing	3D printing of HG matrix with specially planned and strictly defined pore sizes.	Controllable pores size	Insufficient resolution of 3D printers
Electrospinning	Usage of electric charge to obtain porous structure of polymeric HG.	Microscale and macroscale process	Relatively slow process
Freeze-drying	Preparation of an oil-in-water emulsion from which water phase is removed during freeze-drying (formation of pores).	Good pore size control	Water insoluble polymers
Inverse opal hydrogelation	Preparation of a 3D pattern from colloidal particles, between which polymer solution is poured in, followed by removal of the template after polymerization	Interconnected pores	Selection of colloidal particles