Two different hydrogels, blank-SA and 2dDR-SA, were prepared by simple manual mixing of the constituents in autoclaved sterilized water at RT by using a spatula.

The 2dDR-SA hydrogel was composed of 1.4 g sodium alginate (6.416% w/w), 250 mg propylene glycol (1.146% w/w), 82.5 mg of 2-phenoxyethanol (0.375% w/w), and 86.62 mg of 2-deoxy-D-ribose sugar (0.394% w/w) in 20 mL water. The prepared hydrogels (blank-SA and 2dDR-SA) were stored in glass vials at RT.

For FTIR and 2dDR release studies, 10 mL each of blank-SA and 2dDR-SA hydrogels were poured into Petri dishes (100 × 15 mm) and covered with a perforated aluminum foil. These were frozen at −20°C for 20 h and then placed in Labconco’s FreeZone (4.5 L) at −105°C for 24 h. These freeze-dried hydrogels (FD-blank-SA and FD-2dDR-SA) were stored at RT until used for analyses.

I don't know why they freeze-dried them.