Optimizing microgreen cultivation through post-crosslinked alginate-gellan gum hydrogel substrates with enhanced...

Hydrogels present a promising alternative to soil and traditional hydroponic substrates for indoor and space farming due to their high water retention capacity, non-toxicity, biodegradability, and capacity to fix microgreen seeds in place. However, conventional hydrogels typically suffer from poor porosity in their native state and often experience mechanical instability, which leads to the collapse of critical porous networks necessary for root zone oxygenation. To address these challenges, we developed a hydrogel-based growth substrate from alginate and gellan gum, using a sequential process involving directional freezing, lyophilization, and post-crosslinking in a CaCl₂ solution. The as-prepared substrate retained the porous architecture upon rehydration as evidenced by SEM and Micro-CT imaging, as well as a 14-fold increase in the median pore diameter compared to pre-crosslinked control at hydrated state. The post-crosslinked hydrogels also exhibited superior mechanical stability, with an average compression stress of 6.08 kPa compared to 3.35 kPa for the control. In a growth study using Brassica juncea microgreens, the as-prepared hydrogel sustained a 12-day growth cycle without additional watering, achieving similar germination rates and fresh weights to regularly hydrated rockwool. Notably, the hydrogels treated with slow freezing and crosslinked in 5 % CaCl₂ solution yielded higher fresh weight of harvested microgreens than those prepared under other conditions. This work presents a simple and effective approach to developing a plant growth substrate with significant potential for low-maintenance crop production in future space exploration missions.