General method for the production of hydrogel droplets from uniformly sized smart shell membranes†
A smart solid-state liquid crystal (LCsolid) shell membrane template prepared by a microfluidic method with a reactive mesogen mixture of RMM727 (from Merck) doped with a nematic liquid crystal of 4′-pentyl-4-biphenylcarbonitrile (5CB) as a porogen was used for a facile and general method to produce uniformly sized hydrogel droplets. The reactive hydrogel monomers were incorporated into the core of the LCsolid shell through the shell pores by immersing the LCsolid shells in the reactive hydrogel monomer mixture. Subsequently, the hydrogel monomer mixture outside the LCsolid shells was replaced with a non-reactive medium that can close the pores because of its poor solvent nature with respect to the LCsolid shell, confining the hydrogel monomer mixture at the core during UV curing. After UV curing, uniformly sized hydrogel droplets were hatched out from the LCsolid shell when the hydrogel droplet-containing LCsolid shells were put into a solvent, causing the hydrogel droplets to swell more than the LCsolid shells. The hydrogel droplets were separated from the broken LCsolid shell using two immiscible solvents, each of which contained hydrogel droplets and broken LCsolid shells. The hydrogel droplets generated based on the LCsolid shell template were poly(acrylic acid) (PAA), poly(N-isopropylacrylamide) (PNIPAM), and P(AA-co-NIPAM), which denote the pH-, temperature-, and dual (both pH and temperature)-responsive properties. In particular, the molar ratio of the P(AA-co-NIPAM) hydrogel droplet composition could be easily altered by changing the mixing ratio of the AA/NIPAM monomer mixture without tuning the processing conditions for droplet production, which is usually needed when using microfluidic devices. The proposed method can be used to obtain uniformly sized hydrogel droplets without using any instrument (e.g., microfluidic devices) or stabilizing material (e.g., surfactants) at any convenient time.