Novel Synthesis of Macroporous Poly(N-isopropylacrylamide) Hydrogels Using Oil-in-Water Emulsions

Abstract

Porous N-isopropylacrylamide (NIPA) hydrogels having a unique structure, that is, spherelike cavities distributed randomly and a homogeneous network in the gel phase, were successfully synthesized by means of an emulsion templating method; this method involves the synthesis of NIPA gels in an oil-in-water (O/W) emulsion by free radical copolymerization with a cross-linker, followed by washing (removal) of the dispersed oil as a pore template (porogen). The synthesis conditions, O/W volume ratio, amount of added surfactant, and monomer concentration affect the internal pore structure, equilibrium swelling, and swelling/shrinking kinetics. A porous hydrogel swollen at 10 degrees C has a pore diameter distribution in the range of 1-40 microm, which was observed with a scanning electron microscope. Scanning electron micrographs and swelling degree reveal that the pore size and porosity can be adjusted by varying the O/W volume ratios and surfactant amounts. The porous hydrogels show very rapid swelling/shrinking in accordance with the temperature swing. The fast response is attributed to the convection flow of water through the macropores. In addition to a faster response gel, the emulsion templating method can yield potentially intelligent gels in which the pores function as spaces for reaction, separation, and storage.

Preparation of macroporous poly(N-isopropylacrylamide) hydrogels using a suspension–gelation method

“…The diameters of all the types of hydrogels decreased with an increase in temperature, and a drastic change occurred around 33°C owing to the hydrophilic/ hydrophobic transition of the NIPA polymer. The swelling patterns of the semi-IPN hydrogel (/ m = 0) and the composite and macroporous hydrogels were similar to those seen in the results of previously conducted studies on conventional non-porous NIPA hydrogels [15,27].…”

“…For the semi-IPN hydrogel (/ m = 0), composite hydrogels, and macroporous hydrogels (except for / m = 0.7) the swelling equilibrium was attained within a few minutes. On the other hand, the swelling equilibrium for the conventional non-porous NIPA hydrogels with 6 mm diameters was attained in about a day in our previous study [15,28], where a dense and shrunken network (so-called skin layer [28,29]) was formed on the hydrogel surface in the shrinking process because of the quick temperature rise across the LCST.…”

“…The shear modulus and compressive strength of cylindrical hydrogel were measured by the compression technique [2,15,27] using a rheometer. Prior to the measurements, the hydrogel specimen was immersed in water at 20°C.…”

“…In our earlier studies, we developed macroporous hydrogels that entrapped enzymes in the macropores [1] and hydrogels containing oil microdroplets of an extractant used for metal adsorption [2,3]. Porous polymeric hydrogels have been prepared by polymerization in the presence of inert diluents as pore-forming agents (porogens), such as surfactants [4][5][6], polymers (polyethylene glycol [7,8], poly(L-lactide) [9], poly(dimethylsiloxane) [10], sucrose [11], and alginate [12]), silica particles [13,14], and oils [15,16].…”

Micro-structured smart hydrogels with enhanced protein loading and release efficiency

“…The emulsion template technique has been extensively investigated for preparation of macroporous polymeric or inorganic materials because the preparation, deformability and removal of liquid templates are very easy, which allows the materials to withstand the stresses of shrinkage during gelation without cracking [24]. It is also reported that the emulsion templating method yields porous hydrogels with unique features including a homogeneous network structure in the gel phase similar to that in nonporous hydrogels and easily controllable porosity by changing the template volume and the amount of added surfactant [25].…”