The physical characterization of supermacroporous poly(N-isopropylacrylamide) cryogel: Mechanical strength and swelling/de-swelling kinetics

Poly(N-isopropylacrylamide) [poly(NiPAAm)] and poly(acrylamide) [poly(AAm)] cryogels were synthesized by radical polymerization at −12 °C for 12 h using monomers of N-isopropylacrylamide (NiPAAm) and acrylamide (AAm) with N,N-methylene bisacrylamide (MBAAm) as cross-linking agent, respectively. The cryogels synthesized in freezing conditions provided spongy, elastic and supermacroporous character as compared to the hydrogels synthesized at ambient temperatures. Our earlier observations revealed that the elastic deformation of cryogels either by external forces (mechanical deformation) or internal forces (shrinkage-swelling of poly(NiPAAm) cryogels) led to detachment of affinity bound bioparticles to these gels, which promises great potential in understanding cell interactions on elastic matrices [M.B. Dainiak, A. Kumar, I.Y. Galaev, B. Mattiasson, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 849–854]. The deformation characteristic of cryogels as measured by Young's modulus indicates that the modulus of elasticity of poly(NiPAAm) cryogel (33–65 kPa) is comparatively lower than the Young's modulus for poly(AAm) cryogel (42–86 kPa). The Young's modulus of both the cryogels was found to be dependent on monomer concentration in cryogels and increases with the increase in concentration. Thus, poly(AAm) cryogel are mechanically more rigid than poly(NiPAAm) cryogel. Further, the swelling/de-swelling kinetics study on poly(NiPAAm) cryogel and hydrogel showed, higher swelling ratios for cryogels in the range of 13–16 as compared to poly(NiPAAm) hydrogels which were in the range of 7–10. However, the extent of de-swelling is more in the case of poly(NiPAAm) hydrogels.