Response behaviors and mechanical strength of thermal responsive hydrogels fabricated by electrospinning

Abstract

A UV-initiated crosslinking agent was incorporated in a poly(N-isopropylacrylamide) (PNIPAM)-based copolymer. Thermally-responsive hydrogels were then fabricated by the electrospinning of a PNIPAM-based copolymer and subsequently crosslinked under UV-treatment. The morphology of the fibrous structure was well maintained after many temperature induced swelling and de-swelling cycles. Both considerable degree of swelling and de-swelling ratios, along with an excellent response rate, were achieved with temperature variation. The influence of the crosslinker incorporated in the copolymer was investigated in terms of swelling/de-swelling ratios and rate, as well as the compressive strength and viscoelasticity of the hydrogels. Incorporation of crosslinker led to a 15% increase in copolymer molecular weight and 2 °C decrease of the volume phase transition temperature (VPTT). The diameters of fully swollen and shrunk electrospun fibers were around 10 times and 3 times those of as-spun fibers, respectively. Also, compared to as-spun fiber membrane, 4 times swelling in thickness and an unusual 0.6 time shrinking in the length and width are reported and explained for the first time. In terms of response time, both swelling and de-swelling equilibrium were reached within 10 min for samples with 0.7 mm thickness; negligible impact on the response rate was observed when the crosslinker to monomer ratio increased from 100:1 to 100:5. However, the increase of crosslinker ratio showed significant negative impact on equilibrium swelling and de-swelling ratio (about 700% and 15% decrease respectively). A viscoelastic, but close to elastic, behavior in both swelling and de-swelling equilibrium states under small-amplitude oscillatory shear was observed. The results of this work are expected to accelerate research in applications such as actuators and soft robotics.