Thermally-induced softening of PNIPAm-based nanopillar arrays

Abstract

The surface properties of soft nanostructured hydrogels are crucial in the design of responsive materials that can be used as platforms to create adaptive devices. The lower critical solution temperature (LCST) of thermo-responsive hydrogels such as poly(N-isopropylacrylamide) (PNIPAm) can be modified by introducing a hydrophilic monomer to create a wide range of thermo-responsive micro-/nano-structures in a large temperature range. Using surface initiation atom-transfer radical polymerization in synthesized anodized aluminum oxide templates, we designed, fabricated, and characterized thermo-responsive nanopillars based on PNIPAm hydrogels with tunable mechanical properties by incorporating acrylamide monomers (AAm). In addition to their LCST, the incorporation of a hydrophilic entity in the nanopillars based on PNIPAm has abruptly changed the topological and mechanical properties of our system. To gain an insight into the mechanical properties of the nanostructure, its hydrophilic/hydrophobic behavior and topological characteristics, atomic force microscopy, molecular dynamics simulations and water contact angle studies were combined. When changing the nanopillar composition, a significant and opposite variation was observed in their mechanical properties. As temperature increased above the LCST, the stiffness of PNIPAm nanopillars, as expected, did so too, in contrast to the stiffness of PNIPAm-AAm nanopillars that decreased significantly. The molecular dynamics simulations proposed a local molecular rearrangement in our nanosystems at the LCST. The local aggregation of NIPAm segments near the center of the nanopillars displaced the hydrophilic AAm units towards the surface of the structure leading to contact with the aqueous environment. This behavior was confirmed via contact angle measurements below and above the LCST.

Fundamentals and applications of acrylamide based microgels and their hybrids: a review

“…4 Shen et al also reported that P(NIPAM-Am) microgels with NIPAM : Am mol ratio of 9 : 1 has VPTT at 35 C with a broaden transition range. 19 Similar tuning of VPTT of P(NIPAM-Am) microgels has been also reported by Zhang et al 37 and Sanz et al 61 Thermo-sensitivity of microgel was decreased with increase of Am feed contents. The value of equilibrium deswelling ratio of P(NIPAM-Am) polymer microgels measured by Wang et al 1 was decreased with increase of temperature of the medium as shown in Table 1.…”

Poly(N-isopropylacrylamide) and Copolymers: A Review on Recent Progresses in Biomedical Applications

“…PNIPAAm-based hydrogels are extensively investigated for applications in the controlled delivery of active molecules [121,122], in self-healing materials [45], regenerative medicine [123], tissue engineering [39,124], or in smart encapsulation of cells [125], as mentioned before. However, the widespread commercial availability of end-functionalized PNIPAAm (with a variety of M w ), and PNIPAAm copolymers (with different comonomers) has contributed to the recent explosive growth of innovative materials fabricated with this thermally responsive gel; for example, in obtaining smart surfaces [51,126], nanodevices [127,128], and in vivo 4D-printing systems (also called “4D-bioprinting”) [38,129,130].…”

The mechanism of adhesion and graft polymerization of a PNIPAAm thermoresponsive hydrogel to polypropylene meshes

“…Among the large variety of stimuli-responsive soft materials, NIPAAm polymer (PNIPAAm)-based hydrogels, with triggerable phase transition behaviour, 8,26 represent the quintessence of thermoresponsive materials, displaying adaptive changes in mechanical properties related to the amount of water expelled or absorbed from the hydrogel network. 8,27 However, AP-PiCVD technique of solid NIPAAm monomer has some drawback due to its low vapor pressure (2.32×10 -2 mm•Hg at 25°C).…”