Supramolecular Hydrogel Based on pNIPAm Microgels Connected via Host⁻Guest Interactions

Abstract

In this work, host⁻guest supramolecular hydrogels were prepared from poly(N-isopropylacrylamide) (pNIPAm) microgels utilizing electrostatic and host/guest self-assembly. First, pNIPAm microgels bearing a poly(acrylic acid) (pAAc) shell were coated with positively charged β-cyclodextrin polymers. Addition of adamantane-substituted dextrans (Dex-Ada) allowed us to establish interparticle connections through β-cyclodextrin-adamantane (βCD-Ada) inclusion complex formation, and thus to prepare hierarchical hydrogels. Under the conditions of hydrogel formation, close contact between the microgels was ensured. To the best of our knowledge, this is the first example of doubly crosslinked microgels prepared by noncovalent crosslinking via host⁻guest interactions. The prepared macrogels were studied with rheology, and fast mechanical response to temperature variation was found. Furthermore, the hydrogels exhibit fully reversible temperature-induced gel⁻sol transition at the physiological temperature range (37⁻41 °C), due to the synergetic effect between shrinking of the microgels and dissociation of βCD-Ada crosslinks at higher temperatures. This opens up attractive prospects of their potential use in biomedical applications.

Keywords: host–guest polymer complex; hydrogel; microgel; rheology; temperature-induced sol–gel transition; β-cyclodextrin polymer.

Figure 1

Schematic illustration of the strategy for host–guest-driven crosslinking of pCD-coated pNIPAm core (red)/AAc shell (blue) microgels into a 3D DX microgel network.

Figure 2

Cryo-transmission electron microscopy (cryo-TEM) image of microgel/pCD complexes prepared using pCD excess (c(+)/c(−) = 2). The scale bar indicates 100 nm.

Figure 3

Oscillatory rheological measurements of hydrogel samples with varying pCD coverage of the microgel particles. Storage G′ and loss G″ moduli obtained from frequency sweep performed at 0.1% strain for (a) G2; (b) G3; (c) Storage modulus G′ of G2 and G3 samples obtained from amplitude-strain sweep performed at f = 10 Hz. All measurements were performed at 25 °C.

Figure 4

Evolution of storage G′ and loss G″ moduli of supramolecular gels under heating and cooling temperature ramps (temperature change rate = 2 °C/min, $f$ = 10 Hz, γ = 0.1%) for (a) G3; (b) G5; (c) R3 hydrogel samples.

Figure 5

Oscillatory rheological measurements of G3 under temperature ramps (f = 10 Hz, γ = 0.1%).