Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics

Abstract

The swelling and collapsing of thermo-responsive poly( N -isopropylacrylamide)-based polymer (pNIPAAm) networks are investigated in order to reveal the dependency on their kinetics and maximum possible actuation speed. The pNIPAAm-based network was attached as thin hydrogel film to lithographically prepared gold nanoparticle arrays to exploit their localized surface plasmon resonance (LSPR) for rapid local heating. The same substrate also served for LSPR-based monitoring of the reversible collapsing and swelling of the pNIPAAm network through its pronounced refractive index changes. The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of water molecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogel film in the swollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on the tuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in the time constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient of polymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs and micromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogel materials.

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties—Thermoresponsive and Non-Thermoresponsive Gels

“…Comparing these results to previous observations of collapse and swelling behaviour of these photocrosslinked PNiPAAm hydrogel layer systems, which show similar volume transitions and kinetics upon thermal stimulation [ 28 ], it follows that thermoresponsiveness is a crucial factor in photoactuation of these azobenzene-containing gels. While non-thermoresponsive gels demonstrate a lower degree of motion upon irradiation, the effect in the thermoresponsive gel is magnitudes stronger.…”

Microstructuring of Thermoresponsive Biofunctional Hydrogels by Multiphoton Photocrosslinking

“…[33] In short, 4methylbenzophenone was brominated by N-bromosuccinimide via radical initiation with azobisisobutyronitrile (AIBN), yielding 84% of 4-bromomethyl benzophenone. In the second step, substitution of 4-bromomethyl benzophenone with N- [3-(dimethylamino)propyl]acrylamide in methyl tert-butyl ether led to precipitation of the final product BPQAAm as quaternary ammonium bromide in a yield of 90% ( 1 H and 13 C NMR spectra of the BPQAAm are shown in Figures S1 and S2, see SI). The anthraquinone-based crosslinker AAHAQ was prepared according to the literature by modification of 2-amino-3-hydroxyanthraquinone with an excess of acryloyl chloride in yields of up to 81%.…”

“…[7][8][9][10] Among these, poly(N-isopropyl acrylamide) (pNIPAAm) is one of the most studied responsive polymers. This polymer can be toggled between a coil and globule state in an aqueous solution by changing the temperature below or above the cloud point temperature that occurs close to the human body temperature, [1,[11][12][13] which is a result of its lower critical solution temperature (LCST) characteristics. This material is commonly used for the preparation of networks that act as thermoresponsive hydrogels in the form of free-standing or surface-attached elements.…”

Thermoresponsive and Photocrosslinkable Poly(2‐alkyl‐2‐oxazoline) Toolbox – Customizable Ultralow‐Fouling Hydrogel Coatings for Blood Plasma Environments

“…[22] Such volume transitions resulting from the LCST behavior have been exploited for reversible actuation in ad-vanced biosensor applications and are particularly suitable for responsive binding matrices. [23][24][25][26][27][28][29] Poly(2-alkyl-2-oxazoline)s are prepared by cationic ringopening polymerization (CROP), which allows control over the target molar mass while retaining a narrow mass dispersity, and enables the design of advanced polymer architectures via variation of the comonomer types and sequence. However, this preparation technique is sensitive toward nucleophilic substances or side groups and the cationic chain end can be easily terminated by those.…”

“…[ 22 ] Such volume transitions resulting from the LCST behavior have been exploited for reversible actuation in advanced biosensor applications and are particularly suitable for responsive binding matrices. [ 23–29 ]…”