Thermoresponsive Shape‐Memory Hydrogel Actuators Made by Phototriggered Click Chemistry

This article describes the design and synthesis of a new series of hydrogel membranes composed of trialkyne derivatives of glycerol ethoxylate and bisphenol A diazide (BA‐diazide) or diazide‐terminated PEG600 monomer via a Cu(I)‐catalyzed photoclick reaction. The water‐swollen hydrogel membranes display thermoresponsive actuation and their lower critical solution temperature (LCST) values are determined by differential scanning calorimetry. Glycerol ethoxylate moiety serves as the thermoresponsive component and hydrophilic part, while the azide‐based component acts as the hydrophobic comonomer and most likely provides a critical hydrophobic/hydrophilic balance contributing also to the significant mechanical strength of the membranes. These hydrogels exhibit a reversible shape‐memory effect in response to temperature through a defined phase transition. The swelling and deswelling behavior of the membranes are systematically examined. Due to the click nature of the reaction, easy availability of azide and alkyne functional‐monomers, and the polymer architecture, the glass transition temperature (T_g) is easily controlled through monomer design and crosslink density by varying the feed ratio of different monomers. The mechanical properties of the membranes are studied by universal tensile testing measurements. Moreover, the hydrogels show the ability to absorb a dye and release it in a controlled manner by applying heat below and above the LCST.     

Shape‐Memory Microfluidics

Materials with embedded vascular networks afford rapid and enhanced control over bulk material properties including thermoregulation and distribution of active compounds such as healing agents or stimuli. Vascularized materials have a wide range of potential applications in self‐healing systems and tissue engineering constructs. Here, the application of vascularized materials for accelerated phase transitions in stimuli‐responsive microfluidic networks is reported. Poly(ester amide) elastomers are hygroscopic and exhibit thermo‐mechanical properties (T_g ≈ 37 °C) that enable heating or hydration to be used as stimuli to induce glassy‐rubbery transitions. Hydration‐dependent elasticity serves as the basis for stimuli‐responsive shape‐memory microfluidic networks. Recovery kinetics in shape‐memory microfluidics are measured under several operating modes. Perfusion‐assisted delivery of stimulus to the bulk volume of shape‐memory microfluidics dramatically accelerates shape recovery kinetics compared to devices that are not perfused. The recovery times are 4.2 ± 0.1 h and 8.0 ± 0.3 h in the perfused and non‐perfused cases, respectively. The recovery kinetics of the shape‐memory microfluidic devices operating in various modes of stimuli delivery can be accurately predicted through finite element simulations. This work demonstrates the utility of vascularized materials as a strategy to reduce the characteristic length scale for diffusion, thereby accelerating the actuation of stimuli‐responsive bulk materials.