Shape memory polymer with programmable recovery onset.

Stimulus-responsive shape-shifting polymers1-3 have shown unique promise in emerging applications, including soft robotics4-7, medical devices8, aerospace structures9 and flexible electronics10. Their externally triggered shape-shifting behaviour offers on-demand controllability essential for many device applications. Ironically, accessing external triggers (for example, heating or light) under realistic scenarios has become the greatest bottleneck in demanding applications such as implantable medical devices8. Certain shape-shifting polymers rely on naturally present stimuli (for example, human body temperature for implantable devices)8 as triggers. Although they forgo the need for external stimulation, the ability to control recovery onset is also lost. Naturally triggered, yet actively controllable, shape-shifting behaviour is highly desirable but these two attributes are conflicting. Here we achieved this goal with a four-dimensional printable shape memory hydrogel that operates via phase separation, with its shape-shifting kinetics dominated by internal mass diffusion rather than by heat transport used for common shape memory polymers8-11. This hydrogel can undergo shape transformation at natural ambient temperature, critically with a recovery onset delay. This delay is programmable by altering the degree of phase separation during device programming, which offers a unique mechanism for shape-shifting control. Our naturally triggered shape memory polymer with a tunable recovery onset markedly lowers the barrier for device implementation.

Homeostatic growth of dynamic covalent polymer network toward ultrafast direct soft lithography.

Soft lithography is a complementary extension of classical photolithography, which involves a multistep operation that is environmentally unfriendly and intrinsically limited to planar surfaces. Inspired by homeostasis processes in biology, we report a self-growth strategy toward direct soft lithography, bypassing conventional photolithography and its limitations. Our process uses a paraffin swollen light responsive dynamic polymer network. Selective light exposure activates the network locally, causing stress imbalance. This drives the internal redistribution of the paraffin liquid, yielding controllable formation of microstructures. This single-step process is completed in 10 seconds, does not involve any volatile solvents/reactants, and can be adapted to three-dimensional complex surfaces. The living nature of the network further allows sequential growth of hierarchical microstructures. The versatility and efficiency of our approach offer possibilities for future nanotechnologies beyond conventional microfabrication techniques.

Bioinspired Dual-Mode Temporal Communication via Digitally Programmable Phase-Change Materials.

Switchable optical properties are essential for numerous technologies including communication, anticounterfeiting, camouflage, and imaging/sensing. Typically, the switching is enabled by applying external stimulation such as UV light for fluorescence detection. In contrast, ground squirrels utilize spontaneous live infrared emission for fencing off predators as a unique way of communication. Inspired by this, live evolution of both optical and thermal images for temporal communication in which time is the encoded information is demonstrated. This system is based on a digitally light-cured polymeric phase-change material for which the crystallization kinetics can be controlled in a pixelated manner. Consequently, live evolution in optical transparency during the crystallization process enables temporal optical communication. Additionally, by harnessing the dynamic evolution of the thermal enthalpy, multiple sets of time-specific information can be reversibly retrieved as self-evolving infrared thermal images. The versatility of this dual-mode temporal system expands the scope for secured communication, with potential implications for various other areas including optics, thermal regulation, and 3D/4D printing.

Rapid Open-Air Digital Light 3D Printing of Thermoplastic Polymer.

3D printing has witnessed a new era in which highly complexed customized products become reality. Realizing its ultimate potential requires simultaneous attainment of both printing speed and product versatility. Among various printing techniques, digital light processing (DLP) stands out in its high speed but is limited to intractable light curable thermosets. Thermoplastic polymers, despite their reprocessibility that allows more options for further manipulation, are restricted to intrinsically slow printing methods such as fused deposition modeling. Extending DLP to thermoplastics is highly desirable, but is challenging due to the need to reach rapid liquid-solid separation during the printing process. Here, a successful attempt at DLP printing of thermoplastic polymers is reported, realized by controlling two competing kinetic processes (polymerization and polymer dissolution) simultaneously occurring during printing. With a selected monomer, 4-acryloylmorpholine (ACMO), printing of thermoplastic 3D scaffolds is demonstrated, which can be further converted into various materials/devices utilizing its unique water-soluble characteristic. The ultralow viscosity of ACMO, along with surface oxygen inhibition, allows rapid liquid flow toward high-speed open-air printing. The process simplicity, enabling mechanism, and material versatility broaden the scope of 3D printing in constructing functional 3D devices including reconfigurable antenna, shape-shifting structures, and microfluidics.

Logic-Controlled Radical Polymerization with Heat and Light: Multiple-Stimuli Switching of Polymer Chain Growth via a Recyclable, Thermally Responsive Gel Photoredox Catalyst.

Strategies for switching polymerizations between "ON" and "OFF" states offer new possibilities for materials design and fabrication. While switching of controlled radical polymerization has been achieve using light, applied voltage, allosteric effects, chemical reagents, pH, and mechanical force, it is still challenging to introduce multiple external switches using the same catalyst to achieve logic gating of controlled polymerization reactions. Herein, we report an easy-to-synthesize thermally responsive organo-/hydro-gel that features covalently bound 10-phenylphenothiazine (PTH). With this "Gel-PTH", we demonstrate switching of controlled radical polymerization reactions using temperature "LOW"/"HIGH", light "ON"/"OFF", and catalyst presence "IN"/"OUT". Various iniferters/initiators and a wide range of monomers including acrylates, methacrylates, acrylamides, vinyl esters, and vinyl amides were polymerized by RAFT/iniferter and ATRP methods using Gel-PTH and a readily available compact fluorescent light (CFL) source. In all cases, polymer molar masses increased linearly with conversion, and narrow molar mass distributions were obtained. To further highlight the utility of Gel-PTH, we achieved "AND" gating of controlled radical polymerization wherein various combinations of three stimuli were required to induce polymer chain growth. Finally, block copolymer synthesis and catalyst recycling were demonstrated. Logic-controlled polymerization with Gel-PTH offers a straightforward approach to achieve multiplexed external switching of polymer chain growth using a single catalyst without the need for addition of exogenous reagents.