Stimuli-responsive Polymeric Materials Research Articles

Liquid crystalline elastomer self-oscillating fiber actuators fabricated from soft tubular molds.

The self-oscillation of objects that perform continuous and periodic motions upon unchanging and constant stimuli is highly important for intelligent actuators, advanced robotics, and biomedical machines. Liquid crystalline elastomer (LCE) materials are superior to traditional stimuli-responsive polymeric materials in the development of self-oscillators because of their reversible, large and anisotropic shape-changing ability, fast response ability and versatile structural design. In addition, fiber-shaped oscillators have attracted much interest due to their agility, flexibility and diverse oscillation modes. Herein, we present a strategy for fabricating fiber-shaped LCE self-oscillators using soft tubes as molds. Through the settlement of different configuration states of the soft tubes, the prepared fiber-shaped LCE oscillators can perform continuous rotational self-oscillation or up-and-down shifting self-oscillation under constant light stimuli, which are realized by photoinduced repetitive self-winding motion and self-waving motion, respectively. The mechanism of self-oscillating movements is attributed to the local temperature oscillation of LCE fibers caused by repetitive self-shadowing effects. LCE self-oscillators can operate stably over many oscillating cycles without obvious performance attenuation, revealing good robustness. Our work offers a versatile way by which LCE self-oscillators can be conveniently designed and fabricated in bulk and at low cost, and broadens the road for developing self-oscillating materials for biological robotics and health care machines.

Evaluation of Low-Loss Polymer Switches for Multinuclear MRI/S.

Implementation of multinuclear MRI/S as a diagnostic tool in clinical settings faces many challenges. One of those challenges is the development of highly sensitive multinuclear RF coils. Current multi-tuning techniques incorporate lossy components that impact the highest achievable SNR for at least one of the coil frequencies. As a result, optimization of multinuclear coil designs continues to be a priority for RF hardware engineers. To address this challenge, a new frequency switching technology that incorporates stimuli-responsive polymer materials was explored. Q measurements were used as a comparison metric between single-tuned, a standard switching network, and the proposed switching technology. The Q losses measured in the new switching method remained below 38% when compared to single-tuned coils. These results are consistent with low loss values reported using traditional switching networks. Furthermore, preliminary testing indicates that there is potential for improvement. These results establish the new technology as a promising alternative to traditional switching techniques.Clinical Relevance- A low loss multi-tuning technique for MRI radiofrequency coils has the potential of improving the study and diagnosis of disease.

LCST/UCST transition of acrylate copolymer with cosolvency behaviors in alcohol aqueous solutions

Though the mechanism of temperature-induced lower critical solution temperature (LCST) in water is clear for the classic homopolymer poly(N-isopropylacrylamide) (PNIPAM), the LCST of other homopolymer or copolymer remains to be understood. In this work, a novel random copolymer poly(2-hydroxyethyl-co-methyl acrylate) (P(HEA-co-MA)) was prepared. The LCST of the copolymer P(HEA-co-MA) can vary from 11 °C to 65 °C by adjusting flexibly the ratio of the two monomers, showing a broad tunable range. The experimental results showed that the balance of hydrogen bond bridge formed by P(HEA-co-MA) in water and the hydrophobic interaction between polymers can cause the LCST transition of P(HEA-co-MA). An interesting observation is that the sharp phase transition (cosolvency) of P(HEA-co-MA) exists when a rather small amount of alcohol or water is added in polymer solutions at constant temperatures. Moreover, it was found that the increase of LCST is correlated with the increase of alcohol concentration in the water-rich regime, but the decrease of upper critical solution temperature (UCST) is related to the increase of water concentration in the alcohol-rich regime. This work shed light on the understanding of cosolvency by studying the solution behavior of copolymer P(HEA-co-MA) in mixed solvents, which can be of interest for the design of stimuli-responsive polymer materials by variation in solvent composition.

Toward Plant Cyborgs: Hydrogels Incorporated onto Plant Tissues Enable Programmable Shape Control.

Engineered living materials (ELMs) that incorporate living organisms and synthetic materials enable advanced functional properties. Here, we seek to create plant cyborgs by combining plants or plant tissues with stimuli-responsive polymeric materials. Plant tissues with integrated shape control may find applications in regenerative medicine, and the shape control of living plants enables another dimension of adaptability and response to environmental threats, which can be applied to next-generation precision farming. In this work, we develop chemistry to integrate stimuli-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels with decellularized plant tissues assisted by 3D printing. We demonstrate programmable shape morphing in response to thermal cues and ultraviolet (UV) light. Specifically, by taking advantage of the extrusion-based 3D printing method, we deposit nanocomposite PNIPAM precursors onto silane-treated decellularized leaf surface with prescribed shapes and spatial control. When subjected to external stimuli, the strain mismatch generated between the swellable nanocomposite PNIPAM and nonswellable decellularized leaf enables folding and bending to occur. This strategy to integrate the plant tissues with stimuli-responsive hydrogels allows the control of leaf morphology, opening avenues for plant-based biosensors and soft actuators to enhance food security; such materials also may find applications in biomedicine as tissue-engineering scaffolds.

(Digital Presentation) A Shape-Adjustable, Flexible Lithium Battery

Integrating stimuli-responsive materials into energy storage technologies opens a new approach to introduce novel functionalities as well as addressing some of the crucial and unresolved issues. As an example, current flexible lithium-ion batteries (LIBs) still suffer from a fragile or susceptible nature and limited electrochemical performance recovery against severe or repeated mechanical deformations. Thus, integrating a stimuli-responsive material with strong ability to recover from severe and repeated mechanical deformations into the flexible LIBs can be an effective approach to address the shape and performance recovery issue concerning the flexible LIBs [1-3].Herein, we design and synthesize a solid polymer electrolyte (SPE) based on a shape memory polymer (SMP) to integrate into flexible lithium batteries for smart applications, for example, shape recovery from severe mechanical deformation. SMPs are attractive class of programmable, stimuli-responsive polymer materials demonstrating shape-memory behavior. A SMP has the ability to memorize its original or permanent shape; deform, fix into a temporary or secondary shape; and recover its original form via applying an external stimulus, for example, heat, magnetic or electrical field,light [3]. In fact, when mechanical deformation occurs, batteries typically suffer from reduced power, efficiency, capacity and recovery from the deformation without sacrificing the electrochemical performance would be highly desirable. The shape-memory SPE is made based on a cross-linkable polyethylene oxide (PEO) with controlled crystallinity and the ability to indue shape memory behavior above the melting point. PEO is chosen as a model shape-memory polymer due to its unique properties including low-cost, high dielectric constant and Li+-ion solvating ability, high electrochemical and mechanical stability, semi-crystalline nature, and ability to introduce cross-linkable functionalities. In response to a temperature exceeding the melting point of the polymer matrix (~70 oC), the engineered shape memory SPE can recover its original shape and size from mechanical deformations, for example, bending and folding. The all-solid-state shape-memory SPEs also show excellent electrochemical performance in Li/Li and Li/LFP cells at ambient temperature. The Li/LFP cell made using the shape memory SPE delivers a specific capacity ~140 mAh g‒1 with ~92% capacity retention after 100 cycles at 0.2C charge/discharge rate and ~99.85% Coulombic efficiency. Besides excellent electrochemical performance, we demonstrate that a flexible Li based battery made using the shape memory SPE can recover from severe mechanical deformations, for example, bending and folding, upon applying heat. This proof-of-concept study opens up a new approach to design and integrate smart functionalities into energy storage technologies. Reference [1] Z. Fang, J. Wang, H. Wu, Q. Li, S. Fan, J. Wang, J. Power Sources 454, 227932 (2020).[2] M. Koo, K. Park, S. H. Lee, M. Suh, D. Y. Jeon, J. W. Choi, K. Kang, K. J. Lee, Nano Lett. 12, 4810–4816 (2012).[3] V. Jabbari, V. Yurkiv, M. G. Rasul, M. Cheng, P. Griffin, F. Mashayek, R. Shahbazian-Yassar, Small 2021, 2102666, DOI: 10.1002/smll.202102666.

Integrating Boronic Esters and Anthracene into Covalent Adaptable Networks toward Stimuli-Responsive Elastomers.

Stimuli-responsive polymer materials have a promising potential application in many areas. However, integrating multi-stimuli into one elastomer is still a challenge. Here, we utilized boronic esters and anthracene to prepare a cross-linked poly(styrene-butadiene-styrene) (SBS) which was endowed with responsiveness to three stimuli (light, heat, and alcohols). SBS was first functionalized with a certain amount of dihydroxyl groups via a thiol-ene “click” reaction between unsaturated double bonds in PB block and thioglycerol. Then, 9-anthraceneboronic acid was applied to form a cross-linked SBS network upon heat and ultraviolet radiation (λ = 365 nm). The prepared elastomer was demonstrated to be stimuli-responsive based on the dynamic nature of boronic esters and the reversible dimerization of anthracene. In addition, the mechanical properties of the elastomer could be regulated continuously owing to the stimulus responsiveness to ultraviolet or heat.

Triple‐Responsive Polyampholytic Graft Copolymers as Smart Sensors with Varying Output

Inside cover: In article number 2000671 by Felix H. Schacher and co-workers, triple stimuli-responsive polymeric materials are obtained via a straightforward synthetic route. Exhibiting a thermosensitive side chain and a polyampholytic backbone, these materials react to changes in environmental pH, temperature, as well as the presence and the amount of metal ions. As this results in simple outputs like turbidity or a shift of absorption, a high potential is foreseen in sensing applications.

A “nucleophilic” iodine in a halogen-bonded iodonium complex manifests an unprecedented I+···Ag+ interaction

Summary When an electron is removed from a halogen atom, it forms a halenium ion X+ (X = I, Br, Cl). In halogen bonding (XB), X+ is considered as a strong XB donor, and when interacting with two XB acceptors (e.g., pyridine), it forms a halonium XB complex with a [N–I–N] three-center-four-electron bond with the two XB acceptors. An unprecedented I+···Ag+ interaction occurs between a [L1–I–L1]+ halogen-bonded complex and a [L2–Ag–L2]+ complex in which the iodonium ion acts like a nucleophile and donates electrons to the silver(I) cation. The X-ray diffraction analysis reveals a short contact [3.4608(3) A] between the I+ and Ag+ cations, and ITC measurements give a ΔG of −6.321 kcal/mol and Ka∼37,000 M−1 for the 1:1 complex. The DFT computational study on the nature of the I+···Ag+ interaction indicates that the I+ is nucleophilic in character, manifesting an unusual and strongly attractive interaction between the I+ and Ag+ cations.

Grafting with RAFT-gRAFT Strategies to Prepare Hybrid Nanocarriers with Core-shell Architecture.

Stimuli-responsive polymer materials are used in smart nanocarriers to provide the stimuli-actuated mechanical and chemical changes that modulate cargo delivery. To take full advantage of the potential of stimuli-responsive polymers for controlled delivery applications, these have been grafted to the surface of mesoporous silica particles (MSNs), which are mechanically robust, have very large surface areas and available pore volumes, uniform and tunable pore sizes and a large diversity of surface functionalization options. Here, we explore the impact of different RAFT-based grafting strategies on the amount of a pH-responsive polymer incorporated in the shell of MSNs. Using a “grafting to” (gRAFT-to) approach we studied the effect of polymer chain size on the amount of polymer in the shell. This was compared with the results obtained with a “grafting from” (gRAFT-from) approach, which yield slightly better polymer incorporation values. These two traditional grafting methods yield relatively limited amounts of polymer incorporation, due to steric hindrance between free chains in “grafting to” and to termination reactions between growing chains in “grafting from.” To increase the amount of polymer in the nanocarrier shell, we developed two strategies to improve the “grafting from” process. In the first, we added a cross-linking agent (gRAFT-cross) to limit the mobility of the growing polymer and thus decrease termination reactions at the MSN surface. On the second, we tested a hybrid grafting process (gRAFT-hybrid) where we added MSNs functionalized with chain transfer agent to the reaction media containing monomer and growing free polymer chains. Our results show that both modifications yield a significative increase in the amount of grafted polymer.

Stimuli-Responsive Rifampicin-Based Macromolecules.

This paper presents the modification of the antibiotic rifampicin by an anionic polyelectrolyte using a simplified electrochemically mediated atom transfer radical polymerization (seATRP) technique to receive stimuli-responsive polymer materials. Initially, a supramolecular ATRP initiator was prepared by an esterification reaction of rifampicin hydroxyl groups with α-bromoisobutyryl bromide (BriBBr). The structure of the initiator was successfully proved by nuclear magnetic resonance (1H and 13C NMR), Fourier-transform infrared (FT-IR) and ultraviolet–visible (UV-vis) spectroscopy. The prepared rifampicin-based macroinitiator was electrochemically investigated among various ATRP catalytic complexes, by a series of cyclic voltammetry (CV) measurements, determining the rate constants of electrochemical catalytic (EC’) process. Macromolecules with rifampicin core and hydrophobic poly (n-butyl acrylate) (PnBA) and poly(tert-butyl acrylate) (PtBA) side chains were synthesized in a controlled manner, receiving polymers with narrow molecular weight distribution (Mw/Mn = 1.29 and 1.58, respectively). “Smart” polymer materials sensitive to pH changes were provided by transformation of tBA into acrylic acid (AA) moieties in a facile route by acidic hydrolysis. The pH-dependent behavior of prepared macromolecules was investigated by dynamic light scattering (DLS) determining a hydrodynamic radius of polymers upon pH changes, followed by a control release of quercetin as a model active substance upon pH changes.

Photo-responsive Self-Reducible Polymers: Overcoming the Spatiotemporal Barriers for Hypersensitivity

To overcome the spatiotemporal barriers to conventional stimuli-responsive polymeric materials, we report a photo-responsive self-reducible polymer with a built-in photo-locked stimulus precursor d...

Poly-Saora robotic jellyfish: swimming underwater by twisted and coiled polymer actuators

Jellyfish are energy-efficient swimmers due to the muscle-powered flapping of their soft bell that facilitates a unique energy recapture mechanism. In this paper, we present a bio-inspired jellyfish robot named Poly-Saora that mimics the swimming behavior of the jellyfish species Black sea nettle (Chrysaora achlyos). An assembly-based fabrication method is used to create the Poly-Saora that is developed mainly with polymeric materials (95% of the robot by volume). Twisted and coiled polymer (TCP) actuators are successfully implemented in this robot and show great potential for underwater applications. The influence of different parameters such as the amplitude of the input power, the actuation frequency, and the lifecycle of the actuator are investigated underwater. A full characterization of 6-ply TCP muscles is demonstrated. An actuation strain of ∼10% is achieved in water at a frequency of 0.1 Hz and 50 kPa load. When integrated into the jellyfish, the TCP was able to bend a single bell by 17°. Poly-Saora was able to swim a vertical distance of 180 mm in 220 s with four TCP actuators each confined in a separate conduit. The robot mimics the swimming behavior of a real jellyfish by contracting the bell segments through the activation of the actuators, which generates forced water circulation under the bell in a pulsating rhythm, consequently creating a vertical movement of the robot. Overall, Poly-Saora presents a model of an underwater system that is driven by stimuli-responsive polymer materials and has unique advantages over conventional rigid robots due to their lightweight, muscle-like structures, silent actuation and ease of manufacturing. This robot can be used for safe interaction with other underwater species and their natural habitats when fully developed.

Reversible formation of multiple stimuli-responsive polymeric materials through processing control of trifunctional amphiphilic molecules.

A nonionic amphiphile consisting of thermo-responsive tetraethylene glycol chains, redox-responsive thiol groups, and photo-responsive anthracene units was developed. By combining the thermal and redox stimuli in water and bulk to control the assembling processes, two pairs of polymeric materials possessing contrasting responses to thermal treatment and photoirradiation were prepared.

Recent progress in 4D printing of stimuli-responsive polymeric materials

4D printing is proposed based on the additive manufacturing of stimuli-responsive materials and structures, which can realize shape changing upon external stimuli. This article reviews the 4D printing methods and actuating performances of 4D printing structures based on shape memory polymers, hydrogels, liquid crystal elastomers, and electroactive polymers. This article shows that the shape morphing properties of single materials are limited, while 4D printing of composites can integrate the various driving modes of different smart materials. In the end, challenges facing 4D printing such as broadening the scope of smart materials, improving printing processes, the compatibility of printing different materials have been discussed.

Progress in Photo-Responsive Polypeptide Derived Nano-Assemblies.

Stimuli-responsive polymeric materials have attracted significant attention in a variety of high-value-added and industrial applications during the past decade. Among various stimuli, light is of particular interest as a stimulus because of its unique advantages, such as precisely spatiotemporal control, mild conditions, ease of use, and tunability. In recent years, a lot of effort towards the synthesis of a biocompatible and biodegradable polypeptide has resulted in many examples of photo-responsive nanoparticles. Depending on the specific photochemistry, those polypeptide derived nano-assemblies are capable of crosslinking, disassembling, or morphing into other shapes upon light irradiation. In this review, we aim to assess the current state of photo-responsive polypeptide based nanomaterials. Firstly, those ‘smart’ nanomaterials will be categorized by their photo-triggered events (i.e., crosslinking, degradation, and isomerization), which are inherently governed by photo-sensitive functionalities, including O-nitrobenzyl, coumarin, azobenzene, cinnamyl, and spiropyran. In addition, the properties and applications of those polypeptide nanomaterials will be highlighted as well. Finally, the current challenges and future directions of this subject will be evaluated.

Charge-Determined LCST/UCST Behavior in Ionic Polypeptoids.

Stimuli-responsive polymers have received increasing interest for a variety of applications. Here, we report a series of unique charge-determined thermoresponsive polypeptoids synthesized by a combination of ring-opening polymerization and click chemistry. The LCST-type and UCST-type behavior is mainly dominated by the charge state on the side chain. Further, the phase transition temperature highly depends on the degree of polymerization, the side-chain architecture, the pH value, and so on. The obtained polypeptoid solutions exhibit good stability against temperature and salt concentration. To our knowledge, this report presents the first charge-determined LCST/UCST-type polymer from identical homopolymer backbone that displays a wide range of tunable cloudy points in aqueous media. We propose the hydrogen-bonding interaction plays a critical part on the solution behavior. These features make polypeptoids ideal candidates for highly designable stimuli-responsive polymeric materials.

Autonomous and Continuous Stimuli‐Responsive Polymer Surface for Antibacterial Application through Enzymatic Self‐Propagating Reactions

Stimuli-responsive polymer materials inspired by biological materials have invoked increasing research interest; however, they still suffer from limitations such as finite amplified responses and poor sensitivity of the unstimulated parts. Herein, a new strategy for creating H+ -responsive polymer surfaces that are capable of transforming specific local fleeting stimuli into global macroscopic changes is described. The introduction of self-propagating reactions into the polymer-surface systems endows them with excellent stimuli-amplifying properties and response of the unstimulated parts. On the basis of this design, a polymer and enzymatic reaction were employed to enable a specific response to a stimulus and then lead to macroscopic changes of the surface. It is further shown that the prepared H+ -responsive polymer surfaces can be employed for antibacterial application. This work provides a good example of achieving autonomously reconfigurable materials that respond to local fleeting stimuli.

Stimuli-responsive polymeric materials functioning via host–guest interactions

Molecular recognition is essential for realizing functional supramolecular materials. Non-covalent host–guest interactions are an effective tool to introduce switching and functional properties into materials. This review focuses on the achievement of selective molecular adhesion, self-healing, toughness, and actuation properties. These functions have been achieved by reversible bond formation with cyclodextrins (CDs). Self-healing materials with host–guest interactions involving CDs have been used to achieve redox-responsive healing properties and healing efficiency. Furthermore, the materials, which undergo self-healing by chemical and physical mechanisms, exhibit rapid and efficient self-healing properties under semi-dry conditions. To prepare a supramolecular actuator using host–guest complexes, two approaches have been introduced. The first is the functionalization of a supramolecular gel actuator by changing the cross-linking density, and the second is the functionalization of a topological gel actuator by changing distances between the cross-linking points. Both actuators exhibit contractive bending behavior. This review summarizes advancements within the past 10 years in supramolecular materials that function via the chemical mechanism of host–guest interactions and the physical mechanism of the sliding motion of ring molecules.

Reversible Shaping of Microwells by Polarized Light Irradiation

In the last years, stimuli-responsive polymeric materials have attracted great interest, due to their low cost and ease of structuration over large areas combined with the possibility to actively manipulate their properties. In this work, we propose a polymeric pattern of soft-imprinted microwells containing azobenzene molecules. The shape of individual elements of the pattern can be controlled after fabrication by irradiation with properly polarized light. By taking advantage of the light responsivity of the azobenzene compound, we demonstrate the possibility to reversibly modulate a contraction-expansion of wells from an initial round shape to very narrow slits. We also show that the initial shape of the microconcavities can be restored by flipping the polarization by 90°. The possibility to reversibly control the final shape of individual elements of structured surfaces offers the opportunity to engineer surface properties dynamically, thus opening new perspectives for several applications.

Synthesis, characterization, properties of N-succinyl chitosan-g-poly (methacrylic acid) hydrogels and in vitro release of theophylline

Although there was great interest in the biocompatible and stimuli responsive polymeric materials in oral drug delivery systems in recent years. The stimuli sensitive and swellable hydrogels have got striking attention as promising materials for site specific controlled drug delivery. In the present study, unique biodegradable, biocompatible, swellable, and pH responsive N-succinyl chitosan-g- Poly (methacrylic acid) hydrogels were prepared through free radical mechanism for oral administration of theophylline. In this method, ammonium persulfate (APS) and N, N′-methylenebisacrylamide (MBA) were used as initiator and crosslinking agent, respectively. The physical properties characterization was performed by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), field emission scanning electron microscope (FESEM), and rheometer. The results clearly confirmed the successful formation of N-succinyl chitosan-g- Poly (methacrylic acid) gel. In vitro degradation of hydrogels was found dependent on the ratio of monomers to crosslinking concentration. The effect of concentration of monomers, initiator, and crosslinking agent, pH and ionic strength of salts on swelling behavior was investigated. The results revealed the strong influence of these parameters, on swelling properties. Hydrogel prepared with high concentration of initiator and low crosslinking agent showed maximum swelling. The swelling showed good fitting to second order rate equation and the swelling kinetics data demonstrated the swelling behavior followed non-Fickian anomalous transport mechanism. Furthermore, theophylline loading and encapsulation efficiency (EE %) were found to be 14.5–23.5% and 58–94%, respectively. However, the in vitro release profile of theophylline was found dependent on pH, concentration of monomers, initiator, and crosslinking agent. The maximum release was found to be 90% in simulated intestinal fluid (SIF). The drug release data showed good fitting to Ritger-Peppas model. Moreover, the chemical activity of theophylline was also investigated and found that the drug maintained its chemical activity after in vitro release.