A Multiresponsive Anisotropic Hydrogel with Macroscopic 3D Complex Deformations

As one of the most promising smart materials, stimuli‐responsive polymer hydrogels (SPHs) can reversibly change volume or shape in response to external stimuli. They thus have shown promising applications in many fields. While considerable progress of 2D deformation of SPHs has been achieved, the realization of 3D or even more complex deformation still remains a significant challenge. Here, a general strategy towards designing multiresponsive, macroscopically anisotropic SPHs (MA‐SPHs) with the ability of 3D complex deformations is reported. Through a local UV‐reduction of graphene oxide sheets (GOs) with a patterned fashion in the GO‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) composite hydrogel sheet, MA‐SPHs can be achieved after the introduction of a second poly(methylacrylic acid) network in the unreduced part of GO‐PNIPAM hydrogel sheet. The resulting 3D MA‐SPHs can provide remote‐controllable light‐driven, as well as thermo‐, pH‐, and ionic strength‐triggered multiresponsive 3D complex deformations. Approaches in this study may provide new insights in designing and fabricating intelligent soft materials for bioinspired applications.     

Evolution‐Based Design of an Injectable Hydrogel

A new class of simple, linear, amphiphilic peptides are developed that have the ability to undergo triggered self‐assembly into self‐supporting hydrogels. Under non‐gelling aqueous conditions, these peptides exist in a random coil conformation and peptide solutions have the viscosity of water. On the addition of a buffered saline solution, the peptides assemble into a β‐sheet rich network of fibrils, ultimately leading to hydrogelation. A family of nine peptides is prepared to study the influence of peptide length and amino acid composition on the rate of self‐assembly and hydrogel material properties. The amino acid composition is modulated by varying residue hydrophobicity and hydrophilicity on the two opposing faces of the amphiphile. The conformation of peptides in their soluble and gel state is studied by circular dichroism (CD), while the resultant material properties of their gels is investigated using oscillatory sheer rheology. One weight percent gels formed under physiological conditions have storage modulus (G′) values that vary from ≈20 to ≈800 Pa, with sequence length and hydrophobic character playing a dominant roll in defining hydrogel rigidity. Based on the structural and functional data provided by the nine‐peptide family members, an optimal sequence, namely LK13, is evolved. LK13 (LKLKLKLKLKLKL‐NH₂) undergoes triggered self‐assembly, affording the most rigid gel of those studied (G′=797 ± 105). It displays shear thin‐recovery behavior, allowing its delivery by syringe and is cytocompatibile as assessed with murine C3H10t1/2 mesenchymal stem cells.     

Programmed Multiresponsive Hydrogel Assemblies with Light‐Tunable Mechanical Properties,Actuation, and Fluorescence

Fabrication strategies for programmed hydrogels that provide precise spatial control with predetermined responses to external stimuli are highly desirable. In this study, a partially reversible light‐driven assembly (PRLDA) method is introduced to construct multiresponsive hydrogels utilizing microgel (MG) particle building blocks (swollen diameter of 107 nm). No other material is required to prepare the gels beyond the MGs themselves. Facile preparation of multiresponsive hydrogels that are reversibly responsive to light, pH, and temperature using phototriggered covalent interlinking of coumarin‐based MGs is demonstrated. The gels have phototuneable moduli and swelling ratios and show light‐assisted healing and reshaping. Remarkably, the intrinsic fluorescence of the gels undergoes a reversible light‐triggered wavelength‐shift. The emission peak blueshifted from 420 to 390 nm upon irradiation with 365 nm light. The PRLDA gels can be constructed using either positive or negative photopatterning. It is shown that the gels can be exploited for multiresponsive cytocompatible actuators, grippers, and ON/OFF circuit components as well as anticounterfeit gels. The PRLDA method provides new insight into programmed gel property control and has excellent potential for biomaterial and optoelectronic applications.     

Multi‐Stimuli Responsive Hydrogel Cilia

Large arrays of high aspect ratio, artificial hydrogel based cilia that can respond to multiple stimuli are produced by means of micro‐fabrication techniques. The cilia operate in aqueous solutions and are sensitive to pH, electric and/or magnetic fields. The biomimetic system combines both sensing and motility. Detection of changes in environment, such as a decrease in pH, triggers a collective response, to an external time‐dependent magnetic field.     

Hole‐Programmed Superfast Multistep Folding of Hydrogel Bilayers

Two important aspects of actuation behavior of stimuli‐responsive hydrogels are the complexity of the shape change and its speed. Here, it is shown that varying the shape of simple polymer bilayers can result in very complex and very fast spontaneous folding. The complexity and high folding rate arise from the choice of the shape and from the presence of inhomogeneous swelling within the thermoresponsive layer entrapped between the top hydrophobic layer and the substrate. In contrast to homogeneous swelling of a freestanding bilayer, which leads to a gradual increase of curvature throughout the whole bilayer, inhomogeneous swelling first results in complete rolling of the periphery of the film, which changes its mechanical properties and affects the subsequent morphing process. Further swelling of the thermoresponsive layer generates more stress that builds up until a buckling threshold is overcome, allowing very fast switching from the flat edge‐rolled configuration into a folded one. The research demonstrates how the introduction of holes into actuating bilayers gives rise not only to a novel geometric control over the folding fate of the films but also adds the ability to tune the rate of folding, through the careful selection of hole size, location, and shape.     

A Facile Synthesis and Characterization of Monodisperse Spherical Pigment Particles with a Core/Shell Structure

In this paper, a facile sol–gel process for producing monodisperse, spherical, and nonaggregated pigment particles with a core/shell structure is reported. Spherical silica particles (245 and 385 nm in diameter) and Cr₂O₃, α‐Fe₂O₃, ZnCo₂O₄, CuFeCrO₄, MgFe₂O₄, and CoAl₂O₄ pigments are selected as cores and shells, respectively. The obtained core/shell‐structured pigment samples, denoted as SiO₂@Cr₂O₃ (green), SiO₂@α‐Fe₂O₃ (red), SiO₂@MgFe₂O₄ (brown), SiO₂@ZnCo₂O₄ (dark green), SiO₂@CoAl₂O₄ (blue), and SiO₂@CuFeCrO₄ (black), are well characterized by using X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and UV‐vis diffuse reflection, as well as by investigating the magnetic properties. The results of XRD and high‐resolution TEM (HRTEM) demonstrate that the pigment shells crystallize well on the surface of SiO₂ particles. The thickness of the pigment shell can be tuned by the number of coatings, to some extent. These pigment particles can be well dispersed in some solvents (such as glycol) to form relatively more stable suspensions than the commercial products. Apart from the color characteristics, some of pigments like SiO₂@Cr₂O₃, SiO₂@MgFe₂O₄, and SiO₂@CuFeCrO₄ also show magnetic properties with coercivities of 1098 Oe (5 K), 648 Oe (5 K), and 91 Oe (298 K), respectively.     

Synthesis and Optical Properties of KYF4/Yb,Er Nanocrystals,and their Surface Modification with Undoped KYF4

KYF₄/Yb³⁺, Er³⁺ nanocrystals with a mean diameter of approximately 13 nm were synthesized at 200 °C in the high boiling organic solvent N‐(2‐hydroxyethyl)ethylenediamine (HEEDA). The particles crystallize in the cubic phase known from α‐NaYF₄ and form transparent colloidal solutions in tetraethylene glycol (TEG) or propanol. Solutions containing 1 wt % of the nanocrystals in TEG display visible upconversion emission upon continuous wave (CW) excitation at 978 nm. Growing undoped KYF₄ on the surface of the KYF₄/Yb³⁺, Er³⁺ nanocrystals increases the upconversion efficiency by more than a factor of 20. The XRD data of these particles, display a slight increase in the mean particle size from 13 to 15.5 nm, indicating that only a part of the subsequently added KYF₄ shell material is deposited onto the particle surface. Nevertheless the performed surface modification obviously leads to core/shell structured particles.     

Bioinspired Multifunctional Hybrid Hydrogel Promotes Wound Healing

Inspired by the coordinated multiple healing mechanism of the organism, a four‐armed benzaldehyde‐terminated polyethylene glycol and dodecyl‐modified chitosan hybrid hydrogel with vascular endothelial growth factor (VEGF) encapsulation are presented for efficient and versatile wound healing. The hybrid hydrogel is formed through the reversible Schiff base and possesses self‐healing capability. As the dodecyl tails can insert themselves into and be anchored onto the lipid bilayer of the cell membrane, the hybrid hydrogel has outstanding tissue adhesion, blood cell coagulation and hemostasis, anti‐infection, and cell recruitment functions. Moreover, by loading in and controllably releasing VEGF from the hybrid hydrogel, the processes of cell proliferation and tissue remodeling in the wound bed can be significantly improved. Based on an in vivo study of the multifunctional hybrid hydrogel, it is demonstrated that acute tissue injuries such as vessel bleeding and liver bleeding can be repaired immediately because of the outstanding adhesion and hemostasis features of the hydrogel. Moreover, the chronic wound‐healing process of an infectious full‐thickness skin defect model can also be significantly enhanced by promoting angiogenesis, collagen deposition, macrophage polarization, and granulation tissue formation. Thus, this multifunctional hybrid hydrogel is potentially valuable for clinical applications.     

Remote‐Controlled Hydrogel Depots for Time‐Scheduled Vaccination

Remote‐controlled drug depots represent a highly valuable tool for the timely controlled administration of pharmaceuticals in a patient compliant manner. Here, the first pharmacologically controlled material that allows for the scheduled induction of a medical response in mice is described. To this aim, a novel, humanized biohybrid material that releases its cargo in response to a small‐molecule stimulus licensed for human use is developed. The functionality of the material in mice is demonstrated by the remote‐controlled delivery of a vaccine against the oncogenic human papillomavirus type 16. It is shown that the biohybrid depot‐mediated immunoprotection is equivalent to the classical multi‐injection‐based vaccination. These results indicate that this material can be used as a universal remote‐controlled vehicle for the patient‐compliant delivery of vaccines and pharmaceuticals.     

A Review of Design and Fabrication Methods for Nanoparticle Network Hydrogels for Biomedical,Environmental, and Industrial Applications

Nanoparticle network hydrogels (NNHs) in which nanoparticles are used as a key building block to build the gel network have attracted significant interest given their potential to leverage the favorable properties of both hydrogels (e.g., hydrophilicity, tunable pore sizes, mechanics, etc.) and a variety of different nanoparticles (e.g., high surface area, chemical activity, independently tunable porosity, mechanics) to create new functional materials. Herein, recent progress in the design and use of NNHs is comprehensively reviewed, with an emphasis on defining the typical gel morphologies/architectures that can be achieved with NNHs, the typical crosslinking approaches used to fabricate NNHs, the fundamental properties and functional benefits of NNHs, and the reported applications of NNHs in electronics (flexible electronics, sensors), environmental (sorbents, separations), agriculture, self-cleaning-materials, and biomedical (drug delivery, tissue engineering) applications. In particular, the way in which the NNH structure is applied to improve the performance of the hydrogel in each application is emphasized, with the aim to develop a set of principles that can be used to rationally design NNHs for future uses.     

Tuning Hydrogel Mechanics Using the Hofmeister Effect

The mechanical properties of hydrogels are commonly modified by changing the concentration of the molecular components. This approach, however, does not only change hydrogel mechanics, but also the microstructure, which in turn alters the macroscopic properties of the gel. Here, the Hofmeister effect is used to change the thermoresponsiveness of polyisocyanide hydrogels. In contrast to previous Hofmeister studies, the effect is used to change the phase transition temperatures and to tailor the mechanics of the thermoresponsive (semiflexible) polymer gels. It is demonstrated that the gel stiffness can be manipulated over more than two orders of magnitude by the addition of salts. Surprisingly, the microstructure of the gels does not change upon salt addition, demonstrating that the Hofmeister effect provides an excellent route to change the mechanical properties without distorting other influential parameters of the gel.     

Hybrids of Magnetic Nanoparticles with Double‐Hydrophilic Core/Shell Cylindrical Polymer Brushes and Their Alignment in a Magnetic Field

The synthesis of double‐hydrophilic core/shell cylindrical polymer brushes (CPBs), their hybrids with magnetite nanoparticles, and the directed alignment of these magnetic hybrid cylinders by a magnetic field are demonstrated. Consecutive grafting from a polyinitiator poly(2‐(2‐bromoisobutyryloxy)ethyl methacrylate) (PBIEM) of tert‐butyl methacrylate (tBMA) and oligo(ethylene glycol) methacrylate (OEGMA) using atom‐transfer radical polymerization (ATRP) and further de‐protection yields core/shell CPBs with poly(methacrylic acid) (PMAA) as the core and POEGMA as the shell, which is evidenced by ¹H NMR, gel permeation chromatography (GPC), and dynamic and static light scattering (DLS and SLS). The resulting core/shell brush is well soluble in water and shows a pH responsiveness because of its weak polyelectrolyte core. Pearl‐necklace structures are observed by cryogenic transmission electron microscopy (cryo‐TEM) at pH 4, while at pH 7, these structures disappear owing to the ionization of the core. A similar morphology is also found for the polychelate of the core/shell CPBs with Fe³⁺ ions. Superparamagnetic magnetite nanoparticles have also been prepared and introduced into the core of the brushes. The hybrid material retains the superparamagnetic property of the magnetite nanoparticles, which is verified by superconducting quantum interference device (SQUID) magnetization measurements. Large‐scale alignment of the hybrid cylinders in relatively low magnetic fields (40–300 mT) can easily be performed when deposited on a surface. which is clearly revealed by the atomic force microscopy (AFM) and TEM measurements.     

pH‐Based Regulation of Hydrogel Mechanical Properties Through Mussel‐Inspired Chemistry and Processing

The mechanical holdfast of the mussel, the byssus, is processed at acidic pH yet functions at alkaline pH. Byssi are enriched in Fe³⁺ and catechol‐containing proteins, species with chemical interactions that vary widely over the pH range of byssal processing. Currently, the link between pH, Fe³⁺‐catechol reactions, and mechanical function is poorly understood. Herein, it is described how pH influences the mechanical performance of materials formed by reacting synthetic catechol polymers with Fe³⁺. Processing Fe³⁺‐catechol polymer materials through a mussel‐mimetic acidic‐to‐alkaline pH change leads to mechanically tough materials based on a covalent network fortified by sacrificial Fe³⁺‐catechol coordination bonds. These findings offer the first direct evidence of Fe³⁺‐induced covalent cross‐linking of catechol polymers, reveal additional insight into the pH dependence and mechanical role of Fe³⁺‐catechol interactions in mussel byssi, and illustrate the wide range of physical properties accessible in synthetic materials through mimicry of mussel‐protein chemistry and processing.     

Development of Thermoinks for 4D Direct Printing of Temperature-Induced Self-Rolling Hydrogel Actuators

4D printing has emerged as an important technique for fabricating 3D objects from programmable materials capable of time-dependent reshaping. In the present investigation, novel 4D thermoinks composed of laponite (LAP), an interpenetrating network of poly(N-isopropylacrylamide) (PNIPAAm), and alginate (ALG) are developed for direct printing of shape-morphing structures. This approach consists of the design and fabrication of 3D honeycomb-patterned hydrogel discs self-rolling into tubular constructs under the stimulus of temperature. The shape morphing behavior of hydrogels is due to shear-induced anisotropy generated via 3D printing. The compositionally tunable hydrogel discs can be programmed to exhibit different actuation behaviors at different temperatures. Upon immersion in 12 °C water, singly crosslinked sheets roll up into a tubular construct. When transferred to 42 °C water, the tubes first rapidly unfold and then slightly curve up in the opposite direction. Through a dual photocrosslinking of PNIPAAm, it is possible to inverse temperature-dependent shape morphing and induce self-folding at higher and unrolling at lower temperatures. The extensive self-assembling motion is essential to developing thermal actuators with broad applications in, e.g., soft robotics and active implantology, whereas controllable self-rolling of planar hydrogels is of the highest interest to biomedical engineering as it allows for effective fabrication of hollow tubes.     

Multiresponsive PEDOT–Ionic Liquid Materials for the Design of Surfaces with Switchable Wettability

Among the different types of stimuli‐responsive polymers, conjugated polymers reveal unique multiresponsive behavior. In this work, the synthesis and characterization of new functional poly(3,4‐ethylenedioxythiophenes) (PEDOT) bearing imidazolium ionic‐liquid moieties (PEDOT‐Im) is reported. PEDOT‐Im polymers show multiresponsive properties to a variety of stimuli, such as temperature, pH, oxidative doping, and presence of anions. These stimuli provoke different changes in PEDOT‐Im, such as changes in color, oxidation state, and, wetting behavior. In all cases, a reversible effect is observed, and the polymers reveal responsive properties in solution as well as in the form of thin films. Whereas sensitiveness to pH and oxidative doping are known phenomena for other PEDOT derivatives, responsiveness to temperature and to anions is a unique property of PEDOT‐Im. The anion exchange is further investigated by means of the Quartz Crystal Microbalance with dissipation. Anion exchanges induce fast, adjustable, and reversible contact angle changes between 24° and 107°. As a potential application, surfaces with switchable wettability triggered by anion solutions are prepared by spin‐coating PEDOT‐Im films onto different substrates.     

Reversible Electronic Patterning of a Dynamically Responsive Hydrogel Medium

A dynamically responsive hydrogel medium is prepared from two self-assembling components, a polysaccharide (chitosan) and a surfactant (sodium dodecyl sulfate; SDS). It is shown that this medium can be patterned using an electrode “pen” to reconfigure supramolecular structure: cathodic writing induces neutral chitosan chains to form a crystalline network, while anodic writing generates cationic chitosan chains that electrostatically crosslink with anionic SDS micelles. Both supramolecular structures are re-configurable and each is stabilized by structure-induced shifts in chitosan's pKa, thus electronically written patterns can be erased, new patterns can be written, and patterns can be written in three dimensions. Further, it is shown that NaCl-induced morphological transitions of the SDS micelles allow patterns to be reversibly concealed or revealed. To demonstrate the versatility of this medium for information storage, a quick response (QR) code is electronically written and it is shown that this code can be recognized by a standard cellphone app. This QR code can be concealed by making the medium opaque (i.e., by obscuring the pattern) or by making the pattern evanescent (i.e., by making pattern invisible). Overall, this work demonstrates that a dynamically responsive medium composed of simple, safe and sustainable components can be reversibly patterned with spatial and quantitative control using top-down electronic inputs.     

Cooperative Switching in Large‐Area Assemblies of Magnetic Janus Particles

Magnetic Janus particles (MJPs) have received considerable attention for their rich assembly behavior and their potential technological role in applications ranging from simple magnetophoretic displays to smart cloaking devices. However, further progress is hampered by the lack of predictive understanding of the cooperative self‐assembly behavior of MJPs and appropriate dynamic control mechanisms. In this paper, a detailed experimental and theoretical investigation into the magnetically directed spatiotemporal self‐assembly and switching of MJPs is presented. For this purpose, a novel type of MJPs with defined hemispherical compartments carrying superparamagnetic iron oxide nanoparticles as well as a novel simulation model to describe their cooperative switching behavior is established. Combination of the theoretical and experimental work culminates in a simple method to direct assemblies of MJPs, even at high particle concentrations. In addition, a magnetophoretic display with switchable MJPs is developed on the basis of the theoretical findings to demonstrate the potential usefulness of controlled large‐area assemblies of magnetic Janus particles.     

Bifunctional Gd2O3/C Nanoshells for MR Imaging and NIR Therapeutic Applications

This paper reports dual function Gd₂O₃/C nanoshells for application in MR contrast images and NIR‐triggered killing cancer cells. The nanoshells are prepared using biological gelatin particles as core templates through a two‐step thermal treatment. The surfaces of the nanoshells can be readily modified by poly(styrene‐alt‐maleic acid) (PSMA) polymer to improve their water‐dispersible properties and increase their biocompatibility. The Gd₂O₃/C nanoshells show brightened images of kidney cortex and liver in mice, whereas the Gd₂O₃/C@PSMA nanoshells show a darkened liver signal. The biodistribution is measured as a function of time and shows that the nanoshells circulate in the vessels and are cleared out gradually from organs. The graphite carbon coated on the Gd₂O₃ nanoshells displays absorbance in the near‐IR (NIR) region. A large extinction coefficient is obtained, indicating the potential of the nanoshells as photothermal agents. The Gd₂O₃/C@PSMA nanoshells conjugated with anti‐epithermal growth factor receptor antibodies are used for targeting and destroying A549 lung cancer cells by means of NIR‐triggered killing capability. Both laser power density and material dose dependence are investigated to evaluate photothermolysis in cancer cells.     

One‐Step Solvent‐Free Synthesis and Characterization of Zn1−xMnxSe@C Nanorods and Nanowires

The carbon‐encapsulated, Mn‐doped ZnSe (Zn_1−xMn_xSe@C) nanowires, nanorods, and nanoparticles are synthesized by the solvent‐free, one‐step RAPET (reactions under autogenic pressure at elevated temperature) approach. The aspect ratio of the nanowires/nanorods is altered according to the Mn/Zn atomic ratio, with the maximum being observed for Mn/Zn = 1:20. A 10–20 nm amorphous carbon shell is evidenced from electron microscopy analysis. The replacement of Zn by Mn in the Zn_1−xMn_xSe lattice is confirmed by the hyperfine splitting values in the electron paramagnetic resonance (EPR) experiments. Raman experiments reveal that the Zn_1−xMn_xSe core is highly crystalline, while the shell consists of disordered graphitic carbon. Variable‐temperature cathodoluminescence measurements are performed for all samples and show distinct ZnSe near‐band‐edge and Mn‐related emissions. An intense and broad Mn‐related emission at the largest Mn alloy composition of 19.9% is further consistent with an efficient incorporation of Mn within the host ZnSe lattice. The formation of the core/shell nanowires and nanorods in the absence of any template or structure‐directing agent is controlled kinetically by the Zn_1−xMn_xSe nucleus formation and subsequent carbon encapsulation. Mn replaces Zn mainly in the (111) plane and catalyzes the nanowire growth in the [111] direction.