Transfer Printing of Cell Layers with an Anisotropic Extracellular Matrix Assembly using Cell‐Interactive and Thermosensitive Hydrogels

The structure of tissue plays a critical role in its function and therefore a great deal of attention has been focused on engineering native tissue‐like constructs for tissue engineering applications. Transfer printing of cell layers is a new technology that allows controlled transfer of cell layers cultured on smart substrates with defined shape and size onto tissue‐specific defect sites. Here, the temperature‐responsive swelling‐deswelling of the hydrogels with groove patterns and their versatile and simple use as a template to harvest cell layers with anisotropic extracellular matrix assembly is reported. The hydrogels with a cell‐interactive peptide and anisotropic groove patterns are obtained via enzymatic polymerization. The results show that the cell layer with patterns can be easily transferred to new substrates by lowering the temperature. In addition, multiple cell layers are stacked on the new substrate in a hierarchical manner and the cell layer is easily transplanted onto a subcutaneous region. These results indicate that the evaluated hydrogel can be used as a novel substrate for transfer printing of artificial tissue constructs with controlled structural integrity, which may hold potential to engineer tissue that can closely mimic native tissue architecture.     

Stomata‐Inspired Membrane Produced Through Photopolymerization Patterning

The programmed movements of responsive functional hydrogels have received much attention because of their abundant functions and wide range of engineering applications. In this study, an innovative stomata‐inspired membrane (SIM) is fabricated by using a temperature‐responsive hydrogel through a simple, cost‐effective, and high‐throughput patterned photopolymerization. Polymerization‐induced diffusion on the macroscale surface results in formation of a double‐parted polymer membrane with fine pores after single illumination. After heating the SIM, the less deformable thick frame supports the whole structure and the highly deformable thin base regulates pore shape. Among various SIM types, the slit pores of monocot SIM, which are lined up in parallel, exhibit the largest radius deformation. The morphological configuration of the SIM can be easily controlled by changing the photomask for a given application. As the developed SIM features the sensing‐to‐activation functions of stimuli‐responsive hydrogels and can be easily fabricated, this membrane can be potentially used for numerous practical applications, such as filter membranes with adjustable pores, membrane‐based sensors, membrane‐based actuators, and multifunctional membranes.     

Light‐Controlled Reversible Manipulation of Microgel Particle Size Using Azobenzene‐Containing Surfactant

The light‐induced reversible switching of the swelling of microgel particles triggered by photo‐isomerization and binding/unbinding of a photosensitive azobenzene‐containing surfactant is reported. The interactions between the microgel (N‐isopropylacrylamide, co‐monomer: allyl acetic acid, crosslinker: N,N′‐methylenebisacrylamide) and the surfactant are studied by UV‐Vis spectroscopy, dynamic and electrophoretic light scattering measurements. Addition of the surfactant above a critical concentration leads to contraction/collapse of the microgel. UV light irradiation results in trans‐cis isomerization of the azobenzene unit incorporated into the surfactant tail and causes an unbinding of the more hydrophilic cis isomer from the microgel and its reversible swelling. The reversible contraction can be realized by blue light irradiation that transfers the surfactant back to the more hydrophobic trans conformation, in which it binds to the microgel. The phase diagram of the surfactant‐microgel interaction and transitions (aggregation, contraction, and precipitation) is constructed and allows prediction of changes in the system when the concentration of one or both components is varied. Remote and reversible switching between different states can be realized by either UV or visible light irradiation.     

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.     

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.     

Poly(N‐isopropylacrylamide)‐Clay Nanocomposite Hydrogels with Responsive Bending Property as Temperature‐Controlled Manipulators

Novel poly(N‐isopropylacrylamide)‐clay (PNIPAM‐clay) nanocomposite (NC) hydrogels with both excellent responsive bending and elastic properties are developed as temperature‐controlled manipulators. The PNIPAM‐clay NC structure provides the hydrogel with excellent mechanical property, and the thermoresponsive bending property of the PNIPAM‐clay NC hydrogel is achieved by designing an asymmetrical distribution of nanoclays across the hydrogel thickness. The hydrogel is simply fabricated by a two‐step photo polymerization. The thermoresponsive bending property of the PNIPAM‐clay NC hydrogel is resulted from the unequal forces generated by the thermoinduced asynchronous shrinkage of hydrogel layers with different clay contents. The thermoresponsive bending direction and degree of the PNIPAM‐clay NC hydrogel can be adjusted by controlling the thickness ratio of the hydrogel layers with different clay contents. The prepared PNIPAM‐clay NC hydrogels exhibit rapid, reversible, and repeatable thermoresponsive bending/unbending characteristics upon heating and cooling. The proposed PNIPAM‐clay NC hydrogels with excellent responsive bending property are demonstrated as temperature‐controlled manipulators for various applications including encapsulation, capture, and transportation of targeted objects. They are highly attractive material candidates for stimuli‐responsive “smart” soft robots in myriad fields such as manipulators, grippers, and cantilever sensors.     

Hofmeister Effect‐Assisted One Step Fabrication of Ductile and Strong Gelatin Hydrogels

Hydrogels with high strength and ductility are normally prepared from synthetic polymers, and most protein‐based hydrogels are soft and brittle. Here, a strong, ductile gelatin hydrogel is prepared by simply soaking a virgin gelatin gel in an ammonium sulfate solution. The polymer chains in the covalent, crosslink‐free network can freely move to homogeneously distribute stress, and more importantly, the highly kosmotropic ammonium sulfate ions greatly enhance the hydrophobic interactions and chain bundling within the gelatin gels. As a result, the treated hydrogels have an extraordinary ultimate strength (compressive and tensile strains of over 99% and 500%, respectively, and stresses of 12 and 3 MPa) superior to that of common protein gels. The physical crosslinks introduced by the Hofmeister effect can rapidly absorb energy and sustain large deformations via decrosslinking and dissociation, which result in energy dissipation and antifatigue properties. The effects of the gelatin and (NH₄)₂SO₄ concentrations on the hydrogel mechanics are evaluated, and the possible strengthening mechanism is discussed. The effect of various ions in the Hofmeister series on the gelatin hydrogel is also investigated. Kosmotropic ions enhance the mechanical properties, whereas chaotropic ions soften and dissolve the gel.     

Solid‐State NMR Investigations of the Unusual Effects Resulting from the Nanoconfinement of Water within Amphiphilic Crosslinked Polymer Networks

Two types of solid‐state ¹⁹F NMR spectroscopy experiments are used to characterize phase‐separated hyperbranched fluoropolymer–poly(ethylene glycol) (HBFP–PEG) crosslinked networks. Mobile (soft) domains are detected in the HBFP phase by a rotor‐synchronized Hahn echo under magic‐angle spinning conditions, and rigid (hard) domains by a solid echo with no magic‐angle spinning. The mobility of chains is detected in the PEG phase by ¹H → ¹³C cross‐polarization transfers with ¹H spin‐lock filters with and without magic‐angle spinning. The interface between HBFP and PEG phases is detected by a third experiment, which utilized a ¹⁹F → ¹H–(spin diffusion)–¹H → ¹³C double transfer with ¹³C solid‐echo detection. The results of these experiments show that composition‐dependent PEG inclusions in the HBFP glass rigidify on hydration, consistent with an increase in macroscopic tensile strength.     

Compact Saloplastic Poly(Acrylic Acid)/Poly(Allylamine) Complexes: Kinetic Control Over Composition,Microstructure, and Mechanical Properties

Durable compact polyelectrolyte complexes (CoPECs) with controlled porosity and mechanical properties are prepared by ultracentrifugation. Because the starting materials, poly(allylamine hydrochloride) (PAH) and poly(acrylic acid sodium salt) (PAA), are weak acids/bases, both composition and morphology are controlled by solution pH. In addition, the nonequilibrium nature of polyelectrolyte complexation can be exploited to provide a range of compositions and porosities under the influence of polyelectrolyte addition order and speed, and concentration. Confocal microscopy shows these “saloplastic” materials to be highly porous, where pore formation is attributed to a combination of deswelling of the polyelectrolyte matrix and expansion of small inhomogenities by osmotic pressure. The porosity (15–70%) and the pore size (<5 μm to >70 μm) of these materials can be tuned by adjusting the PAA to PAH ratio, the salt concentration, and the pH. The modulus of these CoPECs depends on the ratio of the two polyelectrolytes, with stoichiometric complexes being the stiffest due to optimized charge pairing, which correlates with maximized crosslinking density. Mechanical properties, pore sizes, and pore density of these materials make them well suited to three dimensional supports for tissue engineering applications.     

A Gene Therapy Technology‐Based Biomaterial for the Trigger‐Inducible Release of Biopharmaceuticals in Mice

Gene therapy scientists have developed expression systems for therapeutic transgenes within patients, which must be seamlessly integrated into the patient's physiology by developing sophisticated control mechanisms to titrate expression levels of the transgenes into the therapeutic window. However, despite these efforts, gene‐based medicine still faces security concerns related to the administration of the therapeutic transgene vector. Here, molecular tools developed for therapeutic transgene expression can readily be transferred to materials science to design a humanized drug depot that can be implanted into mice and enables the trigger‐inducible release of a therapeutic protein in response to a small‐molecule inducer. The drug depot is constructed by embedding the vascular endothelial growth factor (VEGF₁₂₁) as model therapeutic protein into a hydrogel consisting of linear polyacrylamide crosslinked with a homodimeric variant of the human FK‐binding protein 12 (F_M), originally developed for gene therapeutic applications, as well as with dimethylsuberimidate. Administrating increasing concentrations of the inducer molecule FK506 triggers the dissociation of F_M thereby loosening the hydrogel structure and releasing the VEGF₁₂₁ payload in a dose‐adjustable manner. Subcutaneous implantation of the drug depot into mice and subsequent administration of the inducer by injection or by oral intake triggers the release of VEGF₁₂₁ as monitored in the mouse serum. This study is the first demonstration of a stimuli‐responsive hydrogel that can be used in mammals to release a therapeutic protein on demand by the application of a small‐molecule stimulus. This trigger‐inducible release is a starting point for the further development of externally controlled drug depots for patient‐compliant administration of biopharmaceuticals.     

Reinforcement of Shear Thinning Protein Hydrogels by Responsive Block Copolymer Self‐Assembly

Shear thinning hydrogels are promising materials that exhibit rapid self‐healing following the cessation of shear, making them attractive for applications including injectable biomaterials. Here, self‐assembly is demonstrated as a strategy to introduce a reinforcing network within shear thinning artificially engineered protein gels, enabling a responsive transition from an injectable state at low temperatures with a low yield stress to a stiffened state at physiological temperatures with resistance to shear thinning, higher toughness, and reduced erosion rates and creep compliance. Protein‐polymer triblock copolymers capable of the responsive self‐assembly of two orthogonal networks are synthesized. Midblock association forms a shear‐thinning network, while endblock aggregation at elevated temperatures introduces a second, independent physical network into the protein hydrogel. These reversible crosslinks introduce extremely long relaxation times and lead to a five‐fold increase in the elastic modulus, significantly larger than is expected from transient network theory. Thermoresponsive reinforcement reduces the high temperature creep compliance by over four orders of magnitude, decreases the erosion rate by at least a factor of five, and increases the yield stress by up to a factor of seven. Combined with the demonstrated potential of shear thinning artificial protein hydrogels for various uses, this reinforcement mechanism broadens the range of applications that can be addressed with shear‐thinning physical gels.     

Tuning Specific Biomolecular Interactions Using Electro‐Switchable Oligopeptide Surfaces

The ability to regulate biomolecular interactions on surfaces driven by an external stimuli is of great theoretical interest and practical impact in the biomedical and biotechnology fields. Herein, a new class of responsive surfaces that rely on electro‐switchable peptides to control biomolecular interactions on gold surfaces is presented. This system is based upon the conformational switching of positively charged oligolysine peptides that are tethered to a gold surface, such that bioactive molecular moieties (biotin) incorporated on the oligolysines can be reversibly exposed (bio‐active state) or concealed (bio‐inactive state) on demand, as a function of surface potential. The dynamics of switching the biological properties is studied by observing the binding events between biotin and fluorescently labeled NeutrAvidin. Fluorescence microscope images and surface plasmon resonance spectral data clearly reveal opposite binding behaviors when +0.3 V or ?0.4 V vs. SCE are applied to the surface. High fluorescence intensities are observed for an applied positive potential, while minimal fluorescence is detected for an applied negative potential. Surface plasmon resonance spectroscopy (SPR) results provided further evidence that NeutrAvidin binding to the surface is controlled by the applied potential. A large SPR response is observed when a positive potential is applied on the surface, while a negative applied potential induces over 90% reduction in NeutrAvidin binding.     

Hofmeister Effect Assisted Dual-Dynamic-Bond Cross-Linked Organohydrogels with Enhanced Ionic Conductivity and Balanced Mechanical Properties for Flexible Sensors

For soft electronic applications, the simultaneous incorporation of conductivity and mechanical robustness remains a grand constraint, not to mention being able to operate at wide temperatures ranges. Herein, a novel conductive platform is proposed by designing skin-inspired ionic organohydrogels based on Hofmeister effect and glycerol/water system, which simultaneously realize balanced conductivity, mechanical strength, and versatile properties. The comprehensive performances are broadly and simultaneously altered via tuning the aggregation states of polymer chains by kosmotropes or chaotropes. With various ions, the conductivity and mechanical strength are continuously in situ modulated over a large window: conductivity from 0.08 to 4.8 S m⁻¹, strength from 0.01 to 17.30 MPa, toughness from 5.4 to 9236.9 kJ m⁻³, and modulus from 5.1 to 2258.9 kPa. The ion transport process is inseparable from the changes of water content and pore structures caused by cross-linking density. Meanwhile, the mechanical properties greatly depend on the densification or loosing of polymer chains and crystalline domains. Furthermore, oil/water system exhibits low temperature tolerance at ≈−65–15 °C and long-term stability. Finally, the champion organohydrogels are applied as wearable electronic sensors and artificial skins. The mechanism proposed in this work advances the understanding of the ions contribution to organohydrogels for electronic applications.     

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.     

Digital Plasmonic Patterning for Localized Tuning of Hydrogel Stiffness

The mechanical properties of the extracellular matrix (ECM) can dictate cell fate in biological systems. In tissue engineering, varying the stiffness of hydrogels—water‐swollen polymeric networks that act as ECM substrates—has previously been demonstrated to control cell migration, proliferation, and differentiation. Here, “digital plasmonic patterning” (DPP) is developed to mechanically alter a hydrogel encapsulated with gold nanorods using a near‐infrared laser, according to a digital (computer‐generated) pattern. DPP can provide orders of magnitude changes in stiffness, and can be tuned by laser intensity and speed of writing. In vitro cellular experiments using A7R5 smooth muscle cells confirm cell migration and alignment according to these patterns, making DPP a useful technique for mechanically patterning hydrogels for various biomedical applications.     

Surface Patterning of Hydrogels for Programmable and Complex Shape Deformations by Ion Inkjet Printing

Convenient patterning and precisely programmable shape deformations are crucial for the practical applications of shape deformable hydrogels. Here, a facile and versatile computer‐assisted ion inkjet printing technique is described that enables the direct printing of batched, very complicated patterns, especially those with well‐defined, programmable variation in cross‐linking densities, on one or both surfaces of a large‐sized hydrogel sample. A mechanically strong hydrogel containing poly(sodium acrylate) is first prepared, and then digital patterns are printed onto the hydrogel surfaces by using a commercial inkjet printer and an aqueous ferric solution. The complexation between the polyelectrolyte and ferric ions increases the cross‐linking density of the printed regions, and hence the gel sample can undergo shape deformation upon swelling/deswelling. The deformation rates and degrees of the hydrogels can be conveniently adjusted by changing the printing times or the different/gradient grayscale distribution of designed patterns. By printing appropriate patterns on one or both surfaces of the hydrogel sheets, many complex 3D shapes are obtained from shape deformations upon swelling/deswelling, such as cylindrical shell and forsythia flower (patterns on one surface), ding (patterns on both surfaces), blooming flower (different/gradient grayscale distributive patterns on one surface), and non‐Euclidean plates (different/gradient grayscale distributive patterns on both surfaces).