Bilayer Hydrogels by Reactive-Induced Macrophase Separation

Bilayer hydrogels encoded with smart functions have emerged as promising soft materials for engineered biological tissues and human-machine interfaces, due to the versatility and flexibility in designing their mechanical and chemical properties. However, conventional fabrication strategies often require multiple complicated steps to create an anisotropic bilayer structure with poor interfaces, which significantly limit the scope of bilayer hydrogel applications. Here, we reported a general, one-pot, macrophase separation strategy to fabricate a family of bilayer hydrogels made of vinyl and styryl monomers with a seamless interface and a controllable layer separation efficiency (20–99%). The working principle of a macrophase separation strategy allows for the decoupling of the two gelation processes to form distinct vinyl- and styryl-enriched layers by manipulating competitive polymerization reactions between vinyl and styryl monomers. This work presents a straightforward approach and a diverse range of radical monomers, which can be utilized to create next-generation bilayer hydrogels, beyond a few available today

Probing the Structural Dependence of Carbon Space Lengths of Poly(<i>N</i>‑hydroxyalkyl acrylamide)-Based Brushes on Antifouling Performance

Numerous biocompatible antifouling polymers have been developed for a wide variety of fundamental and practical applications in drug delivery, biosensors, marine coatings, and many other areas. Several antifouling mechanisms have been proposed, but the exact relationship among molecular structure, surface hydration property, and antifouling performance of antifouling polymers still remains elusive. Here this work strives to provide a better understanding of the structure–property relationship of poly­(<i>N</i>-hydroxyalkyl acrylamide)-based materials. We have designed, synthesized, and characterized a series of polyHAAA brushes of various carbon spacer lengths (CSLs), that is, poly­(<i>N</i>-hydroxymethyl acrylamide) (polyHMAA), poly­(<i>N</i>-(2-hydroxyethyl)­acrylamide) (polyHEAA), poly­(<i>N</i>-(3-hydroxypropyl)­acrylamide) (polyHPAA), and poly­(<i>N</i>-(5-hydroxypentyl)­acrylamide) (polyHPenAA), to study the structural dependence of CSLs on their antifouling performance. HMAA, HEAA, HPAA, and HPenAA monomers contained one, two, three, and five methylene groups between hydroxyl and amide groups, while the other groups in polymer backbones were the same as each other. The relation of such small structural differences of polymer brushes to their surface hydration and antifouling performance was studied by combined experimental and computational methods including surface plasmon resonance sensors, sum frequency generation (SFG) vibrational spectroscopy, cell adhesion assay, and molecular simulations. Antifouling results showed that all polyHAAA-based brushes were highly surface resistant to protein adsorption from single protein solutions, undiluted blood serum and plasma, as well as cell adhesion up to 7 days. In particular, polyHMAA and polyHEAA with the shorter CSLs exhibited higher surface hydration and better antifouling ability than polyHPMA and polyHPenAA. SFG and molecular simulations further revealed that the variation of CSLs changed the ratio of hydrophobicity/hydrophilicity of polymers, resulting in different hydration characteristics. Among them, polyHMAA and polyHEAA with the shorter CSLs showed the highest potency for surface hydration and antifouling abilities, while polyHPenAA showed the lowest potency. The combination of both hydroxyl and amide groups in the same polymer chain provides a promising structural motif for the design of new effective antifouling materials

Salt-Responsive Bilayer Hydrogels with Pseudo-Double-Network Structure Actuated by Polyelectrolyte and Antipolyelectrolyte Effects

Development of stimuli-responsive, shape-transformable materials is fundamentally and practically important for smart actuators. Herein, we design and synthesize a bilayer hydrogel by assembling a polycationic (polyMETAC/HEAA) layer with polyelectrolyte effect and a polyzwitterionic (polyVBIPS) layer with antipolyelectrolyte effect together. The bilayer hydrogels adopt a pseudo-double-network structure, and both polyelectrolyte and polyzwitterionic layers have salt-responsive swelling and shrinkage properties, but in a completely opposite way. The resulting polyMETAC/HEAA–polyVBIPS bilayer hydrogels exhibit bidirectional bending in response to salt solutions, salt concentrations, and counterion types. Such bidirectional bending of this bilayer hydrogel is fully reversible and triggered between salt solution and pure water multiple times. The bending orientation and degree of the bilayer hydrogel is driven by the opposite volume changes between the volume shrinking (swelling) of polyMETAC/HEAA layer and the volume swelling (shrinking) of polyVBIPS layer. Such cooperative, not competitive, salt-responsive swelling–shrinking properties of the two layers are contributed to by the polyelectrolyte and antipolyelectrolyte effects from the respective layers. Moreover, an eight-arm gripper made of this bilayer hydrogel is fabricated and demonstrates its ability to grasp an object in salt solution and release the object in water. This work provides a new shape-regulated, stimuli-responsive asymmetric hydrogel for actuator-based applications

Biomimetically Structured Poly(lactic acid)/Poly(butylene-adipate-<i>co</i>-terephthalate) Blends with Ultrahigh Strength and Toughness for Structural Application

The preparation of ultrastrong and super-tough polymeric materials for structural application remains a considerable challenge. To simultaneously enhance the strength and toughness of poly(lactic acid) (PLA), we successfully prepared ultrastrong and super-tough PLA/poly(butylene-adipate-co-terephthalate) (PBAT) blends by biomimetically constructing a mussel nacre-like hierarchically ordered superstructure through a simple pressure-induced flow (PIF) processing technique. The morphology, crystallization, and mechanical properties of the blends were studied. During PIF processing, the immiscible PLA/PBAT blends were forced to undergo plastic deformation, which enhanced the interaction between PBAT and PLA and resulted in the formation of an oriented nanohybrid shish-kebab-like hierarchical structure. The combination of scanning electron microscopy and two-dimensional wide-angle X-ray diffraction results demonstrated that the nanohybrid shish-kebab-like crystalline structure was formed in the blends. Due to the hierarchical structure, the PLA/PBAT (90/10) blend prepared by PIF processing at 110 °C and 100 MPa exhibited ultrahigh tensile strength (209.5 MPa), good elongation at break (91.4%), high tensile modulus (2446.7 MPa), and excellent toughness (142.6 MJ/m3), which were much higher than those prepared by an injection-molded pure PLA and PLA-based blend. According to these results, the facile, effective, and practical method employed in this study was able to fabricate high-performance polymeric materials and may be used as a viable alternative to engineering plastics for structural application

Salt-Responsive Zwitterionic Polymer Brushes with Tunable Friction and Antifouling Properties

Development of smart, multifunction materials is challenging but important for many fundamental and industrial applications. Here, we synthesized and characterized zwitterionic poly­(3-(1-(4-vinyl­benzyl)-1<i>H</i>-imidazol-3-ium-3-yl)­propane-1-sulfonate) (polyVBIPS) brushes as ion-responsive smart surfaces via the surface-initiated atom transfer radical polymerization. PolyVBIPS brushes were carefully characterized for their surface morphologies, compositions, wettability, and film thicknesses by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle, and ellipsometer, respectively. Salt-responsive, switching properties of polyVBIPS brushes on surface hydration, friction, and antifouling properties were further examined and compared both in water and in salt solutions with different salt concentrations and counterion types. Collective data showed that polyVBIPS brushes exhibited reversible surface wettability switching between in water and saturated NaCl solution. PolyVBIPS brushes in water induced the larger protein absorption, higher surface friction, and lower surface hydration than those in salt solutions, exhibiting “anti-polyelectrolyte effect” salt responsive behaviors. At appropriate ionic conditions, polyVBIPs brushes were able to switch to superlow fouling surfaces (<0.3 ng/cm² protein adsorption) and superlow friction surfaces (<i>u</i> ∼ 10^–3). The relationship between brush structure and its salt-responsive performance was also discussed. This work provides new zwitterionic surface-responsive materials with controllable antifouling and friction capabilities for multifunctional applications

A Simple, Low-Cost, and Green Method for Preparing Strong, Tough, and Ductile Poly(lactic acid) Materials with Good Transparency and Heat Resistance

Notwithstanding that unprecedented progress has been achieved in strengthening and toughening PLA materials, it is still a challenge to find a facile and low-cost way to improve poly(lactic acid) (PLA) strength, toughness, and ductility while maintaining transparency and biodegradability. Herein, strong, tough, transparent, and highly heat-resistant PLA materials with a nacre-like lamellar structure were fabricated via a simple, low-cost and additive-free pressure-driven flow process. PLA powders with a size under 500 μm and heterogeneous size distribution obtained from ball milling and size sieving were used to yield a PLA material with a dense and ordered crystalline structure after pressure-driven flow treatment. The newly formed structured PLA material exhibited exceptional mechanical properties, with tensile strength, elongation at break, impact strength, and tensile toughness reaching 88.9 MPa, 102.5%, 45.1 KJ/m2, and 81.2 MJ/m3, respectively. The refined strength, toughness, and ductility were attributed to more particles undergoing uniform plastic deformation during the pressure-driven flow treatment. The interface between particles in the powder was much larger and more tortuous, and the particles were firmer. Additionally, the newly formed tightly stacked crystal structure consisting of densely and orderly arranged nanosized crystals played an important role in improving the mechanical properties of PLA. Moreover, the newly formed structured PLA materials exhibited enhanced heat resistance and retained good transparency, with a visible light transmission of over 80%. Overall, this work presents a simple and efficient method for fabricating high-performance PLA materials that are strong, tough, ductile, transparent, heat-resistant, and easily recyclable

Structural Dependence of Salt-Responsive Polyzwitterionic Brushes with an Anti-Polyelectrolyte Effect

Some polyzwitterionic brushes exhibit a strong “anti-polyelectrolyte effect” and ionic specificity that make them versatile platforms to build smart surfaces for many applications. However, the structure–property relationship of zwitterionic polymer brushes still remains to be elucidated. Herein, we aim to study the structure-dependent relationship between different zwitterionic polymers and the anti-polyelectrolyte effect. To this end, a series of polyzwitterionic brushes with different cationic moieties (e.g., imidazolium, ammonium, and pyridinium) in their monomeric units and with different carbon spacer lengths (e.g., CSL = 1, 3, and 4) between the cation and anion were designed and synthesized to form polymer brushes via the surface-initiated atom transfer radical polymerization. All zwitterionic brushes were carefully characterized for their surface morphologies, compositions, wettability, and film thicknesses by atomic force microscopy, contact angle measurement, and ellipsometry, respectively. The salt-responsiveness of all zwitterionic brushes to surface hydration and friction was further examined and compared both in water and in salt solutions with different salt concentrations and counterion types. The collective data showed that zwitterionic brushes with different cationic moieties and shorter CSLs in salt solution induced higher surface friction and lower surface hydration than those in water, exhibiting strong anti-polyelectrolyte effect salt-responsive behaviors. By tuning the CSLs, cationic moieties, and salt concentrations and types, the surface wettability can be changed from a highly hydrophobic surface (∼60°) to a highly hydrophilic surface (∼9°), while interfacial friction can be changed from ultrahigh friction (μ ≈ 4.5) to superior lubrication (μ ≈ 10^–3). This work provides important structural insights into how subtle structural changes in zwitterionic polymers can yield great changes in the salt-responsive properties at the interface, which could be used for the development of smart surfaces for different applications

Supplemental Material, Revised_Supplementary_Information_20180210 - Enhanced sound insulation and mechanical properties based on inorganic fillers/thermoplastic elastomer composites

<p>Supplemental Material, Revised_Supplementary_Information_20180210 for Enhanced sound insulation and mechanical properties based on inorganic fillers/thermoplastic elastomer composites by Wei Fang, Yanpei Fei, Huanqin Lu, Jiangming Jin, Mingqiang Zhong, Ping Fan, Jintao Yang, Zhengdong Fei, Feng Chen and Tairong Kuang in Journal of Thermoplastic Composite Materials</p

Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures

Development of smart soft actuators is highly important for fundamental research and industrial applications but has proved to be extremely challenging. In this work, we present a facile, one-pot, one-step method to prepare dual-responsive bilayer hydrogels, consisting of a thermoresponsive poly­(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly­(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)­propane-1-sulfonate) (polyVBIPS) layer. Both polyNIPAM and polyVBIPS layers exhibit a completely opposite swelling/shrinking behavior, where polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional bending in response to temperatures, salt concentrations, and salt types. Such bidirectional bending, bending orientation, and degree can be reversibly, repeatedly, and precisely controlled by salt- or temperature-induced cooperative swelling–shrinking properties from both layers. Based on their fast, reversible, and bidirectional bending behavior, we further design two conceptual hybrid hydrogel actuators, serving as a six-arm gripper to capture, transport, and release an object and an electrical circuit switch to turn on-and-off a lamp. Different from the conventional two- or multistep methods for preparation of bilayer hydrogels, our simple, one-pot, one-step method and a new bilayer hydrogel system provide an innovative concept to explore new hydrogel-based actuators through combining different responsive materials that allow us to program different stimuli for soft and intelligent materials applications

Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures

Development of smart soft actuators is highly important for fundamental research and industrial applications but has proved to be extremely challenging. In this work, we present a facile, one-pot, one-step method to prepare dual-responsive bilayer hydrogels, consisting of a thermoresponsive poly­(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly­(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)­propane-1-sulfonate) (polyVBIPS) layer. Both polyNIPAM and polyVBIPS layers exhibit a completely opposite swelling/shrinking behavior, where polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional bending in response to temperatures, salt concentrations, and salt types. Such bidirectional bending, bending orientation, and degree can be reversibly, repeatedly, and precisely controlled by salt- or temperature-induced cooperative swelling–shrinking properties from both layers. Based on their fast, reversible, and bidirectional bending behavior, we further design two conceptual hybrid hydrogel actuators, serving as a six-arm gripper to capture, transport, and release an object and an electrical circuit switch to turn on-and-off a lamp. Different from the conventional two- or multistep methods for preparation of bilayer hydrogels, our simple, one-pot, one-step method and a new bilayer hydrogel system provide an innovative concept to explore new hydrogel-based actuators through combining different responsive materials that allow us to program different stimuli for soft and intelligent materials applications