Rippling Colloidal Polyelectrolyte Complex for Customized Fingerprints with High Tactile Perception.

Fingerprints possess wide applications in personal identification, tactile perception, access control, and anti-counterfeiting. However, latent fingerprints are usually left on touched surfaces, leading to the leakage of personal information. Furthermore, tactile perception greatly decreases when fingerprints are covered by gloves. Customized fingerprints are developed to solve these issues, but it is a challenge to develop fingerprints with various customized patterns using traditional techniques due to their requiring special templates, materials, or instruments. Inspired by ripples on the lake, blowing air is used to generate surface waves on a colloidal polyelectrolyte complex, leading to vertical stratification and the accumulation of particles near the top of the film layer. As water rapidly evaporates, the viscosity of these particles significantly increases and the wave is solidified, forming fingerprint patterns. These customized fingerprints integrate functions of grasping objects, personal identification without leaving latent fingerprints and tactile perception enhancement, which can be applied in information security, anti-counterfeiting, tactile sensors, and biological engineering.

Dual-Cross-Linked PEI/PVA Hydrogel for pH-Responsive Drug Delivery.

Herein, a pH-responsive dual cross-linked hydrogel for controlled drug release is presented. The hydrogel was constructed with reversible borate ester bonds and crystalline poly(vinyl alcohol). By changing the environmental pH, its physicochemical characteristics, including rheological properties, mechanical properties, microstructural features, and the biocompatibility of the gels, were evaluated. The gels at tumor acidic conditions exhibited swelling and lower compressive strength and modulus than those in a physiological environment, which was attributed to the pH-responsive borate ester bonds and the protonation of amine groups on the PEI polyelectrolyte. Importantly, the drug-encapsulated biocompatible hydrogel showed sustained and increased release under an acidic environment, and it followed the Fickian diffusion mechanism. Therefore, it exemplifies that borate ester bond-based pH-responsive biomaterials have high promise in biomedical research, especially for drug delivery.

Synergetic Combination of Interfacial Engineering and Shape-Changing Modulation for Biomimetic Soft Robotic Devices.

Robotics is a frontal interdisciplinary subject across the fields of mechanical engineering, chemical and materials engineering, artificial intelligence, and nanotechnology. Robotic devices with a variety of frameworks, functionalities, and actuation modes have been developed and employed in the manufacture of advanced materials and devices with improved efficiency and automation. In recent years, soft robots have attracted a significant amount of interest among scientific researchers and technological engineers because they can offer the desired safety, adaptability, sensibility, and dexterity that conventional robotics cannot deliver. To date, emulating living creatures in nature has been a promising approach to design soft robots. For living creatures, both body deformation and their surface characteristic are essential for them to function in dynamic ecological environments. Body deformation offers athletic ability while surface characteristics provide extraordinary adaptable interactions with the environment. In this article, we discuss the recent progress of emulating the body deformation of living creatures such as shrinking/expanding, bending, and twisting and programmable deformations based on the manipulation of shape-changing behaviors of liquid-crystal polymeric materials (LCPs) and the interfacial technologies to build up various microstructures similar to the interface of living creatures. We further review the pioneering work that integrates interfacial engineering and the shape-changing modulation of LCPs to develop biomimetic soft robotic devices. We also provide an outlook for opportunities and challenges in the design and fabrication of advanced biomimetic soft robots based on the synergetic combination of interfacial engineering and shape-changing modulation.

3D coaxial printing of porous construct stacked with hollow filaments for heavy metal removal.

Biopolymers-based hydrogels/aerogels have been widely used for removing heavy metals from water. However, the adsorption efficiency is limited by the monolithic macroscopic structure due to low exposed adsorption sites and ion diffusion rate. In this study, we report a proof-of-concept micro-extrusion-based 3D coaxial printing technology to solve the aforementioned problems in heavy metal removal. The 3D printed grid-like architectures stacked with hollow filaments prepared by alginate and cellulose nanocrystal showed excellent heavy metals (including Cu, Zn, Cr, and Cd) removal performance over other common adsorbents. The Cu(II) adsorption was greatly influenced by the initial pH and ionic strength. It followed pseudo-second order kinetics and Langmuir isotherms models. The maximum adsorption capacity of the porous construct was found to be 97.22 mg∙g-1 at room temperature. Moreover, the equilibrium adsorption capacity and adsorption rate of Cu(II) in the hierarchical porous adsorbents (~68 mg∙g-1, 2.29×10-3 g∙mg-1∙min-1) were much higher than that in its corresponding solid chunk hydrogel (~48 mg∙g-1, 7.27×10-4 g∙mg-1∙min-1). It exemplifies that the 3D coaxial printing strategy for wastewater treatment enables shape-specific applications of functional hierarchical porous adsorbents.

Coacervation-Based Method for Constructing a Multifunctional Strain-Stiffening Crystalline Polyvinylamine Hydrogel.

Strain-stiffening hydrogels are essential in the development of ionic skin, as human skin possesses a strain-stiffening property for self-protection. Semicrystalline polymers such as poly(vinyl alcohol) (PVA) have been widely investigated to fabricate strain-stiffening hydrogels via freeze-thaw cycling or chemical cross-linking but with limited adjustable properties. Compared with PVA, polyvinylamine (PVAm) has a higher reactive activity, making it easier to achieve multifunctionalities including strain-stiffening in a PVAm hydrogel. However, the amine moieties in the backbone tend to be ionized and form strong ionic hydrogen bonds with water, resulting in difficulties in forming crystalline hydrogels by conventional methods. Herein, a one-pot method to induce crystallinity and achieve multifunctional hydrogel is devised via coacervation of PVAm. Different from a published coacervation method to fabricate hydrogels with various properties via noncovalent interactions between different chemicals, coacervation occurs between PVAm to form aggregated and loose PVAm in our devised system. Such a strategy lowers the amine-water binding energy in the polymer-dense phase to achieve crystallinity and subsequently the strain-stiffening property; meanwhile, self-healability, self-adhesion, and ionic conductivity can be realized in the polymer-loose phase. The obtained hydrogel integrates stretchability (∼1300% elongation), toughness (227 kPa), the strain-stiffening property (∼10 times increase), self-adhesion (90 J m-2), self-healability (∼80% healing efficiency in toughness), and ionic conductivity (0.22 mS m-1). This convenient strategy will open a new horizon to design multifunctional skin-mimic materials.

Nature-inspired robust hydrochromic film for dual anticounterfeiting.

Nature-inspired materials have been actively developed for anticounterfeiting applications. Among a variety of stimuli-responsive anticounterfeiting strategies, hydrochromic materials exhibit reversible color change in response to moisture or water and have the advantage of being easy to authenticate. However, the security level of current hydrochromic anticounterfeiting materials is not sufficient for practical applications since they only exhibit a single anticounterfeiting function, where the information switches between visible and invisible. To improve the security level and efficiency of hydrochromic anticounterfeiting materials, here we developed a robust dual hydrochromic material via the self-assembly of polyurethane (PU)-polyelectrolytes colloids with which the desired information can not only switch between visible and invisible but also transform from one pattern to another within 3 s without the need of any external instruments. The bio-inspiration, material design and demonstrated hydrochromic properties might have profound implications for using colloidal complexes to make advanced anticounterfeiting materials.

Polymer Binders: Characterization and Development toward Aqueous Electrode Fabrication for Sustainability.

Binders play an important role in electrode processing for energy storage systems. While conventional binders often require hazardous and costly organic solvents, there has been increasing development toward greener and less expensive binders, with a focus on those that can be processed in aqueous conditions. Due to their functional groups, many of these aqueous binders offer further beneficial properties, such as higher adhesion to withstand the large volume changes of several high-capacity electrode materials. In this review, we first discuss the roles of binders in the construction of electrodes, particularly for energy storage systems, summarize typical binder characterization techniques, and then highlight the recent advances on aqueous binder systems, aiming to provide a stepping stone for the development of polymer binders with better sustainability and improved functionalities.

Poly Methacrylic Acid Sodium Salt (PMANa)/Polyurethane (PU) Latex-Polyelectrolyte Colloid Systems Enabling One-Pot Fabrication of Nonperiodic Structured Mechanoresponsive Smart Windows.

Mechanoresponsive smart windows can modulate optical transparency by mechanical actuation, showing the advantages of low cost, energy efficiency, and chemical stability. To date, mechanoresponsive smart windows are mainly built on periodic nano- or microstructures such as arrays, wrinkles, 3D photonic crystals, and polymeric nanocomposites. However, the production of periodic structures requires multiple and complicated processes, which restricts large-scale manufacturing of mechanoresponsive smart windows for practical application. Here, we report one-pot fabrication of poly methacrylic acid sodium salt (PMANa)/polyurethane (PU) mechanoresponsive smart windows with a nonperiodic naturally occurring microsized structure. The obtained smart windows display a high contrast ratio and stable transparency switching behavior under 80% tensile strain over 1000 cycles and have a large breaking elongation of 820%. It offers a promising platform for designing and fabricating multifunctional optical devices including anticounterfeiting labels and dynamical light gratings.

Dual Colorimetric and Conductometric Responses of Silver-Decorated Polypyrrole Nanowires for Sensing Organic Solvents of Varied Polarities.

We report on the discovery of an interesting dual colorimetric and conductometric response of synthesized water-based silver-decorated polypyrrole (Ag/PPy) nanowire to organic solvents, and the possible fabrication of a volatile organic compounds (VOCs) sensor based on the stimuli-responsive Ag/PPy nanocomposites. The dual-responsive nature of this material allows for a straightforward way to differentiate various VOCs within a broad polarity range by noncontact optical means via color transition, but also can provide highly accurate quantitative identification of each VOC by electronic detection.