Xuemin Du

Water as a colorful ink: transparent, rewritable photonic coatings based on colloidal crystals embedded in chitosan hydrogel

The invention of paper has greatly contributed to the development of information storage and spreading. However, the vast production and consumption have also brought significant environment problems to our society. Although several kinds of rewritable papers have been reported, their practical application is rare. Herein, a new rewritable paper with water as a colorful ink is proposed and demonstrated successfully by using water as the trigger to tune the band-gap of photonic coatings on solid substrates. Various colors are achieved by adjusting the particle size of photonic coatings, or writing with aqueous solutions of certain pH. The photonic coatings can be erased and rewritten multiple times with no significant loss in color quality. Furthermore, the photonic coatings, which are transparent, enable fast and convenient visualization of the invisible photonic patterns with good tenability and reproducibility, offering potential applications for steganography, identification marking and anti-counterfeiting purposes. This transparent and rewritable paper can serve as an environmentally friendly information storage device to meet the sustainability of modern society.

Vapomechanically Responsive Motion of Microchannel-Programmed Actuators

Materials that respond rapidly and reversibly to external stimuli currently stand among the top choices as actuators for real-world applications. Here, a series of programmable actuators fabricated as single- or bilayer elements is described that can reversibly respond to minute concentrations of acetone vapors. By using templates, microchannel structures are replicated onto the surface of two highly elastic polymers, polyvinylidene fluoride (PVDF) and polyvinyl alcohol, to induce chiral coiling upon exposure to acetone vapors. The vapomechanical coiling is reversible and can be conducted repeatedly over 100 times without apparent fatigue. If they are immersed in liquid acetone, the actuators are saturated with the solvent and temporarily lose their motility but regain their shape and activity within seconds after the solvent evaporates. The desorption of acetone from the PVDF layer is four times faster than its adsorption, and the actuator composed of a single PVDF layer maintains its ability to move over an acetone-soaked filter paper even after several days. The controllable and reproducible sensing capability of this smart material can be utilized for actuating dynamic elements in soft robotics.

Light-Powered Micro/Nanomotors

Designed micro/nanomotors are micro/nanoscale machines capable of autonomous motion in fluids, which have been emerging in recent decades owing to their great potential for biomedical and environmental applications. Among them, light-powered micro/nanomotors, in which motion is driven by light, exhibit various advantages in their precise motion manipulation and thereby a superior scope for application. This review summarizes recent advances in the design, manufacture and motion manipulation of different types of light-powered micro/nanomotors. Their structural features and motion performance are reviewed and compared. The challenges and opportunities of light-powered micro/nanomotors are also discussed. With rapidly increasing innovation, advanced, intelligent and multifunctional light-powered micro/nanomachines will certainly bring profound impacts and changes for human life in the future.

Non-invasive in vivo imaging of the ionic regimes along the gastrointestinal tract of a freshwater vertebrate model organism (Japanese medaka) using responsive photonic crystal beads

Microspherical photonic colloidal crystalline beads that are responsive to media ionic strength of cationic electrolytes have been developed for in vivo imaging of the morphology and concentration gradient of cationic electrolytes along the gastrointestinal (GI) tract of live Japanese medaka (Oryzias latipes). These responsive photonic beads were assembled from core-shell nano-sized particles with polystyrene-co-polyacrylic acid (PS-co-PAA) cores and poly(hydroxyethyl methacrylate-co-p-styrene sulfonate) (PHEMA-co-PSS) hydrogel shells. The three-dimensional orderly packing of these nano-sized core-shell particles gave rise to the photonic properties of the resultant colloidal crystalline array of microspheres. The cationic electrolyte-induced volume phase transition of the sulfonate-laden hydrogel shells of the nano-sized particles altered the lattice spacing among those particles and brought about the photonic responses of the colloidal crystalline beads. Unambiguous changes in the diffraction colour of the colloidal crystalline beads were observable under ordinary ambient light in solution media of increasing concentration of sodium chloride up to 500 mM. These photonic colloidal crystalline beads were found to possess enough structural integrity for in vivo imaging of the GI tract of live Japanese medaka. With the use of a conventional optical microscope, the gradient in the ionic strength of cationic electrolytes along the GI tract of live Japanese medaka larvae was readily revealed, with a lower electrolyte concentration in the mid-intestine (<50 mM) compared to that of the posterior-intestine (≥50 mM). Our results demonstrated the potential of stimuli-responsive photonic materials in bio-imaging applications.

Poly(N-isopropylacrylamide) hydrogel-based shape-adjustable polyimide films triggered by near-human-body temperature

Hydrogel-based shape-adjustable films were successfully fabricated via grafting poly(N-isopropylacrylamide) (PNIPAM) onto one side of polyimide (PI) films. The prepared PI-g-PNIPAM films exhibited rapid, reversible, and repeatable bending/unbending property by heating to near-human-body temperature (37 °C) or cooling to 25 °C. The excellent property of PI-g-PNIPAM films resulted from a lower critical solution temperature (LCST) of PNIPAM at about 32 °C. Varying the thickness of PNIPAM hydrogel layer regulated the thermo-responsive shape bending degree and response speed of PI-g-PNIPAM films. The thermo-induced shrinkage of hydrogel layers can tune the curvature of PI films, which have potential applications in the field of wearable and implantable devices.Hydrogel-based shape-adjustable films were successfully fabricated via grafting poly(N-isopropylacrylamide) (PNIPAM) onto one side of polyimide (PI) films. The prepared PI-g-PNIPAM films exhibited rapid, reversible, and repeatable bending/unbending property by heating to near-human-body temperature (37 °C) or cooling to 25 °C. The excellent property of PI-g-PNIPAM films resulted from a lower critical solution temperature (LCST) of PNIPAM at about 32 °C. Varying the thickness of PNIPAM hydrogel layer regulated the thermo-responsive shape bending degree and response speed of PI-g-PNIPAM films. The thermo-induced shrinkage of hydrogel layers can tune the curvature of PI films, which have potential applications in the field of wearable and implantable devices.

Tunable shape memory polymer mold for multiple microarray replications

Microarrays have attracted great interest due to their widespread applications in anti-icing, cell manipulation, and antibioadhesion. However, such arrays are typically fabricated with high-cost and time-consuming lithography techniques. Various kinds of new polymers have been developed to solve this problem. Nevertheless, most of the microarrays fabricated with these materials are usually static, lacking sufficient intelligence and therefore hindering their practical applications. Here, a new strategy is reported that introduces shape memory polymer (SMP) into microarray fabrication. The deformation and recovery of SMP microarrays was investigated, and they show robust controllability and stable surface wettability, as well as a high shape recovery ratio (∼91%) at microscale, even after more than ten cycles of “stretch-recovery” process. In addition, various microarrays with a series of continuously changeable microscales were replicated by using the patterned SMP film as a tunable mold. Specifically, the SMP mold not only shows excellent ability to replicate multiple microarrays with controlled and ordered microstructures via one specific SMP mold, but it also can be applied to various polymers with more than ten replicas. This universal strategy paves an avenue for the broad applications of SMP arrays in cell manipulation, water droplet manipulation and smart dry adhesives.

Regulation Effects of Biomimetic Hybrid Scaffolds on Vascular Endothelium Remodeling

The formation of complete and well-functioning endothelium is critical for the success of tissue-engineered vascular grafts yet remaining a fundamental challenge. Endothelium remodeling onto the lumen of tissue-engineered vascular grafts is affected by their topographical, mechanical, and biochemical characteristics. For meeting multiple requirements, composite strategies have recently emerged for fabricating hybrid scaffolds, where the integrated properties are tuned by varying their compositions. However, the underlying principle how the integrated properties of hybrid scaffolds regulate vascular endothelium remodeling remains unclear. To uncover the regulation effects of hybrid scaffolds on vascular endothelium remodeling, we prepared different biomimetic hybrid scaffolds using gelatin methacrylamide (GelMA) and poly-ε-caprolactone (PCL) and then investigated vascular endothelial cell responses on them. GelMA and PCL, respectively, conferred the resulting scaffolds with biomimetic bioactivity and mechanical properties, which were tuned by varying GelMA/PCL mass ratios (3:1, 1:1, or 1:3). On different GelMA/PCL hybrid scaffolds, distinct vascular endothelial cell responses were observed. Firm cell-scaffold/cell-cell interactions were rapidly established on the hybrid scaffolds with the highest mass ratio of bioactive GelMA. However, they were mechanically insufficient as vascular grafts. On the contrary, the scaffolds with the highest mass ratio of PCL showed significantly reinforced mechanical properties but poor biological performance. Between the two extremes, the scaffolds with the same GelMA/PCL mass ratio balanced the pros and cons of two materials. Therefore, they could meet the mechanical requirements of vascular grafts and support the early-stage vascular endothelial cell remodeling by appropriate biological signaling and mechanotransduction. This investigation experimentally proves that scaffold bioactivity is the dominant factor affecting vascular endothelial cell adhesion and remodeling, whereas mechanical properties are crucial factors for the integrity of endothelium. This work offers a universal design strategy for desirable vascular grafts for improved endothelium remodeling.

Thermal-induced three-dimensional shape transformations of hydrogel sheets

Self-constructing materials, which can spontaneously change their shapes from ordinary two-dimensional (2D) planes into complex three-dimensional (3D) structures, are always an attractive subject of intelligent materials. Herein, we report a simple yet effective strategy for the fabrication of programmable, thermally responsive and self-constructing hydrogels which can undergo various types of transformation from an ordinary sheet into different complex 3D shapes. By designing the surface patterns of a thermo-responsive hydrogel sheet, planar-to-tubular, planar-to-helical and planar-to-cylindrical transformation can be readily achieved upon temperature change. Integrating this kind of materials into semiconductors, self-constructive, flexible and functional electronic devices may be developed as a leading topic of smart materials.

Fabrication of inverse opal beads based on biocompatible and biodegradable polymer

Photonic crystals have been widely used in various fields, however, it is still challenging to employ photonic crystals for in vivo application due to the poor biocompatible and biodegradable properties. In this work, we fabricated inverse opal beads based on biocompatible and biodegradable chitosan. Not only these chitosan-based inverse opal beads exhibited hollow structure together with a uniform interconnected pore structure, but also showed excellent Bragg diffraction in the near-infrared region. These polysaccharide-based inverse opal beads have attractive applications in bio-imaging, drug delivery and so on.

Vapor-condensation-assisted reverse display for anti-counterfeiting applications

We propose a simple yet effective method for the display based on vapor condensation on the hydrophilic/hydrophobic patterned surfaces. It was found that supersaturated water vapor first formed nanometer-sized water droplet on the condensation nuclei on the patterned surfaces, then the water droplet grew bigger and therefore scatter more light to visualize the outline of the patterns for naked eyes. This vapor-condensation-assisted display is applicable to a variety of substrates from Si wafers, glass to mirrors. This method does not induce impurities and essentially low-cost and efficient, which may inspire interesting applications in various fields.