Supramolecular polymer networks are materials crosslinked by reversible supramolecular interactions, such as hydrogen bonding or electrostatic interactions. Supramolecular materials show very interesting and useful properties resulting from their dynamic nature, such as self-healing, stimuli-responsiveness and adaptability. Here we will discuss recent progress in polymer-based supramolecular networks for the formation of hydrogels and bulk materials.

Supramolecular polymer networks: hydrogels and bulk materials

3D printing will revolutionize the manufacturing industry. Significant advances in computer aided design, additive manufacturing and materials science have opened up the possibilities of self-assembly systems, self-healing and material property alterations. Printing layer by layer allows complex geometries to be built, previously difficult under conventional manufacturing routes. This paper is a review of the current materials available for 3D printing that enable the emergence of 4D printing, a ‘smart material’ that responds in a programmed way to an external stimuli. The outlook is towards potential space applications, in all areas including deployable structures, antennas and medical supplies.

Additive manufacturing — A review of 4D printing and future applications

Materials with switchable mechanical properties are widespread in living organisms and endow many species with traits that are essential for their survival. Many of the mechanically morphing materials systems found in nature are based on hierarchical structures, which are the basis for mechanical robustness and often also the key to responsive behavior. Many “operating principles” involve cascades of events that translate cues from the environment into changes of the overall structure and/or the connectivity of the constituting building blocks at various levels. These concepts permit dramatic property variations without significant compositional changes. Inspired by the function and the growing understanding of the operating principles at play in biological materials with the capability to change their mechanical properties, significant efforts have been made toward mimicking such architectures and functions in artificial materials. Research in this domain has rapidly grown in the last two decades and aff...

Bioinspired Polymer Systems with Stimuli-Responsive Mechanical Properties

Four-dimensional (4D) bioprinting, encompassing a wide range of disciplines including bioengineering, materials science, chemistry, and computer sciences, is emerging as the next-generation biofabrication technology. By utilizing stimuli-responsive materials and advanced three-dimensional (3D) bioprinting strategies, 4D bioprinting aims to create dynamic 3D patterned biological structures that can transform their shapes or behavior under various stimuli. In this review, we highlight the potential use of various stimuli-responsive materials for 4D printing and their extension into biofabrication. We first discuss the state of the art and limitations associated with current 3D printing modalities and their transition into the inclusion of the additional time dimension. We then suggest the potential use of different stimuli-responsive biomaterials as the bioink that may achieve 4D bioprinting where transformation of fabricated biological constructs can be realized. We finally conclude with future perspectives.

https://iopscience.iop.org/article/10.1088/1758-5090/9/1/012001/pdf

4D bioprinting: the next-generation technology for biofabrication enabled by stimuli-responsive materials.

Invented by Charles Goodyear, chemical cross-linking of rubbers by sulfur vulcanization is the only method by which modern automobile tires are manufactured. The formation of these cross-linked network structures leads to highly elastic properties, which substantially reduces the viscous properties of these materials. Here, we describe a simple approach to converting commercially available and widely used bromobutyl rubber (BIIR) into a highly elastic material with extraordinary self-healing properties without using conventional cross-linking or vulcanising agents. Transformation of the bromine functionalities of BIIR into ionic imidazolium bromide groups results in the formation of reversible ionic associates that exhibit physical cross-linking ability. The reversibility of the ionic association facilitates the healing processes by temperature- or stress-induced rearrangements, thereby enabling a fully cut sample to retain its original properties after application of the self-healing process. Other mecha...

Ionic Modification Turns Commercial Rubber into a Self-Healing Material.

A strategy for combining covalent and non-covalent cross-links to construct multifunctional rubber materials with intelligent self-healing and shape memory ability is demonstrated. Rubbers were prepared by self-assembly of complementary polybutadiene oligomers bearing carboxylic acid and amine groups through reversible ionic hydrogen bonds via the acid–base reaction, and then further covalently cross-linked by tri-functional thiol via the thiol-ene reaction. The resulting polymers exhibit self-healing and shape memory functions owing to the reversible ionic hydrogen bonds. The covalent cross-linking density can be tuned to achieve tailorable mechanical and stimuli-responsive properties: a low covalent cross-linking density maintains the remarkable self-healing capability of rubber at ambient temperature without any external stimulus, while a high covalent cross-linking density improves the mechanical strength and induces shape memory behavior, but effective self-healing needs to be triggered at high temperature. This strategy might open a promising pathway to fabricate intelligent multifunctional polymers with versatile functions.

Intelligent rubber with tailored properties for self-healing and shape memory

Dynamic cross-links to facilitate recyclable polybutadiene elastomer with excellent toughness and stretchability

Designing gradient structures, mimicking biological materials, such as pummelo peels and tendon, is a promising strategy for developing advanced materials with superior energy damping capacities. Here a facile and effective approach for fabricating polymers with composition gradients at millimeter length scale is presented. The gradient thiol-ene polymers (TEPs) are created by the use of density difference of ternary thiol-ene-ene precursors and the subsequent photo-crosslinking via thiol-ene reaction. The compositional gradients are analyzed via differential scanning calorimeter (DSC), compressive modulus testing, atomic force microscopy (AFM) indentation, and swelling measurements. In contrast to homogeneous TEPs networks, the resultant gradient polymer shows a broader effective damping temperature range combining with good mechanical properties. The present result provides an effective route toward high damping materials by the fabrication of gradient structures.

Biomimetic Gradient Polymers with Enhanced Damping Capacities

It is challenging to create artificial catalysts that approach enzymes with regard to catalytic efficiency and selectivity. The enantioselective catalysis ranks the privileged characteristic of enzymatic transformations. Here, we report two pyridine-linked bis(β-cyclodextrin) (bisCD) copper(II) complexes that enantioselectively hydrolyse chiral esters. Hydrolytic kinetic resolution of three pairs of amino acid ester enantiomers (S1–S3) at neutral pH indicated that the “back-to-back” bisCD complex CuL1 favoured higher catalytic efficiency and more pronounced enantioselectivity than the “face-to-face” complex CuL2. The best enantioselectivity was observed for N-Boc-phenylalanine 4-nitrophenyl ester (S2) enantiomers promoted by CuL1, which exhibited an enantiomer selectivity of 15.7. We observed preferential hydrolysis of L-S2 by CuL1, even in racemic S2, through chiral high-performance liquid chromatography (HPLC). We demonstrated that the enantioselective hydrolysis was related to the cooperative roles of the intramolecular flanking chiral CD cavities with the coordinated copper ion, according to the results of electrospray ionization mass spectrometry (ESI-MS), inhibition experiments, rotating-frame nuclear Overhauser effect spectroscopy (ROESY), and theoretical calculations. Although the catalytic parameters lag behind the level of enzymatic transformation, this study confirms the cooperative effect of the first and second coordination spheres of artificial catalysts in enantioselectivity and provides hints that may guide future explorations of enzyme mimics.

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Enantioselective Hydrolysis of Amino Acid Esters Promoted by Bis(β-cyclodextrin) Copper Complexes.

A facile cross‐linking strategy of using small molecules as physcial crosslinkers to facilitate recyclable polybutadiene (PB) elastomer with excellent toughness and stretchability is demonstrated. Carboxylic acid groups were incorporated along the PB backbone via thiol‐ene reaction, and then the polymer can be cross‐linked by ionic hydrogen bonds between the carboxylic acid groups from PB and the amine groups of the cross‐linkers. The ionic hydrogen bonds can dynamiclly break and reconstruct upon deformation, thus endowing the resultant polymer with not only high toughness and stretchability (∼1800%), but also good self‐recovery and enhanced damping properties. Remarkably, the dynamically cross‐linked PB elastomer can be thermally recycled owing to the thermal reversibility of the ionic hydrogen bonds and the mechanical properties can be largely recovered after reprocessing. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1357‐1366

Beichen Cheng

Papers

Intelligent rubber with tailored properties for self-healing and shape memory

Dynamic cross-links to facilitate recyclable polybutadiene elastomer with excellent toughness and stretchability

Biomimetic Gradient Polymers with Enhanced Damping Capacities

Enantioselective Hydrolysis of Amino Acid Esters Promoted by Bis(β-cyclodextrin) Copper Complexes.

Dynamic cross‐links to facilitate recyclable polybutadiene elastomer with excellent toughness and stretchability Part A Polymer chemistry