Iron Tris(bipyridine) PEG Hydrogels with Covalent and Metal Coordinate Cross-Links

Self-Healing Polymeric Hydrogel Formed by Metal-Ligand Coordination Assembly: Design, Fabrication, and Biomedical Applications

Self-healing hydrogels based on metal-ligand coordination chemistry provide new and exciting properties that improve injectability, rheological behaviors, and even biological functionalities. The inherent reversibility of coordination bonds improves on the covalent cross-linking employed previously, allowing for the preparation of completely self-healing hydrogels. In this article, recent advances in the development of this class of hydrogels are summarized and their applications in biology and medicine are discussed. Various chelating ligands such as bisphosphonate, catechol, histidine, thiolate, carboxylate, pyridines (including bipyridine and terpyridine), and iminodiacetate conjugated onto polymeric backbones, as well as the chelated metal ions and metal ions containing inorganic particles, which are used to form dynamic networks, are highlighted. This article provides general ideas and methods for the design of self-healing hydrogel biomaterials based on coordination chemistry.

Functional Hyper-Crosslinkers

Crosslinking is an essential architecture for polymers to improve mechanical and thermal properties and to form three-dimensional (3D) structures. Although conventional synthetic crosslinkers usually only serve as bridges between different polymeric chains, biological crosslinkers are multifunctional. Recently, crosslinkers with active functionality after polymerization are promising a new dimension of molecular design for generating smart materials as they may exhibit dramatic changes in structures and properties in response to external stimuli. This concept article introduces several different designs of hyper-crosslinkers (i.e., multifunctional crosslinkers that provide three or more sites for generating networks) and their applications in tailoring stimuli-responsive and self-healing soft materials. Particularly, the polymeric materials that contain functional hyper-crosslinkers are exhibiting anti-intuitive redox or thermal-pH responses. The goal of this article is to illustrate the concept that functional hyper-crosslinkers not only improve the physical and chemical performance of the polymers, but more importantly, may introduce new architectures and functionalities for soft materials.

Flame-retardant, flexible vermiculite–polymer hybrid film

Flame-retardant, flexible polymer thin film and coating materials are in large demand for various applications.

Supramolecular hydrogel capsules based on PEG: a step toward degradable biomaterials with rational design

Supramolecular microgel capsules based on polyethylene glycol (PEG) are a promising class of soft particulate scaffolds with tailored properties. An approach to fabricate such particles with exquisite control by droplet-based microfluidics is presented. Linear PEG precursor polymers that carry bipyridine moieties on both chain termini are gelled by complexation to iron(II) ions. To investigate the biocompatibility of the microgels, living mammalian cells are encapsulated within them. The microgel elasticity is controlled by using PEG precursors of different molecular weights at different concentrations and the influence of these parameters on the cell viabilities, which can be optimized to exceed 90% is studied. Reversion of the supramolecular polymer cross-linking allows the microcapsules to be degraded at mild conditions with no effect on the viability of the encapsulated and released cells.

Free radical polymerization of poly(ethylene glycol) diacrylate macromers: impact of macromer hydrophobicity and initiator chemistry on polymerization efficiency

A series of poly(ethylene glycol)-co-poly(lactide) diacrylate macromers was synthesized with variable PEG molecular weights (10 or 20 kDa) and lactate contents (0 or 6 lactates per end group). These macromers were polymerized to form hydrogels by free radical polymerization using either redox or photochemical initiators. The extent of polymerization was determined by monitoring the compressive modulus of the resulting hydrogels and by quantitative determination of unreacted acrylate after exhaustive hydrolysis of the gel. Polymerization efficiency was found to depend on the lactate content of the macromer, with higher lactate macromers giving more efficient polymerization. For redox-initiated polymerization using ferrous gluconate/t-butyl hydroperoxide initiator, macromers containing approximately six lactate repeats per end group required lower concentrations of initiator to reach high conversion than lactate-free macromers. Photochemical polymerization with α,α-dimethoxy-α-phenylacetophenone (Irgacure 651(®)) was found to be less efficient than redox polymerization, requiring the addition of N-vinyl-2- pyrrolidone (NVP) as a co-monomer to achieve conversions comparable with redox polymerization. When conditions were optimized to provide near complete conversion for all gels, the presence of lactate repeat units in the hydrogel was generally found to reduce swelling and increase the compressive modulus. Calculated values of molecular weight between cross-links (M(c)) and mesh size using Flory-Rehner theory showed that macromer molecular weight had the greatest impact on the network structure of the gel.