Hyaluronic acid (HA), an immunoneutral polysaccharide that is ubiquitous in the human body, is crucial for many cellular and tissue functions and has been in clinical use for over thirty years. When chemically modified, HA can be transformed into many physical forms-viscoelastic solutions, soft or stiff hydrogels, electrospun fibers, non-woven meshes, macroporous and fibrillar sponges, flexible sheets, and nanoparticulate fluids-for use in a range of preclinical and clinical settings. Many of these forms are derived from the chemical crosslinking of pendant reactive groups by addition/condensation chemistry or by radical polymerization. Clinical products for cell therapy and regenerative medicine require crosslinking chemistry that is compatible with the encapsulation of cells and injection into tissues. Moreover, an injectable clinical biomaterial must meet marketing, regulatory, and financial constraints to provide affordable products that can be approved, deployed to the clinic, and used by physicians. Many HA-derived hydrogels meet these criteria, and can deliver cells and therapeutic agents for tissue repair and regeneration. This progress report covers both basic concepts and recent advances in the development of HA-based hydrogels for biomedical applications.

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Hyaluronic acid hydrogels for biomedical applications.

Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials including adhesion, self-assembly, toughness, and hardness without mineralization [Rubin DJ, Miserez A, Waite JH (2010) Advances in Insect Physiology: Insect Integument and Color, eds Jerome C, Stephen JS (Academic Press, London), pp 75–133]. Coordination between Fe and catechol ligands has recently been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties [Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P (2010) Science 328:216–220]. Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe3+ interpolymer cross-linking via pH. The resonance Raman signature of catechol-Fe3+ cross-linked polymer gels at high pH was similar to that from native mussel thread cuticle and the gels displayed elastic moduli (G′) that approach covalently cross-linked gels as well as self-healing properties.

https://www.pnas.org/content/pnas/108/7/2651.full.pdf

pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli

Encapsulating cells in biodegradable hydrogels offers numerous attractive features for tissue engineering, including ease of handling, a highly hydrated tissue-like environment for cell and tissue growth, and the ability to form in vivo. Many properties important to the design of a hydrogel scaffold, such as swelling, mechanical properties, degradation, and diffusion, are closely linked to the crosslinked structure of the hydrogel, which is controlled through a variety of different processing conditions. Degradation may be tuned by incorporating hydrolytically or enzymatically labile segments into the hydrogel or by using natural biopolymers that are susceptible to enzymatic degradation. Because cells are present during the gelation process, the number of suitable chemistries and formulations are limited. In this review, we describe important considerations for designing biodegradable hydrogels for cell encapsulation and highlight recent advances in material design and their applications in tissue engineering.

Cell Encapsulation in Biodegradable Hydrogels for Tissue Engineering Applications

Polysaccharide hydrogels for modified release formulations.

The knee meniscus: structure-function, pathophysiology, current repair techniques, and prospects for regeneration.

Disulfide Cross-Linked Hyaluronan Hydrogels

Dynamically Restructuring Hydrogel Networks Formed with Reversible Covalent Crosslinks

Described herein are polymeric compositions that comprise at least one polymer residue and at least one crosslinking moiety, wherein the polymer residue is crosslinked by the crosslinking moiety and wherein the crosslinking moiety is formed from a reaction between a boronic acid moiety and a hydroxamic acid moiety. Also, described are methods of making and using such polymeric compositions.

Polymeric compositions and methods of making and using thereof

Temperature and pH Sensitive Hydrogels: An Approach Towards Smart Semen-Triggered Vaginal Microbicidal Vehicles

Hydrogel networks crosslinked with polymer-bound phenylboronic acid (PBA) and salicylhydroxamic acid (SHA) demonstrate pH-reversible gel behavior due to the pH-dependent equilibrium of the crosslinking moieties that form the gel network. Furthermore, the pH at which gels behave dynamically can be controlled by use of a polyelectrolyte backbone. Here we report on the frequency-dependent chemorheological characterization of PBA-SHA crosslinked hydrogel networks with a sulfonated polymer backbone. Our results suggest that the anionic nature of the polymers allows reversible crosslinking at neutral pH that an otherwise neutral-backboned PBA-SHA crosslinked network cannot, and that these charge-induced dynamics can be effectively screened by ions in solution. Moreover, moduli-frequency data can effectively be reduced into a single master curve with a neutral-backboned PBA-SHA gel data set as the reference condition.

/pdf/chemorheology-of-phenylboronate-salicylhydroxamate-cross-4v7hxemp77.pdf

Meredith C. Roberts

Papers

Disulfide Cross-Linked Hyaluronan Hydrogels

Dynamically Restructuring Hydrogel Networks Formed with Reversible Covalent Crosslinks

Polymeric compositions and methods of making and using thereof

Temperature and pH Sensitive Hydrogels: An Approach Towards Smart Semen-Triggered Vaginal Microbicidal Vehicles

Chemorheology of Phenylboronate−Salicylhydroxamate Cross-Linked Hydrogel Networks with a Sulfonated Polymer Backbone