Readily prepared dynamic hydrogels by combining phenyl boronic acid- and maltose-modified anionic polysaccharides at neutral pH.

Dynamic covalent hydrogels are facilely prepared from biocompatible polysaccharides in physiological conditions by the formation of phenylboronate ester cross-links. This is based on the simple mixing of carboxylate-containing polysaccharides (i.e., hyaluronic acid or carboxymethylcellulose) modified with phenylboronic acid and maltose moieties according to mild coupling reactions performed in aqueous solution. The formation of dynamic networks based on reversible boronic-ester cross-links is demonstrated by analyzing their rheological behavior. This study shows that these gels can adapt their structure in response to chemical stimuli such as variations in pH or addition of glucose and self-heal.

Thiol-ene clickable hyaluronans: from macro-to nanogels.

The fabrication of hyaluronic acid (HA) nanogels using a thiol-ene reaction has been demonstrated. HA was modified with pentenoate groups and then cross-linked with poly(ethylene glycol)-bis(thiol) by exposure to UV light. The cross-linking density and thereby the rigidity of the obtained gels were precisely controlled by the degree of substitution of pentenoate-modified HA. Their swelling properties also depended on cross-linking density. To produce hydrogels at the nanoscale, hyaluronic acid precursors were solely confined inside liposomes before cross-linking and purified after cross-linking. The size of the resulting nanogels followed their swelling properties and was also affected by their cross-linking density. Such bionanogels with tunable mechanical and swelling properties have potential in drug delivery.

Photochemical crosslinking of hyaluronic acid confined in nanoemulsions: towards nanogels with a controlled structure.

We present the preparation of nanogels made of hyaluronic acid (HA) with a well-controlled structure. To this end, HA precursors with polymerizable methacrylate groups (HA-MA) were confined within water-in-oil nanoemulsion droplets as nanoreactors and further photopolymerized under UV. Particular attention was paid to the preparation of a stable nanoemulsion template with a homogeneous droplet size. Upon UV irradiation of the emulsion containing HA-MA, crosslinked HA-MA particles with a well-defined size were obtained. Moreover, by varying the photopolymerization conditions, i.e. the number of received photons, we could control the conversion rate of the polymerization, as proved by 1H-NMR. Nanogels with controlled cross-linking densities were thus obtained. Not only could their crosslinking densities be controlled by the number of incident photons, but also by the degree of methacrylation (DM) of HA-MA derivatives. In addition, the swelling properties of the nanogels depended on external factors, showing their pH and ionic strength responsiveness. We show that these structures were highly biocompatible, stable under storage and enzymatically biodegradable, which opens the route for their application as drug delivery systems. Finally, insulin was loaded in the nanogels and its pH-dependent release was demonstrated. This versatile method of nanogel preparation, which can be applied to every type of hydrophilic precursor, offers a potential synthetic route to design other types of fully biocompatible drug delivery systems.

Design of biomimetic cell-interactive substrates using hyaluronic acid hydrogels with tunable mechanical properties.

Hyaluronic acid (HA) is a natural polysaccharide abundant in biological tissues with excellent potential for constructing synthetic extracellular matrix analogues. In this work, we established a simple and dependable approach to prepare hyaluronic acid-based hydrogels with controlled stiffness and cell recognition properties for use as cell-interactive substrates. This approach relied on a new procedure for the synthesis of methacrylate-modified HA macromers (HA-MA) and, on photorheometry allowing real time monitoring of gelation during photopolymerization. We showed in this way the ability to obtain gels that encompass the range of physiologically relevant elastic moduli while still maintaining the recognition properties of HA by specific cell surface receptors. These hydrogels were prepared from HA macromers having a degree of methacrylation <0.5, which allows to minimize compromising effects on the binding affinity of HA to its cell receptors due to high substitution on the one hand, and to achieve nearly 100% conversion of the methacrylate groups on the other. When the HA hydrogels were immobilized on glass substrates, it was observed that the attachment and the spreading of a variety of mammalian cells rely on CD44 and its coreceptor RHAMM. The attachment and spreading were also shown to be modulated by the elastic properties of the HA matrix. All together, these results highlight the biological potential of these HA hydrogel systems and the needs of controlling their chemical and physical properties for applications in cell culture and tissue engineering.