Tuning the Properties of Hydrogel Microspheres by Adding Chemical Cross-linking Functionality to Sodium Alginate

Two novel types of hydrogel microspheres (MS) are presented. First, one-component microspheres (1-comMS) were produced from sodium alginate (Na-alg) equipped with thiolfunctionalized hydroxyl groups. The functionalization pathway included the conversion of Na-alg into tetrabutylammonium alginate, insertion of new carboxyl groups, grafting of α-amine-ω- thiol poly(ethylene glycol), and restoration of the sodium salt. This modification conserves all original carboxyl groups of Na-alg and allows for covalent disulfide bond formation in addition to ionic cross-linking. Second, two-component microspheres (2-comMS) were obtained from a mixture of Na-alg and Na-alg functionalized with cysteamine. This functionalization was achieved by grafting cystamine dihydrochloride on some carboxyl groups followed by the reduction to cysteamine. Using the one-step MS formation process developed for both MS types, very fast ionic gelation with calcium ions conserves the spherical shape of the polymer solution droplets upon extrusion into the gelation bath, while simultaneously occurring slow covalent cross-linking reinforces the hydrogels. The physical properties of both MS types are adjustable by varying the polymer concentration, the degree of grafting, and the mixing ratio. In vitro cell microencapsulation studies confirmed the cytocompatibility of 1-comMS and 2-comMS.

Synthesis strategies to extend the variety of alginate-based hybrid hydrogels for cell microencapsulation

Sandrine Gerber, François Rémi Pierre Noverraz, Solène Marcelle Françoise Marie Passemard, Luca Szabó, Christine Wandrey

The production of hydrogel microspheres (MS) for cell immobilization, maintaining the favorable properties of alginate gels but presenting enhanced performance in terms of in vivo durability and physical properties, is desirable to extend the therapeutic potential of cell transplantation. A novel type of hydrogel MS was produced by straightforward functionalization of sodium alginate (Na-alg) with heterotelechelic poly(ethylene glycol) (PEG) derivatives equipped with either end thiol or 1,2-dithiolane moieties. Activation of the hydroxyl moieties of the alginate backbone in the form of imidazolide intermediate allowed for fast conjugation to PEG oligomers through a covalent carbamate linkage. Evaluation of the modified alginates for the preparation of MS combining fast ionic gelation ability of the alginate carboxylate groups and slow covalentcross-linking provided by the PEG-end functionalities highlighted the influence of the chemical composition of the PEG-grafting units on the physical characteristics of the MS. The mechanical properties of the MS (resistance and shape recovery) and durability of PEG-grafted alginates in physiological environment can be adjusted by varying the nature of the end functionalities and the length of the PEG chains. In vitro cell microencapsulation studies and preliminary in vivo assessment suggested the potential of these hydrogels for cell transplantation applications.

Amer Chemical Soc2017

Development of functionalized alginate-based hydrogels to reduce fibrosis in the transplantation of microencapsulated cells

François Rémi Pierre Noverraz

Nowadays, the development of cell therapy relies mainly on the advances made toward cell microencapsulation which allows to evade the need for immunosuppression during cell transplantation. Among the materials considered for cell encapsulation, hydrogels distinguish themselves by their exceptional properties. Due to its gelling properties in contact with divalent cations, the biopolymer sodium alginate (Na-alg), has been widely studied for the microencapsulation and transplantation of cells. However, several defects notably in permselectivity and durability in vivo is limiting the translation of alginate-based hydrogels in clinical applications. In addition, the transplantation of cells without immunosuppressive treatment have to face adverse host responses such as inflammation and fibrosis. At the moment, several approaches have been considered for the development of an ideal material for cell encapsulation; however many challenges still remain in this domain. Hence, this research project focused on the development of hydrogels in order to improve their properties toward cell encapsulation. The first part of this research focused on the development of new types of one-component hybrid alginate-based hydrogels. These hydrogels combine the gelling properties of Na-alg with covalent crosslinking within the same polymeric structure. In order to do so, a new synthetic approach was developed allowing the functionalization on the hydroxyl position of alginate with heterobifunctional poly(ethylene glycol) (PEG) linkers through carbamate bond. This allowed to maintain the carboxyl moieties available for the formation of electrostatic interactions. Two types of one-component hybrid hydrogel were developed with this method adding either thiols or lipoyl moieties on alginate leading to an improvement of mechanical properties while preserving a good biocompatibility. Yet, transplantation of encapsulated cells in a foreign host body have to face immune response, in particular inflammation and pericapsular fibrosis overgrowth (PFO) which ultimately leads to necrosis of the encapsulated cells and loss of graft functionality. Therefore, the second part of this research project focused on tuning the composition of the polymeric components of the hydrogels with anti-inflammatory agents which would reduce PFO in vivo. Different anti-inflammatory agents were considered for this purpose. The best candidate (ketoprofen) was then PEGylated via either ester or amide bond and grafted on the backbone of alginate to ensure a controlled release at the site of transplantation. By analyzing the fibrotic tissue around the microspheres, it was observed that the incorporation of ketopreofen on the structure of the hydrogel significantly reduced PFO 30 days after transplantation. In summary, Na-alg was functionalized with a new synthetic methodology allowing the incorporation of either functionalities reinforcing the mechanical resistance of the hydrogel or anti-inflammatory compounds improving the biocompatibility of the graft.

EPFL2018

Reversibly Cross-Linking Polymer Brushes Using Interchain Disulfide Bonds

Harm-Anton Klok, Piotr Mocny

The introduction of interchain cross-links in surface-grafted polymer brushes increases the robustness and mechanical properties of these thin polymer films. In most cases, cross-linked polymer brushes contain permanent interchain cross-links. The use of reversible interchain cross-links, in contrast, provides opportunities to dynamically modulate the cross-link density and properties of surface-grafted polymer brushes. This study explores the use of disulfide bonds to reversibly cross-link poly(2-(dimethylamino)ethyl methacrylate) copolymer brushes. These brushes were prepared via surface-initiated atom transfer radical copolymerization of 2-(dimethylamino)ethyl methacrylate and an azide-containing comonomer, 3-azido-2-hydroxypropyl methacrylate, followed by copper(I)-catalyzed azide-alkyne cycloaddition of S-propargyl thioacetate and subsequent deprotection of the thioacetate moieties. Heating the polymer brushes to 60 degrees C under air for 2 h resulted in the formation of disulfide cross-links, which could be reduced to generate the corresponding free thiol groups upon brief exposure to tris(2-carboxyethyl)phosphine hydrochloride. The formation and cleavage of the interchain disulfide cross-links has a profound influence on the swelling and viscoelastic properties of the brushes. Cross-linking leads to a decrease in swelling ratio and a concomitant dehydration and loss in dissipative properties of the brush film. These changes were observed using ellipsometry and quartz crystal microbalance with dissipation monitoring experiments by exposing the polymer brushes to a sequence of successive cross-linking and uncross-linking steps. These experiments indicated that while cross-linking and uncross-linking were fully reversible during the first few cycles, the response of the brush films became less pronounced upon prolonged oxidation/reduction, which was attributed to the oxidation of thiol side-chain functional groups and a concomitant reduction in the cross-link density of the polymer brushes. The results presented in this study show that the incorporation of disulfide interchain cross-links allows access to polymer brush films that can be reversibly cross-linked and uncross-linked over many cycles.