Michael-Type Addition Reactions for the In Situ Formation of Poly(vinyl alcohol)-Based Hydrogels Mariarosaria Tortora, Francesca Cavalieri, Ester Chiessi, and Gaio Paradossi* Dipartimento di Scienze e Tecnologie Chimiche, Universita ` di Roma Tor Vergata, Via della Ricerca Scientifica, 00133 Roma, Italy Received July 25, 2006; Revised Manuscript Received October 6, 2006 Michael-type addition reactions offer the possibility to obtain in situ formation of polymeric hydrogels in the absence of a radical mechanism for the networking process. We explored such a synthetic route for obtaining a poly(vinyl alcohol) (PVA)-based hydrogel as a potential biomaterial for applications in vitro-retinal replacement surgery. The presence of radicals in the reaction medium can represent a risk for in situ surgical treatment. To circumvent this problem we have applied nucleophilic addition to ad hoc modified PVA macromers. The gel formation has been studied with respect to the timing required in this surgery and in terms of the structural characteristics of the obtained network. Introduction The formulation of new biomaterials displaying tailored features is the main issue in the present scenario of drug delivery, tissue engineering, and substitution. 1 The properties of a polymeric biomaterial can be addressed by suitable chemical modifications of the starting polymer structure and by the choice of the most convenient type of cross-linking reaction. Our activity is focused on the formulation and characterization of new polymeric materials for vitreous replacement, a major demand of vitro-retinal surgery. 2 The requirements of an ideal material for vitreous substitution are dictated not only by the tamponade and shock absorption functions of the material but also by its capability to sustain the delivery of metabolites and drugs. The vitreous body structure is complex despite the fact that most of its weight is made of water. The main polymeric components, collagen and hyaluronate, are responsible for the confinement of water and for the gel-like properties of vitreous by forming a double network 3 of randomly distributed collagen fibers and random-coiled high molecular weight hyaluronate, as demonstrated by transmission electron microscopy. The interaction pattern occurring in this multicomponent scaffold, not yet completely disclosed, is made of ionic bridges and entanglements although the presence of chemical cross-links has been hypothesized. Vitreous replacement has been used as a treatment in tissue reconstructive surgery for several retinal pathologies, systemic diseases, degenerative processes, and trauma caused by me- chanical, chemical, and thermal injuries. Several kinds of materials have been used for the posterior segment of eye replacement including gases, silicone oil, biopolymers, and synthetic polymers, the selection rule being the requirements of transparency, biocompatibility, and permeability to gases and metabolites to assure retina functions. In the design of a vitreous substitute a benchmark is certainly a satisfactory simulation of the viscoelastic behavior of the biomaterial. The viscoelastic properties of the vitreous are a direct consequence of the dual nature of the polymeric con- stituents as the collagen fiber scaffold provides the elastic behavior and the hyaluronate moiety is the damping element for the vitreous shock-absorbing function. 4 The determination of the rheological properties of the vitreous is difficult due to its structural complexity and its deterioration in the preparation of the experiment and during the measure- ment. Experiments on intact bovine vitreous were carried out by measuring the dynamic compression moduli, 4 revealing different viscoelastic behaviors depending on the probed vitreal region. Modern vitro-retinal surgery is oriented toward the formulation of vitreous substitution treatments with reduced invasiveness where the replacement of the vitreous is carried out by injecting the material in the posterior of the eye and in situ triggering of the process leading to the shaping of the mechanical and chemical functionalities of the material. Biopolymers are a natural choice for the design of a vitreous substitute. 5-7 However, they are subjected to chemical and enzymatic hydrolysis with a fast deterioration of the gel properties. Modified biopolymers and synthetic biocompatible polymers have been used in the past. As far as PVA is concerned, in vivo viability tests showed that PVA cross-linked by γ-irradiation in dilute aqueous solution and injected in the vitreal cavity was a suitable material for vitreous substitution. 8 A toxic response was observed with commercial PVA obtained by methanolysis. 9 In the context we have recently formulated a hydrogel based on the in situ cross-linking of methacryloyl-PVA upon irradia- tion that can be envisaged as a potential surgical treatment for vitreous replacement. A possible bias to this synthetic route is a transient and low concentration of free radicals triggered by the photoinitiation of the cross-linking reaction that may damage the vitreal cavity tissues. To circumvent this difficulty, we have devised a synthetic route based on the Michael addition reaction and applied by the Hubbell group to several molecules and polymer functionalities. 10 This approach allows the in situ formation of a PVA-based network, avoiding the drawbacks of the photopolymerization of the methacryloyl-PVA system. Scheme 1 depicts the synthetic route followed for obtaining the polymer network by mixing two macromolecular compo- nents, and it is based on the coupling of (i) an end-capped thiol- PVA macromer with (ii) a methacryloyl derivative of PVA. * Author to whom correspondence should be addressed. E-mail: paradossi@stc.uniroma2.it. 209 Biomacromolecules 2007, 8, 209-214 10.1021/bm0607269 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006