Tuning the Interactions in Multiresponsive Complex Coacervate-Based Underwater Adhesives.

In this work, we report the systematic investigation of a multiresponsive complex coacervate-based underwater adhesive, obtained by combining polyelectrolyte domains and thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) units. This material exhibits a transition from liquid to solid but, differently from most reactive glues, is completely held together by non-covalent interactions, i.e., electrostatic and hydrophobic. Because the solidification results in a kinetically trapped morphology, the final mechanical properties strongly depend on the preparation conditions and on the surrounding environment. A systematic study is performed to assess the effect of ionic strength and of PNIPAM content on the thermal, rheological and adhesive properties. This study enables the optimization of polymer composition and environmental conditions for this underwater adhesive system. The best performance with a work of adhesion of 6.5 J/m2 was found for the complex coacervates prepared at high ionic strength (0.75 M NaCl) and at an optimal PNIPAM content around 30% mol/mol. The high ionic strength enables injectability, while the hydrated PNIPAM domains provide additional dissipation, without softening the material so much that it becomes too weak to resist detaching stress.

Thermoresponsive Complex Coacervate-Based Underwater Adhesive.

Sandcastle worms have developed protein-based adhesives, which they use to construct protective tubes from sand grains and shell bits. A key element in the adhesive delivery is the formation of a fluidic complex coacervate phase. After delivery, the adhesive transforms into a solid upon an external trigger. In this work, a fully synthetic in situ setting adhesive based on complex coacervation is reported by mimicking the main features of the sandcastle worm's glue. The adhesive consists of oppositely charged polyelectrolytes grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains and starts out as a fluid complex coacervate that can be injected at room temperature. Upon increasing the temperature above the lower critical solution temperature of PNIPAM, the complex coacervate transitions into a nonflowing hydrogel while preserving its volume-the water content in the material stays constant. The adhesive functions in the presence of water and bonds to different surfaces regardless of their charge. This type of adhesive avoids many of the problems of current underwater adhesives and may be useful to bond biological tissues.

Design and Evaluation of a Structural Reinforced Small Intestinal Submucosa Vascular Graft for Hemodialysis Access in a Porcine Model.

Synthetic vascular access for hemodialysis exhibits biological and mechanical material properties mismatch with the native vessels. These limitations prevent infiltration of endothelial cells and decrease grafts long-term patency, particularly in small diameter vessels. We aimed to design a curved structural reinforced small intestinal submucosa (SIS) vascular graft for hemodialysis access and to evaluate in a porcine animal model graft patency by Doppler ultrasonography, tissue remodeling by histology, and vascular wall Young's modulus after implantation by biaxial tensile test. Curved 4 mm inner diameter, 0.5 mm thickness, and 150 mm length SIS grafts were designed. Small intestinal submucosa vascular grafts were preliminary tested in vivo in a porcine animal model (n=3) constructing an arteriovenous fistula between the carotid artery and the jugular vein; GORE-TEX grafts were implanted as control. Small intestinal submucosa grafts remained patent 46 ± 7 days against the control, 30 ± 3 days. Histology showed thrombus formation on the lumen (80% to 100% surface area) of all explanted grafts. Small intestinal submucosa grafts exhibited neovascularization and endothelial cells alignment on the graft wall, indicating regeneration. Biaxial tensile tests demonstrated no significant differences in Young's moduli between SIS grafts (ECirc = 2.5 ± 1.0 MPa, ELong = 5.7 ± 2.6 MPa) and native artery (ECirc = 1.4 ± 0.8 MPa, ELong = 5.5 ± 1.1 MPa), indicating similar wall stiffness. This study proposes an innovative design of a tissue-engineered vascular graft for hemodialysis access that, besides its structural characteristics similar to those of current synthetic grafts, could enhance biological performance because of its composition.