PEG-Based Photo-Cross-Linked Networks with Adjustable Topologies and Mechanical Properties.

We report the synthesis of networks having adjustable topologies and mechanical properties. Our approach consists of photopolymerizing poly(ethylene glycol) diacrylates (PEG-DA) in the presence of mixtures of mono- and multifunctional thiols. We show that the introduction of monothiols as non-cross-linking transfer agents provides a simple way to tune the topology of the networks and produce soft extensible networks. In a systematic study with model short PEG-DA (Mn = 700 g·mol-1), we explored how the gel point and network properties, such as the swelling ratio, the soluble fraction, the viscoelastic moduli, and the ultimate stress and strain, can be adjusted by varying the ratio of thiol to acrylate functions and the average functionality of the thiol mixture. We applied this strategy to longer chains of PEG-DA (Mn = 2300 and 3200 g·mol-1) and varied the viscoelastic and tensile responses of these networks to optimize their adhesive performance. This simple and robust approach further enriches the toolbox of thiol-acrylate polymerization and expands the application scope of PEG-based hydrogels.

Coatings of hydroxyapatite-bioactive glass microparticles for adhesion to biological tissues.

Adsorption of particles across interfaces has been proposed as a way to create adhesion between hydrogels and biological tissues. Here, we explore how this particle bridging approach can be applied to attach a soft polymer substrate to biological tissues, using bioresorbable and nanostructured hydroxyapatite-bioactive glass microparticles. For this, microparticles of aggregated flower-like hydroxyapatite and bioactive glass (HA-BG) were synthesized via a bioinspired route. A deposition technique using suspension spreading was developed to tune the coverage of HA-BG coatings at the surface of weakly cross-linked poly(beta-thioester) films. By varying the concentration of the deposited suspensions, we produced coatings having surface coverages ranging from 4% to 100% and coating densities ranging from 0.02 to 1.0 mg cm-2. The progressive dissolution of these coatings within 21 days in phosphate-buffered saline was followed by SEM. Ex vivo peeling experiments on pig liver capsules demonstrated that HA-BG coatings produce an up-to-two-fold increase in adhesion energy (9.8 ± 1.5 J m-2) as compared to the uncoated film (4.6 ± 0.8 J m-2). Adhesion energy was found to increase with increasing coating density until a maximum at 0.2 mg cm-2, well below full surface coverage, and then it decreased for larger coating densities. Using microscopy observations during and after peeling, we show that this maximum in adhesion corresponds to the appearance of particle stacks, which are easily separated and transferred onto the tissue. Such bioresorbable HA-BG coatings give the possibility of combining particle bridging with the storage and release of active compounds, therefore offering opportunities to design functional bioadhesive surfaces.

Hydrogel-Tissue Adhesion Using Blood Coagulation Induced by Silica Nanoparticle Coatings.

The fixation of hydrogels to biological tissues is a major challenge conditioning the development of implants and surgical techniques. Here, coatings of procoagulant nanoparticles are devised which use the presence of blood to create adhesion between hydrogels and soft internal organs. Those nanostructured coatings are simply adsorbed at the hydrogel surfaces and can rapidly activate the formation of an interfacial blood clot acting as an adhesive joint. This concept is demonstrated on pig liver capsules with model poly(ethylene-glycol) membranes that are intrinsically poorly adhesive. In the absence of blood, ex vivo peeling tests show that coatings with aggregates of bare silica nanoparticles induce a 2- to 4-fold increase in adhesion energy as compared to the uncoated membrane (3 ± 2 J m-2). This effect is found to scale with the specific surface area of the coating. The highest adhesion energies produced by these nanoparticle-coated membranes (10 ± 5 J m-2) approach the value obtained with cyanoacrylate glue (33 ± 11 J m-2) for which tearing of the tissue is observed. Ex vivo pull-off tests show an adhesion strength of coated membranes around 5 ± 1 kPa, which is significantly reduced when operating in vivo (1.0 ± 0.5 kPa). Nevertheless, when blood is introduced at the interface, the in vivo adhesion strength can be improved remarkably with silica coatings, reaching 4 ± 2 kPa after 40 min contact. In addition, these silica-coated membranes can seal and stop the bleeding produced by liver biopsies very rapidly (<30 s). Such a combination of coagulation and particle bridging opens promising routes for better biointegrated hydrogel implants and improved surgical adhesives, hemostats, and sealants.