In this study we investigated the use of a fibrin hydrogel to improve the potential of a polyurethane (PU) scaffold-based system for articular cartilage tissue engineering. PU-only ("no-fibrin") and PU-fibrin ("fibrin") composites were cultured for up to 28 days and analyzed for DNA content, glycosaminoglycan (GAG) content, type II collagen content, GAG release, and gene expression of aggrecan, collagen I, and collagen II. The use of fibrin allowed for higher viable cell-seeding efficiency (10% higher DNA content on day 2 in fibrin versus no-fibrin composites) and more even cell distribution on seeding, a more than 3-fold increase in the percentage of newly synthesized GAG retained in the constructs, and 2- to 6-fold higher levels of type II collagen and aggrecan gene expression through day 14. Addition of aprotinin to the medium inhibited fibrin degradation, most noticeably in the center of the constructs, but had little effect on biochemical composition or gene expression. Short-term mechanical compression (0-10% sinusoidal strain at 0.1 Hz for 1 h, applied twice daily for 3 days) doubled the rate of GAG release from the constructs, but had little effect on gene expression, regardless of the presence of fibrin. Although further work is needed to optimize this system, the addition of fibrin hydrogel to encapsulate cells in the stiff, macroporous PU scaffold is a step forward in our approach to articular cartilage tissue engineering.

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A hydrogel system that is designed to gel due to oxidation by molecular oxygen is described. This is achieved by the use of a branched poly(ethylene glycol) modified with thiol end groups. A stable precursor molecule, starPEG thioacetate, was synthesized by radical addition of thioacetic acid to an intermediate starPEG allyl ether. Rapid deprotection of the thiol can be achieved using a base, e.g. sodium hydroxide, quantitatively liberating a thiol group and a non-toxic acetate ion. This step can be carried out under anaerobic conditions, yielding a solution with known thiol content that can be stored. The reaction with oxygen is accelerated by the use of a catalyst based on Fenton chemistry, which makes the material useful for biomedical applications where in situ polymerization of an injectable material is beneficial. This gelation takes place under near physiological conditions without the need for a cross-linking agent.