Effect of Nitro-Functionalization on the Cross-Linking and Bioadhesion of Biomimetic Adhesive Moiety

Dopamine mimics the exceptional moisture-resistant adhesive properties of the amino acid, DOPA, found in adhesive proteins secreted by marine mussels. The catechol side chain of dopamine was functionalized with a nitro-group, and the effect of the electron withdrawing group modification on the cross-linking chemistry and bioadhesive properties of the adhesive moiety was evaluated. Both nitrodopamine and dopamine were covalently attached as a terminal group onto an inert, 4-armed poly(ethylene glygol) (PEG-ND and PEG-D, respectively). PEG-ND and PEG-D exhibited different dependence on the concentration of NaIO4 and pH, which affected the curing rate, mechanical properties, and adhesive performance of these biomimetic adhesives differently. PEG-ND cured instantly and its bioadhesive properties were minimally affected by the change in pH (5.7–8) within the physiological range. Under mildly acidic conditions (pH 5.7 and 6.7), PEG-ND outperformed PEG-D in lap shear adhesion testing using wetted pericardium tissues. However, nitrodopamine only formed dimers, which resulted in the formation of loosely cross-linked network and adhesive with reduced cohesive properties. UV–vis spectroscopy further confirmed nitrodopamine's ability for rapid dimer formation. The ability for nitrodopamine to rapidly cure and adhere to biological substrates in an acidic pH make it suitable for designing adhesive biomaterials targeted at tissues that are more acidic (i.e., subcutaneous, dysoxic, or tumor tissues).