To enable attachment to underwater surfaces, aquatic fauna such as mussels and sandcastle worms utilize the advantages of coacervation to deliver concentrated protein-rich adhesive cocktails in an aqueous environment onto underwater surfaces. Recently, a mussel adhesive protein Mfp-3s, was shown to exhibit a coacervation-based adhesion mechanism. Current synthetic strategies to mimic Mfp-3s often involve complexation of oppositely charged polymers. Such complex coacervates are more sensitive to changes in pH and salt, thereby limiting their utility to narrow ranges of pH and ionic strength. In this study, by taking advantage of the lower critical solution temperature-driven coacervation, we have created mussel foot protein-inspired, tropoelastin-like, bioabsorbable, nonionic, self-coacervating polyesters for the delivery of photo-cross-linkable adhesives underwater and to overcome the challenges of adhesion in wet or underwater environments. We describe the rationale for their design and the underwater adhesive properties of these nonionic adhesives. Compared to previously reported coacervate adhesives, these "charge-free" polyesters coacervate in wide ranges of pH (3−12) and ionic strength (0−1 M NaCl) and rapidly (<300 s) adhere to substrates submerged underwater. The study introduces smart materials that mimic the self-coacervation and environmental stability of Mfp-3s and demonstrate the potential for biological adhesive applications where high water content, salts, and pH changes can be expected.

Three-dimensional (3D) printing offers the unprecedented ability to create medical devices with complex architectures matched to the patient’s anatomy. However, the development of 3D printable synthetic polymers for biomedical applications has been relatively slow. Here, we present the synthesis and characterization of a library of single-component, undiluted, modular multifunctional polyesters for extrusion-based direct-write 3D printing (EDP). The polyesters were synthesized using carbodiimide-mediated polyesterification of pendant functionalized diols and succinic acid and characterized using 1H NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and rheology. The rheology was characterized by using small amplitude oscillatory shear rheology and at steady-state shear flow conditions. The viscoelasticity of the polyesters was characterized by plotting master curves using the time–temperature superposition (TTS) principle, which were then validated by Van Gurp-Palmen and Cole–Cole plots. The 3D printability of the polyesters was assessed on the basis of several key parameters including the ability to extrude as continuous filaments, retain the printed shape, form multilayer constructs, and form bridge-spanning filaments without significant sagging or collapse. The rheological characterization suggests that the polyesters are unentangled melts that facilitate printing at ambient temperatures without the use of external additives or solvents. The presence of supramolecular interactions inducing pendant functional groups forms a temporary, physical cross-link-like network that enables 3D shape retention. The insights from this study will further assist in the design and characterization of 3D printable polymer melts for biomedical applications and standardizing the assessment of polymer 3D printability.

Inspired from the one-component self-coacervation of tropoelastin and mussel foot protein-3s, we created the first non-ionic, single component coacervates that can coacervate in a all ranges of pH (acidic to basic) and wide range of ionic strengths with degradability, rapid curing and strong underwater adhesion. In contrast to the complex coacervates, these ‘charge-free’ coacervates are potential candidates as tissue adhesives and sealants, adhesives for sensor attachment to wet skin, and as sprayable adhesives. Their potential use in the clinic arises from their enhanced stability to changes in external conditions, cytocompatibility, biodegradability and modular nature in incorporating various functional groups and crosslinkers.

Inspired from the one-component self-coacervation of tropoelastin and mussel foot protein-3s, we created the first non-ionic, single component coacervates that can coacervate in a all ranges of pH (acidic to basic) and wide range of ionic strengths with degradability, rapid curing and strong underwater adhesion. In contrast to the complex coacervates, these ‘charge-free’ coacervates are potential candidates as tissue adhesives and sealants, adhesives for sensor attachment to wet skin, and as sprayable adhesives. Their potential use in the clinic arises from their enhanced stability to changes in external conditions, cytocompatibility, biodegradability and modular nature in incorporating various functional groups and crosslinkers.