Marine-Inspired Polymers in Medical Adhesion.

Medical adhesives that are strong, easy to apply and biocompatible are promising alternatives to sutures and staples in a large variety of surgical and clinical procedures. Despite progress in the development and regulatory approval of adhesives for use in the clinic, adhesion to wet tissue remains challenging. Marine organisms have evolved a diverse set of highly effective wet adhesive approaches that have inspired the design of new medical adhesives. Here we provide an overview of selected marine animals and their chemical and physical adhesion strategies, the state of clinical translation of adhesives inspired by these organisms, and target applications where marine-inspired adhesives can have a significant impact. We will focus on medical adhesive polymers inspired by mussels, sandcastle worms, and cephalopods, emphasize the history of bioinspired medical adhesives from the peer reviewed and patent literature, and explore future directions including overlooked sources of bioinspiration and materials that exploit multiple bioinspired strategies.

Functional Polymers Through Mechanochemistry.

While coupling mechanical and chemical processes is widespread in living organisms, the idea to harness the mechanically induced dissociation of weak covalent and non-covalent bonds to create artificial materials that respond to mechanical stimulation has only recently gained attention. Here we summarize our activities that mainly revolve around the exploitation of non-covalent interactions in (supramolecular) polymeric materials with the goal to translate mechanical stresses into useful, pre-defined events. Focusing on mechano- chromic polymers that alter their optical absorption or fluorescence properties, several new operating principles, mechanosensitive entities, and materials systems were developed. Such materials are expected to be useful for technical applications that range from the detection of very small forces in biological systems to the monitoring of degradation processes and damage in coatings and structural objects.

Mechano- and Thermoresponsive Photoluminescent Supramolecular Polymer.

Mechanoresponsive luminescent (MRL) materials change their emission color upon application of external forces. Many dyes with MRL behavior are known, but they normally do not display useful mechanical properties. Here, we introduce a new approach to overcome this problem, which relies on combining MRL compounds with the concept of supramolecular polymerization. As a first embodiment, a cyano-substituted oligo(p-phenylenevinylene), whose MRL behavior is associated with different solid-state assemblies, was derivatized with two ureido-4-pyrimidinone groups, which support the formation of a dynamic supramolecular polymer. The new material displays the thermomechanical characteristics of a supramolecular polymer glass, offers three different emission colors in the solid state, and exhibits both MRL and thermoresponsive luminescent behavior.

Mechanochemistry with metallosupramolecular polymers.

The transduction of mechanical force into useful chemical reactions is an emerging design approach to impart soft materials with new functions. Here, we report that mechanochemical transductions can be achieved in metallosupramolecular polymers. We show that both reversible and irreversible reactions are possible and useful to create mechanically responsive materials that display new functions. The metallopolymer studied was a cross-linked network assembled from a europium salt and a telechelic poly(ethylene-co-butylene) with 2,6-bis(1'-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini. The Eu(3+) complexes serve both as mechanically responsive binding motifs and as built-in optical probes that can monitor the extent of (dis)assembly due to their characteristic photoluminescent properties. Indeed, dose-dependent and reversible metal-ligand dissociation occurs upon exposure to ultrasound in solution. The absence of ultrasound-induced dissociation of a low-molecular weight model complex and in-depth studies of temperature effects confirm that the dissociation is indeed the result of mechanical activation. The influence of the strength of the metal-ligand interactions on the mechanically induced dissociation was also explored. Metallopolymers in which the Mebip ligands were substituted with more strongly coordinating dipicolinate (dpa) ligands do not dissociate upon exposure to ultrasound. Finally, we show that mechanochemical transduction in metallosupramolecular polymers is also possible in the solid state. We demonstrate mending of damaged objects through ultrasound as well as mechanochromic behavior based on metal-exchange reactions in metallopolymers imbibed with an auxiliary metal salt.

Supramolecular Cross-Links in Mussel-Inspired Tissue Adhesives.

Here we introduce a tissue-adhesive patch with orthogonal cohesive and adhesive chemistries; supramolecular ureido-4-pyrimidinone (UPy) cross-links provide cohesive strength, and catechols provide mussel-inspired tissue adhesion. In the development of tissue-adhesive biomaterials, prior research has focused on forming strong adhesive interfaces in wet conditions, leaving the use of supramolecular cross-links for cohesive strength underexplored. In developing this adhesive patch, the influence of the comonomers' composition and amphiphilicity on adhesion was investigated by lap shear adhesion to wet tissue. We determined failed lap joints' failure mechanism using catechol-specific Arnow's stain and identified formulations with improved cohesive strength. The adhesive materials were cytocompatible in mammalian cell conditioned media viability studies. We found that using orthogonal motifs to independently control adhesives' cohesive and adhesive strengths resulted in stronger tissue adhesion. The design principles presented here advance the development of wet tissue adhesives and could allow for the future design of biomaterials with desirable stimuli-responsive properties.

Conformal Bacterial Cellulose Coatings as Lubricious Surfaces.

We report a versatile method to form bacterial cellulose coatings through simple dip-coating of 3D objects in suspensions of cellulose-producing bacteria. The adhesion of cellulose-secreting bacteria on objects was promoted through surface roughness and chemistry. Immobilized bacteria secreted highly porous hydrogels with high water content directly from the surface of a variety of materials. The out-of-plane orientation of cellulose fibers present in this coating leads to high mechanical stability and energy dissipation with minimal cellulose concentration. The conformal, biocompatible, and lubricious nature of the in situ grown cellulose surfaces makes the coated 3D objects attractive for biomedical applications.

Nanopatterning of a Stimuli-Responsive Fluorescent Supramolecular Polymer by Thermal Scanning Probe Lithography.

The miniaturization of nanometer-sized multicolor fluorescent features is of continuous significance for counterfeit security features, data storage, and sensors. Recent advances in engineering of stimuli-responsive supramolecular polymeric materials that respond upon exposure to heat or mechanical force by changing their fluorescence characteristics open new opportunities as functional lithographic resists. Here, we demonstrate the patterning of a thermochromic supramolecular material by thermal scanning probe lithography (t-SPL), an emerging nanofabrication technique, which allows for ultrafast indentation with a heated probe, resulting in both fluorescent and topographic nanofeatures. t-SPL indentation reveals a linear relationship between the temperature at which material softening occurs and the indentation force in the range from 200 to 500 nN. The softening temperature decreases as the heating time increases from 4 µs to 1 ms, following time-temperature superposition behavior. Our results herein confirm that the fluorescence contrast, perceivable as a shift from red to green, was obtained by kinetic trapping of the dissociated state due to ultrarapid cooling when the probe is removed. We use t-SPL to create highly customized fluorescence patterns up to 40 × 40 µm2 in size with a spatial resolution of 86 nm and change the pitch size to modify the fluorescence intensity when observed by fluorescence microscopy. As an application, multifaceted security features with nanometer resolution are explored.

Optically responsive supramolecular polymer glasses.

The reversible and dynamic nature of non-covalent interactions between the constituting building blocks renders many supramolecular polymers stimuli-responsive. This was previously exploited to create thermally and optically healable polymers, but it proved challenging to achieve high stiffness and good healability. Here we present a glass-forming supramolecular material that is based on a trifunctional low-molecular-weight monomer ((UPyU)3TMP). Carrying three ureido-4-pyrimidinone (UPy) groups, (UPyU)3TMP forms a dynamic supramolecular polymer network, whose properties are governed by its cross-linked architecture and the large content of the binding motif. This design promotes the formation of a disordered glass, which, in spite of the low molecular weight of the building block, displays typical polymeric behaviour. The material exhibits a high stiffness and offers excellent coating and adhesive properties. On account of reversible dissociation and the formation of a low-viscosity liquid upon irradiation with ultraviolet light, rapid optical healing as well as (de)bonding on demand is possible.

The effect of isotopic substitution on the chirality of a self-assembled helix.

N,N',N″-trialkylbenzene-1,3,5-tricarboxamides (BTAs) self-assemble by means of strong, threefold α-helix-type intermolecular hydrogen bonding into well-defined, helical, one-dimensional columnar aggregates. When a stereogenic centre is introduced into the alkyl side chains of these BTAs, strong Cotton effects are observed in dilute apolar solutions, indicating the preference for one helical conformation over the other. Here, we report the creation of a helical sense preference in self-assembled BTAs by introducing deuterium/hydrogen isotope chirality into the alkyl side chains. We determine the relative stabilities of the left- and right-handed helical conformations of these deuterated supramolecular polymers by performing a conformational analysis. Our findings show that the results of deuterium/hydrogen substitution in BTA-based supramolecular polymers and helical polyisocyanates are very similar, although the formation mechanisms differ. The selectively deuterated BTAs discussed here represent the first example of supramolecular chirality resulting from isotope substitution.