Polydopamine is the first adhesive polymer that can functionalize surfaces made of virtually all material chemistries. The material‐independent surface modification properties of polydopamine allow the functionalization of various types of medical and energy devices. However, the mechanism of dopamine polymerization has not yet been clearly demonstrated. Covalent oxidative polymerization via 5,6‐dihydroxyindole (DHI), which is similar to the mechanism for synthetic melanin synthesis, has been the clue. Here, it is reported that a physical, self‐assembled trimer of (dopamine)2/DHI exists in polydopamine, which has been known to be formed only by covalent polymerization. It is also found that the trimeric complex is tightly entrapped within polydopamine and barely escapes from the polydopamine complex. The result explains the previously reported in vitro and in vivo biocompatibility. The study reveals a different perspective of polydopamine formation, where it forms in part by the self‐assembly of dopamine and DHI, providing a new clue toward understanding the structures of catecholamines such as melanin.
Nature has developed materials that are integrated and effective at controlling their properties of adhesiveness and cohesiveness; the chemistry of these materials has been optimized during evolution. For example, a catechol moiety found in the adhesive proteins of marine mussels regulates its properties between adhesion and cohesion, rapidly adapting to environmental conditions. However, in synthetic materials chemistry, introduced chemical moieties are usually monofunctional, either being adhesive or cohesive; typically, this is not effective compared to natural materials. Herein, it is demonstrated that hyaluronic acid‐catechol (HA‐catechol) conjugates can exhibit either adhesiveness, functionalizing the surface of materials, or cohesiveness, building 3D hydrogels. Up to now, catechol‐conjugated polymers have shown to be useful in one of these two functions. The usefulness of the polymer in stem cell engineering is demonstrated. A platform for neural stem cell culture may be prepared, utilizing the adhesive property of HA‐catechol, and hydrogels are fabricated to encapsulate the neural stem cells, utilizing the cohesive property of the HA conjugate. Moreover, the HA‐catechol hydrogels are highly neural stem cell compatible, showing better viability compared to existing methods based on HA hydrogels.
Sandcastle worms, Phragmatopoma Californica (Fewkes), live along the western coast of North America. Individual worms build tubular shells under seawater by gluing together sandgrains and biomineral particles with a multipart, rapid-set, self-initiating adhesive. The glue comprises distinct sets of condensed, oppositely charged polyelectrolytic components—polyphosphates, polysulfates, and polyamines—that are separately granulated and stored at high concentration in distinct cell types. The pre-organized adhesive modules are secreted separately and intact, but rapidly fuse with minimal mixing and expand into a crack-penetrating complex fluid. Within 30 s of secretion into seawater, the fluid adhesive transitions (sets) into a porous solid adhesive joint. The nano- and microporous structure of the foamy solid adhesive contributes to the strength and toughness of the adhesive joint through several mechanisms. A curing agent (catechol oxidase), co-packaged into both types of adhesive granules, covalently crosslinks the adhesive and becomes a structural component of the final adhesive joint. The overall effectiveness of the granulated sandcastle glue is as much a product of the cellular sorting and packaging mechanisms, the transition from fluid to solid following secretion, and its final biphasic porous structure as it is of its composition or any particular amino acid modification.
In
general, mechanical properties and gelation kinetics exhibit
a positive correlation with the amount of gelation reagents used.
Similarly, for catechol-containing hydrogels, which have attracted
significant attention, because of their unique dual properties of
cohesion and adhesion, increased amounts of cross-linking agents,
such as organic oxidants and/or transition metals (Fe3+), result in enhanced mechanical strength and more rapid gelation
kinetics. Here, we report a new metal–ligand cross-linking
chemistry, inspired by mussels and ascidians, that defies the aforementioned
conventional stoichiometric concept. When a small amount of vanadium
is present in the catechol-functionalized polymer solution (i.e.,
[V] ≪ [catechol]), organic radicals are rapidly generated that
trigger the gelation reaction. However, when a large amount of the
ion is added to the same solution (i.e., [V] ≫ [catechol]),
the catechol remains chemically intact by coordination that inhibits
gelation. Thus, a large amount of cross-linking agent is not necessary
to prepare mechanically strong, biocompatible hydrogels using this
system. This new chemistry may provide insight into the biological
roles of vanadium and its interaction with catechol-containing molecules
(i.e., determination of the liquid state versus the solid state).
Excess amounts of vanadium ([V] ≫ [catechol]) coordinate with
catechol, which may result in a liquid state for ascidian blood, whereas
excess amounts of catechol ([V] ≪ [catechol]) generate an organic
radical-mediated chemical reaction, which may result in solid-state
conversion of the mussel byssal threads.
Sticky stuff: A versatile strategy to spatially control gene expressions of mammalian cells is developed. A catecholamine polymer (PEI‐C) is used to functionalize surfaces of adeno‐associated viruses (AAV). Because of the underwater adhesive property of catechol, AAV/PEI‐C hybrid vectors become highly “sticky”, resulting in spatially patterned viral attachment onto substrates by the simple “gene‐vector drawing” technique (see picture).
Antifouling
surfaces have been widely studied for their importance in medical
devices and industry. Antifouling surfaces mostly achieved by methoxy-poly(ethylene
glycol) (mPEG) have shown biomolecular adsorption less than 1 ng/cm2 which was measured by surface analytical tools such as surface
plasmon resonance (SPR) spectroscopy, quartz crystal microbalance
(QCM), or optical waveguide lightmode (OWL) spectroscopy. Herein,
we utilize a single-molecule imaging technique (i.e., an ultimate
resolution) to study antifouling properties of functionalized surfaces.
We found that about 600 immunoglobulin G (IgG) molecules are adsorbed.
This result corresponds to ∼5 pg/cm2 adsorption,
which is far below amount for the detection limit of the conventional
tools. Furthermore, we developed a new antifouling platform that exhibits
improved antifouling performance that shows only 78 IgG molecules
adsorbed (∼0.5 pg/cm2). The antifouling platform
consists of forming 1 nm TiO2 thin layer, on which peptidomimetic
antifouling polymer (PMAP) is robustly anchored. The unprecedented
antifouling performance can potentially revolutionize a variety of
research fields such as single-molecule imaging, medical devices,
biosensors, and others.
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