Surface-initiated self-healing of polymers in aqueous media

Abstract

Polymeric materials that intrinsically heal at damage sites under wet or moist conditions are urgently needed for biomedical and environmental applications [1][2][3][4][5][6] . Although hydrogels with self-mending properties have been engineered by means of mussel-inspired metal-chelating catechol-functionalized polymer networks 7-10 , biological self-healing in wet conditions, as occurs in self-assembled holdfast proteins in mussels and other marine organisms 11,12 , is generally thought to involve more than reversible metal chelates. Here we demonstrate self-mending in metal-free water of synthetic polyacrylate and polymethacrylate materials that are surface-functionalized with mussel-inspired catechols. Wet self-mending of scission in these polymers is initiated and accelerated by hydrogen bonding between interfacial catechol moieties, and consolidated by the recruitment of other non-covalent interactions contributed by subsurface moieties. The repaired and pristine samples show similar mechanical properties, suggesting that the triggering of complete self-healing is enabled underwater by the formation of extensive catechol-mediated interfacial hydrogen bonds.All polymeric materials suffer damage in the course of their functional lifetimes. Few, if any, completely heal at damage sites. Despite recent progress in the design of self-mending polymeric materials based on crack-activated crosslinking 1 , light 2 , heat 3 or other external stimuli 4 , these remain less than perfectly healed, and, in the case of polymers in wet environments, self-healing technologies are even more limited than those engineered for dry conditions. Mussel adhesive holdfasts exhibit significant self-healing capabilities 11,12 , although the molecular mechanisms involved are poorly understood. Notwithstanding this, the selfmending adhesion and cohesion of isolated dopa (3,4-dihydroxyphenyl-L-alanine)-containing adhesive proteins were shown to rely critically on maintaining dopa in an acidic and reducing environment 13,14 . Significantly different conditions are required to recapitulate the self-healing cohesion of tris-dopa-Fe 3+ -mediated complexes in proteins and polymers [7][8][9]15 . Such results increasingly suggest the importance of dopa, but also its subtle and diverse interfacial reactivity vis-à-vis the traditional and still widely held view that dopa, and catechols generally, function primarily as crosslinkers after their 2-electron oxidation to quinones 16 . To better assess the contribution of catechol to polymer selfhealing in a reducing (pH 3), metal-free wet environment, we prepared a material from common, water-insoluble synthetic acrylic polymers having a catechol-functionalized surface. These materials are completely self-healing in a process initiated by catecholmediated interfacial hydrogen bonding, and consolidated by followup interactions (for example, hydrophobic and steric) after a brief compression (∼6 × 10 4 Pa). The crucial and robust roles played by

Mussel adhesion – essential footwork

“…4B): (1) bidentate intercatechol H-bonding is robust in aqueous solution (Ahn et al, 2014); (2) cation-π interactions between Lys and Dopa, Phe, Tyr or Trp, which are surprisingly cohesive in the Mfps (Lu et al, 2013a); (3) Ca 2+ salt bridges between paired pSer residues (i.e. electrostatic interactions), which were first noted in P. californica cement but are reckoned to be more widespread (Zhao et al, 2005;Ashton et al, 2011); and (4) hydrophobic interactions .…”

“…Some of the H-bonds, cation-π and electrostatic interactions, and coordination complexes that exist between Mfps at acidic pH are different at pH 8. In particular, the H-bonds between Dopa residues are surprisingly robust and can, in the absence of oxidation, maintain noncovalent cohesion across a range of pH (Ahn et al, 2014). With >15 mol% lysine and Dopa in many Mfps, cation-π interactions are well-supported as participants in Mfp-Mfp cohesion (Lu et al, 2013a), but these are unlikely to change significantly between pH 2 and pH 8.…”

Recent Progress of Mussel‐Inspired Underwater Adhesives

“…[19,48] A surface-initiated self-healing hydrogel was fabricated by synthetic polyacrylate and polymethacrylate materials surface-functionalized with mussel-inspired catechols (Figure 8). [49] When bisecting the synthesized polymer rods with a clean razor blade, the silyl-protecting groups of the catechols at the polymer surface were conveniently removed at low pH (pH＝3 buffer) to form phenolic-OH groups, thus realizing in-situ self-mending. The repaired and pristine samples show similar mechanical properties, suggesting that the triggering of complete self-healing is enabled underwater by the formation of extensive catechol-mediated interfacial hydrogen bonds.…”

“…The repaired and pristine samples show similar mechanical properties, suggesting that the triggering of complete self-healing is enabled underwater by the formation of extensive catechol-mediated interfacial hydrogen bonds. [49] Barrett et al [50] fabricated mechanically tough and zero-or negative-swelling mussel-inspired surgical adhesives on account of catechol-modified amphiphilic poly(propylene oxide)-poly(ethylene oxide) block copolymers (PPO-PEO). Catechol oxidation occurred to form chemically cross-linked network rapidly at or below room temperature and a thermal hydrophobic transition of PPO provided volumetric reduction as temperature rose.…”

Self-Healing Hydrogels as Biomedical Scaffolds for Cell, Gene and Drug Delivery

“…The catechol moiety is capable of forming strong complexes with metal ions. The structure provides locus for hydrogen bonding, and can be consolidated by follow up interactions (for example, hydrophobic and steric) [66] . With DOPA or catechol analogues (for example, dopamine) has the very good application prospects in embryonic membrane seal, Achilles tendon repair, wound suture technology, cell and drug delivery.…”