Introduction: In nature, catecholamine reactions are ubiquitously found and especially are abundant in insects and fruits in which the reactions occur at liquid/air interfaces. For insects, they have an exoskeletal cuticle, forming at in interface between their body fluid and environmental air. Catecholamines such as N-acetyldopamine are actively involved for the solidification reaction also called as tanning processes, which occurs specifically at air/water interfaces. For fruit browning, surface color of fruit body (e.g. apple, banana, and peach) turns gradually out to be brown when peeled or cut. Insects and fruits have catechol or catecholamine derivatives and enzymes (e.g. tyrosinase and laccase), which facilitates oxidation of the aforementioned catecholamines. Dopamine (DA), a representative catecholamine, has been extensively studied in various fields including medical devices, bio-sensors, and batteries. Up to now, the studies have been focused on the reactions at solid/liquid interfaces (i.e. underwater coating). We found that DA film was also formed at liquid/air interface when only DA solution was exposed to air. Herein, we reported the unique properties of an interfacial catecholamine film. The interfacial reaction results in stimuli-responsive, self-healing, and self-regenerating films with micrometer thickness. In addition, the film also has the adhesiveness like mussel foot protein. The edge of the film sticks to the substrates while the film is formed at liquid/air interface. From these properties, the film can be applied to self-healable sealant biomaterials.
Materials and Methods: High molecular weight branched polyethylenimine (b-PEI, 750 kDa, 50% (w/v) solution) was diluted with deionized and distilled water (DDW) to 20 % (w/v) solution and dopamine (37.9 mg, 0.2 mmol) was dissolved in DDW (1mL). b-PEI solution and dopamine solution were mixed with 1 to 1 ratio and then exposed to air without any stimulus including stirring and shaking.
Results and Discussions: We found that a dopamine (DA) film was formed at air/liquid interface when dopamine solution was exposed to air, but DA film was very thin and brittle (Figure 1A, left). To make this brittle film stronger, b-PEI was mixed with dopamine solution (Figure 1A, right). Due to the formation at the interface between water and air, b-PEI/DA film has Janus-faced characters. The air-exposed region is a DA-rich domain because DA is easily to be oxidized and polymerized where oxygen is abundant. This region is a hydrophobic domain and exhibits solid-like morphology, whereas the opposite side is a hydrophilic domain and exhibits porous microchannel structure, which mainly consists of b-PEI (Figure 1B).
Fig. 1. Structure of b-PEI/DA film and it interesting properties; stickiness and self-healing
As shown in Figure 1C, b-PEI/DA film shows a strong in situ adhesiveness. After the b-PEI/DA solution was exposed to air for 2 hour, the edge of the film was stuck to the surface of glass vial as strongly as the film endured the weight of the solution. Another unique property of the film is a self-healing property at liquid/air interface. When parts of the film were removed, the b-PEI/DA solution was re-exposed to air and the cross-linking reaction re-triggered at the damaged region. After 2 hours, new film was regenerated and subsequently bonded to the edge of the pre-existing film (Figure 1D).
Conclusion: b-PEI/DA film inspired by insect cuticle and fruits browning is formed at liquid/air interface and it has unique properties; freestanding, stimuli-responsive, self-healing, and sticky properties. The b-PEI/DA film can be used as a self-healable sealant.