An Epidermis-like Hierarchical Smart Coating with a Hardness of Tooth Enamel

Abstract

We overcome the fundamental dilemma in achieving hard materials with self-healing capability by integrating an epidermis-like hierarchical stratified structure with attractive mechanical and barrier properties of graphene oxide and show that such biomimetic design enables a smart hierarchical coating system with a synergetic healing effect and a record-high stiffness (31.4 ± 1.8 GPa)/hardness (2.27 ± 0.09 GPa) among all self-healable polymeric films even comparable to that of tooth enamel. A quasi-linear layer-by-layer (LBL) film with constituent graphene oxide is deposited on top of an exponential LBL counterpart as a protective hard layer, forming a hierarchical stratified assembly mimicking the structure of epidermis. The hybrid multilayers can achieve a complete restoration after scratching thanks to the mutual benefit: The soft underneath cushion can provide additional polymers to assist the recovery of the outer hard layer, which in turn can be a sealing barrier promoting the self-healing of the soft layer during stimulated polymer diffusion. The presenting hybridization mode of LBL assembly represents a promising tool for integrating seemingly contradictory properties in artificial materials with potential performances surpassing those in nature.

Scalable Synthesis of Multifunctional Epidermis‐Like Smart Coatings

“…Compared with other 2D fillers, MTM nanosheets have many advantages such as high modulus, low cost, insulating characteristic, and fire‐retardant properties . In particular, different from graphene oxide (GO) we used before in epidermis‐like smart coatings, MTM nanosheets can be obtained simply by dispersing clay powders into water without harsh oxidation treatment, forming few‐layer nanosheets with a thickness of ≈2 nm (Figure b and its inset). The dope solutions for the top layer containing different amounts of MTM nanosheets are obtained by adding certain amounts of PVA‐TA solutions (160 mg mL −1 ) into the MTM nanosheet dispersion (5 mg mL −1 ).…”

“…The transition from LBL assembly previously used for making epidermis‐like smart coatings to DBC is however not straightforward. LBL assembly involves repetitive dipping in polyvinyl alcohol (PVA) and tannic acid (TA) solutions to establish film growth through hydrogen bonding and the solution pH is controlled at 2 to maximize the interactions . Instead, for DBC, PVA and TA need to be premixed to form a uniform PVA‐TA solution.…”

“…The thickness of each layer can be facilely increased by applying additional coating and drying processes. The total time spent during DBC is typically less than one hour, much efficient than LBL assembly . Despite the largely improved production efficiency, the epidermis‐like design characteristic of the brick‐and‐mortar structure at the top and smooth cross‐section at the bottom can be achieved with a seamless interface (Figure d,e), similar to LBL assembly .…”

“…To maintain the high flowability during self‐healing, these smart coatings are typically soft with sacrificed robustness . We claim that such fundamental dilemma between hardness and healing ability could be solved by creating a stratified structure using layer‐by‐layer (LBL) assembly . The resultant smart coating is an epidermis‐like bilayer with a brick‐and‐mortar structure on the top of a sublayer supplying healing agents.…”

“…The resultant smart coating is an epidermis‐like bilayer with a brick‐and‐mortar structure on the top of a sublayer supplying healing agents. Such design enables the combination of hardness comparable to tooth enamel with complete self‐healing for several cycles of scratching . However, despite the molecular‐level control over film architecture, LBL assembly is typically energy‐intensive and only suitable for small‐scale operations.…”