Abstract: Magnesium (Mg)-based implants have been clinically proven to fulfill long-term service requirements, but their passive degradation periods remain to be uncontrollable. Herein, we developed a novel near infrared (NIR)-responsive coating on a Mg-Ag-Mn alloy with controllable biodegradation enhanced by air release. The coating exhibits a bi-layered structure, in which the outer layer consists of polycaprolactone (PCL) with the addition of nano-sized polypyrrole (PPy) particles for NIR response, whereas the inner layer is a porous ceramic film produced via plasma electrolytic oxidation (PEO). In particular, the porous structure of PEO film was proposed as a carrier for entrapped air to form the “air bomb”. Without NIR irradiation, the coating possesses a dense and homogeneous microstructure and exhibits excellent long-term durability in saline. Under the NIR irradiation, the PCL resin transforms from a rubbery state to a viscous state promoted by the photothermal action of PPy, while the thermal-expanded air in PEO film escapes from the PCL resin, resulting in macroscopic defects across the coating. This phenomenon leads to a change in the function of Mg alloy from "anti-corrosion" to "biodegradation". This work is expected to provide a new strategy for optimizing the service time of Mg-based implants.

Key words: Near infrared, Air bomb, Controllable biodegradation, Plasma electrolytic oxidation, Mg-Ag-Mn alloy