Laser-induced graphitization of polydopamine leads to enhanced mechanical performance while preserving multifunctionality

Polydopamine (PDA) is a simple and versatile conformal coating material that has been proposed for a variety of uses; however in practice its performance is often hindered by poor mechanical properties and high roughness. Here, we show that blue-diode laser annealing dramatically improves mechanical performance and reduces roughness of PDA coatings. Laser-annealed PDA (LAPDA) was shown to be >100-fold more scratch resistant than pristine PDA and even better than hard inorganic substrates, which we attribute to partial graphitization and covalent coupling between PDA subunits during annealing. Moreover, laser annealing provides these benefits while preserving other attractive properties of PDA, as demonstrated by the superior biofouling resistance of antifouling polymer-grafted LAPDA compared to PDA modified with the same polymer. Our work suggests that laser annealing may allow the use of PDA in mechanically demanding applications previously considered inaccessible, without sacrificing the functional versatility that is so characteristic of PDA.

Supplementary Table 3. The atomic percentages of elements composing the pristine PDA and the LAPDA films. Source data are provided as a Source Data file.
Supplementary  volumetrically heated by the blue diode laser. Furthermore, most of the heat is dissipated into the substrate as the thermal penetration depth is much bigger than the film thickness. Therefore, the pristine PDA film was modeled as a simplified two-dimensional layer whereas the quartz substrate was modeled in three-dimensional 4 . Furthermore, all surfaces were modeled as an adiabatic boundary condition, as presented below.
where, ⍴ is the density, Cp is the heat capacity, and T is the temperature at time t, is the thickness of the PDA film, α is absorbance of the PDA, and I is the laser intensity. of the phase change mechanism from amorphous to poly-crystalline graphitization was not included herein. The actual laser profile measured by the knife-edge experiment was utilized for the simulation. As the structure of the PDA is expected to be comparable with that of melanin, thermal and physical properties of melanin (thermal conductivity ~ 0.63 W m^-1 K^-1, density ~ 2000 kg m^-3 and heat capacity ~2500 J kg^-1 K^-1) were alternatively utilized 5 , while properties of quartz were found in literature 6 . substantial effect on the graphitization, but laser powers higher than 1.2 W started inducing partial graphitization. To further put the estimated temperature in perspective, numerical simulations at 0.9 W, 1.2 W, and 1.9 W with 18 mm s^-1 were performed and presented in Supplementary Figure   6b. As a result, the maximum temperature under 0.9 W was estimated at 922 K (below 1000 K), and above 1000 K at 1.2 W, and 1.9 W, which is consonant with the predictions and experimental results of the 50 mm s^-1 annealing. Therefore, when combining all experimental and numerical analysis, it is believed that the main mechanism of the LAPDA process rendered by BLA and leading to partial graphitization is photothermal rather than photochemical. In order to verify the ablation mechanism for PDA NPs, the high-speed recording system with 8000 fps (125 µs acquisition per frame) was utilized to probe in-situ the removal process of NPs under 1.9 W laser power and 50 mm s^-1 scanning speed conditions (Supplementary Figure 11a).

Supplementary
Specifically, the NP marked as A has a size of 3 µm, and the NP indicated as B has a size of 4.5 µm. During the BLA, these aggregated NPs were being ablated as shown in Supplementary Figure   11a. After the processing, NPs in group A were totally delaminated, but a trace of NPs in group B still remained. Through these observations, a lumped capacitance analysis was used to estimate the order of magnitude of the annealing temperature for the PDA NPs during the BLA process.
where, Qabs is the absorption efficiency, G is the cross-section of the NP, qs is the laser fluence, ⍴ is the density, is the Stefan-Boltzmann constant, As and V are the surface area and the volume of the NP, Cp is the heat capacity, and T is the transient temperature at time t. As in Supplementary