Evaluation of Titanium Dioxide Nanoparticle Suspensions as a Low-Cost Surface Coating to Improve Optical Profilometry of Transparent 3D-Printed Microdevices

Abstract

Improved resolution of stereolithography (SLA) 3D printers is accelerating the rapid prototyping of microdevices and has highlighted the need to evaluate their dimensional accuracy. Optical profilometry using structured light allows for rapid 3D scanning of devices with micrometer resolution but requires part surfaces with sufficient opacity and reflectivity for accurate measurement. Microfluidic devices are often made with transparent materials (e.g., clear SLA resins, PDMS, and glass), which poorly reflect the projected light, making them difficult to optically measure. To address the poor reflectivity of transparent objects, a low-cost titanium dioxide (TiO₂) nanoparticle suspension was formulated to coat and opacify the surface of an object using a simple handheld airbrush. PDMS microdevices were cast from SLA printed molds to evaluate part geometry accuracy, surface roughness, and coating thickness between varying concentrations of the custom TiO₂ spray, as well as commercially available 3D scanning sprays. TiO₂ suspensions of 10 and 100 mg/mL in ethanol permitted accurate interrogation of parts of the features, yielding comparable results to commercial treatments. The performance of the treatments on different surface materials and channel designs was analyzed based on their intrinsic properties (roughness, thickness, and carrier solvent). The lower TiO₂ concentration was preferable for microdevices with constricted features due to its lower coating thickness, while the higher concentration was favored for features with smaller z-heights due to its lower coating roughness, highlighting the need for tunable coating formulations. Cost, ease of use, and customization of the surface treatments were compared. The commercial treatments, in both the aerosol canister and microemulsion formats, were more time-effective due to minimal setup and cleaning requirements, whereas the custom TiO₂ coatings were more cost-effective and customizable due to tunable properties and known composition.