Detachable hyaluronic acid tattoo microneedles of various colorants

Here, we used a detachable hyaluronic acid (HA) microneedle platform to deposit a diverse set of colorants into the skin. Microneedle patches with 1000 μm needle length were fabricated from high-molecular-weight HA (2000 kDa) and loaded with fifteen colorants, including three black pigments, three conventional synthetic pigments, three Food, Drug, and Cosmetic (FD&C) dyes, and six natural colorants. All formulations demonstrated sufficient mechanical strength, withstanding compressive forces of 55–100 N, with needle displacement values of 0.005–0.01 mm N⁻¹, enabling reliable insertion and complete needle transfer within 1–2 min. When tested on fresh ex vivo porcine skin, these 1000 μm tattoo microneedles could be effectively detached from the base within 2 min and deposited into the skin, reaching depths of approximately 500–700 μm from the stratum corneum surface. Comparative assessments using CIELAB color parameters obtained from stereomicroscopic images of the tattoo patterns on the skin surface and cross-sectional skin samples revealed that carbon black and conventional red and blue tattoo pigments produced the most vivid and spatially stable patterns, with minimal lateral or vertical diffusion observed over a 7-day period. In contrast, molecular dyes exhibited broader diffusion, as quantified by the CIELAB color parameters. Furthermore, cross-linking the HA matrix with BDDE at 5.313 mmol per 100 mol of HA significantly restricted the diffusion of hydrophobic dyes such as curcumin. These findings provide specific design guidelines for tailoring microneedle systems for various applications.