Metallic glass coating for improved needle tattooing performance in reducing trauma: analysis on porcine and pig skins

Tattooing inserts pigment into the dermal skin layer (~2 mm) via thousands of punctures, yet is often not regulated as a medical procedure. Complications such as infections, allergic reactions to pigments, needle-induced trauma, and pigment overload are common and can cause acute injury and chronic issues. Metallic glass (MG) coatings are amorphous multi-component metallic alloys with high strength, low friction, smooth surfaces, and hydrophobicity, making them promising for medical devices. Previous work showed MG coatings can reduce insertion/retraction resistance and needle-induced trauma. This study investigates whether coating tattoo needles with MG can reduce trauma, accelerate healing, and improve tattoo performance (pigment retention and line resolution) using pork skin and live pig models. A Zr-based MG system was evaluated for hydrophobicity across common pigments and compared with Al- and W-based MG, then used to assess pigment spread, tissue adhesion, and in vivo healing outcomes.

Prior reports document tattoo-related complications including bacterial infection and systemic/cutaneous adverse events linked to pigments and needle trauma. Hydrophobic and low-friction coatings on medical instruments can reduce drag and tissue damage. Previous studies on MG-coated medical needles demonstrated 60–70% reductions in insertion/retraction resistance and up to 44% reduction in trauma area in porcine tissue and rubber. Metallic glass coatings on cutting blades and other devices have enhanced performance (sharpness, durability). Pigment fate studies suggest inks can migrate to lymph nodes and liver via the bloodstream, underscoring the importance of minimizing tissue trauma and pigment overload. Building on these findings, the present study applies MG coatings to tattoo needles to evaluate effects on pigment spread, tissue adhesion, and healing.

Needles: 316 stainless-steel #12-gauge (350 µm) round-liner needles (Aerolite, BELLEZA TATTOO AGENCY) in two configurations: 1205RL (5-needle) and 1209RL (9-needle). Coating deposition: Metallic-glass (MG) coatings applied via high-power impulse magnetron sputtering (HiPIMS; Highpulse Bipolar 4002 G2, TRUMPF Hüttinger) under base pressure <9.3×10^-4 Pa and working pressure 5.0×10^-1 Pa; power 1–2.5 kW; target-to-substrate distance 8.68 cm from 6-inch alloy targets; deposition rate 7.1 nm/min; nominal thickness 270 nm. Needles were mounted on a planetary rotation stage for uniform coating. Coating composition (at.%): Zr 62.4, Cu 22.4, Al 9.8, Ni 5.4. Characterization: XRD (BRUKER D8 DISCOVER, Cu Kα) for amorphous structure; SEM (Quanta 3D FEG, FEI in SEM mode) with FIB cross-sectioning to assess coating integrity pre/post tattooing. Hydrophobicity: Contact angle measurements (Phoenix, SEO) using seven tattoo pigments (black, red, blue, green, orange, purple, yellow). Zr-based MG was compared with Al- and W-based MG; hydrophobicity assessed on coated steel sheets; pigments from DYNAMIC COLOR (black) and RADIANT COLORS (others). Tattooing apparatus: Coil-driven tattoo machine (Sidewinder 7, DK ROTARY), stroke length 1.5–3 mm, 50–200 Hz, operated by a professional tattoo artist. Non-animal tests: Pork skin (CAS Taiwan inspected) used with a material testing system (MTS Criterion 42.503) for insertion at 20 mm/s to 3 mm depth. Surface topography characterized by laser confocal microscopy (OLS5000, OLYMPUS). Pigment distribution quantified from photographs via ImageJ (gray-scale line scans; Gaussian fits; FWHM). Needle fouling assessed by SEM and fluorescence microscopy (LEICA DM2000) after DAPI staining to visualize attached biological material. Animal tests: Six 10-week-old pigs (~30 kg) under IACUC-approved protocol (NPUST-109-001). Anesthesia: Zoletil 15 mg/kg intramuscular. Tattoos applied to ventral skin (~25 cm² area) using black, blue, and red pigments; two path patterns used (lines and circles). Excess surface pigment removed for photographs. Post-tattoo evaluations at 2 h, 6 h, 1 d, 2 d, 3 d, and 5 d. Histopathology: After sacrifice, tissue from epidermis to subcutis of black tattoo regions collected, fixed, sectioned at 4 µm, stained with hematoxylin and eosin, and imaged (OLYMPUS DP71). Image analysis performed in ImageJ for pigment concentration (pixel values; 1 pixel ≈1.92 µm). For non-animal tattoo area analysis, 1 pixel ≈25.6 µm was used.

• Zr-based MG coating exhibited higher contact angles than Al- and W-based MG across all tested pigments, indicating superior hydrophobicity and lower pigment adhesion; selected for subsequent experiments. XRD showed a broad hump (2θ = 25–45°) confirming amorphous structure. FIB-SEM revealed coating thickness decreased from ~275 nm to ~250 nm after tattooing a 25 cm² area on live pig skin, demonstrating good adhesion under use.
• Pork skin (non-animal): Confocal microscopy after single insertions showed bare needles caused raised skin due to friction during retraction, especially with 9-needle arrays; MG-coated needles left smooth, flat surfaces with well-defined punctures. After 30 insertions per tattoo across multiple positions (10 sets each of 5-needle and 9-needle; 40 sets total), coated needles left less residual pigment on the surface. ImageJ line-scan FWHM of gray-level intensity profiles (mean±SD; 1 pixel ≈25.6 µm): • 5-needle: Bare 39.4±5.3 pixels vs Coated 21.5±3.0 pixels (45% reduction). • 9-needle: Bare 49.0±3.5 pixels vs Coated 21.3±3.1 pixels (57% reduction). Interpretation: MG-coated needles reduced pigment spread and produced narrower, denser tattoo lines.
• Needle fouling: SEM and DAPI fluorescence showed substantially more biological tissue adhered to bare needles than to MG-coated needles after 30 insertions, for both 5- and 9-needle arrays. More tissue accumulated on 9-needle sets and on posterior needle regions than at the tips, consistent with repeated insertions wiping tips clean. Reduced tissue adhesion supports reduced friction/stickiness of MG-coated needles.
• Live pig tattoos: Photographs (after removing excess pigment) showed bare needles induced obvious skin secretions (indicative of trauma), particularly in circular fill regions and at turning points of lines; MG-coated needles produced minimal visible damage and preserved skin texture. Coated-needle tattoos displayed more continuous color fill without requiring extensive line overlap, consistent with denser pigment deposition per puncture.
• Histopathology: At 2 h, bare-needle tattoos had numerous open puncture wounds (20–75 µm) and wider pigment dispersion; coated-needle tattoos showed few signs of trauma with smaller punctures. By 6 h, bare-needle wounds healed more slowly and exhibited less retained pigment, while coated-needle sites showed better coverage and pigment retention. Insets at 6 h showed inflammatory cell infiltration (mainly neutrophils) with necrotic debris; bare-needle wounds had marked eosinophil infiltration into epidermis and dermal papilla, whereas coated-needle sites had smaller affected epidermal areas. Day 1: bare wounds showed ongoing inflammation; coated wounds showed new skin formation. Day 2: clear wound/epidermis boundary in both; coated scabs already peeling (restoring normal skin). Day 3: bare wounds still had many eosinophils, necrotic cells, and abnormal dermal bleeding (not observed with coated). Day 5: coated wounds fully healed; many bare wounds still scabbed.
• Pigment retention (ImageJ pixel values; 1 pixel ≈1.92 µm): • 2 h: Bare 144,342; Coated 138,561 (bare ~9% higher initially). • 6 h: Bare 54,840 (−62% vs 2 h); Coated 130,984 (~94% of 2 h retained). • 1 day: Bare 44,128; Coated 41,269. • 2 days: Bare 10,900; Coated 93,515. • 3 days: Bare 2,555; Coated 19,152. • 5 days: Bare 2,095; Coated 13,974. Overall, bare-needle tattoos showed continual pigment loss, becoming nearly indistinguishable by day 5, whereas coated-needle tattoos retained substantially more pigment throughout recovery. Reduced trauma with MG-coated needles correlated with higher pigment retention.

The study addressed whether MG coatings can mitigate needle-induced trauma and improve tattooing outcomes. The Zr-based MG’s hydrophobic and low-friction properties translated into reduced tissue drag, less skin deformation during retraction, and decreased tissue adhesion to needles. These mechanical and surface effects led to smaller, cleaner punctures and reduced pigment spread in ex vivo pork skin, yielding narrower, higher-density lines suitable for high-resolution work. In vivo, coated needles caused markedly less acute trauma (fewer secretions, fewer open punctures at 2 h) and accelerated healing (wounds closing within 2 h and fully healed by day 5), while bare needles produced prolonged inflammation and delayed re-epithelialization. Importantly, improved tissue preservation with coated needles was associated with superior pigment retention over time, whereas severe trauma with bare needles coincided with rapid pigment loss (likely via exudate and later clearance from inflamed regions). Thus, MG coatings not only enhance procedural performance (smoother insertions, potential reduction in discomfort and operator perturbations) but also improve medical safety metrics (reduced trauma and inflammation) and aesthetic outcomes (line precision and pigment longevity).

Applying a thin Zr-based metallic-glass coating to tattoo needles significantly reduces needle-induced trauma, decreases pigment spread (by up to 57%), produces narrower, denser lines, and enhances pigment retention in skin. Ex vivo tests showed less tissue adhesion and smaller deformation of skin on retraction, while in vivo pig studies demonstrated fewer open wounds at 2 h, faster healing with full closure by day 5 for coated needles, and less inflammation compared to bare needles. These findings indicate MG-coated needles can improve both the safety and quality of tattooing and may be particularly beneficial for high-resolution applications. Future work could assess long-term pigment fate and durability in clinical (human) settings, evaluate broader needle geometries and pigments, and optimize coating thickness/longevity under extended use.

Limitations were not explicitly discussed, but the experiments were conducted on pork skin and six live pigs with follow-up limited to 5 days. Only two round-liner configurations (5- and 9-needle) and selected pigments (black, blue, red in vivo) were evaluated. The final experiments focused on Zr-based MG; broader comparisons across MG chemistries and long-term clinical outcomes in humans were not assessed. Coating wear under extended use was only indirectly evaluated (thickness decreased ~25 nm after tattooing a 25 cm² area).