Deep subsurface cavities in skin utilizing mechanical optical clearing and femtosecond laser ablation

High precision subsurface ablation can be produced in transparent materials using femtosecond laser pulses and multiphoton absorption. Light scattering limits application of the same technique to most biological tissues. Previously, subsurface ablation was demonstrated at superficial depths (50-250 µm) in highly scattering tissues including murine skin and human sclera. We report application of mechanical optical clearing to produce deeper subsurface femtosecond ablation in rodent skin. Ability to target deeper structures in skin using subsurface ablation may allow novel clinical applications for dermatological laser surgery.

Operation of a prototype tissue optical clearing device (TOCD) was verified with white light photography in ex vivo rodent skin. A focused femtosecond beam transmitted through the TOCD and was scanned across rodent skin to produce subsurface ablation at increasing focal depths. Histological sections with H&E staining of the laser irradiated rodent skin were examined for subsurface ablation features following laser irradiation.

Subsurface cavities were observed as deep as 1.7 mm below the skin surface in histological tissue sections. Diameter of subsurface cavities varied from tens of microns to over 100 μm. Subsurface cavities produced by scanning the focused femtosecond beam were contiguous and formed a continuous cut. Mechanical disruption of the overlying tissues was not observed.

Mechanical optical clearing can be applied directly to in situ rodent skin and produces an optical clearing effect. High precision subsurface ablation can be produced at positions substantially deeper than previously demonstrated. Future studies may be targeted in in vivo human skin to investigate potential clinical applications of subsurface femtosecond ablation using mechanical optical clearing.