The resulting jet of two interacting laser-induced cavitation bubbles is optimized and studied as a technique for micro-scale targeting of soft materials. High controllability of double-bubble microjets can make such configurations favorable over single bubbles for applications where risk of ablation or thermal damage should be minimized such as in soft biological structures. In this study, double-bubble jets are directed toward an agar gel-based skin phantom to explore the application of micro-scale injection and toward a soft paraffin to quantify the targeting effectiveness of double-bubble over single-bubble jetting. The sharp elongation during the double-bubble process leads to fast, focused jets reaching average magnitudes of U_jet = 87.6 ± 9.9 m/s. When directed to agar, the penetration length and injected volume increase at ∼250 μm and 5 nl per subsequent jets. Such values are achieved without the use of fabricated micro-nozzles seen in existing needle-free laser injection systems. In soft paraffin, double-bubble jetting produces the same penetration length as single-bubble jetting, but with ∼45% reduction in damage area at a 3× greater target distance. Thus, double-bubble jetting can achieve smaller impact areas and greater target distances, potentially reducing collateral thermal damage and effects of strong shockwave pressures.

Background and Objective Molecules native to tissue that fluoresce upon light excitation can serve as reporters of cellular activity and protein structure. In skin, the fluorescence ascribed to tryptophan is a marker of cellular proliferation, whereas the fluorescence ascribed to cross-links of collagen is a structural marker. In this work, we introduce and demonstrate a simple but robust optical method to image the functional process of epithelialization and the exposed dermal collagen in wound healing of human skin in an organ culture model. Materials and Methods Non-closing non-grafted, partial closing non-grafted, and grafted wounds were created in ex vivo human skin and kept in culture. A wide-field UV fluorescence excitation imaging system was used to visualize epithelialization of the exposed dermis and quantitate wound area, closure, and gap. Histology (H&E staining) was also used to evaluate epithelialization. Results The endogenous fluorescence excitation of cross-links of collagen at 335 nm clearly shows the dermis missing epithelium, while the endogenous fluorescence excitation of tryptophan at 295 nm shows keratinocytes in higher proliferating state. The size of the non-closing wound was 11.4 ± 1.8 mm and remained constant during the observation period, while the partial-close wound reached 65.5 ± 4.9% closure by day 16. Evaluations of wound gaps using fluorescence excitation images and histology images are in agreement. Conclusions We have established a fluorescence imaging method for studying epithelialization processes, evaluating keratinocyte proliferation, and quantitating closure during wound healing of skin in an organ culture model: the dermal fluorescence of pepsin-digestible collagen cross-links can be used to quantitate wound size, closure extents, and gaps; and, the epidermal fluorescence ascribed to tryptophan can be used to monitor and quantitate functional states of epithelialization. UV fluorescence excitation imaging has the potential to become a valuable tool for research, diagnostic and educational purposes on evaluating the healing of wounds. Lasers Surg. Med. 48:678–685, 2016. © 2016 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.