LASER TISSUE WELDING/RAMAN SPECTROSCOPY: Light is the only catalyst in this wound-stitching approach

A number of groups have demonstrated laser tissue welding (LTW)—a technique that aims to make obsolete the practice of stitching wounds with needle and thread. Generally the approaches involve using a protein or dye as an agent to facilitate the tissue bonding. What's different about the work of a team at the Institute for Ultrafast Spectroscopy and Lasers (IUSL) at the City College of New York (CCNY) is that it involves nothing but near infrared (NIR) light. IUSL researchers demonstrated their method of laser-based wound repair first with in-vitro guinea pig tissue and arteries, and later on live piglets to excite water in tissue.1,2

The IUSL team's method works by photoexciting the molecules in the tissue. The researchers first used a tunable Fosterite laser covering 1150 to 1350 nm; the result showed that tensile strength followed the spectral shape of the absorption of water. Water absorption in this NIR spectral range relates to combinations of the three primary vibrational modes of water molecules. Thanks to the rapid transfer of energy from water to collagen fibers in the tissue, the mechanism produces almost invisible scars. The collagen fibers undergo a number of molecular transformations, resulting, in the end, in the creation of hydrogen and covalent bonds.

Click to Enlarge
Two 40X histology images of in-vivo skin samples on a guinea pig after seven postoperative days show a well-opposed surgical incision and no significant inflammatory cell infiltration. However, serial sections of the area "welded" using a 1535 nm femtosecond laser (top left) show almost complete wound healing in the epidermis and granulation tissue in the dermis, while the sutured control (left) serial sections show very minimal wound healing in epidermis and minimal granulation tissue in dermis.

The team reports satisfactory welding on guinea pig aorta and skin tissues using a pulsed-mode fiber laser at 1535 nm. The tensile strength of the welded tissue is very similar to original tissue samples and shows histologically, very little or no evidence of scarring in the welded area. "At the present time, our limiting factor is the speed of the welding, which may be addressed in a variety of ways," says IUSL director Robert Alfano.

The researchers were able to qualitatively assess laser welded tissue on the millimeter (mm) depth scale using Raman spectroscopy, which enables monitoring of changes in key protein and lipid molecules in real time during the LTW process. The Raman spectrum indicates average contributions from different layers of the tissue, and gives an average probe of skin, arteries and aorta.

The Raman approach monitors underlying molecular changes from the degree of thermal damage and protein denaturation before and after laser application—enabling evaluation of changes in the local structure of collagen and elastin and the role of water in the collagen helical structure. The method revealed that "the Amide I peak intensity and height for collagen and gelatin are different," according to researcher Stephane Lubicz. "Changes in corneal collagen due to aging and heat denaturation are characterized by the broadening of the amide I and III bands; conversion of reducible cross-links to non-reducible cross-links in collagen resulted in an increase in the Raman intensity at specific wavelengths."

The team is working with Intuitive Surgical Corp., maker of the Da Vinci surgical system, on possible incorporation of LTW into robotic surgery. Without the addition of foreign agents into the surgical field, the probability of tissue breakdown is low, as is risk of infection. And the key benefits of LTW are: tight seal, less scarring, and fast closure and healing.

1. A. Alimova et al., J. Photochemistry and Photobiology B: Biology 96, 178-183 (2009)
2. J. Tang et al., J. Clinical Laser Medicine and Surg. 18, 117-123 (2000)
3. R. R. Alfano et al., #7, 033,348 B2, Apr. 25, 2006

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