To the best of our knowledge, the technique optimized and proposed by our research team is the only one laser welding application which has reached the preclinical and clinical phases. It is based on the use of a near infrared diode laser emitting at 810 nm and the topical application of the chromophore ICG , which shows high optical absorption at the laser wavelength emission [12–15]. The procedure consists in a preliminary staining phase with the chromophore, followed by an irradiation phase. ICG has been chosen because of its biocompatibility, which has already favored its exploitation in several biomedical applications. In practice, the chromophore is prepared in the form of an aqueous saturated solution of commercially available Indocyanine Green for biomedical applications (e.g., IC-GREEN Akorn, Buffalo Grove, IL or ICG -Pulsion Medical Systems AG, Germany). This solution is accurately positioned in the tissue area to be welded, using particular care to avoid the staining of surrounding tissues, and thus their accidental absorption of laser light. Then the wound edges are approximated and laser welding is performed under a surgical microscope.

The laser used in preclinical tests and in the clinical applications is typically an AlGaAs diode laser (e.g., Mod. WELD 800 by El.En. SpA, Italy) emitting at 810 nm and equipped with a fiber-optic delivery system.

We proposed two different protocols, to be used in penetrating keratoplasty [12, 13, 16] and in endothelial transplantation [17]. The first one is used in keratoconus patients, in combination with the femtosecond laser to cut donor and recipient tissues.

In penetrating keratoplasty, the technique has been named the continuous wave laser welding (CWLW ). Noncontact, CW diode laser irradiation is used for the welding of corneal wounds, in substitution or in conjunction with traditional suturing procedures. The CWLW procedure developed to weld human corneal tissues in penetrating keratoplasty is as follows. The donor and recipient cornea are trephined by the use of a femtosecond laser . The donor cornea is then applied onto the patient’s eye and secured by 8–16 interrupted stitches or continuous suturing. The ICG chromophore solution is prepared in sterile water (10 % w/w) in the surgery room, soon before its use. The surgeon places a small quantity of chromophore solution inside the corneal cut, using an anterior chamber cannula, in an attempt to stain the walls of the cut in depth. A bubble of air is injected into the anterior chamber prior to the application of the staining solution, so as to avoid perfusion of the dye. A few minutes after the application, the solution is washed out with abundant water. The stained walls of the cut appear greenish, indicating that ICG has been absorbed by the stroma. Lastly, the whole length of the cut is subjected to laser treatment. Laser energy is transferred to the tissue in a noncontact configuration, through a 300-μm core diameter fiber. The fiber is mounted in a handpiece and moved by the surgeon as a pencil. A typical value of the laser power density clinically used is around 10 W/cm², which results in a good welding effect. During irradiation, the fiber tip is kept at a working distance of about 1 mm, and at an angle of 20°–30° with respect to the corneal surface (side irradiation technique). This particular fiber position provides in-depth homogenous irradiation of the wound and prevents accidental irradiation of deeper ocular structures. The fiber tip is continuously moved over the tissue to be welded, with an overall laser irradiation time of about 120 s for a 360°. This procedure has been performed up to now on 300 patients with very satisfactory results [2, 12, 16]. The position of the apposed margins has been found to be stable over time, thus assuring optimal results in terms of postoperatively induced astigmatism after cataract and keratoplasty surgery. The lower number of stitches reduces the incidence of foreign body reactions, thus improving the healing process. Objective observations on treated patients have proved that the laser-welded tissues regain a good morphology (without scar formation) and pristine functionality (clarity and good mechanical load resistance, see Figs. 24.2 and 24.3).

In endothelium keratoplasty, the technique is called Pulsed Laser Welding (PLW) . In this protocol, single laser spots (lasting tens of milliseconds) are delivered to the tissue, resulting in a photothermal effect localized within the spot dimension (a few hundreds of micrometers in diameter): the induced effect is a hard laser welding, consisting in a photocoagulation of the collagen confined at the donor/host interface. The result of the collagen denaturation at the welded site is a strong adhesion between the donor and host tissues, thus providing a suturing effect that is impossible to obtain with standard technique. The tissue regains his natural appearance in a short follow-up (1 month) and the adhesion between donor/host tissue is improved by the welding provided in the very early stage of the healing phase. However, this protocol is not used in keratoconus cases.

The proposed approach has been characterized by the use of different experimental and theoretical studies, such as thermal modeling and microscopic analyses [11–15]. These include traditional methods as optical and fluorescence microscopy which allow for an investigation at the micron scale [18] and transmission and scanning electron microscopy (TEM and SEM, respectively) which are useful when studying nanometric structures [19–21]. Other used techniques are atomic force microscopy (AFM) and second-harmonic generation (SHG) microscopy which provide complementary information [22–24]. These studies have been helpful (although not exhaustive) in elucidating the different dynamics behind the sealing process.