Biomedical Engineering Reference
In-Depth Information
future clinical applications of the laser-assisted procedure are concerned. In
fact, as water, haemoglobin, and melanin are the main absorbers of laser light
within tissue, the heating effect is not selectively limited to a target area, and
all irradiated tissues are heated. For instance, the CO
2
laser has been used for
laser repairs of thin tissues because of its short penetration depth (
<
20
m).
However, for thicker tissues, welding has been achieved only by irradiating
with high laser power and longer exposure times, thus inducing high levels of
heat damage [3]. The emissions of other near-infrared lasers, such as Nd:YAG
and diode lasers, are more suited to the welding of thicker tissues. In any case,
control of the dosimetry of laser irradiation and of corresponding temperature
rise is crucial to minimize the risk of heat damage to the tissue and to generate
strong welds.
Two advances have been useful in addressing the issues associated with
laser tissue welding: the application of laser-wavelength-specific chromophores
and the addition of endogenous and exogenous material to be used as solder.
The use of wavelength-specific chromophores enables differential absorp-
tion between the stained region and the surrounding tissue. The advantage
is primarily a selective absorption of laser radiation by the target, without
the need for a precise focusing of the laser beam. Moreover, lower laser ir-
radiances can be used because of the increased absorption of stained tissues.
Various chromophores have been employed as laser absorbers, including in-
docyanine green (ICG) [4], fluorescein [5], basic fuchsin, and fen 6 [6]. The
use of a near-infrared laser - which is poorly absorbed by biological tissues -
in conjunction with the topical application of a dye with an absorption peak
overlapping the laser emission, is a very popular setting for the laser welding
technique. Diode lasers emitting around 800 nm and ICG have been used in
corneal tissue welding in cataract surgery and corneal transplant [7,8], vascu-
lar tissue welding [9-11], skin welding [4, 12], and in laryngotracheal mucosa
transplant [13].
Laser welding by means of solders, namely “laser soldering,” makes use
of exogenous solders as topical protein preparations. This makes possible a
bonding of the adjoining and underlying tissues when activated by laser light.
The extrinsic agents provide a large surface area over which fusion with the
tissue can occur, thus favoring the approximation of the wound edges that
eventually heal together in the postoperative period. Useful welding materials
include blood [14], plasma [15], fibrinogen [16] and albumin, which is the
one most frequently employed [5, 17]. Several studies have demonstrated that
the addition of an albumin solder to reinforce laser tissue repairs significantly
improves postoperative results [5,18]. Moreover, incorporation into the protein
solder of a laser-absorbing chromophore makes it possible to confine the heat
into the area of solder application, which reduces the extent of collateral heat
damage to adjacent tissues. ICG-doped albumin has become an increasingly
popular choice in the last decade [17]. The laser is used to denature the protein
immediately after application of the protein solder to the wound site, thus
yielding a bond at the solder-tissue interface.
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