Biomedical Engineering Reference
In-Depth Information
Fig. 15.5.
Left
: histological section of a rabbit cornea after laser welding on post-
operative day 30 (Hematoxillin and Eosin,
×
50); the architecture of the cornea
regained an almost physiological appearance (the original cut is indicated by the
dashed line
).
Right
: histological section of a rabbit cornea after conventional stay
suturing on postoperative day 30; the cut is still clearly detectable and large lacunae
are evident in the corneal stroma
15.2.2 Combing Femtosecond Laser Microsculpturing
of the Cornea with Laser Welding
In recent decades, femtosecond (FS) lasers have been developed, tested, and
optimized for miniinvasive intratissue surgery and cell manipulation [62-64].
In particular, eye tissue characteristics are well suited to FS lasers applica-
tions, as they provide precise intrastromal cuts that are the basis of the new
corneal and refractive surgery techniques. With the term FS lasers, we indi-
cate lasers that emit pulses in the near-infrared spectral region with durations
ranging between a few femtoseconds and hundreds of femtoseconds. With the
use of these ultra-short pulses, it is possible to induce photo-disruption inside
semitransparent and transparent media, such as corneal tissue. To induce this
effect, the laser beam is focused in a focal region of a few micrometers in diam-
eter. High intensities
10
13
Wcm
−
2
are achieved, which induce multiphoton
nonlinear absorption, followed by plasma formation. The laser-induced plasma
expands rapidly with high pressure (GPa), and this is followed by development
and further collapse of cavitation bubbles, accompanied by formation of de-
structive shock waves. To obtain a highly-localized destructive effect without
significant collateral damage, small bubble diameters and low photomechani-
cal effects in the surroundings are required. These features are satisfied in FS
laser-induced photo-disruption, first because initiation of nonlinear absorp-
tion of laser radiation requires tight focusing of the light, which assures effect
confinement in the central portion of the focus volume (where really high
∼
