Biomedical Engineering Reference
In-Depth Information
other advantages over continuous wave (cw) illumination. At a given mean
power, a pulsed laser has a higher peak power than a cw laser, the peak power
rising as the pulsewidth decreases. Where focused laser light meets a living
cell, the mean power of the laser is a measure of how much overall power the
cell will be subjected to. We can increase the peak or instantaneous pulse
power while retaining the same mean power by decreasing the pulsewidth.
Femtosecond (fs) lasers may have pulsewidths smaller than 80
10
−
15
s.
Focused femtosecond pulses transfer their energy to a cell on a time scale that
is faster than the time taken for heat to thermally diffuse out of the focal
volume [33], effectively confining the absorbed energy to the illumination
zone.
×
1.5.2
Laser-Induced Disruption in Biomaterials
If sucient energy is delivered to the sample, an irreversible transition occurs.
Material inside the focal volume can be completely ionized to form plasma,
and although some residual material remains, the focal volume is effectively
destroyed. If the laser is focused at the surface of the sample, the high local
temperature causes some of the material to be ejected at high speed. If, ho-
wever, the focus is beneath the surface of the sample, material is not ejected,
but still undergoes a transition, and the contents of the focal volume are
destroyed.
These effects can be observed at peak power densities on the order of
terawatts/cm
2
and exhibit fairly consistent threshold behavior in the depen-
dence of damage on peak laser intensity. Laser ablation thresholds have been
extensively studied in a variety of materials. However, biological materials
are particularly dicult to characterize due to their wide variation in com-
position.
1.5.3
Cell Nanosurgery by Focused Light
Laser-induced disruption can be used as a tool to dissect or modify parts of a
cell by focusing the laser in the region to be modified. Figure 1.30 shows inci-
sions made in the surface of red blood cells using 800 nm 140 fs illumination
at a mean power of 30 mW.
The damage threshold is higher in some areas of cells than in others.
It also increases with depth beneath the cell surface: however, multi-photon
absorption tends to minimize this effect. For characterization purposes, we
measured the dependence of the damage threshold in dye-stained collagen
on the depth beneath the sample surface [36]. Figure 1.31 shows the relation
between the exposure time required to cause damage and the incident pulse
energy (or peak power) for two different depths in a collagen sample. Alt-
hough the damage threshold changes with depth, the functional relationship
is the same, allowing well-controlled micro- or nanoscale manipulation within
a cell.
