Biomedical Engineering Reference
In-Depth Information
process leads to rapid and efficient plasma generation at the focal point of the
beam, thereby providing only a few nanojoules for subcellular structure abla-
tion. Damage from this technique depends on the pulse intensity, total number
of pulses, and repetition rate of the laser but the wavelength has very minimal
effect (see
Table 6.1
).
64
Disection effect on the tissue depends upon the density
of free electrons.
65
One of the applications of nanosurgery is for analysis of cell regulation.
16
At this molecular-level ablation process, there is a need to use low-repetition rate,
low-energy femtosecond laser pulses. Using tightly focused pulses beneath the
cell membrane, cellular material inside the cell is ablated through nonlinear
processes. Using this technique, Mazur's group
16
selectively removed sub-
micrometer regions of the cytoskeleton and individual mitochondria without
altering neighboring structures or compromising cell viability. This nanoscissor
technique enables a noninvasive manipulation of the living cells with several-
hundred-nanometer resolution. This technique was used to demonstrate that the
mitochondria are structurally independent functional units.
The distribution of contractile stresses across the extracellular matrix
(ECM) in a cell in a spatially heterogeneous fashion underlies many cellular
behaviors such as motility and tissue assembly.
17
Tanner et al.
17
investigated
the biophysical basis of this process using femtosecond laser nanosurgery.
They used nanosurgery to measure the viscoelastic recoil and the contributions
of cell shape of contractile stress fibers (SFs,
Figure 6.1
). Right after laser
nanosurgery and recoil, myosin light-chain kinase-dependent SFs along the
cell periphery displayed less effective elasticities with severing of peripheral
TABLE 6.1 Sources of Damage during Ultrafast Laser Surgery
64
Photochemical
damage
Thermoelastic stress
confinement
Plasma-mediated
ablation
Parameter
Intensity
threshold
0.26 × 10
12
W/cm
2
5.1 × 10
12
W/cm
2
6.54 × 10
12
W/cm
2
Electron
density at
threshold
2.1 × 10
23
/cm
3
0.24 × 10
21
/cm
3
1.0 × 10
21
/cm
3
One free electron
in the focal volume
Induced thermal stress
overcomes tensile
Critical electron
density for optical
breakdown
Description
of damage
Free electrons
cause formation
of reactive oxygen
species that break
chemical bonds
Thermalization of the
plasma occurs faster
than accoustic relaxation
time; hence, confine-
ment of thermal stress
leads to nanoscale
transient bubbles
Damage caused by
high-pressure and
high temperature
plasma plus the
accompanying
wave and cavitation
bubble
