Application of sub-microsecond positive pulses via the
microelectrode in physiological medium (saline) with voltages above 1
kV (and pulse energies >10 μJ) resulted in formation of narrow
plasma streamers
(Fig. 1) . The width of the plasma streamers (∼2μ
m) did not change with increasing pulse voltage and energy, but the
length extended up to ∼200 μm when voltage was increased to 3 kV
and energy to 670 μJ. With further increases in pulse energy, the
streamers multiplied and branched but did not significantly elongate.
Each plasma streamer generated an explosive evaporation of liquid
medium, thereby forming multiple vapor cavities, as can be seen in
Figure 2 . Vapor bubbles growing around each streamer fuse into a spherical
bubble, which expands and then collapses, with its lifetime and maximal
size dependent on the pulse energy.
19 For example,
at a discharge energy of 50 μJ, the bubble reaches a maximal radius
of 0.19 mm 16 μs after the pulse and then collapses during the same
period. To test the cutting characteristics of PEAK, we used several
types of ocular tissue extracted from bovine eyes. The energy
requirements and pulse repetition rate that would cut at a practical
speed varied greatly among different tissues. For example, we
found that to dissect the iris and lens at a rate of 1 mm/sec to a
depth of 100 μm/scan, pulse energies of ∼500 μJ and a repetition
rate of ∼50 Hz were required. On the other hand, bovine retina could
be dissected in vitreous to full depth (∼200 μm) by a single pulse
at an energy level of ∼90 μJ, with only minor damage to the
underlying retinal pigment epithelium cell layer (see
Fig. 3 ). In all these experiments the probe was kept in contact with the
tissue. Histologic examination revealed no signs of
coagulation at the edges of the crater. With pulses of 50μ
J at a repetition rate of 10 Hz, we could cut bovine retina at
a linear rate of 1 mm/sec, while sparing the retinal blood vessels
(Fig. 4) . At a pulse energy of 90 μJ, large retinal blood vessels could also
be incised, which results in bleeding, because PEAK cutting
is not accompanied by coagulation.
Full-depth dissection of rabbit retina in vivo was achieved at a pulse
energy of 17 μJ and repetition rate of 10 Hz, with linear cutting
rate of 0.5 mm/sec. With this energy level, the cut is not visible
clinically but is full-thickness on histology (see
Fig. 5 ). Although retina was fully cut, bleeding from the choroid only
occurred at one point along one of four 4-mm-long cuts. To better
evaluate the ability of PEAK to cut tissue at the microsurgical level,
we examined its action on polyacrylamide gels of various densities. The
transparency of these gels, in contrast to most biological tissue,
facilitates imaging and therefore allowed more detailed illustration
and characterization of the effects of the instrument. We found that
the PEAK’s pattern of dissection depended heavily on the density of
the gel being cut. Additionally, we found that a 10% gel displayed
many of the same cutting properties as the relatively hard tissues in
the eye (such as lens capsule and lens cortex). Application of the PEAK
instrument to lens capsule or cortex or a 10% gel was quite
reproducible and typically produced a cut without ejection of bulk
material. (as seen in
Fig. 6 for the gel). If no forward movement of the instrument occurred,
repetitive pulses were progressively less efficient in deepening the
gel’s fracture zone, and maximal depth was reached after ∼15 pulses
(
Fig. 6 , horizontal sequence). However, the depth of the ruptured zone
increased almost linearly with pulse energy (i.e., from 55 μm
at 27 μJ to 220 μm at 141 μJ on average), as seen in vertical
sequence in
Figure 6 .
Polyacrylamide gel at a 6% concentration shared many of the same
cutting properties as softer ocular tissue. Application of PEAK to 6%
gel or bovine retina resulted in ejection of the fractured material and
formation of a crater. The maximal depth of the crater (saturation
level) was still determined by the pulse energy, as shown in
Figure 7 . Scanning the gel or tissue with forward movement of the probe allowed
for removal of the material layer by layer.