| UV radiation is extremely strongly absorbed by most biological molecules in a wavelength band between 200 and
360nm. This feature has recently been exploited to produce well defined, non-necrotic{1} photoablative cuts of very small widths (~50µm) by exposure
to excimer laser wavelengths with short pulses (~10ns) focused on tissue (~10e8W/cm2).
Excimer Laser Series (the photo-ablators):
- ArF (Argon fluoride) at 193nm [6.4eV]{2}
- KrF (Krypton fluoride) at 248nm [5 eV]
- XeCl (Xenon chloride) at 308nm [4 eV]
![[image]](../images/ph_ablat.gif)
The photoablative process (figure 1) consists in a photo-dissociation, (the direct breaking of intra-molecular bonds in
polymeric chains) caused by absorption of incoming photons. Biopolymers such as collagen{3} may dissociate by absorption of a single photon.
The microscopic mechanisms correspond to transitions of a macromolecule AB that is promoted to a repulsive electronic state
that yields photo-products A and B. This can be shown in the mathematical expression:
hv + AB ----> A+B
The strongest UV absorption occurs at 193nm. Ablation of corneal stroma with 193nm radiation seems to produce the most
precise cuts, as narrow as 20µm, without the ragged edges produced at 248nm and other UV laser wavelengths. Hence, the Argon fluoride (193nm) is the
laser of choice for laser RK/PRK correction of myopia.
Footnotes:
- Control of thermal damage is clinically important since it generally produces undesirable biological effects.
- This value corresponds to the Energy per Photon in Electron Volts (eV). Also known as "Damage Potency".
- The chemical structure of collagen is almost identical to that of polymer films used as photo-resists in
semi-conductor processing.
|
