- Photoablation: When electrons in a molecule absorb high-energy UV photons,
the electrons can be raised to such high non-bonding orbitals that the molecule
breaks up immediately. This molecular breakup causes a rapid expansion of the
irradiated tissue volume with an accompanying ejection of tissue molecules. The
tissue is removed in a very clean and precise fashion without the tissue damage
that can arise from thermal effects in coagulation and vaporization processes.
Note that this photoablation process differs from the ablation due to a thermal
vaporization effect. - Plasma-Induced Photoablation: When intense irradiation levels of greater than
1011 W/cm^2 impinge on biological tissue in pulses of less than one nanosecond
duration, molecules are torn apart and form a plasma. As the plasma expands it
ablates tissue material in a clean and precise fashion. One application is in the
treatment of cataracts. - Photodisruption: Similar to plasma-induced photoablation, in photodisruption
very high irradiance levels result in plasma formation. In this case, mechanical
effects such as shock waves, jetting of material, and bubble formation followed
by cavitation, accompany the plasma-creation process. Photodisruption is
widely used for minimally invasive surgery, for breaking up kidney stones or
gallstones, and in ophthalmology for drilling holes in the cornea or lenses.
The following subsections give some details on these light-tissue interaction
categories. Table6.3summarizes their characteristics.
6.5.1 Photobiomodulation
Photobiomodulation[also calledbiostimulationorlow-level light therapy(LLLT)]
refers to the stimulation of biological systems by means of low light levels with the
aim to provide a therapeutic, healing, or cosmetic effect [ 32 – 38 ]. In photobiomod-
ulation, cells or tissue are exposed to low levels of red to near-IR light from
low-power lasers or high-radiance LEDs to either stimulate or (in some occasions) to
inhibit cellular functions. This procedure is used in numerous medical areas to reduce
cell and tissue death, relive acute and chronic pain and inflammation, accelerate
wound healing, treat inflamed and ulcerated oral tissues, alleviate tinnitus and other
nerve injury conditions, and treat edema (swellings), indolent (non-healing) ulcers,
burns, and dermatitis.
Light sources commonly employed for photobiomodulation emit at wavelengths
between 600 and 1070 nm. Normally optical power levels range from 1 to
1000 mW with irradiance levels varying from 1 mW/cm^2 to 5 W/cm^2. The light
sources typically run in a pulsed mode, but sometimes continuous-wave beams are
delivered. Treatment time ranges are 30–60 s per treatment point a few times a
week for up to several weeks. The skin layers that are relevant to photobiomodu-
lation consists of the upper epidermis and lower dermis. These layers are approx-
imately 100– 150 μm and 1500– 3000 μm thick, respectively, and are separated by a
174 6 Light-Tissue Interactions