How do lasers work on the skin?
Laser light has four properties that distinguishes it from other light sources (like sunlight or the light from an intense pulsed light (IPL) device).
1. Coherence - all the light waves are in step with each other
2. Directionality - all the light waves travel parallel to each other
3. Monochromatic - the light waves in a laser beam are of the same wavelength
4. High intensity - laser light waves can carry a lot of energy
These properties allow the laser to precisely target specific molecules in the skin, delivering higher amounts of energy to that particular target. In contrast, IPL devices shine a bright incoherent light consisting of a much broader range of wavelengths. Thus, lower energies are delivered to the target. However, IPL still has many useful applications in cosmetic medicine.
Laser light interacts with skin in specific ways.
The diagram below identifies the three major forms of interaction (2, 3 and 4) by which laser can treat various skin conditions in cosmetic medicine.
The two predominant forms of laser and light-based treatments are commonly known as ablative and non-ablative.
1. Ablative Laser Treatments
Ablative lasers were the first type of laser used for non-surgical cosmetic procedures and are still used today for the same purpose.
Ablative resurfacing lasers work generally by interaction 2 (photoablation) targeting water molecules in the skin. By removing thin layers of the upper skin (epidermis), layer by layer, it is grossly similar to other peeling techniques such as chemical peels and microdermabrasion, but is more precise and controlled.
So-called laser peels can remove nearly all fine wrinkles while significantly improving deeper, more dynamic wrinkles.
The laser beam gently vaporises micron-thin layers of the epidermis. This effect can also transmit heat to the deeper skin layers to stimulate the production of new collagen in the dermis. Laser peels can remove age spots and various ageing-related skin lesions, as well as improving the tone of your skin, giving you a natural, rejuvenated look.
Ablative treatments are more aggressive, resulting in redness and potentially oozing, crusting and blistering for a few days. As the skin heals, it remains red for a further few days to weeks. However, the overall result is that your skin will look smoother and younger as well as feeling tighter and more supple as well.
2. Non-ablative Laser Treatments
Non-ablative laser skin rejuvenation and skin resurfacing treatments also work predominantly by interaction 2 (photothermal) but these lasers target specific molecules in the skin other than water.
The theory of selective photothermolysis explains that specific targets in the skin preferentially absorb light at certain wavelengths, raising their temperature and causing them to break down.
Identifiable targets in the skin include melanin (the pigment causing skin and hair colour, pigmented skin lesions such as moles, freckles and melasma), haemoglobin (the molecule that carries oxygen in the red blood cells) and water.
Laser skin rejuvenation works by targeting melanin and haemoglobin that allows for treatments of skin conditions such as acne, excessive hair and visible blood vessels.
Non-ablative laser facial resurfacing uses lasers to target blood vessels and water raising the temperature in the deeper skin and promotes the development of new, more healthy collagen, helping to restore your skin’s contour and appearance with minimal down time.
Unlike ablative treatments, non-ablative treatments cause minimal discomfort following the procedure. Most people are generally able to return to their daily activities immediately, although they may appear “flushed.”
Even though the impact on the skin is less dramatic, non-ablative lasers are still able to achieve substantial improvement. For the best results, multiple non-ablative treatments may be necessary, usually spaced several weeks apart. In addition, continued treatments are often required to maintain results.
3. A note about Q-switched lasers for pigment and tattoo removal
There are a third group of lasers that are called Q-switched (or "quality-switched") lasers. These lasers are able to generate very short pulses (nano- or pico-second pulse durations) that interact with targets in the skin by interaction 3.
As the pulse durations are very short, the targets absorb the laser energy and heat up so rapidly that the target is physically disrupted (photodisruption). This is the mechanism, for example, by which large tattoo ink particles are broken down into much smaller sized particles, allowing the body to clear them.
The skin is usually intact during Q-switched treatments so they may be classified as non-ablative treatments as well.