The Engineering Behind Laser Hair Removal

Understanding laser hair removal requires knowledge of many disciplines. Quantum and optical physics, the biology behind the growth cycles of hair, and the historical and modern rationale for depilation (hair removal) contribute to this interesting cosmetic procedure. The study of Laser Hair Removal reveals the interconnection between societal values and science while also providing an interesting vantage point from which to view engineering as a part of beauty culture.

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Introduction

Engineers must consider the unintended consequences of their actions. The promotion of a new source of energy could cause riots in Mexico, the discovery of hydrogen spectra could create a 244 million-dollar cosmetic industry, smart robots could take over the world [1]. While the robot revolution has yet to come, the unmitigated disaster of corn-based ethanol [2] still haunts the alternative energy world while a cosmetic industry based on the atomic spectrum has flourished in America. That industry happens to be laser hair removal. In , 60% of adults age 18-26 responded that they want to get laser hair removal, and more than 1.2 million laser hair removal procedures occurred in [1]. The allure of laser hair removal is understandable: the procedure is billed as &#;virtually painless, safe and long lasting,&#; and a high-tech halo still surrounds laser technology.

It is important &#; as with any medical procedure &#; to understand how laser hair removal works, to evaluate the science on its own merits, and to weigh the potential side effects. However, this is not a simple task; the engineering behind laser hair removal incorporates a wide range of disciplines from quantum mechanics to biology to the elusive science that is human attraction.

The History of Hair Removal

Humans have been removing hair since &#; and possibly before &#; the ancient civilizations of Egypt. Both men and women in Ancient Egypt shaved the hair from their bodies and heads, for the dual reasons of cleanliness and fashion. Lice were a frequent problem, and it was difficult to keep hair clean in the hot Egyptian climate; additionally, wigs were seen as both fashionable and cleaner than natural hair [3]. The Ancient Egyptians waxed off hair using caramelized sugar and shaved using the precursor to the straight razor, shown in Fig. 1.

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Hair removal remained important as a sign of civilization and social status. Roman soldiers &#; who shaved their heads to prevent hair-pulling in battle &#; coined the term &#;barbarian&#; in reference to the un-barbered enemy.

Although it is relatively easy to keep hair clean today, hairlessness continues to be important as a cultural phenomenon. Skin is considered &#;flawless&#; if it is without acne, blemishes, and hair. This is especially true for women in America, where popular magazines and websites often publish articles with titles like &#;See Hairy Celebrity Slips: Armpits, Legs, Guess Who?&#; [4]. These articles emphasize that it is socially unacceptable to have body hair. This social message created the impetus to invest in new hair-removing technologies.

Advanced technologies include the invention of the modern razor by K. C. Gillette in the late nineteenth century [5]. More futuristic methods include electrolysis, in which an electric current is applied to the hair follicle. This method &#; not laser hair removal &#; is the only method approved by the FDA to remove hair permanently.

Lasers have always captured the public imagination, and laser hair removal was attempted almost as soon as lasers were invented. These early lasers were impractical and often caused skin damage due to ineffective hair targeting and lack of control over the power of the laser. It took forty years &#; from the invention of the laser in to &#; for scientists and dermatologists to engineer a practical laser hair removal system.

The Biology of Human Hair

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Human hair is made up of three parts:

the medulla, the cortex, and the cuticle, as shown in Fig. 2.
The medulla makes up the center of the hair, the cortex surrounds the medulla and is made out of roughly straight keratin structures, and the cuticle is the outside of the hair, made up of flat cells laid out like roof shingles [6]. The hair itself is part of a biological system that includes the follicle, where the hair grows, the sebaceous gland, which when infected produces pimples, and the arrector pili muscle, which causes goose bumps.

Two types of melanin, eumelanin and phomelanin, contribute to the pigment of human hair. Eumelanin colors dark hair, while phomelanin contributes to red hair. The melanin in hair is a chromophore &#; the part of a molecule responsible for its color [7]. Color, in human hair and in general, is caused by the absorption of wavelengths of light and the reflection or transmission of others. This property is what allows laser light to be selectively absorbed by hair and not the surrounding skin. However, to know exactly why this happens, you have to go back to quantum physics.

The Physics behind Laser Technology

In , Neils Bohr postulated that electrons in atoms could only have discrete (quantized) orbits and energies [8]. He stated that electrons could jump from one orbit to another by emitting or absorbing energy in amounts predetermined by the orbits themselves. Bohr proved his theory by discovering the atomic spectrum of the Hydrogen atom, which contains one electron. When the electron lost energy, it would emit a wavelength of light corresponding to the change in energy between the orbits. The emitted wavelength of light &#; if in the visible band of the electromagnetic spectrum &#; would correspond to a specific color. Although Bohr was not completely correct in his assessment of atomic structure, the concept of discrete spectra allowed for the development of lasers.

What is a Laser?

The term laser is an acronym for Light Amplification by Stimulated Emission of Radiation. In the most basic sense, a laser is a device that emits light of a specific wavelength. Charles Townes and Arthur Schawlow invented the modern laser in . At the time, Townes had previously discovered a method of amplifying electromagnetic waves; however, he had used microwave radiation instead of visible or infrared radiation [9]. The theory behind lasers &#; exciting atoms in pure gases or solids to produce specific wavelengths of light &#; had been known for years, but Schawlow put the theory into practice. The first laser was comprised of a long narrow cavity containing the desired atoms with silvered mirrors at each end. The reflected light rays caused more atoms to be energized and consequently radiate energy at the specific wavelength.

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At the center of a modern laser is the Gain Medium

, a material through which specific wavelengths of light can be amplified. The other essential parts of the laser include a mechanism to apply energy to the Gain Medium, and a reflector or other way to provide optical feedback. Energy is applied to the Gain Medium, the electrons in the medium are excited to a higher orbital, and then they fall to a lower orbital, emitting light as a byproduct. The purity of the Gain Medium affects the purity of the spectrum emitted. A high quality laser should emit a very narrow spectrum [10]. This narrow spectrum is characterized graphically in Fig. 3, which shows that the spectrum of a helium-neon laser is confined to a small range of frequencies centered around 632 nanometers.

How Laser Hair Removal Works

In laser hair removal, the laser light is applied to the entire area over which a person wants to lose hair &#; the hairs are not individually &#;zapped&#; by the laser (unlike in electrolysis). By not zapping individual hairs, the process is much faster than electrolysis; however, the laser light may harm the skin if not applied correctly [11]. As shown in Fig. 4 and Fig. 5, the laser hair removal patient is prepped for their treatment by cutting the hair short and applying a cooling gel to the skin to reduce the chance of overheating; then, the laser light is applied to the skin.

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Laser hair removal works by targeting areas with specific chromophores &#; in this case, the melanin found in the hair (See previous section for a description of chromophores). The laser emits a wavelength of light that should be primarily absorbed by the hair and primarily reflected by skin, causing the hair follicle to heat up faster than the surrounding skin. This causes inflammation, which will kill the follicle that produces the hair. However, one should not expect to go in for a treatment and come out hairless &#; it can take up to three weeks for the affected follicles to shed the treated hair.

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No single laser works on the complete range of skin and hair types (skin color is less relevant than melanin content), since the laser hair removal system works based upon a difference between skin and hair color in order for the hair to absorb the light without burning the skin. Melanin under the skin &#; even if inactivated, will absorb rather than reflect light at the same rate as melanin of the same type in the hair. If the skin and hair absorb the light at the same rate, the laser will not be able to &#;target&#; the hair. For this reason, for many years laser hair removal was only possible for people with light skin and dark hair. However, new lasers have been invented that work for different skin and hair combinations. For example, the alexandrite laser &#; which has a relatively short wavelength (775 nanometers) &#; is effective on patients with lighter skin, although it is dangerous for patients with dark skin, while the neodymium-doped yttrium aluminum garnet laser &#; developed to have a longer wavelength ( nanometers) &#; works well on darker-skinned patients [12].

Although scientists have made advances in the selection of laser gain medium, there are still many other factors that must considered in the process of creating a safe and effective laser hair removal system. These parameters include the pulse width, the width of the laser beam, and the energy of the laser light. A shorter pulse width with higher energy and a wider laser beam has been shown to be more effective at disabling hair growth but also runs the risk of collateral skin damage.

Even if scientists engineered a laser that was perfectly absorbed by hair and perfectly reflected by skin, patients would still have to make several treatments to become hairless in the desired region. There are three phases of hair growth: anagen, catagen, and telogen, in which follicles are either active or dormant. Laser hair removal only works on active, growing hair follicles, since this is when the follicle is most sensitive. (At this point in the hair growth cycle, the early Anagen stage, the hair is completely under the skin.) In order to have a high probability of treating most hair follicles at the time they are active, dermatologists suggest seven treatments spaced anywhere from three to eight weeks apart [13].

Dangers of Laser Hair Removal

Laser hair removal &#; marketed as permanent and painless &#; was not well understood or well engineered for the first four decades of its development. The first laser hair removal systems in the s were not only impractical but also harmful to patients, damaging the skin more than the hair. Side effects included itching, redness, acne, hypo or hyper-pigmentation, and burning of the skin similar to sunburn [10]. However, laser hair removal continued to grow in popularity; patients volunteered as test subjects for clinical trials [14]. Today, the science of laser hair removal is much older and better understood, although side effects continue to be a problem for laser hair removal clinics and injuries can still occur if technicians are not trained or if systems are improperly calibrated.

In , the company ThermoLase created the first laser for hair removal approved by the FDA; however, four years later, patients won a class action lawsuit against the company for false advertising. ThermoLase clients were angered when the laser hair removal they received was not permanent &#; many clients experienced &#;full regrowth of all hair&#; [14]. Although the technologies have improved, laser hair removal does not guarantee permanent results or complete elimination of hair, even with multiple treatments. Today, laser hair removal companies are careful to market their product as a &#;reduction&#; in hair growth and regard successful treatments as ones that have reduced the thickness or quantity of hair.

Physics, Culture and Laser Hair Removal

The cosmetic procedure of laser hair removal has probably made a small impact in the lives of today&#;s physicists &#; the intellectual successors of the scientists and engineers who discovered the theories and technologies that make laser hair removal possible. However, laser hair removal has &#; unintentionally &#; helped to shape the study of optical physics. The use of lasers in medical applications continues to grow, from minimally invasive surgeries to LASIK to laser fat removal [15]. The laser hair removal industry&#;s funding and interest in the science of lasers for biological applications contributed at least a small part to these discoveries.

In the end, society decides what scientific research survives, which projects receive funding and which fade into obscurity. In order to be successful, engineers must create things that make life easier, things that people desire. The continuing story of laser hair removal shows us that people want the same thing they desired thousands of years ago &#; to conform to a standard of beauty that few can achieve without the help of the unintentional consequences of physics.

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