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Light has become a powerful medical tool. Continuing advances in the understanding of biological interactions with light, and newly developing light technologies, have given rise to a wide range of light-based therapies. As a physicist, I use light to study and modify the properties of materials down to the nanoscale. My strong interest in biophysics and medical technologies has now inspired me to explore the world of photomedicine. There are many varied interactions between biological processes and light, hugely dependent on wavelength and intensity. Medical treatments and choice of light sources are dictated by the desired treatment outcome.

Photothermal

A beam of light transfers energy. The more photons there are, the higher the light intensity and the more energy is transferred. The invention of the laser in 1960 introduced a light source capable of supplying high-intensity light with surgical precision. A laser can be used in place of a scalpel as a surgical instrument. The intense light tool can seal blood vessels and nerves as it makes an incision – reducing bleeding, pain and inflammation. High-intensity laser light is also used to destroy and remove cells and tissues through a process called photoablation. The absorption of laser light can raise the temperature of a cell so high and so rapidly that it is vaporised while leaving other tissue nearby unharmed. Laser ablation  can be used to treat superficial and early stage cancers including some forms of skin cancer.

Large tumours which are causing blockages – e.g. in the throat – can be partially removed using an endoscope combined with a laser. Light can even be applied inside tissues and tumours in places which are hard to reach with traditional surgical techniques – for example, brain tumours. Live magnetic resonance imaging (MRI) can be used to guide a catheter containing a fibre-optic cable to a tumour. Once correctly situated, laser emission is engaged to heat the tumour, causing cell death by hyperthermia. An even greater degree of treatment localisation is becoming possible using nanoparticles, guided to cancerous cells by conjugation with specific antibodies. Designed to interact strongly with light for efficient conversion of light energy into heat, the nanoparticles allow for treatment with lower intensity light and with increased precision.

Photochemistry

Photochemical processes are those which are triggered by the absorption of light. Perhaps the most famous photochemical process is photosynthesis, by which all plants generate chemical energy from sunlight. Human skin also demonstrates some photochemical interactions on the absorption of short-wavelength ultraviolet (UV) light – some negative and some positive. Famously, UV absorption in the skin enables the generation of vitamin D which is important for maintaining the health of bone, teeth and muscles. Sunburn is caused by direct photochemical damage to DNA in skin cells by UVB (280-315 nm) light absorption.  UVA (315-400 nm) absorption can also be harmful indirectly by increasing the generation reactive oxygen species. Both damage mechanisms can increase the risk of developing skin cancer. Balancing the positive and negative impacts of UV exposure can be key to designing an effective treatment. UV light is often used to treat skin conditions including psoriasis by slowing the production of skin cells and suppressing the immune system. UV exposure is also used for repigmentation of skin in conditions which cause skin cells to lose their natural colour. UV illumination may also be used to stimulate wound healing and as an antibacterial agent – the latter is particularly relevant at time when resistance to antibiotic drugs is increasing.

Cancer therapy is also a large area in photochemical photomedicine. In a treatment called photodynamic therapy (PDT), photosensitive cancer drugs can be administered which are activated only when exposed to light. Limiting light exposure to the target treatment regions reduces side effects in healthy tissue. In late 2016, a trial of a prostate cancer treatment using a drug which is only activated when exposed to laser light was reported with very encouraging improvements in cure rates compared with traditional treatments. PDT may also be used to treat blood cancers by exposing blood treated with a photosensitising agent to light outside of the body.

Photoperiodism

Controlling exposure to light, particularly blue light, can be used to treat some sleep disorders and circadian rhythm (‘body clock’) disruption. Eyes contain photoreceptors which communicate time-of-day information to the brain’s central timekeeping zone. Increased artificial blue light exposure from device screens, for example, is suspected of causing disruption, and most laptops and smartphones now feature a ‘blue light’ mode to reduce blue light intensity according to the time of day. Products to increase ‘natural’ light exposure are also available – which claim to improve sleep patterns and treat seasonal affective disorder.

Conclusion

Developments in our understanding of the interaction of the human body with light, and advances in light sources and light guiding technologies, have generated the enormous field of photomedicine. Surgical procedures and cancer therapies look set to continue to make use of laser light to improve precision and to extend treatments beyond traditional techniques. Advances in technology and continued innovation should lower the barriers to use, including the requirement of specialist training, access to equipment and cost. The market for natural light exposure management may mature as evidence of links between biological processes and artificial light exposure increases.

For light in medicine, the future is bright.


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