Light and optical techniques have made profound impacts on modern medicine, with numerous lasers and optical devices currently being used in clinical practice to assess health and treat disease. Recent advances in biomedical optics have enabled increasingly sophisticated technologies — in particular, those that integrate photonics with nanotechnology, biomaterials and genetic engineering. In this Review, we revisit the fundamentals of light–matter interactions, describe the applications of light in imaging, diagnosis, therapy and surgery, overview their clinical use, and discuss the promise of emerging light-based technologies.
The modern use of light in medicine began in the nineteenth century, with rapid improvements in the understanding of both the physical nature of light and fundamental light–matter interactions. A notable example of an early triumph in phototherapy is the ultraviolet (UV)-induced treatment of lupus vulgaris invented by physician Niels Finsen, who in 1903 was awarded the Nobel Prize in Physiology and Medicine for this application. Since 1960, the development of lasers has opened new medical avenues. Today, numerous laser-based therapeutic and diagnostic devices are routinely used in the clinic.
Within tissue, photons interact with biological matter via various processes, which can be broadly categorized into scattering and absorption (Fig. 1a and Box 1). Scattering can alter the propagation path, polarization and spectrum of incident light. The states of the scattered light can be analysed and mapped for diagnosis and imaging. In light absorption, the energy of photons is converted to electronic or vibrational energy in the absorbing molecule. Some of that energy can be re-emitted through luminescence (for example, fluorescence), inelastic scattering, or acoustomechanical waves. Such emission from the tissue carries information about its microstructure and molecular contents, and serves as the basis for optical diagnostics and imaging. On the other hand, photoexcitation of intrinsic molecules or exogenous light-sensitive agents introduced in the body can affect the tissues and cells within in various ways, via the generation of heat (photothermal), chemical reactions (photochemical) and biological processes (photobiological or optogenetic). Optical therapy and laser surgery use these effects in a controlled manner.
X-rays and gamma rays have transformed modern medicine by enabling computed tomography (CT) and positron emission tomography (PET), as well as radiation therapy. Radio waves and magnetic fields have made magnetic resonance imaging (MRI) possible. Among the wide spectrum of electromagnetic waves, light comprises a region from the deep blue to the near-infrared (NIR) and provides a distinct advantage due to its unique photon energy range of 0.5–3 eV. These energies fall into a window that permits rich yet safe interactions with organic molecules. At higher energies, bond dissociation (>3.6 eV for C–C and C–H bonds) and ionization (>7 eV) can occur; and at lower energies, water absorption dominates, preventing any specific targeting of molecules. Light-based technologies in medicine take advantage of the variety of light–molecule interactions in this optical window (Box 2).
In what follows, we overview the major applications of light in three categories: optical diagnosis, laser surgery and light-activated therapy (Fig. 2a). We also discuss optical imaging, which plays important roles in diagnosis, surgical guidance and therapy monitoring. For each technology, we briefly highlight its principle of operation, advantages and limitations, and current clinical utilities. We then describe emerging light-based technologies, with particular emphasis on opportunities in nanomedicine, optogenetics and implantable devices.
The development of lasers expanded the therapeutic applications of light well beyond the long-known effects of sunlight and focused lamps. With lasers, emission intensities can be several orders of magnitude higher than that of sunlight, short pulses are readily generated and wavelengths can be precisely selected. Physicians started exploring the medical applications of lasers after the first demonstration of the ruby laser by Maiman in 1960, when biohazards of high-intensity laser pulses to the eye and skin became appreciated. Photocoagulation in the retina1, destruction of skin lesions2 and removal of dental caries and cardiovascular plaques are examples of early pioneering works. Today, medical lasers are routinely used in many applications, including surgeries in ophthalmology, the treatment of cutaneous disorders, and tissue ablation in internal organs through fibre-optic delivery. This has led to a global market for therapeutic lasers estimated to be over US$3 billion (Fig. 2b,c).
Laser surgery in ophthalmology. The ablative ability of UV photons from an ArF excimer laser (193 nm) to reshape the cornea is widely used for refractive error correction3. UV radiation breaks the peptide bonds of collagen fibres within the cornea, expelling a discrete volume of corneal tissue from the surface4 (Fig. 3a). For example, a single excimer pulse (0.25 J cm−2) with a spot size of 1 mm removes about 0.25 μm of tissue. A customized scanning pattern generated by optical wavefront analysis of the patient is used to precisely reshape the patient's cornea. Unlike photorefractive keratectomy, where the corneal epithelium is irradiated, laser-assisted in situ keratomileusis (LASIK) ablates corneal stroma through an ∼160-μm-thick corneal flap that is prepared by using femtosecond Nd:glass laser pulses (600 fs; 1,053 nm) focused to a spot size of 2–3 μm. Millions of LASIK procedures have been performed in the past two decades, with a 95.4% overall patient satisfaction in a recent meta-analysis5. Laser thermal keratoplasty uses holmium:YAG (2,100 nm) laser energy to reshape the cornea but is less used due to plastic regression after treatment.
A cataract in the lens is the leading cause of preventable blindness worldwide; in fact, over 20 million cataract surgeries are performed annually. In these, a laser is used to create an incision on the lens capsule and gain access to the cataract. Nd:YAG laser capsulotomy is used to treat posterior capsule opacification, which occurs in about 20% of patients following cataract surgery. Although the disruption of the crystalline lens is done by ultrasound, femtosecond laser technology for cataract surgery may offer a more reliable and improved safety profile6.
Laser coagulation therapy is a common procedure to seal a retinal tear or small retinal detachment7 (Fig. 3b), shrink abnormal blood vessels in proliferative diabetic retinopathy and seal blood vessels in retinal edema8. Green and yellow wavelengths, well absorbed by oxyhaemoglobin and melanin, are suitable for photocoagulation. Typically, this procedure uses a potassium titanyl phosphate (KTP)-based frequency-doubled Nd:YAG laser (532 nm), krypton ion laser (531 or 568 nm), or a diode-pumped dye laser (577 nm) at a typical fluence of 100 J cm−2 and duration of 10–100 ms per pulse.
Dermatological and aesthetic treatments. Since the laser was invented, efforts have been made to develop more effective lasers for the removal of unwanted skin markings, including tattoos, birth marks, stretch marks, port-wine stains, acne scars and leg veins9. Cutaneous laser surgery was revolutionized by the concept of selective photothermolysis10, which states that optical pulses with optimal parameters — wavelength, duration, beam size and energy — can selectively destruct a target within the skin, minimizing the risk of scarring and damage to normal tissue. A pulsed dye laser (PDL) tuned at 585 nm (0.4–2 ms; 5–10 J cm−2) removes skin markings by selectively targeting haemoglobin within red blood cells, with minimal thermal damage to other structures11. Spot sizes of 2–10 mm are used to permit deeper dermal penetration and the destruction of larger blood vessels. A PDL is useful in the treatment of port-wine stains, superficial haemangiomas and other vascular lesions. A cryogen-cooling device is used to limit the temperature rise at the surface of the skin while allowing heat accumulation at the target region underneath12. A PDL is also used to treat non-vascular lesions such as hypertrophic scars, keloids and striae.
Laser skin resurfacing can reduce facial wrinkles, scars and pimples. Indeed, ablative therapy using CO2 lasers (10.6 μm; 1–10 ns; 5 J cm−2) is the current gold standard13. However, the Er:YAG laser (2,940 nm) has a higher water absorption (12,800 cm−1) than the CO2 laser (800 cm−1), ablating 5–20 μm of tissue at 5 J cm−2. Alternatively, non-ablative resurfacing can minimize risk and shorten recovery times by producing dermal thermal injury to reduce rhytides (skin wrinkles) and photodamage while preserving the epidermis. Moreover, fractional resurfacing thermally coagulates or ablates microscopic columns of epidermal and dermal tissue in a regularly spaced array that is followed by rapid re-epithelization and dermal remodelling, which reduces the risk of scarring, improves treatment efficacy and shortens recovery time14. Fractionated lasers are divided into non-ablative NIR lasers (typically 1,550 nm)15 and ablative infrared lasers (typically 10.6 μm)16 (Fig. 3c).
When targeting melanin, laser treatment can lighten or remove benign epidermal and dermal pigmented lesions as well as tattoos. To induce rapid ablative heating, nanosecond Q-switched pulses (10–100 ns; 3–7 J cm−2; 1–10 Hz) are the usual choice. The optimal laser wavelength depends on specific skin types, target depth and pigment absorption. Q-switched ruby (694 nm), alexandrite (755 nm) and Nd:YAG (1,064 nm) lasers are typically used17. Superficial lesions are treated with shorter wavelengths (510 nm for PDL; 532 nm for KTP). Also, a picosecond alexandrite laser has been developed for targeting smaller pigments18.
Laser hair removal uses light pulses targeting melanin in the hair shaft and hair follicle to achieve long-lasting removal of excessive hairs in cosmetically undesirable locations19. Because of the presence of melanin in the epidermis, active cooling of the skin by spray or contact cooling is critical to minimize unwanted thermal injury of the tissue. A variety of lasers emitting long pulses (1–600 ms; 10–50 J cm−2), such as the ruby laser, the alexandrite laser, semiconductor diode lasers (800–810 nm) and flash lamp-based intense pulsed light (IPL) sources (550–1,200 nm) are suited for photoepilation.
Recently, a number of hand-held light sources have become available as consumer cosmetic products for home use in applications including photorejuvenation, hair growth, hair removal and acne treatment20. Advances in compact fibre lasers in the NIR range may also replace some of the existing solid-state Nd:YAG and holmium:YAG lasers used in the clinic21.
Laser surgery in urology and gastroenterology. Benign prostate hyperplasia is a non-cancerous condition affecting about 50% of men by the age of 50 and up to 90% older than 80, in which an enlarged prostate squeezes or partially blocks the surrounding urethra. A high-power laser (80–180 W; 20 ms; 400 J cm−2) is used to remove the excess tissue. Holmium-laser enucleation or ablation of the prostate22, developed in the 1990s, is less popular compared with transurethral resection using electrical loops. A rapidly growing laser-surgery technique for the prostate is photoselective vaporization using a frequency-doubled Nd:YAG laser (532 nm) delivered fibre-optically through a cystoscope23 (Fig. 3d). Medicare estimated that ∼72,000 photoselective vaporization procedures were performed in the United States in 2008 (accounting for 27% of all benign prostate-hyperplasia surgeries), whereas transurethral resection amounted to 64% of surgeries.
Lithotripsy is a surgical procedure to remove stones from the urinary tract (kidney, ureter, bladder or urethra) in patients with urinary calculi (stones). When ultrasound-based shock-wave lithotripsy cannot be used, lithotripsy is performed using a holmium:YAG laser through a flexible ureteroscope24. Laser lithotripsy is also used for the fragmentation of intraductal gallstones, particularly stones that cannot be targeted by conventional mechanical lithotripsy25.
Lasers in cardiology and vascular surgery. Laser-assisted lead extraction is a highly effective procedure for the extraction of lead wires of cardiac pacemakers and defibrillators from coronary blood vessels26. This method uses a special sheath that conducts laser pulses to the distal tip as a ring of light to break up scar tissue around the lead, facilitating removal. Typically, XeCl laser pulses (100–200 ns; 3–6 J cm−2) are used, causing localized ablation of the ∼100 μm of tissue in front of the tip27.
An early approach for angioplasty in the treatment of coronary artery disease was excimer laser ablation of plaques through a fibre-optic catheter28. However, its use has declined in recent years. Endovenous laser treatment is effective for treating lower-extremity venous insufficiency such as great saphenous vein reflux29. High-power laser light, typically from continuous-wave NIR diode lasers (810, 940 or 1,470 nm; 10–30 W), is intravenously delivered through a fibre-optic catheter to ablate varicose veins30. For asymptomatic lower-extremity telangiectasias, lasers (Nd:YAG, diode, KTP) are applied externally, particularly when sclerotherapy is not feasible31.
Other applications. Various dental lasers have been used to remove and modify soft and hard tissues in the oral cavity. For instance, CO2, Nd:YAG and diode lasers are used for soft-tissue removal such as in gingivectomy, for killing bacteria and for promoting the regrowth of tissues. Applications to hard tissue (enamel and dentin), such as dental drilling using highly ablative Er:YAG lasers32, are also growing33.
In otolaryngology, the CO2 laser is routinely used because of its minimal lateral tissue damage particularly in larynx surgery34. Interstitial laser coagulation has shown potential for causing necrosis of small tumour metastases35. Endoscopic laser therapy can be used for the palliation of advanced esophageal cancers36. And Nd:YAG lasers are used in bronchoscopic laser resection to relieve intraluminal airway obstruction caused by bronchogenic carcinoma or foreign bodies37.
Light-activated therapeutic applications harness the versatility of light in manipulating photoactive molecules, proteins and cells. Phototherapies, which require only an appropriately selected light source, are commonplace in the clinic and are first-line treatments for some conditions. For instance, since the 1960s, blue-light phototherapy has been used to treat severe hyperbilirubinemia in millions of infants38. More broadly, the use of exogenous photosensitizers offers capabilities beyond intrinsic light-induced signalling, such as oxidative killing of unwanted cells in photodynamic therapy (PDT). Today, PDT is an established modality used in oncology, dermatology, ophthalmology, dentistry, cosmetics and other fields. The combined global markets for drugs and devices used in phototherapy and PDT were estimated to be US$630 million in 2014 (Fig. 2b).
Ultraviolet light (100–400 nm). Exposure to the Sun's UV radiation, consisting of 95% UVA (320–400 nm) and 5% UVB (290–320 nm), is a major environmental risk factor for skin cancer, which afflicts more than 2 million people worldwide every year. UVB radiation can directly damage DNA, and thus is particularly carcinogenic. UVA can also cause cancer via the generation of DNA-damaging free radicals. Despite the harmful effects, UV-light exposure may have potential beneficial effects through the modulation of immune responses39. Indeed, UVB radiation can activate innate immune responses via release of antimicrobial peptides and vitamin D production40,41, which may play a role in Finsen's phototherapy for lupus vulgaris. UVB light also suppresses the adaptive immune system by activation of regulatory T cells, B cells and mast cells42. UVA light can also be immunosuppressive through the upregulation of the alternative complement pathway43. These mechanistic findings have suggested a physiological role for low, non-carcinogenic doses of UV light: induction of innate immunity to protect against microbial attacks, and suppression of adaptive immunity to inhibit autoimmune or allergic responses. The latter mechanism forms the basis of UV phototherapy, which is used to treat a wide range of dermatoses in the clinic44.
Narrow-band UVB therapy. For over 90 years, UVB radiation has been used to treat psoriasis44,45 — a chronic, immune-mediated inflammatory skin disorder affecting tens of millions of people worldwide. It is also used to treat vitiligo, atopic dermatitis and other inflammatory dermatoses. With emission at 311 ± 2 nm, narrow-band UVB is preferable to standard broadband UVB (290–320 nm) because of improved efficacy and the reduced risk of erythema (superficial reddening of the skin).
Psoralen and UVA therapy (PUVA). This is another frequent treatment for psoriasis and vitiligo. It requires oral or topical administration of psoralen, exposure to UVA light, and is commonly used to treat patients for whom narrow-band UVB is unsuccessful46. The therapy also works through immunosuppressive and antiproliferative effects.
Other effects. Several epidemiology studies have noted that increased sun exposure is associated with decreased risk of multiple sclerosis47. A study in mice substantiated these findings, showing that UVB radiation suppresses the progression of experimental autoimmune encephalomyelitis48. Another mouse study suggested that UVB radiation may suppress the development of obesity and metabolic syndrome independent of vitamin D supplementation49.
Visible light (400–700 nm). Phototherapy for neonatal jaundice. Jaundiced infants are commonly treated with phototherapy at 460–490 nm, preventing serious sequelae of hyperbilirubinemia, such as permanent neurological damage38 (Fig. 3e). Blue light isomerizes bilirubin to facilitate its excretion. In the United States, 0.5–4% of term and late-preterm infants receive phototherapy before discharge from the nursery. In low-resource settings, filtered sunlight using canopies is a promising and affordable option for treating neonatal jaundice50.
Bright light therapy for mood disorders. Seasonal affective disorder is a common form of depression that coincides with shorter days during the winter months and has a prevalence of 0.4–9.9% in the Northern Hemisphere51. This disorder and other conditions involving irregular light exposure, such as jet lag, have been linked to mood and cognitive alterations either by modulation of sleep and circadian rhythms, or by direct activation of neural pathways52. These mechanisms are mediated by intrinsically photosensitive retinal ganglion cells that express the photopigment melanopsin. Bright-light therapy has proven to be as effective as most antidepressant drugs for treating seasonal affective disorder53. In fact, activation of the ganglion cells by bright blue light (480 nm) conveys signals to the brain that suppress melatonin secretion and reset circadian rhythms54. Bright-light therapy has also shown promise for the treatment of non-seasonal major depressive disorder55, but the mechanism of action is still unclear.
Antimicrobial treatment. Antibiotic resistance increasingly threatens the ability to effectively treat bacterial infections. Blue light at 405–470 nm has been effective in treating a broad range of bacterial infections, including Propionibacterium acnes-associated acne vulgaris and Helicobacter pylori gastritis in humans, as well as wound infections with methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa in mouse models56,57. The antimicrobial effect of blue light is thought to arise from the excitation of endogenous porphyrins within the bacterial cells. Before it is accepted for mainstream clinical practice, however, the possible development of microbial resistance to phototherapy needs to be evaluated.
Other effects. Insufficient outdoor exposure to sunlight has been implicated with the growing prevalence of myopia58 (an estimated 2.5 billion people worldwide will be affected by the condition by 2020). Supporting this observation, high ambient lighting was found to prevent the development of myopia in macaque models59. Another potential application is the use of non-ablative lasers to adjuvant vaccines. Pre-treatment of the site of vaccine administration with visible or NIR light significantly enhanced vaccine efficiency and augmented immune responses in mouse models60.
Near-infrared light (700–1,800 nm). Photobiomodulation. Also known as low-level laser-light therapy (LLLT), photobiomodulation uses red and NIR light at 600–1,000 nm, at fluences of 1–10 J cm−2 with intensity of 3–90 mW cm−2. Clinical studies have suggested that light can stimulate epidermal stem cells in the hair follicle bulge to promote hair growth61. Photobiomodulation for wound healing, tissue repair and anti-inflammatory therapy62 has been shown to be efficacious in animal models. Results from clinical trials have been mixed, yet have shown therapeutic potential for neck pain63 and chronic traumatic brain injury64. Because its underlying mechanisms are not well understood, adoption of photobiomodulation has been controversial, and its use largely empirical. The mechanisms are largely attributed to the absorption of NIR light by cytochrome C oxidase in mitochondria. This triggers the dissociation of inhibitory nitric oxide from the protein complex, thus increasing adenosine triphosphate synthesis, which can have direct beneficial effects on compromised and hypoxic cells. Therapeutic effects of photobiomodulation may also be mediated by the generation of reactive oxygen species (ROS) and the induction of gene transcription. In fact, photobiomodulation on tooth pulps in rats with 810 nm continuous-wave light at a total dose of 3 J cm−2 was shown to form tertiary dentin via direct dental stem cell differentiation through light-induced generation of ROS65.
Thermal neuromodulation. Optical modulation of neural or muscle activity is increasingly explored as a therapeutic option for neurological and cardiovascular diseases. Thermal neuromodulation by infrared light does not require any exogenous agents or genetic interventions66. The mechanism involves absorption of infrared light (1.8–2.2 μm) by water, producing a transient increase in temperature that alters the membrane capacitance and depolarizes the target cell. Pulsed laser light (for example, 0.25 ms and 1–2 J cm−2) has been used to stimulate and control heart beating in intact quail embryos67 and cranial nerves for nerve monitoring during surgery in gerbil models68, without causing apparent thermal damage. Although thermal neuromodulation lacks the precision and selectivity afforded by optogenetics, its non-invasive nature could make it more amenable to clinical translation.
Photochemical crosslinking. Photoinduced crosslinking has a variety of applications in biomaterials engineering, biochip fabrication, drug synthesis and dental-composite curing. In situ polymerization of UV-epoxy is a common technique in the dental clinic. Corneal collagen crosslinking with riboflavin and UVA light (365 nm; 5 J cm−2) is a treatment for corneal ectasia, such as keratoconus69 (crosslinking increases the stiffness of corneal stroma by 50–100%). Its application may expand to strengthening a corneal flap, or to refractive error correction by patterned light delivery or multiphoton absorption. Photochemical crosslinking could also be used to bond native collagen fibres to aid wound closure, and to enable the use of photopolymerizable biomaterials as surgical glue70,71. In tissue engineering, photopolymerization can be used to fabricate 3D tissue constructs72, or employed in situ to enable restoration of soft tissues73.
Photodynamic therapy. Three components are required for PDT: a photosensitizer, light energy and oxygen. Following absorption of light by an exogenously delivered photosensitizer, energy is transferred from it to molecular oxygen, generating highly cytotoxic singlet oxygen and ROS. The efficacy of PDT is dependent on its ability to target diseased cells while sparing healthy cells, which requires selective accumulation of the photosensitizer and sufficient delivery of light energy to the target tissue. The antitumour effects of PDT result from the direct killing of tumour cells, the damage to tumour vasculature restricting oxygen and nutrient supply, and the induction of inflammatory responses that can result in systemic antitumour immunity74,75. Importantly, the mechanisms of cytotoxicity by PDT are fundamentally distinct from those of conventional chemotherapeutics; this may be leveraged in combinatorial therapies of PDT and chemotherapy in order to overcome resistance to cancer drugs76.
PDT is now clinically approved to treat a number of cancers, including bladder, lung, skin, esophageal, brain, ovarian and bile duct75 (Fig. 3f). In dermatology, PDT is commonly used for actinic keratosis and acne vulgaris, in addition to superficial non-melanoma skin cancers77. PDT is also approved to treat neovascular age-related macular degeneration, but its use has declined since the availability of anti-vascular-endothelial-growth-factor (anti-VEGF) therapy. Outside the clinic, PDT is used to eradicate viruses and other pathogens in blood products. For example, PDT with methylene blue is effective for inactivating viruses, including hepatitis C, HIV-1 and West Nile, and since 1991 it has been used to treat over 4.4 million units of fresh frozen plasma in Europe78.
The poor penetration depth of visible light required for photosensitizer activation has limited clinical indications of PDT, which is currently only used for the treatment of superficial lesions, or of lesions accessible during surgery or through endoscopy. To extend the therapeutic depth, photosensitizers that can be activated with NIR light are under development75,79. Upconversion nanoparticles that convert incident NIR light to visible light for indirect activation of conjugated photosensitizers have shown promise for treating deep-seated and solid tumours in animal models80. Another strategy is to deliver bioluminescent molecules directly to target tissues that are inaccessible by external illumination. For example, bioluminescent nanoparticles can activate photosensitizers via resonant energy transfer to treat lymph node metastases in mice81.
Photothermal therapy. Since the early 1990s, hyperthermic therapy using externally applied radiofrequency, microwave or ultrasound to heat tumours has been used as an adjunct to chemotherapy and radiotherapy to enhance treatment response82. In more recent years, the use of thermal agents such as magnetic nanoparticles to selectively localize heat to target tumour cells has shown considerable potential (photothermal therapy can be effective for treating hypoxic tumours, as it does not require oxygen83). Strongly absorbing nanoparticles, such as gold nanocages, can induce hyperthermia or thermal ablation. Among NIR-absorbing noble metal nanoparticles utilizing surface plasmon resonances84, only gold nanoshells have been tested in patients (in ongoing clinical trials for lung and head-and-neck tumours). In fact, the clinical translation of these biodegradable inorganic nanoparticles has been slowed by concerns over their biocompatibility, clearance and long-term toxicity. This has spurred the development of biodegradable organic photothermal agents85, such as porphysome nanoparticles86.
Optical diagnostics and imaging
Optical technologies are used along the entire spectrum of diagnostic medicine, from point-of-care and laboratory testing, to screening and diagnostic imaging, to treatment monitoring, to intraoperative imaging. Optical imaging enables real-time visualization of tissues and cells at high spatial resolutions with instrumentation that is relatively inexpensive and portable, especially compared with MRI and CT. A variety of macroscopic and microscopic imaging technologies have been adopted in clinical practice as standard of care, are currently in clinical adoption phases, or are under development for clinical translation (Fig. 4). In the operating room, surgical techniques that rely on optical guidance, such as laparoscopic surgery, have reduced the risk of haemorrhage and shortened patient recovery time. Intraoperative optical imaging provides tissue contrast beyond what can be perceived by the human eye, resulting in improved surgical outcomes. In this section, we discuss optical technologies that have enhanced the physician's ability to screen for, diagnose and monitor disease, with a focus on applications that have addressed previously unmet clinical needs.
Point-of-care testing. Optical technologies are an integral part of patient care, from routine physical examinations to bedside diagnostic testing. Relatively simple instruments such as the otoscope87 and the ophthalmoscope enhance visual inspection of tissues. Probing light absorption and scattering by tissue constituents enables measurement of important diagnostic indicators, including haemodynamics and the presence of specific biomarkers. The most prominent example is the pulse oximeter, which relies on absorption differences between oxygenated and deoxygenated haemoglobin to estimate the patient's arterial oxygen saturation. Beyond its universal use in emergency medicine, pulse oximetry has also become an effective screening tool for congenital heart defects in newborns88. NIR spectroscopy, also relying on haemoglobin absorption, is an emerging tool for non-invasive monitoring of brain function89. Spectroscopic techniques sensitive to changes in cellular chromophore concentrations are also being developed for cancer screening, such as for the diagnosis of oral cancer90. Dynamic light scattering techniques are increasingly adopted in the clinic for rapid, real-time assessment of microvascular function91, such as for burn wounds or atherosclerosis. Two techniques relying on NIR laser light scattered by moving red blood cells are laser Doppler imaging, which detects frequency-shifted light, and laser speckle contrast imaging92, which measures motion-induced intensity fluctuations.
Extensive efforts have been made in the development of optical technologies for point-of-care testing in resource-limited settings93. Lens-free computational imaging has eliminated the need for costly optical components, enabling the development of low-cost, portable, on-chip diagnostic platforms94. Ubiquitous optical devices such as smartphone cameras may be modified to add diagnostic functionalities such as calorimetric detection95, multiplexed detection96, video microscopy97 and high-resolution spectroscopy98. With the continued development of optical technologies, such as fibre optics, optoelectronics and optical microelectromechanical systems, various optical-imaging platforms may be miniaturized for widespread use in point-of-care settings99.
Molecular diagnostics. Optical techniques have been a mainstay of pathology analysis in the laboratory for over a century. For example, light microscopy is routinely used to analyse peripheral blood smears for haematologic disorders. Flow cytometry, typically through fluorescence-activated cell sorting (FACS), is commonly used for molecular phenotyping of haematologic malignancies and immune disorders, as well as for purifying cellular subpopulations for cell-based therapies. The enzyme-linked immunosorbent assay (ELISA), in which a colour change indicates the binding of specific proteins, is the gold standard for detection of serum antibodies and various biomarkers of disease. To improve sensitivity of protein detection, a number of optical approaches have been proposed, including surface plasmon resonance100, surface-enhanced Raman scattering101 and resonant optical cavity sensing102,103. Recent innovations in genetic sequencing have ushered in a new era of precision medicine, in which treatment regimens are tailored to patient genotypes. Optical probes are integral tools in genetic analyses, including fluorescence in situ hybridization, DNA microarrays and high-throughput sequencing. Future diagnostic tools are likely to combine both optical imaging and sequencing capabilities to fully capture the genetic and phenotypic complexity needed to treat heterogeneous diseases such as cancer104,105.
Diagnostic imaging. Owing to its non-invasiveness, high-resolution and rich contrast mechanisms, optical imaging is an established and rapidly growing modality for screening and diagnosis. In what follows, we briefly discuss the most clinically relevant optical-imaging methods.
Endoscopy. The standard white-light endoscope is one of the most basic yet essential optical instruments used by physicians to examine the inner organs of patients, such as the gastrointestinal and respiratory tracts106. Improvements in optical technologies have greatly enhanced the capabilities of endoscopic imaging, which has coincided with the rapid growth of the endoscopy equipment market (Fig. 2d), projected to reach US$38 billion by 2018. Chromoendoscopy uses dyes or stains to aid tissue characterization such as to identify dysplatic mucosa107. Narrow-band imaging, which does not require dyes, relies on the light-penetration difference between blue and green light (typically, 415 and 540 nm) to enhance the visibility of superficial and deeper blood vessels.
Capsule endoscopy. Over the past decade, wireless video capsule endoscopy has become an increasingly popular non-invasive tool for the diagnosis of small bowel disorders108. Approved capsule endoscopes consist of a camera sensor, a light-emitting-diode light source, a battery and a wireless radiofrequency transmitter. The capsule is swallowed with water, and excreted with bowel movements. Capsule endoscopes are often the treatment of choice for obscure gastrointestinal bleeding in adults, as they enable the examination of the entire length of the small bowel109. They are used increasingly for the assessment of Crohn's disease and for the detection of small bowel tumours.
Ophthalmic imaging. A variety of optical instruments are routinely employed to examine a patient's vision. For example, the slit lamp is used to visualize the anterior and posterior segments of the eye. Rotating the slit illumination in conjunction with a Scheimpflug camera allows for the measurement of corneal topography and of intraocular pressure by analysing corneal deformation in response to an applied air puff. Other commonly used instruments include wavefront sensors, confocal ophthalmoscopes and fundus cameras. Optical coherence tomography (OCT) is used to obtain high-resolution images of corneal and retina morphology. OCT is the standard of care for diagnosis of pathologies such as glaucoma and age-related macular degeneration110. OCT imaging of retinal anatomy is also being explored as a marker for neurological disorders, including multiple sclerosis and Alzheimer's disease.
Optical coherence tomography. OCT is an established modality for high-resolution, label-free, non-invasive imaging111. The global market for OCT equipment is growing, and is expected to reach US$1.4 billion by 2019. OCT provides cross-sectional images of tissue microstructure by measuring the echo time delay and intensity of backscattered light at different depths (up to 2–3 mm) at a resolution of 3–15 μm, typically using a broadband light source (800–900 nm) or a wavelength-swept laser (1,200–1,400 nm)112. Beyond ophthalmic applications, OCT is clinically approved for catheter-based intracoronary imaging113, and has been used for the monitoring of outcomes of intracoronary stenting, for assessing coronary plaques in the management of patients with acute coronary syndrome, and for the accurate quantification of fibrous cap thickness as a marker of plaque vulnerability114. Another target for OCT is endoscopic imaging of the upper gastrointestinal tract115. Technical innovations that may improve the diagnostic capabilities of OCT include cellular resolution116, high frame rates, birefringence measurement and multimodality117.
Diffuse optical tomography. Diffuse optical tomography (DOT) is a label-free method that uses NIR light to probe the concentration of haemoglobins, water and lipids in tissues by spectroscopic analysis of optical absorption by these molecules. It has been successful in monitoring the response to neoadjuvant chemotherapy in breast cancer patients118,119. Non-responders and responders could be differentiated as early as the first day after chemotherapy, which would allow non-responders to change treatment strategies early, thus avoiding unnecessary toxicities from additional chemotherapy. The chromophore signatures obtained by DOT can also be used to predict the response to neoadjuvant chemotherapy in breast cancer patients prior to treatment120. DOT combined with X-ray mammography has shown potential in reducing the false-positive rate of conventional mammography121. Recent DOT technology has also enabled functional neuroimaging of the superficial cortex, with images that are of similar quality to those from functional MRI (fMRI)122. DOT could thus be advantageous for patients with MRI-incompatible implants or for neonates in intensive care123.
Fluorescence imaging. Confocal laser endomicroscopy (CLE)124 uses fluorescence contrast and fibre-bundle probes to generate high-magnification images of the gastrointestinal mucosa. Confocal detection, or collection of fluorescence light only from the illuminated focal plane, enhances spatial resolution compared with conventional endoscopy, enabling assessment of mucosal histology. CLE has shown promise in the detection of neoplasia in patients with Barrett's oesophagus, as well as improved sensitivity over chromoendoscopy in the classification of colorectal polyps125, in the assessment of inflammatory bowel disease126 and in the diagnosis of bladder cancer127. For molecular imaging, fluorescently labelled peptides that specifically bind to dysplastic tissue have been used to detect colonic dysplasia128 and esophageal neoplasia129. Fluorescent lectins that bind to normal tissue can also be used as negative contrast agents130. Although the high resolution of CLE enables accurate image quantification, it is limited by the small field of view, which precludes efficient examination of large areas to detect lesions. This has driven the development of molecular imaging with wide-field fluorescence endoscopy. For example, blue-light cystoscopy with fluorescently labelled CD47 antibodies enabled the detection of bladder cancer in ex vivo specimens131, and neoplastic colorectal polyps in humans were detected through fluorescent peptides targeting c-Met132. Recently, fluorescence angiography has been used to visualize tissue perfusion in extremity wounds, such as diabetic foot ulcers133. Continued development of molecular imaging agents and optical technologies is likely to fulfil the goal of characterizing lesions in vivo, improving diagnostic accuracy and reducing pathology costs.
Nonlinear microscopies. Two-photon microscopy offers the benefit of deeply penetrating NIR light as well as light absorption confined to the focal plane, improving optical sectioning in tissue. However, its use has generally been limited to tissue specimens and small-animal imaging134 due to the requirement of a femtosecond laser with high peak power. Nonetheless, two-photon microscopy has been used for the diagnosis of malignant melanoma in humans through the imaging of endogenous chromophores135. Another target is the non-invasive imaging of retinal pigment epithelium, in particular retinosomes containing fluorescent metabolic products associated with age-related macular degeneration136. Nonlinear Raman scattering techniques enable label-free imaging of intrinsic molecular vibrations, generating optical contrast between lipids and proteins137. Stimulated Raman scattering microscopy has been used to detect brain-tumour infiltration in fresh specimens from neurosurgical patients, demonstrating near-perfect agreement with standard haematoxylin and eosin light microscopy138.
Photoacoustic imaging. Photoacoustic or optoacoustic imaging relies on the photoacoustic effect, in which absorption of a periodic train of nanosecond laser pulses induces cyclic, localized heating and cooling, generating pressure waves that can be detected by ultrasound transducers139. Its main advantages include enhanced depth penetration compared with optical imaging and multispectral imaging of multiple endogenous or exogenous absorbers. This technique can be readily integrated with commercial ultrasound probes to enhance image contrast through the detection of vasculature or contrast agents140. The sensitivity can also be leveraged for the in vivo detection of circulating tumour cells in rodent models141. In initial patient studies of breast cancer imaging, it has shown promise for the imaging of tumour vasculature and oxygenation with higher spatial resolution than DOT142,143.
Other techniques. Reflectance confocal microscopy is an accurate non-invasive technique for the diagnosis of basal cell carcinoma144, and may be able to diagnose melanoma145. Brillouin microscopy relies on light scattering from spontaneous acoustic waves in tissue to provide biomechanical information of transparent ocular tissues. It has been used in humans in vivo to map stiffness in healthy and keratoconus corneas146.
Imaging in surgery and therapy. Optical imaging during surgical operation has become increasingly indispensable, guiding the surgeon's eye through narrow endoscopic channels, and providing greater contrast and higher resolution between healthy and diseased tissues than perceived by the naked eye.
Endoscopic imaging. Minimally invasive surgeries, or endoscopic surgeries, have dramatically changed the way operations are performed. In the United States, 96% of cholecystectomies are performed by laparoscopy147, whereas many other procedures increasingly use minimally invasive techniques148. Compared with open procedures, laparoscopic techniques have resulted in superior surgical outcomes due to smaller scars, fewer complications and quicker recovery. Advances in optical devices and technologies have enabled surgeons to operate effectively without direct visualization of the target tissue. Stereoscopic vision has been employed in precision robotic surgery149.
Intraoperative imaging. For open-field procedures, surgeons rely on visual inspection and palpation to differentiate between healthy and diseased tissue. However, human vision through white-light reflectance cannot easily discriminate between different tissue types, and is limited by haemoglobin absorption and tissue scattering. For cancer surgeries, the ability to discriminate between malignant and normal tissue is paramount for determining which tissues to resect or preserve. A growing number of clinical trials in recent years have showcased the promise of fluorescence-guided surgery in improving surgical outcomes, decreasing surgical time and lowering overall healthcare costs150,151.
Fluorescence-guided surgery has been adopted for the resection of high-grade gliomas152. Orally administered 5-aminolevulinic acid (ALA) is metabolically activated to form protoporphyrin IX, which accumulates in tumour tissues and exhibits red fluorescence. A phase-III clinical trial showed significantly improved complete resection and higher 6-month progression-free survival when ALA guidance was used instead of white-light illumination153. Similarly, ALA has been used to guide tumour resection of bladder cancer, resulting in improved outcomes.
Exogenous NIR fluorescent probes are ideal for intraoperative imaging because of minimal background autofluorescence and enhanced tissue penetration (when compared with visible light). Currently, the only NIR probe approved by the Food and Drug Administration and the European Medicines Agency is indocyanine green (ICG)151. Other probes with reduced background154 and high tissue specificity155 are under development. The clinical utility of ICG for surgical guidance has been demonstrated for the mapping of sentinel lymph nodes156 and in the detection of previously undetectable small liver metastases157.
Molecular imaging. The next frontier for fluorescence-guided surgery involves tumour-targeting, molecular contrast agents. In 2011, the first in-human trial of tumour-specific fluorescence-guided surgery was conducted in patients with ovarian cancer158. Using folate–fluorescein isothiocyanate to target folate receptor-α, which is expressed in 90–95% of patients with epithelial ovarian cancer, surgeons were able to identify significantly more tumour deposits than with visual inspection alone. However, folate receptor-α is overexpressed only in certain cancers, which limits the general applicability of folate-conjugated dyes. Another approach currently in development uses activatable probes that become fluorescent when exposed to proteases overexpressed in tumours, thus enabling optical contrast between normal and tumour tissues159,
Microscopic and spectroscopic guidance. Although large-field-of-view, fluorescence-based imaging is most suited for surgical guidance in the operating room, optical microscopy techniques with high resolution and reduced field of view have also demonstrated intraoperative utility. OCT has been widely used for ophthalmologic surgeries163 and in conjunction with angiography for percutaneous coronary intervention164. OCT and stimulated Raman scattering microscopy have been used to delineate brain tumours in mice in vivo165,166. Raman scattering spectroscopy has been used to detect brain cancer cells in humans during brain surgery167. Rapid detection of cancer cells by optical methods could therefore become an invaluable tool for surgical decision-making.
Optical imaging in theranostics. Broadly defined, theranostics refers to the combination of therapeutic modalities and diagnostic imaging to adjust treatment protocols to the needs of individual patients. Optical imaging can facilitate the detection of pathologies, the determination of target location and dosimetry, and the monitoring of therapeutic effects in situ. Optical imaging is particularly well suited for guiding light-activated therapies, which require light sources. Detection of photosensitizer fluorescence is commonly used for the monitoring and optimization of PDT168,169. In vivo fluorescence molecular imaging can also be used to predict therapeutic outcomes, as has been shown for cancer170 and Crohn's disease171. And optical Cherenkov emission from radioisotopes could be useful for the dosimetry of X-ray external-beam radiation therapy172.
With the success of medical lasers and optical technologies and the recognition of the needs and opportunities for better healthcare (Table 1), considerable efforts have been made to develop improved or new light-based technologies. Technological innovations continue to push the envelope of optical imaging, including super-resolution imaging beyond the diffraction limit173 and rapid 3D imaging with light-sheet microscopy174. Tissue-clearing techniques that render entire organs optically transparent while preserving tissue microstructure175 may enhance clinical histological evaluation. Spatial light modulation that counteracts scattering in tissues for improved light penetration and resolution could lead to practical tools for imaging, endoscopy and light-activated therapies176. Besides advances in optics and photonics, progress in other fields, such as nanotechnology, bioinspired engineering, genome editing and advanced materials, has opened up opportunities for further innovation in photomedicine. Injectable multifunctional nanoparticles enable light energy to be relayed to specific cells of interest while providing spatiotemporal control of light activation. Genetic integration of fluorescent proteins enables the identification of molecular targets, whereas optogenetics offers precise control of cellular function in the nervous system. These technologies harness the molecular specificity of light–tissue interactions to meet the need for more precise and personalized therapies, such as the targeting of vulnerable cancer cells identified by molecular profiling. At the same time, advances in biomaterials and optoelectronics have led to novel optical devices that can be worn or safely integrated in the human body, allowing continuous health monitoring and providing new opportunities to deliver light to targets previously inaccessible by external illumination.
Light-activated nanomedicine. Nanoparticles based on lipid, polymeric and inorganic materials have advantages over molecular-based therapeutics, such as tunability of size and shape, versatile functionalization capabilities for active targeting and unique optical properties177. Light-triggered drug release from nanoparticles enables the on-demand, spatiotemporally controlled delivery of therapeutic agents. This allows dosing regimens to be tailored to patient needs while increasing drug concentration at target tissues and reducing systemic toxicities. For cancer patients, the optimal timing of a chemotherapeutic treatment could enhance antitumour efficacy and reduce metastatic spread178. Strategies for nanoparticle-mediated drug delivery include NIR-to-UV light transduction by upconversion nanoparticles179, NIR-light-induced heating of thermosensitive polymers180 (Fig. 5a) and permeabilization of drug-containing liposomes181.
Multifunctional nanoparticles that bear a combination of diagnostic and therapeutic functionalities — theranostic agents — are of particular interest for precision medicine, as they combine both biomarker identification and therapy in a single platform. Theranostic capabilities can span a wide array of modalities, including fluorescence and photoacoustic imaging, PET, MRI, photodynamic and photothermal therapy, and drug delivery182,
A major barrier to systemically administered nanomedicines, particularly for cancer therapy, is the inefficient delivery of nanoparticles to the tumour or target site190. Overcoming this barrier will require both improvements in nanoparticle design and better understanding of biological responses to nanoparticle delivery191. In future, applications of light-activated nanoparticles are likely to extend beyond imaging and the delivery of direct therapeutic payloads192. Indeed, light activation may be useful for modulating biological behaviours at the systems level for therapeutic benefit; for example, NIR-light-induced heating of gold nanorods has been used to cause a clotting cascade in tumours, which then amplified the delivery of chemotherapeutics targeted to a by-product of the coagulation cascade193. Another potential avenue is the manipulation of nanorobots to perform complex tasks in vivo (such as precise, non-invasive surgeries). Towards this end, a recently developed light-controlled, programmable artificial microswimmer sensed and oriented its migration to the direction of external illumination194 (Fig. 5b).
Optogenetic therapies. Optogenetics has revolutionized the way light can be used to control cellular activity195. In a typical protocol, target cells are rendered photoactive via genetic integration of microbial opsins (light-gated ion channels), thus enabling optical control of electrical activity. Optogenetic techniques have enabled unprecedented insights into disease circuitry, which led to immediate clinical impact. For example, optical dissection of brain circuitry in mouse models revealed mechanisms of deep brain stimulation in the treatment of Parkinson's disease196, and helped to design new deep-brain-stimulation protocols for treating cocaine addiction197.
Because of its capabilities for neuromodulation therapy, optogenetics has enticing clinical potential. However, significant barriers to use in the clinic remain, such as the need for genetic transfection and proper illumination198,199. Optogenetic therapy for conditions such as depression200, chronic pain201 and laryngeal paralysis202 has been tested in rodent models and may one day be translated for use in humans. Orphan status was recently granted to a viral-vector-based optogenetic therapy (from RetroSense Therapeutics) for retinitis pigmentosa, and initial clinical trials to evaluate safety are underway. In this case, optogenetic transfection can impart light-sensing ability to relevant cell types (such as ON bipolar cells) located downstream of the damaged photoreceptors203. A more recent concept is the coupling of an optogenetic retinal prosthesis with a high-radiance, optical-stimulation display device204 (Fig. 5c).
A long-term limitation of optogenetic therapies concerns the potential adverse immune responses resulting from the xenotransplantation of microbial proteins into humans. Light-activated control of native ion channels can also be achieved through exogenously delivered caged molecules or photoswitchable compounds205. Indeed, photoisomerizable photoswitches have been used to restore visual responses in blind mice206, and to reversibly control nociception in rats207. To enable genetic targeting to specific cells, mammalian ion channels can be engineered to impart light-gated functionality208, enabling restoration of retinal function in blind mice and dogs209. Melanopsin — a mammalian opsin that controls G protein signalling pathways — can also be introduced to render cells photosensitive210. These optogenetic tools can create synthetic gene circuits with therapeutic applications. For example, an optogenetic circuit that regulates the expression of glucagon-like peptide-1 was developed for the treatment of type 2 diabetes. When implanted in diabetic mice, the engineered optogenetic cells improved blood-glucose homeostasis on treatment with blue light211,212. Therapies involving the implantation of genetically modified cells rather than in vivo gene therapy could have an easier path to clinical translation.
Wearable, personal healthcare devices. The skin offers a promising site for measuring physiological information (such as body temperature) for continuous health monitoring213 (Fig. 5d). The development of low-cost, stretchable and flexible optoelectronics214,215, such as polymer light-emitting diodes (LEDs), may enable a range of wearable health-monitoring devices in both clinical and at-home settings. Examples include a flexible, all-organic optoelectronic pulse oximeter that has comparable efficiency to commercially available oximeters216, and a wireless, stretchable optoelectronics device worn on the skin for monitoring blood flow and sweat217 (Fig. 5d). Future wearable devices could incorporate therapeutic functionalities, such as antibacterial blue-light therapy or low-level light therapy. Flexible optoelectronic devices may also be incorporated into endoscopes for multimodal theranostic functionality218 (Fig. 5e).
Implantable optoelectronic devices. A significant portion of the more than US$200-billion-per-year global medical device market is for devices that are implanted in the human body. Artificial intraocular lenses make up the largest number of implanted medical devices, accounting for nearly 40% of the market in 2011. Prosthetic retinas promise to restore vision in patients blinded by the degeneration of retinal photoreceptor cells, and their development is being fuelled by advances in optoelectronic technologies (Fig. 5f)219. With rapid advances in bio-optic interface technologies and heightened recognition of light versatility in diagnostics and therapies, many new implantable devices based on light-based functionalities are likely to emerge. Novel device concepts and designs include implantable LEDs220, miniaturized wireless optoelectronic devices for optogenetic applications221,
Biomaterial photonic devices. To fully realize the diversity in light–tissue interactions for medical applications, light must be delivered to target tissues with sufficient energy and specificity. Biomaterial waveguides may be used for long-term light delivery229 and need not be removed if they are made of biodegradable materials, such as silk and poly(l-lactide-co-glycolide)230 (Fig. 5h). Light-controlled therapy and sensing have been demonstrated by using fibre-optic hydrogel implants with fluorescent reporters and optogenetic cells212 (Fig. 5h). Multifunctional optical fibres may be implanted for neuromodulation of the nervous system in the body231. Cell-based lasers may offer new ways of delivering light by exploiting the intrinsic capabilities of the cells carrying intracellular lasers232 (Fig. 5i) to target, for example, sites of inflammation, and to permit highly multiplexed imaging based on narrow-band coherent emission233.
Laser surgery and treatments, particularly in dermatology and ophthalmology, are clinically established yet steadily growing areas. Development of new, compact and cost-effective lasers delivering desired output characteristics will expand clinical utilities and enhance treatment outcomes. Phototherapies and photoactivated drug therapies have a long history, yet only a handful of modalities have been adopted as first-line treatments in modern medicine. Nevertheless, recent progress in the understanding of light–tissue interactions, drug delivery and nanotechnologies will accelerate clinical translation. Optical diagnostics and imaging have enjoyed growing clinical adoption in many areas and will find increasing utilities. The wide variety of strategies for enhancing imaging contrast, resolution and molecular specificity make optical imaging a powerful modality, especially in conjunction with conventional radiologic imaging. Given the versatile role of light in nature, the many ways that light-based approaches can sense, monitor and manipulate biological processes is not surprising. In the years to come, increased integration of light-sensitive functionalities in patients through genetic engineering, implantable optoelectronics and intracellular devices such as micro- and nanolasers, will further enrich the role of light in medicine. As noted in 1909 by the radiographer Harvey van Allen234, “Many other things have been claimed for light treatment, but other methods have proved better in those cases in my hands. We need to look at these things carefully and not be carried away by our enthusiasm.” More than a century later, the excitement in photomedicine is still high.
This work was supported by the US National Institutes of Health Director's Pioneer Award (DP1-OD022296) and grants P41-EB015903, R01-EY025454 and R01-CA192878, and National Science Foundation grants ECCS-1505569 and CMMI-1562863.