Absorption of photon energy by cells or tissue can evoke photothermal, photomechanical, and photochemical effects, depending on the density of the deposited energy. Photochemical effects require a low energy density and can be used for reversible modulation of biological functions. Ultrashort-pulsed lasers have a high intensity due to the short pulse duration, despite its low average energy. Through nonlinear absorption, these lasers can deliver very high peak energy into the submicrometer focus area without causing collateral damage. Absorbed energy delivered by ultrashort-pulsed laser irradiation induces free electrons, which can be readily converted to reactive oxygen species (ROS) and related free radicals in the localized region. Free radicals are best known to induce irreversible biological effects via oxidative modification; however, they have also been proposed to modulate biological functions by releasing calcium ions from intracellular organelles. Calcium can evoke variable biological effects in both excitable and nonexcitable cell types. Controlled stimulation by ultrashort laser pulses generate intracellular calcium waves that can modulate many biological functions, such as cardiomyocyte beat rate, muscle contractility, and blood–brain barrier (BBB) permeability. This article presents optical methods that are useful therapeutic and research tools in the biomedical field and discuss the possible mechanisms responsible for biological modulation by ultrashort-pulsed lasers, especially femtosecond-pulsed lasers.
Since the development of bright-field microscopy and the first observation of cells in the 17th century by Leeuwenhoek and Hooke , light has become an indispensable tool for biological research. A variety of biomedical applications have used light to restore or manipulate biological functions (Table 1) . Light has been applied in tumor treatment, a method known as photodynamic therapy (PDT), by producing singlet oxygen or free radicals that have toxic effects on tumor cells or tumor-associated vasculatures [3-6]. Recently, a new therapeutic modality, called low-level light therapy (LLLT) has been developed and applied to regenerate wounds or alleviate pain through its photothermal or photochemical effects [7-9]. Lasers, especially ultrashort-pulsed lasers, can disrupt the materials it contacts due to the photomechanical effects occurring at high peak intensity. This property can be used for material engineering, laser surgery such as laser-assisted in situ keratomileusis (LASIK) or subcellular nanosurgery [10-14]. Optogenetics has recently emerged as a powerful tool for studying cellular activities, and requires photoactivatable receptors that react to light by changing their permeability; this facilitates manipulation of the cellular functions of neurons or cardiomyocytes. However, optogenetics has the drawback that it requires genetic modification to produce photoactivatable receptors on target cells [15-18]. A new optical method has been reported recently, in which ultrashort laser pulses can be used to modulate various biological functions without the need for genetic modification or exogenous molecules [19-24].
Multiphoton microscopy is a type of laser-scanning microscopy that utilizes nonlinear effects of ultrashort laser pulses. Most commonly, a near-infrared femtosecond-pulsed laser is used as a primary source due to its deep penetration, low scattering, and localized nonlinear absorption . The probability of nonlinear absorption is extremely small and proportional to Ik (I = laser intensity, k = the number of photons absorbed) [26, 27]. Thus, nonlinear multiphoton absorption occurs only on a tightly focused region without out-of-focus fluorescence or phototoxicity . These advantages allow nondestructive three-dimensional deep-tissue imaging even in highly scattering samples. Multiphoton microscopy is widely used for biomedical research such as neuronal imaging , vascular imaging , or in vivo investigations of tumor physiology .
Ultrashort-pulsed lasers have been widely used for manipulation of biomedical samples. The optical tweezer technique is the method that manipulates nano- to micrometer-sized particles in three spatial dimensions by using forces generated by focused lasers . Continuous-wave (CW) lasers are commonly used in the optical tweezer technique; however, recently ultrashort-pulsed lasers, especially in the femtosecond range of 100 fs or less in pulse duration have been applied to trap particles . The combination of the multiphoton imaging and optical trapping techniques proves to be a valuable tool for biophotonics and cell study . Ultrashort-pulsed lasers have also been used to ablate intracellular organelle structures via laser-induced production of low-density plasma [26, 34]. Lastly, targeted ultrashort laser pulses have been used to modulate various biological functions by controlling the intracellular Ca2+ concentration [19-22, 24, 35]. Here, we briefly describe the phenomenon of pulsed laser–tissue interactions and discuss the possible mechanisms. We summarize our observations and current applications of the optical modulation method using ultrashort-pulsed lasers, especially femtosecond-pulsed lasers.
2. Direct optical modulation of biological function using femtosecond-pulsed lasers
2.1. Laser–tissue interaction
A highly focused pulsed laser induces multiphoton absorption, which can result in multiphoton ionization in transparent materials, such as biological tissues or cells [36-38]. The electron overcomes the bandgap energy and becomes a free- or quasi-free electron via multiphoton absorption (Fig. 1a). Once a quasi-free electron is produced, it obtains kinetic energy by absorbing photons, a process known as “inverse Bremsstrahlung (antibraking) absorption.” When the kinetic energy of the excited electron reaches the bandgap energy, it can ionize another electron in the ground state by molecular collision, which is known as “impact ionization.” The recurring sequence of inverse Bremsstrahlung absorption and impact ionization leads to an ionization cascade, called “avalanche ionization” (Fig. 1b) . Multiphoton absorption generates large numbers of quasi-free electrons that can initiate avalanche ionization, and this process generates a dense electron cloud, called a “plasma.” This plasma generates cavitation bubbles, which produce a rupture in the material due to violent mechanical effects . Therefore, this process is used mainly for ablative applications, such as LASIK surgery .
The plasma formation process differs depending on pulse duration and photon density. Nanosecond laser pulses below the plasma formation threshold intensity of 1011 W/cm2 do not produce free electrons. For the production of seed electrons by multiphoton ionization and subsequent avalanche ionization, irradiance values must reach the optical breakdown threshold value for a nanosecond pulse. Nanosecond laser pulses at the intensity of the over-irradiance threshold produce too many electrons, which induces a rapid increase in avalanche ionization rate. Thus, free electrons generated by nanosecond laser pulses results in steep plasma formation, which has a detrimental effect on biological functions due to the high kinetic energy. Femtosecond laser pulses generate plasmas with an intensity of over 1013 W/cm2 in pure water. Unlike nanosecond laser pulses, femtosecond laser pulses below the optical breakdown threshold can generate free electrons via multiphoton ionization, which are not sufficient to initiate avalanche ionization. The density of free electrons rises smoothly with increases in irradiance. Thus, free electrons induced by femtosecond laser pulses below the optical breakdown threshold value have a lower density than conventional plasma; this specific dense cloud of free electrons is called low-density plasma. While this low-density plasma has little destructive effect due to its low kinetic energy, it can induce photochemical effects that break chemical bonds or alter molecular compositions .
2.2. Laser-induced plasma formation and reactive oxygen species (ROS)
Free electrons generated by laser irradiation induce ionization or dissociation of water and other molecules, and subsequently produce reactive oxygen species (ROS: superoxide, hydrogen peroxide, and hydroxyl radicals) by electron delivery [40, 41]. Highly reactive oxygen radicals induce oxidative modification of cellular macromolecules, including proteins, lipids, and DNA and cause irreversible damage and subsequent cell death. Oxidative modification of the various ion-transport proteins underlying ion channels changes the permeability of channels and initiates ion release .
The levels of intracellular ROS should be tightly regulated, and cells have developed strong antioxidant defense systems to protect macromolecules from oxidative modification. The first ROS produced in mitochondria is the highly reactive superoxide (O−2), which superoxide dismutase (SOD) converts into a much more stable, and therefore relatively inert, ROS, hydrogen peroxide (H2O2). H2O2 can be further reduced to water (H2O) by many antioxidant enzymes such as catalase, peroxiredoxin (Prx), and glutathione peroxidase (Gpx) [43, 44]. However, production of ROS in mitochondria is accelerated by ROS themselves. Given oxidative stress, ROS generation in only small numbers of mitochondria can affect neighboring mitochondria, eventually propagating an ROS surge throughout the cell via this positive feedback loop [45, 46]. This phenomenon is called ROS-induced ROS release (RIRR), and several studies have revealed how loss of function in a small number of mitochondria can influence overall cell functioning [47-49]. Based on current knowledge, mitochondria-driven RIRR represents a mechanism of amplifying optically generated ROS (Fig. 2).
2.3. Irreversible effect of femtosecond-pulsed lasers on biological samples
Laser pulses that have energy above the plasma formation threshold induce submicrometer-sized bubbles of plasma within a diffraction-limited volume . For the high-energy laser pulse, optical amplifiers are used to increase the energy of each pulse while maintaining the average energy, and thus, amplified femtosecond-pulsed lasers show low repetition rates less than 10 kHz . This laser-induced plasma can precisely ablate diffraction-limited volumes in tissue or specific organelles in the cell. Plasma-mediated ablation provides the opportunity to study the role of specific biological structures, including axons, microglial, mitochondria, and microvessels by ablating without any significant heat damage [39, 50, 51].
Below the plasma formation threshold energy, laser pulses can also induce irreversible damage to the biological sample, especially in cells. Tirlapur et al.  found that a mean power over 7 mW of unamplified 80-MHz 170-fs laser pulses generated ROS in scanned regions. Laser-induced ROS resulted in impaired cell division or initiated apoptotic cell death. Scanning with low laser power and relatively long beam dwell time (60–120 µs per pixel) induced a cytotoxic effect; whereas brief exposures of high laser power with a short beam dwell time (∼2 µs per pixel) on a diffraction-limited volume (∼femtoliter) in cells also evoked damage . This optical stimulation induced whole mitochondrial fragmentation even though the cytosolic laser-exposed region was less than 1 µm2 in area, suggesting the involvement of the intermitochondria network.
2.4. Optical modulation of various biological functions
The cytoplasm is a restrictive medium for the diffusion of charged compounds and ions such as ROS because of its highly reducing environment. Thus, intracellular signaling systems utilizing ROS frequently operate via local communication between the sources and targets [53, 54]. The endoplasmic reticulum (ER) can be influenced by the ROS produced by mitochondria, due to its close proximity to mitochondria and abundance throughout the cytoplasm [55, 56]. Established ROS-dependent regulators include Ca2+ channels (ryanodine receptors; RyRs and inositol 1,4,5-triphosphate receptors; InsP3Rs), cAMP-dependent kinases (PKA), and Ca2+/calmodulin-dependent kinases (CaMK), that can associate with Ca2+ transport proteins via anchoring proteins (Fig. 2) [57-60]. All of these scenarios suggest that ER-mitochondrial coupling serve as the center stage for ROS-Ca2+ cascade.
Recent biophotonic studies have indicated that femtosecond-pulsed lasers stimulation could modulate many biological functions, regardless of cell type by controlling intracellular Ca2+ concentrations [20, 22, 25, 35]. Localized Ca2+ release from the ER through Ca2+ channels such as InsP3 or RyRs, initiates calcium-induced calcium release (CICR) . CICR, which is related to ER Ca2+ release channels, plays a role in generation of intracellular Ca2+ waves and is important in the excitation of muscle cells and neurons [62-64]. The intracellular Ca2+ wave propagates to adjacent cells though gap junctions, which are intercellular connections that allow various molecules and ions to pass freely between cells . The processes mentioned above modulate Ca2+-dependent signaling in tissues. Cells are classified into two types: excitable cells that are able to produce and respond to electrical signals, called action potentials; and nonexcitable cells that also react to electrical signals, but cannot produce action potentials. Although the excitability of these cell types differs, the intracellular Ca2+ level of both is tightly regulated due to its physiological importance [66, 67].
2.4.1. Optical modulation of cellular functions in excitable cells
Excitable cell types including neurons, muscles, and secretory cells, require electrical signals to regulate their functions. The difference in calcium ion concentration between the inside and outside of the cell induces an electrical potential, leading to changes in cellular activities. The entry of calcium ions into neurons causes action potentials and neurotransmitter release. Ca2+ is also an essential molecule for regulation of contraction of all types of muscle .
Recently, optical methods of inducing action potentials on neurons have emerged. One such method is use of photoactivatable chemical molecules. Many chemical compounds that could release bioactive molecules such as glutamate or Ca2+ have been developed, and such compounds are metaphorically termed “caged compounds” or “caged molecules” . The exposure of light alters the chemical structure of caged compounds liberating (“uncaging”) the caged bioactive molecule. Caged compounds along with focused light irradiation could directly trigger membrane depolarization and action potential in the neuron. However, the compounds should be introduced into the neuron, and they have off-target effects that limit functional specificity. Hence, caged compounds are largely limited to in vivo applications [69, 70].
Another optical method for manipulation of neuronal activity using femtosecond-pulsed lasers has been developed. Hirase et al.  showed that exposure of an 76-MHz, 13-fs laser pulses of 780–800 nm produced an action potential on a pyramidal neuron in the absence of any exogenous molecules like caged compounds. The authors found that only mode-lock pulsed laser stimulation could induce this action potential on the neuron, but CW laser irradiation could not. This result indicated that the nonlinear effect, especially multiphoton absorption, is crucial to depolarization. Stimulation at low intensity and for a longer exposure duration showed different responses compared to a higher-intensity and shorter-duration exposure. The former showed induction of sustained depolarization, which was mediated by ROS, but the latter produced rapid depolarization, which might be the result of membrane pore formation due to photomechanical effects. Liu et al.  found that femtosecond-pulsed laser stimulation triggered a calcium wave in the irradiated hippocampal neuron. Then, the laser-induced calcium wave propagated to adjacent neurons, which allowed the authors to identify neural circuits ex vivo. Because there was no need for an exogenous probe or genetic modification, the femtosecond-pulsed laser was proposed as a useful optical tool for neurophysiology studies.
Femtosecond-pulsed laser stimulation can modulate muscle contractility. There have been many reports that femtosecond laser pulses generate intracellular calcium waves in a variety of cell types [20, 24, 73]. By controlling intracellular Ca2+ concentration, femtosecond-pulsed laser stimulation could induce muscle contraction, in which Ca2+ plays a critical role. Muscle is classified into three types; skeletal, smooth, and cardiac muscle. Skeletal muscle involves a voluntary action that has a distinct series of alternating light and dark bands perpendicular to the long axis. Skeletal muscle fibers can be detected by label-free imaging techniques, using autofluorescence or second-harmonic generation . After laser stimulation, skeletal muscle shows rapid twitch contraction and returns to its basal length within several minutes (Fig. 3a). Smooth muscle, located within the walls of blood vessels, the urinary bladder, and respiratory tract, lacks the distinct banding pattern found in skeletal muscle, and nerves innervating smooth muscle are derived from the autonomic division. Thus, smooth muscle is not normally under direct voluntary control. Femtosecond-pulsed laser irradiation changes the cytosolic Ca2+ concentration, leading to smooth muscle contraction without nerve activity. This method can induce the contraction of arterial blood vessels without use of exogenous probes, such as caged molecules. Laser irradiation focused in the brain artery wall caused localized circular contraction; the artery recovered its basal lumen diameter within a few minutes (Fig. 3b) . Laser irradiation also caused bladder smooth muscle contraction (Fig. 3c) . The bladder wall has a smooth muscle layer that controls its capacity. Laser irradiation of dissected bladder smooth muscle tissue induced localized increases in calcium ion concentration, followed by whole smooth muscle tissue contraction. The bladder smooth muscle fibers recovered to their basal length within a few minutes. This optical method for modulation of muscle contractility can be used as an alternative therapeutic tool in neuromuscular diseases.
Femtosecond laser pulses can alter the intracellular Ca2+ concentrations in cardiomyocytes, a type of muscle cell that provides contractility to the heart. Using an 82-MHz, 80-fs laser pulses of 780 nm, an 8-ms exposure induced an intracellular Ca2+ wave, and the Ca2+ signal propagated to nearby regions. Cardiomyocyte beat rate was synchronized to laser irradiation frequency [23, 24]. Jenkins et al.  showed that a pulsed infrared diode laser (λ = 1.875 µm) coupled light into a multimode fiber 400 µm in diameter and modulated pacing of the embryonic quail heart in vivo. This study used relatively long (millisecond) pulses and a long wavelength, without sufficient photon energy to overcome the bandgap energy. Thus, the laser intensity was insufficient to generate low-density plasma directly. Although the mechanisms remain unclear, both studies were remarkable in that optical stimulation was shown to have the potential to act as a pacemaker.
2.4.2. Optical modulation of cellular functions in nonexcitable cells
The role of Ca2+ in nonexcitable cell types including epithelial cells, endothelial cells, and astrocytes, is different from that in excitable cell types. Ca2+ signaling in nonexcitable cells is closely involved in cell death, migration, and cell differentiation. Thus, nonexcitable cells maintain low intracellular Ca2+ concentrations by tightly regulating the flux of Ca2+ between cellular compartments . Despite their nonexcitability, femtosecond-pulsed lasers can control the Ca2+ signaling and subsequent cellular functions of nonexcitable cells.
Femtosecond-pulsed laser stimulation can modulate blood–brain barrier (BBB) permeability in vivo . Brain microvascular endothelial cells are linked by tight junctions that interconnect adjacent endothelial cells, forming a physiological barrier, called the BBB. Most endogenous and exogenous macromolecules do not cross the blood vessel wall due to BBB. Thus, exogenous delivery of molecular probes or drugs is widely used for in vivo brain research and brain-disease therapy. Recent studies showed that unamplified 80-MHz, 120-fs pulsed laser stimulation could modulate BBB permeability in vivo. Brief laser exposure of the brain vein wall caused a transient break in tight junctions and extravasation of plasma into the brain parenchyma (Fig. 4a). We have observed that the irradiated blood vessel wall and BBB were recovered within several minutes after stimulation . By combining this method with systemic injection, laser-induced extravasation can be used for local delivery of functional molecular probes, such as the astrocyte staining dye SR101 (Fig. 4b), nuclear staining probe Hoechst 33342 (Fig. 4c), nanoparticles, and adenovirus, into the brain. This optical method has the advantages of noninvasive introduction of macromolecules into the brain without opening the skull.
Astrocytes are the most abundant cell type in the central nervous system (CNS). Unamplified 80-MHz femtosecond-pulsed laser irradiation focused on a single astrocyte induced intracellular calcium wave generation in the irradiated astrocyte in vitro and in vivo [21, 35]. In response to elevation of intracellular calcium, astrocytes release neuromodulatory signaling molecules that modulate vasomotion of the brain arteries. After a short femtosecond-pulsed laser irradiation exposure of an astrocyte wrapped around an artery, the astrocyte showed rapid increases in levels of intracellular calcium ions, followed by artery dilation in vivo (Fig. 5). This optical method has drawbacks compared to other techniques, such as caged molecules or optogenetics, because it does not target specific molecular events. However, it has the advantages of being label-free, noninvasive, and does not show deleterious effects, such as microglial activation.
Optical modulation of biological functions using femtosecond-pulsed lasers has become an important method in various biomedical fields. By allowing instantaneous high energy delivery to a three-dimensional localized area, femtosecond-pulsed lasers can generate low-density plasma. One effect of low-density plasma is ROS production. ROS induce calcium ion release through ER calcium channels, generating a calcium wave that modulates many biological functions. Optical approaches have advantages with regard to both precision and minimal invasiveness compared to chemical and electrical methods. Optical methods for modulating biological functions using femtosecond-pulsed lasers provide new opportunities in areas ranging from basic biological studies to the treatment of human disease.
Ultrashort-pulsed lasers have many applications other than modulation of biological functions described above. These lasers can be applied to study functional neural circuits by Ca2+ propagation, which is caused by optical stimulation . In addition, these lasers are utilized in tumor treatment by targeting the vasculature formation that results from tumor-associated aberrant angiogenesis. Laser irradiation generates a high dose of ROS, which induces cytotoxic effects that result in destruction of the blood vessels that supply the nutrients and oxygen required for tumor survival . With the further development of laser technology, the application of ultrashort-pulsed lasers in biomedical fields will expand. In particular, biological modulation methods combined with imaging systems can be useful as both therapeutic and research tools.
This research was supported by a grant (2011K000286) from the Brain Research Center of the 21st Century Frontier Research Program, funded by the Ministry of Education, Science and Technology, the Republic of Korea (to C.C.).
Chulhee Choi is Professor and Chair of the Optical Bioimaging Center at Korea Advanced Institute of Science and Technology (KAIST). His researches are focused on developing in vivo imaging technique and system, and discovering potential drugable targets of malignant cancers using in vivo-mimetic tumor models. Recently, he is delineating the molecular mechanisms of the tissue-photon interaction induced by ultra-short pulsed lasers as a novel tool for modulation of multiple cellular functionsa.
Myunghwan Choi is a post-doctoral research fellow at Harvard Medical School and Wellman Center for Photomedicine, Massachusetts General Hospital. He has worked on in vivo modulatory effect of ultrashort pulsed lasers. He mainly contributed on vascular permeability control, muscular contraction, and astrocyte activity control using ultrashort pulsed lasers.
Junseong Park is a post-doctoral research fellow at the Information & Electronics Research Institute of KAIST. He has worked extensively on many aspects of cell biology and systems biology using both wet work and computational methods. He mainly contributed to elucidation of cell signaling network and progression of diseases including hepatitis C and cancer, and identified many drug targets.
Won Jong Choi received his B.S. degree in biomedical engineering and applied mathematics & statistics at Johns Hopkins University (Baltimore). After graduation, he joined Cell Signaling and Bio-Imaging lab at Korea Advanced Institute of Science and Technology (KAIST). He studied the functions of femtosecond laser and its relationship with calcium signaling in muscle contraction.
Jonghee Yoon is a Ph.D. candidate in bio and brain engineering department at the Korea Advanced Institute of Science and Technology (KAIST). He has studied biophotonics using ultrashort-pulsed lasers. He mainly contributed to mechanisms of laser-induced calcium wave generation and applications for biomodulation such as muscle contraction and cell death using ultrashort-pulsed lasers.