Method for optical capturing of particle in soft biological tissue

FIELD: physics.

SUBSTANCE: method for optical capturing of a particle in soft biological tissue is based on irradiating the surface of the tissue with a parallel beam of coherent laser radiation and determining the depth z of the captured particle in the tissue. The radiation wavelength λ* is selected depending on the depth z - for z<0.1 mm λ*=450 nm, for z≥0.1 mm λ*=1250·[1-exp(-z/1.35)], where λ* is given in nm and z in mm.

EFFECT: invention provides maximum particle capturing force with minimal heating of the tissue.

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The invention relates to the creation of optical traps (laser tweezers) to capture particles or aggregate particles within soft biological tissues. It can be used in the study of the structural, biophysical, morphological and optical properties of particles of biological tissue in vivo and their interaction with the environment for retention of the particles in a certain place tissue or manipulation.

It is known [1] that changing the flux density of the I of the light field along an arbitrary axis x there is a force Fgradelectromagnetic nature, which acts on a dielectric particle that enters the spatial region of the specified changes. The absolute value of the force Fgraddepends on the gradient dI/dx in the x direction, and the optical and structural parameters of the particles and the environment in which it is located. Force Fgradis called the gradient and is used in optical traps (laser tweezers) to capture, move and other contactless transactions with small particles.

Known [2] method of capture of dielectric particles in an optical trap created by the forces FPof light pressure on the particle, which is formed by one or more laser sources. These forces act in the direction of radiation propagation, and their absolute value is

where A is the coefficient of reflection of light by the particle.

The disadvantages of this method [2] are the formation of weak forces (1) and its unsuitability for the capture of particles within biological tissues because of the need to use the high power density of the E0irradiation of the surface of the fabric to provide the desired force FP.The increase in E0causes excessive heating of the tissue and can lead to its damage or destruction.

Also known method [3], in which the density gradient of the luminous flux is created by interference of two-mode laser beams with wavelength λ=632.8 nm. These beams are directed into the cell with the particles due to the action of gradient forces are captured near the highs (bright areas) interference pattern.

The disadvantage of this method [3] is its inapplicability to biological tissue in vivo, since light with a wavelength of 632.8 nm does not provide the required magnitude of force Fgradin a wide range of depths z of the fabric, where it can be captured particle. In addition, because of light scattering in biological tissue interference pattern formed in the depth of the medium, usually severely diluted in space that leads to a noticeable reduction in the gradient of the flux density and, consequently, the force Fgrad.

Closest to the proposed method is with�persons [4] capture biological particles in the optical trap, created an infrared laser, which generates light with a fixed wavelength in the range from 800 to 1800 nm. The laser radiation is incident on a converging lens of small focal length and focus (beam waist) is formed by a large gradient of the light field and the corresponding force Fgrad. Converging the laser beam is directed into the sample cell containing the particle that is captured within a specified focal point of the converging lens.

The disadvantage of this method [4] is its inapplicability to biological tissue in vivo, since infrared radiation with these wavelengths does not provide the required magnitude of force Fgradin a wide range of depths z of the fabric, where it can be captured particle. In addition, due to strong light scattering in biological tissue the waist region of the beam is usually very blurred in space that leads to a noticeable reduction in the gradient of the flux density and, consequently, the force Fgrad. Note also that the method of [4] does not answer the question, what is the wavelength of laser irradiation for optimal trapping of particles at a given depth z in the tissue from the point of view of providing the maximum grip force of the particle.

The object of the present invention is the ability to capture particles or aggregate particles in a wide range of depths z of biological tissue by Optim�form of further selection of wavelength λ *irradiation of its surface, forming the maximum force capture particles with minimal heating of tissue.

The solution of this problem is achieved in that in the method of optical capture particles in soft biological tissues, based on the irradiation surface of the tissue with laser light to form a parallel beam of coherent laser radiation, determine the depth z of the location of the captured particles in the tissue and, depending on the depth z is chosen wavelength λ*exposure - when z<0.1 mm λ*=450 nm, at z≥0.1 mm λ*=1250[1-exp(-z/1.35)], where λ*in nm, z in mm.

The essence of the invention is illustrated by drawings.

Fig.1 shows the radial structure of the flux density I(r) in the soft tissue on the example of the dermis of the skin at wavelengths of λ=600 nm (solid curves) and 700 nm (dashed) when the degree of blood oxygenation S=0.5 (a, b) and 0.97 (b, g), volumetric blood concentration (Cb=0.04 (a, b) and 0.02 (b, g); volume concentration of melanin Cm=0.08, z=1 mm, E0=1 W/cm2.

Fig.2 shows the dependence of the gradient of the force F exerted by a laser beam of light at depth z=0.16 (curves 7), 0.2 (2), 0.5 (3), 1 (4), 2 (5), 4 (6) and 8 mm (7) upon irradiation of the skin surface at various wavelengths λ=400-1800 nm.

Fig.3 shows the calculated (solid curve) and approximate (dashed) dependence of wavelength λ*exposure, about�ensuring maximum gradient force F maxfrom the depths of the z position of the captured particles within the dermis of the skin.

It is known that the processes of scattering biological tissue coherent beam of radiation lead to the formation within the environment of the speckle pattern of the light field. The speckle structure is the result of interference of radiation scattered under small angles relative to the direction of light incidence [5]. In a radial plane or the plane perpendicular to this direction, it represents the alternating bright and dark regions, called speckles. This change in the density of the luminous flux of I leads to the formation of the gradient force Fgradthat can be calculated by the formula

where C=3·1010/n cm/s is the speed of light in the environment,

n is the absolute value of the refractive index of the medium,

α=3(m2-1)/(m2+2) - specific polarizability of the particle,

m=np/n is the relative refractive index of the particle,

np- the absolute value of the refractive index of the particle,

R is the radius of the sphere of the same volume as the particle,

dI/dx is the gradient of the density of luminous flux (watts/cm3),

B - constant of proportionality, depending on the parameters of particles (npand R) and environment (n), in which it is located.

From (2) that the force Fgradis directed along the x-axis in the direction of increasing (if > 1) or decrease (if m<1) density of the luminous flux. For particles of biological tissue typically m≈1.05.

The characteristic radius L speckle dependent on the wavelength λ of irradiation of the surface and depth z, is given by the formula [5, 6]

where D(λ,z) is the variance of the angular intensity distribution of light propagating at small angles relative to the direction of illumination of the surface.

The full density of the luminous flux at a depth z in the radial plane can be calculated by the formula [6]

where E0- illumination of the surface of the fabric,

r is the distance measured from the beam axis,

φ is a random phase,

Ec(λ,z) and Enc(λ,z) is a normalized luminance values generated respectively coherent and incoherent scattered light at a depth z in the said plane during irradiation of the surface at wavelength λ.

Note that in the right part of (4) the rst term gives the component of the light field, depending on r and the second is the incoherent background that does not depend on r. Therefore, in the formation of gradient forces in the depth of the environment is the contribution of only the first term.

Fig.1 shows the radial structure of the flux density I(λ,z,r), calculated by the authors according to the formula (4) at two values of λ=600 and 700 nm at a depth of z=1 mm. as an example biotinylated human skin. Structural and optical parameters is given in [7], and the method of calculation of the characteristics of E(λ,z) and L(λ,z) of the light field in [5, 6, 8]. Selected typical values of structural and biophysical parameters of the skin. Here the degree of blood oxygenation S=0.5 (a, b) and 0.97 (b, g), volumetric blood concentration (Cb=0.04 (a, b) and 0.02 (b, g), volumetric melanin concentration (Cm=0.08, thickness 20 microns of the stratum corneum and epidermis 100 µm. The authors have performed calculations for other values of the specified parameters of the skin. They varied, typical for this type of tissue [7]. It turned out that the variable on the y component (the rst term in the right part of (4) depends only weakly on these changes and is determined mainly by the values of λ and z.

From the formula (4) find the gradient of the density of luminous flux dI/dr, which produces a force Fgradacting on a particle located at a depth of z:

The minus sign indicates the direction of the force is in the direction of decreasing or increasing r. As can be seen from (1) and (5), the gradient force Fgradin a radial plane takes the absolute value of the maximum value corresponding to the equality|sin[πr/L(λ,z)+ϕ] |=1. From (1) and (5) it also follows that the maximum absolute value of force Fgrad

depends on λ and z through the characteristics of speckles Ec(λ,z) and L(λ,z).

Comparable maximum values of the gradient strength (6) and the pressure force (2) at the same power density of the irradiation surface. For this example, particles with R=3 µm consider the relation Fmax(λ,z)/FPmax={8παR[3(1+A)Imax(λ,z,r)]}(dI/dr)maxwhere the index max means the maximum value of the respective quantities. Let A=1. The calculations showed that this ratio lies in the range of 150-800 at z<2 cm. in Other words, a maximum gradient strength of about 2-3 orders of magnitude higher than the highest pressure force, so that the latter can safely be neglected. Similar calculations showed that it is possible because of the smallness in comparison with dlldr not take into account the density gradient of the luminous flux dI/dz in the z axis direction, i.e. in the direction of light propagation.

Fig.2 presents the values of the gradient of the force F(λ,z) (in H) created a parallel laser light beam with E0=1 W/cm2at depths z=0.16 (curves 1), 0.2 (2), 0.5 (3), 1 (4), 2 (5), 4 (6) and 8 mm (7) upon irradiation of the surface of the tissue at different wavelengths λ=400-1900 nm. In calculations used are typical for soft tissue PA�Amery n=1.33, m=1.05 and R=3 µm. As can be seen, with increasing z, the maximum force Fmaxprovides exposure to increasing wavelength λ*. So, in the upper layers of the dermis at z=0.12 mm, the largest value of Fmaxoccurs when λ*≈450 nm, at z≈0.5 mm - λ*≈700 nm, at z≈1 mm, λ*≈850 nm, etc. features of the dependence of the force F of the wavelength λ shown in Fig.2, and the presence of the maximum at λ=λ*depends on the spectral behavior of the absorption characteristics and scattering components of the soft tissues, primarily derivatives of hemoglobin of blood and water.

Using the data of Fig.2, compare the value of force Fmaxacting on particles within the tissue during irradiation of the surface environment of the proposed method and the prototype [4] (λ=800-1800 nm). Suppose for concreteness the irradiation is carried out at a wavelength of λ=1000 nm. As can be seen from Fig.2, the upper layer of fabric at z≤0.5 mm (curves 1-3) and depth at z≥4 mm (curves 6 and 7) the values of Fmaxthe proposed method is about 2-4 times higher than the maximum power according to [4] at the same power density E0. At z=1-2 mm (curves 4 and 5), both methods give approximately the same maximum force. Similar conclusions can be drawn for other wavelengths of radiation to the surface of the fabric from the range 800-1800 nm, was proposed in [4]. Note that the calculation results in Fig.2 shows �La the case of irradiating the surface of the fabric parallel beam of light. If the beam is converging, as in [4], the excess of the force Fmaxthe proposed method over [4] will be in a wide range of depths z even more pronounced, because in this case falling on the surface of the laser energy will be in the depth is distributed over a larger area.

Fig.3 illustrates how to find the wave length λ*for maximum force Fmaxat a given depth z in the tissue. Here shows the calculated dependence of λ*(z) obtained from the graphs of Fig.2 (solid curve) and its approximation (dashed curve) by the formula λ*=1250[1-exp(-z/1.35)], where λ*in nm, z in mm. Minor differences between these curves lead to a small deviation of the force Fmaxfrom its maximum value. However, such a deviation does not exceed 5%.

Thus, the proposed method allows for a wide range of depths z in the tissue to generate maximum force Fmaxcapture of a particle or aggregate of particles due to the optimal selection of the wavelength of irradiation of the surface of the fabric. The specified force in 2 and more times the gripping power of a particle in accordance with the prototype.

Sources of information

1. B. M. Yavorsky, A. A. Detlaf. Handbook of physics. M.: Science, 3rd edition. 1965. Pp. 347-348.

2. A. Ashkin. Apparatuses for trapping and accelerating neutral particles. US Patent No.370279. H01S 3/06, 3/09. 09.01.1973.

3. A. A. Afanas'ev, V. M. Tatarkiewicz, A. N. Rubies, T. S. Efendiev. Fashion�AZIA concentration of particles in the interference field of laser radiation // Phys. applied. spectrosc. 2002. T. 69. No. 5. S. 675-679.

4. A. Ashkin. Non-destructive optical trap for biological particles and method of doing same. US Patent No.4893886. G02B 27/00. 16.01.1990.

5. Ivanov A. P., Katsev I. L. On the speckle structure of the light field in the dispersion medium, illuminated by a laser beam // Quantum electronics. 2005. Vol. 35. No. 7. Pp. 670-674.

6. N. D. Abramovich, V. V. Barun, S. K., Dick, A. S. Cierech. Analytical method of estimating the contrast of the speckle pattern of the light field, diffuse, soft biological tissues // 5th Trinity conference "Medical physics and innovations in medicine". The collection of materials. 2012. Vol. 1. P. 212-214.

7. V. V. Barun, A. P. Ivanov, A. V. Volotovsky, V. S. Ulashchik. The absorption spectra and the depth of penetration of light in normal and diseased human skin // Journal of applied spectroscopy. 2007. T. 74. No. 3. Pp. 387-394.

8. V. V. Barun, A. P. Ivanov. The absorption of light by blood when low-intensity laser irradiation of the skin // Quantum electronics. 2010. T. 40. No. 4. Pp. 371-376.

Method of optical capture particles in soft biological tissues, based on the irradiation surface of the tissue by laser light, characterized in that form a parallel beam of coherent laser radiation, determine the depth z of the location of the captured particles in the tissue and, depending on the depth z is chosen wavelength λ*exposure - when z<0.1 mm λ*=450 nm, at z≥0.1 mm λ*=1250·[1-exp(-z/1.35)], where λ*in nm, z Tim.



 

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