2.1

Fiber Bragg grating sensing

For temperature measurements during LA, we used fiber optic sensors based on the FBG phenomenon. An FBG is a wavelength-dependent reflector resulting from a periodic variation of the refractive index in the fiber core: it reflects only a specific wavelength while transmitting others. The reflected wavelength, λ_B, is proportional to the grating period Λ (distance between two regions with high refractive indices) [24]:

Temperature or strain applied to an FBG changes the grating period Λ, that, in his turn, alters λ_B. Under constant strain conditions, the measurement of this Bragg shift, Δλ_B, can be used to reconstruct temperature change, ΔT, applied to the grating, making each FBG to be a temperature point sensor:

where k [nm/°C] is the thermal sensitivity of the grating.

It is possible to inscribe a chain of FBGs with different grating periods in one fiber. This chain, also called an FBG array, reflects a set of different λ_B, analysis of which allows multi-point temperature measurement across the fiber.

For our experiments, we used highly dense FBG arrays inscribed in a special polyimide-coated single-mode optical fiber using the femtosecond point-by-point writing technology [25]. Polyimide coating provides higher temperature resistance (short term up to 400 °C and continuous operation at 300 °C) in comparison with acrylate one (up to 85 °C) [25], [26]. Each array has 25 FBGs with a 0.9 mm grating length and a 0.1 mm edge-to-edge distance between gratings. In order to have a wavelength-division multiplexing of the array (to discriminate FBGs in the spectral region) the periods of the gratings and related Bragg wavelengths were changed along the array. Fig. 1a depicts the reflection spectrum of the array measured by the Micron Optics si255 interrogation unit (Micron Optics, Atlanta, USA). Temperature sensitivity coefficients of the FBG arrays were obtained after the calibration in a thermal chamber for 30 °C to 140 °C temperature range. As alternative sensors, we used commercially available acrylate-coated FBG arrays containing 5 FBGs with 1 mm grating length and 2 mm edge-to-edge spacing (AtGrating Technologies, Shenzhen, China).

Figure 1.

(a) Reflection spectrum of the fabricated highly dense FBG array with the femtosecond point-by-point writing technology: 25 peak Bragg reflections equidistant in 1464.5-1614.5 nm wavelength range; (b) Cryo-TEM image of Au/Cu@PDA nanocomposite (scale bar is 500 nm).

2.2

Preparation of gold/copper–polydopamine nanocomposite

NC consisting of mussel-inspired polydopamine (PDA), gold NPs and copper species, was prepared by in situ co-reduction approach as described in our recently published work [27]. First, the precursor of copper oxide NPs was loaded during oxidative polymerization of dopamine in the presence of air and Tris buffer (pH 8.5). Then, the acidic solution of HAuCl₄ was added to initiate a redox reactions between the PDA and metal ions (Au³⁺ and Cu²⁺). As a result, water soluble Au/Cu@PDA NC was formed. Cryogenic transmission electron microscopy (cryo-TEM, FEI Talos 200C) was used to visualize the presence of the gold NPs embedded in the PDA matrix, the two components that mainly provide the photothermal behavior (Fig. 1b).

2.3

Experimental setup

The main experimental setup is shown in Fig. 2: continuous wave (CW) LA was performed by a diode laser LuOcean Mini 4 (Lumics GmbH, Berlin, Germany) with a wavelength of λ=808 nm, an output power of 2.2 W and a treatment time of 10 s guided by a 300 μm-diameter quartz optical fiber immersed in the ex vivo phantom placed in a 1.5 ml quartz cuvette. The phantom consisted of blended porcine pancreas tissue mixed with the PDA NCs embedded with gold NPs and copper species; tissue mixed with water was used for control measurements. The laser-guiding fiber and arrays were immersed in the phantom parallel to each other, and LA was repeated three times. FBG arrays were positioned at fixed distances (2 mm and 4 mm) from the applicator tip. The acrylate-coated FBG array with 5 gratings was not placed at 2 mm distance in order to prevent possible damage of the fiber due to high temperatures. Spectra measured during ablation were analyzed to reconstruct temperature profiles across the FBG arrays.

Figure 2.

(a) Schematic representation of the laser ablation of the porcine phantom placed in quartz cuvette: FBG arrays were positioned at 2 mm (the highly dense array) and 4 mm (the highly dense array and the acrylate-coated FBG array with 5 gratings) from the applicator tip; (b) Zoom-in of the schematic illustrating dimension characteristics of the gratings.

Also, a comparison of the highly dense FBG array and the thermal camera measurements was done during the laser irradiation of a distilled water in the cuvette. In this case, only one array of 25 FBGs was positioned at a 2 mm distance from the laser tip. The infrared thermal imaging camera (FLIR T540, FLIR® Systems, Inc), positioned at a 30 cm distance, measured the surface temperature of the cuvette walls at a 10 Hz rate.

3.1

The highly dense FBG array and the infrared thermal camera measurements comparison

For this experiment, the temperature during laser irradiation of water in the quartz cuvette was measured with the FBG arrays and the thermal camera. Both techniques were not able to measure the real maximum temperature of the water because the FBG arrays were positioned at 2 mm distance from the tip, while the camera measured only the surface temperature of the cuvette walls. Fig. 3a reports the highest temperatures measured by both techniques during the irradiation. As can be seen, the camera significantly underestimates the maximum temperature of the solution as it measures the temperature of the walls. Moreover, this underestimation can be even higher in the case of confined temperature increase near the laser tip (for example, during irradiation of the solution with lower heat conduction to the cuvette walls).

Figure 3.

(a) The maximum temperatures of the solution in cuvette measured by the FBG sensors and the thermal camera; (b) peak temperature profiles measured during laser ablation of the control sample (pancreas tissue mixed with water) and experimental one (pancreas tissue mixed with the PDA NC embedded with gold NPs and copper species). The array with 5 FBGs (blue color) underestimates the maximum temperature due to the lower resolution of the array.

3.2

The effect of the NCs presence during laser irradiation of the pancreas phantom

Fig. 3b illustrates peak temperature profiles measured during the laser irradiation. The NCs (Au/CuO@PDA) presence increased the maximum temperature reached during LA from 48 °C (control) to 90 °C (NCs) at 2 mm. While at 4 mm distance, the NCs presence increased temperature from 33 °C to 36 °C (measured by 25 FBGs), and from 31 °C to 35 °C (measured by 5 FBGs). The low spatial resolution of the commercial arrays produced an underestimation (in comparison with the highly dense arrays) of the peak temperature by 2 °C (control), and by 1 °C (NCs) at 4 mm.

Fig. 4 reports thermal maps (distance along the sensor vs time) during the laser irradiation of the experimental sample measured by the commercial and the highly dense arrays at a 4 mm distance from the applicator tip. After the irradiation starts at 20 s, the temperature increases, and the heat distributes towards the edges of the sensing length of the array. When ablation is over, the temperature starts to decrease, but the heat dissipation is continuing towards the edges. The highly dense array in comparison with the commercial one has a higher spatial resolution (1 mm vs 2 mm) and longer sensing length (25 mm vs 9 mm), that results in better thermal maps.

Figure 4.

Two-dimensional thermal maps (distance along the sensor vs time) during laser irradiation of the pancreas tissue blended with NCs at a 4 mm distance measured by: (a) the commercial array with 5 FBGs; (b) the highly dense array with 25 FBGs.