**3.1 Laser-induced generation of micro-bubbles in a free water**

Formation of micro-bubbles in a free water was studied with the aid of the optical methods using a water filled plastic cell (the horizontal dimensions are 150 × 100 mm and the height is 15 mm) and glass capillaries with an inner diameter of 1 mm. In the most of experiments, the working fiber tip is preliminary blackened by a short (~1 s) contact of the fiber tip with a wooden plate at a laser power of about 3 W. The fiber tip surface thus covers by a thin carbon layer owing to the wood burning. Such a procedure is well reproduced, so that from 10 to 20% of the laser power is absorbed in the thin carbon layer. Computer controlled fiber lasers (LS-0.97 and LS-1.55 of IRE–Polus, Russia) with the wavelengths of 0.97 μm and 1.55 μm, 1–10 W in power were interfaced with a 400 μm core diameter silica fiber. Low intensity (up to 1 mW) green pilot beam from the built in diode laser was used to highlight the laser irradiated zone in the cell. The fiber is horizontally fixed in the cell, which is placed on the worktable of a MICROS MC300 microscope equipped with a Vision digital color camera interfaced with PC. The water cell was also placed on the table with illumination, and the processes in the vicinity of the heated fiber tip were visualized using a Photron Fastcam SA-3 camera at rates of 2000 or 10000 frames per second. To control the laser induced spectrum, an Ocean Optics USB4000 fiber spectrum analyzer was used, which is interfaced with PC and has an optical resolution of about 1.5 nm and 200–1100 nm wavelength range. For better visualization of hydrodynamic flows the collargol (albumin coated Ag nanoparticles) have been added to water in the cell (Yusupov et al., 2011b).

Hydrodynamic flows taking place nearby the fiber tip when laser power is on, can be clearly seen in a scattering mode using illumination with green light of pilot laser beam through the same transport fiber (Fig. 4). Such flows result in intrusion of collargol from neighboring area into the area in front of the fiber tip. One can also see here the initial process of new intrusion formation (outlined with a dashed line). The rate of rise-up front of a given intrusion (which is about 150 μm in average thickness) is found to be described by exponential low (Yusupov et al., 2011b)

$$V = 0.6 \cdot \exp(-1.5 \cdot r) \,, \tag{1}$$

where *r* is the distance from fiber tip: at 1 mm from fiber tip *V* = 150 μm/s, while at 2 mm from fiber tip *V* falls down to 30 μm/s.

The bubbles don't occur up to laser power of 10 W with non-blackened fiber tip and for 0.97 μm laser radiation, while for 1.56 µm laser radiation (which is much stronger absorbed by water) the bubbles are generated at about 1 W of laser power. Blackening of fiber tip results in generation of bubbles for both 0.97 µm and 1.56 µm laser wavelengths.

We believe that *effective hydrodynamic processes* play dominant role for the effect of a medium power laser-induced regeneration and healing of connective tissues diseases (intervertebral hernia, osteomyelitis and some other diseases) using laser puncture procedures (Chudnovskii & Yusupov, 2008; Chudnovskii et al., 2010a, 2010b). Main features of these

The key process for the mechanism of medium power laser-induced regeneration and healing of musculoskeletal system diseases is the generation of micro-bubbles in inter-tissue

Formation of micro-bubbles in a free water was studied with the aid of the optical methods using a water filled plastic cell (the horizontal dimensions are 150 × 100 mm and the height is 15 mm) and glass capillaries with an inner diameter of 1 mm. In the most of experiments, the working fiber tip is preliminary blackened by a short (~1 s) contact of the fiber tip with a wooden plate at a laser power of about 3 W. The fiber tip surface thus covers by a thin carbon layer owing to the wood burning. Such a procedure is well reproduced, so that from 10 to 20% of the laser power is absorbed in the thin carbon layer. Computer controlled fiber lasers (LS-0.97 and LS-1.55 of IRE–Polus, Russia) with the wavelengths of 0.97 μm and 1.55 μm, 1–10 W in power were interfaced with a 400 μm core diameter silica fiber. Low intensity (up to 1 mW) green pilot beam from the built in diode laser was used to highlight the laser irradiated zone in the cell. The fiber is horizontally fixed in the cell, which is placed on the worktable of a MICROS MC300 microscope equipped with a Vision digital color camera interfaced with PC. The water cell was also placed on the table with illumination, and the processes in the vicinity of the heated fiber tip were visualized using a Photron Fastcam SA-3 camera at rates of 2000 or 10000 frames per second. To control the laser induced spectrum, an Ocean Optics USB4000 fiber spectrum analyzer was used, which is interfaced with PC and has an optical resolution of about 1.5 nm and 200–1100 nm wavelength range. For better visualization of hydrodynamic flows the collargol (albumin coated Ag nanoparticles) have

Hydrodynamic flows taking place nearby the fiber tip when laser power is on, can be clearly seen in a scattering mode using illumination with green light of pilot laser beam through the same transport fiber (Fig. 4). Such flows result in intrusion of collargol from neighboring area into the area in front of the fiber tip. One can also see here the initial process of new intrusion formation (outlined with a dashed line). The rate of rise-up front of a given intrusion (which is about 150 μm in average thickness) is found to be described by

where *r* is the distance from fiber tip: at 1 mm from fiber tip *V* = 150 μm/s, while at 2 mm

The bubbles don't occur up to laser power of 10 W with non-blackened fiber tip and for 0.97 μm laser radiation, while for 1.56 µm laser radiation (which is much stronger absorbed by water) the bubbles are generated at about 1 W of laser power. Blackening of fiber tip results

in generation of bubbles for both 0.97 µm and 1.56 µm laser wavelengths.

*V r* 0.6 exp( 1.5 ) , (1)

processes will be considered below.

water (Yusupov et al., 2010).

**3. Laser-induced generation of micro-bubbles in water**

**3.1 Laser-induced generation of micro-bubbles in a free water**

been added to water in the cell (Yusupov et al., 2011b).

exponential low (Yusupov et al., 2011b)

from fiber tip *V* falls down to 30 μm/s.

Fig. 4. Microscope pictures (in scattering mode) of intrusions of Ag nanoparticles in water (outlined with dashed line) stimulated by laser induced hydrodynamics nearby optical fiber tip at 1.0 W of 0.97 µm laser power in 6 s (a), 12 s (b), and 18 s (c) of laser irradiation. Fiber tip is shown by dashed line (Yusupov et al., 2011b).

Energy of incident laser light is partly (10–20%) absorbed by the carbon layer on the blackened fiber, so that the fiber is heated. When laser radiation with a power of greater than 3 W is transmitted by the fiber tip in air, the spectrum of the optical radiation from the fiber tip contains the fundamental line (0.97 µm or 1.56 µm) and the broadband visible and near-IR radiation caused by the heating of the tip surface to relatively high temperatures. When a blackened tip is placed into water, the tip surface is effectively cooled and the absence of the broadband radiation means the substantially lower temperatures of the tip surface. However, a medium power laser radiation (1–5 W) is sufficient for surface heating and generation of vapor-gas bubbles. When water is heated, the dissolved gases are liberated in the vicinity of the tip surface and gas bubbles emerge. Water is evaporated inside the bubbles, so that the bubbles are filled with vapor and, consequently, increase in size. At the lower boundary of the above power interval, the bubbles increase in size residing on the tip surface (Fig. 5a). When a critical size is reached, the bubbles are detached and move to the surface.

Water molecules which approach the heated tip surface acquire additional kinetic energy and momentum. The component of the total momentum of vapor molecules that is directed perpendicularly to the tip surface of the fiber towards water appears insufficient for the detachment of the bubble. Figure 5a shows that the bubbles sizes can be close to the diameter of the silica fiber core (400 µm). In the experiments, the bubbles normally emerge at same spots on a tip surface, which correspond to a high temperature areas. Evidently, the presence of such spots is related to the nonuniformity of the carbon layer: the absorbed energy (and, hence, the temperature) is greater for thicker regions. The stabilization (i.e., the

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 101

fiber to water (Fig. 5b) and, then, the velocity decreases due to viscosity. At a finite exposure time the tracks of bubbles moving in water was observed. Notice that the track length corresponds to the mean velocity of the bubble over the exposure time. Bright spots in the vicinity of the tip surface (Fig. 5) are related to stray light: the Vision video camera is

The side measurements (Fig. 6a) show that the bubbles come to the surface at a certain distance from the fiber. Knowing the vertical velocity of the bubbles (about 5 mm/s in accordance with visual observations) and the trajectories, we can estimate the horizontal velocity (Fig. 6b). The analysis of the trajectories yields an exponential decrease in the horizontal velocity with increasing distance from the fiber: for the slowest and fastest

respectively, where *V* is the horizontal velocity in mm/s and *r* is the distance from the fiber tip surface in millimeters. The relationships show that the velocity of bubbles at the moment

Fig. 6. a - Side view of the tracks of microbubbles in the vicinity of the blackened optical fiber tip surface in water; b - Plots of the horizontal velocity vs. distance from the end surface for slowest (1) and fastest (2) bubbles at a laser wavelength of 0.97 µm and a power

We have directly observed motion of bubbles even in the immediate vicinity of the surface tip (at the maximum velocities) in the experiments on the generation of microbubbles performed with the aid of the Photron Fastcam SA3. Fig. 7 shows the bubbles as dark circles with different sizes. Previous (at time step *Δt*) positions and sizes are shown as open circles, and the trajectories are shown as rectilinear segments. Table 1 presents the calculated sizes and velocities of the bubbles shown in Fig. 7. It is seen that the bubble with a diameter of 47 μm (bubble 7 in Fig. 7a and Table 1), which is initially located at a distance of about 100 μm from the fiber tip, moves at a mean velocity of 97 mm/s over the

of the detachment from the fiber tip (*r* = 0) ranges from 67 to 101 mm/s.

*V r* 67 exp( 0,82 ) (4)

*V r* 101 exp( 0,74 ) , (5)

sensitive to the near-IR laser radiation.

of 5 W (Yusupov et al., 2010).

observation interval (4.4 ms).

and

bubbles, we obtain the dependences(Yusupov et al, 2010)

Fig. 5. Laser-induced generation of microbubbles in the vicinity of the blackened end surface of the optical fiber in water for the laser radiation with a wavelength of 0.97 µm and a power of (a) 1 and (b) 5 W. The photograph is taken from above at an exposure time of 250 ms.

attachment of the vapor-gas bubbles to the high temperature spots) can be caused by two reasons. First, the temperature at the hot spot additionally increases owing to the formation of the bubble and the consequent decrease in the local heat sink to water. The second reason is related to the Marangoni effect (Berry et al., 2000): the temperature gradient gives rise to the gradient of surface tension, so that convective flows emerge on the surface of the bubble and cause the force that presses the bubble to the hot spot. The experiments on the growth of the bubbles in the vicinity of the tip surface show that the rate of growth gradually decreases and, finally, the growth is terminated. At a laser power of 1 W, the duration of a relatively fast growth is about 200 ms. Bubble size increases at this stage from zero to 25% of the maximum size. Then, over a few seconds, the growth is well described with the formula (Yusupov et al, 2010):

$$D \propto t^{4/5}\,,$$

where *D* is the diameter of bubble and *t* is time. When laser light is terminated (Fig. 5a), the size of bubble gradually decreases (the bubble remains attached to the tip surface of the fiber) and, finally, the bubble vanishes. Note that a decrease in the size is also nonmonotonic. At the first stage with a duration of less than 1 s, the diameter decreases by 8– 10%. Then, the slowing takes place. Such a non-monotonic behavior must be related to the fact that the size of bubble decreases at the first stage predominantly, due to a decrease in the temperature of the vapor-gas mixture inside the bubble to the temperature of water in the cell, whereas the second stage is isothermal. The lifetime of such bubbles ranges from 3 to 8 h, and the rate of a decrease in the diameter with time always monotonically increases. At the second stage, the dependence of the diameter on time is well approximated with the formula(Yusupov et al., 2010):

$$D = D\_0 \cdot (1 - t \;/\; \tau\_0)^{\alpha} \; , \tag{3}$$

where *D0* is the initial diameter, τ0 is the lifetime, and *α* = 0.1–0.5 is the empirical parameter. Note a similar decrease in the diameter with time at *α* = 0.5 in (Taylor & Hnatovsky, 2004). A qualitatively different scenario corresponds to higher laser powers. The explosive boiling of water is observed in the vicinity of the hot end: the vapor-gas bubbles are ejected from the fiber to water (Fig. 5b) and, then, the velocity decreases due to viscosity. At a finite exposure time the tracks of bubbles moving in water was observed. Notice that the track length corresponds to the mean velocity of the bubble over the exposure time. Bright spots in the vicinity of the tip surface (Fig. 5) are related to stray light: the Vision video camera is sensitive to the near-IR laser radiation.

The side measurements (Fig. 6a) show that the bubbles come to the surface at a certain distance from the fiber. Knowing the vertical velocity of the bubbles (about 5 mm/s in accordance with visual observations) and the trajectories, we can estimate the horizontal velocity (Fig. 6b). The analysis of the trajectories yields an exponential decrease in the horizontal velocity with increasing distance from the fiber: for the slowest and fastest bubbles, we obtain the dependences(Yusupov et al, 2010)

$$V = 67 \cdot \exp(-0.82 \cdot r) \tag{4}$$

and

100 Hydrodynamics – Advanced Topics

Fig. 5. Laser-induced generation of microbubbles in the vicinity of the blackened end surface of the optical fiber in water for the laser radiation with a wavelength of 0.97 µm and a power of (a) 1 and (b) 5 W. The photograph is taken from above at an exposure time of 250 ms.

attachment of the vapor-gas bubbles to the high temperature spots) can be caused by two reasons. First, the temperature at the hot spot additionally increases owing to the formation of the bubble and the consequent decrease in the local heat sink to water. The second reason is related to the Marangoni effect (Berry et al., 2000): the temperature gradient gives rise to the gradient of surface tension, so that convective flows emerge on the surface of the bubble and cause the force that presses the bubble to the hot spot. The experiments on the growth of the bubbles in the vicinity of the tip surface show that the rate of growth gradually decreases and, finally, the growth is terminated. At a laser power of 1 W, the duration of a relatively fast growth is about 200 ms. Bubble size increases at this stage from zero to 25% of the maximum size. Then, over a few seconds, the growth is well described with the formula

where *D* is the diameter of bubble and *t* is time. When laser light is terminated (Fig. 5a), the size of bubble gradually decreases (the bubble remains attached to the tip surface of the fiber) and, finally, the bubble vanishes. Note that a decrease in the size is also nonmonotonic. At the first stage with a duration of less than 1 s, the diameter decreases by 8– 10%. Then, the slowing takes place. Such a non-monotonic behavior must be related to the fact that the size of bubble decreases at the first stage predominantly, due to a decrease in the temperature of the vapor-gas mixture inside the bubble to the temperature of water in the cell, whereas the second stage is isothermal. The lifetime of such bubbles ranges from 3 to 8 h, and the rate of a decrease in the diameter with time always monotonically increases. At the second stage, the dependence of the diameter on time is well approximated with the

0 0 (1 / )

where *D0* is the initial diameter, τ0 is the lifetime, and *α* = 0.1–0.5 is the empirical parameter. Note a similar decrease in the diameter with time at *α* = 0.5 in (Taylor & Hnatovsky, 2004). A qualitatively different scenario corresponds to higher laser powers. The explosive boiling of water is observed in the vicinity of the hot end: the vapor-gas bubbles are ejected from the

*DD t*

4/5 *D t* , (2)

, (3)

(Yusupov et al, 2010):

formula(Yusupov et al., 2010):

$$V = 101 \cdot \exp(-0.74 \cdot r) \,, \tag{5}$$

respectively, where *V* is the horizontal velocity in mm/s and *r* is the distance from the fiber tip surface in millimeters. The relationships show that the velocity of bubbles at the moment of the detachment from the fiber tip (*r* = 0) ranges from 67 to 101 mm/s.

Fig. 6. a - Side view of the tracks of microbubbles in the vicinity of the blackened optical fiber tip surface in water; b - Plots of the horizontal velocity vs. distance from the end surface for slowest (1) and fastest (2) bubbles at a laser wavelength of 0.97 µm and a power of 5 W (Yusupov et al., 2010).

We have directly observed motion of bubbles even in the immediate vicinity of the surface tip (at the maximum velocities) in the experiments on the generation of microbubbles performed with the aid of the Photron Fastcam SA3. Fig. 7 shows the bubbles as dark circles with different sizes. Previous (at time step *Δt*) positions and sizes are shown as open circles, and the trajectories are shown as rectilinear segments. Table 1 presents the calculated sizes and velocities of the bubbles shown in Fig. 7. It is seen that the bubble with a diameter of 47 μm (bubble 7 in Fig. 7a and Table 1), which is initially located at a distance of about 100 μm from the fiber tip, moves at a mean velocity of 97 mm/s over the observation interval (4.4 ms).

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 103

Figure 7b and Table 1 show that a short laser pulse with power of 6 W causes generation of many bubbles, whose diameters range from 10 to 41 μm. The velocities of bubbles are 60 and 20 mm/s in the vicinity of the fiber and at a distance of 300 and 800 µm, respectively. In spite of a twofold increase in the laser power, the maximum velocities of the bubbles in the vicinity of the fiber under the pulsed irradiation are significantly less than the velocities corresponding to the continuous wave irradiation. At a relatively large distance from the fiber end, the velocities corresponding to the pulsed irradiation are also less than the velocities corresponding to the continuous wave irradiation: the velocity of bubble 4 in Fig. 7a is almost equal to the velocity of bubble 5 in Fig. 7b, whose distance from the fiber tip is almost two times shorter. Such result indicates to the presence of water flows in the case of the continuous wave laser irradiation and shows that the flow velocity is comparable with

 Such liquid flows are more clearly observed in the microscopic measurements of the laserinduced hydrodynamic effects in the vicinity of the fiber tip surface of the fiber that is

Liquid flows are more clearly observed in the microscopic measurements of the laserinduced hydrodynamic effects in the vicinity of the fiber tip surface of the laser fiber that is

As it follows from Fig. 8, the attached vapor-gas bubbles at a laser power of 1–2 W emerge at the tip surface and the convective motion is observed in the liquid. A qualitatively different scenario corresponds to a power of 3 W: the microscopic bubbles ejected from the fiber tip move along arc shaped trajectories and entrain liquid flows (Fig. 8a). The intensity of the resulting vortices rapidly increases with increasing radiation power (Fig. 8b). In accordance with the estimations based on the frame-to-frame analysis of the video records, the period of the typical circulating liquid flows at laser powers of 3– 5 W ranges from 0.2 to 1 s. Note that the above effects can be observed in the experiments with the blackened fiber tip at both laser wavelengths (0.97 µm and 1.55 μm). In the absence of the preliminary blackening, the effects are observed only for a radiation wavelength of 1.55 μm. Such a difference is caused by the fact that the radiation with a wavelength of 1.55 μm (unlike the short wavelength

Fig. 8. Water flows that actively circulate inside the glass capillary (with a diameter of 1 mm) in the vicinity of the blackened tip surface I heated by the laser radiation with a wavelength

**3.2 Laser-induced generation of micro-bubbles in a glass capillary**

placed in the glass capillary filled with water (model of the laser channel).

the mean velocity of bubbles.

placed in a glass capillary filled with water.

of 0.97 µm and a power of 3 W (a) and 5 W (b)

Closed circles 1–7 show positions of bubbles, open circles show previous positions, and rectilinear segments show bubbles trajectories.

The images are taken from above at rates of (a) 10000 and (b) 2000 frames per second.

Laser powers of (a) 3 and (b) 6 W, time intervals *Δt* = (a) 4.4 and (b) 2.0 ms, and a laser wavelength of 0.97 µm.

The pulse duration is 50 ms and the interval between pulses is 500 ms.

Fig. 7. Displacements of microbubbles (that are generated in the vicinity of the schematically shown blackened tip surface of quartz fiber **I**) over short time intervals *Δt* in the presence of laser radiation (Yusupov et al., 2010). **a** - CW laser radiation. **b** - Pulsed laser radiation.

Such result is in good agreement with the above estimations of the initial velocities in the vicinity of the fiber tip. The velocities of the bubbles rises rapidly with increasing distance from the fiber: the velocities are not higher than 50 and 20 mm/s at distances of 0.5 mm and 2 mm, respectively (Table 1). When bubbles are generated in a viscous liquid over a relatively long time interval the steady-state flow results in increase of the bubbles velocities. To determine the relative contribution of such a flow, we have measured the motion of microbubbles under the pulsed laser irradiation (Fig. 7b). It is seen that the bubbles predominantly move at relatively large angles relative to the fiber axis. That is caused by the features of the tip surface and hydrodynamic effects. Note that the asymmetry also corresponds to the motion of microbubbles under the continuous wave laser irradiation.

Table 1. Parameters of the bubbles shown at Fig. 7

Closed circles 1–7 show positions of bubbles, open circles show previous positions, and rectilinear

Laser powers of (a) 3 and (b) 6 W, time intervals *Δt* = (a) 4.4 and (b) 2.0 ms, and a laser wavelength of

Fig. 7. Displacements of microbubbles (that are generated in the vicinity of the schematically shown blackened tip surface of quartz fiber **I**) over short time intervals *Δt* in the presence of laser radiation (Yusupov et al., 2010). **a** - CW laser radiation. **b** - Pulsed laser radiation.

Such result is in good agreement with the above estimations of the initial velocities in the vicinity of the fiber tip. The velocities of the bubbles rises rapidly with increasing distance from the fiber: the velocities are not higher than 50 and 20 mm/s at distances of 0.5 mm and 2 mm, respectively (Table 1). When bubbles are generated in a viscous liquid over a relatively long time interval the steady-state flow results in increase of the bubbles velocities. To determine the relative contribution of such a flow, we have measured the motion of microbubbles under the pulsed laser irradiation (Fig. 7b). It is seen that the bubbles predominantly move at relatively large angles relative to the fiber axis. That is caused by the features of the tip surface and hydrodynamic effects. Note that the asymmetry also corresponds to the motion of microbubbles under the continuous wave laser irradiation.

Velocity,

1 26 9 17 38 2 26 9 10 37 3 200 3 10 5 4 58 16 41 60 5 42 12 21 20 6 63 48 21 52 7 47 97 27 32

Parameters of radiation

mm/s Diameter, μ<sup>m</sup>

pulsed radiation, 6W (Fig. 7b)

> Velocity, mm/s

The images are taken from above at rates of (a) 10000 and (b) 2000 frames per second.

CW radiation, 3 W (Fig. 7a)

Diameter, μm

Table 1. Parameters of the bubbles shown at Fig. 7

The pulse duration is 50 ms and the interval between pulses is 500 ms.

segments show bubbles trajectories.

0.97 µm.

Number of the bubble (Fig. 7)

Figure 7b and Table 1 show that a short laser pulse with power of 6 W causes generation of many bubbles, whose diameters range from 10 to 41 μm. The velocities of bubbles are 60 and 20 mm/s in the vicinity of the fiber and at a distance of 300 and 800 µm, respectively. In spite of a twofold increase in the laser power, the maximum velocities of the bubbles in the vicinity of the fiber under the pulsed irradiation are significantly less than the velocities corresponding to the continuous wave irradiation. At a relatively large distance from the fiber end, the velocities corresponding to the pulsed irradiation are also less than the velocities corresponding to the continuous wave irradiation: the velocity of bubble 4 in Fig. 7a is almost equal to the velocity of bubble 5 in Fig. 7b, whose distance from the fiber tip is almost two times shorter. Such result indicates to the presence of water flows in the case of the continuous wave laser irradiation and shows that the flow velocity is comparable with the mean velocity of bubbles.

 Such liquid flows are more clearly observed in the microscopic measurements of the laserinduced hydrodynamic effects in the vicinity of the fiber tip surface of the fiber that is placed in a glass capillary filled with water.


*6.11.2 Correlated failures*

*Disasters* that affect a specific area can lead to vast service interruption, and consequently, many customers have to be recovered by CSPs. In this case, it is possible that related servers cannot handle all the customers' requests. Therefore, it can be critical to multiplex customers' data of the same area in different servers. One major challenge in this case is how to distribute customers' traffic and data between cloud servers to minimize correlated failure risks with respect to required QoS for each server and also cloud SLAs [14].


**1. Introduction**

The main goal of the use of music in intensive care units (ICU) is to reduce anxiety and pain [1]. Music in ICU has been shown to reduce respiratory rate, blood pressure and heart rate [2]. Music interventions are also associated with decreased pain, sedation and post-traumatic stress disorder in children operated on for congenital heart disease [3].

According to the American Music Therapy Association, music therapy intervention (MT) is "the clinical and evidence-based use of music interventions to accomplish individualized goals within a therapeutic relationship by a credentialed professional who completed and approved music therapy program" (available online: https://www.musictherapy.org/ about/musictherapy/ (accessed on 14 June 2022)). Music therapy intervention has the potential to alleviate pain, promote physical rehabilitation and help manage stress. Its use is recommended by the Society of Critical Care Medicine in both pediatric and adult ICU [4,5]. The use of MT in neonatal ICUs also suggests that MT is feasible and might have a positive impact on premature children. Little has been reported about the effect of MT in the pediatric ICU (PICU) setting and especially how parents respond to MT [6–9]. Therefore, the aim of this study was to describe the perception of MT by parents of children who were exposed to MT in PICU.

**Citation:** Cousin, V.L.; Colau, H.; Barcos-Munoz, F.; Rimensberger, P.C.; Polito, A. Parents' Views with Music Therapy in the Pediatric Intensive Care Unit: A Retrospective Cohort Study. *Children* **2022**, *9*, 958. https:// doi.org/10.3390/children9070958

Academic Editor: Dafna Regev

Received: 12 May 2022 Accepted: 21 June 2022 Published: 26 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **2. Methods**

This quality improvement initiative was supported by the Foundation Alta Mane. It was implemented in April 2019 in a tertiary 12-bed PICU at the Geneva Children's Hospital, Switzerland, and is still running as of today. As a retrospective study on a quality improvement project, and in agreement with institutional policy, this study was exempt from ethics committee review. Nonetheless, parents' oral consent was required for participation in the study.

We aimed to evaluate the perception of such an initiative by the parents of a sample of children admitted to our unit. An evaluation of the program was performed by means of two five-point Likert scale questionnaires. The questionnaires were designed and created by the authors to collect views from parents whose children received music intervention in the PICU. This questionnaire was adapted from Moss et al. [10,11], but without previous validation. It was addressed to parents whose children were exposed to MT throughout a five-month survey period (February–June 2021, Supplementary Material). Only patients who were fully responsive and not receiving sedation were exposed to MT. A personalized approach to MT was used whenever possible. In order to avoid possible MT side effects (e.g., anxiety, overstimulation and unwanted memory triggering), MT was tailored to the patients' needs and constantly adapted to the child's responses to music and sound. Two versions of the questionnaire (French and English translation) were given to parents. Participants were encouraged to add comments and suggestions at the end of each questionnaire.

Hospitalized patients participated in personalized MT with a certified music therapist (HC). The therapist was present 4 days per week, for a total of 15 h per week. Music therapy interventions were provided at the bedside in each patient's room. The duration of sessions varied between 25 and 60 min based on the patient's needs, preferences and clinical conditions. Parent's and child's direct participation are considered key components of MT in our PICU, and were encouraged whenever possible. No psychotherapy was offered during MT. Several different instruments were used during MT sessions (kalimba, rain stick, guitar, ukulele, metallophone, small djembe and other percussions).

Descriptive analyses were performed by VLC and AP, using Microsoft Excel 2013 (Microsoft Corporation, Redmond, WA, USA). As our study is solely descriptive, no statistical comparisons were performed.

#### **3. Results**

During the study period, 263 patients received a total of 603 h of interventions. The main primary diagnoses were represented by congenital heart diseases (155 patients, 59%), respiratory diseases (42 patients, 16%), central nervous system diseases (24 patients, 9%), post-surgical interventions (16 patients, 6%) and solid organ transplantations (8 patients, 3%).

Parents' views on MT were studied over a 4-month period (February–June 2021); a total of 25 questionnaires were handed over to the parents of PICU patients whose mean age was 7 years (SD+/ −6). The number of sessions per patient was 1 to 2, 2 to 4 or ≥5 in 50%, 39% and 11% of patients, respectively.

One parent per patient answered the questionnaire, with a response rate of 76% (19/25). The results from the parents' questionnaires are summarized in Figure 1. The main reasons for PICU admissions of children whose parents responded to the questionnaire were congenital heart diseases (3/19), central nervous system diseases (3/19), liver insufficiency (3/19) and respiratory insufficiency (3/19). All parents thought that MT was beneficial for their child. Specific benefits of MT, as stated by parents, included helping to communicate (89%), feel less isolated (100%) and cope with being in hospital (100%). Overall, 100% parents felt that their child was respected and supported in MT. The majority of parents also thought that MT contributed to physical recovery (90%) and alleviated feelings of anxiety (90%). Of the parents surveyed, 63% thought that MT facilitated communication with the PICU team. The majority of parents (93%) also believed that MT should be offered as an out-patient service. A few examples of open-ended responses are shown in Figure 1.

**Figure 1.** Parents' answers to questionnaire. *Y* axis: proportion in %. MT: music therapy. **Figure 1.** Parents' answers to questionnaire. *Y* axis: proportion in %. MT: music therapy.

#### **4. Discussion 4. Discussion**

sponses are shown in Figure 1.

In this single-center study, we showed that MT could introduce alternative ways of supporting children's and parents' psychological and possibly physical well-being. We also found that MT might promote constructive communication between families and the PICU team. In this single-center study, we showed that MT could introduce alternative ways of supporting children's and parents' psychological and possibly physical well-being. We also found that MT might promote constructive communication between families and the PICU team.

that MT should be offered as an out-patient service. A few examples of open-ended re-

Our results are in accordance with recent literature showing that MT is feasible and is accompanied by a high level of appreciation from staff and families in the PICU setting [12,13]. Music therapy interventions might also reduce anxiety and pain in critically ill children [14–16]. Our results underlined the complex role of MT with a potential impact on multiple aspects of patients' and parents' perceptions during their stay in the PICU. Actually, the Society of Critical Care Medicine strongly recommended MT as part of a multicomponent intervention aimed at reducing analgesic use and improving pain management in pediatric patients in ICU [5]. Our results are in accordance with recent literature showing that MT is feasible and is accompanied by a high level of appreciation from staff and families in the PICU setting [12,13]. Music therapy interventions might also reduce anxiety and pain in critically ill children [14–16]. Our results underlined the complex role of MT with a potential impact on multiple aspects of patients' and parents' perceptions during their stay in the PICU. Actually, the Society of Critical Care Medicine strongly recommended MT as part of a multicomponent intervention aimed at reducing analgesic use and improving pain management in pediatric patients in ICU [5].

Our report has several limitations. Our results are based on a small sample size. Although it is based on an already published questionnaire, the survey used in this study was created by the authors for the sake of this analysis and has never been validated. We chose the survey approach as our experience with the qualitative interview approach is limited. We believed that, although less informative, the use of an already published survey might have helped us to overcome the lack of familiarity with the qualitative interview approach. However, similarly to humanities in general, MT is a complex phenomenon and is more than just listening to music [17]. As such, the real impact of MT on patients and parents might be only partially captured by objective measurable components [18]. We were able to confirm the usefulness and feasibility of MT in the PICU in terms of patient-centered outcomes and the team's acceptance. Further research looking at patients' and their parents' experiences is warranted to accurately adapt MT interventions to patients' needs and help promote children's autonomy. Our report has several limitations. Our results are based on a small sample size. Although it is based on an already published questionnaire, the survey used in this study was created by the authors for the sake of this analysis and has never been validated. We chose the survey approach as our experience with the qualitative interview approach is limited. We believed that, although less informative, the use of an already published survey might have helped us to overcome the lack of familiarity with the qualitative interview approach. However, similarly to humanities in general, MT is a complex phenomenon and is more than just listening to music [17]. As such, the real impact of MT on patients and parents might be only partially captured by objective measurable components [18]. We were able to confirm the usefulness and feasibility of MT in the PICU in terms of patientcentered outcomes and the team's acceptance. Further research looking at patients' and their parents' experiences is warranted to accurately adapt MT interventions to patients' needs and help promote children's autonomy.

**Supplementary Materials:** The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: parents' questionnaires. **Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/children9070958/s1, Figure S1: parents' questionnaires.

**Author Contributions:** V.L.C. drafted the initial manuscript. H.C. collected the data. A.P. conceived and designed the analysis. All authors critically revised the manuscript. All authors have read and agreed to the published version of the manuscript. **Author Contributions:** V.L.C. drafted the initial manuscript. H.C. collected the data. A.P. conceived and designed the analysis. All authors critically revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This quality improvement initiative was supported by the Foundation Alta Mane Italia. **Funding:** This quality improvement initiative was supported by the Foundation Alta Mane.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki. As a retrospective study on a quality improvement project, and in agreement with institutional policy, this study was exempt from ethics committee review **Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki. As a retrospective study on a quality improvement project, and in agreement with institutional policy, this study was exempt from ethics committee review.

**Informed Consent Statement:** Informed and written consent was obtained from all subjects involved in the study **Informed Consent Statement:** Informed and written consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Data are accessible upon reasonable request to the corresponding author.

**Acknowledgments:** We thank the Foundation Alta Mane for the financial support of our initiative. In particular, we are indebted to the Director of the Foundation Alta Mane, Cristina Tango, whose support was essential for the project.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**

MT music therapy

PICU pediatric intensive care unit

#### **References**


**1.4.2 Structural organization**

384 Non-Viral Gene Therapy

and degree of protonation of polycationic vectors depend on their amount of primary, secondary, and tertiary amines, which greatly influences the cell toxicity, the escape of

Gene transfer to eukaryotic cells is a long process encompassing several successive steps. Plasmid DNA must be packaged into complexes/particles first. Next, the DNA-containing complexes/particles must associate with cells and become internalized into them by cellular uptake processes. Following uptake, DNA-containing complexes/particles must escape the endosomal compartment into the cytoplasm and release their DNA-cargo. Finally, DNA must translocate into the cell nucleus to be transcribed into mRNA and subsequently translated into protein antigen (Nguyen et al., 2009). For all these steps to successfully occur, the final characteristics of the lipo- and polyplexes to be employed must, undoubtedly, be studied and tailored. Of especial importance for complex formation and gene delivery, we

The ability of cationic vectors to condense DNA into nano-sized complexes is believed to be crucial for gene therapy (Sahay et al., 2010; Tros de Ilarduya et al., 2010). DNA compaction, also known as condensation, is a reversible coil to globule transition favored by the binding of cationic vectors to the negatively charged DNA phosphate groups (Bloomfield, 1997). When the number of neutralized charges reaches a critical value, DNA undergoes localized bending or distortion, which facilitates the formation of complexes with sizes much smaller (in the range of nanometers) than that of the DNA coil conformation (in the range of microns) (De Smedt et al., 2000; Wilson & Bloomfield, 1979). Analyzing the impact of experimental conditions on the resulting complex dimensions, several studies have demonstrated that parameters like type, size and modification of the cationic vector, the carrier/DNA charge ratio, and also the protocol of complex formation, can all exert a strong influence (Ogris et al., 1998). On the other hand, with respect to the role of complex size on the cellular internalization mechanism, it has been demonstrated that lipo- and polyplexes with sizes up to 200 nm are taken up by the clathrin-dependent pathway, whereas aggregates larger than 500 nm are internalized via clathrin-independent mechanisms

Apart from a reduction in size, binding of cationic vectors to DNA also imparts a positive charge. Although this positive charge is important for both cellular-binding and internalization, it might also be a cause for concern for in vivo applications since cationic complexes readily bind with serum proteins such as serum albumin, promoting aggregation and blood clearance (Tros de Ilarduya et al., 2010). Additionally, an excess of positive charge, commonly reflected by complexes formed at high vector to DNA ratios, might lead to cytotoxicity provided that negatively charged cell membranes are prone to be damaged in the presence of cationic, extracellular compounds (Thomas & Klibanov, 2003). Thus, a successful gene delivery procedure, pursuing a high transfection efficiency at the lowest possible cytotoxicity, should find a delicate balance in the complex surface charge, that is, the complex must possess a high enough positive charge so as to ensure a proper cellcomplex association, but at the same time this necessary positive charge must not cause a

polyplexes from endosomes, and the transfection efficiency (Behr 1997).

**1.4 Physicochemical aspects of relevance for gene therapy**

can enumerate the following physicochemical aspects.

**1.4.1 Size and surface charge**

(Rejman et al., 2004; Sahay et al., 2010).

lethal damage to the cell.

The DNA ordering inside lipoplexes has been reported in the form of four main conformations: one with a short-range lamellar structure composed of flat lipid bilayers and DNA packed between them (Battersby et al., 1998; Dias et al., 2002; Lasic et al., 1997; Radler et al., 1997; Salditt et al., 1997), another where the DNA molecules are encapsulated inside a lipid bilayer forming cylindrical complexes that are closely packed on a hexagonal network (Koltover et al., 1998), another where positively charged vesicles attach to the extended DNA molecule, the so-called ''beads on a string'' model (Felgner et al., 1987; Gershon et al., 1993; Ruozi et al., 2007; Sternberg et al., 1994), and a final one where DNA is expected to collapse and attach in the form of a globule into the outer surface of positively charged vesicles (Miguel et al., 2003). What can be drawn from all four cases, whatever the nature of the interactions, is that at dilute concentrations no structural change in the systems is present, namely vesicle or bilayer disruption, whereas at high concentrations the situation becomes different with vesicles tending to disrupt and flocculate (Dias et al., 2002; Radler et al., 1997; Salditt et al., 1997;). Concerning polyplexes, rod-like, globular, and toroidal DNA condensates are the morphologies most commonly observed (Carnerup et al., 2009; Danielsen et al., 2004).

### **1.4.3 Binding affinity**

The capability of lipo- and polyplexes to avoid premature dissociation and promote the release of genetic material to the cytoplasm once inside the cell is strongly related to the binding affinity between the DNA and the vector in question (Prevette et al., 2007). Indeed, a strong binding affinity between DNA and its carrier entails a high DNA compaction and protection against degradation in the extracellular environment. However, following their escape from endosomes, the complexes need to approach the nucleus, as well as dissociate; as such, a high DNA-vector binding affinity might constitute a limiting step for transfection considering the difficulty in the separation of the DNA from its gene carrier (Tros de Ilarduya et al., 2010). For the case of polyplexes, presenting by far the highest degree of DNA binding affinity (and condensation), it is well accepted that the molecular weight (Mw) of the polycation (directly related to the cationic valence) is a key factor controlling the DNA-vector binding affinity and subsequent transfection. In general, lower Mw polycations yield higher DNA transfection efficiencies (Ziady et al., 1999) as the DNA dissociation from them is faster (Schaffer et al., 2000).


*2.2. Evaluation of Cellular Viability*

Qualitative analysis of the cell spheroids for cellular viability was done using the Live/Dead assay kit (Molecular Probes, Eugene, OR, USA). These spheroids were washed twice with the growth media

before calcein acetoxymethyl (Molecular Probes) and ethidium homodimer-1 (Molecular Probes) were added for an incubation period of 30 min at room temperature. Then, the spheroids were evaluated using a fluorescence microscope (Axiovert 200; Zeiss, Oberkochen, Germany) on Days 3 and 7.

Quantitative analysis of cell viability was performed using the cell counting kit-8 (CCK-8; Dojindo, Tokyo, Japan) on Days 1, 3, 5, and 7. WST-8 solution was added to the stem cell spheroids, which were then cultured for 45 min at 37 ◦C. Absorbance was measured using a microplate reader at 450 nm (BioTek, Winooski, VT, USA).


**2. Electromechanical impedance based concrete strength estimation**

As already described, in order to manage effectively the construction process of the concrete structures, online monitoring of the concrete strength-development is strongly required. To end this, this study employs the electromechanical impedance-based structural health monitoring (SHM) methodology because the electromechanical impedance can represent the mechanical properties of a host structure. The strength variation developed during the curing process can be observed throughout the electromechanical impedance measurements.

#### **2.1 Strength development due to concrete curing process**

Concrete achieves its strength through a hydraulic process known as hydration. With the addition of the correct amount of water, the cement gels into a paste that glues sand and aggregates together to form hardened concrete. The curing of concrete involves maintaining a proper moisture vapor transmission rate (2% mvtr) immediately after concrete placement

accessibility of the impedance method for in-field measurements. Lynch et al. (2004) designed a wireless active sensing unit to monitor civil structures, which was constructed of off-the-shelf components and had the ability to command active sensors and actuators from a computational core combined with wireless transmission and sensing circuits, embedded algorithm to process the acquired data, and structural status broadcasting. Grisso and Inman (2005) designed a DSP (Digital Signal Processor) based prototype to provide wireless assessment of thermal protection systems. It was able to directly detect damages by analyzing variations of the electrical impedance of PZT sensors bonded to the structure. The obtained impedance signals were compared with the pre-stored baseline and a statistical damage index was calculated. Mascarenas et al. (2007) proposed a wireless sensor node which consists of a miniaturized impedance measuring chip, a microprocessor, and a radiofrequency identification (RFID) module. Low cost impedance measuring chip actuated the structure through a PZT and measured the structural impedance response, and RFID module delivered the diagnostic result to a base station. Park S. et al. (2009) improved the wireless sensor node of Mascarenas et al. (2007) by adding two multiplexer IC chips for 16 channels for the cases of multiple sensors in a small region and by embedding signal processing algorithms in the microcontroller unit (MCU) for both structural damage identification and sensor self-diagnosis. Research groups at Los Alamos National Lab recently developed a compact impedance-based wireless sensing device (WID3) for low power operation (Overly et al. 2007, 2008), which requires around 60 mW of power to operate. Here, a wake-up capability was combined for low power operation. Taylor et al. (2009a, b) extended the capability of the WID3 by implementing a module with lowfrequency A/D and D/A converters to measure low-frequency vibration data for multiple SHM techniques. It is wirelessly triggered by a mobile agent for use in the mobile-hostbased wireless sensing network. Min et al. (2010) developed a multi-functional wireless sensor node integrating piezoelectric actuating, sensing, signal processing, temperature

In this context, this study presents a series of efforts to confirm the applicability of the electromechanical impedance technique using both wired and wireless systems for online

As already described, in order to manage effectively the construction process of the concrete structures, online monitoring of the concrete strength-development is strongly required. To end this, this study employs the electromechanical impedance-based structural health monitoring (SHM) methodology because the electromechanical impedance can represent the mechanical properties of a host structure. The strength variation developed during the curing process can be observed throughout the electromechanical impedance

Concrete achieves its strength through a hydraulic process known as hydration. With the addition of the correct amount of water, the cement gels into a paste that glues sand and aggregates together to form hardened concrete. The curing of concrete involves maintaining a proper moisture vapor transmission rate (2% mvtr) immediately after concrete placement

monitoring on the strength-development during the curing process of concrete.

**2.1 Strength development due to concrete curing process**

**2. Electromechanical impedance based concrete strength estimation**

compensating, and energy harvesting modules.

measurements.

as well as throughout the ensuing period of approximately 28 days. Fig. 1 shows a typical strength development curve (Shariq et al., 2010).

Fig. 1. Typical Strength Development Curve

#### **2.2 Electromechanical impedance modeling**

The electromechanical impedance-based SHM techniques have been developed as a promising tool for real-time structural damage assessment on critical members of large structural systems (Park G. et al., 2003, Koo et al., 2009, Taylor et al., 2009a, 2009b, Mascarenas et al., 2009). They make use of piezoelectric sensors such as piezoceramic (PZT) and macro-fiber composite (MFC) patches, which form a collocated sensor and actuator, often referred to as a self-sensing actuator (Giurgiutiu, 2007). The basis of this active sensing technology is the energy transfer between the actuator and the host mechanical system. If a PZT attached on a structure is driven with a sinusoidal voltage, it causes the local area of the structure to vibrate (the converse piezoelectric effect). And the structural response causes an electrical response in the PZT (the direct piezoelectric effect). Liang et al. (1996) first proposed a one-dimensional analytical model of this setup as in Fig. 2, and showed that the electrical admittance (inverse of the electrical impedance), Y(ω), of a PZT is directly correlated to the local mechanical impedance of the host structure, Zs(ω), and that of a PZT patch, Za(ω), in most applications as

$$Y(\alpha) = G(\alpha) + jB(\alpha) = j\alpha \mathbb{C} \left( 1 - \kappa\_{31}^2 \frac{Z\_s(\alpha)}{Z\_s(\alpha) + Z\_a(\alpha)} \right) \tag{1}$$

where G is the conductance (real part); B is the susceptance (imaginary part); C is the zeroload capacitance of a PZT; and κ31 is the electromechanical coupling coefficient of a PZT. Given that the mechanical impedance and the material properties of the PZT stay constant, the equation shows that a change in the structure's mechanical impedance directly results in a change in the electrical impedance measured by the PZT. Since damages cause a change in the structure's local mass, stiffness, or damping properties and consequently its mechanical impedance, the structure's mechanical integrity can be assessed by monitoring the PZT's electrical impedance. It should be noted that the admittance function, Y(ω), is a complex number. Bhalla et al. (2002) demonstrated that the real part of the measured admittance is more sensitively changed due to the structural damage condition as compared to the

Ubiquitous Piezoelectric Sensor Network

Fig. 3. Schematic diagram of a self-sensing circuit

**3. Development of wireless impedance sensor nodes**

**3.1 Subsystems of wireless impedance sensor nodes**

functional wireless sensor node developed by Min et al. (2010) was utilized.

(UPSN)-Based Concrete Curing Monitoring for u-Construction 79

The recent advances in wireless online monitoring integrating actuation and sensing, onboard computing, and radio-frequency (RF) telemetry improved the accessibility of the electromechanical impedance method for in-field measurements. In this study, the multi-

To measure the electromechanical impedances, impedance analyzers such as HP4294A are conventionally used. However, they are not quite suitable for field applications to online SHM because they are bulky (approximately 25 kg) and expensive (approximately 40,000 USD). Thus, research on the impedance based SHM technique trends toward development of self-contained sensors and wireless active sensor nodes with all required functions including actuating/sensing, data processing, damage assessments and sensor selfdiagnostics on the sensor board as well as power management with energy harvesters. Recently, Analog Devices© developed an integrated impedance converters, AD5933 (www.analog.com). It is equipped with a 12-bit analog-to-digital converter (ADC), a digitalto-analog converter (DAC) and a discrete Fourier transform (DFT) functionality. The frequency generator allows an external complex impedance with range of 100 Ω to 10MΩ to be excited with a known frequency of up to 100 kHz. AD5933 is just of a penny size, thus it provides a solution for self-contained miniaturized impedance measuring. Therefore, AD5933 has been used as a core component in developing a wireless impedance sensor node for SHM applications (Mascarenas et al. 2007, 2009, Overly et al. 2007, 2008, Taylor et al. 2009a, b, Park S. et al. 2009, Min et al. 2010). The wireless sensor node, proposed in this study, has extended the previous researches for multifunctional and environment-friendly uses in the impedance-based SHM (Mascarenas et al. 2007, Park S. et al. 2009). It was designed by adding: (1) optimal arrangement of each chip for low power consumption, (2) energy harvester equipped with solar panels, (3) peer-to-peer communication by using a RF transceiver of CC2420, which enables to construct the ubiquitous sensor network, (4) internal algorithms for operations, which are optimized by using microcontroller-dependent

imaginary part. On the other hand, Park G. et al. (2006) found out that the imaginary part can be more effectively used for piezoelectric sensor self-diagnosis.

Fig. 2. 1-D Model used to derive electromechanical admittance of a PZT bonded to a structure (Liang et al. 1996)

#### **2.3 Self-sensing based wired impedance measurement method**

In this study, impedance measurement systems are based on a self-sensing technique, as shown in Fig. 3. A self-sensing circuit as described in Fig. 3 is suitable for use in cast-in-site concrete because it is inexpensive and has sufficient accuracy to measure the development of strength, even though the impedance signal is less accurate than other impedance measurement methods. The self-sensing circuit board consists of a single PZT patch, and a voltage divider, such as a resister or capacitor to acquire the output voltage.

The impedance is measured in three steps based on the self-sensing circuit as follows: (1) the input voltage (Vi ) generated from an arbitrary waveform generator (AWG) is applied to the free surface of the PZT sensor; (2) the output voltage from the self-sensing circuit (Vo) is measured using a digitizer (DIG); and (3) the admittance, which is the inverse of impedance, is derived from the input voltage, output voltage, and reference capacitance (Cr). The output voltage from the self-sensing circuit consists of the input voltage and mechanical response of the structure (Vr). Although the amplitude of the mechanical responses of the structure is small enough to ignore, the output voltage is dominated by the input voltage and can be approximated as follows (Lee and Sohn, 2006)

$$V\_o(t) = \frac{C\_p}{C\_p + C\_r} \left[ V\_i(t) + V\_p(t) \right] = \frac{C\_p}{C\_p + C\_r} V\_i(t) \tag{2}$$

where Cp is the PZT capacitance and Cr is the reference capacitance of the self-sensing circuit. From Eq. (2), the impedance of structure (*Z*) is derived as follows:

$$Z(o) = \frac{V\_p(o)}{I(o)} = [i\nu C\_p]^{-1} = \left[ i\alpha C\_r \left( \frac{V\_o(o)}{V\_i(o) - V\_o(o)} \right) \right]^{-1} \tag{3}$$

Thus, the impedance explained at Eq. (1) could be measured by Eq. (3) using the self-sensing circuit displayed in Fig. 3.

imaginary part. On the other hand, Park G. et al. (2006) found out that the imaginary part

Fig. 2. 1-D Model used to derive electromechanical admittance of a PZT bonded to a

In this study, impedance measurement systems are based on a self-sensing technique, as shown in Fig. 3. A self-sensing circuit as described in Fig. 3 is suitable for use in cast-in-site concrete because it is inexpensive and has sufficient accuracy to measure the development of strength, even though the impedance signal is less accurate than other impedance measurement methods. The self-sensing circuit board consists of a single PZT patch, and a

The impedance is measured in three steps based on the self-sensing circuit as follows: (1) the input voltage (Vi ) generated from an arbitrary waveform generator (AWG) is applied to the free surface of the PZT sensor; (2) the output voltage from the self-sensing circuit (Vo) is measured using a digitizer (DIG); and (3) the admittance, which is the inverse of impedance, is derived from the input voltage, output voltage, and reference capacitance (Cr). The output voltage from the self-sensing circuit consists of the input voltage and mechanical response of the structure (Vr). Although the amplitude of the mechanical responses of the structure is small enough to ignore, the output voltage is dominated by the input voltage and can be

> ( ) () () ( ) *p p o i p i p r p r C C Vt Vt V t Vt C C C C*

where Cp is the PZT capacitance and Cr is the reference capacitance of the self-sensing

1 ( ) ( ) () [ ] ( ) () () *p o p r*

*I V V*

Thus, the impedance explained at Eq. (1) could be measured by Eq. (3) using the self-sensing

 ω

− = = −

+ + (2)

*i o*

ω

ω

 ω 1

−

(3)

= +

*V V Z iwC i C*

circuit. From Eq. (2), the impedance of structure (*Z*) is derived as follows:

ω

ω

**2.3 Self-sensing based wired impedance measurement method**

voltage divider, such as a resister or capacitor to acquire the output voltage.

can be more effectively used for piezoelectric sensor self-diagnosis.

structure (Liang et al. 1996)

approximated as follows (Lee and Sohn, 2006)

ω

circuit displayed in Fig. 3.

Fig. 3. Schematic diagram of a self-sensing circuit