Vibrating Micro-Capsule as an Autonomous Device for in-situ Endoscopic

Scan and Body Tissue Characterization

WEHBE H.1, RABIH A.2, LEFEBVRE F.1, NASSAR G.1

1Institute of Electronic, Microelectronic & Nanotechnology, INSA (HdF), UPHF

Le Mont-Houy-59300 Valenciennes,

FRANCE

2Faculty of Technology, Lebanese University,

Saïda,

LEBANON

Abstract: – This work concerns the implementation of a diagnostic technique for the digestive system using a

device designed based on a spherical vibrating capsule of 1 cm in diameter and autonomous by its onboard

electronics. For local scanning, this device is also equipped with a network of high-frequency optimized sensors.

Depending on the desired diagnosis, this device can adapt its electromechanical characteristics to the nature of the

application in transmission and/or simultaneous reception. It is also designed for network configuration to provide

a 3D mapping of the acoustic properties of the environment under investigation (biological tissues).

Key-Words: - Pill resonator, Acoustic Sensor, Tissues Characterization, Endoscopic scan, Embedded electronic,

Soft diagnosis, Micro-System.

Received: June 12, 2023. Revised: November 23, 2023. Accepted: December 27, 2023. Published: February 12, 2024.

1 Introduction

Today, our lives are assisted by intelligent concepts

thus guiding our gestures, our movements, and even

our way of life. In multiple areas, studies have

focused on the adaptation and optimization of devices

integrating into new models of socio-technological

interaction involving the different actors of society in

areas as diverse as health, comfort, and safety. Indeed,

considerable advances in digital technologies coupled

with the needs of the population are leading us to

redefine our modern environment. This context is

subject to constant evolution, both on a daily level and

on a technological level. Estimating “socio-

technological” coherence means being able to respond

to growing demand for specific needs in care, well-

being, and comfort which are the virtues sought for

the development of cross-skills and harmonization

between the actors concerned. In the case of this study,

the distribution of a biocompatible sensor network

within the body is at the center point of concern in

order to understand the behavior of tissues in

interaction with the mechanical wave. The clinical

environment makes it difficult to acquire multiple

physiological data except a few techniques, often

wired (probes) or radiation (MRI) which require

patient specificity to allow continuous monitoring of a

dynamic process in its entirety. Many of the studies

have addressed endoscopic ultrasound applications,

the majority of which used wired systems, [1], [2], [3],

[4]. It is in this context that this study aims to

implement a non-wired diagnostic micro-system

adapted to specific needs such as the effects of

environmental aggressiveness to correlate wave/tissue

interactions with possible metabolic modifications.

Indeed, the need for a technology exploiting

autonomous sensors led us to develop a miniaturized

spherical resonator to characterize in-vivo the

mechanical behavior of biological tissues under a

wide frequency band. The study aims to develop an

acoustic/ultrasonic system based on the design and

implementation of a miniaturized biocompatible

capsule with on-board electronics (Figure 1) which

will serve as an ultrasound pill as well for the

characterization of tissues in a low-frequency band for

endoscopic exploration at higher frequencies.

The study of this device was carried out step by

step, [5], [6], starting first with the evaluation of the

vibrational behavior of the pill. Then, energy

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Wehbe H., Rabih A., Lefebvre F., Nassar G.

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autonomy was addressed by the integration of

electronics dedicated to energy harvesting and storage,

focusing on the energy balance and conversion

(Figure 2). This approach made it possible to logically

schedule the operation of several tasks in parallel or

sequentially such as the selection of the vibration

mode of the structure, the transmission/reception

mode (characterization and scanning), data storage,

and acoustic telecommunications through the media as

well as an energy optimization which gives it

operating autonomy rarely found in current screening

systems.

The technology of resonators concept makes their

use in a three-dimensional network configuration

possible. Each sensor is identifiable either by the

choice of the resonance frequency of its structure or

by digital coding of the acoustic transmission frames.

A network of capsules randomly distributed in

complex environments could provide useful

information on the properties of a dynamic bio-system.

Fig. 1: Schematic illustration showing the principle of

the acoustic pill: the red dot is the endoscopic position

of the transmitting vibrating pill and the blue dots

represent the external annular resonators used as

receivers

2 Resonator Devices

2.1 Integrated Transmitter Capsule

For macroscopic characterization which requires a

low frequency to avoid the attenuation phenomenon in

transmission waves, the structure of the capsule is

excited by a ring having piezoelectric properties set

under stress on sandwiched between the two bio-

compatible half-spheres representing the hollow

capsule in its entirety. This arrangement allowed us to

resonate a pill 1 cm in diameter in the low frequency

band, which transforms the capsule into a point

vibrational source generating a spherical wave in the

medium.

The tests focused on a hollow plexiglass capsule

with 8mm as an internal diameter and 2 mm as a

thickness. A 0.5 mm thick piezoelectric ring is

wedged between the two hemispheres (Figure 2)

which offers a good compromise for a resonance in

breathing mode at 32 kHz. This frequency allows the

vibrational exploration of complex, highly dispersive

environments and the transmission of information

acoustically.

Fig. 2: Components of the vibrating pill, [6]

2.2 Vibration by Elongation: Fundamental

Mode

1) State of the art on the use of a spherical shape.

Few works have exploited the vibrational effects of a

spherical or almost spherical resonator. Some of their,

[7], [8], are among those who were interested in the

miniaturization of a quasi-spherical vibration sensor

for an application relating to the gastrointestinal tract

by ultrasound. They focused on an elliptical-shaped

resonator measuring 1 cm x 2.5 cm whose active

elements are actuated by an external source. For the

same purpose, some researchers used a wired

ultrasound concept in his work to evaluate the

feasibility of endoscopy using a miniaturized

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ultrasound capsule, [9], and other was oriented

towards the development of a theoretical formalism

by finite elements of an ultrasound concept based on

spherical resonators, [10], [11].

2) Analytical approach

Numerous studies have been carried out on the

mechanical behavior of spherical shells, particularly

on axisymmetric modes. Studies have concerned the

field of macrostructure, [12], [13], [14], [15], [16],

[17], [18], [19], targeting the effects of the vibration

of a macrospherical shell subjected to transverse

shears and rotational inertias.

From this work, by using the Lagrangian

formulation resulting from the fundamental theory of

Love, we have established the equations of motion of

a spherical shell. In free motion without constraints on

an undamped structure, the Lagrangian feature L is

expressed by the kinetic energy T and the potential

energy U under the following expression:

     (1)

Based on Hamilton's approach applied to a

dynamic system in free vibration over t1 ≤ t ≤ t2, the

associated equations of motion satisfy the following

condition:

  



 󰇛  󰇜  



 (2)

In the case of a spherical shape, T is given by:

  

   (3)

Whose physical characteristics of the sphere are

given by:

h : thickness of the shell

 : mass density of material used

(u, v, w) represents the strain components in

spherical coordinates.

The expression of the potential energy U can be

found in [20].

The analytical equations make it possible to

calculate the values of the frequencies of the

fundamental axisymmetric resonance modes,

providing the natural radial frequencies of breathing

and the nth mode of closed spherical shells:



 󰇣

󰇛󰇜󰇤

 (4)

With R the radius of shell, E is an elastic

component,  define the Poisson’s ratio, n vibrational

mode rank and  is a parameter linked at the

frequency for the nth mode and given by:

 󰇛󰇜

 (5)

The curves of Figure 3 give the solution of the first

frequency components of the membrane modes using

a spherical pill shape.

Fig. 3: Curves showing natural frequencies of

tortionless motion of the sphere having a radius of

R=6.7 mm, thickness h=2.6 mm, E=3300Mpa, ρ =

1190 Kg/m3, and ν = 0.39

For values of n ≥ 1, with n integer, the analytical

solutions show the existence of two distinct frequency

branches (lower and higher) corresponding to the

membrane and bending vibrational modes. For n = 0,

the structure is subjected to a resonance in spherical

elongation mode (breathing) which is the fundamental

mode. Results are in good agreement both in the

literature such, [21], [22] and those given by a

numerical approach in ANSYS modal analysis (Figure

4).

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Fig. 4: Comparative results of two resonance

frequencies showing the good agreement between the

experimental response (impedance analysis in blue

line) and the numerical simulation

3) Embedded electronics architecture

The spherical capsule is made autonomous by

integrating an energy management and task

scheduling device. This unit, as shown in Figure 5, is

programmed to perform the following tasks:

 Control of capsule resonance in simple or coded

emission mode

 Acquisition, signal processing, and data storage

 Optimal management of onboard energy

 Transmission of information in the sensor

network

Fig. 5: Electrical architecture of the embedded unit

(left) to ensure different tasks associated with the

main functions of the spherical resonator sensor

(right)

As shown in Figure 5, the embedded electronic

chip includes a central processing unit (CPU) and

subunits each of which performs a dedicated function

such as the digital oscillator (DCO), a scheduler (IT),

a controlled mechanical wave generator (PWMG), an

energy management module (PMU), an amplifier

(OB) for the output balance and a button battery

(BAT). Control of the autonomy of the capsule in its

entirety is managed by the energy management

module (PMU) which ensures the power supply of the

different elements of the chain with minimal energy

consumption for maximum autonomy.

2.3 Wide-band Low-Frequency Receiver

Sensor

In the design of classic ultrasonic sensors, the choice

of the resonance frequency strongly determines the

dimension of the vibrating electromechanical element.

For an electroactive element to vibrate at resonance,

its thickness must be inversely proportional to the

frequency. This imposes a size restriction for the low

frequencies concept, thus strongly impacting the

sensor integration approach. To overcome this

drawback, we proceeded in this work with an

innovative approach relating to the resonance of a fine

structure in concentric rings produced by laser

ablation. The nature of the material and its physical

characteristics were chosen to align its resonance

frequency with that emitted by the vibrating capsule

(Figure 6).

Fig. 6: Illustration showing the receiver device based

on the coupling of the vibratory movement of

different concentric rings. The fundamental resonance

frequency of 34.2 KHz was obtained by choosing the

characteristics of the vibrating element

On the theoretical level, the proposed model was

the subject of a double validation both by an

analytical calculation (Figure 7) using an equivalent

mechanical model and a numerical simulation by

numerical discrete method. The convergence of the

results made it possible to evaluate the vibrational

modes of the resonator and to visualize the dynamic

motion (distortions). Finally, the real experimental

concept was developed thanks to the strong agreement

observed between the different approaches used.

Identified Resonance Frequency

f0 = 34.2 KHz

f1 = 58.1 KHz

f3 = 5.7 MHz

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Fig. 7: Equivalent mechanical behavior by coupling

the dynamic motion of an assembly of masses (mi)

split on a system of springs (ki) and dampers (ci)

Where k = 18.104 N/m and c = 0.5 are the

physical characteristics of the elements constituting

the dynamic concept (spring & dampers). And for the

considered mass we give:

m1 = 4,835.10-6 Kg, m2 = 1,088.10-8 Kg & m3 =

1,813.10-8 Kg

The dynamic motion of the system satisfies the

following differential equation

- + 15.107j+ 4.1015 – 33.1020j

+ 1.1027 + 2.1032j + 25.1036 = 0 (6)

whose solution gives access to different frequencies fn

as: f0 = 35 KHz, f1 = 56 KHz and f2 = 5,25

MHz.

2.4 Experimental Validation and Results

1) Metrological approach:

a- Evaluate Sensor stability

The application of the complete system (spherical

transmitter and concentric ring receivers) in

transmit/receive mode in biological tissues required

validation to ensure its stability and sensitivity. Its

performance has been validated in thermos-regulated

water (25°C +/- 0.3°C) as a reference medium. As an

example, Figure 8 shows the diagram upon reception

of a 32 KHz frequency wave emitted by a spherical

emitter placed in a network sensors configuration.

Fig. 8: Time and frequency diagram at the reception

for a wave (in water as reference) emitted by a short

electrical pulse exciting the capsule sensor as emitter

3 Material, Methods and Results

From a practical point of view, the speed, attenuation,

and dispersion of the propagated wave were

considered as the desired characteristics to estimate

their variations affected by biological tissue samples

from animal origin. The samples were standardized in

thickness with varied properties (muscle, skin, fat,

etc.). These samples were incorporated into Agar

considered a coupling medium with known properties

(Figure 9).

To evaluate the characteristics of the waves

received by the different receptors and then to

estimate information on the bio-physical state of the

tissues, it is necessary to know the instantaneous

position of the receptors. For this, the spatial

identification of the transmitter is ensured at any time

by trilateration protocol. Thus, the analysis of the

wave upon reception made it possible to quantify the

variation of its characteristics giving access to the

desired quantities of the medium. It should be noted

that a propagation speed of 1545±7m/s at 25 °C in a

reference solution as reconstituted agar having

controlled and perfectly known physical properties.

Fig. 9: Unit of measurement protocol based on a

central position of the pill emitter frozen in the agar at

varying distances from some receivers implanted on

the different samples

The experimental configuration was conducted

using different animal tissue samples, equipped with

suitable receptors, and placed equidistant (10 cm)

from the central position of the emitting pill.

The different receivers (Ri; i = 1, 2, 3, 4) are

distributed to ensure varied reception depending on

the nature and biophysical characteristics of the

different samples:

1. Reception R1 issues from 3 mm thick pure skin

sample

2. Reception R2 issued from composite 10 mm skin

+ muscle sample (3 mm skin & 7 mm muscle)

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3. R3 is the reception from 10 mm pure muscle

sample

4. R4 is considered as reference reception in agar

solution (coupling without tissue)

The physical properties of the coupling medium

(agar) and the different samples are shown in the

Table 1 at the initial time t0 of the experiment.

Table 1. Physical characteristics of different coupling

medium

Agar (0.5%)

Skin

Muscle

25KPa

30 MPa

480 MPa

148 KPa

0.58MPa

0,14 MPa

 (Kg/m3)

 1

1.3

1.57



0.5

0.3

0.31

In this application, we have chosen to quantify the

variations in the characteristics of the wave in

transmission through samples at a constant

temperature while considering the aging phenomenon

as a variable acting as physical constraints on

biological metabolism. The strong dependence of the

wave propagation characteristics (speed and

attenuation) on the viscoelastic properties of the

medium allowed us to measure and follow over time

the evolution of both the Elastic (E) and the shear

modulus (G). The curves in Figure 10 show a notable

decrease in the measured values over 10 days.

Fig. 10: Comparative results showing the agreement

between the experimental measurements (solid curve)

and the analytical calculation (dashed curve) of the

elastic and shear modulus in various samples (agar,

skin, and muscle) over a period of 10 days at a

constant temperature at 25°C ±0.2°C

The measured values were continuously compared

with those resulting from a matrix rigidity formalism,

[23]. For the latter, results analysis shows both the

local (by layer) and the global behavior of the

apparent medium, and this by respect to the boundary

conditions on the successive interfaces’ layers. (agar-

muscle-epidermis).

4 High-Frequency Endoscopic Scanning

Application

For applications requiring high-frequency local

scanning (4 MHz), the capsule will be equipped, by

grafting, with ultrasonic transmitters/receivers

distributed over six generators partially covering the

overall structure (Figure 10a). By spatiotemporal

sampling of the endo position of the capsule, this

function will have the role of ensuring the acquisition

of ultrasound once the Sij sensor is in intimate contact

with the internal walls of the intestines. Figure 11

shows the results of the numerical model. Indeed, by

correlating the signals we can see the impact of the

presence of heterogeneity affecting the walls of the

intestines about a healthy environment. The coupling

of the signals received, simultaneously at low and

high frequencies, will provide access to the

localization and evaluation of the heterogeneities of

the nodules. This will help identify areas at risk or

those infected by the potential development of

carcinogenic tumors.

Fig. 10a: Schematic illustration of the high-frequency

function of the µ-capsule showing the position of the

embedded PZT sensors working on an

emission/reception mode at 4 MHz of the resonance

frequency. The Sij (i = 1,2,3 & j = 0, …, 6) represents

the rank of the active sensor when it is in contact with

internal tissues

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Figure 11(a) shows the principle of detecting a

zone of heterogeneity when the endoscopic capsule in

internal circulation in the intestinal tract is in a

position where one of the sensors being scanned

ensures intimate contact with the internal walls. The

associated numerical results show the curves relating

to the emission signals (Figure 11(b)), reception

signals in healthy areas (Figure 11(c)), and those that

are affected by areas with detected cancerous

development (Figure 11(d)).

Fig. 11: (a) Synthetic image showing the passage of

the capsule past different areas of potential

physiopathological states of the small intestine, (b)

ultrasonic signal emitted by a sensor identified Sij, (c)

shape of the signal upon reception for healthy walls

(green dotted square) and (d) signal disrupted by the

presence of an area supposedly affected by a tumor

(red dotted square)

Note that the emission of low-frequency signals

from the capsule allows, the various receiver sensors

distributed over the body (Figure 1), to locate the

capsule inside the body by trilateration and therefore

identify the pathological zone.

5 Conclusion

Measurement using an autonomous ultrasound pill

capable of contributing to the physiological and/or

pathophysiological diagnosis of the digestive system

was the main objective of this work. To understand

certain biophysical phenomena, we have attempted

through this work to couple two measurement scales:

a macroscopic (low-frequency system) and a

microscopic (high-frequency scanning). These two

scales were proposed by a unique concept based on a

vibrational pill coupled to an adapted receiver for low-

frequency characterization and to a surface sensor

module dedicated to high-frequency endo scanning.

The analytical results of the vibrational behavior

of the overall concept (transmission/reception units),

consolidated by the numerical approach, enabled the

design of the experimental prototype as an

autonomous vibratory element for medical diagnosis.

To validate our approach in a 3D space, this

mechanical concept is coupled with an embedded

trilateration algorithm for the localization step. Using

this method, we can quantify and localize

physiological changes in tissues inside the human

body. However, the analysis of the curves resulting

from the experimental results shows the sensitivity

and dependence of the mechanical behavior of the

concept on tissue aging factors over time. They also

show all the critical phases in a complex evolutionary

environment, knowing that there are few techniques

that, under similar conditions, provide access to the

desired local physical properties. We also highlighted

ongoing work aimed at finalizing a high-frequency

imaging module carried by this capsule. The

promising results of the digital approach show the

clinical potential of such a device when its coupling

with computing capabilities makes it possible to

record data and trigger alerts in the event of exceeding

the limit of a critical value.

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Contribution of Individual Authors to the Creation

of a Scientific Article (Ghostwriting Policy)

- Wehbe H. carried out the analytical approach and

numerical simulation of the concept

- Rabih A. focused on optimizing the choice and

physical nature of used sensors

- Lefevbre F. & Nassar G. equally contributed in the

present research, at all stages from the formulation

of the problem to the final findings and solution.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

No funding was received for conducting this study.

Conflict of Interest

The authors have no conflicts of interest to declare.

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en_

WSEAS TRANSACTIONS on SYSTEMS

DOI: 10.37394/23202.2024.23.10

Wehbe H., Rabih A., Lefebvre F., Nassar G.

E-ISSN: 2224-2678

Volume 23, 2024