A Plasma Modified Fiber Sensor for Breath Rate Monitoring A.Vallan, A. Carullo, M.L. Casalicchio, A. Penna, G. Perrone
N. De Vietro, A. Milella, F. Fracassi Università degli Studi di Bari “Aldo Moro” Department of Chemistry Bari, Italy
[email protected]
Politecnico di Torino Department of Electronics and Telecommunications Torino, Italy
[email protected] Abstract—The paper describes a new sensor for breath activity monitoring that, being based on optical fibers, is specifically conceived for applications where the intrinsic safety and the immunity to strong electromagnetic fields are mandatory, such as during MRI. The sensor is based on a plastic optical fiber (POF) in contact with the patient chest and properly modified to be extremely sensitive to deformations induced by normal breathing activity. The paper presents the sensor development procedure, the fiber modification process based on a cold plasma facility, and experimental results carried out during rest and sleeping conditions.
[10]. Some of these contact approaches are reliable and well assessed, but cannot be employed in the presence of strong electromagnetic fields, like during MRI, or when the patient safety from electrical injury must be guaranteed even in critical situations, such as after surgical or during intensive treatments. These safety imposed limitations can be overcome with sensors based on optical fibers: indeed, both plastic and glass fibers have been employed to measure quantity related to the breath activity, such as chest volume [11] and circumference [12], the exhaled air humidity [13] and the bed pressure changes [14], just to mention a few.
Keywords—fiber sensors, breath monitoring, plastic optical fibers, plasma etching.
The measurement approach proposed in this paper exploits an inexpensive fiber sensor made with Plastic Optical Fibers (POF), suitably modified to enhance the sensitivity to bending so it is possible to detect the changes of the chest shape due to respiration activity. This way a breath rate sensor with all the advantages typical of fiber optic sensors, but with cost so low to make it a disposable device, can be easily realized. Since the sensor performs a local measurement of the body curvature, unlike other similar fiber sensors for dimensional measurements, it does not require to be embedded in elastic belts or T-shirts and this results in greater flexibility of applications.
I.
INTRODUCTION
Breath activity monitoring is of great importance in several medical fields such as in the diagnosis of different lung pathologies and sleep disorders [1, 2], so quite a number of different measuring systems have been devised in the last decades. Among them, non-contact approaches are very attractive since they enable measuring several breath related parameters while working at a certain distance from the patient. However, despite applications based on vision techniques [3], laser doppler vibrometry [4], radio radar [5] and thermal imaging [6] have been deeply investigated, still they are rather complex and expensive and suffer from some practical limitations related to the constrains on the patient position and to the interference with other medical devices. Therefore, measuring systems for practical clinical usage are mainly based on sensors that work in contact with the patient, because they have higher reliability and reduced cost and complexity. The most exploited sensing approaches take advantage of the dimensional changes of the chest and abdomen that occur during inspiratory and expiratory phases. Respiratory Inductance Plethysmograph [7] method, for instance, employs sensors embedded in an elastic belt around the patient chest and abdomen and measures the thorax crosssectional area changes, typically through inductive [7], capacitive [8] or resistive [9] approaches. Other solutions have been devised to indirectly obtain information about the breath activity by measuring other physiological quantities, such as the blood oxygenation and the humidity of the exhaled air This work has been carried out within the project PRIN2009 ”Innovative plastic optical fiber sensors” supported by Italian Ministry of Education, University and Research.
978-1-4799-2921-4/14/$31.00 ©2014 IEEE
II.
SENSOR WORKING PRINCIPLE AND REALIZATION
The proposed fiber sensor is a deformation transducer sensitive to bending, so it is basically an intensity-based sensor whose working principle exploits the changes in the light propagation loss that occur when the fiber is bent. Light propagation inside large core fibers, such as POF, can be modeled as a group of rays that are reflected or refracted at the core-cladding interface, depending of the incident angle. Rays having an incident angle larger than the critical angle at the core-cladding boundary are reflected, whereas rays having a smaller angle are refracted outside the core and, from the point of view of the received power through the fiber link, are lost. According to this simplified model, any fiber bend changes the incident angle and thus affects the amount of rays (and thus of optical power) propagating down the fiber. This way the fiber curvature can be experimentally related to the optical power at the fiber output and by monitoring its variations it is thus possible to evaluate the fiber deformations.
Standard commercial fibers, however, have low sensitivity to bending since their main application is in telecommunications, where any environmental-dependent loss is detrimental. This is especially true for glass fibers, but also plastic fibers do not exhibit a high enough bending sensitivity to be used as they are for realizing the proposed transducer. Hence, the first step in the sensor development is to properly modify the fiber in order to enhance its sensitivity to bending. This approach has already been exploited for structural monitoring in civil and industrial engineering [15,16], and therefore several techniques have been studied to significantly increase the fiber bending sensitivity [17]. Among them, the simplest and most effective are probably those based on the modification of the fiber structure [18], which can be obtained in many ways, such as through chemical etching, mechanical milling, laser ablation, etc. Given the use of POF, the modification technique investigated in this paper is based on cold plasma because it is known to be a particularly effective and reproducible way to remove polymers. Therefore the etching process has been optimized to remove the fiber cladding and part of the core, increasing the core surface roughness to obtain an enhancement of the surface scattering and, as shown in [19], an improvement of the fiber sensitivity. Then, since the fiber etching has been carried out on a single fiber longitudinal surface only, it is also possible to distinguish between positive and negative bending. Fig. 2 shows the sensor realization process. A piece of bare POF is firstly etched for a length of about 5 cm in order to remove the cladding and increase the surface roughness. The picture shows the scattered light before (Fig. 1a) and after the etching (Fig. 1b). The fiber is then permanently folded in a Ubent shape using a gentle thermal treatment to soften the fiber, obtaining a compact sensor embodiment that has both fiber ends on the same side. The sensor is then connected to two spans of unmodified fibers using an optical-grade resin (Fig. 1c). Eventually, the modified part is protected using adhesive tape thus obtaining the prototype shown in Fig. 1d that has the advantage to be thick and flexible. III. PRELIMINARY EXPERIMENTAL RESULTS Some sensors developed as previously described have been arranged using a step-index 1 mm POF, which is typically employed as transmission media in short-range communication systems and data networks. Plasma treatments have been carried out in a parallel plate stainless steel reactor with the upper electrode connected to a radio frequency generator (13.56 MHz, Advanced Energy) through a matching network. The lower electrode is grounded and acts as sample holder. Vacuum in the reactor chamber is achieved by a rotary pump, which allows a minimum pressure of about 0.1 Pa. The pressure in the vacuum chamber is monitored by a baratron gauge (MKS Instruments). Gasses are injected in the reactor by mass flow controllers (MKS Instruments). The thickness of the fiber cladding (10 μm ± 3 μm) has been measured by FEG-SEM (SUPRA 40, Zeiss) analyzing the fiber cross-sections. The cladding was effectively removed feeding the plasma with a mixture of O2 (30 sccm) and CF4 (5 sccm) at a power of 100 W and a
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Fig. 1. Fabrication of a fiber sensor: a) bare POF without jacket b) the same fiber after the plasma etching process c) fiber permanently folded and joined d) the final embodiment.
pressure of 10 Pa, and treating the fiber for 5 minutes. Nanotexturing of the fiber core (Polymetylmetacrylate, PMMA) was accomplished in the same reactor with a pure O2 plasma (20 sccm, 120 mTorr, 80 W, 2-10 min). Results show that the treatment allows a homogeneous roughening of the fiber to be obtained, with features tunable in shape, spacing and height depending on the treatment duration (see Fig. 2). Two fiber spans having a length of about 2 m are employed to connect the sensor to the interrogation system, whose structure is shown in Fig. 3. The sensor is interrogated using an LED, which emits a constant optical power of about 150 μW at a wavelength of about 650 nm, and the bending is measured by monitoring the light intensity changes using a traditional scheme, where a photodetector is followed by a transimpedance amplifier having an overall amplification of about 56 MΩ. A band-pass filter removes the unwanted signal components located below 0.1 Hz and above 10 Hz, which are mainly due to the source and to the ambient light collected by the fiber spans that, in our setup, are left unprotected. The output signal is eventually sampled at 100 Hz and acquired by means of a general purpose digital acquisition board (DAQ)
200 nm
Fig. 2. Nanotexturing of the fiber core by O2 plasma etching, effect of treatment time: 2 min (left); 5 min (centre); 10 min (right).
to the PC
PD
POF
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Fig. 3. The interrogation system block diagram.
Fig. 4. The interrogation system and two fiber sensor prototypes.
interfaced to a personal computer for data storing and processing. Fig. 4 shows a picture of the arranged interrogation system and two prototypes of the proposed sensor. The figure, taken in low light conditions, also shows a remarkable light scattering at the beginning of the fiber span due to the non-optimal LED launching conditions. Anyway, this aspect does not affect the sensor sensitivity. The sensor has been characterized using a setup devised to test and calibrate curvature sensors [16]. The setup is based on a metallic cantilever that can assume a know curvature. The characterization has shown that the sensor is able to detect a curvature radius greater than 500 m, which is much larger than what required for the target application. Some tests have been carried out recording the respiratory movements of a 30 year old man while breathing normally in rest conditions. Special attention has been paid in order to locate the best position where to fix the sensor. During the respiratory movement, chest and abdomen expand and relax to change their volume causing a pressure change inside the lungs. Both the central area of the thorax and of the abdomen wall approximately move up and down and remain almost flat; on the contrary, the area located in the lower part of the chest is subjected to larger curvature changes even during normal breathing because the movement of chest and abdomen have different amplitudes and phases. The change in curvature becomes more evident during paradoxical breathing when chest and abdomen move out of phase [7]. During the tests, the contact between the sensor and the patient skin has been maintained using common adhesive tape. It
This limitation has been further highlighted during another test in which the patient respiratory movement has been recorder during overnight sleep. Fig. 7 shows the signal recorded for about 100 minutes; the breath related oscillations can be clearly identified, but there are also some large disturbances that can be ascribed both to artifacts and to unwanted bending of the fiber spans employed to connect the sensor to the interrogation system. In order to overcome this drawback, the authors are investigating different optical fibers, like fibers having different diameter or structure, such as the multi-core fibers that are known to be almost insensitive to bending [20]. As a demonstration, a further sensor has been developed using a POF having 0.5 mm diameter. The sensor has been 200 150 Output (mV)
LED
should be noted that, contrary to other deformation fiber sensors that are sensitive to strain or elongation, the proposed sensor does not require to be embedded in elastic belt or Tshirt because it does not require axial stress to properly respond and this is a great simplification for developing disposable devices. Fig. 5 shows the recorded response when the sensor is fixed between the chest and the abdomen at the bottom of the sternum, and Fig. 6 shows another alternative position for the sensor located at the level of the 7th and 8th rib on the costal cartilage. Thanks to the very high sensitivity, a good signal to noise ratio has been obtained, which potentially allows the breath rate to be measured with good accuracy. The signal portion related to inspiration, expiration and pause phases can be clearly identified. The sensor, however, like other dimensional sensors, is not able to distinguish artifacts due to the chest deformation that are caused by the patient movements, and this aspect represents the main limitation to the breath rate accuracy.
100 50 0 -50 -100 -150 0
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Fig. 5. Example of signal recorder when the sensor is located between the chest and the abdomen. 200 150 Output (mV)
Interrogation System
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Fig. 6. Example of signal recorder when the sensor is located in a different position.
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Fig. 7. Signal as recorded during sleeping using a sensor based on a 1 mm POF sensor.
Fig. 9. Normalized Power Spectral Density (PSD) of the acquired signal at time 0.5 h (blue), 1 h (red), and 1.5 (green).
III.
Fig. 8. Signal recorded during sleeping using a 0.5 mm POF sensor.
developed and characterized using the previously shown procedures. The sensor has shown almost the same sensitivity of the prototype arranged using a 1 mm POF, but with a reduced sensitivity to bending for the unmodified fiber span. The sensor has been tested again recording the respiratory movement in sleeping conditions: Fig. 8 reports an example of the recorded signals, which now presents a sensible reduction of the disturbances. The breath rate has been obtained analyzing some frames of the acquired signal in the frequency domain. To this aim, the power spectral density has been estimated processing 20 minutes wide signal frames by means of the Welch’s method. The results concerning the processing of three frames extracted at time 0.5 h, 1 h and 1.5 h are shown in Fig. 9, where the frequency axis has been expressed in breaths per minutes (BPM). The breath rate has been estimated as the frequency of the fundamental component and is of about 15.5 BPM, almost constant during the acquisitions.
CONCLUSION
In this work, an application of a bending fiber sensor for breath rate monitoring has been developed. The sensor is made with plastic optical fibers with surface modified via plasma etching in order to enhance the sensitivity to bending. The sensor is mainly suited for applications where traditional electrical sensors cannot be employed because of safety reason or in the presence of strong electromagnetic fields. Moreover, thanks to the usage of POF and intensiometric interrogation system, it also presents the advantage of a very low cost and simplified use, since it can be easily handled and replaced, so it is suitable to be employed in applications where a disposable device is needed. The sensor presents better performance when it works in contact with the lower part of the chest, where the body surface presents a large curvature change during breathing activity. Since the sensor is not sensitive to strain or elongation, it does not require to be embedded in elastic belts. Preliminary tests performed both during normal breathing and in sleeping conditions have confirmed the feasibility of the proposed approach, even though the measuring system still has to be improved in order to reduce the spurious signals due the unwanted bending of the fiber spans employed to connect the sensor to the interrogation system. The sensor output provides a measure of the chest bending that occur during the respiratory movement and it can be employed to measure the breath rate and to detect the presence/absence of respiratory activity. A measurement of the volume changes cannot be obtained processing a single sensor output since the relation between the chest/abdomen curvature and the lung volume depends on the sensor position and on the chest/abdomen activity. Anyway, the volume changes can be measured using an array of sensors and a subsequent processing for the reconstruction of the thorax shape.
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