the capillaries where the starling equation predicts an increase in capillary filtration as a result. Fluid buildup in the extremities is known as peripheral edema.
Citation: ASME Journal of Medical Devices, 10(2), 020953 (2016); doi: 10.1115/1.4033149 View online: http://dx.doi.org/10.1115/1.4033149
Wearable Coplanar Capacitive Sensor for Measurement of Water Content – A Preliminary Endeavor
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Methods
2.1 Sensor Design. A noninvasive coplanar sensor intended to be mounted on a sock for monitoring the lower leg edema is designed. The basic principle of coplanar capacitors is illustrated in Fig. 2.
Brant Axt Song Zhang Rajesh Rajamani Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455
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Fig. 2 The concept behind coplanar capacitor design. The solid lines are high conductivity charge-carrying elements, and the dashed lines are electric field lines
Background
Capacitance in general is determined by the ability of an insulating material between two conductors to store charge. In the common parallel plate design, the charge is stored mainly in an insulator between the two plates. However, in a coplanar design, the charge is stored in the material on top of and below the two plates. The depth of penetration of the electric field depends upon the geometry of the two plates and the distance between them. Copper tape was used to manufacture the two electrodes of this coplanar capacitive sensor, shown in Fig. 3(a).
Congestive heart failure affects 5.1 million people in the U.S. [1]. In right-ventricular heart failure, peripheral pitting edema is a key symptom used in identifying the disease. More rapid identification of edema symptoms would lead to better patient outcomes with earlier diagnoses. In families with a history of heart failure, it can become even more important that symptoms are caught early on. When the right ventricle fails, it is unable to pump enough venous blood to the lungs, which leads to increased right atrial pressure and subsequently an increase in pressure in the systemic circulation. This rise in pressure travels all the way to the capillaries where the starling equation predicts an increase in capillary filtration as a result. Fluid buildup in the extremities is known as peripheral edema. Skin tissue is made up of 3 major layers: the epidermis, the dermis, and the hypodermis. The epidermis layer, 50 ~ 85µm in thickness [2, 3], is not vascularized and consists majorly of keratinized skin cells which act as a barrier between the body and the outside world. The dermis layer, 1.5 ~ 2mm thick [4], is largely connective tissue like collagen, but it also contains a large amount of capillaries in the region closest to the epidermis. The hypodermis on the other hand contains fat storage and larger blood vessels. When peripheral edema occurs, the fluid is contained in the dermis and hypodermis layers of tissue. Since fats and oils are hydrophobic in nature though, most of the fluid is likely kept in the dermis layer. Fluid buildup in tissues causes a change in the moisture content of the tissue as well as a size change. In Fig. 1, the blue hash lines represent the percent of water in the tissue. In edema cases, the tissue becomes both saturated, and increases in thickness. Before Edema
1mm
Cup 6mm
Water
29mm
Tissue Phantom
HDPE Rod PI Layer
Electrodes 10mm
(a)
Delrin
(b)
Fig. 3 (a) Copper tape-based coplanar capacitive sensor (b) In vitro test setup schematic
2.2 In Vitro Test. For the purpose of in vitro test, a tissue phantom to represent different levels of edema was constructed and shown in Fig. 3(b). A sheet of 50µm polyimide (PI) was hot glued to the top of a 4 oz plastic cup in order to create a water tight seal. This PI layer represents the thickness of epidermis layer. The adhesive side of the copper tape was used to adhere the two electrodes to one side of this PI layer. On the other side of the PI layer, the bottom of the cup was removed and there was water to represent the fluid accumulation in the dermis layer and a high-density polyethylene (HDPE) rod to represent the fat in the hypodermis layer since they have similar level of dielectric constant (HDPE: 𝜀𝑟 = 1 ~ 5, fat: 𝜀𝑟 = 9). The HDPE rod was attached to an XY linear stage so that it can move vertically with 0.05mm step and change the amount of water between itself and the PI layer as a simulation of different levels of edema. The two electrodes of the coplanar capacitive sensor were connected to an L/C meter (Model IIB by Almost All Digital Electronics). As the water depth changes, the according capacitance readouts were recorded. A slab of Dupont Delrin was placed beneath the sensor to ensure that the change in capacitance is only caused by this change of water depth. Fig. 4 shows the coplanar capacitive sensor and in vitro test setup:
During Edema Epidermis Dermis
Hypodermis
Fig. 1 The proposed effects of edema on tissue
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PI Layer
the HDPE rod and the PI layer increases by merely 0.05mm, more free charges from the water are available to be stored between the two electrodes, thus the capacitance increased. For the in vivo test, however, the saline has much lower dielectric constant than the chicken tissue (chicken: 𝜀𝑟 ≈ 400 ~ 1000, saline: 𝜀𝑟 ≈ 80 [5, 6]), thus when the percentage of saline inside the tissue increased, the sensor capacitance dropped tremendously.
HDPE Rod
XY Linear Stage
Plastic Cup, PI Layer, Sensor, and Delrin
L/C Meter
Electrodes Wires for connection to L/C Meter
(a)
Capacitance Measure
(b)
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Fig. 4 (a) Sensor (b) In vitro test setup 30
Capacitance (pF)
2.3 In Vivo Test. The coplanar capacitive sensor was also tested on a chicken breast tissue as a preliminary in vivo test to verify the sensor idea. The experimental setup is shown below: Electrodes
Tissue (Chicken)
25 20 15
Physiological Saline
10
0
2
4 Depth (mm)
6
8
(a)
Wires
(a)
Camera Stand
(b) Fig. 6 (a) In vitro test result (b) In vivo test result
Physiological Saline Tissue
References
L/C Meter
[1]Go, A. S., Mozaffarian, D. and Roger, V. L., 2014, "Heart disease and stroke statistics—2014 update: a report from the American Heart Association," Circulation, 129(3), pp. 28-292. [2]Sandby-Møller, J., Poulsen, T., and Wulf, H. C., 2003, "Epidermal thickness at different body sites: relationship to age, gender, pigmentation, blood content, skin type and smoking habits," Acta Dermato-Venereologica, 83(6), pp. 410-413. [3]National Cancer Institute, "Layers of the Skin," accessed May 1, 2015, http://training.seer.cancer.gov/melanoma/anatomy/layers.html. [4]Derraik, J. G. B., Rademaker, M., Cutfield, W. S., Pinto, T. E., Tregurtha, S., Faherty, A., Peart, J. M., Drury, P. L. and Hofman, P. L., 2014, "Effects of Age, Gender, BMI, and Anatomical Site on Skin Thickness in Children and Adults with Diabetes," PLOS ONE, (9)1, p. e86637. [5]Adam, A. B. and Nasukha, N. H. M., 2011, "Analysis of Dielectric Constants of Slaughtered and Non-Slaughtered Chicken," Proceedings of IEEE Electronics, Communications and Photonics Conference, Saudi Arabia, April 24-26, 2011. [6]Golombeck, M.-A., Riedel, C. H. and Dössel, O., 2002, "Calculation of the dielectric properties of biological tissue using simple models of cell patches," Biomed Tech (Berlin), 47(Suppl 1 Pt 1), pp. 253256.
(b) Fig. 5 (a) Capacitive sensor on the tissue (b) In vivo test setup
As shown in Fig. 5, two electrodes were attached to the surface of the tissue, which was then submerged in the physiological saline (the top part, however, was kept dry) and left static for over 7 hours, during which period, the saline was assumed to gradually permeate into the tissue and the capacitance value was read intermittently from the L/C meter.
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Results Results from in vitro and in vivo tests are shown in Fig. 6.
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Interpretation
Both in vitro and in vivo tests show monotonic relationship between water content and sensor capacitance value. For the in vitro test, as the water depth or the amount of water between
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