Design a Wireless Pressure Sensor With an Ellipse and a Circular Shape to Monitor the Pressure Within the Coronary Artery

This paper put forward the design and modeling pertaining to a wireless pressure sensor that can monitor blood pressure within the coronary artery with the help of a capacitive pressure sensor. To adjust the resonance frequency (26.78–27.09 MHz) with regards to this sensor’s applied pressure (0–30 KPa), sensor’s dimension as well as human blood pressure (0–220 mmHg), a variable capacitor in the pressure sensor has been put forward. The shape of the capacitive pressure sensor was designed to look like an Ellipse and a circle, which were assessed for their performance with regards to capacitive sensitivity and diaphragm deflection. Also, the diaphragm thickness pertaining to the capacitive pressure sensor with Ellipse and circular shapes was altered (<inline-formula> <tex-math notation="LaTeX">$0.1~\mu \text{m}$ </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">$0.5~\mu \text{m}$ </tex-math></inline-formula>) while the cross-section area was kept constant at 1 mm<sup>2</sup>. As per the results, an invasive sensor with circular and Ellipse capacitive pressure shape that had smaller size <xref rid="deqn1" ref-type="disp-formula">(1)</xref> mm<sup>2</sup> displayed high sensitivity. The sensitivity readings for Ellipse and circular shape capacitive pressure sensor were 7.73 ff mmHg<sup>−1</sup> and 9.94 ff mmHg<sup>−1</sup>, respectively. Diaphragm deflection was simulated based on the COMSOL Multiphysics software, while MATLAB was employed for simulation with regards to changes in resonance frequency, capacitance, and capacitive sensitivity.

stents would widen the diameter of blood artery. On the other 23 hand, the mechanical expansion pertaining to the metal stent 24 The associate editor coordinating the review of this manuscript and approving it for publication was Alon Kuperman . could lead to many harmful impacts for the patient. In most 25 of these cases, formation of neointima forms occurs due to 26 thrombus in the peripheral section of the blood vessel, which 27 could lead to a new constriction of the inner diameter pertain-28 ing to the blood vessel [2], [3]. In addition, diabetic patients 29 do not feel chest pain when the narrowing in the arteries 30 returns, and this may lead to a sudden and dangerous deterio-31 ration of the health condition. X-ray-based inspection devices 32 can be employed to continuously monitor the metal stents 33 but is costly. Thus, it is important to identify an alternative 34 approach for monitoring these metal stents as well as blood 35 pressure in real time [4]. MEMS pressure sensors are better 36 than traditional pressure monitoring systems due to their 37 area, with the help of an inductor and MEMS. This was done 66 by maintaining 50 MHz as the frequency of opera-tion [11]. 67 The MEMS pressure sensor was designed by employing a and, at the same time, made it compliant with the generally 94 employed balloon catheter stenting technique. Electroplating 95 of the stent was done with a gold layer to minimize its 96 series resistance. The length and diameter of the stent were 97 20 mm and 5 mm, respectively. In free space, its sensitivity 98 was found to be 302-335 ppm mmHg −1 accommodating a 99 pressure of up to 250 mmHg, while the in vitro sensitivity 100 was found to be 146 ppm mmHg −1 when frequency was 101 maintained in the range of 30-80 MHz and the size of the 102 sensor was (1.5 × 1.5 mm 2 × 200 µm) [14]. The design of the 103 smart stent included an integrated capacitive MEMS pressure 104 sensor based on stainless steel, which was electroplated with 105 a layer of gold to minimize stent's series resistance. Parylene 106 C layer was employed to passivize the stent and to make its 107 sur-face biocompatible and electrically isolated. At frequency 108 of 10 MHz in free space and in vivo, the chip sensor size was 109 (1.5 × 1.5 mm 2 × 200 µm) and stent length was 30 mm 110 [15], [16]. Despite the efforts made by the researcher, the 111 pressure sensor is still faced with issues such as large size 112 and low sensitivity.

113
This paper puts forward an approach of employing 114 COMSOL Multiphysics to develop a biocompatible pressure 115 sensor as well as smart stent. The pressure sensor employed 116 on the smart stent platform can track pressure of the blood 117 vessel in real time. The unique design of the wireless pres-118 sure sensor helps decrease the number of routine medical 119 tests. The preliminary investigations employing simulation 120 rate showed that the proposed design can also be integrated 121 and used for health monitoring applications.

123
The wireless pressure sensor has been designed based on the 124 inductor-capacitor resonant circuit technique. The schematic 125 design pertaining to the put forward wire-less pressure sensor 126 system is shown in Figure 1. The wireless pressure sensor 127 system is made of two parts, internal and external compo-128 nents. The external component is placed outside the body, 129 but it touches the chest; while the internal component is 130 placed within the coronary artery inside the human body. The 131 internal part has length of the inductor coils (stent) is 30 mm 132 and has been designed to be helical shape when expanded by 133 a balloon placed within the artery along with a diameter of 134 5 mm, and a zigzag helical shape with a diameter of 2 mm 135 prior to expansion and when inserted via a catheter. The 136 basic principle behind this system is to reach a resonance 137 frequency as well as sufficient transfer power from the outer 138 circuit towards the inner circuit that is implanted within the 139 coronary artery to enable it to work without a battery. It can 140 measure the pressure by altering the resonance frequency 141 because of change in the capacitor value, which happens due 142 to the deflection of diaphragm sensor caused by the applied 143 pressure. This, in turn, results in altering of the distance 144 between the two plates, thereby leading to variation in the 145 resonance frequency as well as capacitance value. 156 Since the coronary artery's diameter does not exceed 5 mm,

158
it includes certain re-strictions pertaining to the use of spiral 159 coil that are rectangular or circular in shape, which impacts 160 the flow of the blood within the artery. Thus, the helical coil is 161 regarded to be the optimum solution that can be placed within 162 the coronary artery. Mathematically, equation (2) can be 163 employed to compute the helical coil and self-inductance [18] 164 where µ is the free-space permeability, r=d stent /2, and T is where d stent wire is the diameter of the wire used in the coil, Stent inserts were carried out via PCI operation with a 176 size of 2 mm, which is expanded by the balloon to reach 177 5 mm. Gold is used as the material since it is biocompatible 178 and possesses a good electric conductivity to allow wireless 179 transfer of power. Based on Equations (6-9), calculation of the 180 stent parameter is done prior to expansion and post expansion 181 as demonstrated in Figure 2 182 l one revolution = H 2 + C 2 (6) 183 where H=2πr and present the rise of helical in one revolution 184 (pitch), where r is the radius of helical coil where N strut is number of struts in one revolution The parameter of stent before expansion when the shape 194 like zigzag helical shown in Table 1.  There is an incremental increase in applied pressure by 5 kPa. capacitance, between the plates, which can be calculated by 220 employing equation (10).
where ε 0 is the air permittivity, ε r the dielectric permittiv- where P denotes the applied pressure and D represents the 237 flexural stiffness, as defined by: where v is Poisson's Ratio of Diaphragm, and E is Young's 240 Modulus.

241
Calculation of the capacitive pressure sensor's sensitivity 242 is carried out mechanically. Mechanical sensitivity can be 243 defined as the difference in capacitance values to the applied 244 pressure values. If the device possesses higher sensitivity, 245 it demonstrates more accurate reading of the output. For any 246 device, it is always favourable to have a high sensitivity value. 247 Calculation of the sensitivity can be done theoretically based 248 on the following equation (14) [19] For a sensor with ellipse shape, the substrate material is 251 polyamide that possesses dimensions of 0.750 × 0.424 × 252 200 µm 3 . The dimensions of the diaphragm would be 253 0.750 × 0.424 × 0.1 µm 3 , and gold is selected as the material 254 due to its biocompatibility and highest capacitance value. The 255 bottom electrode includes identical material that is used in the 256 diaphragm, with dimensions of 0.550 × 0.224 × 0.1 µm 3 . 257 The same material is employed by the circular capacitive 258 pressure sensor as those used in the ellipse capacitive pres-259 sure sensor, but it differs in size. The substrate dimensions 260 are 0.564 × 0.564 × 200 µm 3 , the bottom electrode with 261 0.364 × 0.364 × 0.1 µm 3 and the diaphragm with 262 0.564 × 0.564 × 0.1 µm 3 , as shown in Table 2.  Table 3 illustrates the results of the simulated diaphragm  Compared to elliptical diaphragms, deflection is more sig-288 nificant in circular diaphragms. The most noteworthy deflec-289 tion can be seen in the design with the thinnest diaphragm. 290 Figure 5 demonstrates how thickness increases as pressure 291 sensitivity de-creases. For a particular pressure range, the 292 optimum diaphragm deflection can be reached by striking a 293 balance between diaphragm thickness, plate separation gap, 294 and maximum diaphragm deflection.

295
If no pressure is applied, the value of the base capacitance 296 is 97.41pf. While this value does not factor in the thick-297 ness of the diaphragm, it is dependent on the size of the 298 diaphragm's area and the separation gap between the plates. 299   9.94 ff/mmHg. Compared to other diaphragm thicknesses, 319 this is the greatest value. As for an Ellipse diaphragm with 320 a similar thickness (0.1 µm), its capacitive sensitivity is 321 7.73 ff/mmHg; this diaphragm thickness is also the greatest 322 among the said values as exhibited in Table 5.

323
The dissimilarities in capacitance values generated 324 between the plates for Ellipse and circular diaphragms with 325 different thicknesses at varying pressure levels are being 326 demonstrated. The design model that shows the best potential 327 output is the circular diaphragm shape: since it demonstrates 328 more sensitivity to pressure, it is the most recommended 329 method for coronary artery blood pressure monitoring. 330  In addition, the coronary artery pressure is measured utilizing 331 a wireless pressure sensor that has different frequencies 332 which are different from the resonance frequency of the heart 333 that have a low frequency of 0.1Hz (6 breaths per minute); 334 this method alters the capacitance value upon application of 335 pressure. In Figure 7, the shifts in frequencies for the wireless 336 pressure sensor are shown with the application of varying 337 pressures that ranges from 0 to 240 mmHg. As for the stent, 338 its inductance value before expansion is 0.350 µH, while after 339 expansion, the value decreased to 0.320 µH.  sharp corners or edges to be safer in increasing sensitivity 349 and minimizing restenosis.

351
This research provides a description of how a capacitive 352 pressure sensor (MEMS) in circular and Ellipse geometries is 353 designed, analyzed, and modelled. This MEMS, which may 354 be utilized for measuring blood pressure, operates efficiently 355 within the coronary artery in the 0-30 KPa range. The unique 356 stent design can also result in good mutual coupling and 357 staying within fracture limits in times of balloon expansions. 358 Capacitive pressure sensors are employed since they are 359 more sensitive to pressure and dependable in measuring low-360 pressure levels. COMSOL Multiphysics is employed during 361 the design and simulation for the sensor and its electrome-362 chanical analysis. With its 0.1 m diaphragm thickness, the 363 circular capacitive type is identified to have a 9.94 ff/mmHg 364 maximum sensitivity and a 97.776 PF capacitance value. 365 Consequently, the proposed design is found to be more suit-366 able for measuring coronary artery blood pressure.