# Design sensor for COVID-19

#### TheRoBoTBiLL

Joined May 26, 2020
7

I have a homework for my studies.
I need to design a sensor with capacitive measurement for COVID-19 with measurement in air and saliva. I will need to calculate the geometric features of sensor, design the stimulation circuit and measurement circuit.

Have anyone idea how to start?

This is the picture of sensor

#### crutschow

Joined Mar 14, 2008
25,126
How do expect that sensor to differentiate between COVID-19 and the other billions of virus's out there?

#### TheRoBoTBiLL

Joined May 26, 2020
7
I also have these dielectric constants

#### OBW0549

Joined Mar 2, 2015
3,454
Try as I might, I cannot imagine any way that COVID-19 could appreciably affect the capacitance of that device. By what physical mechanism is this alleged effect supposed to happen?

I have a lot of experience with capacitive sensors in the low-picofarads range, and the signal conditioning circuits needed to use them, and I can tell you that designing the circuits for exciting these sensors and accurately measuring minute changes in capacitance (which is what you would get, unless you're operating in some "let's just suppose" universe where large capacitance changes would somehow magically occur) is an extreme challenge even for an experienced designer. And I do mean EXTREME.

#### Analog Ground

Joined Apr 24, 2019
407
It is just a homework exercise. Start with the calculations for a parallel plate capacitor. Apply this to the geometry of your sensor capacitance. It is multiple parallel plate capacitors in parallel. Then, move on to calculating the dielectric constant of your test sample based on the concentration of Covid-19. Then, you can calculate the change in capacitance of your sensor for difference concentrations of Covid-19. Next, look for circuits to measure the capacitance. Here is a hint. There is an IC out there which can measure with a resolution of a few femtofarads. Finally, you will need to have a calibration strategy. The concentration of Covid-19 virus will be very, very low. So, such a device is probably not practical. However, going through the design will allow you to show WHY it is not practical and meet your homework assignment.

#### jpanhalt

Joined Jan 18, 2008
9,755
Must it be capacitance?

If so, read about rapid methods to detect septicemia (bacteria in blood). There is a review of the subject of rapid detection using such indicators a capacitance and conductivity. I cannot find a good link to the chapter. It is in this: J. A. Washington II (ed.), The detection of bacteremia. CRC Press, West Palm Beach, Fla. Published about 1976-1978.

If your library doesn't have it, it is not worth buying. Unfortunately, it is rather dated.

Advice (based only on the use of capacitance):

1) You will not be able to detect the "free" virus in an sample. The titers are too low. (Of course nucleic acid amplification and antibody-based methods do work.)
2) You will need to detect the effects of the virus on cell cultures in which it is grown.
3) Do not use human cell lines.
4) Be sure you have a BSL4 level laboratory and appropriate licenses.

I suspect you are joking. What have you really been asked to do?

#### MrAl

Joined Jun 17, 2014
7,669
I cant imagine this working unless you are dealing with some sort of highly controlled samples, so this must be more a theoretical question than a practical one.
Given that, we would have to know the variables and constants involved in that sample so we would even know what to look for. It would also have to be calibrated, at least in theory.
This tells me that maybe you were given some other constraints that would be good to know here.
Also, i guess that sample would be used as the dielectric.

It is a coincidence that yesterday i was talking with someone about detecting the virus, but that was more of a surface type detector idea that could detect the virus on surfaces so there would be no guesswork. Unfortunately, even high powered microscopes cant do that so we have no starting point, at least not yet. But we might all start thinking about this and see what we can collectively come up with. This could help the entire world.

It might get a little scary too when we can finally see in real time what is on every surface we check, maybe we dont want to know

#### TheRoBoTBiLL

Joined May 26, 2020
7
I apologize for the confusion. I had to describe a bit better my approach. It is related to an exercise in which we suppose (with imaginary values) that the dielectric constant (ε0) of different air particles are as follows:
ε0=1,
εr=8,85x10^-12
ACE II: ε=15,
COVID-19: e=16,
H20: e=80
There is also this form in order to calculate the capacitance:
C=(εr*ε0*S)/d

In this case, I have to make a proposal of a Capacitor – Inductor circuit which based on the change of the Capacitor Resistance (due to the corresponding dielectric constant) should be able to identify the air quality in each case. I was thinking of identifying this by the phase change between the current and voltage (i.e. cos(f) value). To design the values of the capacitor and coil to have a resistive behavior while in clear air, and detect changes while having other particles present. In this case, I would like to ask you if you have any idea of what kind of circuit or different approach that I could follow to do that. Thank you in advance.

#### jpanhalt

Joined Jan 18, 2008
9,755
OK. Now, let's do a test for reasonableness. The SARS-CoV-2 virus is approximately 100 nm in diameter and spherical. Viral "concentrations" are hard to define because of the presence of lots of defective virus particles. The quickest result I could find is a letter to the editor of NEJM:
Lirong Zou et al. (letter), https://www.nejm.org/doi/pdf/10.1056/NEJMc2001737
Ct values are inversely related to viral RNA copy number, with Ct values of 30.76, 27.67, 24.56, and 21.48 corresponding to 1.5×104, 1.5×105, 1.5×106, and 1.5×107 copies per milliliter.
They tested swabs from various locations of patients with clinical disease. Now, take the highest of those viral loads (1.5 x 10^7/mL) and derive an approximate ppb (or ppm or %, whichever unit fits best) for virus vs. air or water (whatever you chose). That will get you started on defining the sensitivity you need.

EDIT: Ct is the number of amplification cycles needed to detect.

#### MrAl

Joined Jun 17, 2014
7,669
I apologize for the confusion. I had to describe a bit better my approach. It is related to an exercise in which we suppose (with imaginary values) that the dielectric constant (ε0) of different air particles are as follows:
ε0=1,
εr=8,85x10^-12
ACE II: ε=15,
COVID-19: e=16,
H20: e=80
There is also this form in order to calculate the capacitance:
C=(εr*ε0*S)/d

In this case, I have to make a proposal of a Capacitor – Inductor circuit which based on the change of the Capacitor Resistance (due to the corresponding dielectric constant) should be able to identify the air quality in each case. I was thinking of identifying this by the phase change between the current and voltage (i.e. cos(f) value). To design the values of the capacitor and coil to have a resistive behavior while in clear air, and detect changes while having other particles present. In this case, I would like to ask you if you have any idea of what kind of circuit or different approach that I could follow to do that. Thank you in advance.
Hi,

First, i think you have your εr and ε0 swapped.

Ok so you are working on simple straightforward principles, that's good.
So what you need is a fairly sensitive capacitance measuring circuit because some of dielectric constants are not that different. And that is ALL you need right?

#### BobaMosfet

Joined Jul 1, 2009
1,121

I have a homework for my studies.
I need to design a sensor with capacitive measurement for COVID-19 with measurement in air and saliva. I will need to calculate the geometric features of sensor, design the stimulation circuit and measurement circuit.

Have anyone idea how to start?

This is the picture of sensor
View attachment 211100
You can't do this with a capacitor- you require a MEMs device designed with several sections and circuits to do this.

#### jpanhalt

Joined Jan 18, 2008
9,755
You can't do this with a capacitor- you require a MEMs device designed with several sections and circuits to do this.

#### TheRoBoTBiLL

Joined May 26, 2020
7
A lot of answers, thank you!!!
I'm confused!!!
So, let's start again!!
Firstly, I want to calculate the geometric features of this sensor.
(I knew this formula for calculate the capacitance, where εr is the permittivity in air which is 1, ε0 is the permittivity of free space which is 8,85x10^-12, but I'm not sure what they are S and d.

Secondly, I want to design the electronic trigger circuit and electronic measurement circuit.

I have also these dielectric constant

How does capacitor sensor call? Interdigital or co-planar?

#### jpanhalt

Joined Jan 18, 2008
9,755
It looks like all you have done is copy some formulas from a text. What have you done toward making the design yourself?

Have you calculated what the maximum change in capacitance/dielectric constant will be given the maximum number of viral particles per mL? It seems you would need to know that to start designing your capacitor.

#### BobaMosfet

Joined Jul 1, 2009
1,121
@jpanhalt - Google is your friend. Use it. MEMS is the only way anything like this can be done. Biomedical companies world-wide use MEMs routinely for everything like this that they can do. Government provided grant to company to do it:

https://www.eejournal.com/article/using-mems-to-thrash-covid-19/

Scroll down to the 'HEMEMICS' and 'Rogue Valley Microdevices' sections. for more erudition on MEMS in the biomedical segment.

#### OBW0549

Joined Mar 2, 2015
3,454
(I knew this formula for calculate the capacitance, where εr is the permittivity in air which is 1, ε0 is the permittivity of free space which is 8,85x10^-12, but I'm not sure what they are S and d.
εr is the relative permittivity of a dielectric, the ratio of that dielectric's permittivity to that of free space (i.e., a vacuum). It is a dimensionless quantity, a simple ratio.

On the other hand, ε0 is not dimensionless: it has the dimensions of Farads/meter.

In the given formula for capacitance, S represents the mutually-exposed plate area in square meters, while d represents the distance between the plates in meters. Square meters divided by meters equals meters, and that gets multiplied by ε0 and εr to yield the capacitance in Farads.

#### jpanhalt

Joined Jan 18, 2008
9,755
@jpanhalt - Google is your friend. Use it. MEMS is the only way anything like this can be done. Biomedical companies world-wide use MEMs routinely for everything like this that they can do. Government provided grant to company to do it:

https://www.eejournal.com/article/using-mems-to-thrash-covid-19/

Scroll down to the 'HEMEMICS' and 'Rogue Valley Microdevices' sections. for more erudition on MEMS in the biomedical segment.
The TS specified that he was using/must use capacitance. Immuno-biosensors and nucleic acid probes have been around a long time. In fact, the latter is what is currently being used to detect the virus.

#### TheRoBoTBiLL

Joined May 26, 2020
7
So, I calculated the capacitance when it is on the vacuum and when there are on it the virus particles.
Now, I want to design a measurement circuit with base on 555IC and arduino. I found this link. Could I follow these instructions, are these ok or missing anything?

#### jpanhalt

Joined Jan 18, 2008
9,755
So, I calculated the capacitance when it is on the vacuum and when there are on it the virus particles.
Now, I want to design a measurement circuit with base on 555IC and arduino. I found this link. Could I follow these instructions, are these ok or missing anything?
Please show your calculations. How are you going to operate it in a vacuum?

Without seeing your calculations, it is impossible to tell whether the design you copied will work. I seriously doubt you will perceive and change in the rate at which the LED flashes. Is Arduino required?