Make With Mouser IoT Design Contest - Baby Vital Monitor_04 Impedance

This is my 4th blog entry regarding the project "Baby Vital Monitor" for "Make With Mouser IOT Design Contest".
In this blog post I am sharing my work on Bio-impedance measurement. The basic idea or goal is to relate the skin impedance with body hydration parameters. I'll share my experience of measuring Bio-impedance using "tetra polar - four electrode" method in a conventional way. I have used constant current sources and bio-amp. I'll show a possible relation of skin impedance and hydration. At the end of the blog I'll share my plan, on how ADPD4100 can be used for measuring impedance to provide hydration parameter or respiration rate when focused on lungs.

disclaimer: ** I have taken most of the images by searching goggle and other sources from internet. I am sorry I cannot provide the reference.
** the basic experiment was also published in another blog of mine in Element14 forum

Working principle:


** CC-> constant current carrier
PU -> potential measuring unit

A limited current at certain frequency is passed through the body and the impedance is calculated by measuring the voltage output across the surface. Impedance varies inversely with water presence in the body tissue and electrolyte content, both of them depend on hydration. There are several techniques of measuring impedance ->

  • two electrode - bipolar
  • three electrode - tripolar
  • four electrode method - tetrapolar

Among them four electrode method is immune to contact impedance from the current injecting electrodes. Therefore, it gives better result and I am going to use it for my experiment.

The electrodes will be placed on back of the wrist.

Hydration from Skin Impedance:

Bio-electrical Impedance Analysis [BIA] is a well-known technique used in hydration measurement. Whereas, skin impedance is not an easy way to directly measure the Hydration, but can give a rough relative difference of impedance due to variation of hydration within a certain period of time for a certain subject. We don't need to have a precise output which can be used for diagnosis purpose. Rather than my intention is to use it for primary screening.

In BIA technique, current is passed through the body organs where electrodes are generally placed on wrist and ankle. But skin impedance works on the surface level.

Working Steps:

  1. variable frequency wave generation
  2. constant current source at surface electrode
  3. potential measurement from surface electrode using bio-amp
  4. passing the output through a rectifier and filter circuit
  5. impedance measruement
  6. relation with hydration

Tetrapolar system:
skin electrode.png

Image: Cole-Cole equivalent electrical model of skin

For tetrapolar system, the sensitivity of the system depends on the distance between the electrodes.

Our region of interest is the level where we have the Subcutaneous Tissue.
Considering, Epidermis at .05 mm to 1.5 mm and Dermis from 0.3 mm to 3.0 mm -
Our target depth to reach at Subcutaneous tissue should be about 5mm.
So, distance between electrodes = 5mm * 3 = 15mm.

For my preliminary experiment, I have selected lower frequency [oscillation up to 10KHz] as it's easier to implement. I took several data and different frequencies.

Current is limited to 1mA using a constant current source [Howland] to maintain the current limit independent of the load resistance.


Table1: Measurement of skin contact impedance before Hydration;
where,Vin =2V p-p sine wave at a range of frequencies
R=3.26 KΩ


Table2: Measurement of skin contact impedance after Hydration;
where,Vin =2V p-p sine wave at a range of frequencies
R=3.26 KΩ

Figure: plotting experimental data


Figure: sample plot for comparison

The plot shows variation at lower frequencies. I have compared this plot with a paper published on "Epidermal Impedance Sensing Sheets for Precision Hydration Assessment and Spatial Mapping". Their plot for different frequencies at different hydration level shows the same shape of impedance variation.

current source and bioamp.jpg

Image: Constant Current Source and Bio-amp circuit

rectifier and filter.jpg

Image: Rectifier circuit

ADPD4100 for Impedance measurement:

ADPD4100 can be used for impedance measurement which can provide hydration or even respiration rate. Output depends on the focused area set by the distance of the electrodes and supplied current. If the electrodes are placed around lungs with three times the depth then we can have a representation of respiration.
lung resp.jpg

A combined ECG and Respiration Rate configuration is presented below -
[source: Multiparameter Vital Signs Monitoring Is Easier Than Ever Before by Electronics Maker November 21, 2020]
combined ECG respiration layout.jpg
combined respiration plot.jpg

I'll try out this configuration and compare the output of ADPD4100 in a separate blog.

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