Confounding Factors and Their Mitigation in Measurements of Bioelectrical Impedance at the Skin Interface
Abstract
1. Overview of Physical Principles of Bioelectrical Impedance Techniques
1.1. Introduction
1.2. Basic Principles of Bioelectrical Impedance Techniques
1.2.1. Electric Impedance Spectroscopy (EIS)
1.2.2. Electric Impedance Tomography (EIT)
1.3. General Considerations About the Skin
1.4. Challenges in Bioelectrical Impedance Measurements
2. Confounding Factors in Bioelectrical Impedance Measurements at the Skin Interface
2.1. Overall Physiological and Psychological Condition of the Participants
2.1.1. Health Status of Participants
2.1.2. Relevant Medical History
2.1.3. Core Temperature
2.1.4. Body Mass Index (BMI)
2.1.5. Age
2.1.6. Sex
2.1.7. Menstrual Cycle
2.1.8. Ethnicity
2.1.9. General Hydration Status
2.1.10. Feeding (Prandial) Status
2.1.11. Medication or Substance Use
2.1.12. Sleeping Status
2.1.13. Fitness
2.1.14. Exercise
2.1.15. Emotional Status
2.2. Environmental Conditions
2.2.1. Ambient Temperature
2.2.2. Ambient Relative Humidity
2.2.3. Stability of Ambient Conditions
2.2.4. Accommodation to Laboratory Conditions
2.2.5. Posture and Postural Accommodation
2.2.6. Season
2.3. Time Constraints
2.3.1. Time of Day
2.3.2. Timing of the Measurement: Immediate or Delayed After Electrode Placement
2.3.3. Measurement Repetitions in Time
2.4. Skin Condition
2.4.1. Tattoos, Piercings
2.4.2. Items in Contact with Skin: Jewelry and Conductive Clothing Accessories
2.4.3. Items in Contact with Skin: Clothing, Belts, etc.
2.4.4. Local Health of the Skin
2.5. Skin Preparation
2.5.1. Topical Hygiene Products
2.5.2. Skin Hygiene
- Water-and-soap: effective, but has several downsides. Harsh surfactants can cause skin dryness and irritation mainly due to damage to skin proteins [123]. As a recommendation, less irritating, “mild” surfactants exist, designed to alleviate this problem [124,125]. Additional care should be paid to the pH of the soap mixtures with water, as strong alkaline ones (pH ~ 10) increase SC thickness (by swelling and increasing lipid rigidity), even in the absence of surfactants [123]. Therefore, recommended soaps should have a neutral pH (~7) or close to the pH of SC (~5.5). Some soaps also have moisturizing agents (see above). Another downside is that it exposes the skin to water, which transiently increases the water content of the superficial layer (increases conductivity) or might leave a thin water film on the surface. Tap water might contain variable levels of oxidizers or reduced residues from water management (ozone, chlorine, potassium permanganate, polyphosphates, etc.) that might alter redox equilibrium at the electrode interface [126]. Countermeasures: (i) rinse effectively (maybe with normal saline instead of water); (ii) dry effectively (and include a timed interval between the skin preparation and electrode attachment).
- Rubbing alcohol (either ethanol or isopropyl alcohol in various concentrations): It cannot effectively remove proteinaceous residues that might clog sweat ducts, as proteins denature in concentrated alcohol solutions. Also, it removes skin oils in the hair follicles, which will slightly reduce the impedance temporarily [4] and might cause irritations. Therefore, cleaning with alcohol should be avoided if possible.
- Gel-based hygienic agents: These off-the-shelf products can contain various thickening agents that can attach to the SC as a thin additional layer; we recommend avoiding them.
- Single use cleaning wipes: These can be effective but check for the presence of moisturizing agents and pH; abrasive wipes are commercially available for slightly abrading the SC.
- Sterile woven cotton gauze (surgical cotton cloth) moistened in normal saline. When rubbed against the skin, this appears to be a safe option that we recommend, with the following observations. The texture appears to be effective for gentle, non-traumatic exfoliation, and the absorbent fibers of the cotton mop up oils and debris. We advise using normal saline (sterile 0.9% sodium chloride solution) and not water to further minimize osmotic water flux through microscopic cracks in the skin. The number of rubs per electrode location should be standardized in the research protocol (i.e., “each location was rubbed four times before with a woven cotton gauze moistened in normal saline”). For each electrode location, use a new cotton pad (to prevent cross-transfer of debris between locations).
2.5.3. Stratum Corneum: Modifying or Not
- For stripping, the most used method seems to be the use of adhesive tape (taped to the skin, lifted rapidly), several times (one to four times most common). Some authors specify the type of tape used (generic plastic based/cellulose based), others do not; also, there are commercially available tapes specifically marketed for the purpose of stripping the SC. Other authors used other stripping methods (for instance, fine-grained sandpaper rubbed on the skin). Mechanical abrasion seems to be the most effective method to reduce impedance [127].
- Other authors prefer not to use stripping, for the following reasons: because it is unpleasant, cannot be applied on hairy skin easily, reduces participation, modifies the natural state of the skin, severe stripping (~20 times) denudes completely the skin protection and increases the risk of infections. Accordingly, lack of stripping will induce a variability between subjects. We want to note that even if the study authors did not use stripping, it might be done inadvertently by the participants, if they, in their hygienic habits at home, used cosmetic exfoliating soaps or gels or hard sponges or brushes (that can have a stripping effect). If the choice of the researcher is to avoid stripping, the participants should perhaps be instructed to refrain from using exfoliating cosmetics for the duration of the study (especially if the study design has repeated measures in time). This could be an easy to miss confounding factor in longer duration studies.
- Another mechanical abrasion recommended by some researchers involves the use of an abrasive electrolytic gel, rubbed on the skin [95]; the application should be carefully performed, not to spill the gel into adjacent areas (it could create an electrical shorting between neighboring electrodes).
- Other modifications could be chemical exfoliation with different agents or microperforation with microneedles [127], but these appear to be less effective than stripping.
- “Penetration enhancers” are substances that increase the permeability of the skin for certain species. Surfactants (like sodium lauryl sulfate, commonly used in shampoos, soaps, etc), solvents (like dimethylsulphoxide), esters of organic acids, and aromatic compounds seem effective in reducing the parallel resistance of SC [95], but their use is severely restricted by their toxicity, or risks of irritations and allergies.
2.5.4. Shaving
2.6. Electrodes
2.6.1. “Wet” Electrodes
- Advantages: The presence of the liquid/gel media increases the effective surface area by eliminating random tiny air pockets, resulting in a stabler impedance signal over time. The initial electrical contact is usually stable and reliable. The electrodes based on adhesive gels are very easy to apply and are used extensively in hospitals in electrocardiography (ECG) recordings; their ubiquity made them a popular choice for many impedance research projects.
- Disadvantages: The presence of a gel/conductive saline solution moistens the skin, thus the skin impedance is lowered. This process is not stable because over time the conductive liquid can infiltrate between the tiny cracks in the stratum corneum, making spurious conductive bridges with the underlying layer. Also, the outer layers of the gel can dry, modifying conductance; thus, the gel–skin interface can actually generate an unwanted noise over longer recording [80]. Most of these electrodes are single use; some participants might be allergic to a particular gel formulation.
2.6.2. “Dry” and Other Contact Electrodes
- Advantages: Most types are designed to overcome the problems of wet electrodes (easy to use, reusable, comfortable) and can be used easily in any location of the body, but they come with their own challenges. Flexible contact electrodes have the advantage of high conformability to skin shape (even on hairy regions) and if they are elastic are able to maintain the interface even in motion (like breathing or exercising) [138]. In general, because of the lack of a contact liquid or gel, the absolute impedance values recorded are generally higher than those recorded with wet electrodes. Newer conformal electrodes appear to maintain stable electrical contact for longer durations in EIT applications [141].
- Disadvantages: Even if they are not “wet” in a chemical sense, an electrical double layer forms in time at the contact with the skin (from natural sweat/oils that the underlying skin might secrete during the usage). This results in a variable contact impedance that “drifts” over time from the initial value [136]. There are attempts to minimize this effect via porous mesh-like electrodes [142]. Dry contact electrodes (especially solid ones) are highly sensitive to motion artifacts [143]. The motion artifacts have a large variation between subjects and appear with overt motion and also with involuntary movements (like breathing) [22].
2.6.3. “Non-Contact” Electrodes
2.6.4. “Inserted” Electrodes
- Advantages: They avoid the variability of the contact between the applied electrode and the skin surface, discussed above. They appear to have a better signal-to-noise ratio than the rest of the electrodes [147].
- Disadvantages: (i) The geometrical arrangement of the needle electrode surface influences the recordings: at lower frequencies of the electrical signal, they have a high, significant electrode polarization impedance; the impedance of the surrounding tissue was thus best recorded at higher frequencies above 10 kHz [148]. (ii) As an additional source of error, most of these electrodes have to be “activated” before measurement with different procedures (typically by dipping them in saline solution with wetting agents or on-site electrolytical treatments). This allows a stabilization of the electrode surface area, reducing the variation of electrode impedance observed during the in vivo recording. This phase must be followed carefully before the measurement; the results (from the same electrode) are markedly different with or without this pre-treatment [148]. (iii) Needle insertion is a medical procedure that breaks the integrity of the skin barrier, and proper sanitary precautions must be followed to avoid infections or medical complications. It might be painful and less likely to be accepted by the participants/patients than alternatives. A promising solution to these problems appears to be “microneedles” arranged in an array that can penetrate the nonconducting stratum corneum of the skin without pain and can even be arranged in multichannel setups [149].
2.6.5. Electrode Surface Area Considerations
- “Small” electrode area. Advantage: It leads to higher density current lines below the electrode, which has a higher chance to intercept the interest area (for instance, an unknown low impedance anatomical structure that was to be investigated in an impedance spectroscopy study or impedance tomography study). Disadvantage: Higher current density means a higher chance for the sudden appearance of burned-through low impedance paths in the epidermal layer (and thus a false low impedance reading) [99].
- “Larger” electrode area. Advantage: It is easier to use and attach. Lower current density beneath the electrode reduces the chances that the tissue is adversely affected by the measurement, but also lowers the signal-to-noise ratio. Also, a larger perimeter of the electrode is a source of other confounding factors (noise at the electrode–skin interface [150] and, especially in the case of wet electrodes, a greater surface area of evaporation, so the gel will desiccate faster).Newer generations of “compound electrodes” [151,152] that have a large outer electrode area to inject current and a smaller inner area to sense voltage are actively designed to work around the above described problems. “Active electrodes” are a newer generation of electrodes that incorporate parts of the controlling electronics for better signal-to-noise ratio [153,154].
2.7. Electrode Placement
2.7.1. Pressure on Electrodes
2.7.2. Electrode Location
2.8. General Problems Related to Hardware
2.8.1. Wires
2.8.2. Equipment Location
2.9. Sources of Bias in Human Trials
2.10. Data Modeling
3. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
AC | Alternating Current |
BIA | Bioelectric Impedance Analysis |
BMI | Body Mass Index |
EIS | Electric Impedance Spectroscopy |
EIT | Electric Impedance Tomography |
GAM | General Additive Modeling |
GLMM | General Linear Mixed Models |
LED | Light Emitting Diodes |
PCA | Principal Component Analysis |
SC | Stratum Corneum |
UPS | Uninterruptible Power Supply |
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Confounding Factor | Accounted for and Mitigated in the Experimental Design? | |||
---|---|---|---|---|
Yes | No | Partially | N/A | |
Overall physiological and psychological condition of the participants | ||||
Health status of participants | □ | □ | □ | □ |
Relevant medical history | □ | □ | □ | □ |
Core temperature | □ | □ | □ | □ |
Body mass index (BMI) | □ | □ | □ | □ |
Age | □ | □ | □ | □ |
Sex | □ | □ | □ | □ |
Menstrual cycle | □ | □ | □ | □ |
Ethnicity | □ | □ | □ | □ |
General hydration status | □ | □ | □ | □ |
Feeding (prandial) status | □ | □ | □ | □ |
Medication or substance use | □ | □ | □ | □ |
Sleeping status | □ | □ | □ | □ |
Fitness | □ | □ | □ | □ |
Exercise | □ | □ | □ | □ |
Emotional status | □ | □ | □ | □ |
Environmental conditions | ||||
Ambient temperature | □ | □ | □ | □ |
Ambient relative humidity | □ | □ | □ | □ |
Stability of ambient conditions | □ | □ | □ | □ |
Accommodation to laboratory conditions | □ | □ | □ | □ |
Posture and Postural accommodation | □ | □ | □ | □ |
Season | □ | □ | □ | □ |
Time constraints | ||||
Time of day | □ | □ | □ | □ |
Timing of the measurement: immediate or delayed after electrode placement | □ | □ | □ | □ |
Measurement repetitions in time | □ | □ | □ | □ |
Skin condition | ||||
Tattoos, piercings | □ | □ | □ | □ |
Items in contact with skin: jewelry and conductive clothing accessories | □ | □ | □ | □ |
Items in contact with skin: clothing, belts, etc. | □ | □ | □ | □ |
Local health of the skin | □ | □ | □ | □ |
Skin preparation | ||||
Topical hygiene products | □ | □ | □ | □ |
Skin hygiene | □ | □ | □ | □ |
Stratum corneum: modifying or not | □ | □ | □ | □ |
Shaving | □ | □ | □ | □ |
Electrodes | ||||
Electrode type | □ | □ | □ | □ |
Electrode surface area considerations | □ | □ | □ | □ |
Electrode placement | ||||
Pressure on electrodes | □ | □ | □ | □ |
Electrode location | □ | □ | □ | □ |
General problems related to hardware | ||||
Wires | □ | □ | □ | □ |
Equipment location | □ | □ | □ | □ |
Sources of bias in human trials | ||||
Check for potential biases | □ | □ | □ | □ |
Data modeling | ||||
Check for appropriate modeling | □ | □ | □ | □ |
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Iftime, A.; Scheau, C.; Babeș, R.-M.; Ionescu, D.; Periferakis, A.; Călinescu, O. Confounding Factors and Their Mitigation in Measurements of Bioelectrical Impedance at the Skin Interface. Bioengineering 2025, 12, 926. https://doi.org/10.3390/bioengineering12090926
Iftime A, Scheau C, Babeș R-M, Ionescu D, Periferakis A, Călinescu O. Confounding Factors and Their Mitigation in Measurements of Bioelectrical Impedance at the Skin Interface. Bioengineering. 2025; 12(9):926. https://doi.org/10.3390/bioengineering12090926
Chicago/Turabian StyleIftime, Adrian, Cristian Scheau, Ramona-Madalina Babeș, Diana Ionescu, Argyrios Periferakis, and Octavian Călinescu. 2025. "Confounding Factors and Their Mitigation in Measurements of Bioelectrical Impedance at the Skin Interface" Bioengineering 12, no. 9: 926. https://doi.org/10.3390/bioengineering12090926
APA StyleIftime, A., Scheau, C., Babeș, R.-M., Ionescu, D., Periferakis, A., & Călinescu, O. (2025). Confounding Factors and Their Mitigation in Measurements of Bioelectrical Impedance at the Skin Interface. Bioengineering, 12(9), 926. https://doi.org/10.3390/bioengineering12090926