Wearable Inductive Damping Sensors for Skin Edema Quantification

Citation

Chou, Tzu-Chieh (2022) Wearable Inductive Damping Sensors for Skin Edema Quantification. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/q77f-me73. https://resolver.caltech.edu/CaltechTHESIS:01152020-143207091

Abstract

The electrical conductivity of human organs is closely related to the physiological or pathological changes occurring within the organ. For example, metastatic liver tumors significantly increase electrical conductivity compared to healthy liver tissues over a wide frequency range. Therefore, knowing when and where these conductivity changes happen within an organ is highly valuable for disease monitoring.

Skin is the largest human organ by surface area, and under its large surface, there are numerous tiny blood and lymphatic vessels that circulate body fluid and dissipate heat. Therefore, it contains critical information about systemic circulation. Diseases such as congestive heart failure, acute renal injury, and liver failure disturb the systemic circulation and allow extra interstitial fluid to accumulate in the form of peripheral skin edema. As the interstitial fluid is highly conductive, the overall skin conductivity significantly increases when edema occurs.

Consequently, quantification of skin edema allows us to track the progression of these diseases and is the main goal to pursue in this study. The current clinical standard uses a 0-to-4 grade system to quantify the severity of edema based on how the skin responds to a pressing force. However, it requires in-person examination and has relatively large inter-examiner variations, making it less suitable for real-time edema monitoring.

To solve the unmet need to quantify edema in real-time, I present a skin edema model that relates skin conductivity to the interstitial fluid volume fraction. The latter is used to quantify the severity of edema. Furthermore, I developed a wearable coil sensor that provides accurate real-time conductivity measurements on subcutis, a significant portion of the skin where edema typically occurs. The coil sensor uses alternating magnetic fields to induce eddy currents in the skin and measures the skin conductivity as a function of coil resistance change. The experimental results suggested that when grade-1 edema occurs, the subcutis conductivity increases from the average value of 0.09 S/m to 0.25 S/m. This change corresponds to an increase of interstitial volume fraction from 10% to 20% in the subcutis. These quantitative results are consistent with finite element simulations and allow direct comparison with ultrasonography measurements. Due to its high accuracy and portability, the proposed wearable sensor opens a new possibility for continuous monitoring of skin edema.

Thesis Files