Minimally Invasive Wearable Patch Device for Measurement of Interstitial Fluid Content
Authors: Kubota Jun , Nagamine Kuniaki , Kai Hiroyuki , Kaji Hirokazu , Nishizawa Matsuhiko 
Published: February 2020

Introduction
Edema, which is the medical term of swelling, is a condition in which excess interstitial fluid derived from a capillary lumen accumulates in a subcutaneous tissue. For example, edema can be caused daily by the effect of gravity mainly in lower limb of a healthy person during prolonged sitting on a chair. Such daily edema often accompany pain and languor, resulting in reducing quality of life. On the other hand, there is serious edema attributed from circulatory diseases such as cardiac arrest, myocardial infraction, expansion type cardiomyopathy, arteriosclerosis et al. The position where edema develops varies depending on the kind of primary disease. Conventionally, edema is diagnosed by non-quantitative technique such as direct observation of appearance or confirmation of an indent in the swollen skin after following finger pressure. Therefore, it is necessary to develop the devices which enable quantitative evaluation of the degree of edema in the local site of body not only for monitoring daily edema of healthy people, but also diagnosis of the patients with circulatory diseases and monitoring the effect of therapeutic treatment of the diseases. The electrical technique to apply an electric current to the living body through the skin is effective to quantify physiological information in a non- or low-invasive manner. In this research, we examined the relationship between interstitial water content and its electric resistance by applying the electric current to a self-made human body model.
Experiments
Surface structure of a human body is composed of epidermal layer which is outermost layer of the skin, dermic layer, and subcutaneous tissue. Edema is caused by accumulation of the excess interstitial fluid in dermic layer and subcutaneous tissue that are under the epidermis layer. In this research, we constructed a human body model by laminating a porcine-derived epidermal layer on a surface of a gelatin hydrogel which mimics a dermic layer. The gelatin hydrogel was prepared as follows. The gelatin powder was dissolved in a heated 1 wt% saline solution. The gelatin solution was poured into a mold and cooled to room temperature to facilitate gelation of the solution. Two kinds of gelatin hydrogels with different water content were prepared. The one included 70 wt% of water in the gel to simulate the dermic layer of healthy person, and the other was 80 wt% to simulate extreme swelling condition. Then, the electric current was applied to this model to examine the relationship between the water content of gelatin hydrogel and its electric resistance.
Result and discussion
Relationship between the water content of the gelatin hydrogel and its electric resistance is shown in Figure 1. Horizontal axis of Figure 1 represents the water content of gelatin hydrogel, and vertical axis the ratio of change in electric resistance in which 1 means the standardized electric resistance of the gel with 70 wt% of water content. From this result, we found that the electric resistance decreased with increase in water content of the gelatin hydrogel. From this, it was suggested that there is possibility to determine the degree of swelling in the body from the change of electric resistance by applying electric current to the body.

Figure 1