Using Soil Moisture Sensors on Humans?
Dr. Stuart Campbell, professor of Biomedical Engineering at Yale University has been toying with the idea of using soil moisture sensors to measure tissue edema in human subjects.
He says he got the idea from Dr. Ken Campbell, former professor of Bioengineering at Washington State University: “I was explaining to Dr. Campbell about the sensors METER makes, and he pointed out that there are many diseases where you might want to measure someone’s tissue edema, and it would be interesting to see if you could use a soil sensor in a wearable device to help doctors monitor swelling in their patients, much like a heart monitor monitors heart activity.”
Tissue edema occurs when too much fluid leaks from your capillaries into your tissue. Capillaries, the smallest blood vessels in your body, are somewhat leaky, allowing the exchange of nutrients and waste between the tissue and the blood. The fluid that surrounds the blood cells is free to exchange across the capillaries, and edema will occur when too much fluid leaks out of the circulatory system into the tissue. Edema can be caused by things like heart disease, pregnancy, or standing on your feet all day.
What Makes the Fluid Leak?
In soil, water moves from high water potential to low water potential. Similarly, there are forces inside the circulatory system that cause the transfer of fluid between capillaries. Your blood vessels have a certain amount of pressure that is generated by your heart. If your blood pressure goes up, it can cause edema. Dr. Stuart Campbell says, “The actual fluid pressure is part of what decides how much fluid is pushed out, but it’s not that simple. Your blood has large proteins that are too big to get out of the capillary. That means the more water that leaves the capillary and moves into the tissue, the more concentrated those proteins become, which lowers the water potential (or osmotic potential) of the blood. This delicate balance is what prevents too much water from leaking out. However, if you have a disease that tips this balance, either through high blood pressure or a condition that allows those proteins to leak out of the capillary, edema would occur because you don’t have the osmotic potential pulling the water back into the capillary and keeping the proper balance.”
The Heart Experiment:
Dr. Campbell decided to see if a soil sensor would work to measure animal tissue when he was working as a summer student in the Visible Heart Lab at the University of Minnesota. Campbell says, “Similar to a human heart transplant, this lab is able to keep pig hearts alive outside the body. The problem, however, is that they use a manmade solution instead of blood, and that imitation blood is not ideal. If the composition of the fluid is not perfectly adjusted, you can have problems with your experiments. I thought it would be interesting to figure out if we could monitor the edema of the heart tissue during one of the procedures. I hooked up the soil probe and used it in one experiment where I put it in contact with the heart while it was beating. There was, in fact, a change in output of that signal during the experiment. But, because I only got one chance at it, it was inconclusive as to whether this was indicative of an imbalance in the composition of our artificial blood substitute.”
An Anecdotal Experiment:
Still curious to see if the idea would work, Dr. Campbell decided to try one more experiment: this time on his wife who was experiencing edema symptoms after childbirth. He says, “It occurred to me that this was an opportunity to try out the soil moisture probe one more time to measure tissue edema. So each day, I would measure her ankles, putting the probe in flat contact with her skin while tightening a strap gently.” Dr. Campbell says he watched the swelling go down as the numbers on the probe got smaller, and comments, “It was anecdotal evidence that at least in extreme cases, you might be able to get the soil probe to work. But I still have questions, such as, how would you make sure that the probe was always touching the skin in the same way? And, if the person got sweaty, would that change the soil probe reading?”
Why the Experiments Should Continue:
Though Dr. Campbell hasn’t had time to pursue the experiment further, he feels that if the idea works, it has the potential to improve lives and save our nation billions of dollars. He says, “There are millions of people in this country who have heart failure. Maybe they’ve had a blockage in one of their coronary arteries, or perhaps their heart is worn out because of age. You can tell when someone is in heart failure because when they lie down to go to sleep at night, all that fluid makes its way slowly from the ankles, through the legs, the torso, and eventually into the chest. The problem is that the lungs are very delicate, and when you have edema in the lungs, it’s almost like you have pneumonia. This type of sensor could be an easy way for people to monitor themselves and manage their fluid intake and diet after they get home from the hospital.” Dr. Campbell says this helps the economy because if people don’t manage their fluids, they have to return to the hospital so they can be supervised to eat correctly and regain the proper fluid balance. This ends up costing the economy billions of dollars unnecessarily. He concludes, “Perhaps people just need to follow instructions, but it’s possible with better monitoring that the situation can be improved.”
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