Recall those early days when the family physician was called home to treat a patient. The first thing he’d do was to touch the skin on your face, temples and chest. This would let him diagnose quickly. If the skin feels hotter than usual, you have fever; if it is paler than the usual, you are dehydrated and must drink more water; if it has a bluish tinge, you need to breathe more oxygen; and if it feels wet, you need to exercise less or cut your physical stress and so on. Then you are given what he considers the appropriate medicine as pills or potion, or an injection. Alas, we have now replaced him with a doctor sitting in a clinic, who asks you to go to a commercial centre for diagnostics and prescribes the medicine based on the report. Skin-based diagnosis is a gone thing for general practitioners.
These days, skin specialists do an interesting procedure, in which they attach a thin polymer-based sheet which contains the desired drug, stick it to the skin on your arm or chest and deliver the drug past the sweat fluid directly into the body, using a tiny electric current on the patch. This is thus a wearable technology for personalised medicine — no pills or potions. And with the advent of microelectronics and bio-compatible polymers, we now have ‘electronic skin’ (e-skin), and nanoscale wires that can be attached and an external electric power supply using micro-scale batteries.
Role of sweat
Notice in all this, the active body fluid, namely, the sweat, is ignored or treated as an inert carrier of no other value. The role of sweat fluid in our body and the chemicals it contains are becoming increasingly understood and utilised only recently. Sweat comes out of three types of glands distributed across all over our skin, secreting water and substances that help keep our body at the optimum temperature of 37 degrees C (or 98.4 degrees F). Our brain has temperature-sensitive nerve cells (neurons) which control the sweat glands in releasing the fluid depending on the temperature and physical and metabolic activity of the body. Sweat is thus our body’s thermo-regulator.
What does sweat contain? It is 99% water containing sodium, potassium, calcium, magnesium and chloride ions, ammonium ions, urea, lactic acid, glucose and other minor components. An analysis of the sweat fluid in a patient and how it compares with that of a ‘normal’ individual will thus be of diagnostic value (just as much as other body fluids do). For example, in the illness called cystic fibrosis, the ratio of the sodium to chloride ions in the sweat is different from that of a normal individual. Likewise, the amount of glucose in the sweat of a diabetic is higher than normal. But the problem is the amount of sweat available from the skin.
Diagnosis based on e-skin
It is here that modern-day technology has become of value. Now that microelectronics and e-skin patches are both available, scientists have been using them for real-time measurements of some chosen component in the sweat, using the appropriate probe (sensor) in the patch in order to detect and measure the level of the component. But would it not be much better if we can measure as many components as we desire if the e-skin patch be loaded with probes not for one but several components simultaneously? A breakthrough on this was made by a group of biologists, material scientists, computer experts and electrical engineers from California (Stanford and Berkeley), and was published with the title: “Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis” ( Nature 2016 January 28; 529(7587): 509-514. doi:10.1038/nature16521. free access on the web).They put in not just one but six sensor probes — for Na, K, Cl ions, lactate, glucose and the temperature of the sweat — all six of them embedded on a e-skin patch, such that a stable sensor-skin contact is maintained. Signals coming from each sensor measuring the sweat component as a tiny electrical signal are then converted into a digital form, and sent to a micro-controller, and from there to a Bluetooth transceiver, which can be seen on a mobile phone or other screen, and passed on through SMS, email, or uploaded to the Cloud interface.
The Californian Group followed it up with another paper in 2017 titled: “Autonomous sweat extraction and analysis applied to cystic fibrosis and glucose monitoring using a fully integrated wearable platform” in Proceedings of the National Academy of Science, 114(18), 4525-4630; https://www.pnas.org/content/114/18/4625. Since the amount of sweat accessible in sedentary people is too low, the group resorted to what is called iontophoresis, wherein one can stimulate local secretion of sweat at chosen sites, thus getting enough of the fluid, analysing its relevant components in normal (control) individuals, and people with cystic fibrosis and also monitoring glucose levels in the sweat — all this in a similar integrated platform as was used in their Nature paper. In a control individual, they found 26.7 mM of Na ions and 21.2 mM of Cl ions (note that the Na level is higher than Cl level here), while in a CF patient, Na level was 2.3 mM and Cl level 95.7 mM (far higher than the Na level), in keeping with what CF specialists have found in their routine (“classical”) practice. The group further found that oral glucose consumption while fasting led to increased glucose levels in sweat and in blood.
Sweat as power supply
Note that in all these assays, the probes and sensors need to be powered externally using microbatteries. If these e-skin platforms are to be used in robotics and other devices, can we do away with this external, and have the material in the sweat itself be used as a biofuel generator of electric power? The group showed in their paper that just appeared 10 days ago in the journal Science Robotics (Yu et al., Sci. Robot. 5, eaaz7946(2020)). On a patch on the individual’s e-skin patch they added the enzyme Lox which would react with the lactate in the sweat and oxidise it to pyruvate in a bioanode, and reduce the oxygen into water in a biocathode, thus generating electrical energy that is sufficient to drive the patch with no external energy source — what a brilliant idea!
Finally, in these COVID-19 days, it is good to know that sweat does not carry any pathogen (bacteria or virus); on the contrary it carries a germ-killer protein called dermcidin (Schittek et al, BMJ 2001, 323(7323);1206). One wonders if dermcidin or its modification can be anti-viral.
dbala@lvpei.org
