The February 2023 issue of IEEE Spectrum is here!

Close bar

How Your Smart Phone Can See You Sweat

With this thin microfluidic patch and an app, you’ll know if you’re staying hydrated

3 min read
A man holding a smartphone scans his Gx Sweat Patch.
Photo: PepsiCo

Into this stay-at-home era of DIY personal training comes the first sweat-monitoring patch intended for broad consumer use. Rolling out this week from PepsiCo is the Gx Sweat Patch, developed by startup Epicore Biosystems in partnership with PepsiCo subsidiary Gatorade. It measures the rate of perspiration and the sodium chloride concentration in that sweat.

The aim? To track sweat loss during physical activity and heat stress and use that information on a personalized basis to recommend exactly how much and how often the athlete should drink to properly replace fluids and electrolytes to avoid dehydration and impaired performance. In the future, its developers predict, information from the patch will be used to help athletes determine their optimal diet and sleep patterns.

Gx Sweat PatchPhoto: PepsiCo

The patch will go on sale today, in sporting goods stores and online, at a suggested price of US $24.99 for a pack of two. It represents Gatorade’s first move into the world of digital products and apps for athletes.

Epicore has been testing its flexible, stretchable, single use patch on athletes for some time. The device routes sweat through microfluidic channels cut into stacks of thin-film polymers. In one of the microchannels, used to track sweat rate and volume, the excreted sweat is dyed orange to make it visible as it moves through the pathways. In the other, chemical reagents react with the chloride in the sweat and turn it purple, with the intensity of the purple color corresponding to the concentration of the chlorine ions detected.

Close up of the Gx Sweat PatchPhoto: PepsiCo

After wearing the device on the inner left forearm for the duration of a workout, the user scans the patch with a smartphone. Then the Gx app uses that image, along with previously input data like weight, sex, workout type, and the environment, to create what the company calls a “sweat profile” and make recommendations about the individual’s fluid intake.

In recent clinical trials involving 60 players in the National Basketball Association’s G-League, PepsiCo and Epicore tested the patch, worn on the left forearm of each athlete, against an absorbent pad worn on the athlete’s right forearm. The researchers compared the snapshot reading from the patch against laboratory tests conducted on the sweat collected by the pad, demonstrating comparable results.

Previously, the Gatorade Sports Science Institute and Epicore published the results of a similar large scale trial of the patch on more than 300 bicyclists, track and field athletes, and others exercising in real world conditions, outside of the laboratory environments.

Microfluidic technologies have been used in many applications including DNA chips, point of care diagnostics, and inkjet printing. These devices tend to be rigid, and not suitable for use in a wearable.

“The Gx Sweat patch is the first soft, conformal, and skin-interfaced microfluidic patch that has entered the consumer health and fitness arena,” says Epicore co-Founder and CEO Roozbeh Ghaffari.

After getting a sample patch from PepsiCo this past weekend, I can attest that it is indeed comfortable to wear. It feels like the kind of sticker store clerks sometimes hand out to children, and, a few minutes after attaching it to my skin, I no longer felt it. The accompanying app was simple enough to use, and the scan feature found and photographed the patch almost as soon as I got my arm into the camera’s view. I took a long, brisk walk, hoping I would work up enough of a sweat to get a reading, however, the cool February weather worked against me, and I literally came up dry. I’ll try it again on a warmer day, or when I can get back into a gym post-pandemic.

The Conversation (0)

The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.