A Self-Powered HMI That’s Also Waterproof

Using the giant magnetoelastic effect to generate electricity from ambient mechanical motion

3 min read
A hand covered in water droplets shows a square sheet with 4 circular sensors affixed to the top of the hand.

This magnetoelastic sensor array conforms to human skin and can function even when exposed to liquid. It can interact with a music speaker’s command components: play, pause, next, and previous.

Jun Chen Research Group/UCLA

Over the past few decades, human-machine interface (HMI) technologies, especially driven by the demands of wearables, have progressed rapidly—into smaller, lighter, more efficient, more robust forms. Today, we also have various self-powered HMIs, using piezoelectricity and triboelectricity, for example, for power generation. Despite these advances, most wearable HMIs still remain relatively defenseless to moisture, such as sweating and weather conditions.

To address this issue, a research team at the Samueli School of Engineering at the University of California, Los Angeles, have come up with a prototype of a waterproof, flexible, and low-cost skin-integrated HMI. The device comprises a magnetoelastic sensor array that converts ambient biomechanical motion into electrical signals. Their study has been published in Applied Physics Reviews.

The device is intrinsically waterproof, biocompatible, and able to convert the force induced by pressing a finger on it into a continuous electrical signal. The team demonstrated this by controlling a music player and turning an electric lamp on and off. They tested it in a variety of real-world conditions as well, including in a water spray, in heavy rain, and during vigorous physical activity.

The researchers, lead by Jun Chen of UCLA’s Wearable Bioelectronics Research Group, used what is known as the magnetoelastic effect (also known as inverse magnetorestriction), discovered in the 1860s, describing the variation in magnetization of a material under mechanical stress. In fact, the current study expands upon the group’s earlier work demonstrating the magnetoelastic effect in a soft polymer system.

The effect is traditionally observed in rigid metals and metal alloys, Chen says, but in 2021, the researchers observed the effect in his lab, in a soft system composed of magnets ranging in size from micro to nano and a polymer matrix. “[It showed] a measured magnetoelastic effect with a five times enhancement compared to that in metals and metal alloys.” As a result, they dubbed it the “giant magnetoelastic effect.” Speaking of their current work, he adds, “we harnessed this discovery to develop a fundamentally new and wearable HMI.”

The prototype was a 4-by-4-centimeter array comprising two components: a layer of nanomagnets in a porous silicone rubber matrix that converts biomechanical pressure (touch of a finger) into a magnetic response; and a magnetic induction layer of patterned liquid metal coils. The latter responds to the magnetic variations and generates electricity (electromagnetic induction).

“In this way, [our] device works in a self-powered manner,” Chen says. “Compared with conventional HMI devices, which are non-self-powered, our device will [result in] much less power consumption.”

Two fingers strecth the magnetoelastic HMI out.Jun Chen Research Group/UCLA

The fabrication processes involved in making magnetoelastic HMI devices are straightforward, Chen says. The research team used methods like 3D printing, the reverse-mold process, and laser patterning, but, he adds, costs can be further reduced by exploring more economical range of micromagnets to nanomagnets and polymer-matrix materials, or replacing the liquid metal coils layer with a fine-design solid conductive wire layer that is suitable for large-scale fabrication.

The material was flexible, elastic, and durable, generating stable power even when rolled, folded, and stretched. Additionally, the magnetic field was not particularly affected by the device being wet. It was unperturbed by a strain of up to 150 percent, and exhibited wide pressure sensitivity as well as a response time of 0.2 seconds at a 1-hertz frequency. These characteristics make it conducive to controlling electronic devices in real time. In their demonstration, the researchers integrated a circuit with buttons to operate a desk lamp and a music player.

Chen is interested in commercializing the technology. “Since the devices are soft, wearable, and biocompatible, [they] could be widely adopted for HMI applications,” he says. Among the possible uses are mechanosensitive electronics for electronic skins and soft actuators. He also foresees applications in virtual platforms. “Another appealing potential application is for body motion sensing, in which it can operate as a self-powered stretchable strain sensor, capturing the motion of limbs, muscles, and vital-sign signals for lifestyle, fitness, and health-related applications,” he adds.

Meanwhile, Chen notes, the fundamental science behind the technology needs more basic research so that scientists and engineers can develop a deeper understanding of its attributes and physical limits. He would also like to work on the voltage output: “Faraday’s law [says] the output voltage is linearly proportional to the number of the coils and the variation of total magnetic flux through the coil. Therefore, one possible direction [of research is] more advanced fabrication techniques to increase the magnetomechanical coupling factor and the number of coils.”

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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.